1

New Departures in Structural Geology and

Tectonics
A White Paper resulting from a workshop held at Denver Colorado,
September 22nd and 23rd, 2002 sponsored by the Tectonics Program, Earth
Sciences Division, and National Science Foundation (GEO/EAR).

April 2003
Website and downloadable pdf file available at:
http://www.pangea.stanford.edu/~dpollard/NSF/

Executive Summary
Recognition of exciting opportunities for research in structural geology and tectonics
(SG&T), brought into focus by recent technological developments, new quantitative data
sets, and both conceptual and theoretical advances, motivated a workshop to consider the
future directions (New Departures) for science in this field over the next ten years. Four
topical areas of research were identified at the workshop:
1) Beyond Plate Tectonics: Rheology and Orogenesis of the Continents;
2) The Missing Link: From Earthquakes to Orogenesis;
3) Dynamic Interactions between Tectonics, Climate, and Earth Surface Processes;
4) Co-evolution of Earth and Life.
Descriptions of these research themes along with sections on research facilities and
education form the major sections of this white paper. A major theme of this document is
the integrated nature of research and teaching in SG&T.
Research funded by the NSF in this area has contributed to our fundamental
understanding of how the lithosphere works, helped to build the scientific workforce of
the nation, fostered international collaborations, and spun off knowledge that has had a
direct economic benefit to the nation. Researchers in structural geology and tectonics
have integrated field-based analyses, laboratory work on rock deformation, fluid-rock
interactions, metamorphic reactions, continuum and fracture mechanics, geochemistry,
geochronology, isotope geochemistry, and a variety of other disciplines, across vast
ranges of time and space to develop new ways to examine and understand our planet.
However, total funding and the number of awards in SG&T has remained essentially
level for the last 17 years while collaborative proposals have increased significantly. The
new research opportunities and strong basis for collaborative research on interdisciplinary
problems argue for increased levels of support for the SG&T community.
Plate tectonics emerged as a unifying theory of the solid Earth from observations of
the ocean floors where deformation, seismicity, and volcanism are localized at
boundaries between rigid plates. On the other hand, more pervasive deformation and
wide continental mountain belts indicate weak rheological behavior typifies continental
orogenesis. Thus we must move beyond plate tectonics and seek significant
improvement to our understanding of continental tectonics through a detailed and
comprehensive study of the rheology of continental crust and mantle rocks. Deformation
of the Earth's crust is unevenly distributed in space and time: a single earthquake may
produce substantial deformation and damage in just tens of seconds while tectonic events
such as mountain building occur over millions to tens of millions of years. Deformation
appears very different from these two perspectives, and the temporal and conceptual
missing link between them is one of the most fertile areas for future research.
Topography represents the net product of tectonic and surficial processes, and unraveling
the intricacies of this coupled system represents a primary challenge in this field, with the
opportunity to gain new insights concerning tectonic processes operating within the
Earth. Tectonicists are poised to help usher in new paradigms for understanding the
motions and deformation of continents in deep time. This will require forging a new
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understanding of pre-Pangean supercontinents and the supercontinent cycle and will

lead to newly realized connections between Earth processes and the evolution of life.
The support for research facilities plays a major role in the path toward scientific
discovery outlined about. Cosmogenic isotope studies provide important constraints on
tectonic processes that have affected the Earth's surface during the past several hundred
thousand years, a time range that has previously been difficult to study. Geochronology
and thermochronology provide critical constraints on the ages of geological events and on
the rates of geological processes. The fundamental understanding of dynamic processes
as diverse as the earthquake cycle, fluid transport through the crust, and sedimentary
basin development hinges upon critical input from laboratory measurements of
mechanical and transport properties of rocks. There is an urgent need to support the
infrastructure that would nurture the necessary technological advances, provide the wider
availability of existing experimental and analytical facilities, and encourage synergistic
collaborations among researchers in rock mechanics, field geology, numerical simulation
and materials science.
The Global Positioning System (GPS) not only provides a vital tool for detailed,
precise and quantitative mapping of geological structures, the products of tectonic
processes operating in Earths crust. Furthermore, geodetic grade GPS permits us to
measure the relative velocity of any two points on the earth's surface to within a fraction
of a millimeter per year, after just 2 to 3 years of monitoring. Similarly, interferometric
synthetic aperture radar (InSAR) allows us to create maps showing the movement of
broad areas of Earths surface over time (surface velocity fields). Combined, these two
methods will yield seamless, high spatial- and temporal-resolution maps of surface
movement across plate boundaries and throughout the continental interiors. There is an
urgent need to support instrumentation for both structural mapping and geodetic studies
of crustal deformation.
Educational priorities in SG&T parallel those of the broader Earth Science
community, as articulated in a rich literature of ongoing publications, web sites and
workshops. Fundamental scientific literacy, rigorous scientific methodology, and the
development of quantitative skills at all levels, from K-12 through undergraduate and
graduate education, underlie these priorities. There is a need for integrating quantitative
methods into geoscience classes, interactive exercises, involvement of students in
research, and computer visualization tools as viable strategies for increasing the
effectiveness of earth science education. Geoscience education based on such a
foundation will play an important role in preparing the next generation of researchers,
teachers, and citizens. We encourage college and university earth science departments to
be flexible in defining programs of study that facilitate collaborative and interdisciplinary
work. Tuition and credit should be given for course work in mathematics, physics and
chemistry, as well as related engineering disciplines. For the development of quantitative
skills at the graduate level we point to the role of differential geometry in characterizing
geological structures, and cite continuum mechanics as an example of the fundamental
scientific literacy necessary for mechanical modeling in structural geology and tectonics.

Table of Contents
New Departures in Structural Geology and Tectonics ....................................................... 2
Executive Summary..................................................................................................... 3
Table of Contents......................................................................................................... 5
Participants .................................................................................................................. 7
Preface ......................................................................................................................... 8
Priorities in Solid Earth Sciences: A Related Workshop ..................................... 9
Research in Structural Geology and Tectonics at NSF ...................................... 10
Beyond Plate Tectonics: Rheology and Orogenesis of the Continents ..................... 13
Introduction ........................................................................................................ 13
Research Questions and Opportunities............................................................... 16
Studies of Transitional Brittle-Ductile Deformation and Strain Localization.... 19
Coupled Mechanical-Hydrologic Systems ......................................................... 20
Integrated Studies of Material-Specific, Mechanism-based Rheologies............ 20
Formulation and Evaluation of Rate and State Constitutive Relations that
Describe Transient and Steady-State Behavior .................................................. 20
Fabric Development and Anisotropy at Large Shear Strains ............................. 21
Chemical Weakening and Hardening................................................................. 21
The Missing Link: From Earthquakes to Orogenesis ................................................ 22
Introduction ........................................................................................................ 22
Research Questions ............................................................................................ 24
Relating Elastic and Permanent Deformation .................................................... 24
Spatial and Temporal Distributions of Earthquakes........................................... 26
Localization of Deformation .............................................................................. 26
The Role of Fluids .............................................................................................. 27
Fault Zone Processes and Materials ................................................................... 27
Research Opportunities ...................................................................................... 28
Dynamic Interactions Between Tectonics, Climate and Earth Surface Processes .... 32
Introduction ........................................................................................................ 32
Research Questions and Opportunities............................................................... 33
Coupled Surface Process and Tectonic Models ................................................. 34
Study of the Mechanics of Erosion .................................................................... 35

Studies of Fluvial Terrace Systems .................................................................... 36

Low Temperature Thermochronometry and Cosmogenic Dating...................... 37
Analysis of Digital Elevation Models ................................................................ 38
Sedimentary Basin Architecture and Facies....................................................... 39
Remote Sensing / Neotectonics .......................................................................... 40
Co-evolution of Earth and Life.................................................................................. 41
Introduction ........................................................................................................ 41
Research Questions and Opportunities............................................................... 42
Rodinia ............................................................................................................... 43
Rapid Continental Motions................................................................................. 45
Low-latitude Glaciations .................................................................................... 46
Questions for the Next Decade........................................................................... 47
New Technological Opportunities...................................................................... 48
Facilities..................................................................................................................... 48
Cosmogenic Isotope Studies............................................................................... 48
Geochronology and Thermochronology............................................................. 49
Studies of Rock Deformation ............................................................................. 49
Geodetic Studies................................................................................................. 51
Educational Departures in Structural Geology and Tectonics................................... 52
Introduction ........................................................................................................ 52
Interdisciplinary Education ................................................................................ 53
Undergraduate Education ................................................................................... 53
Graduate Education ............................................................................................ 55
Characterization of Geological Structures using Differential Geometry ...... 55
Analysis of Tectonic Processes Using a Complete Continuum Mechanics .. 56
Contact Information................................................................................................... 59

Participants
David D. Pollard (Chair), Stanford University
Rick Allmendinger, Cornell University
Mark T. Brandon, Yale University
Roland Bergmann, University of California, Berkeley
Rebecca J. Dorsey, University of Oregon
David A. Evans, Yale University
James P. Evans, Utah State University
Raymond C. Fletcher, University of Colorado
David M. Fountain, National Science Foundation
George E. Gehrels, The University of Arizona
Laurel B. Goodwin, New Mexico Institute of Mining & Technology
Arthur Goldstein, National Science Foundation
Karl Karlstrom, University of New Mexico
Andreas K. Kronenberg, Texas A&M University
Tracy Rushmer, University of Vermont
Jane Selverstone, University of New Mexico
John H. Shaw, Harvard University
Carol Simpson, Boston University
Bruno Vendeville, The University of Texas
Brian P. Wernicke, California Institute of Technology
Kelin Whipple, Massachusetts Institute of Technology
Sean Willett, University of Washington
Martha Withjack, Rutgers University
Teng-fong Wong, State University of New York at Stony Brook
An Yin, University of California, Los Angeles
Herman B. Zimmerman, National Science Foundation

Preface
Over the past 30 years research in structural geology and tectonics (SG&T),
supported by the National Science Foundation (NSF), has generated an impressive body
of knowledge about the physical and chemical processes that shape the lithosphere of our
planet. Work funded by the Tectonics Program has increased our fundamental
understanding of how the lithosphere evolves, contributed to building the scientific
workforce of the nation, fostered international collaborations, and spun off knowledge
that has directly benefited the economy of the nation. Well before other fields discovered
the concept of multidisciplinary approaches to science, researchers in structural geology
and tectonics have integrated field-based analyses, laboratory work on rock deformation,
fluid-rock interactions, metamorphic reactions, continuum and fracture mechanics,
geochemistry, geochronology, isotope geochemistry, and a variety of other fields, across
vast ranges of time and space to develop new ways of examining and understanding
Earth's lithosphere.
In light of recent changes at the NSF, including the signing of the act to double the
budget, changes in the managers of the programs that affect the structural geology and
tectonics communities, new initiatives in topics such as biocomplexity, information
technologies, engineering, earth systems science, and the increased emphasis on large
integrative projects such as Earthscope, combined with the need to support single PI
projects, it is timely that the structural geology and tectonics community reflect on their
recent accomplishments and future prospects, and communicate these to the broader
community.
In order for the program managers within NSF to make a case for increased funding,
we must provide them with the evidence of past successes and our visions for the future.
This document is one effort at providing an overview of some of the recent exciting
advances we have made, and an attempt to provide an overview of where some of our
discipline is headed in the next 10 years. This document is not intended to tell NSF what
to fund, nor is it inclusive of every significant advance made by SG&T researchers.
Rather, this white paper is the best effort of a group of researchers from a diverse
spectrum of our community to reflect upon some of the recent highlights and suggest
where this research might lead.
This white paper is the outgrowth of a workshop organized by David Pollard of
Stanford University and held in Denver, in September 2002. During this two-day
workshop, twenty members of the SG&T academic community presented brief overviews
of topics related to their research specialties, and discussed future objectives and needs.
Representing the NSF were David Fountain, Arthur Goldstein, and Herman Zimmerman.
From this meeting draft chapters were written by working groups. Edited versions were
circulated amongst the whole group, and the final document was collated and edited by
David Pollard. In an intellectually diverse community, there will be some who disagree
with some of this document. Our intent was to begin to provide some guidelines for our
science in the near future. A major theme of this document is the integrated nature of our
work, and the implications this has for future funding of the research and teaching we do.
We look forward to fostering a stronger structural geology and tectonics program at the
NSF, and appreciate the opportunity to craft this document.
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Recognition of new opportunities for research in structural geology and tectonics,

brought into focus by recent technological developments, new quantitative data sets, and
both conceptual and theoretical advances, motivated a workshop to consider the future
directions of science in this field. Four broad topical areas of research were identified at
the workshop:
1) Beyond Plate Tectonics: Rheology and Orogenesis of the Continents;
2) The Missing Link: From Earthquakes to Orogenesis;
3) Dynamic Interactions between Tectonics, Climate, and Earth Surface Processes;
4) Co-evolution of Earth and Life.
Descriptions of these research themes along with sections on research facilities and
education form the major sections of this white paper. These new opportunities also bring
into focus long standing questions in this area of research about the relative merits of
descriptive versus quantitative methods, case studies of particular regions versus
experimental studies of particular mechanisms, kinematic versus mechanical modeling,
and historical versus process-oriented objectives.
This White Paper is meant to encourage a broader segment of the Earth Science
community to apply for grants from the Tectonics Program based on identification with
one of the themes described herein. While an increase in proposals is likely to lead to a
decrease in the percentage of proposals that can be funded, it is through this kind of
proposal pressure that new funding may be earmarked for the program. In addition
members of the community are encouraged to use this White Paper as a calling card
when visiting the National Science Foundation or communicating with the program
directors.

Priorities in Solid Earth Sciences: A Related Workshop

A broadly based, NSF-sponsored workshop on "Setting Priorities in the Solid Earth
Sciences" was conducted by members of the Earth sciences community in Denver on
October 26, 2002, immediately prior to the national Geological Society of America
meetings. The organizing committee consisted of Mike Brown (University of Maryland),
Cathy Manduca (Carleton College), Tracy Rushmer (University of Vermont), Basil
Tikoff (University of Wisconsin), and Ben van der Pluijm (University of Michigan).
Attendance was open to everyone and approximately 100 geologists from a variety of
disciplines attended, including structural geology, petrology, geophysics, geochemistry
and sedimentology.
The goals of the workshop were: 1) To foster integration of sub-disciplines within
the solid earth sciences; 2) Recognition of important research areas and merging needs;
3) Integration of teaching and research; and 4) Discussion of needs and methods of
interaction within the Solid Earth Sciences community. Given the wide range of interests
and the limited time (1 day), the discussion focused on general approaches rather than
specific research agendas. By the end of the workshop, there was a collective sense that
an approach (and perhaps an organization) that represents scientists in a range of fields in
Solid Earth Sciences is needed to advance future research goals. Further, there was a

clear recognition of the importance to organize in order to contribute to other Earth

science initiatives.

Research in Structural Geology and Tectonics at NSF

Mark T. Brandon
The Tectonics Program is commonly identified as the core source of funding for
tectonics research in NSF. In actuality, tectonics research is funded by 5 out of the 6
programs in the Earth Science Division at NSF. The Tectonics program largely supports
research in structural geology but some of this also spills over into other programs. The
remarkable breadth of this research makes it an integral part of the Earth Science
Division.
The Tectonics Program provides a useful vantage from which to view proposal
activity and funding for research in structural geology and tectonics overall. The plots in
Figures 1 through 4 provide an informal summary. They were constructed using webbased archival data from NSF for the Tectonics Program over the period from 1985 to
2002. We did not distinguish between different types of awards, although there were only
a minor number that provided support for activities other than research (e.g. conference
support, etc.). It is also important to note that some awards were made by shared funds
from different programs, so the boundaries between different EAR programs are slightly
blurry. Thus, the total amount awarded per year is not necessarily equivalent to the total
annual budget for the Tectonics Program. Dollar amounts have been converted to '2002
dollars' using the Federal Consumer Price Index. Awards were recorded as awards to
institutions, which obscures the fact that some awards involved more than two PIs at the
same institution. We asked the Tectonic Program to make a comparison with their more
detailed internal records. They found that the numbers summarized here are reasonably
similar, to their records for the last couple of years.
Figure A shows that funding has varied from about 6 to 14 million dollars with no
clear trends for the last 17 years with an average of about 10 million per year in 2002
dollars. The average award (Fig. 2) to an institution has remained approximately flat, at
about $140,000 per award. The number of awards has (Fig.3) varied from about 50 to 130
with no clear trend and an average of about 78 awards per year. The number of
collaborative proposals (Fig. 4) has increased significantly over the past 17 years, from
about 15 percent in 1985 to 60 percent in 2002. In this summary, collaborative research
was identified by the fact that the grants were part of a larger multi-institutional project.
Collaborative research between PIs within an institution is not captured in this statistic.
This strong trend towards increasing collaboration reflects, to a large extent, the
increasing importance of interdisciplinary research in tectonics.

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Figure 1. Annual amount of funds awarded in the Tectonics Program from 1985 to
2002.

Figure 2. Average award in the Tectonics Program. Awards are enumerated on the
basis of awards to institutions. Some awards may fund several PIs at a single
institution.

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Figure 3. Number of awards per year in the Tectonics Program.

Figure 4. Percentage of awards in Tectonics Program that are part of a larger

collaborative project. The designation "Collaborative Research", which NSF
requires in the title of the proposal, is used to judge which grants are part a larger
multi-institutional collaborative project.

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Beyond Plate Tectonics: Rheology and Orogenesis of

the Continents

Andy Kronenberg (Coordinator)

Mark T. Brandon
Raymond Fletcher
Karl Karlstrom
Tracy Rushmer
Carol Simpson
An Yin

Introduction
Plate tectonics emerged as a unifying theory of the solid Earth from observations of
the ocean floors long after geologists had described the orogenic belts of the continents.
Deformation, seismicity, and volcanism in the oceans are localized at boundaries between
rigid oceanic plates overlying viscous asthenosphere. Continental geology, on the other
hand, does not fit the plate tectonic model as well. The pervasive deformation and
internal structure of wide continental mountain belts (Figure 5) indicate non-rigid
behavior. Continents appear to be weak relative to oceanic plates, and quantitatively
significant deformation within continental lithosphere interiors and margins is
documented by GPS, seismic and stratigraphic methods. This weak rheological behavior
typifies continental orogenesis, in all of its manifestations, including regions of
convergence, divergence, and transform faulting. Significant improvement to our
understanding of continental tectonics will require a detailed and comprehensive study of
the rheology of continental crust and mantle rocks.
The response of lithosphere and asthenosphere to tectonic, gravitational and thermal
loads is strongly dependent on rheological behavior. We know that oceanic plates owe
their mechanical character to the friction and flow laws of crust and mantle, and their
dependence on lithostatic load and temperature. Oceanic plate thickness and effective
rigidity are well explained by the properties of oceanic gabbro and an upper mantle
dominated by olivine. Continental lithosphere is much more varied in composition and
physical properties. The maximum depth of seismicity in oceanic plates matches the
thermally defined transition from brittle dilatant processes to ductile viscous processes.
We do not have such a clear correlation in continental lithosphere. Recently, the
importance of trace hydrous components of olivine to oceanic lithosphere rheology has
been recognized; extraction of water in melts generated at divergent rifts leads to a

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marked strengthening of oceanic lithosphere. Does water play a similar role in the
strength of continental lithosphere?

Figure 5. Field example illustrating structures and fabrics associated with pervasive
ductile flow in the lower crust. Photograph shows the north face (2000 meter high)
of Dickson Fjord (72o 52N, 26o 37W), with large folds exposed in Precambrian
gneisses of the Greenland Caledonides (courtesy of Jane Gilotti).
The contrast between oceanic and continental deformation is highlighted by the
remarkable range of models currently being considered for mantle return flow associated
with convergent deformation of continental lithosphere. At one extreme (Figure 6a),
continental mantle lithosphere is relatively strong; thus it is subducted in a plate-like
fashion. The weaker, buoyant crust is left behind to form a thickened orogen. At the other
extreme (Figure 6b), the mantle lithosphere behaves in a more fluid fashion. Return flow
may involve steady down welling in a symmetric or asymmetric fashion (as illustrated by
Figure 6b), or it may involve episodic drip-like return flow. Seismic tomography provides
tentative evidence for the full range of behavior. Modeling indicates that the mode of the
return flow is dependent on convergence rate and the rheologies of the crust and mantle.
Mantle return flow causes the crust to thicken, forming a large orogenic wedge, much as
observed for the Alps, Himalayas, and Rocky Mountains.

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Figure 6. Numerical models illustrating different modes of mantle return flow

associated with continent-continent collision. (a) Continental lithospheric mantle
subducts in a plate-like fashion. (b) Fluid-like, symmetrical return flow with
episodic drip-like pulses. The models use temperature-dependent viscous-plastic
rheologies. Deformed tracking mesh shows accumulated strain and arrows are
relative velocity vectors (from Pysklywec, R.N., 2001, Evolution of subducting
mantle lithosphere at a continental plate boundary, Geophys. Res. Lett., 28, 43994402; Pysklywec, R. N., C. Beaumont, and P. Fullsack, 2002, Lithospheric
deformation during the early stages of continental collision: Numerical experiments
and comparison with South Island, New Zealand, J. Geophys. Res., 107(B7), ETG 3
1-19).
Another exciting problem concerns the relative strengths of the lower crust and
mantle. Despite some 40 years of study of rock strength, and the implications for
continental tectonics, we are still debating the relative contributions of crust and mantle
strength to lithosphere properties (Figure 7). The idea of a weak and highly fluid lower
crust dominated tectonics research for the last 15 years. There is now a school of thought
that continental lithosphere consists of weak middle crust and a stronger lower crust,
which in places may exceed underlying mantle strengths. These arguments are based on
inferences from earthquake distributions and estimates of elastic thickness of the
lithosphere using topographic loads. These kinds of arguments are indirect and carry with
them assumptions that have yet to be corroborated. Nonetheless, they highlight how little
we know about the composite rheology of the continents and the presence of weak layers
that interrupt stronger load-bearing layers. Resolution of the rheological structure of
continental regions will significantly improve our understanding of continental
orogenesis in a wide range of tectonic settings.
These diverse conceptual models illustrate the excitement and potential of
rheological contibutions to tectonics. We are now able to integrate field-based studies of
the structure and evolution of collisional orogens with geophysical studies of the crust
and underlying mantle. Numerical models are essential for this work because they allow
us to examine the implications of crust and mantle rheology, as extrapolated from
laboratory experiments. Field studies of deeply exhumed rocks that have been through
the orogenic wedge are vitally important for the validation of predictions from the lab and
geodynamic modeling. But most importantly, experimental studies are the underpinning
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for this area of research. Traditionally, the United States has been the leader in the study
of rheology of the crust and mantle. This situation has shifted dramatically over the last
10 years, with the result that labs in Europe are now dominating the field. Experimental
rock deformation has flourished in Europe due to better funding, and better institutional
support for the facilities that are required for this work.

Figure 7. Strength envelopes with depth for continental crust and mantle illustrating
competing interpretations for the relative strength of the lower crust and underlying
mantle. In all cases, upper crust strengths are represented by Byerlee's frictional
strength and a thermally activated flow law for wet quartz. Lower crust strengths
are predicted by wet and dry rheologies for diabase (MD) and granulite (WC).
Mantle strengths are given by wet and dry olivine rheologies (from Jackson, J.,
2002, Strength of the continental lithosphere: Time to abandon the jelly sandwich?,
GSA Today, 12, 4-9).

Research Questions and Opportunities

Unlike the behavior of oceanic crust, continental deformation depends markedly on
the strengths of both crustal and upper mantle rocks. Continental crust does not have a
common mode of origin, and is an assemblage of heterogeneous compositional elements
with widely varying tectonic and thermal histories. Also interaction with fluids and melts
can greatly modify the rheological structure. Studies of mechanical properties and
evaluations of deformation mechanisms of key mineral phases will provide an accurate
picture of the dependence of continental strength, faulting, and depth of seismicity on
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thermal and compositional structure. Outstanding questions that need to be addressed

include:
1) How does the rheological structure of the continents lead to non-plate-like
behavior?
2) Is the lower crust the strongest element of the lithospheric column? What are the
relative strengths of the continental lower crust and mantle?
3) How do mechanical properties change with strain? How do these changing properties
influence localization of deformation?
4) How distributed or localized is deformation in shallow, middle and lower crust and
mantle?
5) What constitutive relations govern the mechanics of faults and underlying, persistently
weak shear zones?
6) What are the feedbacks between dilatant rock deformation, fluid transport, and
elevated fluid pressures?
7) How does lithology and fluid chemistry influence continental rheology and how is this
related to tectonic setting?
8) What underlying physics and chemistry, from atomic to grain scale mechanisms,
govern the macroscopic rock rheology? What is their dependence on temperature,
pressure and chemical activities?
9) How do rheologies determined for small, homogeneous specimens at rapid
experimental strain rates scale to deformations of composite continental rocks at geologic
rates?
10) Does partial melt play a major role in the flow of deep crustal rocks?
11) What are the feedbacks between deformation and reaction during dynamic
metamorphism?
12) How do deformation and metamorphism determine rheological anisotropy and
fabric?
Rheological behavior can be determined by direct measurement in experiments in
which stress, strain, strain rate and relevant thermodynamic variables are controlled.
However, two major problems must be solved to apply these results to the study of
continental orogenesis. First, a reliable extrapolation must be made to natural strain rates
that are five to seven orders of magnitude lower than those of laboratory experiments.
Second, reliable methods are required to estimate the bulk rheological behavior of large
representative volumes of composite and structured rock from data on its components in
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small laboratory specimens. Deforming rocks are non-equilibrium systems and the
observed flow relationships, microstructures, and textures depend on competing rates of
deformation and recovery processes. Studies of the physics and chemistry governing
mechanical properties are required if we are to formulate mechanism-based flow laws
and to recognize microstructures that result from the same processes in experimental
specimens and naturally deformed rocks.
Experimental studies are needed to evaluate the rheologies of a wide range of crustal
lithologies, with and without fluids present, and across conditions that favor dilatant
brittle failure and frictional sliding, thermally activated plastic and diffusional flow, and
transitional brittle-ductile deformation. However, experiments alone cannot answer all
questions posed. Physical and chemical processes of rock deformation operate at scales
ranging from failure of crystalline bonds at crack tips and motion of lattice defects, to
grain-scale deformation in multiphase rocks, to displacement on map-scale faults and
shear zones (Figures 5 and 8).

Figure 8. Dislocation loops in feldspar deformed at greenschist conditions, within a

granodiorite shear zone, Sierra Nevada near Lake Edison - Mount Abbott
quadrangle (from Kronenberg, A.K., P. Segall, and G.H. Wolf, 1990, Hydrolytic
weakening and penetrative deformation within a natural shear zone, in Geophysical
Monograph 56, AGU, 21-36).
Thus, constraints on rheology are equally needed from careful field and observational
studies and mechanical modeling. Integration of detailed field observation and
mechanical modeling allow naturally deformed rocks and active tectonic regions to be
used as natural laboratories. Estimates of rheological behavior from experimental studies
may be tested against field examples selected for the availability of constraints on strain,
strain rate, thermodynamic conditions, and tectonic & thermal history.

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Field and theoretical studies of small-scale structure and fabric may be used to
examine simultaneous deformation and reaction at conditions where fluid-rock
interaction, mineral growth and dissolution, and melting are important (Figure 9).
Modeling of large-scale active tectonics may be used to examine the sensitivity of
rheology to thermal and lithologic structure, and to examine the consequences of
gravitational loading, uplift, erosion, and magmatic events. In addition to mechanical
constraints and boundary conditions, these models require petrological, geochronological,
and stratigraphic constraints on pressure and temperature histories, and information on
conductive and advective heat flow, and rates of rock exhumation and burial.

Figure 9. Synchronous deformational and magmatic processes operating in the

lower part of the continental crust. The example shown here, from exposures in
Fiordland, New Zealand, highlights what can be learned from direct study of deeply
exhumed continental crust. (a) The section shows a vertically stratified crust
consisting of a meta-sedimentary middle crust (green), a late-stage ten km thick
mafic to intermediate batholith (yellow, Western Fiordland Orthogneiss) that was
emplaced into the crustal section at 126-119 Ma), and a mafic lower crust (purple)
with lenses of dioritic partial melt (red). Outcrop photographs show (b) the dioritic
partial melts in an older gabbroic unit (mafic lower crust), and (c) melt-enhanced
deformation in a shear zone developed at the base of the batholith (courtesy of Keith
Klepeis).

Studies of Transitional Brittle-Ductile Deformation and Strain

Localization
Yield envelopes based on purely brittle processes of friction and failure at shallow
crustal depths and purely ductile processes of plasticity and creep at deeper levels grossly
over-estimate strengths at intermediate crustal depths. Geophysical measures of shear
stress on the San Andreas and other crustal-scale faults indicate that experimentally
derived yield envelopes may be inappropriate for analysis of faulting and seismicity.
Close inspection of laboratory results for mechanical behavior and mechanisms of

19

deformation indicates that complex combinations of brittle and ductile mechanisms

operate over a wide range of conditions.
Application of the observed transitional behavior to deformation in the crust has
been hindered by a lack of quantitative formulation, especially that bearing on its
extrapolation to natural conditions. Better models are needed that incorporate interaction
between dilatant brittle mechanisms, crystal plasticity, diffusion, and solution transport.
These must be guided by key microstructural observations that clarify the physics and
chemistry involved. Such relations will yield lower integrated strengths for the crust than
are predicted by simple end-member friction and flow laws.

Coupled Mechanical-Hydrologic Systems

Transitional brittle-ductile deformation appears closely linked to the role of fluids
during faulting through mechanisms affecting positive and negative dilatation of fault and
host rocks. Grain size reduction and lithologic mixing within fault gouge alter
permeability in the fault-host system. Fluid flow affects dissolution at grain contacts,
transport, and precipitation of cements. Field observations indicate that many processes
that affect fault mechanics have not been replicated or quantified in laboratory
experiments.

Integrated Studies of Material-Specific, Mechanism-based Rheologies

Improvements in experimental methods have led to better data quality. These include
closer control on thermodynamic conditions, refined measurement of stress, and digital
data acquisition. However, with few exceptions, creep data have been fit to theoretical
flow laws, borrowed from materials science, that were developed to model metals and
simple oxides. These relations only approximate the behavior of more complex silicates
and carbonates. Material-specific flow laws are needed that fully incorporate the
dependencies of defect populations, deformation mechanisms and recovery processes on
temperature, stress, and chemical activity. In turn, these relations form the basis for
realistic tectonic models of continental deformation.

Formulation and Evaluation of Rate and State Constitutive Relations

that Describe Transient and Steady-State Behavior
Formulation of improved crustal flow laws can also come from studies of transient
creep through load relaxation experiments and high strain torsion experiments and the
development of rate and state constitutive relations. Steady-state flow laws have been
obtained for many important silicates of the continental crust. Few studies have addressed
the transient creep with strain-hardening that precedes a steady state, or with strain
softening that results in strain localization and development of shear zones. Recent
experiments using torsion apparatus have shown that relations that appear to represent
steady-state creep over small strain increments change significantly with increasing shear
strain. These longer, more gradual transients correspond to changes in microstructure and
texture. Quantitative determinations of transient creep require new types of creep laws
and new types of experiments. These creep laws have important implications for the
evolution of intracontinental fault systems and continued displacement on mature faults.

20

Fabric Development and Anisotropy at Large Shear Strains

Deformation of upper mantle rocks by dislocation creep gives rise to strong foliation
and lattice preferred orientations that result in the seismic anisotropy of oceanic plates.
Equally strong fabric and lattice preferred orientation are seen in rocks exhumed from the
middle to lower continental crust. However, owing to longer and varied deformation
histories, fabrics and anisotropy of the middle and lower continental crust are more varied
and complex.
Recent technical advances improve our ability to study lattice preferred orientations
in deformed rocks in the field and the laboratory, and to interpret the patterns on the basis
of crystal plastic slip systems. New scanning electron microscope methods based on
electron backscattered diffraction allow measurements of lattice preferred orientation on
large populations of grains in fine-grained and heterogeneous rocks. Together with
torsion apparatus experiments, they allow comparison of naturally deformed rocks and
experimentally deformed specimens at equivalent strains. Self-consistent theoretical
models of texture development contribute to our understanding of lattice preferred
orientations and predict anisotropy in rheology that develops with strain.

Chemical Weakening and Hardening

Mechanical properties of rocks are strongly influenced by chemistry. In well-studied
minerals such as olivine, quartz and halite, trace element chemistry critically affects the
structural defect populations and deformation mechanisms. In particular, trace levels of
hydrous defects within these nominally anhydrous minerals and hydrous complexes at
their surfaces greatly weaken them. We have much more to learn about deformation of
minerals in the presence of reactive fluids, as well as the effects of major component
variation in minerals. Flow laws have generally only been determined for end-member
compositions and little is known about the effects on strength of solid solution, cation
ordering, or exsolved phases within minerals.
Additional chemistry and physics must be introduced to rheological laws to describe
the bulk rheology of reacting multiphase systems. Discontinuous reactions alter the
modal compositions of rocks and contribute to volumetric strains, dissolution and growth.
Continuous reactions may contribute to driving forces for recrystallization. Recent
experiments involving partial melts have been successful in describing processes of flow,
melt segregation, and transport. Reactions that evolve or consume fluids may affect local
effective pressures, facilitating dilatant brittle processes at nominally high pressure and
temperature as well as diffusional solution transfer creep. Much more work is needed to
understand how changes in mineral chemistry affect the strength of the lithosphere.

21

The Missing Link: From Earthquakes to Orogenesis

Laurel Goodwin (Coordinator)

Rick Allmendinger
Roland Burgmann
Jim Evans
David Pollard
John Shaw
Bruno Vendeville
Martha Withjack
Teng-fong Wong

Introduction
Deformation of the Earth's crust is unevenly distributed in space and time and thus
its study has become partitioned into disciplines with tools of markedly different
precision populated by practitioners with markedly different perspectives (Fig. 10). A
single earthquake may produce substantial deformation and significant damage of
relevance to society in just tens of seconds (10-6 years). Seismologists and geodesists
have developed highly accurate methods and tools (seismometers, strong motion
instruments, creep meters, GPS, InSAR, etc.) to measure deformation during and between
major earthquakes (up to 102 years if we also utilize historic leveling and triangulation
data). The majority of the signal captured by GPS, InSAR, and similar instrumentation
may be modeled as linear elastic deformation. However, the time span of decades is too
short to capture more than a few major earthquakes on the important seismogenic faults
in an actively deforming volume of the Earth's crust and may miss much of the associated
inelastic deformation.
Tectonic events such as mountain building, development of major transform
systems, and rifting occur over millions to tens of millions of years. The structures
produced on these longer time scales also directly affect society. For example, faults
provide both conduits and barriers to fluid flow, and thereby influence the distribution
and production of water and hydrocarbons as well as contaminant transport; mountain
belts affect global atmospheric circulation patterns, thereby impacting climate. Field
relations seldom allow age constraints tighter than 1 m.y. (106 yr) to be placed on
individual structures, though growth strata imaged by seismic reflection permit some
additional resolution. Deformation at these time scales is decidedly permanent; the
transient elastic strain measured at 101 yrs and less is an insignificant fraction of the total
finite strain recorded in a mountain belt.
22

Figure 10. Earth scientists study deformation across more than 15 orders of
magnitude of temporal sampling windows. Processes, rheologies, types of studies,
and methods used vary enormously across this range. The white band,
corresponding to times from decades to a million years, is the least understood part
of the range and thus the missing link in relating earthquakes to orogenesis.
Deformation appears very different from these two perspectives. The temporal and
conceptual gap between them is one of the most fertile areas for future research: How
does deformation at the time scale of decades and less which includes elastic strain as
well as permanent deformation accomplished by seismic and aseismic fault movement,
integrate over hundreds to millions of years to form a major tectonic province? The time
range from 102 to 106 years is the missing link. Without studies at these time scales, we
are unlikely to understand the spatial and temporal distribution of earthquakes, which is
fundamental for evaluating seismic hazards. The life cycle of faults is initiated and their
mechanical character set over this time range. If we wish to use map- to micro-scale
observations to read the rock record of earthquakes we must understand this missing link.
Finally, to understand fully the rheological character of the upper crust we must bridge
this conceptual gap between elastic and inelastic deformation.
We expect that tangible results from this line of research will include: 1) a much
clearer understanding of rates, durations, and the episodic nature of deformation in the
crust, as well as the mechanisms controlling these factors; 2) physically based and
mechanically plausible models for earthquake sources and the complex process of rupture
propagation, along with an understanding of the driving forces and energy balance of
earthquakes and the earthquake cycle; 3) a better understanding of the role of chemical
and hydrologic processes in rock deformation; 4) characterization of material properties
23

of rocks in the seismogenic zone and the architecture of faults in different lithologies and
tectonic settings; and 5) characterization of structures in the upper few kilometers of the
crust and a better understanding of their relationship to such factors as topography and
tectonic geomorphology.

Research Questions
1) How do the elastic strains measured by techniques such as GPS and InSAR bear on
earthquake distribution and how does this elastic signal relate to permanent deformation
at the scale of millions of years?
2) Are the repeat times for earthquakes quasi-periodic or more complex?
3) How and why does deformation localize into faults?
4) What role do fluids play in these phenomena?
5) What is the structural/microstructural record of fault-zone processs and how does this
record reflect the mechanical behavior of fault-zone materials?

Relating Elastic and Permanent Deformation

Geodetic data such as GPS and InSAR record predominantly an elastic signal that
can yield non-intuitive results, such as evidence of crustal shortening in domains
enclosed within the regionally extending Basin and Range Province. In other areas, GPS
velocity vectors in active zones of continental deformation display rates startlingly
similar to geologically measured shortening rates (Fig. 11), even though the GPS signal is
ephemeral and may be related in part to distant processes such as subduction-zone
locking. Because we don't have a 102 to 103 year record of GPS observations to cover a
more complete seismic cycle along an entire major fault zone, we do not know how the
short-term elastic deformation is converted into long-term permanent strain. This
conceptual gap can be filled by studies focused on the 102 to 106 year time interval.
Study of this temporal window will be facilitated by a variety of geochronometers
that have been / are being developed to constrain the timing and duration of events in this
interval (Fig. 10) and the growing number of GPS networks and availability of InSAR
data around the world (Fig. 11). These data alone will not be sufficient to determine how
short-term elastic deformation is integrated into long-term inelastic strain; that requires
examination of the geologic record of deformation, particularly that which has accrued
over 102 to 106 years. In addition to paleoseismological and tectonic geomorphology
studies to address recurrence intervals, studies of the structures and microstructures
associated with young faults in poorly lithified sediment can illuminate deformation
processes and strain rates at these time intervals.
Structures in sediments may provide the sole record of recent slip sense (and perhaps
slip style, i.e., stick-slip versus creep) where seismologic data are not available. For
example, rotation and cataclasis of elongate pebbles, cobbles, and sand grains during
faulting of poorly lithified sand and gravel produces lineations that record slip in normal
faults of the Rio Grande rift (Fig. 12). Such observational and descriptive studies can not
24

alone answer the questions posed. Integration of these studies with mechanical modeling
will permit regions of active tectonics to be utilized as crustal-scale laboratories within
which conceptual models can be tested and refined.

Figure 11. An example from active foreland thrust belt in western Argentina of
studying deformation at several time scales. (A) Location of study area in western
South America, shown with red box. (B) Landsat Thematic Mapper base showing
active thin-skinned Precordillera thrust belt and active thick-skinned Sierras
Pampeanas. Dashed white line marks reversal of structural vergence; the thickskinned structures to the east verge west whereas the thin-skinned structures to the
west verge east. White letters highlighted with red show the approximate locations
of two large historic earthquakes and their magnitudes; another M 7.4 earthquake
occurred just east of San Juan in 1977. Yellow lines and ellipses: velocity vectors
(relative to a fixed South America) and uncertainties from the MATE GPS network
(data courtesy of B. Brooks, R. Smalley, M. Bevis, & E. Kendrick). (C) Shortening
rate history of the Precordillera and western Sierras Pampeanas for the last 20 Ma
from Jordan et al. (1993) and Zapata & Allmendinger (1996). Red curves show rates
based on minimum permissible duration of motion for each thrust plate; blue curves
show rates based on maximum permissible duration. Note the striking similarity of
million year rates to GPS velocities, even though the latter are dominated by an
elastic signal from a locked plate boundary.

25

Figure 12. Clast lineations recording slip direction in the Sand Hill fault, Rio
Grande rift, New Mexico. Images show deformed sediments in the hanging wall
mixed zone that were subsequently cemented with calcite (Heynekamp et al., 1999).
A. View looking east at fault core bounding surface. Elongate clasts ('c') are
parrallel to subsequently formed slickenside striaw. Shrubs are roughly 1 m high. B.
Photomicrograph taken in the plane light, ~6 mm wide in longest dimension.
Fragmented sand grains ('c') in zone of deformation bands are demonstrably
aligned parallel to macroscopically visible lineations in 3-d (Goodwin and Tikoff,
2002). This zone of deformation bands os juxtaposed against colluvial wedge
sediments, preserved in the hanging wall, that record paleo ground rupture.

Spatial and Temporal Distributions of Earthquakes

The spatial and temporal distribution of earthquakes reflects the distribution and
transfer of stress within the crust as well as local source mechanics. The frequent
occurrence of major earthquakes in clusters or sequences (e.g., 1954 Fairview Peak-Dixie
Valley, 1987 Superstition Hills, and 1992 Landers earthquake sequences in the western
U.S.; the 1944 San Juan and 1977 Caucete earthquakes in western Argentina; or 1999
Izmit, Duzce and earlier earthquakes in Turkey) suggests that earlier events may trigger
later ones. When and how this occurs are two of the most promising avenues to
understanding (and possibly predicting) earthquakes. Recent studies suggest that the
conceptual model of foreshocks, mainshock, aftershocks may be inadequate to explain
the triggering phenomenon.

Localization of Deformation
Deformation can be distributed throughout a region of interest or localized within
discrete zones. Many mature, major faults, like the San Andreas, appear to be weaker
than rock mechanics experiments would predict. Processes of nucleation and growth and
the mechanical behavior of faults - which bear on localization and fault reactivation - are
poorly understood. These processes can be best investigated by the integration of
experimental deformation studies, analytical and numerical modeling, and structural and
microstructural analyses of faults and fault-zone materials in the field and laboratory.
Investigation of how fault segments become linked in space and time, for example, will
help us to understand the significance of the geometric complexity of fault systems.
Understanding the spectrum of failure and localization modes recorded by fault-zone
structures (e.g., opening and shear fractures, shear and compactive deformation bands,
foliations and lineations) would further elucidate the faulting process.
26

Figure 13. Space-time patterns of strain release in a hypothetical plate boundary

fault system with total horizontal displacement rate of 2 mm/yr, 2 m of horizontal
slip per event, and a long-term average of 0.25 mm/yr displacement on each fault.
Courtesy of B. Wernicke and colleagues.

The Role of Fluids

Research into fluid-fault interactions generally falls into two categories: 1) the
impact of faults on fluid flow (typically based on static conceptual models of fault-zone
architecture) and 2) the impact of fluids on fault-zone mechanics (in which dynamic
mechanical processes are considered in detail but the hydrology of the fault-protolith
system is not considered). Bridging the gap between these approaches requires
consideration of three-dimensional spatial and temporal variations in fault-zone character,
and their resulting impact on both the hydrology and mechanics of fault-protolith
systems.
The fact that faults span different crustal levels also needs to be fully considered.
This emphasizes the importance of integrating structural investigations with hydrologic,
petrologic, and geochemical studies of fault systems. For example, variations in flow
pathways over time leave a geochemical signature (e.g., stable isotope, fluid inclusion)
that can help us understand how fluids evolved and how their changing compositions
affect the rheology of fault systems. In addition, fluid-fault interactions can cause
significant chemical changes within and adjacent to fault zones; these chemical changes
can in turn affect the mechanical behavior of the fault zone rocks.

Fault Zone Processes and Materials

Faults can rupture dynamically, produce slow earthquakes, and/or creep
aseismically. Studies of fault-zone architecture and microstructures that might provide
evidence of different modes of failure/ strain rates have been handicapped by the nature
of the geologic record, which integrates the signal of different events over time,
producing a composite fault rock that records every event of its history. In addition, field
and laboratory data demonstrate that fault rock strength and rheology evolve with both
27

rock microstructure and mineralogy, emphasizing the fact that the mechanical response of
an individual fault can change over time.
Recent developments suggest ways to exploit new and old technologies to better
understand the geologic record of fault-zone processes. Combined geodetic and
seismologic data offer the opportunity to target faults that record different deformation
histories, including different strain rates. New experimental approaches should allow
structures formed in the lab to be compared with those developed in the field and suggest
new research directions. For example, during dynamic rupture and coseismic slip,
dramatic changes in frictional strength may arise from frictional melting and thermal
pressurization. Recent lab measurements argue for the coefficient of friction attaining
values as low as 0.1 at slip velocities ~0.1 m/s, possibly due to the generation of a gellike microstructure. Field observations indicate that seismic slip may be highly localized;
theoretical models would then imply that the microstructure associated with partial
melting, flash heating and thermal transients can be resolved only at a very fine scale.

Research Opportunities
Answering the questions we pose will require investigation in a variety of tectonic
environments. We therefore recommend taking advantage of, but not restricting research
to, the focused initiatives mentioned herein. Activities such as EarthScope leverage, but
do not replace, research into these fundamental questions. The integration of field,
geophysical, laboratory, and theoretical studies when feasible and appropriate will
facilitate the proposed work. We therefore support efforts to remove artificial boundaries
between disciplines, to which deformation processes do not adhere. This recognition that
other disciplines may contribute to research in structural geology and tectonics is
intended to encourage collaborations with geophysicists, hydrologists, geochemists, and
petrologists.

GPS and InSAR (addressing questions 1-3) Continued GPS monitoring and
development of new techniques such as InSAR will assist in understanding the
nature and spatial distribution of surface movements on the time scale of years.
Present GPS arrays in large diffuse plate boundary zones (Tibet, Andes, and
western U.S.) are, however, too sparse - we get an averaged, continuous velocity
field but miss important information on the motion and deformation near
individual faults. Geodesy becomes particularly useful when integrated with
seismic paleoseismic, and structural geologic constraints. For example, we can
consider a large earthquake as the beginning of a lithosphere-scale rock
mechanics experiment. The boundary conditions imposed by the coseismic
rupture on the surrounding crust and upper mantle can be defined. It is possible to
constrain the geometry of the experimental 'apparatus' - fault geometry, crustal
layering or heterogeneity, topography - and gauge the response of the system
using surface displacement data and the results of a mechanical model of elastic
strain associated with fault slip (Fig. 14).

28

Figure 14. InSAR and model interferograms from the 1999 Mw 7.1 Hector Mine
earthquake, CA. One fringe cycle = 28.3 mm displacement in the look direction of
the satellite. Model developed by Laurent Maerten using the Poly3Dinv code utilizes
the boundary element method solution for elastic deformation associated with fault
slip.
The elastic models reproduce the pattern of deformation recorded by the InSAR
data, demonstrating a powerful tool for investigating such phenomena as triggered
earthquakes. Geodetic monitoring on scales from major plate boundaries to
individual faults is therefore an important goal. Finally, models can be
constructed to help interpret the processes and their rates, and ultimately to
constrain rheologic parameters of fault zones and lithospheric rocks at different
depths.

Reflection Seismic Characterization of 3-D Structures (addressing questions 1-3).

Research into the areas discussed above requires the understanding that
deformation is typically three-dimensional. The longer-term geologic record (e.g.,
as interpreted in 3-D seismic data from the Gulf of Mexico) shows us that faults
that appear as independent segments at one structural level may be linked at
another or may be linked laterally. We therefore support efforts to collect,
analyze, and model data in three dimensions to characterize structurally complex
areas, and would like to see more efforts on the part of individuals and the federal
government and industry to transfer data from industry to academia. The Flexible
Array associated with U.S. Array also offers potential for extending seismic
coverage of key areas and better understanding of 3-D fault geometries.

Geophysical and Geological Characterization of Active Faults (addressing

questions 1-5). SAFOD will allow sampling and instrumentation of the
seismogenic zone of an active strike-slip fault. NanTroSEIZE (the Nankai Trough
Seismogenic Zone experiment), designed to take advantage of a new Japanese
drill ship to sample and instrument a major subduction-zone fault and splay fault
29

at a variety of depths. In-situ measurements can be used to constrain hydrologic

and petrophysical characteristics of fault-zone rocks. A particularly appealing
aspect of NanTroSEIZE is that it will allow a comparison of these parameters
both up dip from and within the seismogenic zone, which will allow investigators
to evaluate which parameters control seismic behavior.

Mapping and Field Structural Analysis (addressing questions 1-5). Mapping and
field structural analysis of both outcrops and core from the active faults
mentioned above are critical to this effort. Identification and quantitative
characterization of 3-D patterns of deformation, observations pertaining to strain
localization, and structural analysis of young faults that extends beyond
paleoseismic investigations are particularly important to answering the questions
detailed earlier. This research must include observations made from regional to
hand-sample scales, where fabrics that may record strain localization or strain
rate, or impact fault strength, are visible.

Microstructural Analysis (addressing questions 1-5). Microstructural analysis (the

characterization of structures at the scale at which processes controlling frictional
failure operate ) must be done in the context of mapping at a larger scale, and
should be integrated with geochemical and geochronologic / thermochronologic
studies to constrain interpretations of the data collected. The goal of
microstructural analyses in the research proposed here is a better understanding of
the geologic record of fault-zone mechanics. Microstructural evidence of strain
rate (e.g., seismic rupture versus creep) and documentation of features that might
affect the dynamic coefficient of friction of fault rock are examples of particular
focus areas. Microstructural analysis will benefit from the continued development
of image analysis tools; notable among these is NIH Image freeware. Improved
electron imaging capabilities have resulted in better electron microprobe, SEM,
and TEM resources (for example, EBSD) for microstructural analysis.

Geochemical and Mineralogical Analyses (addressing questions 3-5). Fluid

inclusion, stable isotope, and major, minor, trace and REE analyses coupled with
mineralogical and microstructural analyses of fault rocks can, when compared
with protolith materials, provide a record of fluid-fault interactions. This is a
record not only of flow pathways, and thus paleohydrology, but also of reactions
resulting in changes in fault-zone porosity, permeability, and strength, therefore
effecting fault-zone weakening or strengthening over time. Chemical processes
operating in fault zones are commonly not well characterized, but may critically
affect such factors as rate-and-state constitutive relationships and pore fluid
pressure.

Hydrologic Analyses (addressing questions 2-5). Although pore fluid pressure

variations have been intimately linked to the seismic cycle, our understanding of
faults as part of a larger hydrologic system is surprisingly limited. Analyses of
fault-zone permeability and permeability variations at a variety of scales, crossfault flow tests, and flow models involving dynamic fault systems, would all
contribute to the proposed efforts.
30

Geochronology / Thermochronology (addressing questions1-3). The exploitation

and development of geochronometers that can help constrain both the ages and
durations of deformational events in the time window from 102 to 106 years are
clearly critical to this effort (Fig. 2).

Experimental Deformation (addressing questions 1-5). Experiments have the

potential to contribute substantially to our understanding of the rheology of the
upper crust, where frictional processes dominate. The successful application of
experimental results to understanding large-scale processes, however, demands:
1) an understanding of the fundamental physics and chemistry underlying
processes of interest; and 2) that experiments are designed with materials,
boundary conditions, and extrinsic parameters that adequately represent natural
systems. The scaling issue is highlighted, for example, by the large gap between
stresses predicted by laboratory experiments and stresses now inferred on the San
Andreas fault. A major obstacle to fully utilizing experimental approaches to
understanding shallow crustal deformation is the dearth of experimental labs in
the U.S. and the limited number of people with appropriate expertise to use these
labs. One solution might be an STC devoted to these processes, designed to bring
together field structural geologists, experimentalists, and theoreticians to conduct
experiments of mutual interest successful examples of this approach include
Institutes in Potsdam and Bayreuth, Germany.

Analytical and Numerical Modeling (addressing questions 1-5). Participants in

this workshop recognize the value of using analytical and numerical models to
test conceptual models generated through field and laboratory research, and to
suggest new directions for data gathering and for studying natural and
experimental deformation. Of potential value to answering the questions outlined
above would be the formulation of numerical models that capture the continuum
and discrete attributes of natural deformation. Lattice Boltzmann and percolation
network models are examples of approaches that offer promise for coupling
hydrologic and mechanical models of fault development. Fracture mechanics
models are examples that capture the continuous and discontinuous nature of a
faulted rock mass. Development of graphical user interfaces that facilitate the
construction of geometrically complex models and the visualization of model
results are a priority.

31

Dynamic Interactions Between Tectonics, Climate and

Earth Surface Processes

Sean Willett (Coordinator)

Mark T. Brandon
Rebecca Dorsey
Bruno Vendeville
Kelin Whipple

Introduction
The Earth's surface represents an important and underexploited source of information
regarding tectonic processes operating within the Earth. The ready availability of digital
topographic data and new data obtained by emerging technologies (STRM, ASTER,
Lidar, GPS Total Station, reflectorless laser rangefinders, etc) thus represent new
opportunities to study structures and tectonic processes within the lithosphere. However,
our ability to infer tectonic information from surface observations is hampered by a lack
of quantification of the causal links between tectonic activity and topography.
Topography represents the net product of tectonic and surficial processes and unraveling
the intricacies of this highly coupled system represents a primary challenge in this field.
We recognize that the geosphere, atmosphere, hydrosphere and biosphere interact in
diverse ways at a variety of spatial and temporal scales. The principal interface between
these spheres is the Earth's surface. Tectonic activity creates the surface relief that
amplifies these interactions. Surface processes destroy relief by redistributing mass.
These processes are coupled insofar as there is a deformational response to surface
change and the resultant gravitational forces. Rates of surface processes are modulated by
climatic factors, primarily precipitation rate and distribution which determines discharge
levels in rivers. Interactions between tectonic and surficial processes are complex and
involve coupling with feedback through diverse mechanisms (Fig. 15). For example,
fluvial incision rates increase in response to a tectonically-driven increase in channel
slope, but may also increase or decrease in response to changes in drainage area as water
divides are moved or created by tectonic activity. Climatic response to increased
elevation includes orographically-enhanced precipitation, but it also includes lower
surface temperatures as mountains rise through the atmospheric lapse rate; lower
temperatures result in periglacial erosion processes and eventually the creation and
growth of alpine glaciers (Fig. 15).
The dynamic system of the Earth's near surface has implications for natural hazards
and the environment in which we live. Floods, landslides, debris flows and even
earthquakes are a consequence of the processes creating dynamic landscapes in
tectonically-active areas. Understanding the rates and variability of mass transport
systems on the earth's surface will lead to better hazard forecasting. There are also
32

implications for climate change; a better understanding of how the Earth's surface has
responded to climate change over geologic time will lead to a better understanding of
how climate change will affect the Earth's surface environment over human timescales.

Figure 15. Feedback loops within the dynamic system defined by tectonics, climate
and erosional surface processes. There are two feedback loops; a direct path (I)
whereby tectonics increases erosion rates by increasing elevation, relief and
drainage basin areas and an indirect loop (II), whereby increased elevation induces
increased erosion rates through changes in climate. Climate change is in the form of
enhanced precipitation or lower temperatures, which lead to glaciation. In each
case, there is feedback in the tectonic response to surface mass redistribution. More
complex processes and pathways are likely to exist.

Research Questions and Opportunities

The last decade has seen considerable growth of interest in the coupled tectonicsurface-climate system. We anticipate that this interest will continue or even accelerate as
new technologies and new theories lead to further progress. We identify here a number of
research questions that exemplify this interest, and are likely to motivate research in
fruitful areas of future study. The list below is not exhaustive, but rather is intended as an
illustrative set of examples.
1) What are the mechanisms and magnitudes of feedbacks among tectonic activity,
structural style, strain partitioning, erosion and sediment deposition at the earths surface?
2) What are the nature and strength of the climatic feedback?
3) How do rates and patterns of erosion influence growth of individual structures, orogen
kinematics, metamorphism and exhumation?

33

4) How do mechanical, climatic, or geomorphic thresholds influence the evolution of the

coupled system?
5) How can we extract quantitative information about climatic and tectonic processes and
histories from the topography of the Earth's surface and the sedimentary record?
6) What is the nature of the coupling between erosional and depositional systems?
7) What is the timescale of response for the coupled system to climatic or tectonic
perturbation?
8) How do we determine the elevation history of a mountain belt?
In the following sections, we highlight some fields of research that present opportunity to
directly address one or more of the questions above.

Coupled Surface Process and Tectonic Models

The quantitative study of large-scale landscape evolution has blossomed over the last
decade, driven by the widespread inter-disciplinary interest in discovering the nature of
potential global-scale interactions among climate, surface processes, and tectonics, and
fueled by the advent of high-speed desktop computers and the availability of new data
resources (digital topography, remote sensing imagery, new isotopic chronometers). The
complexity inherent to dynamic systems with non-linear processes and multiple feedback
mechanisms as described by Fig. 15 has led to extensive development of numerical
models to couple tectonic, climatic and surface processes. Most research has been
focused on surface processes, given our lack of understanding of the physics of
geomorphic processes such as fluvial incision into bedrock, but as the confidence in
surface process models increase, attention will shift towards tectonic and climatic
processes. Tectonic models to date have been applied at an orogen scale (tens to hundreds
of km) where sensitivity to specific geomorphic processes is less (Figure 16).
Applications to individual structures (1 to 10 km in size) are increasingly common and
will become more so as this scale provides the best possibility of constraining the
kinematics of tectonic motions and the causative tectonic stresses.
Figure 16: Model for topographic evolution of a convergent orogen, driven by plate
subduction and accretion (From Willett and Brandon, 2002). Accretionary flux, FA,
and erosional flux, FE, determine material transport (dashed lines), with vertical
and horizontal components u and v, respectively. Upper illustrations show
topography predicted from a surface-process model driven by constant tectonic
uplift and constant horizontal shortening rate. Surface process model includes
uniform precipitation collected into a river network that incises proportional to
stream power and diffusion of hillslopes.

34

Study of the Mechanics of Erosion

As noted above, the field of tectonic geomorphology has been in a productive phase
of initial discovery and exploration; many important new insights have been derived from
first-generation landscape evolution models erected on essentially "generic" process rule
sets. Major advances in the study of long-term landscape evolution in the coming decade
will depend on: (1) the development and testing of refined process laws that more fully
capture the richness of fluvial and hillslope erosion, and (2) initial exploration of the
different dynamics of glacial landscapes. For instance, work is underway to replace the
stream-power erosion model (a widely used "generic" model for river incision) with more
sophisticated treatments that incorporate the effects of sediment flux, threshold shear
stresses, and a probabilistic representation of flood discharges and sediment influxes from
hillslopes.
First-generation generic process models have enjoyed some measure of success,
partly because of their generality. However, these models have so many parameters that
competing theories often can not be discriminated on the basis of available data. Refined
mechanical understanding of erosion processes through a combination of field and
laboratory study will permit independent determination of physical processes and
parameters, thereby reducing the number or range of parameter values. Numerical
experiments with coupled landscape evolution models incorporating these new rule sets
should guide data collection and model testing strategies.
A fuller representation of climate (e.g. representation of the stochastic distribution of
floods via measurable climate variables) in landscape evolution models is a pre-requisite
to a quantitative exploration of the linkages between climate and tectonics, including
feedback loops such as the orographic enhancement of precipitation. Coupling with the
35

vast range of climate and meso-scale meteorogical models is likely to lead to important
new insights.

Studies of Fluvial Terrace Systems

Mapping deformation of fluvial and marine terraces has long been a tool for
neotectonic studies, particularly those that aim to discover the rates and patterns of
deformation and rock uplift associated with active faulting processes. In recent years this
has become a useful tool for older systems as new cosmogenic dating methods have been
applied to terrace straths and treads. Dating based on Be10 and A126 have pushed back the
datable age of fluvial terraces from the 50ky limit of C14 to several hundred thousand
years as well as permitting age control on terraces with no carbon-bearing material. This
has increased the utility of terraces as tools for characterizing tectonic activity.

Figure 17: Elevation of strath terraces along the Bagmati River (From Lave and
Avouac, 1999). The elevation of the strath levels computed from all DEM and field
measurements are indicated by solid thin lines and dashed lines. Solid thick lines
correspond to Holocene terraces, and dashed lines correspond to hypothetical levels
between late Pleistocene terraces correlated by similar color and depth of
weathering. The structural cross section, with the same horizontal scale is shown for
36

comparison. The terraces attest to sustained Holocene and late Pleistocene activity
of the MFT fold. In contrast, they did not record significant offset across the MDT
and suggest little deformation linked to this thrust.
The height of a dated terrace above the modern river channel provides a rate of
incision or, with an assumption of steady channel elevation, provides a rock uplift rate.
For example, Lave and Avouac mapped and dated Holocene and Pleistocene terraces
(Fig. 17) across the Siwaliks along the Himalayan front, noting their relationship with
mapped structures. They were able to infer and quantify motion on the Main Frontal
Thrust of the Himalaya.
Terrace incision rates in space and time will provide important constraints on the
structural style of deformation, geometry of sub-surface structures, and time-averaged
deformation rates at multiple space and time scales. Such information can greatly aid
studies of regional strain partitioning, the interaction and evolution of individual
structures, and can provide a linkage between geologic and geodetic measures of
deformation rates.

Low Temperature Thermochronometry and Cosmogenic Dating

New methods in low-temperature thermochronometry and cosmogenic dating are
providing powerful new constraints on erosion rates, patterns and rates of exhumation
and models of geomorphic evolution. The development of U-Th/He dating of apatite with
its closure temperature of 70C provides a new tool for assessing rock cooling in the
near-surface environment where the thermal regime is dominated by erosion and motion
towards the earth's surface. The low closure temperature of this system implies
sensitivity, not only to erosion rate, but also to the form of the earth's surface, thus
providing the potential for measuring topographic relief at the time of closure. For
example, House et al. found variation in U-Th/He ages that correlated with existing
topography in the Sierra Nevada (Fig. 18), implying that the present relief of the Sierras
has existed since the late Mesozoic. U-Th/He dating of apatite also provides another
constraint on cooling histories of rocks exhumed in orogenic belts, complementing
fission-track dating of apatite and zircon and Ar40/Ar39 dating of micas and feldspars,
which have higher closure temperatures. U-Th/He dating of other minerals including
zircon and sphene will further constrain cooling histories as the kinetics of these systems
are confirmed.
Surface dating by cosmogenic isotope production provides a technique to investigate
rates of processes such as fluvial incision by dating of straths or terrace fill at a timescale
that has been historically problematic (ka to Ma). Another innovative technique that
provides more complete spatial and temporal coverage in sampling for exhumation rate is
based on analysis of the distribution of individual grain ages in a sample of modern or
ancient sediment. Modern sediment provides a sample of an entire drainage basin and the
grain exposure ages thus provide an integrated measure of the time spent in the cosmic
ray exposure zone. This technique has also been applied using thermochronometric ages
such as zircon fission track ages. This is particularly useful in estimating the history of
exhumation rate by using well-dated stratigraphic sections.

37

Figure 18: Apatite U-Th/He ages on an orogen-parallel, 2000 m elevation transect in

the Sierra Nevada, compared with the modern elevation profile, after House et al.
(1998). The correspondence between the location of deep and wide valleys and
higher He ages suggests that He ages were influenced by perturbations in isotherms
associated with paleocanyons located approximately where they are today,
suggesting relief of ~ 3 km at ~ 75 Ma, i.e., the Sierra Nevada were a large mountain
range by Late Cretaceous time.

Analysis of Digital Elevation Models

Where the distribution and nature of active structures are not well known, analysis of
topography, using digital elevation models (DEMs), can provide important first-order
information about the rates and spatial distribution of displacements. Even where
structures are well known, knowledge of lateral variations in slip along faults is
commonly sparse. Approaches to landform analysis include using abandoned geomorphic
surfaces (river terraces, alluvial fans, moraines, etc) as passive strain markers and using
river profiles as dynamic recorders of tectonic activity. As described in the section on
studies of fluvial terraces, the topography of offset geomorphic surfaces can often be used
to define displacement gradients with high precision. These studies traditionally require
time-consuming field surveys of landform morphology. However, in the coming decade,
high-resolution digital topographic data, such as that attainable with airborne laser swath
mapping (ALSM), will provide an invaluable, and highly efficient, tool for quantifying
these deformational patterns with high precision.

38

A less common, but promising, approach is to attempt to extract quantitative

information about deformational patterns in space and time from analysis of landforms as
a dynamic recorder of tectonic activity. Hillslope gradients record information about the
nature of active transport processes and the rate of erosion. However, once erosion rates
exceed the rate of soil production, hillslope gradients reach a maximum set by the
landsliding stability threshold and no longer record information about the erosion rate.
Accordingly, analyses of river profiles are often most promising. Recent examples
ranging from continental scale (Fig. 19) to the scale of a single anticline have
demonstrated the potential of this approach, although quantitative estimates of erosion
rate await refinement and testing of river incision models. With progress in the
quantification of erosional processes, DEM analyses of dynamic landforms will allow a
rapid, inexpensive assessment of active deformational patterns at an unprecedented and
unmatched spatial resolution. Digital analysis also provides opportunity to tap the rich
resource of surface and remotely-sensed climate data important to surficial processes
(Fig. 19).

Figure 19: Shaded relief and precipitation (color scale) of the Andes. Topography is
from the global 30 s GTOPO30 digital elevation model. Precipitation is from
Hoffman (1975). Orography and global climate zonation are clearly visible with wet
(blue) and dry (brown) zones reversing east-west polarity from north to south.
Figure from Montgomery et al. (2001)

Sedimentary Basin Architecture and Facies

Just as topography contains the record of tectonic activity in a mountain belt,
sedimentary basins contain the record of orogenic denudation. As the precision of
estimates of exhumation, cooling and erosion increase, comparison with the sedimentary
record will become increasingly important. In addition, the structure of sedimentary
basins is itself a measure of the accommodation space created by tectonic processes and
vertical loading. Innovative methods of relating sediment to its source have been and will
continue to be critical tools in the evaluation of coupled tectonic and surface processes.
Isotopic and thermochronometric fingerprinting to establish provenance is one example
of a powerful technique. More sophisticated techniques for deriving additional source
information from foreland basin sediments include the calculation of basin-wide
denudation rates as described above.
Sediments also provide one of the few direct measures of paleo-climate. Traditional
means of interpreting facies and depositional environment are now supplemented by
isotopic analyses of stable isotopes such as carbon to determine plant metabolic pathways

39

and oxygen to determine paleo-temperature and, more recently, paleo-altitude through

fractionation of precipitation.

Remote Sensing / Neotectonics

The quantitative characterization of active structures depends primarily on highprecision images of the earth, especially topographic data with cm- to m-scale vertical
resolution. Despite a long history of investigation, a significant fraction of active
structures within active tectonic zones have not yet been identified or precisely located.
Even for those structures that have been identified, little systematic effort has been
invested in quantitative analysis of offset features (e.g., fan surfaces, river terraces) in
order to deduce slip rates. The advent of multispectral airborne and spaceborne cameras
have made available a number of image types, such as Landsat, SPOT, and ASTER that
cover large areas at relatively fine horizontal resolution (10 m to 30 m horizontal posting)
and that have been extremely useful in identifying active structures. In addition, Light
Detection and Ranging (LIDAR) techniques, also known as Airborne Laser Swath
Mapping (ALSM), have recently become commercially available.

Figure 20. LIDAR image of SE Bainbridge island, near Seattle, showing the Toe
Jam Hill scarp of the Seattle fault. Fault was trenched by the USGS in 1998 and
1999.
LIDAR/ALSM is the first practical system for producing high-resolution images of
the earths surface in areas covered by a thick canopy of vegetation. Because the relief
created by the latest movements on active structures is only a small fraction of the
thickness of a forest canopy (of order meters versus tens of meters, respectively), it can
be very difficult to detect by conventional methods. For example, the Seattle fault in the
Puget Sound lowlands, is difficult to detect in the field and completely obscured in aerial
photography. However, LIDAR imagery clearly reveals this structure (Figure 20),
indicating its recent activity and current threat to the Puget Sound population centers. In
addition to this capability, LIDAR/ALSM may also be used to produce high-resolution
40

(25 cm) digital topographic data, which are essential for quantitative analysis of active
structures.

Co-evolution of Earth and Life

David Evans (Coordinator)

George Gehrels
Karl Karlstrom
Brian Wernicke

Introduction
This section summarizes the need for the Earth Sciences community, and tectonicists
in particular, to study the evolution of Earth in 'deep time.' As presently understood,
geological processes of billion-year antiquity appear at once rather similar to those
observed today (the principle of uniformitarianism) and also divulge some important
secular changes in planetary (and biological) evolution. The study of Earth's multibillion-year geological record allows us to assess these factors with numerous 'natural
laboratories' of ancient processes. About 85% of Earth history passed prior to the
Cambrian appearance of skeletal animals, whose rich fossil record in the succeeding 550
million years has permitted precise timekeeping of natural events. Without such a
complete fossil record, Earth scientists who delve into the Precambrian eras must rely on
diverse datasets for even a basic understanding of stratigraphic correlations, let alone
complex and interwoven Earth-system processes. Reconstruction of Archean (older than
2.5 billion years, still nearly half of Earth history) surface environments, home to the
earliest forms of life on this planet, is especially difficult due to a limited number of well
preserved sedimentary successions of that age.
Processes operating on the longest temporal scales can be related to those associated
with the largest spatial scales, and some of the broadest scales of scientific thought. For
example, one of the grandest tectonic problems on Earth concerns the episodic assembly
and fragmentation of supercontinents. The supercontinent of Pangea, which was
assembled about 300 million years ago, during a time of spectacular diversification of
animal and plant life, is now well accepted and its shape and evolution are increasingly
well understood. However, it is instructive to realize that the acceptance that the Pangean
supercontinent existed was directly linked to the acceptance of the Plate Tectonic
41

paradigm. This paradigm, that lithospheric plates move at rates of cm per year and that
tectonic processes are concentrated at plate margins, developed in the late 1960s and
became accepted in the 1970s, and may mark science's most important advance in
understanding our dynamic planet.
As Earth science matures, tectonicists are poised to help usher in new paradigms for
understanding continents in deep time. This will require forging an international
understanding of pre-Pangean supercontinents and the supercontinent cycle and will lead
to newly realized connections between Earth processes and their affects on life that will
be vitally important for humans. Problems of continental evolution bring together and
unify diverse issues such as mantle dynamics, economic geology, paleoclimate, and
surface geochemical cycles; and they also provide important paleogeographic constraints
on significant events in biological evolution. Methods of solving these ancient continental
puzzles involve diverse tools: field observations at the outcrop scale and laboratory
analyses at the microscopic scale. Tectonics, the study of the achitecture, evolution, and
dynamics of the lithosphere, is well suited to address questions of early Earth history, for
it is a discipline that uses every tool available to understand the geological record, over
the broadest ranges of spatial and temporal scales. Tectonicists, trained to assimilate
diverse data sets and integrate diverse analytical methods, will play a key role in
continued efforts to study the co-evolution of our planet and its life.

Research Questions and Opportunities

There are numerous areas for promising research on Earth systems in deep time,
where tectonicists will make important contributions as integrators of diverse data sets.
Several examples are:
1) Mass extinctions and biologial radiations.
2) Evolution of the carbon-cycle.
3) Tectonic and climatic influences on Phanerozoic diversification of life.
4) Lithosphere-asthenosphere interactions and understanding the difference between
continental and oceanic tectonics.
5) Core dynamics and influences on mantle plumes or the geomagnetic field.
6) Changes in thermal evolution through time and possible difference between Archean
Proterozoic, and Phanerozoic tectonic processes.
7) Evolution of the atmosphere and hydrosphere in the context of shifting plate
configurations.
The Neoproterozoic Era (1000 - 543 Ma) is one example of the way that any of these
major issues has potential to blossom onto new paradigms. The Neoproterozoic was
different than any time before or since. It encompassed: (a) Final assembly of Rodinia
and its subsequent breakup, and rearrangement of fragments to form Gondwanaland. (b)
Extremely rapid continental motions (as speedy as 30 cm/yr or faster) attributed either to
42

favorable combinations of plate-driving forces or true polar wander; (c) Low-latitude

glacial deposits and cap carbonates perhaps indicative of several ~10-Myr "Snowball
Earth" events followed by extreme greenhouse conditions; (d) A bolide impact leaving a
crater (at Lake Acraman in South Australia) over half the diameter of the K-T impact scar
at Chicxulub; and amid this tumult; (e) Evolution and rapid diversification of of fungi,
plants, and animals whose genetic diversity already existed, but which had not been able
to express this diversity in terms of different species and different life niches. The 1990s
was a decade of initial discovery and great speculation on many of these features in the
Neoproterozoic record. Students of the Neoproterozoic world now stand poised to make
great strides toward their understanding through theoretical development of the proposed
models, and detailed hypothesis-testing in the field and laboratory. The following
paragraphs illustrate several examples of how tectonics may contribute to solving
important enigmas of the Neoproterozoic interval.

Rodinia
Since the initial conception of a late Precambrian supercontinent in the early 1970s,
serious consideration of Rodinia's paleogeography began only about ten years ago. The
last decade has witnessed many refinements of Rodinias configuration, and some
refutations of long-held tectonic assumptions. Reconstructing Rodinia, and any other
supercontinent from the pre-Pangean era which lacks the high precision afforded by
seafloor magnetic anomalies, involves two fundamental datasets: comparisons of ancient
tectonostratigraphic links among presently fragmented continental terrains, and
paleomagnetic reconstructions in a fixed external reference frame. The former dataset is
generated by detailed field mapping, stratigraphic reconstruction, structural analysis, and
high-precision geochronology, and will increasingly rely on projects of international
cooperation and integration. The latter dataset also requires excellent field geology
combined with precise geochronology and improving paleomagnetic techniques.
Modern conceptions of Rodinia were born of qualitative tectonostratigraphic
comparisons between western North America (at the margin of the Proterozoic craton
Laurentia) and Australia-East Antarctica, combined with a very sparse paleomagnetic
data set. The SWEAT reconstruction initially found some paleomagnetic support,
although it was recognized that reliable data were few in number. Alternative
reconstructions have since arisen, emphasizing different qualitative aspects of Laurentian
geology as compared with the Trans-Antarctic Mountains, central Australia, South China,
and Siberia. Recent high-quality paleomagnetic results from Western Australia continue
to confound the 'classic' reconstructions of Laurentia with Australia and East Antarctica,
leaving substantial freedom for new models. Our present understanding of Rodinian
paleogeography is the subject of healthy and vigorous debate at nearly every geological
and geophysical meeting (Fig. 21).

43

Figure 21. SWEAT, AUSWUS, AUSMEX, or other? Increasing

tectonostratigraphic, geochronological, and paleomagnetic data acquired over the
next ten years are expected to produce a lasting, first-order picture of Rodinian
paleogeography. [After Wingate et al., 2002, Terra Nova, v.14, p.121-128.]
The Rodinian controversies have persisted primarily due to a lack of high-precision
geochronological and high-quality paleomagnetic data among Neoproterozoic and older
rocks. During the past decade of intense scrutiny, many precise U-Pb ages from these
rocks have been obtained, yet many more successions remain essentially undated. The
last decade of Rodinia hypothesis-testing has gained only a handful of reliable
paleomagnetic poles. Better dating of Neoproterozoic rocks, in the context of detailed
field and other supporting studies has provided and will continue to generate many new
opportunities for quantitative paleomagnetic constraints on Rodinia's paleogeography. In
the next decade, the international tectonics community is likely to produce a lasting firstorder map and animated reconstruction of the evolution of Rodinia.

44

Rapid Continental Motions

Velocities of between-plate motions presently attain 18 cm/yr, and although
continent-bearing plates tend to have slower speeds, India nearly achieved that rate
during its northward flight toward Asia in the early Cenozoic. In contrast, rapid motions
of large continents relative to the paleomagnetic reference frame appear commonplace in
Paleozoic and late Precambrian times (Fig. 22). Does this indicate constructive
combinations of plate-driving forces at those times, superimposed on a greater velocity of
mantle convection corresponding with generally higher geothermal heat fluxes in the
earlier parts of Earth history? Alternatively, a greater proportion of true polar wander, as
a component of all continental motions, has been proposed to explain the measured
velocities. If true polar wander was more prevalent in the past, why has it been of
minimal magnitude during Mesozoic-Cenozoic times yet presently responding to
deglaciation at rates of ~10cm/yr? Rapid continental motions, regardless of mechanism,
set the stage for increased marine biological speciation through abrupt changes in oceanic
circulation as well as regional changes in flooding of continental shelves, creating and
eliminating paleogeographic barriers. Better resolution of the Neoproterozoic
paleomagnetic record in the next decade could determine whether the presently inferred
rapid continental motions are real, and what geodynamic processes they represent.

Figure 22. The gently folded sedimentary succession contained within this
photograph, of the Flinders Ranges in South Australia, span two ~45 rotations of
the Australian continent (and any formerly contiguous neighbors) in latest
Precambrian time. Well preserved Neoproterozoic sedimentary-volcanic successions
such as this are found throughout the world, with rapidly increasing precision on
numerical age constraints. Detailed studies of these successions during the next
decade will help answer many of today's enigmas of the Neoproterozoic world.
[Photo: David Evans]
45

Low-latitude Glaciations
The essence of the Neoproterozoic climatic paradox is the widespread abundance of
glacial deposits intimately associated with warm-climate indicators such as marine
carbonates. Unusual isotopic signatures, including shifts in 13C as great as 15%,
characterize those carbonate successions, and paleomagnetic results from several glacial
deposits and bounding units indicate near-equatorial depositional latitudes. Much recent
attention and controversy has surrounded the Snowball Earth hypothesis, a suggestion
that the planet's oceans may have iced over completely for as long as 10 Myr, as many as
five times in the Neoproterozoic Era. Appearance of the enigmatic Ediacaran megascopic
fauna and earliest evidence of animal embryos follow soon in the geological record after
the last of these glaciations, and it has been proposed that enduring global ice cover could
have caused evolutionary 'bottlenecks' that cleared the way for biological advent of
multicellularity (Fig. 23). The Snowball Earth hypothesis, and any competing models for
explaining Neoproterozoic low-latitude glaciations, will be tested through detailed field
mapping (including stratigraphy and structural analysis) and multidisciplinary
combinations of laboratory techniques (including isotopic and paleomagnetic studies) and
forward modeling. The problem is inherently broad in scope, and tectonic studies will
play an important part in defining spatial and temporal boundary conditions at the
regional at local, regional, and global scales.

Figure 23. Could 'freeze-fry' extreme and sudden shifts in paleoclimate have paved
the way for the evolution and rapid diversification of animals? The Snowball Earth
hypothesis invokes global sea ice cover for as long as 5-10 million years, occurring as
many as five times in Neoproterozoic history. [From Hoffman and Schrag, 2002,
Terra Nova, v.14, p.129-155.]

46

Questions for the Next Decade

The Neoproterozoic Era serves as a telling example of how multidisciplinary
approaches, with tectonics playing a central role, will constrain important events in the
long-term history of Earth and life. Substantial advances in our understanding of the
Neoproterozoic world during the last decade have thus far produced many provocative
hypotheses, which can be seriously addressed during the next ten years. As a guideline
toward approaching these issues from the standpoint of tectonics, we focus on the
following questions:
1) As a first-order constraint on global climate and evolution, what were the positions of
oceans, continents, and major mountain ranges in Deep Time?
2) What are the correlations between extreme tectonic, climatic, and extraterrestrial
events and the rapid development of animal life?
3) Were there one or more "Snowball Earth" events in Neoproterozoic time and did the
assembly and dispersal of supercontinents play a role in generating those events? Did
Snowball events also occur in the Paleoproterozoic Era, and do extreme climatic shifts
signify a fundamentally different climatic regime on early Earth?
4) Have rapid Neoproterozoic continental motions and/or true-polar-wandering
profoundly affected surface processes? Can detailed paleomagnetic studies address these
issues as well as elucidate the long-term behavior of the geomagnetic field?
5) Do the extreme rates and magnitudes of changes possibly recorded in these natural
laboratories place limits on the severity of potential global climate change on a human
timescale?
Tectonics can contribute directly to answering these questions through:
a) identification and characterization of key stratigraphic successions through field
mapping, stratigraphy, and structural analysis; in support of concomitant advances in
paleontology, biogeochemistry, and molecular biology;
b) determining the ages and durations of these successions through high-precision
geochronology, in order to develop a global temporal reference frame for key tectonic,
climatic, and biotic events; and
c) reconstructing continental configurations using quantitative paleomagnetic tests of key
tectonic piercing points defined by tectonic syntheses.
We have emphasized the opportunities offered by integrated tectonic studies of the
Neoproterozoic as an example of the key role tectonic studies can play in Earth sciences.
As was true for the creation and testing of the plate tectonics paradigm, the tectonics
community continues to offer integration of diverse data sets, over the broadest ranges of
temporal and spatial scales, toward new paradigms that will span the physical sciences.

47

New Technological Opportunities

A number of technological or methodological innovations in recent years have
created opportunities for significant advances in understanding Neoproterozoic events,
with ready application toward older and younger times in the evolution of planet Earth.
Recent advances in high-resolution geochronology, including better control of procedural
Pb blanks in standard TIMS analysis as well as more routine application of the SHRIMP
and Laser-ablation ICP-MS techniques, have been applied to a growing suite of dateable
minerals. These include the possibility of dating sedimentary rocks through early
diagenetic xenotime overgrowths on detrital zircons and authigenic (diagenetic)
monazite. Greater sensitivities afforded by state-of-the-art mass spectrometers extend the
study of isotopes to individual phases rather than whole-rock analyses.
New methods of measuring mass-independent isotopic fractionation are providing
insights into Precambrian geochemical cycles, and in-situ analyses of stable isotopes
point to new dimensions of investigation in the same way that in-situ dating has helped to
revolutionize geochronology. Standards of paleomagnetic reliability have increased due
to the recognition of widespread partial overprinting, combated by detailed field stability
tests on the ages of magnetization. Just as recent geochemical approaches have
recognized microscopic heterogeneities in rocks or even single crystals, the newly
developed SQuID (magnetometer) Microscope is leading traditional paleomagnetic
directional studies toward ever smaller spatial scales.

Facilities

George Gehrels (Coordinator)

Andreas Kronenberg
Brain Wernicke
Kelin Whipple
Teng-fong Wong

Cosmogenic Isotope Studies

Cosmogenic isotope studies provide important constraints on tectonic processes that
have affected the Earth's surface during the past several hundred thousand years, a time
range that has previously been difficult to study. Dating of geomorphic surfaces has
traditionally been accomplished via carbon-14 dating, which requires scarce
carbonaceous materials, and is limited to features under about 50,000 years old. Although
Ar-Ar dating may be effective down to this age range, it too suffers from difficulty in
48

finding datable materials, such as volcanic ashes, associated with any given surface.
Cosmogenic dating can be applied to most common rocks types, is accurate for features
ranging in age from a few thousand to a few hundred thousand years, and these methods
open up a broad new frontier of investigation of geologic processes at these time scales.
Given the impact of earthquakes, volcanic eruptions, and landslides on society, there is an
urgent need to increase our understanding of the rates and magnitudes of these processes
and their tectonic framework. It is therefore critical that funds are available to develop
this technology as rapidly as possible.
At present, most cosmogenic isotope analyses are conducted with accelerator mass
spectrometer (AMS) facilities at Purdue University, Lawrence Livermore National Labs,
and the University of Arizona. These facilities are able to meet the current demand for
analyses, but future demands for higher sample through-put, better precision, and smaller
sample size will require both improvements in facilties and a larger number of highly
skilled researchers. In particular, sample preparation, which involves routine wet
chemical methods and can be done at a separate facility from the actual AMS analyses, is
currently a major bottleneck in investigations involving cosmogenic dating. A facility
dedicated to both sample preparation and the analysis of 26A1, 10Be and 36C1 would be a
tremendous asset for studies of the age of surficial features and for determinations of
paleo-elevation. In addition to support for large research centers, there is also a need for
refinement of the production rates of cosmogenic nuclides, and for the development and
application of new isotope systems.

Geochronology and Thermochronology

Geochronology and thermochronology provide critical constraints on the ages of
geological events and on the rates of geological processes. This information is central to
many aspects of tectonic analysis, and will become even more so with future attempts to
quantify how our planet has evolved through geologic time and how tectonic processes
affect life on Earth. It is therefore vital to maintain strong support for the application of
existing geochronologic and thermochronologic techniques, and for the development of
new chronometers. Figure 24 shows the array of thermochronometers that are in current
use, and the approximate temperature range below (or within) which each chronometer
begins to record the passage of time.
At present, most geochronologic and thermochronologic information is generated by
individual researchers (and their staff) using facilities at academic institutions. Most of
these researchers and facilities are supported primarily by EAR at NSF. Development of
a national geochronology/thermochronology center may be an effective means of making
this essential information more available to tectonics researchers, and of driving the
development of new geo- and thermochronometers.

Studies of Rock Deformation

The fundamental understanding of dynamic processes as diverse as the earthquake
cycle, fluid transport through the crust, and sedimentary basin development hinges upon
critical input from laboratory measurements. Determination of the mechanical and
transport properties of rocks is a necessary first step in understanding the tectonic
processes and structures at all scales in the Earth. To understand the spatio-temporal
49

complexity of seismicity, it is necessary to investigate the frictional behavior at sliding

rates ranging from quasi-static to seismic and to relatively large slip. To elucidate the
complex interplay of metamorphic reactions, fluid transport, and mechanical
deformation, it is necessary to investigate the rheology and failure mode to relatively
large strain under controlled conditions of nonhydrostatic loading, fluid drainage and
reaction kinetics. Furthermore it is important to systematically characterize the
microstructure of naturally deformed samples, down to TEM-scale, in order to correlate
the micromechanical processes before one has confidence in extrapolating the laboratory
data to relevant spatial and temporal scales.

Figure 24. Thermochronometers that are in current use, and their effective closure
temperatures (adapted from P. Fitzgerald, S. Baldwin, G. Gehrels, P. Reiners, and
M. Ducea).
These questions pose significant technical challenges that necessitate the
development of new experimental and analytical facilities. In the past decade significant
advances have been made in the use of the gas apparatus for determination of rheology
and hydromechanical properties with the advent of the Paterson rig. While there are
currently two such rigs in the U.S., eight others are located in Europe, as a result of
superior funding and prioritization of this research. With the establishment and growth of
50

rock deformation laboratories and analytic facilities such as new scanning electron
microscopes capable of measuring fabrics by EBSD, European countries have taken the
lead in studies of crustal and mantle rheology.
Several laboratories in this country have the shear and triaxial compression rigs
capable of studying frictional instability and failure mode of crustal rocks. However,
there has only been one new rig established in a US academic institution in the past
decade, while five new machines have been developed for such endeavors in Tokyo,
Kyoto and Tsukuba. There is an urgent need to implement the infrastructure that would
nurture the necessary technological advances, provide the wider availability of existing
experimental and analytical facilities, and encourage synergistic collaborations among
researchers in rock mechanics, field geology, numerical simulation and materials science.

Geodetic Studies
There have been dramatic improvements in our ability to measure the position and
relative movement of different regions of the earth's surface. The Global Positioning
System (GPS) permits us to determine the location of any point on the earth's surface to
within a millimeter or so. As a result, we can measure the relative velocity of any two
points on the earth's surface to within a fraction of a millimeter per year, after just 2-3
years of monitoring. Similarly, interferometric synthetic aperture radar (InSAR) allows us
to create maps showing the movement of broad areas of the surface over time (surface
velocity fields). Combined, these two methods will soon yield seamless, high spatial- and
temporal-resolution maps of surface movement across entire plate boundary systems.
Because of the power of these techniques to measure short-term tectonic motion, it is
important that structural geology and tectonics researchers participate in the design of
future experiments to gather and interpret this type of information.
Satellite-based geodesy is one area in which establishment of one or more dedicated
research centers in the U.S. would be more effective than individual or small-group
research programs. At present, the Earthscope initiative is laying plans for dense geodetic
coverage of the western part of the United States. However, a solid geodetic
infrastructure, through continued support of the University Navstar Consortium
(UNAVCO), will be necessary for projects involving active deformation outside of the
western US. This infrastructure should include GPS instrumentation available for
campaign-style geodetic surveys, expertise in the development of continuous GPS
monitoring, and a readily accessible community database of InSAR imagery that may be
processed and analyzed by small teams of investigators.

51

Educational Departures in Structural Geology and

Tectonics

David Pollard (Coordinator)

Rebecca J. Dorsey
James P. Evans
Raymond C. Fletcher
Laurel B. Goodwin
Jane Selverstone

Introduction
Within the context of developments in the fields of structural geology and tectonics
(SG&T), we recognize a number of challenges and opportunities in geoscience education
that range from K-12 and public outreach through undergraduate and graduate education.
While it is beyond the scope of this document to address all of these issues, we have
identified some unifying themes that were raised at the workshop and in subsequent
discussions, which are summarized below.
Educational priorities in SG&T parallel those of the broader Earth Science
community, as articulated in a rich literature of ongoing publications, web sites and
workshops. Fundamental scientific literacy, rigorous scientific methodology, and the
development of quantitative skills at all levels, from K-12 through undergraduate and
graduate education, underlie these priorities. Many scholars have discussed the need for
integrating quantitative methods into geoscience classes, interactive exercises,
involvement of students in research, and computer visualization tools as viable strategies
for increasing the effectiveness of earth science education. Geoscience education based
on such a foundation will play an important role in preparing the next generation of
researchers, teachers, and citizens for future challenges. Here we focus on a few priorities
that are especially relevant for SG&T education.
Research in structural geology and tectonics is becoming increasingly quantitative
while retaining its orientation as a field-based, observationally motivated science. The
integration, for example, of structural, petrologic, geochemical, stratigraphic,
geomorphologic and geochronologic studies permits us to produce conceptual models
that can be used as a foundation for analytical and numerical models that test hypotheses
generated in the field. This process-based approach has already started to yield exciting
52

new results (for example, the use of low-temperature thermochronology in constraining

rates and processes of crustal exhumation in orogenic belts), and will continue to provide
significant insights into Earth's deformation and driving forces in the coming decades.
To support movement toward this goal, we encourage education at all levels that will
unite structural geology, tectonics, petrology, geochemistry, geochronology, geophysics,
and other related disciplines in a broad-based field of inquiry that is motivated by
fundamental questions about processes of Earth deformation. This objective can be
supported with workshops, short courses, and symposia, and through development of
course curricula in the classroom, laboratory, and field settings. Our suggestions for
addressing these educational needs are presented in three areas: (1) interdisciplinary, (2)
undergraduate, and (3) graduate education.

Interdisciplinary Education
We anticipate that the next generation of students will have unprecedented
opportunities and needs to work across disciplinary boundaries. Indeed, many of the
research targets that we have identified elsewhere in this document will require input
from, and interaction with, geoscientists in the fields of hydrology, petrology, low- and
high-temperature geochemistry, stratigraphy, geodesy, seismology, and geomorphology,
as well as with experts from other sciences. Many of the research targets also admit input
from the engineering disciplines including civil and mechanical engineering, chemical
engineering, computer science and materials science. We thus need to ensure that
students are given the tools to facilitate this cross-disciplinary research. The necessary
tools include: (1) an emphasis in all geoscience courses on cross-disciplinary
communication skills, and (2) appropriate course content from both the earth sciences
and from allied disciplines (math, physics, chemistry, and biology). In light of this need,
we make the following recommendations:

Not everyone can be an expert in all topics, and thus collaborative efforts will
continue to be essential to move our science forward. We recommend placing
increased emphasis on collaborative projects, papers, proposals, and
presentations in many geoscience courses, at both the undergraduate and
graduate levels. Although such work can be difficult to initiate and evaluate,
it helps to develop the skills necessary for successful communication with
colleagues.

We encourage college and university earth science departments to be creative

and flexible in defining programs of study. Tuition and credit should be given
for course work in mathematics, physics and chemistry, as well as related
engineering disciplines. We recognize that this may require changes at
departmental and higher levels, particularly at state universities, but feel that
it is essential for adequate training of the next generation of structural
geology and tectonics students.

Undergraduate Education
Here we emphasize the need for improved quantitative skills in mathematics and
statistics, information technology, physics, and chemistry in undergraduate curricula. All
geoscience students entering graduate school require a strong background in ancillary
53

sciences, mathematics and statistics. Anecdotal evidence suggests that our students'
ability to use this background effectively is directly correlated with the degree to which
we successfully integrate quantitative methods into geoscience classes. It is also
important, however, to teach quantitative skills at an accessible level to non-major
undergraduate students, as this contributes to an educated public that can interpret graphs
and plots and evaluate the results of scientific research that bear on public policy.
In addition to the above priorities, we must also keep in mind that what sets us apart
from some other disciplines is our ability to make observations about the natural world
and interpret these observations through critical analysis. It is important that
undergraduate students learn to think critically and solve problems in situations that
simulate the challenges of field work. For this, we encourage teaching of undergraduate
geoscience courses that require students to:

record and interpret primary observations, and collect original data in the
field or lab;

learn how to handle data (i.e. analysis, interpretation, hypothesis testing,

etc.);

work with complex problems that require integration of different methods

and tools;

predict outcomes and understand dynamic relationships of complex systems;

and

manipulate and interpret statistical uncertainties in data and model

calculations.

This approach goes beyond the call for a strengthening of curricula with more
mathematics, physics, and chemistry, and stretches into nontraditional settings (field and
lab) where students learn to think and engage in science through their own experience of
inquiry, hypothesis-testing, analysis, and interpretation. The increasing complexities and
uncertainties of a highly technological world require us to synthesize complex and
sometimes contradictory information, and compile it into a coherent understanding of
dynamic systems. Critical-thinking and problem-solving skills gained in this way will
benefit students whether they go on to graduate school, law school, business, teaching, or
public policy, and will therefore benefit society as a whole.
A few examples of related earth-science web resources and recent workshops
include:
Digital Library for Earth System Education (http://dlesecommunity.carleton.edu/)
Science Education Resource Center (http://serc.carleton.edu)
NAGT 'On the Cutting Edge' professional development workshop series
Building Quantitative Skills of Students in Geoscience Courses (NAGT, 2000)

54

Graduate Education
As is evident from the research areas highlighted in this document, there are many
paths through which we might train students in more quantitative approaches to structural
and tectonic problems. The exact path chosen by a given researcher will depend on the
hypotheses to be tested and both individual and institutional goals and resources. Beyond
advanced courses in structural geology, tectonics and related geological and geophysical
subjects, graduate training might therefore include (but need not be exclusive to) one or
more of the following: differential equations and linear algebra, thermodynamics,
continuum mechanics, hydrogeology, inverse theory, numerical methods, and/or spatial
statistical analysis.
Because it is beyond the scope of this document to address all of the opportunities
available to teachers at the graduate level, we identify particular examples of two of the
themes introduced above. For the development of quantitative skills we point to the role
of differential geometry in characterizing geological structures. We cite continuum
mechanics as an example of the fundamental scientific literacy necessary for mechanical
modeling in structural geology and tectonics. Finally, the adoption of a rigorous scientific
methodology is illustrated with a flow chart applicable to quantitative investigations. It is
recommended that graduate-level curricula in structural geology and tectonics include
lectures and practical exercises that demonstrate the utility of mathematical concepts
(such as differential geometry), and utilize the solutions of relevant problems from
related disciplines (such as continuum mechanics) for the analysis of tectonic processes.

Characterization of Geological Structures using Differential

Geometry
Geological structures are locally classified as linear (e.g. slickenlines, rib marks,
metamorphic lineations, and surface intersections) or planar (e.g. sedimentary bedding,
faults, fractures, metamorphic foliations, and unconformities), but these structures are
inherently three-dimensional: they are, in fact, curves and curved surfaces. As such,
differential geometry is the appropriate tool for their quantitative description. Differential
geometry includes the analytical study of points, curves, and surfaces in threedimensional space using vectors and the methods of calculus. The elementary concepts of
differential geometry enable one to describe the departure of geological lineations from a
straight line and the departures of geological surfaces from a plane.
Data on the local orientation of structures are gathered at scattered outcrops as point
measurements and the locations of these points are geographic coordinates measured, for
example, using the Global Positioning System (GPS). The classical procedure of plotting
the attitudes of a set of structures on a stereographic projection enables one to compare
their orientations. While serving a useful purpose, stereographic projections provide an
inadequate characterization for some applications because they lack spatial information.
It may be clear from a structural map that a particular surface has different strikes and
dips at different outcrops, but what is the shape of the surface? Differential geometry
provides the tools for the quantification and analysis of these shapes.
As an example consider the fact that for over a century structural geologists have
worked to elucidate the mechanisms by which sedimentary strata are deformed during
55

folding. Analogue models of folds (Figure 25) have played an important role in these
investigations. The results of model experiments, whether physical or numerical, are
compared to descriptions of natural folds to test hypotheses about the folding
mechanisms. However, the various geometrical measures of folds in common use today
are inadequate to describe uniquely the three-dimensional spatial variations in fold shape.
Geological surfaces are sufficiently described by the two fundamental forms of
differential geometry, and the unit normal vectors and principal normal curvatures, kmin
and kmax, can be calculated from these quantities to characterize folded surfaces (Figure
26).

Figure 25: Geometry of a multiply-folded surface obtained by scanning an analogue

model. The surface was constructed using 984 data points with 1 mm precision.
Data courtesy of D. Grujic (2002)
Modern technological innovations such as GPS, 3-D seismic reflection, and 3-D scanning
provide data that allow a more rigorous approach to the description and subsequent
analysis of deformed surfaces. Incorporating the elementary concepts and methods of
differential geometry in the curriculum of structural geology and tectonics can provide a
good starting point for productive discussions of the geometry of geological structures.

Analysis of Tectonic Processes Using a Complete Continuum

Mechanics
A complete mechanics, as we understand it, includes a complete sub-set, or 'N
equations in N unknowns', of the laws of conservation of mass, momentum and energy,
of relations describing the kinematics, and of the constitutive relations describing
material behavior. The familiar kinematic quantities of displacement and velocity, and
the associated displacement gradient tensors (strain and rotation) and velocity gradient
tensors (rate of deformation and vorticity) are found throughout these equations, but are
56

by no means the only physical quantities. In its simplest forms a complete mechanics is
represented by linear elastic theory and the dynamics of linear viscous fluids. However,
relevant constitutive laws are not limited by linearity and isotropy, but include non-linear
material behavior, anisotropic materials, and finite deformation. Also, numerical
solutions using the finite element method or boundary element method, coupled with
high-speed computing, admit consideration of heterogeneous and discontinuous bodies of
complex geometry. Therefore, problems with complex constitutive behavior, boundary
conditions, and geometry are tractable.

Figure 26: Principal normal curvatures for the multiply folded surface shown in
Figure 26. a) Contour plot of kmin. b) Contour plot of kmax. White tic marks are
trajectories of kmin. From S. Bergbauer, PhD Thesis, Stanford University, 2002.
Using a complete mechanics does not result in the simulation of all the details in a
process leading to a particular structure or structural type. Instead, it requires that explicit
choices have been made of constitutive relations, boundary conditions, and initial
conditions, which together with the fundamental laws produce a closed set of relations
from which all results follow. If the model results do not conform in all aspects with the
field data, a more refined model may be formulated by a different choice of these mutable
elements. The explanatory power of simpler models is examined first; often providing
significant physical insight. Then robustness and more detailed simulation are pursued,
thereby providing testable hypotheses and refutable outcomes.

57

As an example of a scientific methodology that applies the extensive machinery of

the physics of the processes involved, most centrally rock deformation, to both suggest
observations and to analyze, visualize, and interpret them we offer Figure 27. In this way,
a coherent and self-consistent, if idealized, and non-unique, re-construction of the
development of a geological structure may be achieved. Tectonic processes and their
products will not be completely described by mechanical models; however, for prescribed
initial and boundary conditions, forward models are generated that may produce
likenesses of some of the observed geological structures and fabrics, to some satisfactory
degree of approximation. If the forward model fails to produce satisfactory likenesses we
learn that one or more of the postulates is inappropriate and must be excluded or
modified (i.e., one may conduct an analysis of the sensitivity of the process of interest to
specific intrinsic and extrinsic variables). One cannot, however, exclude or modify the
fundamental laws upon which the mechanical model is based. The general approach
includes the construction of a sequence of quantitative models, graduated in their degree
of detail and successively providing an improved understanding.

Figure 27: A methodology for designing, implementing, and testing complete

mechanical models of rock deformation.

58

Contact Information
Dr. David D. Pollard (Chair)
Dept. of Geol. and Env. Sciences
Stanford University
Stanford CA 94305-2115
Phone: 415-723-4679
Email: dpollard@pangea.stanford.edu
Dr. Rick Allmendinger
Dept. of Earth & Atm. Sciences
Cornell University
Ithaca, NY 14853-1504
Phone: 607-255-3376
E-mail: rwa1@cornell.edu
Dr. Mark T. Brandon
Dept. of Geology and Geophysics
Yale University
P.O. Box 208109
210 Whitney Avenue
New Haven, CT 06520-8109
Phone: 203-432-3135
Email: mark.brandon@yale.edu
Dr. Roland Burgmann
Dept. of Earth and Planetary Science
University of California, Berkeley
307 McCone Hall
Berkeley, CA 94720-4767
Phone: 510-643-9545
Email: burgmann@seismo.berkeley.edu
Dr. Rebecca J. Dorsey
Department of Geological Sciences
1272 University of Oregon
Eugene, OR, 97403-1272
Phone: 541-346-4431
Email: rdorsey@darkwing.uoregon.edu

Dr. Andreas K. Kronenberg

Dept. of Geology & Geophysics
Texas A&M University
3115 Texas A&M University
College Station, TX 77843-3115
Phone: 979-845-0132
Fax: 979-845-3002
Email: a-kronenberg@tamu.edu
Dr. Tracy Rushmer (Priorities Workshop
Rep.)
Dept. of Geology
Perkins Hall
University of Vermont
Burlington, VT 05405-0122
Phone: 802-656-8136
Email: trushmer@zoo.uvm.edu
Dr. Jane Selverstone
Dept. of Earth & Planetary Sciences
Northrop Hall
200 Yale Blvd. NE
University of New Mexico
Albuquerque, NM 87131-1116
Phone: 505-277-6528
Email: selver@unm.edu
Dr. John H. Shaw
Dept. of Earth & Planetary Sciences
Harvard University
20 Oxford St.
Cambridge, MA 02138
Phone: 617-495-8008
Email: shaw@eps.harvard.edu

59

Dr. David A. Evans

Department of Geology and Geophysics
Yale University
P.O. Box 208109
New Haven, CT 06520-8109
Phone: 203-432-3127
Email: david.a.evans@yale.edu
Dr. James P. Evans
Dept. of Geology
Utah State University
4505 Old Main Hill
Logan, UT 84322-4505
Phone: 435-797-1267
Email: JPEVANS@cc.usu.edu
Dr. Raymond C. Fletcher
Dept. of Geological Sciences, Box 399
University of Colorado
Boulder, CO 80309-0399
Phone: 303-735-4723
Email: fletchr@spot.colorado.edu
Dr. David M. Fountain (Geophys. Program
Dir.)
Program Director, Geophysics
National Science Foundation
4201 Wilson Boulevard
Arlington, VA 22230
Phone: (703) 292-8556
Email: dfountai@nsf.gov
Dr. George E. Gehrels
Dept. of Geosciences
The University of Arizona
Gould-Simpson Building, 1040 E. Fourth
St.
Tucson, Arizona 85721-0077
Phone: 520-621-6026
Email: ggehrels@geo.arizona.edu

Dr. Carol Simpson Provost's Office

Boston University
143 Bay State Road
Boston MA 02215
Phone: 617-353-6851
Email: csimpson@bu.edu
Dr. Bruno Vendeville
Bureau of Economic Geology
The University of Texas
University Station, Box X
Austin, Texas 78713-7508
Phone: 512-471-8334
Email: bruno.vendeville@beg.utexas.edu
Dr. Brian P. Wernicke
Div. of Geological and Planetary Science
California Institute of Technology, MC
100-23
Pasadena, CA 91125-0001
Phone: 626-395-6192
Email: brian@gps.caltech.edu
Dr. Kelin Whipple
Dept. of Earth, Atm. and Planetary
Sciences
Massachusetts Institute of Technology
Cambridge, MA 02139
Phone: 617-253-2578
Email: kxw@mit.edu
Dr. Sean Willett
Dept. of Earth and Space Sciences
University of Washington
Box 351310
Seattle, WA 98195
Phone: 206-543-8653
E il
ill tt@
hi t
d
60

Dr. Laurel B. Goodwin

Dept. of Earth & Env. Sciences
New Mexico Institute of Mining &
Technology
801 Leroy Place
Socorro, NM 87801-4796 USA
Phone: (505) 835-5178
Email: lgoodwin@nmt.edu

Dr. Martha Withjack

Dept. of Geological Sciences
Rutgers University
610 Taylor Road
Piscataway NJ 08854-8066
Phone: 732-445-6974
Email: drmeow3@rci.rutgers.edu

Dr. Arthur Goldstein (Tectonics Program

Dir.)
Tectonics Program Director
The National Science Foundation
4201 Wilson Boulevard
Arlington, Virginia 22230, USA
Phone: 703-292-8552
Email: agoldste@nsf.gov

Dr. Teng-fong Wong

Dept. of Geosciences, ESS Building
State University of New York at Stony
Brook
Stony Brook, NY 11794-2100
Phone: 631-632-8212
Email: wong@mantle.geo.sunysb.edu

Dr. Karl Karlstrom

Dept. of Earth and Planetary Sciences
Northrop Hall
University of New Mexico
Albuquerque NM 87131
Phone: 505-277-4346
Email: kek1@unm.edu

Dr. An Yin
Dept. of Earth and Space Sciences
University of California, Los Angeles
595 Charles Young Drive East, Box
951567
Los Angeles, CA 90095-1567
Phone: 310-825-8752
Email: yin@ess.ucla.edu
Dr. Herman B. Zimmerman (EAR Div.
Dir.)
Director, Division of Earth Sciences
The National Science Foundation
4201 Wilson Boulevard
Arlington, Virginia 22230, USA
Email: hzimmerm@nsf.gov

61