Glossary of Normal Faults Peacock Et Al 2000
Glossary of Normal Faults Peacock Et Al 2000
Glossary of Normal Faults Peacock Et Al 2000
STRUCTURAL
GEOLOGY
PERGAMON
Abstract
Increased interest in normal faults and extended terranes has led to the development of an increasingly complex terminology.
The most important terms are defined in this paper, with original references being given wherever possible, along with examples
of current usage. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The large amount of work on normal faults over the
last few decades has led to the introduction of m a n y
new terms and some confusing terminology. For
example, the term relay ramp (Larsen, 1988) has also
been applied to structures in strike-slip fault zones
(e.g. Herzer and Mascle, 1996), and other terms have
been applied to relay ramps in normal fault systems
(e.g. accommodation zone, fault bridge, transfer zone).
The aim of this glossary is to give definitions of the
most important terms currently used in connection
with normal faults (Fig. 1). The glossary is intended
for non-specialists who have not read the large number
of recent papers about normal faults. Other general
terms for faults are available in standard textbooks
(e.g. Ramsay and Huber, 1987; Price and Cosgrove,
1990; Twiss and Moores, 1992), as are general terms
used in structural geology (such as strain and stress).
Similar glossaries for thrusts are given by Butler (1982)
and by McClay (1992), and for strike-slip faults by
Biddle and Christie-Blick (1985). We have attempted
to find the first usage, or the earliest reasonable usage,
of terms. A problem encountered when attempting to
Glossary
Accommodation zone." The area between two subparallel, non-collinear, overlapping faults (e.g.
Reynolds and Rosendahl, 1984). Rosendahl et al.
(1986, fig. 8) define an accommodation zone as
transferring displacement or strain from one halfgraben to another with opposite sense via oblique
shear along an inter-basinal ridge. See transfer zone.
For a discussion of the Faulds and Varga (1998)
redefinition of accommodation zone, see transfer
zone.
Antithetic .fault. Defined by Cloos (1928) and Hills
(1940, fig. 41) as a minor fault that dips in the
opposite direction to the dip direction of the beds
they displace. Antithetic fault is now commonly used
for a fault that dips in the opposite direction to a
related dominant fault or fault set, while antithetic
is the relationship whereby two related faults have
the opposite shear sense (e.g. Gibbs, 1984) (Fig. 1).
For example, a normal fault with a downthrow to
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seam.
Density, .fault. A measure of the number of faults in a
rock mass (e.g. Gillespie et al., 1993). Fault density
may be measured in one, two or three dimensions,
respectively as: (i) the number of faults per unit
distance (usually measured perpendicular to the
fault set); (ii) the total length of fault trace per unit
area; or (iii) the total area of fault plane per unit
volume. The unit of fault density is m -~ and is
inversely proportional to fault spacing.
Detachment: Fault along a basal surface, along which
overlying strata are detached (Pierce, 1963).
Detachment is now commonly used for a regionally
extensive, gently dipping normal fault (Fig. 1); they
are commonly associated with extension in
metamorphic core complexes and can displace
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uplift.
Hard-linkage: The geometry or process whereby two
faults are connected by one or more (usually
smaller) faults that are visible at the scale of
observation
(Walsh and
Watterson,
1991).
See
block, fault.
Horst: Elongate area of uplift mostly bounded by subparallel normal fault zones that dip away from the
area of uplift (e.g. Reid et al., 1913; Dennis, 1967)
(Fig. 1). Horsts are commonly bounded by grabens
or half-grabens.
Interaction, fault: Mechanical interaction between
faults involves the behaviour and development of
one fault being influenced by another fault or faults
(e.g. Segall and Pollard, 1980; Aydin and Schultz,
1990; Bfirgmann et al., 1994). Overstepping normal
faults interact across transfer zones (Morley et al.,
1990). Rift interaction is described by Nelson et al.
(1992).
Inversion." Originally used for the overturning of strata,
so the sequence is reverse (e.g. Murchison, 1859).
Inversion is now commonly used for the reactivation
of a dip-slip fault such that there is a reversal of the
sense of throw (e.g. Stoneley, 1982; Buchanan and
Buchanan, 1995). Reverse-reactivation of a normal
fault or the contraction of a region that previously
underwent extension is termed positive inversion.
Alternatively, negative inversion is the normalreactivation of a reverse fault or the extension of a
region that previously underwent contraction (e.g.
Williams et al., 1989). For a discussion of the usage
of inversion and reactivation, see Needham (1989).
Isolated fault: A fault that was not affected by
interaction with other faults during its propagation
(e.g. Muraoka and Kamata, 1983; Walsh and
Watterson, 1987). An isolated fault is usually
characterised by a displacement maximum near the
centre of the fault trace, with displacement
decreasing approximately linearly towards the tips
(Barnett et al., 1987; Walsh and Watterson, 1987).
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event.
Slip, cumulative: The finite displacements produced by
a number of individual slip events on a fault (Walsh
and Watterson, 1987).
Slip event: A single displacement event on a fault (e.g.
Walsh and Watterson, 1987), i.e. associated with an
earthquake (Cowie and Scholz, 1992b). The net slip
and final displacement profile on a fault is usually
the result of many slip events (Walsh and
Watterson, 1987; Peacock and Sanderson, 1996).
Slip plane: Used in material science for a plane along
which displacement may occur (e.g. Dennis, 1967).
Used in geology for a surface along which
displacement has occurred (e.g. Stewart and
Hancock, 1991). Synonymous with fault plane.
Slip vector." The orientation and magnitude of the
displacement of formerly adjacent points on
opposite sides of a fault measured along the fault
surface (Reid et al., 1913), cf. displacement vector.
Smear: Wall-rock material, usually clay-rich, spread
along a fault surface (Perkins, 1961; Knipe, 1992;
Knott, 1993; Lindsay et al., 1993). See seal.
Soft domino." Block rotation accommodated by ductile
strain, including faults below the scale of the
analysis (Gibson et al., 1989; Walsh and Watterson,
1991, fig. 10). This behaviour implies geometric
coherence.
Soft-linkage." Coherence between faults achieved by
ductile strain of the wall-rocks, i.e. there is no
linkage by faults visible at the scale of observation
(Walsh and Watterson, 1991). A relay ramp is an
example o f soft-linkage. See hard-linkage and soft
domino.
Spacing, fault: The (average) distance between faults,
usually measured in a line perpendicular to the fault
plane or the fault strike. It only has a rigorous
definition, therefore, for sets of sub-parallel faults,
but may be applied to traverses perpendicular to the
strike of conjugate normal fault sets. Discontinuity
spacing is discussed by Priest and Hudson (1976,
1981). See fault density.
Splay, fault: One or more smaller faults that join a
larger fault, to which it is related (e.g. Biddle and
Christie-Blick, 1985, fig. 2). The larger fault splays if
it is connected with one or more splay faults
(DeSitter, 1956). See branch.
Stepover. Synonymous with overstep (Aydin and Nur,
1982).
heave.
Tilting." Rotation of a fault block about a horizontal
axis. Tilting commonly accompanies normal faulting
(e.g. Anders et al., 1993), for example in relay ramps
(Peacock and Sanderson, 1994, fig. 8). Synonymous
with horizontal axis rotation.
Tip: A tip point is a site where fault displacement dies
to zero, e.g. where a fault trace ends on a map (e.g.
Coward and Potts, 1983) (Fig. 1). A tip line is the
line around a fault plane where displacement
becomes zero. Barnett et al. (1987) suggest that
isolated normal faults have elliptical tip lines. A tip
zone is an area of damage at the termination of a
fault. Synonymous with fault termination.
Transfer fault: A fault that links, is at a high angle to,
and that transfers displacement between two normal
faults (Gibbs, 1984, fig. 15) (Fig. 1). Transfer faults
between normal faults in map view usually have a
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Acknowledgements
W o r k for this paper was funded by an N E R C
R O P A award to the Rock Deformation Research
Group, University of Leeds. Comments by Bob
Janssen, Greg Jones and Eddie McAllister are appreciated, and Nick Allin is thanked for drawing Fig. 1.
Careful reviews by G a r y Axen, Ernest Duebendorfer,
James Evans, James Faulds, Susanne Janecke and Jon
Spencer greatly improved this paper.
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