Journal of Structural Geology, Vol. 8, No. 7, pp.

725 to 735, 1986

Printed in Great Britain

0191-8141/86 $03.00 + 0.00

Pergamon Journals Ltd.

Strike-slip duplexes
NIGEL H . WOODCOCK
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.

and
MIKE FISCHER*
Department of Geology, University College, Cardiff CF1 1XL, U.K.

(Received 6 September 1985; accepted in revised form 13 December 1985)

Abstract--Strike-slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate
fans and duplexes in dip-slip, thrust and normal, fault systems. They are evident in map view rather than in
vertical sections. Examples of duplexes are cited from both active and ancient systems and from theoretical and
physical models. Duplexes may form at bends on strike-slip faults by a process kinematically analogous to the
sequential imbrication of ramps on dip-slip faults. However some may form, and many may initiate, as
non-sequential 'Riedel' fractures at fault offsets or on straight fault segments. This process is more marked than
in dip-slip systems where primary anisotropy such as bedding exerts more control on fault geometry.
Strike-slip duplexes may be shunted along the fault system parallel to the regional slip vector. However,
duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the
unconstrained upper surface to the fault system. These factors mean that no section through a strike-slip system
should be expected to area balance. The faults of strike-slip duplexes and imbricate fans may root in kinematically
necessary low-dip faults or may converge downwards and appear in vertical sections as flower structures.

DUPLEX AND FAN GEOMETRY

IMBRICATE fault arrays have long been recognized in

thrust systems (review by Boyer & Elliott 1982). Imbricate faults that splay upwards off a sole thrust form an
imbricate fan, and those that converge upwards again
into a roof thrust form a duplex (Dahlstrom 1970).
Formation of duplexes and fans by sequential contractional collapse of a footwall ramp is understood in theory
and demonstated by natural examples (e.g. Boyer &
Elliott 1982, Butler 1982). Extensional duplexes have
now been recognized at ramps on major normal faults
(Gibbs 1984). A number of previous authors (e.g.
Kingma 1958, Lensen 1958) have recognized imbricate
fault arrays in strike-slip systems, best seen in map view
rather than vertical section. In this paper we explore the
analogy between these strike-slip fans and duplexes and
their better known dip-slip counterparts. The formation
of strike-slip duplexes is best understood as a kinematic
response to imposed boundary constraints, rather than
by the stress-control or bulk strain approaches usually
applied to wrench tectonics.
An idealized strike-slip system (Fig. 1) shows the
terminology used in this paper. In map view faults
comprise straights, segments sub-parallel to the regional
slip vector, and bends oblique to it. Displacements may
be transferred between two faults across an offset. Pure
strike-slip on the straights necessarily causes potential

* Present address: B.P. Petroleum Development Ltd., Farburn

Industrial Estate, Dyce, Aberdeen AB2 0PB, U.K.

overlap at a bend of one sense (restraining bend, Crowell

1974) and a potential void at a bend of the other sense
(releasing bend, Crowell 1974). Contractional duplexes
may form at restraining bends or offsets and extensional
duplexes at releasing bends or offsets. At the lateral fault
tips, splays of smaller faults may form steep imbricate
fans which themselves may be extensional or contractional. Fans may be designated as leading or trailing
depending on the sense in which the major, higher
displacement, strand flanks the fan (cf. Boyer & Elliott
1982). Imbricate faults usually have a dip-slip component (Fig. 1), dominantly with a normal sense in an
extensional fan or duplex and a reverse sense in a
contractional fan or duplex.

EXAMPLES OF STRIKE-SLIP DUPLEXES

Natural examples of strike-slip duplexes (Fig. 2) will

be referred to individually later. However, some general
features are of note here. The duplexes are usually
bounded by two continuous major fault zones, possibly
with large displacement (e.g. Figs. 2d(I), i, k & 1), or two
zones of high fracture density (Figs. 2h(3) & j). Between
these zones smaller en-6chelon faults define the duplex
structure. These faults usually have a component of
strike-slip combined with either normal dip-slip (Figs. 2i
& j) or reverse dip-slip (Fig. 2g). The individual horses
defined by these imbricate faults have lengths varying
between about half and twice the spacing of the major
faults that bound the duplex. This ratio is probably
modified by deformation and rotation of the horse.

725
SG 8 : 7 - A

726

N . H . WOODCOCKand M. FISCHER
releasing
bend

restraining
bend

releasing
offset

overiap

stTaight

restraining
offset

straight

trailing extensional
imbricate fan
leading extensional
imbricate fan

leading contractional
imbricate fan

extensional duplex

JJjj

contractional duplex

trailing contractional
imbricate fan

Fig. 1. Map view of an idealized dextral strike-slip system to illustrate terminology.

Some duplexes develop at major bends on the main fault

(Fig. 2h(3)) or at major offsets (Fig. 2c,a(1)). Others
develop at more subtle offsets (Figs. 2f & g) and some on
essentially straight fault segments (Fig. 2b(1)).

COMPARISON OF DUPLEXES IN STRIKE-SLIP

AND DIP-SLIP SYSTEMS

Despite some geometric similarities strike-slip

duplexes need not behave kinematically like dip-slip
duplexes. Important differences arise from the different
attitude of strike-slip fault planes and the slip vector with
respect to the vertical and thus to the free topographic
surface. The thickening of a contractional duplex or
thinning of an extensional duplex in a dip-slip, reverse or
normal, fault system can be efficiently accommodated
by distortion of the free ground surface (Fig. 3a & b).
This allows plane strain to be maintained in a vertical
plane through the displacement vector. The widening or
narrowing of a strike-slip duplex must be accommodated
by lateral distortion of surrounding rock in order to
maintain plane strain in a horizontal section (Fig. 3c).
This may be geometrically possible by compensation on
a laterally adjacent fault strand. However usually some
vertical accommodation takes place (Fig. 3d) by uplift
around a contractional duplex or subsidence around an
extensional duplex. Plane strain is not maintained. In a
limiting non-plane strain case (Fig. 3e) the area around
the duplex remains unstrained, and volume balance is
maintained by localized uplift or subsidence of the
duplex itself. The duplex faults will have strong dip-slip
components, normal at a releasing bend and reverse at a
restraining bend.
There are further differences. Vertical strike-slip
faults lack the gravitational potential normal to the fault
plane which may be an important factor in establishing
fault propagation sequences in dip-slip duplexes (Boycr
& Elliott 1982). The distinction between footwall and

hangingwall structures is inappropriate. Duplexes in

dip-slip systems usually retain a similar character along
one major fault zone, whereas extensional and contractional duplexes may coexist along the length of a single
strike-slip zone (e.g. Fig. 3c). Finally, strike-slip faults
usually cut material anisotropies such as bedding at high
angles. Weak stratigraphic horizons are badly oriented
for localizing strike-slip faults, and thus isotropic
theological theory is more relevant in strike-slip systems.
This lack of bedding control will in part account for the
braided, disorganized geometry of mature strike-slip
zones.

PLANE STRAIN DUPLEXING

This section details possible processes for forming

strike-slip duplexes in approximate plane strain in a
horizontal surface. Plane strain is probably rarely maintained in real systems, but the assumption serves to
introduce the kinematic models. The additional complications of non-plane strain deformation will be detailed
in a later section. Duplex geometries probably develop
in strike-slip zones by several different kinematic processes. This contrasts with duplexing in thrust belts,
currently thought to result mainly from one process,
progressive forward collapse of footwall ramps (Boyer
& Elliott 1982).
Duplexing at bends

This process (Fig. 4) is analogous to duplex formation

at ramps in dip-slip systems. Successive imbricate slices
(horses) are cut from the major fault walls at the bend by
progressive propagation of new imbricate faults outward
from the initial fault strand. Inward propagation within
the first horse or pair of horses is theoretically possible
but unless the horses are long there is little scope for
progressive imbrication and therefore a low potential for

Strike-slip duplexes

727

' 500m

N~
5842
I

~E

e
Gulf of Elat

Ill

.i

~'%'.,.."~
~:~

~ ~.

i-

::
/

i/

, ~ :,.

- -2~,~_____~_

,/

....

- - ~_

"/

34o02~ N

Red
Sea

N~

100m

5851 E

;
k

.*~ N
lOkm

35Ow

29N

2o5~=W

d
~

Silurian

Ordovician

~ ~P.,C,~v o / c a n j c s ~
~,'~//-~"

L.Ordovician
sandstone

~ n l e y

Fault

P r e c a mbri an
sediments

~V N
' 5OOm'
5234fN

,f'
f

e
- ,

L.Ordovician
~

~l~)v~c~i'an": ,

_~cs/::::::::.::..::..:.L.
. ~ ~ ~ ; 7 : . . : : . : :

s~

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Carboniferous
" ............ ,

,~.. .

~.~

~-<

";..".
-

2o521W

Calaveras Fa'~t
,

5OOm

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, ~.N
1kin

37051N
I

g
I

Coyote
Lake

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Rodger s c r e e ~
Fa_~utt /

-"~-~.,

~_.~ ~ - ~

Hayward <- Fault

.~.~-~._

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. "-"

.i--.
38N
I

.,..~

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~..

1Okra
122Ow 37N
"~ I

Fig. 2. Natural examples of strike-slip duplexes: (a & b) Dasht-e-Bayaz fault, Nimbluk Valley, Iran (Tchalenko &
Ambraseys 1970); (c) Gulf of Elat (Ben Avraham e t a l . 1979); (d & e) Pontesford-Linley fault zone, Wales (Dean & Dineley
1961, Whittard 1979); (f) Calaveras fault, Coyote Lake, California (Aydin & Page 1984); (g) East Bay Hills, California
(Aydin & Page 1984). C o n t i n u e d .

728

N . H . WooococK and M. FISCHER

I/

'

3403 j N

.........

(3)

- - - ~ / ' ~

h
.

.~

~..

~N

-~
5845fE
I

-I

500m '

'

i
lOOIE

_
--~,~

42551N

Glynnwye
Lake
~ --:-----~.~..

42o35~S

fault zone
5kin

'

~,~N

~N

500m '

172251E
i

granite
C

-42o501N

(
calc-mylonite

~Laka

granulite

a~N
lO25tE
I

~ - ~

=
lOOm
173E
I

strike-slip

normal d i p - or o b l i q u e - s l i p

r e v e r s e d i p - or o b l i q u e - s l i p

undifferentiated

faults

j42o30'S

lOkm

.......

stratigraphic contact

--~'"

fold

/ -~

coastline

Fig. 2 continued (h) Dasht-e-Bayaz fault, Nimbluk Valley, Iran (Tchalenko & Ambraseys 1970); (i & k) North Pyrenean
fault zone (Fischer 1984); (j) Glynnwye Lake, New Zealand (Freund 1971) and (1) Marlborough fault system, New Zealand
(Freund 1974).

adjusting to displacement on the major faults. Fault

propagation may be symmetric about the initial bend or
asymmetric with imbricates being cut preferentially from
one wall (Fig. 4). The geometry of an asymmetric system
depends on whether each new horse is cut from a fault
wall which is unstrained (Figs. 4c & e) or from a wall
which has strained as it has negotiated the fault bend
(Figs. 4b & f). In a symmetric system, outward propagating faults must always cut strained wall rocks (Figs. 4a
& d). Whilst symmetric propagation may be possible
during early development of duplexes, asymmetric
propagation will be favoured by developing contrasts in
lithology, structural anisotropy and stress distribution
induced by the displacement itself.
To maintain plane strain and zero bulk volume change
the shortening of a contractional duplex must be
balanced by increase in its width, and lengthening of an
extensional duplex by a width decrease. This bulging
and necking of duplexes causes bending of the bounding
fault strands and requires geometric accommodation in
surrounding rocks (Figs. 3 and 4). This accommodation
is geometrically analogous to hangingwall culminations

and depressions above duplexes in dip-slip systems.

However, in a plane strain strike-slip system the excess
volume at bulges must be balanced by the volume deficit
at thinning duplexes. This balance could theoretically
occur along one major fault zone, but is more likely to
occur between two adjacent zones in a strike-slip system.
These strain compatibility problems contribute to the
complexity of major strike-slip systems.
Duplexing at bends has not been modelled theoretically or physically, though models of fault offsets
described below are relevant and confirm the orientation
of imbricate faults if not their developmental sequence.
A natural example occurs on the active Dasht-e-Bayaz
fault, Iran (at location 3, Fig. 2h). This incipient extensional duplex has a throughgoing central strand flanked
symmetrically by two imbricate fault zones defining
horses with internally lower fracture density. From the
fracture connectivity the central fault zone probably
initiated earlier than the flanking zones. The length of
each horse (parallel to the major strike-slip fault strands)
is about one and a half times the separation of the major
strands at the bend. A possible asymmetric example

Strike-slip duplexes

729

STRIKE-SLIP

DIP-SLIP

DIP-SLIP

Fig. 3. Geometric accommodation of duplexes in dip-slip systems (a & b) by ground surface distortion compared with the
need for lateral distortion to maintain plane strain in a strike-slip system (c). PSS indicates plane-strain section.
Accommodation of strike-slip duplexes usually involves uplift or subsidence around the duplex (d) or, in the extreme,
localized differential uplift or subsidence of the duplex itself (e).

with a horse being cut from the southern wall occurs at a

gentle restraining bend on the same fault (location 4,
Fig. 2h). An example of a contractional duplex occurs at
a restraining bend between the Clarence and Elliott
faults, New Zealand (Fig. 21). A possible ancient example (location 1, Fig. 2d) occurs on the Palaeozoic Pontesford-Linley fault, Wales. This appears to be an extensional duplex compatible with a releasing bend in a
dextral system.

Fault patterns at offsets have previously been modelled using elastic theory. The results of Segall & Pollard
(1980) show that shear fractures will tend to propagate
from the lateral tips of the major faults (Figs. 6a & b)
with synthetic strike-slip faults deflecting progressively
towards the opposite maj or fault strand. Rodgers' (1980)
results also show this concave inward fault pattern (Figs.
6c & d) which matches closely the early stage of duplex
formation at offsets (Fig. 5). Although some of the
offsets in the theoretical models had substantial overlap,
Duplexing at offsets
the results cannot be used to predict likely fault propagation sequences. They show only 'potential' fault
Offset fault traces may reflect two separate faults or directions in isotropic rock. Real systems will be strongly
two en-6chelon strands that curve helicoidally into a influenced by the anisotropy introduced by the new
single fault at depth (Naylor et al. in press). Duplexing at fractures themselves.
offsets (Fig. 5) involves first the isolation of a horse by
The theoretical results for low displacement are cortwo imbricate strike-slip faults that propagate off the roborated by clay-cake models of fault patterns above
lateral tips of the main faults. The kinematic history will underlying offset faults (Hempton & Neher 1986). At all
depend on the amount of overlap between the two main stages of the experiments, two zones of high fracture
fault strands. For small overlap (Fig. 5a) the system can density join the lateral tip of each underlying fault to the
behave as a fault bend as soon as at least one of the new trace of the opposing fault (Fig. 6e). These two shear
faults has linked with the other main fault and is taking zones define an intervening less deformed horse.
most of the displacement. New horses can form by either
Natural examples of duplexes apparently formed at
symmetric or asymmetric outward progression of new offsets occur on the Dasht-e-Bayaz fault (locations 1, 2,
imbricate faults as at bends (Fig. 4). For large overlap, Fig. 2a), on the Hope fault zone, New Zealand (Fig. 2j)
inward progression of fault development is possible and on the Calaveras fault, California (Fig. 2f). How(Fig. 5b) but is only effective where displacement is low ever, the last two probably depart significantly from
plane strain.
relative to the overlap.

N. H. WOODCOCKand M. FISCHER

730

EXTENSION

CONTRACTION

-~- ,qp

o
,g.

collapse of strained wall

e of unstrained wall

II e

collapse of unstrained wall

collapse of strained wall

Fig. 4. Sequences of duplex development at bends in map views of a dextral strike-slip system. Grid shows displacements
within and around horses. Thick lines are faults, dashed where incipient.

SMALL OVERLAP ~
EXTENSION

.~
~

OUTWARD PROPAGATION
CONTRACTION

LARGE OVERLAP
EXTENSION

~ INWARD PROPAGATION
CONTRACTION

continued outward propagation by symmetric

or asymmetric sequences in Fig.4

Fig. 5. Sequences of duplex development at offsets in map views of a dextral strike-slip system. Grid shows displacements
in and around horses. Thick lines are faults.

Strike-slip duplexes
a

e/2~,II

//
/

',,,'C.%?

,,"

",--kv,...

731

"

,'<"

/'..

%R'

b
e
/

*f

-CY

--2.
20cm

-CY

Fig. 6. Some results of theoretical modelling of fault offsets by (a & b)

Segall & Pollard (1980), (c & d) Rodgers (1980) and (e) of claycake
experiments of Hempton & Neher (in press). Solid lines are surface
faults or (a-d) potential faults, coarse dashed lines (e) are subsurface
faults, fine dashed lines (a & b) are contours of shear failure potential.
C

Duplexing on straights

Formation of duplex geometry is not restricted to

marked bends and offsets. Experiments (e.g. Tchalenko
1970, Naylor et al. in press) show how grossly straight
fault segments often comprise en-6chelon conjugate
Riedel shears (R and R', Fig. 7a) at low displacement
and how these become linked together at higher dis- Fig. 7. (a) Ideal fault orientations in a dextral strike-slip system. (b &
c) Sequences of duplex development on straights in map views of
placements by further shears (P and D, Fig. 7a) at a low
dextral strike-slip systems.
angle to the slip vector. Duplex geometries may form by
various interactions of these shears, but the experiments
suggest two common modes. The first (Fig. 7b) involves
the later P or D shears isolating a lozenge already cut by
R shears. Such a duplex will necessarily have an extensional geometry. The second mode (Fig. 7c) involves
imbrication by P shears of a lozenge between pre-existing R shears. This gives a contractional duplex. The
second mode is kinematically similar to duplex formaa
d,~
,
tion between overlapping major faults at a large-scale
offset (Fig. 5). The first mode is kinematically distinct
from the bend or offset models in that the imbricate
faults separating the horses pre-date the bounding faults
to the duplex. Both types of Riedel duplex may be
geometrically indistinguishable from ones formed at
bends and subsequently shunted onto straights, though
b i d "
~0'5ml~
the Riedel duplexes may be smaller than the bend
duplexes within any one fault system. Genetic classification of duplexes is further complicated because the
Riedel duplexes may themselves induce and nucleate
C
bends in the major faults, and because each duplexing
process may operate simultaneously at a wide range of
scales.
The claycake experiments by Tchalenko (1970) provide examples of both extensional and contractional
8. Examples of duplexes formed on straights in (a, b) claycake
Riedel duplexes (Figs. 8a & b). The sandbox experi- Fig.
experiments (Tchalenko 1970, figs. 4d, 10a) and (c) sandbox experiments of Naylor et al. (1986) produced extensional ments (Naylor et al. in press, fig. 2e). Solid lines are faults, dotted lines
are displaced markers.
Riedel duplexes made up of a series of lensoid horses

732

N . H . WOODCOCKand M. FISCHER

(Fig. 8c, shear lenses of Naylor et al.). The active

Dasht-e-Bayaz fault has a number of natural examples
(e.g. Fig. 2b). A possible ancient example occurs along
the Pontesford Lineament, Wales (location 2, Fig. 2d).
This is an extensional duplex compatible with dextral
displacement. It now occurs on a major releasing bend,
but the small size of the horses compared with the bend
size suggests that it may have originated at a smaller
bend or as a Riedel duplex on a straight and was either
shunted into its present position or overprinted by the
developing large bend.

a autochthonous duplex

b cognate duplex

Shunting of duplexes
C

A duplex Will stop forming when displacement ceases

on its parent major fault. It is then autochthonous in that
it is preserved in its original stratigraphic and structural
context. Alternatively, a duplex may die due to changing
geometry of the fault system whilst the parent fault is still
active. The duplex may remain attached to one wall of
the fault zone (Fig. 9a), with all the displacement taken
up on the opposite bounding fault. With continued
strike-slip this fault may juxtapose contrasting rocks
against the duplex (Fig. 9b). However it retains its match
with rocks on the other wall and in this sense is a cognate
duplex. Later still (Fig. 9c) the same duplex, or one
horse from it, may become attached (docked) to the
opposite fault wall with the main strike-slip switching to
a fault on its other flank. Continued displacement may
finally remove any matching rocks (Fig. 9d) leaving an
exotic duplex or horse totally isolated from any of its
parent rocks and from its original structural context
(Fig. 9d). Exotic duplexes and horses are analogous to
'far-travelled horses' in thrust systems (cf. Elliott &
Johnson 1980).
This process of differential shunting along a strike-slip
fault may be responsible for many of the small isolated
lozenges of mismatching rocks in strike-slip systems
(e.g. Crowell 1975). The shunting process is similar to
one envisaged on a larger scale for transporting
allochthonous terranes in orogenic belts (review by
Schermer et al. 1984) and termed sidling by Dewey
(1982).
An example of a cognate duplex occurs on the
Pontesford-Linley fault, Wales (location 1, Fig. 2d).
The duplex lithologically matches Precambrian sediments and volcanics on the SE side of the fault but large
displacements on its NW boundary have juxtaposed
contrasting Lower Ordovician sediments. Further north
on the same fault (Fig. 2e) an exotic horse of the
Precambrian volcanics is isolated between Lower Ordovician sediments to the NW and contrasting Upper
Ordovician and Precambrian sediments to the SE. An
example of a strongly exotic duplex is the Bastard slice
on the North Pyrenean fault zone (Fig. 2k). The granulite facies basement rocks of the duplex only occur in
quantity north of the fault zone some 30 km to the west.
The duplex is bordered to the north by granites and to
the south by calc-mylonites, both of relatively low

i+

;:oio
oio:i:

:o):C C :
c~

d e x o t i c duplex

Fig. 9. Sequence of shunting of a duplex along a dextral strike-slip fault

seen in map view.

metamorphic grade. During Early Cretaceous sinistral

strike-slip movement on the fault the duplex must have
formed against the Castillon massif, then transferred
onto the southern fault wall and been shunted to its
present position. A number of independent kinematic
indicators, such as mineral extension directions in both
the granulites and calc-mylonites, suggest that vertical
movements are minor and that the duplex developed
mainly as a result of lateral shunting (Fischer 1984).

NON-PLANE STRAIN DUPLEXING

Distributed subsidence and uplift

It has already been argued that most strike-slip systems have substantial differential vertical displacements
which complicate the plane strain models discussed so
far. The most obvious vertical effects are at bends and
offsets, and two end-member methods have been drawn
(Figs. 3d & e) of relieving the volume excess at restraining bends and volume deficit at releasing bends. The first
method is by distributed uplift or subsidence of the bend
and its surrounding area (Fig. 3d). This process is
directly recorded on active faults, for instance the uplift
at the 'big bend' of the San Andreas fault near Los
Angeles (Fig. 10a) and more locally of an offset on the
Coyote Creek Fault, California (Fig. 10b). It is predicted
by elastic theory (Rodgers 1980) and modelled in

Strike-slip duplexes

~,~/~,ow

733
dlI

"

"h

Sp

I
rate" =/d.~l
dt

uplift rate= dh
dt

for constant duplex volume

w (I-dl)(h+dh)= w.l.h
120kml

./'

~,

35N

"~'"J

for small displacement

~N

I.dh = h.dl

b
(uplift rate)

C y t e Cre

d.hh = h . d l (slip rate)

dt
I dt

Fig. 11. Numerical estimate of rate of uplift (or similarly of subsidence)

of a duplex or horse at an offset.

Fig. 10. Natural examples of uplift at bends on (a) San Andreas fault
(in cm between 1959 and 1974, Castle et al. 1976) and (b) Coyote Creek
fault, California (present elevations in feet, Sharp & Clark 1972).

claycake experiments (Hempton & Neher 1986). The

maximum vertical displacement at a bend is about 1015% of the magnitude of the strike-slip in the elastic
model, 15-30% in the clay experiments, and averages
about 10% at natural releasing bends (Hempton &
Dunne 1984). The vertical movements may themselves
be taken up on faults with a dip-slip component, departing from the end-member model of continuous deformation.
Local pull-aparts and push-ups
The other end-member process for accommodating
volume changes is the discrete uplift or subsidence of the
duplex (Fig. 3e). Active strike-slip systems show that
subsiding pull-aparts form at releasing bends and uplifting push-ups form at restraining bends (e.g. Crowell
1973, Mann et al. 1983). Some of these areas have a clear
duplex structure and others are isolated horses. Duplexing may play an important role in their geometric accommodation at depth.
A simple volume balance at a fault offset (Fig. 11)
suggests that the instantaneous uplift or subsidence rate
of a duplex could be of the same order as the slip rate on
the bounding master faults. The controlling factor is the
shape of the duplex, particularly the ratio of its vertical
height to its along-strike length. 'Vertical height' will be
determined by the depth to a low-dip detachment (see
later) or to the branch-line of two downward-converging
boundary faults. The calculation gives a maximum rate,
since in natural systems some of the excess volume is
accommodated by lateral bending of the fault walls and
distributed uplift around the bend.
An example of a pull-apart with duplex structure is the
Glynnwye Lake basin along the Hope Fault Zone, New
Zealand (Fig. 2j). The duplex faults are normal oblique-

slip, dipping in under the pull-apart. Other examples

have already been figured from Iran (location 3,
Fig. 2h), California (Fig. 2f) and in theoretical models
(Fig. 6b). The Gulf of Elat (Fig. 2c) is an extreme
example of a pull-apart duplex floored by oceanic crust.
The East Bay Hills area (Fig. 2g) is a push-up with
contractional duplex geometry forming at a gentle offset
between the Calaveras and Rodgers Creek faults. The
duplex faults are reverse oblique-slip faults dipping in
under the push-up. A further well-constrained example
of inward-dipping boundary faults to a duplex is in the
French Pyrenees (Fig. 2i).
Duplexes and flower structures
The inward-dipping geometry of faults under both
pull-apart and push-up duplexes suggests that the faults
may converge at depth into a single shear zone (Fig. 12).
In vertical section the faults define a flower structure
(Sylvester & Smith 1976). This model is compatible with
the duplex geometry observed in some seismic profiles
through strike-slip flower structures (e.g. Harding 1983,
1985). Pull-aparts may commonly be underlain by negative (normal faulted) flower structures and push-ups by
positive (reverse faulted) flower structures.
A link between flower structures (or 'tulip structures')
in section and 'Riedel shear lenses' (either duplexes or
isolated horses) in plan view is seen in sandbox experiments (Naylor et al. in press). Each Riedel shear has a
helicoidal form (similar to that in Fig. 12) so that at depth
they unite into a single steep fault zone. The downward
converging fault geometry can form on straights with no
bulk shortening or extension normal to the fault trace.
Transtension suppresses the en-6chelon nature of the
Riedel shears whereas transpression increases the angle
between the Riedels and the basement fault trend
(Naylor et al. in press). This relationship is also predicted
theoretically (Sanderson & Marchini 1984).
Low-dip detachments
An alternative geometry to, or a component of, flower
structures for maintaining three-dimensional strain corn-

734

N . H . WOODCOCKand M. FISCHER
a

IMPLICATIONS OF THE DUPLEX MODEL

Balanced sections
The technique of constructing cross-sections by
balancing deformed against restored sections is well
established in thrust belts (e.g. Hossack 1979, Elliott
1983) and has now been applied to normal fault systems
(e.g. Gibbs 1984). These sections are drawn in the plane
perpendicular to the faults that also contains the fault
slip vectors. The assumption is that material points
remain within the plane of the section during deformation, and therefore that the area of a deformed unit
b
exactly balances its undeformed area. Some oblique
sections can be balanced if the structures are cylindroidal
and it is assumed that the amount of material leaving the
plane of the section equals the amount entering the
section.
For dip-slip fault systems the appropriate section to
balance is vertical. The analogous section for strike-slip
systems is horizontal. Map views of such systems can be
balanced (e.g. Figs. 4 and 5) if displacements are purely
horizontal. However, natural strike-slip systems
demonstrate the importance of vertical displacements,
as do theoretical and physical models. If these moveFig. 12. Postulated three-dimensional form of (a) an extensional ments, for instance the subsidence and uplift of duplexes
duplex (showing negative flower structure) and (b) a contractional or isolated fault lozenges, occurred on precisely vertical
duplex (showingpositiveflowerstructure).
faults then area balancing might still be valid because
each fault lozenge would maintain a constant crosssectional area as it moved through the map section. The
invalidating factor is that the faults often converge downwards as flower structures, so that subsiding lozenges
patibility in the upper levels of the crust is for steep will progressively increase in map-view area and upliftstrike-slip faults to be associated with or root in low-dip ing lozenges decrease. There is no reason for these two
faults or shear zones. These faults allow various levels in effects to balance out on any one section.
the strike-slip system to move over one another and to
It is tempting to area balance vertical sections through
rotate [strike-slip flaking of Dewey (1982)]. These strike-slip systems, for example from seismic profiles.
kinematically necessary flats are analogous to lateral This will only be valid if material is moving into and out
ramps or transfer faults in dip-slip systems. Flat detach- of the section at the same rate, for instance in an ideal
ment zones must necessarily delimit strike-slip systems ductile shear zone or one comprising faults exactly
at depth as they do dip-slip systems. Two possible sites parallel to the fault zone strike. This condition is most
are a mid-crustal discontinuity (e.g. Sibson 1983) or a unlikely given the complex braided fault pattern of most
sub-lithospheric zone related to a transform fault zone.
strike-slip systems. In particular it is invalidated by
differential shunting of duplexes along strike-slip faults.
Complex duplex subsidence and uplift
Many balanced sections through apparent dip-slip
systems have been constructed without regard for the
The expected location of extensional duplexes at pull- possible strike-slip component. Woodcock (1986) argues
aparts and contractional duplexes at push-ups only holds that about 60% of orogenic belts may have had a signifiat low displacements. Large displacements allow along- cant orogen-parallel strike-slip component.
strike shunting of duplexes so that, in the extreme case,
a contractional duplex might eventually dock at a releas- Kinematic vs dynamic models
ing bend and be reactivated as a pull-apart. The vertical
displacements of duplexes on straights will depend on a
Previous approaches to strike-slip fault systems have
variety of factors: the regional stress state across the been essentially dynamic, considering ideal orientations
fault zone and local effects, particularly from bends and of faults, folds and fabrics with respect to the stresses or
duplexes migrating along adjacent fault strands. The elastic strains within the fault zone (e.g. Tchalenko
tendency of duplexes and of isolated fault lozenges to 1970, Wilcox et al. 1973, Rodgers 1980). This has been a
uplift and subside alternately as they move along a profitable approach and gives a good match with natural
strike-slip system has been called porpoising (Crowell & structures formed in zones with small displacement.
Sylvester 1979).
However, the theory applies to isotropic rocks, which is

Strike-slip duplexes
why it is not so appropriate to dip-slip systems whose
lower-angle faults are more affected by the antisotropy
of the sedimentary bedding. Only rarely (e.g. Crowell
1974) has it been appreciated that in strike-slip systems
the early, ideally oriented fractures themselves introduce an anisotropy which increasingly influences the
geometric evolution of the system. After only modest
displacement the system is pervaded by fractures in a
wide range of orientations (e.g. experiments by
Tchalenko 1970). It then evolves more by slip on suitably
oriented old fractures than by propagation of new ones.
The control is not so much dynamic as a kinematic need
to rearrange the fault blocks in compatibility with the
imposed boundary conditions. The duplex concept is
one of a series of kinematic patterns which need to be
identified before the complexities of strike-slip fault
systems can be understood.
Acknowledgements--This paper was stimulated by field work in Wales
(NHW) funded by N.E.R.C. Research Grant GR3/4406 and in the
Pyrenees (MF) funded by N.E.R.C. Research Studentship GT4/81/
GS/108.

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