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CONTINENTAL TRANSFORMS AND STRIKE-SLIP FAULTS 211

8.1 INTRODUCTION 8.2 FAULT STYLES

AND PHYSIOGRAPHY

Continental transforms, like their oceanic counter-
parts (Section 4.2.1), are conservative plate boundar-
ies where lithosphere is neither created nor destroyed The following fault styles and physiographic features
and strike-slip deformation results in lateral displace- characterize the surface and upper crust of continental
ments across the fault zone. Strike-slip faults gener- transforms and major continental strike-slip faults:
ally may occur at a variety of scales in virtually
1 Linear fault scarps and laterally offset surface
any tectonic setting. Only transform faults represent
features. Large continental strike-slip faults
plate boundaries.
typically display linear scarps and troughs
In contrast to oceanic fracture zones, which are
that result from the differential erosion of
characterized by a relatively simple linear trough
juxtaposed material and the erosion of fault
(Section 6.12), continental transforms exhibit a
gouge (Allen, 1981). Surface features along
structural complexity that reflects differences in the

active or recently active fault traces may be
thickness, composition, and pressure–temperature
displaced laterally due to the strike-slip motion.
profile of oceanic and continental lithosphere (Sec-

The age and magnitude of these offsets provide
tions 2.7, 2.10.4). In the southwestern United
an important means of determining slip rates.
States, for example, relative motion between the
In New Zealand, for example, the Alpine Fault

Pacific and North American plates is distributed
is marked by a nearly continuous, linear fault
across a zone that ranges from hundreds to a
trace that extends across the South Island for a
thousand kilometers wide (Fig. 8.1). Similarly, in
distance of ∼850 km (Fig. 8.2). Glacial moraines,
New Zealand (Fig. 8.2), oblique convergence on
rivers, valleys, lake shores, and other
the South Island has produced a >100-km-wide
topographic features are offset laterally across
zone of deformation on the continental portion of
the fault (Fig. 8.4), suggesting late Pleistocene

the Pacific plate. These diffuse, commonly asym-
−1
slip rates of 21–24 mm a (Sutherland et al.,

metric patterns generally reflect lateral contrasts in
2006). Vertical motion between parallel fault
lithospheric strength and areas where continental
segments also is common and may create areas
lithosphere is especially weak (Section 8.6.2). In
of localized uplift and subsidence that are
areas where continental lithosphere is relatively
expressed as pressure ridges and sag ponds,
cool and strong, transforms tend to display narrow
respectively (Sylvester, 1988).
zones of deformation. The Dead Sea Transform is
an example of this latter type of system where 2 Step-overs, push-ups, and pull-apart basins. Most
deformation has localized into a zone that is only large strike-slip faults are composed of multiple
20–40 km wide (Fig. 8.3). fault segments. Where one active segment
In this chapter, the shallow (Section 8.2) and deep terminates in proximity to another sub-parallel
(Section 8.3) structure of continental transforms and segment, motion is transferred across the
major strike-slip faults is illustrated using examples intervening gap, resulting in zones of localized
from the southwestern U.S., New Zealand, the extension or contraction (Fig. 8.5a). In these
Middle East, and elsewhere. Other topics include the step-overs, the initial geometry and sense of slip
evolution of transform continental margins (Section on the adjacent faults control whether the area
8.4), the use of velocity fields to describe crustal separating them is extended or shortened

motion (Section 8.5), and the mechanisms that (Dooley & McClay, 1997; McClay & Bonora,
control the localization and delocalization of strain 2001). Normal faults and extensional troughs
during strike-slip faulting (Section 8.6). This latter called pull-apart basins characterize step-overs
subject, and the overall strength of large strike-slip where the intervening region is thrown into
faults (Section 8.7), are especially important for tension. Thrust faults, folds, and topographic
explaining how continental transforms accomplish uplifts known as push-ups form where the
large magnitudes of slip. intervening region is compressed. In these