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CONTINENTAL TRANSFORMS AND STRIKE-SLIP FAULTS 239

mation is far from resolved. Many other models have strain during strike-slip faulting. As is the case in other
been proposed for this same region (e.g. Flesch et al., tectonic settings (e.g. Section 7.6.1), competition among

2000) that also fit the geodetic observations and most these processes, and whether they result in a net
investigators agree that the deformation probably weakening or a net strengthening of the lithosphere,
results from a combination of mechanisms rather than ultimately controls the large-scale patterns of the
a single one. deformation.
Part of the problem of determining the specifi c
mechanisms of continental deformation is that success

in fitting geodetic observations neither proves any given 8.6.2 Lithospheric
model nor precludes other possibilities (McCaffrey,
2005). In addition, the results of mechanical modeling heterogeneity
have shown that the steady-state motion of an elastic
upper crust is insensitive to the properties of any fl ow The distribution of strain within deforming continental
field below it (Savage, 2000; Zatman, 2000; Hetland & lithosphere is strongly influenced by horizontal varia-

Hager, 2004). This latter result means that short-term tions in temperature, strength, and thickness (e.g. Sec-
geodetic observations of deformation between large tions 2.10.4, 2.10.5). In New Zealand, for example,
earthquakes (i.e. interseismic deformation) provide no oblique convergence on the central part of the South
diagnostic information about the long-term behavior of Island has resulted in deformation that occurs almost
a viscous layer in the deep crust or mantle. One espe- entirely on the Pacifi c plate side, leaving the Australian
cially promising area of research suggests that transient plate relatively undisturbed (Fig. 8.2a). This asymmetry

deformation following large earthquakes offers the reflects the greater initial crustal thickness and weaker
prospect of inferring the rheology of lower viscous rheology of the Pacific plate compared to that of the

layers (Hetland & Hager, 2004). However, presently, the Australian plate, causing the former to deform more

specific mechanisms and the relative contribution of easily (Gerbault et al., 2002; Van Avendonk et al., 2004).
edge forces, basal tractions, and buoyancy forces to the To investigate the effects of initial variations in
deformation in most regions remain highly speculative, crustal thickness and lithospheric temperature on strike-
with the results of models depending strongly on the slip deformation patterns, Sobolev et al. (2005) con-
imposed boundary conditions. ducted numerical experiments of a simple transform
fault (Fig. 8.21). In these models, the crust consists of
two layers overlying mantle lithosphere. Velocities of
−1
30 mm a are applied to the sides of the lithosphere,
8.6 STRAIN forming a zone of left lateral strike-slip deformation.
Although motion takes place in and out of the plane of
LOCALIZATION AND observation, all other model parameters vary in only
two dimensions. The rheological description of the
DELOCALIZATION crustal layers allows both brittle and ductile styles of
deformation to develop, whichever is the most ener-

MECHANISMS getically efficient. Brittle deformation is approximated
with a Mohr–Coulomb elastic-plastic rheology. Ductile
flow employs a nonlinear, temperature-dependent,

viscous-elastic rheology (see also Section 7.6.6). Both
8.6.1 Introduction rheologies allow for heating during deformation as a
result of friction or ductile fl ow.

One of the most interesting questions about continen- In the first model, (Fig. 8.21a) the lower crust is
tal transforms and major strike-slip faults is how these thicker on the left than on the right and temperature is
structures accomplish the large displacements that are kept constant at the base of the lithosphere. The second
observed on them. To determine the mechanisms that (Fig. 8.21b) shows a constant crustal thickness and a
allow these displacements to occur, geoscientists have thermal perturbation in the central part of the model.
developed mechanical models to investigate the pro- In the third model (Fig. 8.21c), both crustal thickness
cesses that lead to a localization or delocalization of and temperature heterogeneities are present. This latter