Page 255 - Global Tectonics

CONTINENTAL TRANSFORMS AND STRIKE-SLIP FAULTS 241

model mimics the structure of a rifted continental The results of these two experiments show that
margin (Section 7.7) where crustal thickness decreases in both the strong crust and weak crust models,
linearly from right to left and high lithospheric tem- strain localizes into a sub-vertical, lithospheric-scale
peratures occur at shallow depths below the thinnest zone at the margin of the thick shield region, where
crust. In all three models, multiple faults form in the the temperature-controlled lithospheric strength is at

brittle upper crust during the first 1–2 Ma. The number a minimum (Plate 8.1a,b between pp. 244 and 245).
of active faults gradually decreases over time until a In the case of the strong crust, the zone of largest
single fault dominates the upper crust at about 2 Ma. crustal deformation is located above a zone of mantle
Over time a zone of high strain rate in the ductile lower deformation and is mostly symmetric (Plate 8.1c
crust and mantle lithosphere narrows and stabilizes. between pp. 244 and 245). These characteristics result
These results show that, for each model, strain local- from the strong mechanical coupling between the
izes where lithospheric strength is at a minimum, crust and upper mantle layers. In the 15-km-thick
regardless of the cause of the weakening. They also brittle upper crust, shear strain localizes onto a single
show that crustal thickness and the initial thermal state vertical fault. The deformation widens with depth
of the lithosphere play key roles in localizing strike-slip into a zone of diffuse deformation in the middle
deformation. These effects may explain why strike-slip crust and then focuses slightly in the uppermost part
faulting has localized in some areas of the southwestern of the lower crust. In the model with weak crust, the
USA (Figs 7.9, 8.1), such as the Eastern California Shear lower crust is partially decoupled from both the
Zone and Walker Lane, leaving others, such as the upper mantle and the upper and middle crusts (Plate
Great Valley–Sierra Nevada and central Great Basin, 8.1d between pp. 244 and 245). Consequently, the
virtually undeformed (Bennett et al., 2003). In this case, deformation is delocalized, asymmetric, and involves
strain localization may be related to differences in heat more upper crustal faults. The distribution of viscos-
flow between the western Basin and Range Province ity (Plate 8.1e,f between pp. 244 and 245) also illus-

and the Sierra Nevada (Section 7.3). However, it has not trates the mechanical decoupling of layers in the
been demonstrated whether the elevated heat flux is a weak crustal model. This decoupling results because

cause or a product of strain localization. Alternatively, the deforming lithosphere becomes very weak due to
crustal thickness variations and horizontal gradients in the dependency of the viscosity on strain rate and
gravitational potential energy and viscosity may con- temperature, which increases due to strain-induced
centrate the deformation (Section 7.6.3). heating. In several variations of this model, which
In addition to horizontal variations in strength, a involve the addition of a minor component of trans-

vertical stratification of the lithosphere into weak and form-perpendicular extension, second order effects

strong layers greatly influences how strain is accom- appear, such as a small deflection of the Moho, the

modated during strike-slip deformation. To illustrate development of deep sedimentary basins, and asym-
this effect, Sobolev et al. (2005) compared patterns of metric topographic uplift (Sobolev et al., 2005). These
strain localization and delocalization in two models of latter features match observations in the Dead Sea
pure strike-slip deformation that incorporate two differ- Transform (Section 8.3.1).
ent crustal rheologies. In the first model (Plate 8.1a These numerical models illustrate that the localiza-

between pp. 244 and 245), the crust is strong and tion and delocalization of strain during strike-slip defor-
modeled using laboratory data on hydrous quartz and mation is influenced by vertical contrasts in rheology as

plagioclase. Three layers correspond to a brittle upper well as initial horizontal contrasts in crustal thickness
crust, a brittle-ductile middle crust, and a mostly ductile and temperature. The width of the deforming zone is
lower crust. In the second model (Plate 8.1b between controlled mostly by strain-induced heating and the
pp. 244 and 245), the effective viscosity of the crust at a temperature- and strain-rate dependency of the viscos-
fixed strain rate is reduced tenfold. The models also ity of the rock layers. Lithospheric thickness appears to

incorporate a reduction in crustal thickness from right play a minor role in controlling fault zone width. The
(east) to left (west) in a manner similar to that observed results also highlight how the interplay between forces
in the Dead Sea Transform (Fig. 8.11). Lithospheric applied to the edges of plates or blocks and the effects

thickness is defined by the 1200°C isotherm and of ductile flow in the lower crust and mantle result in
increases to the east, simulating the presence of a thick a vertical and horizontal partitioning of strain within
continental shield on the right side of the model. the lithosphere.