Lithospheric heterogeneity

The distribution of strain within deforming continental lithosphere is strongly influenced by horizontal variations in temperature, strength, and thickness (e.g. Sections 2.10.4, 2.10.5). In New Zealand, for example, oblique convergence on the central part of the South Island has resulted in deformation that occurs almost entirely on the Pacific plate side, leaving the Australian plate relatively undisturbed (Fig. 8.2a). This asymmetry reflects the greater initial crustal thickness and weaker rheology of the Pacific plate compared to that of the Australian plate, causing the former to deform more easily (Gerbault et al., 2002; Van Avendonk et al., 2004).

To investigate the effects of initial variations in crustal thickness and lithospheric temperature on strike-slip deformation patterns, Sobolev et al. (2005) conducted numerical experiments of a simple transform fault (Fig. 8.21). In these models, the crust consists of two layers overlying mantle lithosphere. Velocities of 30 mm a-1 are applied to the sides of the lithosphere, forming a zone of left lateral strike-slip deformation. Although motion takes place in and out of the plane of observation, all other model parameters vary in only two dimensions. The rheological description of the crustal layers allows both brittle and ductile styles of deformation to develop, whichever is the most energetically 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 rheologies allow for heating during deformation as a result of friction or ductile flow.

In the first model, (Fig. 8.21a) the lower crust is thicker on the left than on the right and temperature is kept constant at the base of the lithosphere. The second (Fig. 8.21b) shows a constant crustal thickness and a thermal perturbation in the central part of the model. In the third model (Fig. 8.21c), both crustal thickness and temperature heterogeneities are present. This latter

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Figure 8.21 Thermomechanical models of simple strike-slip faulting in a three-layer lithosphere that incorporate initial variations in (a) crustal thickness, (b) lithospheric temperature, and (c) both crustal thickness and lithospheric temperature (after Sobolev et al., 2005, with permission from Elsevier). Top row of diagrams shows model setup geometry with corresponding lithospheric strength curves before deformation below them. Thin black lines are isotherms prior to deformation. Lower two rows of diagrams show snapshots of the distribution of strain rate demonstrating the strain localization process.

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model mimics the structure of a rifted continental margin (Section 7.7) where crustal thickness decreases linearly from right to left and high lithospheric temperatures occur at shallow depths below the thinnest crust. In all three models, multiple faults form in the brittle upper crust during the first 1-2 Ma. The number of active faults gradually decreases over time until a single fault dominates the upper crust at about 2 Ma. Over time a zone of high strain rate in the ductile lower crust and mantle lithosphere narrows and stabilizes.

These results show that, for each model, strain localizes where lithospheric strength is at a minimum, regardless of the cause of the weakening. They also show that crustal thickness and the initial thermal state of the lithosphere play key roles in localizing strike-slip deformation. These effects may explain why strike-slip faulting has localized in some areas of the southwestern USA (Figs 7.9, 8.1), such as the Eastern California Shear Zone and Walker Lane, leaving others, such as the Great Valley-Sierra Nevada and central Great Basin, virtually undeformed (Bennett et al., 2003). In this case, strain localization may be related to differences in heat flow between the western Basin and Range Province and the Sierra Nevada (Section 7.3). However, it has not been demonstrated whether the elevated heat flux is a cause or a product of strain localization. Alternatively, crustal thickness variations and horizontal gradients in gravitational potential energy and viscosity may concentrate the deformation (Section 7.6.3).

In addition to horizontal variations in strength, a vertical stratification of the lithosphere into weak and strong layers greatly influences how strain is accommodated during strike-slip deformation. To illustrate this effect, Sobolev et al. (2005) compared patterns of strain localization and delocalization in two models of pure strike-slip deformation that incorporate two different crustal rheologies. In the first model (Plate 8.1a between pp. 244 and 245), the crust is strong and modeled using laboratory data on hydrous quartz and plagioclase. Three layers correspond to a brittle upper crust, a brittle-ductile middle crust, and a mostly ductile lower crust. In the second model (Plate 8.1b between pp. 244 and 245), the effective viscosity of the crust at a fixed strain rate is reduced tenfold. The models also incorporate a reduction in crustal thickness from right (east) to left (west) in a manner similar to that observed in the Dead Sea Transform (Fig. 8.11). Lithospheric thickness is defined by the 1200 °C isotherm and increases to the east, simulating the presence of a thick continental shield on the right side of the model.

The results of these two experiments show that in both the strong crust and weak crust models, strain localizes into a sub-vertical, lithospheric-scale zone at the margin of the thick shield region, where the temperature-controlled lithospheric strength is at a minimum (Plate 8.1a,b between pp. 244 and 245). In the case of the strong crust, the zone of largest crustal deformation is located above a zone of mantle deformation and is mostly symmetric (Plate 8.1c between pp. 244 and 245). These characteristics result from the strong mechanical coupling between the crust and upper mantle layers. In the 15-km-thick brittle upper crust, shear strain localizes onto a single vertical fault. The deformation widens with depth into a zone of diffuse deformation in the middle crust and then focuses slightly in the uppermost part of the lower crust. In the model with weak crust, the lower crust is partially decoupled from both the upper mantle and the upper and middle crusts (Plate 8.1d between pp. 244 and 245). Consequently, the deformation is delocalized, asymmetric, and involves more upper crustal faults. The distribution of viscosity (Plate 8.1e,f between pp. 244 and 245) also illustrates the mechanical decoupling of layers in the weak crustal model. This decoupling results because the deforming lithosphere becomes very weak due to the dependency of the viscosity on strain rate and temperature, which increases due to strain-induced heating. In several variations of this model, which involve the addition of a minor component of transform-perpendicular extension, second order effects appear, such as a small deflection of the Moho, the development of deep sedimentary basins, and asymmetric topographic uplift (Sobolev et al., 2005). These latter features match observations in the Dead Sea Transform (Section 8.3.1).

These numerical models illustrate that the localization and delocalization of strain during strike-slip deformation is influenced by vertical contrasts in rheology as well as initial horizontal contrasts in crustal thickness and temperature. The width of the deforming zone is controlled mostly by strain-induced heating and the temperature- and strain-rate dependency of the viscosity of the rock layers. Lithospheric thickness appears to play a minor role in controlling fault zone width. The results also highlight how the interplay between forces applied to the edges of plates or blocks and the effects of ductile flow in the lower crust and mantle result in a vertical and horizontal partitioning of strain within the lithosphere.

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