![]() ![]() For n = 3, for example, if τ xy decreased to half its value at x = 0 over some sensible geological distance x 0, the strain rate would decrease by 8 times. ![]() If τ xy decreases slowly with increasing x, and B is constant, also decreases slowly with distance. Localization of Strain Near Heterogeneity in Strength (or Viscosity) The main part of the paper is a discussion of evidence of major faults and their tectonic settings, most of which is relegated to the Appendix. Given that most faults are minor, we see them as surface manifestations of deeper processes, and following the discussion of brittle processes, we ignore them. Brittle fracturing of the upper crust, however, cannot alone account for the existence of major faults. As faults result from brittle deformation, and they clearly are important in the deformation that we see at the surface, we do not neglect them. We then consider brittle faulting in the upper crust, which also is not the main motivation of our study. This form of localization is not the process that we consider here, but the process that allows strain to be localized in narrow zones once some concentration of strain has begun. We then digress to discuss how localization of strain can develop once a shear zone is present and dissipative heating allows localized weakening. We first give a brief discussion of how nonlinear viscosity can enhance the concentration of strain near a strong region in a continuously deforming medium. Thus, we cannot reject the idea that strain localizes in the mantle beneath major faults, as Peltzer and Tapponnier (1988 see also Tapponnier et al., 1986, 1990, 2001) have argued.īefore pursuing these questions, we review the basic reasoning that underlies asking such questions in the first place. ![]() 1), we found that additional weakening must occur to localize deformation in order to mimic the extent to which strain has been concentrated in a narrow zone along the Altyn Tagh fault ( Dayem et al., 2009). Although localized deformation occurs along the boundary between material with different viscosity coefficients ( Fig. We tested the idea that deformation near the Altyn Tagh fault, which bounds the rigid Tarim Basin on the north from the deforming Tibetan Plateau to the south, occurs without additional weakening such as by dissipative heating or grain-size reduction, and hence that the material properties of lithosphere beneath Tibet and beneath the Tarim Basin remained constant as deformation accumulated ( Dayem et al., 2009). Thus, some degree of localization should develop within a homogeneous viscous substance near its boundaries with more viscous material. Strain will concentrate near the boundaries of relatively strong regions immersed within a deforming medium, with the degree to which strain rates concentrate dependent on the constitutive law that relates stress and strain rate (e.g., Bell et al., 1977 Dayem et al., 2009). Thus, if these different views are to be tested, major, not minor, strike-slip faults are likely to provide the necessary evidence (leaving aside for the moment how one decides what defines a major fault). Slip on many major intracontinental strike-slip faults, however, absorb large fractions of relative plate movement between lithospheric plates and deforming regions, e.g., the Alpine fault in New Zealand, the San Andreas fault in California, and the North Anatolian fault in Turkey. Minor faults separating small blocks of crust almost surely do not pass straight down into the lithosphere as narrow shear zones. Others see strike-slip faults, including major ones, not only marking discontinuities in the velocity field, but also serving as passive markers in the deformation field of the brittle upper part of lithosphere, which deforms continuously at depth (e.g., Davis et al., 1997 England and Houseman, 1986 England and Molnar, 1990, 1997 Flesch et al., 2001, 2005 Holt, 2000 Holt et al., 1991). Some imagine that major strike-slip faults in continents also act as plate boundaries and can be defined as narrow shear zones beneath the much thicker crust of continents than that of oceanic regions (e.g., Peltzer and Tapponnier, 1988 Tapponnier et al., 1986, 1990, 2001). Almost by definition, transform faults between oceanic plates pass through the entire lithosphere as narrow shear zones, but the nature of strike-slip faults within continents remains a subject of discussion. ![]()
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