The distributed nature of deformation on the continents compared to most oceanic regions has led to the invention of a unique framework for describing continental deformation (Sections 2.10.5, 5.3). One of the most important aspects of developing this framework involves determining whether the motion is accommodated by the movement of many coherent blocks separated by discrete zones of deformation or by a more spatially continuous process. In some areas, the presence of large aseismic regions such as the Great Valley and Sierra Nevada in the southwestern USA (Figs 7.8, 7.10) imply that part of the continental lithosphere behaves rigidly. However, in other areas, such as the Walker Lane and Eastern California Shear Zone, seis-micity reveals the presence of diffuse zones of deformation that are better approximated by a regional velocity field rather than by the relative motions of rigid blocks.
To distinguish between the possibilities, geoscien-tists use combinations of geologic, geodetic, and seis-mologic data to determine the degree to which deformation is continuous or discontinuous across a region (Thatcher, 2003; McCaffrey, 2005). Determining the characteristics of these regional velocity fields is important for developing accurate kinematic and rheo-logical models of deforming continental lithosphere (Section 8.6), and for estimating where strain is accumulating most rapidly and, thus, where earthquakes are most likely to occur.
In models involving continuous velocity fields, even though the upper brittle crust is broken into faults, the faults are predicted to be relatively closely spaced, have small slip rates, and extend only through the elastic part of the crust. In this view, the velocity field commonly is assumed to represent the average deformation of the whole lithosphere, which consists of a thin layer (1020 km) that deforms by faulting above a thick layer (80-100 km) that deforms by ductile creep (Jackson, 2004). In rigid block models, faults are predicted to be widely spaced, slip rapidly, and extend vertically through the entire lithosphere, terminating as large ductile shear zones in the upper mantle (McCaffrey, 2005). These latter properties suggest that deforming continental lithosphere exhibits a type of behavior that resembles plate tectonics. In both types of model, the deformation may be driven by a combination of forces, including those acting along the edges of crustal blocks, basal tractions due to the flow of the lower crust and upper mantle, and gravity.
Determining the degree to which continental deformation is continuous or discontinuous has proven difficult to achieve with certainty in many areas. One reason for the difficulty is that the short-term (decade-scale) surface velocity field measured with geodetic data usually appears continuous at large scales (kilometers to hundreds of kilometers) (Section 2.10.5). This characteristic results because geodetic positioning techniques provide velocity estimates at specific points in space, with the density of available points depending on the region and the scale of the investigation (Bos & Spakman, 2005). Most interpretation methods start with some interpolation of the geodetic data, with the final resolution depending on the number and distribution of the available stations (Jackson, 2004). In addition, available information on fault slip rates is incomplete and those that are calculated over short tim-escales may not be representative of long-term slip rates (Meade & Hager, 2005; McCaffrey, 2005). If accurate long-term slip rates on all crustal faults were available, then the problem could be solved directly. Furthermore, even though continental deformation is localized along faults over the long term, the steady-state motion of an elastic upper crust over the short-term contains little information about the rheology of the deforming material, so the importance of the faults in the overall mechanical behavior of the lithosphere is unclear. This latter uncertainty clouds the issue of whether deformation along continental transforms is driven mostly by edge forces, basal tractions, or gravitational forces (Savage, 2000; Zatman, 2000; Hetland & Hager, 2004) (Section 8.5.3).
Despite the difficulties involved in quantifying continental deformation, geoscientists have been able to show that elements of both continuous and discontinuous representations fit the observations in many areas. In this section, the results of geodetic measurements and velocity field modeling are discussed in the context of the San Andreas Fault system, whose structural diversity illustrates the variety of ways in which strain may be accommodated along and adjacent to continental transforms.
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