BLOCK VERSUS CONTINUUM DEFORMATION IN THE WESTERN UNITED-STATES

The relative role of block versus continuum deformation of continental lithosphere is a current subject of debate. Continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies. In contrast, block mode ls are favored by geologic studies of displacement in places like Asia . A problem in this debate is a lack of data from which unequivocal co nclusions may be reached. In this paper we apply the techniques of stu dy used in regions such as the Alpine-Himalayan belt to an area with a wealth of instrumental data-the Western United States. By comparing p late rates to seismic moment release rates and assuming a typical seis mogenic layer thickness of 15 km it appears that since 1850 about 60% of the Pacific-North America motion across the plate boundary in Calif ornia and Nevada has occurred seismically and 40% aseismically. The Sa n Francisco Bay area shows similar partitioning between seismic and as eismic deformation, and it can be shown that within the seismogenic de pth range aseismic deformation is concentrated near the surface and at depth. In some cases this deformation can be located on creeping surf ace faults, but elsewhere it is spread over a several kilometer wide z one adjacent to the fault. These superficial creeping deformation zone s may be responsible for the palaeomagnetic rotations that have been a scribed elsewhere to the surface expression of continuum deformation i n the lithosphere. Our results support the dominant role of non-contin uum deformation processes with the implication that deformation locali zation by strain softening must occur in the lower crust and probably the upper mantle. Our conclusions apply only to the regions where the data are good, and even within the Western United States (i.e., the Ba sin-and Range) deformation styles remain poorly resolved. Nonetheless, we maintain that block motion is the deformation style of choice for those continental regions where the data are best.