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Deformation across the Western United States: A Local Estimate of Pacific-North America Transform Deformation

Eugene D. Humphreys, & Ray J. Weldon

Published 1994, SCEC Contribution #292

We obtain a locally based estimate of Pacific-North America relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern North America from east of the Rio Grande Rift to near the continental escarpment. Data are primarily Quaternary geologic slip rate estimates, and resulting deformation determinations therefore are “instantaneous” in a geologic sense but “long term” with respect to earthquake cycles. We deduce a rate of motion of the Pacific plate relative to North America that is 48 ±2 mm/yr, a rate indistinguishable from that predicted by the global kinematic models RM2 and NUVEL-1; however, we obtain an orientation that is 5–9° counterclockwise of these models. A more westerly motion of the Pacific plate, with respect to North America, is calculated from all three paths. The relatively westerly motion of the Pacific plate is accommodated by deformation in the North American continent that includes slip on relatively counterclockwise-oriented strike-slip faults (including the San Andreas fault), which is especially relevant in and south of the Transverse Ranges, and a margin-normal component of net extension across the continent, which is especially relevant north of the Transverse Ranges. Deformation of the SW United States occurs in regionally coherent domains within which the style of deformation is approximately uniform. In the vicinity of the Transverse Ranges, two important shear systems splay from the San Andreas fault: the eastern California shear zone trending NNW from the eastern Transverse Ranges and the trans-Peninsular faults trending SSE from the western and central Transverse Ranges. Within the Transverse Ranges the right-lateral San Andreas fault steps left, seemingly requiring large amounts of convergence there. However, most of this convergence is avoided through a “funneling flow” of the crust toward the western Transverse Ranges and into the relatively narrow central California Coast Ranges and the northern motion of the Mojave. The former process involves an alternation of rotation direction from counterclockwise (in and south of the central Transverse Ranges) to clockwise (in the western Transverse Ranges).

Humphreys, E. D., & Weldon, R. J. (1994). Deformation across the Western United States: A Local Estimate of Pacific-North America Transform Deformation. Journal of Geophysical Research, 99(B10), 19975-20010.