GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 128-4
Presentation Time: 9:00 AM-6:30 PM


SPOTILA, James A., Department of Geosciences, Virginia Tech, Blacksburg, VA 24061 and CHILTON, Kristin D., Geosciences, Virginia Polytechnic Institute and State University, 1405 Perry Street, Blacksburg, VA 24061

Controls on the pattern, geometry, and linkages of faults in the Eastern California shear zone (ECSZ) are not well understood, but are potentially informative for the tectonic evolution and rupture dynamics of complex shear zones. Among the unexplained observations in the ECSZ are the variable orientation between and along main faults, which in the southern Mojave average N37W but have a 20 degree range. Existing tectonic models, including vertical axis rotation, are inadequate to explain this phenomenon. Another problem is the prevalence and nature of stepovers. Although models suggest that faults should smoothen with increasing displacement, the frequency of stepovers does not decrease with net slip, but instead is relatively constant (one per 15 km). Stepovers are also more commonly (~2 to 1) convergent, despite models that suggest faults integrate mainly via transtension. Finally, the stepovers have a characteristic form, including 3:1 aspect ratio, gentle bend angle, smooth lazy-S double-bend shape, and broad whale-back profile, which suggests consistent forcing. We are exploring how these transpressive stepovers evolve and whether the pattern and geometry of faults are related to preexisting crustal weaknesses or gradual slip-strengthening associated with the accumulation of secondary deformation. Results along the Calico-Hidalgo and Emerson faults in the southern ECSZ suggest that stepovers may be a consequence of linkage between faults of different initial orientation. The stepovers also seem to be long-lived, with progressive deformation of one flanking strike-slip strand into a reverse fault to create a one-sided flower structure. We document successive growth of new strike-slip strands towards the core of the bend from the opposing flank, including identification of a likely late-Holocene rupture with several meters right-lateral slip along the Hidalgo fault. This evolution appears to occur regardless of whether lithologic weaknesses are present. The degree to which this process is autogenic, or influenced by secondary deformation that may perturb the primary faults, such as the pervasive long-wavelength folding between the Calico and Bullion faults (Deadman Basin), remains to be determined.