Kinematics of the west-central Walker Lane: Spatially and temporally variable rotations evident in the Late Miocene Stanislaus Group

Abstract

The Walker Lane currently accommodates ∼20% of the dextral motion between the Pacific and North American plates. This accommodation occurs on regional-scale systems of strike-slip and normal faults located between the northwestward-translating Sierra Nevada microplate and the east-west–extending Basin and Range. At the western edge of the central Walker Lane (lat ∼38°–39°N) is a region of crustal blocks bounded by asymmetric basins and normal faults, here defined as the west-central Walker Lane. Although this region is apparently devoid of major active strike-slip faults, the presence of Neogene clockwise vertical-axis tectonic block rotations indicates the accommodation of dextral shear. We measured vertical-axis rotation by comparing remanence directions of widespread members of the Eureka Valley Tuff of the Late Miocene Stanislaus Group within the west-central Walker Lane to the same units on the Sierra Nevada microplate. Results show that the study area is organized into discrete domains with heterogeneous regional distribution of clockwise vertical-axis rotation, ranging from ∼10° to 60°, since ca. 9.5 Ma. The highest measured magnitudes of vertical-axis rotation (∼50°–60° clockwise) are interpreted as a region of high deformation that includes the asymmetric Bridgeport Valley. Previous work underestimated vertical-axis rotation magnitudes in the region because published reference directions for two of the three members of the Eureka Valley Tuff (By-Day Member and Upper member) derive from the rotated region. We recalculate a reference direction for the By-Day Member of declination 353.2°, inclination 43.7°; α 95 = 10.8°. This corroborates a reference direction for the By-Day Member from the Stanislaus Group type section, situated on the relatively stable Sierra Nevada microplate, providing a robust reference direction for paleomagnetic studies. We present a kinematic model in which dextral shear in the west-central Walker Lane is accommodated by ∼30° of clockwise rotation in the Sweetwater Mountains and Bodie Hills since the Late Miocene. This model incorporates rotation magnitudes, paleostress orientations, edge effects, and bounding faults of rotating tectonic blocks to reveal timing, patterns, and mechanisms of crustal deformation. The results and models presented here elucidate the complex and evolving nature of the west-central Walker Lane. The rotational history of dextral shear accommodation demonstrates that the west-central Walker Lane should be included as an important part of the Walker Lane transtensional zone. The results presented in this study not only improve understanding of deformation in the Walker Lane, but illuminate the potentially significant contribution of crustal block vertical-axis rotations in other transtensional regions of the world.