Changes in Central Walker Lane strain accommodation near Bridgeport, California, as told by the Stanislaus Group /by Chad William Carlson.

The Walker Lane (WL) is an elongate, NW-oriented, region of active tectonics presently accommodating ~20-25% of the dextral-motion between the Pacific and North American plates. This portion of dextral-motion is being accommodated on regional-scale systems of predominantly strike-slip faulting between the northwesterly-translating Sierra Nevada (SN) microplate and the east- west extension of the Basin and Range. At the western edge of the central Walker Lane (ca. 38??-39??N latitude) is a region of crustal-scale blocks bounded by wedge- shaped depositional-basins and normal-fault systems, here defined as the west- central Walker Lane (WCWL). Devoid of obvious strike-slip faulting, the presence of tectonic-block vertical-axis rotations in the WCWL represents unrecognized components of dextral-shearing and/or changes of strain- accommodation over time. This study uses paleomagnetic reference directions for Eureka Valley Tuff (EVT) members of the late Miocene Stanislaus Group (SG) as spatial and temporal markers for documentation of tectonic-block vertical-axis rotations near Bridgeport, CA._x000D_Study-site rotations produced discrete rotational-domains of mean vertical- axis rotation ranging from ~10??-30?? with heterogeneous regional distribution. Additionally, the highest measured magnitudes of vertical-axis rotation (~50??-60?? CW) define a ??Region of High Strain?? that includes the Bridgeport Valley (Basin). Unrecognized tectonic modification of reference direction sites for 2 (By-Day and Upper) of the 3 members of the EVT resulted in under-estimates of vertical-axis_x000D_rotations by previous studies. Mean-remanent-directions and virtual geomagnetic- poles from this study yield a recalculated reference direction for the By-Day member of: Dec.=353.2??; Inc.= 43.7??; _95=10.1. This recalculated value confirms the presence of previously unrecognized reference site rotations, and provides an additional reference direction for determining vertical-axis rotation magnitudes. This study presents a kinematic model based on mean rotation magnitudes of ~30?? CW for the Sweetwater Mountains and Bodie Hills that accounts for rotational- strain-accommodation of dextral-shear in the WCWL since the late Miocene. This model considers rotational magnitudes, paleostrain, edge-effects, present-day strain-accommodating structures, and geometric limitations of rotating crustal blocks to represent changes in regional strain accommodation over time. The results and models presented here elucidate the complicated and evolving nature of the WCWL, and further understanding of variations in strain accommodation for the Walker Lane