CHANGES IN CENTRAL WALKER LANE STRAIN ACCOMMODATION NEAR BRIDGEPORT, CA

The Walker Lane (WL) is an elongate, NW oriented, region of active tectonics positioned between the northwesterly-translating Sierra Nevada microplate and the east-west extension of the Basin and Range (Fig. 1). This region of transtension presently accommodates ~20% of the dextral-motion between the Pacific and North American plates on regional-scale strike-slip fault systems. West of the NW oriented dextral fault systems of the central Walker Lane 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. We use paleomagnetic reference directions for Eureka Valley Tuff members of the late Miocene Stanislaus Group as spatial and temporal markers for documentation of tectonic-block vertical-axis rotations near Bridgeport, CA.

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 wedge-shaped Bridgeport Valley (Basin). We present 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 indicators, basin development, and strain-accommodating structures 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.