DEFORMATION ACROSS THE EASTERN MONO BASIN, ADOBE HILLS, CALIFORNIA REVEALED VIA PRELIMINARY PALEOMAGNETIC DATA

Modern deformation between the North American and Pacific plates is distributed across a wide zone of the western margin of North America, from the San Andreas Fault eastward into the western Basin and Range province. Geodetic and geologic studies indicate that the San Andreas Fault system accommodates ~70 - 75% of the total relative plate motion. The residual 25% - 30% of the strain is transferred away from the San Andreas Fault system via the Eastern California Shear Zone northward into the Walker Lane Belt and the Central Nevada Seismic Zone. We propose the hypothesis that during the early to mid-Miocene deformation associated with the transtensional system was located east of the Sierra Mountain front in the area of the Mono Basin prior to stepping east into the Mina Deflection in the Pliocene. Here we report paleomagnetic data from the southern and northern Adobe Hills, CA where Pliocene basalts yield spatially variable components of vertical axis rotation, or lack thereof, depending on geographic location. The structural grain in the southern Adobe Hills is defined by northwest-striking right-lateral oblique slip faults. In the northern Adobe Hills, the structural grain changes to east northeast-striking left-lateral oblique slip faults. Paleomagnetic site locations were established in numerous coherent stratigraphic sections involving four to sixteen individual basalt flows. Preliminary results from the southern zone yield section mean data that are consistent with the latest Miocene to Pliocene time-averaged expected field direction. Demagnetization experiments are underway from the northern zone with field observations and reconnaissance geologic mapping suggesting a transtensional zone likely characterized by modest vertical axis rotation. Due east in Nevada, preliminary paleomagnetic data from the mid-Miocene Jack Springs Tuff indicate clockwise vertical axis rotation ranging from +20° to +60° between multiple fault blocks. We argue that the Adobe Hills represents a key transitional zone between un-rotated crustal blocks to the south while to the north and northwest, the crust has been fragmented into blocks that have experienced vertical axis rotation. Furthermore, we pose that this zone extends north and west as far as the Bodie Hills and the eastern-most Sierra Nevada Mountains.