EFFECTS OF A BLIND STRIKE-SLIP FAULT SYSTEM ON HYDROTHERMAL ACTIVITY AND STRUCTURE IN THE BISHOP TUFF, CA

The Volcanic Tablelands are situated in the Eastern California Shear Zone at the northern end of the Owens Valley. Volcanism from the adjacent Long Valley Caldera led to the emplacement of the ~ 760 ka Bishop Tuff atop the tectonically active valley floor, north of Bishop, CA. Since its emplacement, the Bishop Tuff has been segmented by a network of N-NW striking normal faults, complex vertical and curvilinear cooling joints, and subjected to hydrothermal activity forming fumarolic mounds and ridges. Earthquake epicenter and focal mechanism records from 1971 to present suggest a dextral strike-slip fault system buried beneath the Volcanic Tablelands. In light of these records, structural fieldwork was conducted over an area of ~ 335 square kilometers in order to determine if discontinuities at the surface recorded the motion of this blind strike-slip fault system, and controlled the distribution of hydrothermal activity. Structural orientations were recorded, and mineralized samples were collected from faults, joints, and fumarolic features to characterize the nature of mineralization across the tablelands and better establish the sequence of events following the emplacement of the tuff. We discovered kinematic evidence in the form of slickenlines, fault grooves, and chatter marks that suggest that fault motion on the tablelands is not solely dip-slip. Rather, these indicators include oblique and pure strike-slip components, some of which overprint vertical slickenlines, which likely indicate the influence of the blind strike-slip system. Given their location relative to mapped earthquake epicenters, the spatial distribution of fumarolic mounds and ridges appear to be located atop the trailing quadrants and core of the blind strike-slip system. Based on our findings, the modified sequence of events is as follows: (1) emplacement of the Bishop Tuff; (2) development of vertical joints; (3) initiation of normal to oblique faulting; (4) hydrothermal activity and mineralization along fault and vertical joint surfaces; (5) hydrothermal activity along curvilinear cooling joint and fracture surfaces; (6) post-hydrothermal faulting to form near-horizontal overprints of vertical slickensides and cut fumarolic mounds; (7) erosion and weathering of hydrothermal mineral deposits.