Fracture joints shows extension of the Mojave block acted as a conduit for initiation of the Garlock fault
Mentor:Ann Blythe, Adjunct Assistant Professor of Geology, Occidental College
A geologic field examination of fracture joints in Tertiary igneous rocks adjacent to the Garlock fault in Southern California shows that extension of the northwest Mojave desert aided the initiation of strike-slip motion along the Garlock fault. Prior to fault initiation, North-South extension of the Mojave block resulted in east-west normal faults, fracture joint sets, and crustal thinning. I measured the orientation of the fracture jointing in the Tehachapi Mountains that formed during extension and were later affect by the in initiation of the Garlock fault. Transects across the Garlock show two homogeneous fracture joint sets; a joint with a moderate southern dip, striking subparallel to the Garlock with an average strike of 96° and a vertical fracture joint striking near North. The preliminary data indicates that as the extensional realm transitioned towards the strike-slip system, slip resulted along the joints. I observed quartz veins with micro-normal faults in the Tehachapi Mountains, with North-South extension as the inferred cause. Miocene age rhyolite on the eastern portion of the field area yields evidence of moderate stike-slip faulting, up to ~3m of displacement, along the subparallel joints. With both strike-slip and normal faulting apparent in the jointing, it can be inferred that the fracture zone jointing guided initial strike-slip motion, which later became the dominant regional forcing.