GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 347-36
Presentation Time: 9:00 AM-6:30 PM


MCGREGOR, Ian Scott, Department of Geosciences, San Francisco State University, 1600 Holloway Ave, San Francisco, CA 94132,

In the Central California Coast Ranges, relative plate motion not accounted for by the San Andreas fault is thought to be accommodated, in part, by right-lateral slip along other major strike slip faults of the San Andreas transform system such as the San Gregorio-Hosgri fault zone (SGHFZ) and the Rinconada fault (RF), and by plate normal and/or oblique contraction. Late Pleistocene slip rates along the mostly offshore SGHFZ double from 1-2 mm/yr at San Simeon south of Monterey Bay, to 3-4 mm/yr at Año Nuevo north of Monterey Bay, suggesting either an underestimation of slip rate at San Simeon or a transfer of slip to the SGHFZ from eastern structures between the two study sites. A possibility is that additional right-lateral slip is transferred westward from the RF system through a broad zone of distributed dextral shearing, including NW striking faults in the northern Santa Lucia Range (NSLR) where the RF terminates after carrying 1-2 mm/yr of slip.

In this study, kinematic evidence of late Pleistocene and possibly younger dextral shearing through the NSLR includes fault plane lineations with shallow rake angles on three exposures of the geomorphically prominent, NW striking Tularcitos fault. Positive flower structure of subsidiary shears on vertical fault exposures, observed in outcrop displacing young deposits, suggests a recent, dominantly lateral component of slip along the Tularcitos fault. Previously unmapped NW striking, steeply-dipping fault exposures located approximately 16 km SE of the Tularcitos fault and 11 km northwest of the north end of the RF contain fault plane lineations with rake angles of 20-31°SE, suggesting regional dextral shear and a physical link between the northern Rinconada fault and the Tularcitos fault. One of these fault exposures involves late Pleistocene and younger deposits. Additional fault mapping and relative timing of offset for these previously unidentified fault exposures are supported by geomorphic features indicative of dextral faulting, identified on high-resolution satellite and LiDAR imagery along projected fault strikes. Fold axes orientations in the NSLR and the Monterey-Seaside area, along with new physical data on the Tularcitos and previously unmapped faults suggest that transpressive dextral shearing occurred through the NSLR in the late Pleistocene or later.