2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 25
Presentation Time: 9:00 AM-6:00 PM


JACHENS, Robert C.1, LANGENHEIM, Victoria1, WENTWORTH, Carl M.2, SIMPSON, R.W.3 and GRAYMER, R.W.4, (1)U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, (2)Emeritus, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, (3)U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, (4)U. S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, jachens@usgs.gov

Modern aeromagnetic surveys now cover the entire central California Coast Ranges and provide a basis for defining offsets on strike-slip faults of the San Andreas system. In the Franciscan terranes SW of the Nacimiento Fault, long, linear ophiolitic belts that have been cut by strikeslip faulting produce offset magnetic anomalies. In the Salinian block NE of the Nacimiento Fault, magnetic plutonic rocks of the La Panza Range and isolated magnetic strata also produce magnetic anomalies useful for defining fault offset. Magnetic ophiolites truncate at the east face of the San Gregorio-Hosgri Fault (SGHF) at Cape San Martin, near Pt Buchon (a pair of ophiolites bracketing the point), Pt Sal, and Pt Arguello. Cross-fault counterparts of their associated magnetic anomalies (supported in most cases by geologic relations) west of the fault suggest southward decreasing apparent right-lateral offsets of these ophiolites of 148, 125, 89, and 29 km, respectively, with uncertainties of < 5 km. This southward decreasing offset pattern is similar to the current southward decreasing slip rate along the fault system, suggesting a consistent tectonic behavior over time. Southward decrease in SGHF offset requires balancing offsets on diverging faults or other structures, and the SGHF offset distribution indicates where to look for these structures. The 23 km difference in offset at Cape San Martin and Pt Buchon, for example, could be taken up on east-splaying faults including the major Oceanic-West Huasna system, and the Cambria, Los Osos, and other faults that likely feed into the Oceanic-West Huasna system. Some combination of these faults might transfer the missing offset onto the Oceanic-West Huasna system and into the heart of the Coast Ranges. The actual mode of offset accommodation is likely more complex than simple offset across obvious faults bounding rigid blocks. The 23 km offset difference (Cape San Martin-Pt Buchon) is significantly less than the estimated 33 km right offset across the West Huasna Fault of the SE end of the Pismo syncline and its bounding ophiolites (expressed both in associated magnetic and gravity anomalies). Understanding the total offset budget of the central California Coast Range faults should contribute to an understanding the current tectonic regime.