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The overarching purpose of this project is to better understand how plate boundary deformation is accommodated across the San Andreas fault system in central California. To test how rocks across the area have responded to the relative motion between the Pacific and North American plates, I use paleomagnetism - the ancient rock magnetic signature preserved within rocks. I have focused on understanding deformation adjacent to the Rinconada fault, which is one of several faults in the plate boundary system. The Rinconada fault is ideal for this study because exposures of the Miocene Monterey Formation outcrop along the strike of the fault. This reservoir rock has been used for similar paleomagnetic projects in southern and central California, thus the results from this project can be placed in a larger tectonic context.

In detail, there are two specific objectives for this project. The first is to sample the Monterey Formation throughout the study area to determine whether there is evidence for vertical axis rotations preserved in the rock record. Unlike the Transverse Ranges to the south, rocks in this area in central California are typically assumed to have experienced little to no vertical axis rotation. The second goal is to better understand the pattern of rotations across the study area addressing where, how, and why they have occurred.

To date, two undergraduates from Carleton College have written senior theses based on this project. Zach McGuire ‘08 analyzed data from seventy stations and found promising evidence that (1) there are vertical axis rotations in parts of the study area and (2) these rotations vary in space, both along the Rinconada fault, as well as with distance from the fault. Sarah Crump ‘10 analyzed additional data from one hundred stations. She found more convincing evidence for variable rotations along the Rinconada fault, including an unexpected region of counterclockwise rotation that seems to correlate with a change from aseismic to seismic behavior along the San Andreas fault. Based on this finding, I collected additional data northeast of the San Andreas fault this summer with two undergraduate students – Amanda Yourd and Alice Newman, both x‘11. We focused on a region where this same transition from creeping-to-locked behavior along the San Andreas fault might be expected to influence off-fault deformation. These two new student projects may shed light on the distribution of plate boundary deformation in a different part of central California.