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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 (if rotations have been determined for the region) is to better understand the pattern of rotations across the study area addressing where, how, and why they have occurred.

In the first year of the project, Zach McGuire ‘08 – an undergraduate student from Carleton College – analyzed data from seventy stations for his senior thesis in geology. Zach 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. A second undergraduate student - Sarah Crump x'10 – has been working on additional data that I have collected in the past year from three separate sampling trips to California. Sarah has spent a total of four weeks processing paleomagnetic data with our collaborator Bernard Housen at Western Washington University and will use data from approximately 150 stations as part of her senior thesis this year.

Because the results from these two student projects demonstrate spatially variable rotations, we are planning a new avenue of work with Lisa White at San Francisco State. Lisa is a micropaleontologist and can therefore help us obtain more precise ages of the Monterey Formation throughout the study area. This age-control will allow us to calculate rates of rotation (not just absolute magnitudes of rotation) and to determine whether rotation rates have been constant over the time period sampled. A Carleton undergraduate student will likely be involved in aspects of this new research.