47132-GB8
Using Paleomagnetism to Investigate Distributed Deformation within the San Andreas Fault System, Central California
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. I can test whether rocks in the region have rotated in response to the relative transform motion between the Pacific and North American plates using 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 will be comparable to broader tectonic patterns.
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.
I have been focusing primarily on the first objective over the past year. Prior to the start of this project, I had analyzed samples from 26 stations, of which 14 stations with interpretable results suggested an average rotation of 14 degrees for the region. Over the past year, I have traveled to central California four times to collect samples for paleomagnetic analyses as well as other relevant structural field data. These trips have coincided with breaks from teaching at Carleton College; on two of the trips I had undergraduate students as field assistants and in total, I have samples from more than 100 additional sites.
This past year, Zack McGuire '08 visited the paleomagnetic lab at Western Washington twice to work with our collaborator Bernie Housen. At the lab, he analyzed samples from stations up to #70 for his senior thesis in geology. Zach found promising evidence from ~30 stations of changing patterns of vertical axis rotations along the strike of the Rinconada fault, with greater rotations observed in the northwest than in the southeast. There is also preliminary evidence that the magnitude of rotation changes with distance from the fault. McGuire's results will be useful once we focus on addressing the second primary goal of this project to determine the scale and style of block faulting in the region.
I plan on making several more trips to collect samples in the next year. I am mentoring a junior undergraduate student, Sarah Crump '10, who will work on this project over the next two years. Crump just processed a batch of samples in August 2008 at WWU, with at least three cores analyzed for stations up to #125 (although her pay was not included in this past year's budget). The data from all the paleomagnetic sites will form the basis for her senior thesis in 2010.
