www.acsprf.org

The overarching purpose of this project is to better understand how plate boundary deformation between the Pacific and North American plates is accommodated across the San Andreas fault system in central California. Instead of envisioning this plate boundary as a single fault, it is important to recognize that plate boundary deformation is accommodated by numerous faults and other geologic structures that develop adjacent to faults. The project is focused near one section of the San Andreas fault in central California, known as the creeping segment, where the two plates move past each other at a steady-rate without generating large earthquakes.

The first part of this project was designed to test how rocks across the area responded to the relative motion between the plates, using paleomagnetism - the ancient rock magnetic signature preserved within rocks. In detail, we looked at deformation on the southwest side of the San Andreas fault, near the Rinconada fault. This area is ideal because the Miocene Monterey Formation is well exposed along the Rinconada 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 were two specific paleomagnetic objectives for this project. The first was 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 was to better understand the pattern of rotations across the study area addressing where, how, and why they have occurred.

We addressed both objectives in a recent publication in the journal Geology, authored with two Carleton College undergraduates. This paper was my first publication based solely on research completed after receiving my Ph.D, so it provides important evidence of independent scholarship for my tenure case. By analyzing data from 177 sites, we found that the pattern of vertical axis rotations from paleomagnetic data is spatially variable across the study area. Generally, regions close to the Rinconada fault have the largest clockwise rotations. However, one surprising finding was that some stations in a small region show evidence of counterclockwise rotation, which is not expected given the relative plate motion in this system. By comparing the paleomagnetic data to two other datasets – the GPS velocity field and deformation recorded by folds throughout the region – we were able to make sense of the overall patterns of off-fault deformation, especially the counter-clockwise rotations. Our results suggest that fault creep is not a recent style of deformation for the creeping segment, but has been occurring for several million years.

The hypotheses from our Geology paper, in particular the speculations about the importance of the creeping-to-locked transition, led to a new research direction in central California. This past year, I examined patterns of deformation on the northeast side of the fault, in the complimentary region that we expect to see the off-fault effects of the fault creep. Two Carleton undergraduates from the class of 2011 wrote their senior theses about deformation at Kettleman Hills, a large oil field in this area. They examined small-scale structures within the Kettleman Hills anticline (e.g. faults, deformation bands, joints) and paleomagnetic data, both of which provide a good window on the style of off-fault deformation on the northeast side of the fault. One of the two students presented her results at a national geology conference in 2010; the other student is presenting her results at a national conference in 2011. When paired with the results from the southwest side of the fault, these two projects provide important insight into the distribution of plate boundary deformation, which is ultimately useful for characterizing earthquake hazards in central California.