Kimberly Cate Psencik

February 18, 2007

My research currently has two focus areas, Northern Baja California, Mexico and the Sierra Nevada Block, with one unifying observational method, GPS (Global Positioning System). In Baja California my goal is to identify the source of uplift in the Sierra San Pedro Martir using both the vertical and horizontal components of motion, as measured by our GPS network. In addition, I hope to use GPS coupled with block modeling and finite element modeling to refine the kinematic description of motion of the Sierra Nevada Block and to use this to draw some conclusions about the dynamics of the Pacific-North American system as a whole.

The observed topographic expression in the northern Baja California region is a challenging multipart problem that may give insight into other fault systems and tectonic mechanisms. Northern Baja California is a complex fault system with mechanisms of both strike-slip and normal faulting. This area is positioned close to the Pacific-North America plate boundary and is intimately associated with the spreading center within the Gulf of California as well as the southern extension of the San Andreas Fault. In this study I focus on the normal faulting components of the region. Over the last 7 years our lab has collected campaign GPS data from a network of stations located throughout the area. The data show statistically significant vertical movements of up to 4.8 +/- 1.5 mm/yr, which correlate with high topography. In order to better understand the source of the observed uplift, I calculated the expected isostatic uplift magnitude and distribution in response to unloading that occurs along a normal fault due to crustal thinning.

I obtained the solution using a two stage calculation procedure, the first of which was an elastic half space model utilizing the Poly3D software package. The second stage of calculations applied the calculated mass removal and uplift magnitude from purely kinematic means, to an elastic plate bending model. My modeled results, however, did not fit the observed uplift signal in northern Baja California well. In particular the wavelength of the uplift distribution was misfit. This mismatch indicates an additional unmodeled uplift process, most likely with a deeper source. Using the same elastic plate bending model and a best fit approach I calculated the amplitude and distribution of the additional uplift. My results show that the additional uplift centers 30 km west of the normal fault, beneath the highest topography. The parameters that varied within these models include rigidity, rate, locking depth of the fault, and the position of the center of negative mass for the additional source of isostatic uplift.

The possibility that this uplift signal is due to a systematic error has been explored and eliminated. However, although I don’t yet have a quantitative model that explains this additional uplift, I attribute it to deep processes such as delamination of a dense root, analogous to delamination beneath the Sierra Nevada batholith to the north. I intend to further test these hypotheses by further modeling using the Finite Element Method and collecting more campaign GPS data to refine the data set.

One of our overreaching goals is to present a new surface velocity field for California and Nevada (UM-CANVAS) and use it to solve for a new angular velocity for the rigid Sierra Nevada-Great Valley block. To this end, I use all publicly available GPS data for this region, from both continuous (CGPS) and episodic campaign (EGPS) sites. All data are being re-processed to create a consistent velocity field. Site velocities are referenced to stable North America. The Sierra Nevada block is a relatively long, narrow block, bounded on the west by the San Andreas Fault, on the east by a complex fault system comprising the Eastern California Shear Zone and the Walker Lane Belt, and on the south by the Garlock fault. The northern extent of the block is not well defined. The new data along with a block model and strain accumulation algorithm may help to define this boundary. Published estimates for the motion of Sierra Nevada block include clockwise, counter-clockwise, and no rotation relative to stable North America; the new velocity data has helped to resolve this discrepancy. To better understand the motion of the Sierra Nevada block over time, I compare my model of present day motion to a geologic model of block motion over the last 2-3 Ma (McQuarrie and Wernicke, 2005) where I use our pole calculation software to calculate a geologic, long term pole of rotation.