HR: 0800h
AN: S41A-0972    [Abstracts]
TI: An Unbroken Moho and the Strength of the Lower Crust beneath the San Andreas Fault: A new look at the LARSE-I profile
AU: * Wilson, C K
EM: wilsonck@ldeo.columbia.edu
AF: Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY 10964-1000 United States
AU: Fuis, G
EM: fuis@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94035 United States
AU: Holtzman, B K
EM: benh@ldeo.columbia.edu
AF: Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY 10964-1000 United States
AU: Baher, S
S41A-0972 AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94035 United States
AU: Langenheim, V
S41A-0972 AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94035 United States
AB: The structure of continental crust at large transform faults reveal complex interactions between strain distribution and rheological variations. Seismic data collected by the LARSE-I experiment provided an opportunity to assess strain accommodation and crustal strength beneath the San Andreas Fault (SAF) near Los Angeles. By combining observations and wavespeed models from active source profiling and teleseismic converted wave imaging, we show there is no break of the Moho beneath the San Andreas and deformation is distributed over a broad region (~60 km) in the lower crust. We image the crust and uppermost mantle beneath the San Andreas by depth migrating receiver functions using the refraction wavespeed model. This image differs from previous studies that observed a nearly 10 km offset in the depth of the Moho beneath the fault and was interpreted to mean that the San Andreas extended through the crust as a narrow shear zone. We attribute the differences to a more accurate depth migration used in creation of our image through incorporation of the high-resolution refraction wavespeed model. To demonstrate the effect of inaccurate wavespeed models and to test the resolution capabilities of our teleseismic imaging method, we show images created from synthetic seismograms calculated by propagating plane waves through crustal models with different Moho geometries. As further support for our proposed Moho geometry, we show that a thickened crustal root predicts observed Pmp arrival times that would be mismatched or absent in models that included a Moho step beneath the San Andreas. Two important aspects of our image allow us to infer vertical variations in strength within the crust: 1) the smooth increase in crustal thickness beneath the SAF over a region ~60 km wide, 2) the presence of a mid-crustal sub-horizontal feature previously recognized as a high amplitude, negative polarity reflector interpreted as a shear zone responsible for accommodating relative motion between the upper and lower crust. We use these two features to develop hypotheses on crustal strength profiles beneath the SAF. We assume that the reflector is caused by some change in the rock physical properties (grain size reduction and/or strong mineral alignment) due to strain partitioning. (For initial simplicity, we neglect a concentration of fluids, a good possibility with poorly constrained rheological consequences.) In a simple flow model (constrained by known plate velocities and geotherms), we test various possible combinations of parameters in flow laws appropriate to lower crustal compositions, to seek the conditions that can lead to significant strain localization and the development of a seismically reflective surface. We also calculate various resulting strength profiles that incorporate spatially changing strain rate conditions in the lower crust.
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Seismology [S]
MN: Fall Meeting 2005