HR: 0800h
AN: S41A-0971    [Abstracts]
TI: A Lithospheric Refraction/Reflection/Teleseismic Model of LARSE Line 2: Thrusting of the Santa Monica Mountains-San Fernando Valley Block Beneath the Central Transverse Ranges, Southern California
AU: * Fuis, G S
EM: fuis@usgs.gov
AF: U.S.Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Murphy, J M
EM: murphy@usgs.gov
AF: U.S.Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Baher, S
EM: swavecoda@yahoo.com
AF: U.S.Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Ryberg, T
EM: trond@gfz-potsdam.de
AF: GeoForschungsZentrum, Telegrafenberg, Potsdam, 14473 Germany
AU: Kohler, M D
EM: kohler@ess.ucla.edu
AF: University of California, Los Angeles, Department of Earth and Space Sciences, Los Angeles, CA 90095-1567 United States
AU: Okaya, D A
EM: okaya@usc.edu
AF: University of Southern California, 3651 Trousdale Parkway, Suite 169, Los Angeles, CA 90089-0742 United States
AB: LARSE line 2, a refraction/reflection/teleseismic line, extended from the coast at Santa Monica, California, northward to the Sierra Nevada, passing through or near the epicenters of the 1971 and 1994 M 6.7 San Fernando and Northridge earthquakes. Refraction tomographic models of Lutter et al. (BSSA, 2004) contain tantalizing hints of geologic structure, but are necessarily smoothed and extend no deeper than about 8 km depth. Vertical-incidence reflection data (Fuis et al., Geology, 2003) reveal some important deep structures, but are low-fold and produce the best images below the upper-crustal region modeled by refraction tomography. Teleseismic tomography (Kohler et al., BSSA, 2003) reveals upper mantle structure below 40-km depth. To sharpen the velocity image of the crust and to extend it to the Moho, we have forward-modeled both refraction and wide-angle reflection data and have incorporated surface geologic and sparse drillhole constraints into the model. The resulting image reveals the shapes of sedimentary basins underlying the San Fernando, Santa Clarita, and Antelope (western Mojave) Valleys as well as many active and inactive faults in the upper crust. Importantly, it also reveals a major north-dipping wide-angle-reflective zone extending downward from the 1971 San Fernando hypocenter toward the San Andreas fault (SAF). This zone, coincident with a vertical-incidence-reflective zone, is interpreted as a shear zone separating generally higher-velocity rocks of the Santa Monica Mountains and San Fernando Valley (SMM-SFV) from lower-velocity rocks of the Central Transverse Ranges (CTR). The SMM-SFV represents a block of the Peninsular Ranges terrane that is currently underthrusting the CTR, presumably as it continues to rotate clockwise. There is a symmetrical south-dipping wide-angle-/vertical-incidence-reflective zone on the north side of the San Andreas fault, but it is not clear if this structure is currently active. Moho is depressed to a maximum depth of 36 km beneath the CTR, defining a crustal root similar to that on LARSE line 1, 70 km to the east, but with smaller relief (3-5 km vs. 5-8 km), and it is centered ~ 5 km south of the SAF. The SAF appears to offset all layers above the Moho. Teleseismic tomography reveals a sharp velocity gradient within the upper mantle that extends to at least 160-km depth along the vertical projection of the SAF, separating the faster southern California upper-mantle "drip" south of the SAF from slower upper mantle north of the SAF.
DE: 0905 Continental structures (8109, 8110)
DE: 7205 Continental crust (1219)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8104 Continental margins: convergent
DE: 8106 Continental margins: transform
SC: Seismology [S]
MN: Fall Meeting 2005