HR: 08:20h
AN: T31G-02    [Abstracts]
TI: Structural Control on the Seismotectonics of the Coast Range-Great Valley Boundary Zone
AU: * O'Connell, D R
EM: doconnell@do.usbr.gov
AF: USBR, PO Box 25007 D-8330, Denver, CO 80225
AU: Unruh, J
EM: unruh@lettis.com
AF: William Lettis and Assoc., 1777 Botelho Drive, Suite 262, Walnut Creek, CA 94596
AU: Block, L
EM: lblock@do.usbr.gov
AF: USBR, PO Box 25007 D-8330, Denver, CO 80225
AB: Structural relations inferred from analysis of three-dimensional velocity-hypocenter inversions and seismic reflection data at several locations between lat $36.5\deg$ N and $38.5\deg$ N demonstrate the strong influence of pre-existing crustal structure on the kinematics of active strike slip and thrust faults located in the Coast Range-Great Valley Boundary Zone. The dextral Ortigalita and Greenville faults, which strike about N30W, intersect thick Great Valley crust at their northern limits and step left to the Greenville and Concord-Green Valley strike-slip faults, respectively. These restraining left steps are expressed as the Mt. Oso and Mt. Diablo anticlines and associated blind thrust faults. Strike-slip seismicity occurs beneath the Sacramento-San Joaquin delta at depths of 15-25 km, but steps left across the Potrero Hills and Cannon Hills anticlines to the Cordelia and Wragg Canyon strike-slip faults in the northern Coast Ranges. Active thrust faults extend eastward from the base of these strike-slip faults as right-stepping Reidel shears along pre-existing thrust faults that were the likely sources of the 1892 Winters-Vacaville earthquake sequence. Slip rates on the thrust faults typically are 1/4 or less of the 2-3 mm/yr slip rates of the strike-slip faults. South of $37\deg$ N, however, the Laguna Seca blind thrust has a slip rate of 2 mm/yr while the adjacent southern Ortigalita fault has a slip rate of 0.5 mm/yr or less. Near $37\deg$ N, the dextral Ortigalita fault makes a right step across a $\sim$5-km-wide pull-apart basin that is adjacent to two blind thrust faults beneath the monocline on the east flank of the Diablo Range. The apparently peculiar position of this pull-apart basin adjacent to active thrust faults is explained by systematic variations in crustal strength that appear to control the position and evolution of the Ortigalita fault. The Ortigalita fault is required to step right to work around a locally stiff (high-velocity) body beneath the pull-apart basin. This high-velocity ($\sim$6 km/s) body, possibly a gabbro intrusion responsible for the local outcrop of basalt (Basalt Hill), separates the northern and southern Ortigalita fault segments at depths $>$ 5 km. All segments of the Ortigalita fault appear to be skirting the edges of high-velocity bodies at depths $>$ 5 km in the vicinity of the pull-apart basin. Thus, while most strike slip faults located along the eastern margin of the Coast Ranges step left, the Ortigalita fault is the only one of these strike slip faults with a significant right step and associated pull-apart basin.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 1200 GEODESY AND GRAVITY
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting