HR: 0800h
AN: T31C-1324    [Abstracts]
TI: Fault Trends and the Evolution of the Pacific-North America Transform in Southern California
AU: * Legg, M R
EM: mrlegg@attglobal.net
AF: Legg Geophysical, 16541 Gothard Street, Suite 107, Huntington Beach, CA 92647 United States
AU: Kamerling, M J
EM: MKamerling@venocoinc.com
AF: Venoco, Inc., 5464 Carpinteria Ave., Suite J, Carpinteria, CA 93013-1476 United States
AB: The Pacific-North America (PAC-NOAM) transform boundary evolved during the past 30 Ma, lengthening more than 1000 km and spanning a zone exceeding 200-km across southern California. The relative plate motion vector has been estimated using seafloor magnetic anomaly patterns. Orientations of major transform fault segments within this boundary provide direct evidence of the relative motion at the time these faults formed, where the faults preserve their original orientations. Avoiding areas of known vertical-axis block rotations, we find at least three major fault trends that document past and present tectonic kinematics. A northwest trend of 330 degrees is related to subduction trends in the forearc region that defined the late Mesozoic and early Tertiary coastline and has subsequently controlled the orientation of oblique rifting during the Neogene initiation and growth of the PAC-NOAM transform. This trend is manifest in the San Diego Trough and adjacent coastal rifts and associated fault zones including the Coronado Bank and Newport-Inglewood. The middle Miocene transform orientation appears to be 300-310 degrees, which imparted extensional character to faults reactivated with older subduction trends. Major faults inferred to represent Neogene transform fault segments with this trend include the Whittier, Palos Verdes Hills, Santa Cruz-Catalina Ridge, Catalina Escarpment, and possibly the Mojave segment of the San Andreas fault. In late Miocene time, the plate motion vector rotated clockwise eventually achieving its modern orientation of about 320 degrees. Active faulting showing pure strike-slip character on the San Clemente - San Isidro fault zone and the Imperial Fault show this trend, as do transform faults in the northern Gulf of California. An intermediate trend is apparent in some areas along the San Clemente fault zone in the Borderland, and along the Elsinore and San Jacinto fault zones, which transect the Peninsular Ranges. The intermediate trends may be due to the transition in the plate motion vector, or possibly fault refraction across different crustal rheology. Important consequences of this systematic change in plate motion vector and fault trends regarding the tectonic evolution of southern California include: 1) transpression and the preponderance of restraining bends along northwest-trending right-slip faults; 2) structural inversion of Miocene "pull-apart" basins; 3) tendency to "capture" large elongate blocks of North America crust by the Pacific plate. The latter process continues following the capture of Baja California with the northward rift propagation into the Owens Valley aong the Eastern California Shear Zone.
DE: 8005 Folds and folding
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 8123 Dynamics, seismotectonics
DE: 8158 Plate motions--present and recent (3040)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting