HR: 0800h
AN: T51D-1385    [Abstracts]
TI: A Broad Holocene-Active San Andreas Fault on Durmid Hill, Southern California-A Zone of Interaction with the Brawley Seismic Zone?
AU: * Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 977, Menlo Park, CA 94025 United States
AU: Lindvall, S C
EM: lindvall@lettis.com
AF: William Lettis & Associates, 28470 Avenue Stanford Suite 120, Valencia, CA 91355 United States
AB: We excavated paleoseismic trenches across the southernmost San Andreas fault on Durmid Hill and also immediately north of the community of Bombay Beach, near the Salton Sea. The trenches were in a section of the fault that, at least at the ground surface, bends to a more northerly orientation than the regional trend to the northwest, and where the Brawley seismic zone connects with the San Andreas fault and extends farther to the southeast. Rock types in the Durmid Hill-Bombay Beach area are dominantly highly deformed mid- to late Pleistocene lacustrine mudstone to conglomerate (Brawley Formation), capped by non- to slightly deformed Holocene lacustrine mudstone to conglomerate; on the lower reaches of Durmid Hill, including Bombay Beach, there are also thin lacustrine deposits from the flooding in 1905-1907 that created the Salton Sea. A 58-m-long trench and several small pits excavated on Durmid Hill show tilted Holocene deposits and a wide zone of San Andreas breaks. Holocene faulting consists of discrete breaks and zones of breaks with a total width of at least 50 m. In addition, nine trenches excavated at Bombay Beach reveal highly deformed Brawley Formation strata capped by late Holocene to historic flood deposits and a broad zone (about 40 m) of faulting beneath the historic flood deposits. Also exposed in the Bombay Beach trenches were left-lateral breaks that project to the southwest, under the Salton Sea. Broad zones of right-lateral Holocene fault breaks exposed on Durmid Hill and at Bombay Beach are abnormal in width relative to other sections of the San Andreas fault. We speculate that the broad zone of Holocene faulting on the San Andreas fault, near its southernmost end, may be due to interaction of the Brawley seismic zone and the San Andreas fault zone, a transition from a series of NE-striking left-lateral breaks in the Brawley seismic zone, as defined by seismicity, to a structurally narrow, right-lateral San Andreas fault to the northwest. The presence of left-lateral breaks exposed in trenches at Bombay Beach supports this speculation.
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005