REORGANIZATION OR INITIATION OF THE SAN JACINTO FAULT ZONE AT 1 MA: EVIDENCE FROM SYNTECTONIC DEPOSITS IN THE SAN FELIPE HILLS, WESTERN SALTON TROUGH, CA

The linkage between the Clark strand of the San Jacinto fault zone (SJFZ) and the North American/Pacific plate boundary is unclear. Within the San Felipe Hills region of the Salton Trough, Miocene-Quaternary sediments deposited during both extension and strike-slip tectonics are deformed by a complex series of folds and faults south-east of the termination of the surface trace of the Clark strand of the SFJZ. Mapping shows a change in fold trend from east-west in the San Felipe Hills to north-south in younger sediment near the southwestern margin of the Salton Sea, as well as numerous N-striking normal faults, NW-striking right lateral, E and NE-striking left lateral faults in the highly deformed zone. The change in fold orientation may be the result of a blind or submerged left-stepping strand of the SFJZ located along the southwestern margin of the Salton Sea. Reorganization or initiation of the SFJZ about 1 Ma is recorded by the deposition of the Ocotillo Formation and its finer equivalent, the Brawley Formation, over synrift sedimentary rocks in the San Felipe Hills. A regional disconformity at the base of the fluvial-deltaic Brawley Formation separates it from the underlying lacustrine Borrego Formation, and reflects a major drop in regional base level. This disconformity passes westward into an angular unconformity where the Brawley Formation interfingers with the Ocotillo Formation across the bedrock-cored east-trending San Felipe anticline. Gravity data define the subsurface extent of this anticline from Borrego Mountain east to the central San Felipe Hills. The Brawley Formation has been previously described as the lacustrine lateral equivalent of the coarser Ocotillo Formation (Dibblee, 1954; 1984). However, the Brawley Formation in the San Felipe Hills is approximately 60% fluvial and fluvial-deltaic. Paleocurrents from fluvial channels in the Brawley Formation show a northeast transport direction, at a high angle to earlier trends in the Diablo Formation. Fluvial distributary channel, flood plain, and crevasse-splay deposits dominate over lacustrine and eolian deposits. The angular unconformity and disconformity, a marked increase in locally derived sediment, the new dispersal patterns, and a shift to fluvial facies, imply that a new set of strike-slip faults began to affect the area prior to about 1 Ma.