HR: 10:20h
AN: T42B-01    [Abstracts]
TI: 0.5-0.6 Ma Onset of Uplift and Transpressive Deformation in the Borrego Badlands, Southern California, due to Initiation of the Coyote Creek Fault
AU: * Lutz, A T
EM: alutz1@darkwing.uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AU: Dorsey, R J
EM: rdorsey@uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272 United States
AU: Housen, B A
EM: bernieh@cc.wwu.edu
AF: Dept. of Geology, Western Washington University, 516 High St., Bellingham, WA 98225 United States
AB: Pleistocene sedimentary rocks between the Clark (CF) and Coyote Creek (CCF) faults in the Borrego Badlands (BB), southern California, record evolution of the central San Jacinto fault zone. The Inspiration Point fault (IPF) is a NE-striking left-lateral fault that cuts all units older than Qal and is bounded by the CF and CCF. The Ocotillo Formation ranges in age from 1.1 to 0.5-0.6 Ma, and consists of: (1) a lower member (200-350 m thick) of conglomerate, sandstone and siltstone preserved throughout the BB, and (2) an upper member (50-190 m) of cobble-boulder conglomerate and pebbly sandstone in the NE BB. Detailed mapping reveals no evidence for growth folding or faulting during deposition of the Ocotillo Fm. The Fonts Point Sandstone (FPS) is a widespread, 2- to 10-m-thick unit of conglomeratic sandstone that overlies the Ocotillo Fm and contains a well developed capping Aridosol up to ca. 2 m thick (Ryter, 1999). The Ocotillo-FPS contact changes from a low-angle unconformity NW of the IPF to a conformable contact in most locations SE of the IPF, north of the Fonts Point escarpment. Subcrop map provides evidence for growth of a NE-trending anticline on the NW side of the IPF shortly prior to FPS deposition; conformable contact SE of the fault suggests early NW-side-up movement associated with initiation and early transpressive slip on the IPF. Magnetostratigraphic study of a continuous section that crosses Beckman Wash in the western BB shows that the contact between the Ocotillo Fm and underlying Borrego Fm is a short-lived hiatus dated at slightly post-1.07 Ma. Polarity reversals correspond to the disconformable base and conformable top of the Jaramillo sub-chron (1.07 and 0.99 Ma, respectively) and the Matuyama-Brunhes boundary (0.78 Ma, 18 +/- 13 m below the 0.758-Ma Bishop Ash). Sedimentation rates slowed from 1.6-2.7 mm/yr during Jaramillo time to 0.31-0.41 mm/yr from 0.99 to 0.758 Ma. SE of the IPF, the 0.74-Ma Thermal Canyon Ash is 70 m below the conformable base of the FPS. Application of sedimentation rates from the Beckman Wash section yields an age of 0.51-0.57 Ma for the base of the FPS. Based on structural and stratigraphic relationships described above, the basal FPS dates initiation of the IPF and related folding in the BB. Presence of a thick Aridosol in the FPS and lack of post-FPS deposits indicates that the beginning of transpressive deformation coincided with the end of deposition and onset of uplift throughout the BB, which produced the modern Fonts Point erosional escarpment. Modern uplift in the BB results from oblique shortening across a restraining bend in the CCF south of Fonts Point. We infer that uplift began when the NW strand of the CCF was initiated at 0.6 +/- 0.1 Ma (Dorsey, 2002), and that the age of basal FPS (0.51-0.57 Ma) provides a refined estimate for the timing of this event. These results are consistent with recent studies that document the end of deposition and onset of deformation in the San Felipe Hills at 0.5-0.6 Ma (Kirby et al., 2004; in prep; Janecke et al., 2004). This study thus supports an emerging model for major reorganization of the San Jacinto fault zone, and possibly initiation of the entire Coyote Creek fault system, at 0.5-0.6 Ma.
DE: 8105 Continental margins and sedimentary basins
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting