HR: 1340h
AN: GP43A-0841    [Abstracts]
TI: Magnetostratigraphy and Rotation of Pleistocene Sedimentary Rocks in the San Jacinto Fault zone, Western Salton Trough, CA.
AU: * Housen, B A
EM: bernieh@cc.wwu.edu
AF: Geology Department, Western Washington University, 516 High St, Bellingham, WA 98225 United States
AU: Dorsey, R J
EM: rdorsey@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403 United States
AU: Janecke, S U
EM: sjanecke@cc.usu.edu
AF: Dept. of Geology, Utah State Univ., Logan, UT 84322 United States
AU: Kirby, S M
EM: stefankirby@utah.gov
AF: Dept. of Geology, Utah State Univ., Logan, UT 84322 United States
AU: Lutz, A T
EM: alutz1@darkwing.uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403 United States
AB: Pleistocene sedimentary rocks in the San Felipe Hills (SFH) and Borrego Badlands (BB), Salton Trough, are deformed by the diffuse seismogenic San Jacinto fault zone. The Plio-Pleistocene lacustrine Borrego Fm is sharply overlain by the alluvial Ocotillo Fm in the southwest (BB and Ocotillo Badlands) and its finer-grained equivalent (Brawley Fm) in the northeast (SFH). This contact records rapid progradation of alluvium and termination of paleo Borrego Lake in response to initiation or reorganization of the San Jacinto fault. We sampled these units in measured sections at Oil Well Wash (in the SFH) and Beckman Wash (in the BB), and collected a thick composite section that includes all of the Borrego Fm in the southern BB. Sites were located at 10-50 m intervals in each section. Paleomagnetic results from 31 sites in the Oil Well Wash and Beckman Wash sections have well-defined components of magnetization in most samples. A persistent magnetic overprint, and some mineralogical alteration during thermal demagnetization, hampers analysis of magnetization vectors in some sites. Our results indicate that the age of the Ocotillo and Brawley Fms ranges from 0.5-0.6 Ma to 1.0-1.1 Ma, based on the presence of the Brunhes / Matuyama boundary and Jaramillo sub-chron in both sections, and Bishop ash in the BB. The Borrego-Ocotillo contact coincides with the base of the Jaramillo in both sections and in the nearby Ocotillo Badlands section (Brown et al., 1991). This result is consistent with evidence for a depositional hiatus at the contact in the BB section and an erosional disconformity at Oil Well Wash. Sites with well-defined directions and k$>$16 can be used to determine vertical-axis rotations. In Beckman Wash the tilt-corrected mean is D=360, I=38.2, k=48, \alpha$_{95}$=$8.1\deg$, N=8, indicating little or no rotation. In Oil Well Wash the tilt-corrected mean is D=8.5, I=61.1, k=51.0, \alpha$_{95}$=$9.5\deg$, N=6, indicating a cw rotation of $8.5\deg$\pm$$5.7\deg$. Combined, these data lead us to conclude that only minor ($8\deg$ or less) rotation has occurred in the two areas since 1.0 Ma, despite structural and stratigraphic evidence for significant fault reorganization during this time. The larger ($35\deg$) clockwise rotation documented in the Fish Creek-Vallecitos Basin (Johnson et al, 1983) may have occurred prior to 1.0 Ma, but the timing and amount of rotation is poorly known in that area and will be tested in future work.
DE: 8015 Local crustal structure
DE: 8105 Continental margins and sedimentary basins
DE: 1520 Magnetostratigraphy
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting