HR: 1340h
AN: GP13A-0039    [Abstracts]
TI: Rotation of Plio-Pleistocene Sedimentary Rocks in the Fish Creek Vallecito Basin, Western Salton Trough, CA
AU: * Housen, B A
EM: bernieh@cc.wwu.edu
AF: Dept. of Geology, Western Washington University, 516 High St, Bellingham, WA 98225-9080
AU: Dorsey, R J
EM: rdorsey@uoregon.edu
AF: Dept. of Geological Sciences, University of Oregon, Eugene, OR 97403-1272
AU: Janecke, S U
EM: sjanecke@cc.usu.edu
AF: Dept. of Geology, Utah State Univ, Logan, UT 84322-4505
AU: Axen, G J
EM: gaxen@dutchman.nmt.edu
AF: Dept. Earth & Environmental Sci., New Mexico Tech. Univ., Socorro, NM 87801
AB: Application of paleomagnetism to the study of vertical axis rotations in deformed regions can be used to test estimates of block rotation derived from other methods, such as GPS, with many implications for our understanding of crustal deformation and seismic hazard. We present results of a study that compares magnetization directions obtained by a combination of low-temperature and thermal demagnetization experiments that is inspired by the work of David Dunlop and his colleagues. The magnetostratigraphy of Plio-Pleistocene sedimentary rocks in the Fish Creek-Vallecito Basin (FCVB), southern California, is well documented (Opdyke et al., 1977; Johnson et al., 1983), who found these rocks to contain a complete record of geomagnetic field reversals spanning Plio-Pleistocene time. Johnson et al. (1983) also concluded that the FCVB had undergone 35° of CW rotation during the past 0.9 Ma. We resampled and reanalyzed their section in order to better document the amount of vertical axis rotation recorded by these rocks. Initial results from 29 sites have well-defined magnetizations with two components. Low temperature (77 K) demagnetization produced 5 to 15% drops in NRM intensity. The first removed component in all samples is unblocked between 90 and 220 °C, and the second-removed components are unblocked between 300 and 590 °C. These two components have sharp and well-defined LT-HT junctions in most samples, indicating SD-like magnetizations. We thus interpret the first-removed component to represent a thermo-viscous magnetization acquired during the past 500-700 ka. The mean of this component is D = 358.4, I = 56.8, k =111, α₉₅ = 2.6°, N = 29. The second-removed components have either normal or reverse polarity. Due to gentle and mostly homoclinal bedding dips, paleomagnetic fold tests are statistically inconclusive, but do show stratigraphic variation in degree of maximum clustering as a function of tilting. Sites from the upper portion of the section have directions that are best clustered at 0-40% untilting, and sites from the base of the section have directions that are best-clustered at 80-100% untilting. For this initial interpretation, results are tilt-corrected for maximum degree of clustering. Sites from near the base of the section (Diablo Fm) are predominately reverse and have a mean of D = 204, I = -48.3, k = 37, α₉₅ = 12.7°, N = 5. Sites from the middle of the section (Olla and Tapiado Fms) are predominantly normal and have a mean of D = 8.1, I = 48, k = 32, α₉₅ = 8.7°, N = 10. Sites from the upper portion of the section (Hueso Fm) have predominately reverse polarity with means of D = 179.6, I = -43.4, k = 82, α₉₅ = 10.2°, N = 4. Based on these results and correlations between the magnetostratigraphy of this section with the polarity timescale we tentatively conclude that little or no CW rotation has occurred in the FCVB during the past 2 Ma. Larger (24° CW) rotations are recorded by the 3-4 Ma rocks of the Diablo Fm, indicating that significant rotation occurred between 2 and 3 Ma. We tentatively ascribe slowing of the rotation rate in the FCVB to still poorly understood evolution of the west Salton detachment fault system. The relative lack of rotation since 2 Ma suggests that younger, post-detachment strike-slip faulting has not produced significant CW rotation between the San Jacinto and Elsinore faults.
DE: 1520 Magnetostratigraphy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1540 Rock and mineral magnetism
DE: 8109 Continental tectonics: extensional (0905)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005