Magnetostratigraphy and Block Rotation of the Canyon Sin Nombre Area, Western Salton Trough, CA

Sedimentary basins in southern California record a number of important geological processes including changes in deformation style within the southern San Andreas fault system, and changing rates of tectonic uplift, subsidence, and deposition. Establishing the age and thickness of sedimentary rocks in the Fish Creek - Vallecito basin (FCVB) is important for testing hypotheses about climatic versus tectonic controls on progradation of locally-derived clastic deposits at ca. 2.8-3.0 Ma. This study documents the first magnetostratigraphic results obtained from the Canyon Sin Nombre area in the southwestern FCVB. Previous studies showed that the FCVB contains a continuous magnetostratigraphic record from late Miocene to early Pleistocene (Opdyke et al., 1977; Johnson et al., 1983, Dorsey et al, 2007, 2010). Johnson et al. (1983) concluded that the FCVB has rotated 35° clockwise in the past 0.9 Ma. We sampled the upper 1000 m of the Canyon Sin Nombre section, collecting 93 sites spaced at approximately 11 m intervals. Initial demagnetization results show two well defined components of magnetization with the first removed component unblocking between 80 and 200° C and the second unblocking between 280 and 680° C. Because of consistent bedding along this subsection, fold tests for these components are inconclusive and results are reported at 100% untilting. Our preliminary magnetostratigraphic results indicate that we have captured 4 polarity zones. Ash beds with SHRIMP U-Pb ages of 2.65 ± 0.05 Ma and 2.60 ± 0.06 Ma (Dorsey et al 2010) have been mapped from the central FCVB to the Canyon Sin Nombre section. Using this constraint we tentatively correlate the bottom of our section with the normal subchron C2An.1 (3.04-2.58 Ma) of the Cande and Kent (1995) polarity timescale, and a normal to reversed polarity change near the top of our sampled section with the top of the Olduvai subchron (C2n; 1.78 Ma). Using a subset of sites from the upper 500 m of the section that have sufficiently defined data, we report means of normal and reversed polarity sites from this interval. Sites from the lower part of this subsection have normal polarity and a mean of D = 10.7, I = 48.6, k = 38.94, α95 = 9.8°, N = 7. Sites from the upper part of this subsection have reversed polarity and a mean of D = 196.2, I = -45.6, k = 21.95, α95 = 7.5°, N = 18. Based on these initial results and our preliminary magnetostratigraphic correlation, these data suggest that the Canyon Sin Nombre area records only 10° of clockwise rotation in the past 1.78 Ma. If rotation previously documented for the larger FCVB occurred here, this would imply approximately 20° clockwise rotation prior to ~ 2.5 Ma. Alternatively, the Canyon Sin Nombre area could be partitioned from the rest of the FCVB by strike-slip faults, or the results of Johnson et al. (1983) may require revision.