Shirvell, C.R. 1 , Axen, G.J., 1,2 , Stockli, D.F. 3 , and Grove, M. 1

1. Results Twenty-five (U-Th)/He apparent ages of apatites and zircons record a significant thermal break across the WSDS. (figure 3a). Footwall apatite apparent ages at YR range from ~2-7 Ma, but hanging wall apatite apparent ages are much older, ~25 to 44 Ma. Seven zircon samples at YR also record this thermal break (figure 3b): footwall ages range from ~35 to 51 Ma but one hanging wall sample is 72.7±3.6 Ma. B A • Conclusions • 1. Footwall and hanging wall (U-Th)/He apatite and zircon apparent ages record a significant thermal break across the WSDS. • (U-Th)/He apatite apparent ages from the footwall of the WSDS collected at SY,PR, and YR constrain the onset of rapid cooling and exhumation at ~4.5 Ma. The onset of exhumation at WP may be slightly earlier, ~5-6 Ma. • 3. Preliminary estimates from comparison of similar apatite hanging wall and zircon footwall ages suggest a minimum of ~3 km of exhumation since the Eocene. This estimate of overall exhumation is approximately half of the total exhumation estimated by low-temperature thermochronometers for large normal fault systems father south in Baja California (Axen et al., 2000; Fletcher et al., 2000). • 4. Apatite apparent ages constrain the onset of rapid cooling and exhumation at ~4-6 Ma along the WSDS. Our results also suggest a mid-Miocene cooling event in the Salton Trough. Thermochronometers in the Sierra El Mayor and farther south in the Los Cabos Block also suggest rapid cooling in the mid-Miocene, ~10-12 Ma (Axen et al., 2000; Fletcher et al., 2000). Figure 3. LANDSAT image of YR showing apparent ages in the footwall and hanging wall of the WSDS for apatite (A) and zircon (B). Figure 1.LANDSAT image of the northern Gulf of California and Salton Trough. Green box outlines area in figure 2. Footwall apatite apparent ages from SY, PR, and YR constrain the onset of rapid cooling at ~4.5 Ma (figure 4). The youngest apparent ages are from YR, and may record continued and/or renewed Late Pliocene cooling. Mid-Miocene ages at high elevations on SY suggest a possible cooling event ~10-12 Ma. The onset of rapid cooling at WP may be slightly earlier, ~5-6 Ma (figure 5). Future Work 1. Additional samples from San Ysidro will be analyzed to determine when rapid cooling ended at San Ysidro and, if possible, to obtain the exact timing and magnitude of the ~12 Ma event. 2. Six samples from Whale Peak will be analyzed to establish the exact timing of the onset of rapid cooling at Whale Peak. WSDS 20 km CONSTRAINTS ON EVOLUTION OF THE WEST SALTON DETACHMENT SYSTEM (WSDS), SALTON TROUGH, FROM (U-Th)/He THERMOCHRONOLOGY Shirvell, C.R.1, Axen, G.J.,1,2, Stockli, D.F.3, and Grove, M.1 • Dept. of Earth & Space Sciences, Univ of California Los Angeles, 594 Charles E Young Drive East, Los Angeles, CA 900956 • Dept. of Earth & Environmental Science, New Mexico Tech, Socorro, NM 87801 • (3) Dept. of Geology, Kansas University, 120 Lindley Hall, 1475 Jayhawk Boulevard, Lawrence, KS 66045-7613 Introduction The WSDS is a low-angle normal fault that bounds the eastern Peninsular Ranges and roots E beneath sedimentary and basement rocks of the Salton Trough (figures 1 & 2). It is thought that slip on the WSDS controlled deposition of latest Miocene to Pleistocene marine and fluvial strata in the western Salton trough, but the exact onset of slip and magnitude of exhumation are unknown. The detachment has been folded and faulted by younger strike slip faults in the region. The purpose of this study is to constrain the timing and magnitude of exhumation along the WSDS using (U-Th)/He low-temperature thermochronometry (e.g. Stockli, 2005). Hanging wall apatite apparent ages are similar to unpublished footwall apatite fission track ages (~30-50 Ma) (Stockli and Axen, unpublished data), and footwall (U-Th)/He ages on zircons (this study). These ages presumably record slow cooling during development of the regional Eocene-Oligocene erosion surface. In addition, the zircon apparent age from the hanging wall is consistent with regional rapid cooling following pluton emplacement determined from 40Ar/39Ar cooling ages on biotite and K-feldspar (figure 6) (Grove et al., 2003). Figure 6.Time-temperature history for footwall and hanging wall rocks of the WSDS. See text for references. References Axen, G.J., Grove, M., Stockli, D., Lovera, O.M., Rothstein, D.A., Fletcher, J.M., Farley, K., and Abbot, P.L., 2000. Thermal Evolution of Monte Blanco Dome; Low-angle normal faulting during Gulf of California rifting and late Eocene denudation of the eastern Peninsular Ranges: Tectonics, v.19, no.2, p.197-212. Fletcher, J.M., Kohn, B.P., Foster, D.A., and Gleadow, A.J.W., 2000. Heterogeneous Neogene cooling and exhumation of the Los Cabos block, southern Baja California: Evidence from fission-track thermochronology: Geology, v.28, no.2, p.107-110. Grove, M., Lovera, O.M, and Harrison, T.M., 2003. Late Cretaceous cooling of the east-central Peninsular Ranges Batholith (33 degrees N); Relationship to La Posta Pluton emplacement, Laramide shallow subduction, and forearc sedimentation. Geological Society of America Special Paper, v.374, p.355-379 Stockli, D.F., 2005. Application of low-temperature thermochronometry to extensional tectonic settings: Reviews in Mineralogy & Geochemistry. v.58. p.411-448. Figure 2.LANDSAT image of the western Salton Trough showing location of major strike slip faults (red), WSDS (blue), and, sample locations (yellow). Vertical transects were sampled at two locations in the footwall, Mount San Ysidro (SY) and Whale Peak (WP). In addition, three transects were collected perpendicular to the detachment trace at Pinyon (PR) and Yaqui Ridges (YR). Samples were also collected in both the footwall and hanging wall at the tip of Yaqui Ridge. Figure 4.Apatite apparent age plotted versus sample elevation for SY, PR, and YR. Error bars are standard deviation of analyzed aliquots. Light blue area shows preferred interpretation of cooling history for PR and YR. Blue dashed lines show alternative interpretations for SY. Figure 5.Apatite apparent age plotted versus sample elevation for WP. Error bars are standard deviation of analyzed aliquots. Light blue area shows possible cooling histories. Dash line indicates where cooling history is poorly-constrained.