HR: 15:10h
AN: V33E-06    [Abstracts]
TI: Two-phase Neogene extension of the northwestern Basin and Range deduced from thermochronology of a single sample
AU: * Colgan, J P
EM: jcolgan@usgs.gov
AF: U.S.Geological Survey, 345 Middlefield Rd. MS901, Menlo Park, CA 94025, United States
AU: Shuster, D L
EM: dshuster@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, United States
AU: Reiners, P W
EM: reiners@U.Arizona.EDU
AF: Department of Geosciences, University of Arizona, Tucson, AZ 85721, United States
AB: We use a combination of apatite ⁴He/³He, (U-Th)/He, and fission-track thermochronology to date slip on the Surprise Valley Fault in northeastern California by analyzing a single granitic boulder from a conglomerate lens near the base of the adjacent Warner Range. Apatite from this sample yielded a fission-track age of 11.6 ± 1.4 Ma and a (U-Th)/He age of 3.12 ± 0.2 Ma. Regional geologic relationships indicate that this sample was buried to a depth of about 3.3 km prior to exhumation during slip on the Surprise Valley Fault. Apatite fission-track data indicate that the sample was fully reset (>120°C) prior to exhumation, which began sometime after 14 Ma. A single aliquot of nine apatite grains was step-heated for ⁴He/³He analysis; modeling of the resulting ⁴He distribution indicates cooling from >80°C to ~20°C between 3 and 1 Ma. Internally consistent time-temperature (t-T) solutions to the combined ⁴He/³He, (U-Th)/He, and fission-track data require two periods of cooling, consistent with non-continuous slip on the Surprise Valley Fault. Early cooling and fault slip took place between 14 and 8 Ma, and was followed by a more recent pulse of cooling at about 3 Ma. This timing is consistent with both local geologic relationships and with the regional timing of faulting along the western margin of the Basin and Range. These data demonstrate the resolving power of combined fission-track, (U-Th)/He, and ⁴He/³He thermochronometric data to extract high-resolution and ultra low temperature t-T information from a single sample close to the earth's surface.
DE: 1140 Thermochronology
DE: 8109 Continental tectonics: extensional (0905)
DE: 9350 North America
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting