HR: 1330h
AN: S12B-0396    [PDF]
TI: Low Temperature Thermochronology From the SAFOD Pilot Hole: Constraining the Thermal History With Apatite Fission-Track and (U-Th)/He Analyses
AU: * Blythe, A E
EM: blythe@earth.usc.edu
AF: University of Southern California, Department of Geological Sciences, Los Angeles, CA 90089 United States
AU: D'Alessio, M A
EM: dalessio@seismo.berkeley.edu
AF: University of California, Department of Earth and Planetary Sciences, Berkeley, CA 94720 United States
AU: B\"urgmann, R
EM: burgmann@seismo.berkeley.edu
AF: University of California, Department of Earth and Planetary Sciences, Berkeley, CA 94720 United States
AU: Farley, K A
EM: farley@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AB: The San Andreas Fault Observatory at Depth (SAFOD) pilot drillhole traverses the upper $\sim$2 km of a site 1.8 km west of the San Andreas fault near Parkfield, California. We use apatite fission-track and (U-Th)/He analyses of drillhole cuttings samples to document the thermal signature of downhole samples currently at a temperature range of $\sim$ 20 to 95 $^oC$. Knowing the long-term thermal history of the site will be important for interpreting geochemical, structural, and geophysical observations in the SAFOD project. With these data we evaluate the geothermal gradient and exhumational history of the drillhole site. Preliminary (U-Th)/He analyses from drillhole samples decrease in age with depth, with He ages of $\sim$ 20 Ma for samples at 800 m (where the Pilot Hole first encounters Salinian granitic rocks; current temperature $\sim$ 50 $^oC$) to a He age of 1 Ma in the deepest sample from the base of the hole (at 2.2 km depth; current temperature 95 $^oC$). The pattern of ages is consistent with that expected for the observed geothermal gradient of $\sim$ 35 $^oCkm^{-1}$ and shows little or no evidence of recent exhumation. Preliminary modeling of fission track length distributions also show that the block has been exhumed less than 1 km in the last 5 m.yr., despite its proximity to the active strand of the San Andreas fault and other related faults.
DE: 7203 Body wave propagation
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2003 Fall Meeting