LOW TEMPERATURE THERMAL HISTORY OF THE SAN ANDREAS FAULT OBSERVATORY AT DEPTH (SAFOD) FROM APATITE FISSION TRACK ANALYSES

New apatite fission track analyses from the San Andreas Fault Observatory at depth are used to constrain the thermal history of the site. The six samples analyzed were from hole depths of 1800 to 2500 m, and modern downhole temperatures of ~80 to 107 C. Apatite fission track ages from these samples ranged from 46 to 10 Ma. These ages are in general older than the ages previously obtained from the SAFOD pilot hole (Blythe et al., 2004), in which the deepest sample (from a depth of 2200 m) yielded an apatite fission track age of 3 Ma. The older ages from the main hole appear to be the result of a wider spread in apatite composition than was seen in the pilot hole samples. All of the main hole samples showed a large spread in individual grain ages, which Dpar analyses (measuring track shape) indicate are the result of variations in Cl and F content of the apatite crystals. F-rich apatites have lower closure temperatures than Cl-rich apatites. When the Cl-rich apatite grains are excluded from the sample age calculations, the ages of individual samples closely match the data trends seen in the pilot hole. We are currently in the process of obtaining zircon fission track analyses from these samples in order to better constrain the geothermal gradient as a function of time, and the exhumation history of the Salinian block in Central California.