Constraining the exhumation and burial history of the SAFOD pilot hole with apatite fission track and (U-Th)/He thermochronometry

Abstract

The San Andreas Fault Observatory at Depth (SAFOD) pilot hole traverses the upper 2 km of a site 1.8 km west of the San Andreas fault (SAF) near Parkfield, California. In order to evaluate the burial and exhumation history of the site and its relationship to the kinematics and mechanics of the SAF, we use 15 apatite fission-track (FT) and 5 (U-Th)/He analyses from pilot hole samples to document their thermal history. Sample ages decrease with depth: FT and (U-Th)/He ages range from ∼60 and ∼31 Ma, respectively, in the upper 800 m of the hole to ∼3 and 1 Ma at the base of the hole (2.2 km depth, 93°C). Thermal modeling of the distribution of FT lengths indicates three events in the last 80 Ma: 1) cooling and exhumation of >60°C that culminated at ∼30 Ma; 2) reheating of ∼50°C from ∼30 to 8-4 Ma, probably as the result of basin subsidence and burial by 1-1.5 km of sediments; and 3) cooling of ∼30°C and estimated Coast Range exhumation of ∼1 km since 8-4 Ma. Copyright 2004 by the American Geophysical Union.