Thermal and Structural History of the San Emigdio Mountains, Southern San Joaquin Basin, California, Based on Fission-Track Analysis

Abstract

The thermal and structural history of the San Emigdio Mountain area, southern San Joaquin basin, California, was studied using inverse modeling and geohistory analysis of new fission-track (FT) data from 32 apatite samples from five wells and two outcrop traverses. Apatite composition was determined by microprobe analysis on eight of the samples in order to select appropriate kinetic parameters for modeling. Modeling of seven Eocene to Oligocene samples and a single basement sampled along the Pleito Creek traverse shows that the base of the sequence began to cool from {approximately} 115 {plus minus} 10C during the late Mohnian ({approximately} 13-7 Ma). Ten Eocene to Oligocene samples and a single basement sample along the San Emigdio Creek traverse cooled below {approximately} 85 {plus minus} 20C during latest Mohnian to early Pliocene ({approximately} 8-4 Ma). Assuming paleogeothermal gradients of {approximately} 25 to 30C/km, these data indicate that the San Emigdio Mountains experienced {approximately} 2 km of denudation from the late Mohnian to Pliocene at rates less than 0.5 mm/yr, and {approximately} 1.5 km of Pliocene to Recent denudation at rates less than 1 mm/yr. The study also shows that the area was affected by Miocene heating, probably associated with Tecuya Formation volcanismmore » (22.5 Ma). In addition, ten new zircon FT dates indicate a Sierra Nevada-type provenance from Eocene to Miocene. The apatite microprobe analyses indicate decreasing numbers of chlorine-rich grains from Eocene to Miocene, suggesting a change in the geochemistry of apatite in the source region over time.« less