Print this pageABSTRACT: Thermal and Structural History of the San Emigdio Mountains, Southern San Joaquin Basin, California, Based on Fission-Track Analysis
WHITE, LESLIE A., ARCO Oil and Gas Company, Plano, TX, and University of Texas, Austin, TX, SHARI A. KELLEY, Southern Methodist University, Dallas, TX, and JEFF CORRIGAN, University of Texas, Austin, TX
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 sample along the Pleito Creek traverse shows that the base of the sequence began to cool from approximately 115 plus or minus 10 degrees C 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 or minus 20 degrees C during latest Mohnian to early Pliocene (approximately 8-4 Ma). Assuming paleogeothermal gradients of approximately 25 to 30 degrees C/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. /P>
The study also shows that the area was affected by Miocene heating, probably associated with Tecuya Formation volcanism (22.5 Ma). Ten of the 11 San Emigdio Creek samples show significant but incomplete resetting of detrital fission track ages from approximately 40 to 70 Ma to a mean of 19.4 plus or minus 4.8 Ma. These samples experienced temperature maximums of approximately 110 plus or minus 15 degrees C near 20 Ma, and cooled at a rate of 10 degrees C/my from approximately 20 to 6 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.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)