WAHRMUND, LESLIE A., The University of Texas at Austin, Austin, TX. Currently at Exxon Production Research Company, Houston, TX

ABSTRACT: Provenance Analysis Based on Apatite Fission Track Thermochronology (AFTT): An Example from the Western Transverse and Southern Coast Ranges (WTR/SCR), California

Previous FT provenance studies primarily utilize zircon rather than apatite because (1) its higher uranium content yields more precise single-grain ages, and (2) its greater resistance to post-depositional annealing provides more information on parent rock thermal history. Use of apatite, however, enables resolution of processes occurring at substantially lower temperatures (<90-120 degrees C), such as sediment recycling and smaller-scale tectonic unroofing. Here AFTT was used to characterize the provenance of Pliocene to Late Cretaceous basins in the WRR/SCR region, providing new constraints on (1) the timing and magnitude of cooling of their sedimentary source(s), and (2) paleographic reconstructions of southern California.

A set of 31 Pliocene to Late Cretaceous sedimentary samples that experienced minimal post-depositional annealing were analyzed. The bulk of single-grain AFT ages are middle Eocene to Late Cretaceous (45-70 Ma). These ages are remarkably similar to eight new AFT ages from the Salinian Block (46-62 Ma), and a very large population of early Tertiary to Late Cretaceous cooling ages widely reported throughout southern and central California. This means a southern California origin for the WRR/SCR region is likely; Late Cretaceous to early Tertiary cooling of the source may have been due to regional "refrigeration" and tectonic unroofing.

Younger populations of detrital apatite grains are also present in Oligocene and younger samples, indicating that the source area(s) experienced additional episodes of cooling and/or unroofing. Finally, very few single grain ages >100 Ma were found. This should serve as a distinct and fairly unique "fingerprint" against which possible source areas can be tested.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.