Geochronology of major thrusts, southern Great Basin, California
Sutter, John F
Adams, John A. S.
Master of Arts
Potassium-argon ages of 37 samples were determined in this study. The purpose of the study was to determine apparent age brackets for the major orogenic pulses in the southern Great Basin, California. Mineral ages from the Hunter Mountain piutonic complex indicate that it was emplaced 160-165my ago. Because the piutonic complex intrudes both plates of three major thrust faults, (the Last Chance Thrust, the Racetrack Thrust, and Lemoine Thrust), the faults must have been emplaced previous to this. One hundred fifty miles to the southeast at Clark Mountain several samples which exhibited cross-cutting relationships with thrust faults were collected and analyzed. The Data suggest that the thrusts were emplaced in the younger portion of a bracket of 85-135my ago. The Riggs Thrust in the Silurian Hills also offered possibilities for bracketing its emplacement. Data from the samples collected suggest that the Riggs Thrust was emplaced 85-95my ago, and it was probably contemporaneous with the thrusting at Clark Mountain. In addition to the sampling of rooks showing crosscutting relationships with thrust faults, "(No Suggestions)" formed along the fault planes were also sampled and analyzed. In the two cases where the minutes show complete neomineralization, the whole rock ages determined fell within the limits of the bracket established by the cross-cutting samples. Thrusting in the Hunter Mountain area and the Clark Mountain area was not contemporaneous, and even more important neither fits into a classical orogenic episode. Hunter Mountain thrusting is at least 25my older than classical Laramide and fits better into the Sevier Orogeny described by Armstrong. This, along with other data presented in the text, leads the author to conclude that orogenic activity has more or less continuous in the southern Great Basin in the bracket 185-75my ago.