Fission Track Evidence for a Miocene Cooling Event, Whipple Mountains, Southeastern California

Abstract

Fission track dating of metamorphic rocks from the Whipple Mountains, California indicates a. previously unreported cooling event affected the region during the early Miocene. The timing of this event coincides with basaltic volcanism and the development of an enigmatic low-angle fault terrane that seems to be of regional proportions. Metamorphosed granitic rocks and mylonitic gneisses lying along and structurally below the basal dislocation surface were sampled and their apatite, sphene, and zircon fractions dated. These mineral-thermometers have fission track retention temperatures that are sensitive within the range of 100° to 250° C. Data from all the mineral phases were concordant and therefore, indicate a rapid cooling interval between 18 and 20 m.y. ago. Cooling intervals in the geologic record of the magnitude described here are indicative of a major, rapid uplift of an orogenic terrane. Although at present it is not possible to place the proposed uplift into the tectonic framework of the area, the timing of the event does coincide with ma.jor southern Cordilleran developments such as Basin and Range extension and basaltic volcanism.