Exhumation history of the southern Sierra Nevada mountains from zircon fission track thermochronometry

Abstract

An understanding of the tectonic history of the southern Sierra Nevada Mountains is essential in order to develop better fault models for the two active faults bounding the range. In order to better understand the mechanisms responsible for the formation of the southern Sierra Nevada over the last 100 million years, I am using zircon fission track thermochronometry to constrain the exhumational history of the mountain range. Zircon fission track thermochronometry is used to constrain the time of rock cooling through ~250?C, which is equivalent to when the rocks were exhumed to a depth of ~8 km (at a geothermal gradient of ~30?C/km depth). Nine bedrock samples were collected from a NNW transect across the southern Sierra Nevadas. At 100 million years ago the Sierra Nevada Mountains were a string of volcanoes whose interior was filled with magma at 690-790? C. I hypothesize that by ~85 million years ago these rocks had cooled rapidly to less than 250? C (at mid-crustal levels) as a result of the shallow subduction of the Farallon plate. Previous studies in the area analyzed either the shallow crust, and therefore a more recent history, or the deep crust and therefore an older history. The new zircon fission track data will fill in information for mid-crustal depths and help to better constrain the timing of major tectonic events.