T40 The Mesozoic and Cenozoic Tectonic Evolution of Northwestern Mexico and the Southwestern United States (Posters)

Sunday, 18 October 2009
9:00 AM - 6:00 PM
Hall A

JURASSIC SHORTENING AND EPISODIC EXTENSION IN THE FUNERAL MOUNTAINS CORE COMPLEX, DEATH VALLEY, SOUTHEASTERN CALIFORNIA

Paper number: T40-9
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

New studies in the Funeral Mountains core complex (FMCC) improve our understanding of the timing of thrust burial in the “Sevier” hinterland and the relative roles of Cretaceous and Miocene extension in exhumation of the core complexes. The FMCC comprises metasedimentary rocks that exhibit a metamorphic field gradient from upper amphibolite facies metamorphic rocks in the northwest to lower greenschist facies rocks to the southeast. Exhumation is commonly attributed to the Late Miocene NW-directed Boundary Canyon Detachment (BCD). New Lu-Hf garnet geochronology of middle greenschist facies schist of the Johnnie Formation from the southeastern FMCC resolves the age of the latest Barrovian metamorphism; a 5-point garnet-whole rock isochron yields an age of 158.0 ± 1.2 Ma (2s), consistent with an ~160.0 Ma 40Ar/39Ar muscovite age from nearby. Composite PT paths determined from growth-zoned garnets from the same samples show a nearly isothermal pressure increase of ~2 kbar at ~490°C, suggesting thrust burial at 158 ± 1.2 Ma. 40Ar/39Ar muscovite ages of footwall marbles beneath the BCD along a 32 km transect in the transport direction exhibit a northwest decreasing age pattern. Ages vary from pre-burial Proterozoic detrital ages in the southeast, through 160.0 Ma to 91.0 Ma cooling ages in the Sterling Quartzite towards to northwest. Further northwest, where the BCD truncates the Stirling Quartzite, cooling ages vary from 75.0 Ma to 70.0 Ma in the subjacent Johnnie Formation. The general pattern of 40Ar/39Ar muscovite ages decreasing towards the northwest is consistent with the differences in depth of metamorphism and, for the Late Cretaceous, suggests top-to-northwest motion during exhumation, consistent with mesoscopic and microscopic kinematic studies.

Recognition of early Late Jurassic tectonic burial in the Death Valley region suggests that the distinction between “Sevier” and western older Jurassic deformation provinces evident further north in the Great Basin becomes obscured to the south as the magmatic arc and the miogeoclinal hingeline converge. Preservation of Late Cretaceous muscovite ages from the furthest down-dip position in the footwall of the BCD suggests that the Late Miocene exhumation was modest and confirms the importance of Late Cretaceous extension in exhumation of these metamorphic rocks.