LATE CRETACEOUS AND LATE MIOCENE MOTION on THE BOUNDARY CANYON DETACHMENT FAULT CONSTRAINED BY 40Ar/39Ar AND (U-TH)/HE THERMOCHRONOMETRY: FUNERAL MOUNTAINS, DEATH VALLEY

New thermochronometry studies in the Funeral Mountains core complex (FMCC) improves our understanding of the relative roles of Cretaceous and Miocene extension in exhumation of the core complexes in the Death Valley region. The FMCC comprises metasedimentary rocks that exhibit a metamorphic field gradient from lower greenschist facies rocks to the southeast to upper amphibolite facies metamorphic rocks in the northwest. Exhumation is commonly attributed to the Late Miocene NW-directed Boundary Canyon Detachment (BCD). Muscovite ⁴⁰Ar/³⁹Ar ages of footwall marbles beneath the BCD along a 32 km transect in the transport direction exhibit a northwest decreasing age pattern from pre-burial Proterozoic detrital ages in the southeast, through 160.0 Ma, to 70.0 Ma cooling ages to the northwest. New (U-Th)/He analyses on detrital zircon (ZrHe) from quartzite samples collected in the footwall of the Boundary Canyon detachment fault along the same transect reveal a pattern of ages decreasing towards the northwest, and capture the pre-Miocene extension partial retention zone. ZrHe ages range from 85 Ma in the southeast in rocks preserving detrital ⁴⁰Ar/³⁹Ar muscovite ages, to 7.4 Ma to the northwest in rocks recording 74 to 66 Ma ⁴⁰Ar/³⁹Ar muscovite cooling ages. All 10 samples from the central and northwest portion of the study area, over an 18 km distance in the slip direction, vary from 7.4 to 9.3 Ma (with the exception of one age of 10.9 Ma) suggesting rapid late Miocene slip. An inflection in the array of ZrHe ages plotted against distance suggests inception of the BCD at ~ 10-11 Ma. Preservation of Late Cretaceous muscovite ages from the furthest down-dip position in the footwall of the BCD, suggests that Late Miocene exhumation along the BCD was only modest.