THERMOCHRONOLOGIC CONSTRAINTS OF FAULT RELATED EXHUMATION IN THE AREA OF THE IBEX HILLS, CA

The northern Ibex Hills have, until recently, been mapped as an up-dip component of the Black Mountain Detachment (BMD) which places Pahrump Group and younger rocks atop ~1.8 Ga regional basement. More recent work, however, has suggested that there are at least two kinematically distinct fault systems in the Ibex Hills, making a simple correlation to the BMD more tenuous. New zircon (U-Th)/He data is presented here, taken from the western flank of the northern and central Ibex Hills, which provides further insight into the timing of deformation in the Ibex Hills and, more broadly, of southern Death Valley. Six samples were analyzed for this study, collected along two transects, one trending SW and the other NW. Samples showed a strong bimodal distribution with one family of cooling ages distributed in the Paleocene to the early Eocene, whereas the rest of the samples had younger cooling ages of ~6-7 Ma. The older set of ages is consistent with previous interpretations of the Ibex Hills having an older cooling history than the rest of the Black Mountains to the north. Thermal modeling of this data had initial constraints defined by previously published biotite cooling ages from the northern Ibex Hills. Further constraints were based upon iterative model results of the older set of cooling ages without the input of the younger sample grains. The inflection points of those model paths were then used for the inputs of thermal models of the late Miocene aged samples. The best-fit paths indicate that the basement rock of the Ibex Hills likely remained at or near the zircon He partial retention zone since ~55 Ma until ~35Ma, after which a reheating event is indicated, reaching a maximum of ~220º at ~23 Ma. This reheating event was then followed by relatively slow cooling until rapid cooling began at ~5 Ma. These results imply that if the BMD is continuous into the Ibex Hills, its role in exhumation was limited since such a large regional detachment would likely be reflected by more consistent younger, mid-late-Miocene based on estimates of BMD movement, cooling ages. Additionally, the general spatial pattern of cooling ages support previous interpretations of NE-SW extension in the region.