Thermochronologic Constraints on the Extensional History of the Ibex Hills, Southeastern Black Mountains, Death Valley, California

ABSTRACT This study presents new zircon U‐Th/He geochronology for the Ibex Hills, California, a range located in the southeastern Black Mountains in southern Death Valley. This region has long been an area of debate over the nature of extension and transtension, in particular the influence of detachment faults as opposed to more discrete, strike‐slip bounded basins. The cooling ages from this study range from ~6 Ma to over 60 Ma; however, there does not appear to be any clear spatial pattern to the cooling ages, although ages negatively correlate with elevation. Additionally, there is a negative correlation shown between the cooling ages and effective uranium (eU) values in the Ibex Hills, indicating that the samples spent a significant time in the zircon U‐h/He partial retention zone (PRZ). The samples were modelled in HeFTY and all showed a rapid exhumation event at ~10–7.5 Ma. The modelled Miocene exhumation event is interpreted as the onset of NE–SW extension in the range, which implies a more limited influence of later NW directed detachment faulting in the Ibex Hills. This is consistent with previous interpretations of multiple extensional phases in the range. Modelling results also reveal evidence for earlier exhumation that persisted until ~40 Ma, potentially related to contraction in the region.