Very high slip rates on continental extensional faults: new evidence from (U–Th)/He thermochronometry of the Buckskin Mountains, Arizona

New (U–Th)/He in apatite ages suggest rapid slip rates on the Tertiary Buckskin–Rawhide detachment fault, which crops out in the Buckskin and Rawhide Mountains of western Arizona. Single-crystal (U–Th)/He in apatite ages were determined for a suite of crystalline rock samples from the footwall to this detachment. The samples were collected from a transect parallel to the fault slip direction, and proximal to the fault zone; they show a linear decrease in age with distance, from ∼14 to ∼11 Ma over ∼15 km. The age versus distance relationship is interpreted to show a slip rate of 4.2 (+1.2/−0.8) mm/yr, continuing for a period of >3 Myr. This rate is more-or-less in agreement with, but more precise than, previously published slip rates from this and other large-offset extensional detachments. Such slip rates are alarmingly high, being roughly 5–10 times faster than rates commonly observed on active structures at the Earth’s surface. These rapid slip rates suggest that significant seismic hazard might be posed by some extensional structures, and they raise questions regarding the relationship between such rapidly slipping, large-offset structures and the slower, smaller offset faults that are more commonly observed at the Earth’s surface.