THERMOCHRONOLOGIC CONSTRAINTS ON EXTENSION VIA DETACHMENT FAULTING IN THE WHITE HILLS OF NW ARIZONA AND GOLD BUTTE BLOCK OF SE NEVADA

The South Virgin-White Hills detachment (SVWHD) is the controlling structure for tilting and exhumation of the Gold Butte Block (GBB) of SE Nevada and the White Hills of NW Arizona that together form the east-tilted Lake Mead extensional domain. Samples collected for apatite fission track thermochronology (AFTT) from basement rocks of the lower plate of the SVWHD constrain timing and patterns of cooling, and hence exhumation and extension. In the GBB new AFT data complements existing data, whereas in the White Hills, all data is new.

In the western GBB AFT ages reflect rapid cooling, increasing in age from west to east until the base of an exhumed PAZ is reached, constraining the onset of that rapid cooling and therefore extension at ~17 Ma. In the White Hills AFT ages follow a similar pattern, increasing from west to east and although the base of an exhumed PAZ is not revealed, the data suggests extension was initiated at ~17 Ma. Thus, there is no apparent variation in timing of extension along the SVWHD. AFT ages immediately under the SVWHD in the White Hills are ~13 Ma, but south of the Golden Rule Lineament they appear to be slightly older (~16 Ma). Samples under a newly discovered 20-30 m thick mylonite zone east of the SVWHD may also break the general west-east age trend indicating that movement along this fault zone was contemporaneous with movement along the SVWHD. Some samples, notably those close to Lake Mead, have slightly reduced mean lengths and shortened modal distributions. This reflects a thermal overprint that forward modeling indicates occurred in the Late Miocene, likely associated with post-extensional basaltic volcanism. While all AFT ages west of the Grand Wash Fault in the White Hills reflect rapid exhumation via detachment faulting, all samples east of this fault are older (65-110 Ma) with complex track length distributions reflecting a protracted thermal history with track components from the pre-Laramide, Laramide cooling, residence in a PAZ and later Miocene cooling. Forward modeling of these samples also suggests mid-Tertiary reheating, somewhat problematic in correlation to regional geologic events, but previously noted elsewhere on the Colorado Plateau. AFT ages from basement directly under the Tapeats Sandstone in the northern GBB are ~50 Ma but increase to 110 Ma SE of the White Hills indicating that the unconformity shallows to the south.