THERMOKINEMATIC HISTORY OF THE BADWATER TURTLEBACK IN DEATH VALLEY, CALIFORNIA

The Black Mountains, in Death Valley, California, have been the focus of many studies to better understand high-magnitude extensional tectonics because they contain well-exposed, mid- to deep-crustal rocks exhumed during Late-Cenozoic Basin and Range extension. These rocks make up the footwall of three prominent, high-relief “turtleback” fault surfaces in the western flank of the Black Mountains. They contain ~1.74 Ga mylonitic gniess with abundant fault striations, large-scale folds, and tectonite fabrics containing top-to-the-northwest shear-sense indicators formed during the Basin and Range extension.

It remains unknown if the turtlebacks are separate exposures of an undulating detachment fault surface with a rolling-hinge style of exhumation, or separate, deeply rooted, high-angle normal faults, which were progressively abandoned during tilting and cross-cut by newer, west-dipping normal faults. To attempt to resolve this controversy, we are building a thermokinematic model of the northernmost turtleback, the Badwater turtleback, using (U-Th)/He and Ar-Ar thermochronology and U-Pb geochronology on rock samples collected along an east-west transect with ~1,300 meters of vertical relief. Our model will compliment previously published models in the central and southern Black Mountains, and fill an important, data-poor, spatial gap in order to elucidate the pattern of exhumation and cooling in the Black Mountains. Our preliminary zircon U-Pb data suggest a crystallization age for the gneissic rocks on the Badwater turtleback of 1.74 Ga (²⁰⁷Pb/²⁰⁶Pb, 2σ error=31.8 Ma, n=6) with two younger populations at 1.46 Ga (²⁰⁷Pb/²⁰⁶Pb, 2σ error=51.8 Ma, n=3) and 79.6 Ma (²⁰⁶Pb/²³⁸U, 2σ error=10.0 Ma, n=2). The younger ages possibly reflect post-crystallization alteration. This is in agreement with previously published ages. The zircon ²⁰⁶Pb/²³⁸U ages of the Smith Mountain Granite, exposed at the top of the Badwater Turtleback, range from 3.06-9.71 Ma (n=11). Once the thermochronometric data are available, we will test several assumptions of the two competing hypotheses, such as whether the exhumation/cooling timing and rate were similar across the turtlebacks, and whether there is a horizontal versus vertical (i.e. elevation correlated) pattern of exhumation across the Badwater turtleback.