THE BASEMENT-COVER CONTACT IN THE AMARGOSA CHAOS, DEATH VALLEY, CALIFORNIA: REGIONAL FAULT OR LOCALLY FAULTED UNCONFORMITY?

Since Lauren Wright and Bennie Troxel published their geologic map and report on the Amargosa Chaos in 1984, many researchers have ascribed an unprecedented degree of importance to the basement-cover contact, calling on it to accommodate 10s of km of regional extension. However, Wright and Troxel (1984) found that the contact, here called the “Amargosa Surface”, was not a single continuous fault as required by these models. They mapped scores of localities where the Proterozoic cover sequence rested depositionally on the basement, and noted others where the contact was faulted but not part of a regional structure. They also noted that the Chaos differed considerably southwest and northeast of Desert Hound Peak.

Our mapping in both the southwest and northeast parts of the Chaos confirms and extends Wright and Troxel’s findings. In the southwestern part, at least five episodes of faulting affected both the basement and Proterozoic cover. Most of the deformation appears controlled by at least three presently low-angle faults that affect the cover sequence only. These faults, with a collective slip of <5km, act as local detachment faults. The Amargosa Surface itself forms both a high-angle fault that post-dates these low-angle faults as well as an unconformity. As a fault, it is locally discontinuous and exhibits subhorizontal slickenlines.

In the northeastern part of the Chaos, the Amargosa Surface presents numerous outstanding exposures, including two isolated klippe-like features that present a near three-dimensional view of the structures. Throughout the area, the surface consists primarily of variably oriented low- and high-angle faults, and numerous exposed unconformities. Moreover, the faults exhibit inconsistent (preliminary) kinematic data, with hangingwall transport directions to the west, northwest, north, northeast, and even to the east. Miller and Friedman (2003) reported an age of 10.08 +/- .03 Ma for the “Thermometer Rhyolite” dike, which intrudes one of the largest of these faults.

We concur with Wright and Troxel (1984) that “the contact between the (basement) complex and the overlying later Precambrian units is not a regional surface of dislocation”. Any model requiring considerable detachment of upper crustal rocks from the basement requires a different surface in the southern Black Mountains.