Abstract

More than 7,500 m of upper Precambrian and Paleozoic sedimentary rocks in the area of the Montgomery Mountains and the northern half of the Nopah and Resting Spring Ranges represent a typical Cordilleran miogeosynclinal sequence. During Mesozoic time, after a period of earlier Mesozoic folding and high-angle faulting, these rocks were cut by thrust faults that divided the rock sequence into four structural units in the Resting Spring Range and the Montgomery Mountains. From the top down, the units are: (1) the Montgomery thrust plate, (2) the Baxter thrust plate, (3) the Resting Spring thrust plate, and (4) the Amargosa unit. Beneath the Montgomery thrust fault in the Montgomery Mountains lies the Six Mile thrust plate, which has limited extent but may be a major subunit of the Baxter plate. The Montgomery thrust plate moved eastward along a single thrust, but the Resting Spring thrust plate moved eastward along an anastomosing series of thrust faults, which divided it into a series of lobes. Thrust faults in the Resting Spring Range now dip east because Cenozoic tilting has reversed their original subhorizontal or west dips. In the Nopah Range, there are three major structural units, which are, from the top down: (1) the Chicago Pass thrust plate, (2) the Shaw thrust plate, and (3) the Nopah Range unit. Structural units in the Nopah Range cannot be unequivocally correlated with those in the Resting Spring Range and Montgomery Mountains, and two hypotheses must be presented for correlation: hypothesis 1 correlates the Baxter thrust plate of the Resting Spring Range with the Shaw thrust plate of the Nopah Range, and hypothesis 2 correlates the Baxter thrust plate with the Chicago Pass thrust plate of the Nopah Range.

Post-thrusting structures are related to progressive west or northwest extension. Cenozoic folds around the north end of Stewart Valley are oblique and related to right slip on the northwest-striking Stewart Valley fault. These may be the oldest post-thrusting structural features in the area. Right slip and high-angle dip-slip faults are probably contemporaneous, forming ranges and valleys by pull-aparts. Continued extension caused rotation of earlier high-angle dip-slip faults so that some of them are now subhorizontal. Some low-angle faults are probably gravity slides and form gradations into more chaotic landslides.

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