Uplift and exposure of the Panamint metamorphic complex, California

Abstract

The central Panamint Mountains, Death Valley area, California, comprise three groups of rocks. The first consists of middle Proterozoic gneiss and upper Proterozoic sedimentary rocks that were regionally metamorphosed under low-pressure conditions during Middle Jurassic time, were intruded by the Late Cretaceous Hall Canyon granitic pluton, and were folded along NNW-trending axes during Late Cretaceous time. This group, called the Panamint metamorphic complex, makes up the core of the Panamint Mountains. The complex is cut by numerous west-dipping, low-angle normal faults that are locally intruded by the Miocene Little Chief stock. The second group consists of monolithologic breccias, called the Surprise breccia, derived from the local metamorphic rocks. The Surprise breccia forms the western slope of the Panamint Mountains north of Pleasant Canyon and forms the hanging wall of the west-dipping Surprise fault. Displacements along the fault, estimated from offset structures, are about 2,500 m down dip. The third group consists of the Nova Formation, which comprises fanglomerate, basalt, and minor breccia and which lies in the hanging wall of the west-dipping Emigrant fault. The fanglomerates contain abundant clasts of metamorphic and granitic rocks from the metamorphic complex. The three groups of rocks are separated from each other by intervening faults. The Panamint metamorphic complex is the structurally lowest group and is separated from the Surprise breccia by the Surprise fault. The breccia is separated from the structurally highest Nova Formation by the Emigrant fault. All these rocks lie above the regionally extensive Amargosa fault. The difference in rock character among fanglomerates, breccias, and relatively intact metamorphic core rocks is readily visible in Thematic Mapper (TM) and Shuttle Imaging Radar (SIR-B) images. The TM image, which is sensitive to differences in mineralogic compositions of the rock types, distinguishes the metamorphic complex from the Surprise breccia because the breccias are more highly weathered than the metamorphic complex. The SIR-B image, which is sensitive to differences in surface roughness and topography, readily distinguishes between the breccia and the fanglomerate. The breccia is characterized by closely spaced, parallel drainages, whereas the fanglomerate contains a dense dentate drainage pattern. The combination of the images is a significant mapping aid in the central Panamint Mountains. Most of the uplift of the Panamint metamorphic complex, from a Late Mesozoic depth of about 10 km to ~3 km, probably occurred during displacements along the Amargosa and related faults. Some of these faults were intruded by the late middle Miocene Little Chief stock. The maximum possible uplift rate at this time was 17 mm/yr; the actual rate could have been lower if some uplift occurred prior to faulting. The metamorphic complex was largely unroofed by the time of deposition of the Nova Formation. Uplift of the range prior to, and partly contemporaneous with, Nova deposition resulted in formation of the Surprise breccia. The Emigrant fault formed late. The fanglomerates dip ~25° eastward against the Emigrant fault, but this tilting appears to have resulted from aggregate rotation along several fault surfaces rather than from uniform eastward tilting of the entire Panamint Mountain block. Isolated remnants of Surprise breccia on ridge crests indicate that the unroofing of the metamorphic complex south of Wildrose Canyon probably was not completed until after the breccia-mass development.

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