Abstract

In the Silurian Hills, 15 miles southeast of Death Valley, California, the dominant structure is the Riggs thrust. Beneath the thrust older Precambrian metamorphic rocks are overlain by the later Precambrian Pahrump group. Above the thrust are Paleozoic(?) carbonate rocks (Riggs formation, provisional name) and Tertiary(?) rocks, in part sedimentary and in part volcanic. Unconformably on all these bedrock units are a monolithologic carbonate megabreccia, fan gravels of two ages, and several terrace gravels. The topography is controlled by lithology and structure, drainage is subsequent, and scarps are either erosional or fault line.

The Pahrump group in the Silurian Hills is 11,000 feet thick and is subdivided into 35 mappable members, chiefly coarse clastic rocks derived from the south. Correlation with the three formations of the type Pahrump in the Kingston Range, 15 miles north, is uncertain.

Granitic rocks of two or more ages intrude the Riggs and Pahrump rocks. Distinctive members of the Pahrump group can be traced from unmetamorphosed sedimentary rocks in the west to intensely feldspathized and metamorphosed rocks in the east. The older granitic rock is displaced by the Riggs thrust fault; the younger granitic rock is both localized by the fault and displaced by it. Most of the movement on the north-trending, high-angle faults is prethrust, but contemporaneous and post-thrust movement occurred.

The Riggs thrust was apparently localized along the angular unconformity between the Riggs and Pahrump rocks. The thrust surface is anticlinal or dome-shaped, probably because of post-thrust warping. The thrust zone is a “chaos” similar to the type chaos, the Amargosa chaos described by Noble (particularly the Virgin Spring phase), except that it is composed chiefly of rocks from the footwall of the thrust instead of the plate. Detailed mapping of the chaos, possible because many of the members or beds in the Pahrump group are distinctive, shows that component fragments were carried southwestward as much as 2 miles from their original position in the autochthon and were piled up in an imbricate structure in which normal stratigraphic order is approximately maintained. The amount of movement was not determined, but the distribution of granitic rock in and under the plate suggests a minimum movement of 8 miles. A debris-flow megabreccia covers the eroded trace of the Riggs thrust fault. The Tertiary(?) volcanic rocks are folded and confined to the thrust plate, which suggests that the less intensely folded Riggs thrust is younger.

Chaos structures may have formed by the imbrication and piling up of a whole series of very small thrust plates above an underlying block, which was being shortened under compression. A few strong, competent plates moved out several miles without being broken up appreciably, but most plates broke up into giant lenses and blocks that moved much shorter distances. The incompetent material was ground up and acted as a lubricant. The stratigraphic section, thus “skeletonized” and abbreviated into “chaos”, retained a crude stratigraphic order.

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