Structural geology of the southern Silurian hills, San Bernardino County, Calif

Abstract

In the southern part of the Silurian Hills, seven lithologic units are delineated. They are an earlier Precambrian gneiss complex, the later Precambrian Pahrump Group, a Precambrian (?) metasedimentary complex, a Precambrian (?) metadioritic and metasedimentary complex, the Riggs Formation, basic plutonic rocks, and acidic plutonic rocks. Metamorphism reaches high greenschist to low amphibolite grade with an overprint of contact metamorphism caused by the intrusion of the acidic plutonic rocks. The Riggs thrust fault separates the Pahrump Group, which is in the lower plate, from all of the other units. The upper plate rocks show a more complex folding history than the lower plate rocks. Two quite different types of movement have occurred along the Riggs thrust. The first occurred in late Mesozoic time, was a compressional event, moved in a north or northeast direction, formed a sharp contact with plastic deformation rather than brecciation, and can be seen in the southern Silurian Hills. The second occurred in the Tertiary, was a gravitational event, moved south, formed a chaos structure in the lower plate, and can be seen in the northern Silurian Hills.