Abstract

The northwest-trending Spring Mountains, Nevada, contain a 45-mi-wide (75-km) cross section of the eastern part of the North American Cordilleran orogenic belt and geosyncline. This cross section is probably the most southerly exposed section which exhibits structure and stratigraphy “typical” of the eastern part of the Cordillera.

Stradgraphically, the transition from Paleozoic craton to miogeosyncline is present from east to west across the Spring Mountains. The sedimentary succession through the middle Permian thickens from 8,800 ft (2,660 m) east of the Spring Mountains to approximately 30,000 ft (9,000 m) in the west. Thickening of individual formations accounts for 6,800 ft (2,070 m) of added section, addition of formations at unconformities accounts for 4,600 ft (1,400 m) of added section, and addition of a thick terrigenous late Precambrian sequence accounts for 9,800 ft (3,000 m) of added section.

Three major thrust plates are exposed in the Spring Mountains, each structurally higher plate containing a thicker sequence of Paleozoic rocks. The easternmost thrust is the Keystone thrust, except where the earlier Red Spring thrust plate is present below the Keystone as isolated remnants. The Keystone thrust appears to be a décollement thrust, but complications at depth suggest that additional thrust slices may be present below the thrust or several thousand feet of late Precambrian terrigenous rocks may be present above the thrust. The structurally higher Lee Canyon thrust plate probably contains at least 4,000 ft (1,200 m) of these terrigenous rocks at its base, and the Wheeler Pass thrust plate contains at least 11,000 ft (3,300 m) of these rocks. Pregeosynclinal basement could be involved in some of the higher thrust plates, particularly the Wheeler Pass plate, but depths of exposure are inadequate to determine its role.

Thrust faulting has produced a shortening of from 22 to 45 mi (36.6 to 75 km) in the geosynclinal rocks based on geometric constructions of cross sections at depth. This range probably represents a minimum figure. Some folding and thrusting occurred during the early Late Cretaceous, but data within the Spring Mountains only establish a much wider time bracket, post–Early Jurassic to pre–late Cenozoic for the easternmost thrust faults and post–Early Permian to pre–late Cenozoic for the westernmost thrusts.

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