Abstract

Mesozoic basin development in the Rocky Mountain region and throughout the Cordillera of the western United States is interpreted to have resulted from the initiation and evolution of two temporally distinct subducted lithospheric slabs, a pre-Farallon plate and the Farallon plate, beneath the western margin of North America. Basin tectonics and geometries associated with subduction of the pre-Farallon plate were dominated by the effects of dynamic topography caused by viscous flow in the mantle above the subducted slab. Basin development associated with subduction of the Farallon plate was dominated by shortening in the backarc region and lithospheric flexure adjacent to the attendant supracrustal load.

Subduction of the pre-Farallon slab was initiated in Early Triassic time following the Sonoma orogeny, which marked closure of a marginal basin that had lain west of North America. Following the brief formation of early Early Triassic flexural basins adjacent to the Golconda allochthon, initiation of subduction created a westward-thickening wedge of marine sedimentary rocks and volcanics in western Nevada, a forebulge resulting from viscous flow in the mantle above the descending slab in central Nevada, and a back-bulge basin in the eastern Basin and Range and Rocky Mountain Provinces. Strata of the Early Triassic back-bulge basin accumulated between the forebulge and remnant uplifts of the Ancestral Rocky Mountains. In the Late Triassic, continued subduction created an extensive continental-margin arc in eastern California and western Nevada, a rapidly subsiding, westward-thickening backarc basin east of the arc in northwestern and west-central Nevada, a forebulge in central Nevada, and a nonmarine back-bulge basin between the forebulge and remaining topography of the Ancestral Rocky Mountains. Flattening of the slab angle through the Jurassic resulted in Early Jurassic backarc extension following a cusp in the North American apparent polar wander path, and Middle Jurassic expansion of the arc eastward to the vicinity of the Utah-Nevada border. The forebulge is inferred to have migrated into the Rocky Mountain Province after Early Jurassic time to create a regional (J2) unconformity. The early subduction system was terminated in Late Jurassic time by the Nevadan orogeny, a collision of an island-arc terrane with the continental margin. The collision caused the subducting slab to become detached and forced abandonment of the former locus of subduction. The Morrison Formation was deposited east of the quiescent Middle Jurassic arc and adjacent region, which may have been elevated by dynamic viscous flow in the mantle above the deep, detached slab.

Initiation of Farallon subduction at the margin of North America roughly coincided with a decreased rate of orthogonal convergence between North America and the oceanic plate, as well as a magmatic hiatus in the arc. A subsequent Early Cretaceous (middle to late Neocomian) increase in orthogonal convergence of the oceanic and North American plates was marked by essentially simultaneous initiation of a composite fold and thrust orogen, consisting of the Luning-Fencemaker, Central Nevada, and Sevier thrust belts, across the width of the backarc region. Lower Cretaceous intermontane deposits in northwestern and central Nevada record this deformation, coeval with initial deposition of widespread foreland-basin deposits in the Rocky Mountain Province. In the Late Cretaceous (late Campanian), progressive shallowing of the subducted Farallon slab resulted in isostatic broadening of the foreland basin, followed immediately by wholesale disruption of the basin by intraforeland uplifts at the end of the Campanian.