New perspectives on the construction and extensional collapse of the Nevadaplano: A summary of recent progress
Sean Long, Washington State University
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Orogenic plateaus are testing grounds for resolving debates over the roles of several deformation processes, including crustal shortening, continental underthrusting, and delamination. In eastern Nevada and western Utah, a Cordilleran orogenic plateau (the 'Nevadaplano') is hypothesized to have existed from the Late Cretaceous to the Paleogene, and attained crustal thicknesses of 50-60 km and elevations of 2.5-3.5 km. The Nevadaplano has undergone a complex Cenozoic history of extension, making it an ideal locality for studying the processes that contribute to the collapse of thickened crust. Significant progress has been made in the last decade on understanding the structural evolution of this plateau.
Between the latest Jurassic and Paleocene, ~200 km of shortening was accommodated in the Sevier thrust belt. In the Sevier hinterland, narrow thrust belts in western Utah and central Nevada and a broad region of folds in eastern Nevada deform rocks as young as Early Cretaceous. These hinterland provinces collectively record low-magnitude (10's of km) shortening, interpreted to have accompanied eastward translation of the 15 km-thick Cordilleran passive margin basin section during shortening in the Sevier thrust belt. Low hinterland shortening magnitudes are attributed to the rheological competence of this thick section. However, the westward underthrusting of thick, un-rifted continental crust under eastern Nevada, which is a kinematic requirement of the shortening recorded in the Sevier thrust belt, resulted in significant thickening.
Synorogenic sediments deposited atop the Nevadaplano are sparsely preserved. Early Cretaceous basins in central Nevada record locally-sourced deposition in proximity to active thrust faults and folds. Conodont alteration indices of Pennsylvanian-Triassic sedimentary rocks indicate peak burial temperatures of 50-80°C, which rules out a thick Jurassic-Cretaceous foreland basin section ever having been deposited across the hinterland. Thus, large-scale flexural subsidence was likely limited to the region within and east of the Sevier thrust belt.
The Nevadaplano experienced spatially-isolated, Late Cretaceous-Paleocene, synorogenic extension, which is interpreted as a consequence of isostatic adjustment and thermal weakening following delamination of mantle lithosphere. Following this, several regions experienced extension either during or closely following an Eocene-Oligocene sweep of ignimbrite volcanism, which is attributed to convective heating and a decrease in interplate coupling accompanying post-Laramide slab rollback. Finally, widespread extension that formed the Basin and Range Province, which was responsible for the lowering of surface elevations, did not begin until the Miocene, and is attributed to the reorganization of the Pacific-North American plate boundary to a transform margin.