A SUMMARY OF RECENT PROGRESS ON UNDERSTANDING THE STRUCTURAL EVOLUTION AND PALEOGEOGRAPHY OF THE NEVADAPLANO (Invited Presentation)

The Sevier hinterland in Nevada and western Utah has been interpreted as a Cretaceous-Paleogene ‘Nevadaplano’. Despite a complex overprint of Cenozoic extension, 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 hinterland, structural reconstructions define narrow thrust belts in western Utah and central Nevada and a broad region of folds in eastern Nevada, which deform rocks as young as Early Cretaceous. These hinterland provinces record low-magnitude (10’s of km) shortening, interpreted to have accompanied eastward translation of the Cordilleran passive margin basin section during construction of the Sevier thrust belt. Low hinterland shortening magnitudes are attributed to high rheological competence of this thick section.

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 within and east of the Sevier thrust belt.

The Nevadaplano experienced spatially-isolated, Late Cretaceous-Paleocene, synorogenic extension, including normal faulting, half-graben development, and initial exhumation of mid-crustal rocks. This extension was synchronous with widespread granitic magmatism, and has been interpreted as a consequence of isostatic adjustment and thermal weakening following delamination of mantle lithosphere. The rise of granitic crustal melts resulted in heterogeneously-distributed shallow-crustal metamorphism and ductile deformation.

Restorations of extension suggest average crustal thicknesses of ~50 km, and isotope paleoaltimetry indicates 2.5-3.5 km elevations attained by the end of crustal thickening. Widespread extensional collapse did not occur until the Neogene, and was associated with restructuring of the Pacific-North American plate boundary.