Magnitude and spatial patterns of erosional exhumation in the Nevadaplano, eastern Nevada and western Utah: insights from a Paleogene paleogeologic map

The hinterland of the Sevier orogenic belt in Nevada and westernmost Utah has been interpreted as an ancient, high-elevation orogenic plateau, or 'Nevadaplano' that extensionally collapsed during Tertiary time. The cumulative effects of this extension have obscured evidence for a relict, low-relief region. To illustrate both the pre-extensional structural geometry and structural relief, a paleogeologic (or 'subcrop') map showing the distribution of Neoproterozoic, Paleozoic, and Triassic rocks exposed beneath a regional Eocene to Oligocene unconformity has been compiled for the Nevadaplano region. The map area extends from the trace of the westernmost major Sevier thrust system in western Utah to the Roberts Mountains thrust in central Nevada. Using published thicknesses of sedimentary rocks deposited in Nevada and Utah between the Neoproterozoic and the Triassic, the paleogeologic map becomes a map of exhumation, which contours the magnitude of sedimentary rock removed prior to the Paleocene. The exhumation map highlights a ~150 km wide region of low structural relief and minimal (<3 km) exhumation that, when restored for extension, would correspond to a ~100 km wide low-relief area preserved between two regions of higher exhumation and structural complexity to the east and west. At a first-order, this pattern of structural relief is broadly comparable to the 150 km wide Altiplano basin preserved between the structurally-higher western and eastern Cordilleras of Bolivia. Regional domains defined on the subcrop map include: 1) Cambrian subcrop levels in the hanging wall of the DeLamar-Wah-Wah-Canyon Range (DWC) thrust system and Sevier culmination, which young westward to Mississippian and higher levels; 2) a broad region of eastern Nevada and westernmost Utah that is devoid of regional-scale, surface-breaching thrust faults, and has subcrop levels between Mississippian and Triassic, indicating relatively low structural relief; and 3) subcrop levels varying between Neoproterozoic and Permian in the Central Nevada thrust belt (CNTB), indicating significant structural relief. Isolated segments of CNTB structures can be correlated by their subcrop patterns, and can be traced southward to connect with thrust faults in southern Nevada. The exhumation map can be divided into three zones that exhibit significant across-strike magnitude changes: 1) high exhumation magnitudes (4-8 km) characterize the deeply-eroded hanging wall of the DWC thrust system and Sevier Culmination, which display little variability for a 350 km along-strike distance; 2) the CNTB displays high magnitudes (4-8 km) in its southern extent, and moderate magnitudes (2-5 km) in its northern extent; 3) the DWC thrust system and CNTB bound a large region that exhibits low (1-3 km) exhumation magnitudes. The close spatial association of high exhumation magnitudes with the hanging walls of thrust faults highlights the first-order control deformation and structural elevation has on exhumation patterns, and suggests that erosional exhumation is a response to relief generation accompanying contractional deformation in orogenic plateaus. The lack of significant exhumation variability within each structural zone implies that relief generation was relatively uniform along-strike.