Magnitudes and spatial patterns of erosional exhumation in the Sevier hinterland, eastern Nevada and western Utah, USA: Insights from a Paleogene paleogeologic map

The hinterland of the Sevier orogenic belt in Nevada and western Utah (United States) has been interpreted as an ancient high-elevation orogenic plateau, or Nevadaplano, that collapsed extensionally during Tertiary time. To illustrate the preextensional structural relief of this region, a new paleogeologic (or subcrop) map showing the distribution of Neoproterozoic to Triassic rocks exposed beneath a regional Paleogene unconformity is presented here. The map area extends between the traces of the westernmost major Sevier thrust system and the Roberts Mountains thrust. Three across-strike subcrop domains can be defined: (1) Cambrian to Mississippian subcrop levels in the leading part of the Delamar–Wah Wah–Canyon Range (DWC) thrust sheet, indicating high (as much as 7 km) structural relief; (2) a broad region of eastern Nevada and westernmost Utah devoid of surface-breaching thrust faults, with Mississippian–Triassic subcrop levels, indicating low (2 km) structural relief; and (3) subcrop levels varying between Neoproterozoic and Permian in the central Nevada thrust belt (CNTB), indicating high (as much as 8 km) structural relief. Using published isopach maps of depositional thickness of Neoproterozoic to Triassic sedimentary rocks, the paleogeologic map has been converted into a map that contours the thickness of rock eroded between the top of the Triassic section and the Paleogene subcrop level, and thus illustrates magnitudes and spatial patterns of synorogenic erosional exhumation. Characteristic exhumation magnitudes for the three subcrop domains are: (1) 4–8 km in the leading part of the DWC thrust sheet; (2) 4–10 km, 2 km, and 4–6 km in the southern, central, and northern parts of the CNTB, respectively; and (3) 1–3 km in the intervening low-relief region. Isolated exposures of southern CNTB structures can be correlated by their subcrop and exhumation patterns into two through-going thrust systems that connect with structures of the Sevier thrust belt in southern Nevada. This supports previously suggested correlations, and implies a direct structural link between these two thrust systems. Subcrop and exhumation patterns do not reveal a surface-breaching thrust trace that would represent a southern continuation of the Windermere thrust south of the Pequop or Ruby Mountains. Thus, if the Windermere thrust model is correct, this implies either termination at a lateral structure such as a tear fault, or a transition to a blind geometry. The ∼2 km structural relief that characterizes much of the Sevier hinterland indicates that the majority of high-magnitude (>1–2 km throw), regionally distributed, surface-breaking normal faulting that dismembered the orogenic highland and produced the high structural relief observed today had to be post-Oligocene. Although the traces of 70 normal faults that are overlapped by the Paleogene unconformity are identified, the throw on nearly all of these structures is limited to a maximum of 1–2 km. This further highlights the paradox of extensive Late Cretaceous and Paleogene exhumation of mid-crustal rocks now exposed in metamorphic core complexes without corresponding high-magnitude upper crustal extension. The close spatial association of high exhumation magnitudes with the hanging walls of major thrust faults suggests that erosional exhumation is a response to relief generation accompanying contractional deformation in orogenic plateaus. The lack of significant along-strike exhumation variability within much of the Sevier hinterland implies that relief generation was relatively uniform along strike. The steep across-strike exhumation gradient between the leading part of the DWC sheet and the low-exhumation region to the west is interpreted as the result of shielding of the western area from headward erosion by long-lived uplift, erosion, and relief development through passive eastward translation of the DWC thrust sheet and growth of antiformal culminations at depth. A significant difference between the Sevier and central Andean orogenic plateaus is emphasized here. Much of the interior part of the Sevier plateau was an eroding highland, composed of preorogenic rocks, with low structural relief, while the interior part of the Andean plateau consists of a variably deformed and exhumed, synorogenic hinterland basin as much as 12 km thick, with very high structural relief. Thus, comparison of the structural level of the stratigraphic contact between preorogenic and synorogenic rocks indicates a difference in rock uplift between the interior parts of these two plateaus of as much as 14–15 km.