The Role of Upper-crustal Thickening in Spatially-focusing Synorogenic Extension: A Case Study from the Late Cretaceous-Paleocene Nevadaplano

Abstract:

Synorogenic extension is often interpreted as gravitational spreading of thickened crust or as a response to thrust belt dynamics; however, the processes that spatially-localize extension during orogenesis are not fully understood. Here, we examine additional factors that control the location of synorogenic extension by presenting a case study from the hinterland plateau of the Sevier orogenic belt (or ‘Nevadaplano’). The Eureka culmination, a 20 km-wide, N-trending anticline with 4.5 km of structural relief, was constructed in the plateau interior in eastern Nevada in the Early Cretaceous. After its construction, the culmination underwent 40% extension, accommodated by two sets of N-striking normal faults, which exhumed rocks from depths of 6-8 km, and pre-date a late Eocene unconformity. Well-defined fault geometries, stratigraphic thicknesses, and preservation of erosion surfaces allowed estimation of the contributions of tectonic exhumation versus pre- and post-extensional erosional exhumation. Time-temperature paths determined from zircon and apatite (U-Th)/He and fission-track thermochronology collected from Paleozoic quartzite in the footwalls of two normal faults demonstrate rapid (10 °C/Myr) Late Cretaceous to Paleocene (75–60 Ma) cooling, which we interpret as tectonic exhumation during extension, and which was concurrent with late-stage shortening in the frontal Sevier thrust belt. This example illustrates that structural and topographic relief generated within zones of localized upper-crustal thickening can spatially-focus extension during orogenesis. This observation, when combined with Late Cretaceous exhumation in core complexes in northeast Nevada and northern Utah, which also developed within sites of localized crustal thickening, illustrates that gradients in gravitational potential energy, at different scales and crustal levels, played a primary role in controlling the location of late-stage, synorogenic extension within the Nevadaplano.