Synorogenic extension localized by upper-crustal thickening: An example from the Late Cretaceous Nevadaplano

Abstract

Synconvergent extension within orogenic systems 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, a case study from the United States Cordillera demonstrates that localized upper-crustal thickening can exert a first-order control on the spatial location of synorogenic extension. The Eureka culmination, a 20-km-wide, north-trending anticline with 4.5 km of structural relief in the hinterland plateau of the Sevier orogenic belt (or "Nevadaplano") in eastern Nevada was deformed by two sets of north-striking normal faults that pre-date late Eocene volcanism. (U-Th)/He and fission-track thermochronology data collected from Paleozoic quartzite in the footwalls of two normal faults demonstrate rapid (10 °C/m.y.), 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 uppercrustal thickening can spatially focus extension during orogenesis, and adds to a growing body of evidence that Late CretaceousPaleocene extension in the Nevadaplano occurred at both mid-and upper-crustal levels.

Geometry and kinematics of the Grant Range brittle detachment system, eastern Nevada, U.S.A.: An end‐member style of upper crustal extension

“…During the Late Cretaceous and Paleogene, eastern Nevada has been interpreted as an orogenic plateau [e.g., Coney and Harms , ; Allmendinger , ; DeCelles , ]. Evidence for spatially localized Late Cretaceous and Paleogene extension in this plateau, including exhumation of midcrustal rocks now exposed in metamorphic core complexes [e.g., Hodges and Walker , ; Lewis et al ., ; McGrew et al ., ; Wells and Hoisch , ] and upper crustal normal faulting [e.g., Taylor et al ., ; Gans et al ., , ; Vandervoort and Schmitt , ; Axen et al ., ; Camilleri and Chamberlain , ; Druschke et al ., , ; Long et al ., ], records a protracted, spatially heterogeneous transition to an extensional tectonic regime in eastern Nevada. However, the inception of widespread extension that formed the Basin and Range province (Figure a), which is attributed to reorganization of the Pacific‐North American plate boundary [e.g., Atwater , ], was not until the middle Miocene [e.g., Dickinson , , ; Colgan and Henry , ].…”

Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages

“…Eocene exhumation and erosion of the upper ~3.0-3.5 km of the Paleozoic section from the footwall of a normal fault is comparable to exhumation of the lower plate of the northern Snake Range by ~3 km during the middle Eocene, Lee et al | Timing of ductile extension, northern Snake Range metamorphic core complex GEOSPHERE | Volume 13 | Number 2 the interpreted first episode of Cenozoic exhumation in the lower plate based on MDD model cooling histories (Lee, 1995). This interpretation holds if the geothermal gradient during the Eocene was ~30 °C/km, i.e., the calculated Late Cretaceous to Paleocene geothermal gradient in central Nevada (Long et al, 2015) and the maximum typical for the Basin and Range today (Lachenbruch and Sass, 1978). Paleogene outcrop and exhumation data across east-central Nevada preclude widely distributed kilometer-scale slip on upper crustal normal faults of this age (Long, 2012;Ruksznis, 2015); however, normal fault slip and attendant topographic relief may have been locally significant between the Late Cretaceous and late Eocene (Long, 2012;Druschke et al, 2009aDruschke et al, , 2009bDruschke et al, , 2011.…”

Middle to late Miocene extensional collapse of the North Patagonian Andes (41°30′–42°S)

“…Since Orts et al (2015) recently interpreted the migration of magmatic activity north of 41º45´S as a possible case of a shallowing-retreating cycle of the subducted slab, this could constitute an alternative explanation for the extensional event described in this work. However, this seems to constitute a case of localized upper-crustal synconvergent extension (Long et al, 2015) rather than a crustal-scale orogenic process as expected from rollback hypothesis.…”