HETEROGENOUS LATE MIOCENE EXTENSION IN THE WESTERN BASIN AND RANGE DEMONSTRATES VERTICALLY DECOUPLED CRUSTAL EXTENSION

Cenozoic crustal extension magnitudes and rates vary significantly across the Basin and Range, western United States. Despite spatially and temporally variable Cenozoic extensional strain observed at the Earth’s surface, the present-day crustal thickness of the Great Basin is roughly uniform, signifying apparently well-distributed finite strain across the extensional province. This paradox implies compensation in the middle or lower crust to accommodate heterogenous upper crustal extension, and thus requires decoupling between the upper and lower crust. To further test this interpretation, we integrated extensional records across a ~150-km-long traverse in the western Basin and Range with new geologic mapping and geochronology in the Pine Nut Mountains to better refine Miocene-present extension magnitudes and rates. This synthesis reveals alternating domains of low extensional strain (~15%; i.e., the Carson Range–Pine Nut Mountains and Gillis Range) and high extensional strain (~150-180%; i.e., the Singatse and Wassuk Ranges). Because there is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic prior to extension, or today—extensional strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ∼150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable. This is especially true for interpretations of pre-Cenozoic crustal-thickness variability.