HETEROGENEOUS MIOCENE EXTENSION IN THE BASIN AND RANGE DEMONSTRATES VERTICALLY DECOUPLED EXTENSION WITH IMPLICATIONS FOR PRE-EXTENSIONAL CRUSTAL THICKNESS: A CASE STUDY FROM THE WESTERN BASIN AND RANGE

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. We present a case study along a ~150-km-long traverse across the western Basin and Range to demonstrate 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). Similar heterogeneous extension is observed across the eastern Basin and Rane near the metamorphic core complex belt. Because there is no evidence for irregular crustal thickness variations in these regions—either inferred 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. Based on these simple logical interpretations of surface geological observations, we argue 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 may explain why common reconstructions of Mesozoic crustal thickness across the Nevadaplano invoke crustal welts that are spatially offset from the well-constrained Mesozoic west-east drainage divide.