Uplift, unbuckling, and collapse: Flexural history and isostasy of the Wind River Range and Granite Mountains, Wyoming

Abstract

Two‐dimensional gravity and flexural models constrain the structure and isostatic compensation of the Wind River Range and Granite Mountains, Wyoming. These ranges are asymmetric, basement‐cored Laramide uplifts. Their large positive isostatic gravity anomalies imply that the topography of the uplifts is not locally compensated. For the Wind River Range, gravity data are consistent with thrusting along a listric fault which flattens and soles out at 30 km depth with no detectable offset on the Moho. We model the large Bouguer gravity high over the range with high‐density lower crustal rocks emplaced over lower‐density middle crustal rocks along the Wind River thrust. The high positive gravity anomaly over the Granite Mountains can be modeled as an extension of the South Pass greenstone belt from the southern Wind River Range eastward beneath the Emigrant Trail thrust. A crustal root, as required in local (Airy) compensation, for either range requires unreasonably high‐density material in the upper crust to obtain the observed anomaly. Combined gravity and flexural models show that a flexural rigidity at least 2.55×1022 N m (maximum deflection of 2 km of the Moho) is required to satisfy the gravity data. Our results are consistent with models of uplift due to horizontal compression and support of the resulting topography by lithospheric strength. An otherwise puzzling episode of Oligocene uplift of the Wind River Range can be explained as “unbuckling” (decrease of deflection of lithosphere) accompanying release of Laramide compressional stresses. Flexurally supported uplifts will collapse if the lithosphere is weakened. A number of Laramide uplifts have collapsed, primarily east‐west trending structures such as the Granite Mountains, the southern end of the Wind River Range, and the Uinta Mountains. We propose that collapse has occurred along zones of established weakness in the crust in response to north‐south extension.