Topographic evolution of the western United States since the early Miocene

Abstract

The origin of the high topography within the western United States has been attributed to either crustal/lithospheric isostasy or dynamic topography, but their relative contributions remain unconstrained. Here we investigate this problem using gravity, residual topography and geodynamic modeling. We first evaluate two end-member scenarios of isostatic balance: crustal isostasy and lithospheric isostasy. Both cases lead to prominent negative mantle residual gravity within the tectonically active western U.S. and unrealistic crustal/lithospheric density structures, requiring the presence of low-density mantle underneath. The negative mantle residual gravity is consistent with both the estimated positive residual topography and calculated dynamic uplift due to the presence of hot asthenospheric mantle underneath. Geodynamic modeling further reveals that this landward migrating dynamic uplift originates from the eastward intrusion of the hot Pacific mantle through tears and edges of the Juan de Fuca slab since middle Miocene. The estimated paleotopography maps by combining dynamic topography and lithosphere isostasy over the western U.S. are consistent with several observational constraints, including episodic uplifts of the Sierra Nevada, post-mid-Miocene uplift of the Idaho batholith, the sustaining subsidence within most of the B&R, and the largely stable topography of central Colorado Plateau since 20 Ma.