Cold and wet at the roots of U.S. Cordilleran high elevation

Abstract:

Mechanisms for high elevation and large-scale deformation of the western U.S. Cordillera have long been a source of controversy. Lowry and Pérez-Gussinyé (2011) suggested abundant quartz in the Cordilleran crust, evidenced by low seismic velocity ratio vP/vS, might be one clue. Here we examine thermal transfer processes to look for additional insight. We calculate temperature at the Moho by modeling the conductive 1D geotherm from observed surface heat flow, and compare these estimates to measurements derived from Pn velocity tomography and mineral physics (Schutt et al., 2015). Moho temperature is moderately sensitive to assumptions regarding crustal radiogenic heating and thermal conductivity, but differences between modeled and measured temperatures exceed the range consistent with reasonable variations in these parameters. Residual (measured minus modeled) Moho temperatures are “cold” in regions deformed during the Laramide flat-slab subduction event. A simple model of transient cooling by cold subducting slab at the base of the lithosphere chills the Moho by a small fraction (<20%) of the observed anomaly.

Intriguingly, the “cold-Moho anomaly” strongly correlates (R = 0.71) to high elevation (opposite the relationship expected) and also correlates with crustal vP/vS, which in turn correlates to surface heat flow. Recent analyses (Guerri et al., 2015) suggest low crustal vP/vS and density may signal hydration. Geotherm models are sensitive to assumed advective thermal transfer, and we modeled advection consistent with lithospheric extensional strain (i.e., increasing with depth). If instead volatile transfer dominates advection, geotherm modeling predicts a better match of surface heat flow to Moho temperature. We hypothesize that the “cold-Moho anomaly” actually reflects enhanced volatile flux and advective heat transfer from fluids derived from Laramide subduction, and associated volumetric expansion of the crust contributes to high elevation.

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