Subdued state of tectonism of the Great Basin interior relative to its eastern margin based on deep resistivity structure

Abstract

Lower-crustal fluid content and physical state of the central and eastern subprovinces of the Great Basin are compared on the basis of their deep electrical resistivity structure as derived from two, detailed east-west profiles of magnetotelluric (MT) soundings. A simple, but powerful and effective transformation of each profile's data allows derivation of a single characteristic MT sounding for each subprovince which is essentially free of the effects of middle- to upper-crustal lateral heterogeneity. The characteristic soundings are amenable to layered Earth inversion, yielding broad-scale, model resistivity profiles into the uppermost mantle. A striking difference in lower-crustal electrical resistivity is apparent between the two subprovinces. The tectonically more active eastern domain has a lower-crustal conductance of ∼ 3000 siemens (S) at a model depth around 15 km, while that of the relatively subdued Great Basin interior is only ∼ 750 S with a depth around 22 km. Thicknesses of the two subprovince conductors are resolvable independent of their resistivity (∼ 20 km at 7 Ω-m in the east and ∼ 15 km at 20 Ω-m in the interior). This allows estimates of fluid-filled porosity in the deep crust assuming highly saline brines and predominantly grain-edge interconnection. Nominal values are 0.4 vol% for the eastern subprovince but only ∼ 0.15 vol% for the central region. These porosities are consistent with fluid percolation models for ductile crustal rocks, assuming on-going fluid replenishment by basaltic underplating in the eastern Great Basin and with little fluid input occurring in the central Great Basin in the last 5–10 Ma. Resistivity of the uppermost mantle in both regions is greater than that of the lower crust. This implies relatively lesser fluid/melt contents or poorer interconnection in the shallow mantle.