Altered Mantle Fabric Beneath the Mid‐Continent Rift

Abstract

AbstractWe present new, densely sampled shear‐wave splitting results from southern Minnesota and adjacent areas of neighboring states, sampling the southwestern limit of the Archean Superior Province and straddling the Proterozoic Mid‐Continent Rift (MCR). The new measurements include data from the Earthscope Transportable Array (TA) as well as the Superior Province Rifting Earthscope Experiment (SPREE), yielding 99 new station‐averaged measurements. The split times show a consistent decrease from 1.1 s in the NE to 0.2 s in the SW, with the lowest values being associated with the Minnesota River Valley Terrane (MRVT). From modeling and other geophysical constraints, we interpret the split time variations to represent variations in fabric strength within a thick lithosphere, rather than lithospheric thinning or a multi‐layered effect, and propose that the weak fabric of the MRVT is associated with a different mechanism of formation than elsewhere in the Superior Province. The fast directions we measure range from NNE‐SSW to E‐W and vary on a shorter length scale than the split times, with a pattern of NE‐SW splits that closely follows the axis of the MCR. We interpret this as a perturbation of the net fast direction due to anisotropy in an underplate along the rift.