Distinct crustal structure of the North American Midcontinent Rift from P wave receiver functions

Eighty-two broadband seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) collected 2.5 years of continuous seismic data in the area of the high gravity anomaly associated with the Midcontinent Rift (MCR). Over 100 high-quality teleseismic earthquakes were used for crustal P wave receiver function analysis. Our analysis reveals that the base of the sedimentary layer is shallow outside the MCR, thickens near the flanks where gravity anomalies are low, and shallows again in the MCR's center where the gravity anomalies peak. This pattern is similar to that found from local geophysical studies and is consistent with reverse faulting having accompanied the cessation of rifting at 1.1 Ga. Intermittent intracrustal boundaries imaged by our analysis might represent the bottom of the MCR's mostly buried dense volcanic layers. Outside the MCR, the Moho is strong, sharp, and relatively flat, both beneath the Archean Superior Province and the Proterozoic terranes to its south. Inside the MCR, two weaker candidate Mohos are found at depths up to 25 km apart in the rift's center. The intermediate layer between these discontinuities tapers toward the edges of the MCR. The presence of this transitional layer is remarkably consistent along the strike of the MCR, including beneath its jog in southern Minnesota, near the Belle Plaine Fault. We interpret these results as evidence for extensive underplating as a defining characteristic of the rift, which remains continuous along the Minnesota jog, where due to its orientation, it is minimally affected by the reverse faulting that characterizes the NNE striking parts of the rift.