DISTINCT CRUSTAL STRUCTURE OF THE NORTH AMERICAN MID-CONTINENT RIFT FROM P WAVE RECEIVER FUNCTIONS

Eighty-two flexible-array 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 Mid-continent Rift (MCR). Among earthquakes recorded in this time period are over a hundred teleseismic ones with sufficient data quality for crustal P wave receiver function analysis. Our receiver function 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 active-source seismic and gravity surveys and is consistent with reverse faulting having accompanied the cessation of rifting 1.1 billion years ago. Intermittent intra-crustal boundaries resulting from our analysis might represent the bottom of the MCR’s mostly buried dense volcanic layers.

Outside of 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. This intermediate layer between the candidate Mohos tapers towards the edges of the MCR. The presence of this transitional layer is remarkably consistent along the strike of the MCR, including beneath the 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 and the rift remaining continuous throughout its jog in southern Minnesota, though with less reverse faulting than along the NNE striking parts of the rift.