P Wave Teleseismic Traveltime Tomography of the North American Midcontinent

The remains of the 1.1-Ga Midcontinent Rift (MCR) lie in the middle of the tectonically stable portion of North America. Previous and ongoing studies have imaged strong heterogeneity associated with the MCR in the crust but have not imaged such within the mantle. It is unclear whether this is due to the absence of rift-related mantle structures or these studies had insufficient resolution to image them. To address this issue, we measured 46,374 teleseismic P wave delay times from seismograms recorded by the USArray Transportable Array, Superior Province Rifting EarthScope Experiment, and surrounding permanent stations. We included these and 54,866 delay times from prior studies in our tomographic inversion. We find that high-velocity anomalies are widespread in our study area, but there are also prominent low-velocity anomalies. Two of these are coincident with high-Bouguer gravity anomalies associated with the MCR in Iowa and the Minnesota/Wisconsin border at 50- to 150-km depth. Extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift-related material that “underplated" the crust, although we cannot exclude that the subcrustal mantle lithosphere beneath the MCR is anomalously enriched, hydrated, or warm. Other anomalies occur at syntaxes of the Penokean Orogen. One with the Superior Province and Marshfield Terrane in southern Minnesota and another with the Yavapai and Mazatzal Terranes, both at 100- to 250-km depth. In the midmantle, we image two linear high-velocity anomalies, interpreted as subducted fragments of the Farallon and Kula plates.