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.

The assembly of Laurentia by Precambrian accretion is also believed to have formed the underlying lithosphere. Accretionary signatures are detectable by seismic observations but subject to modification by later processes, e.g., orogeny, rifting, and plumes. We examine the Archean Superior Province (SP) and environs using a set of teleseismic P wave arrivals from Canadian and American instruments. The resulting tomographic model has high resolution beneath the Dakotas and Minnesota, provides a first look at the lithosphere beneath Manitoba, and sharpens previously documented features in Ontario. From the model and previous anisotropy observations, we detect (i) a large high‐velocity feature beneath the western SP, associated with elevated lithospheric anisotropy. The high‐velocity feature does not match crustal boundaries; notably, its western edge lies approximately 200 km east of the contact with the Proterozoic Trans‐Hudson Orogen (THO). (ii) A low‐velocity channel‐shaped feature strikes northwest through Minnesota and the Dakotas, associated with weakening anisotropy. (iii) High velocities southwest of (ii), beneath the Minnesota River Valley terrane (MRVT), associated with low anisotropy. We interpret (i) to be accretionary, and contemporaneous with Superior assembly; similar velocity but weaker anisotropy of the MRVT is consistent with vertical‐tectonic mechanisms. The inboard location of the THO contact may indicate modification of the Superior root. The low‐velocity channel has no obvious crustal expression but connects to an offset in the Proterozoic Mid‐Continent Rift (MCR) and may be rift related. The MCR is not well imaged but will be examined via the temporary Superior Province Rifting EarthScope Experiment, in progress.

The Superior Province Rifting Earthscope Experiment (SPREE) recorded continuous seismic data over the Midcontinent Rift from April 2011 through October 2013. Analysis of power spectral density (PSD) estimates shows that horizontal noise levels at periods > 20 s vary seasonally and diurnally. During winter, horizontal noise power at many SPREE stations is within 5 dB of nearby Transportable Array (TA) stations. As the ground thaws, SPREE stations in fine-grained material such as silt or clay become noisier due to changes in the mechanical properties of the soil. During summer, the daily mean PSD value of stations in fine-grained material is approximately 10-20 dB higher than in the winter, and daytime noise levels are 20-30 dB higher than nights. Stations in sandy material also show diurnal variations of 20-30 dB during summer, but the daily mean PSD value varies no more than 5-10 dB during the year. Most neighboring TA stations have relatively constant daily mean PSDs, and their horizontal components show summer diurnal variations of 10-15 dB. Some very quiet TA stations, such as SPMN, show a 5-10 dB increase in horizontal noise power during winter. The timing and amplitude of horizontal noise power variations between 20 and 800 s correlate with variations in atmospheric pressure PSDs. We propose that the grain size and pore water content of the material surrounding a shallow seismic station influences the local response to atmospheric pressure. Stations that must be placed in soft sediments should be installed in sandy, well-drained material to minimize long-period noise generated by atmospheric pressure variations.

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.

We present shear-wave splitting analyses of SKS and SKKS waves recorded at sixteen Superior Province Rifting Earthscope Experiment (SPREE) seismic stations on the north shore of Lake Superior, as well as fifteen selected Earthscope Transportable Array instruments south of the lake. These instruments bracket the Mid-Continent Rift (MCR) and sample the Superior, Penokean, Yavapai and Mazatzal tectonic provinces. The data set can be