SEISMIC ANISOTROPY ACROSS AND ALONG THE MID-CONTINENT RIFT

Seismic anisotropy in the lithosphere provides a record of strain history related to past tectonic events. The ca. 1.1 Gz Mid-Continent Rift (MCR) in central North America cross-cuts a series of Archean and Proterozoic tectonic provinces with varying tectonic styles; from 2011-2013 it was instrumented by a dense seismic array via the Superior Province Rifting Earthscope Experiment (SPREE). We perform shear-wave splitting analysis on SPREE stations following three transects across and along the MCR. The northern (SN) line shows an average split time of 1.03 s and an average fast azimuth of 73°, while the southern (SS) line has an average split time of 0.52 s and an average fast direction of 84°; the north-south line (SM) connecting them shows a steady northward increase in split time. The fast directions show modest change near the MCR boundaries, with the 65° fast-direction contour bracketing the rift axis. The modest changes in split time related to the MCR are consistent with tomographic images showing no major lithospheric anomaly associated with the MCR, though the slight rotation in fast axis suggests that a rifting did affect the net fabric of the lithosphere, either through recrystallization of lithospheric mantle or due to a contribution from the Moho underplate detected in receiver-function studies of the MCR. The large change between the SN and SS transects suggests that they sample different pre-existing lithospheric fabrics, with the northern line being more characteristic of western Superior Province lithosphere, and the southern line resembling the lithosphere beneath the Minnesota River Valley Terrane.