Varying lithospheric fabric in midcontinental North America: the Superior Province and the Mid-Continent Rift

Thick continental regions with a strong accretionary fabric have been found to exhibit strong seismic anisotropy, as detected by shear-wave splitting measurements. A good example of this phenomenon is the western Superior Province, central North America, where a well-developed accretionary pattern of thin tectonic belts aligns with strong SKS splits in previous studies, in places exceeding 2 s split time. However, the boundaries of the preserved lithospheric fabric may not necessarily correspond to current crustal contacts. We examine the edges of the western Superior mantle anomaly along its western, southern and eastern edges, through a data set from the Earthscope Transportable Array, the Manitoba Teleseismic Array, and some preliminary measurements from the Canadian portion of the Superior Province Rifting Earthscope Experiment (SPREE). We find that the boundaries of the Superior feature correspond to the edges of a high-velocity lithospheric feature seen in teleseismic tomography; the western edge lies ca. 200 km east of the crustal contact, while to the south there are rapid changes in split time over a short distance. Within the Minnesota River Valley Terrane (MRVT), the southernmost subprovince of the Superior, splitting is very weak (less than 0.5 s), suggesting that little or no lithospheric fabric is present, and supporting the hypothesis that the MRVT developed through vertical tectonics rather than accretion. Preliminary results from the SPREE data set showing the influence of the Mid-Continent Rift on lithospheric fabric will also be shown.