We present new, densely sampled shear-wave splitting results from southern Minnesota and adjacent areas of neighboring states, sampling the southwestern limit of the Archean Superior Province and straddling the Proterozoic Mid-Continent Rift (MCR). The new measurements include data from the Earthscope Transportable Array (TA) as well as the Superior Province Rifting Earthscope Experiment (SPREE), yielding 99 new station-averaged measurements. The split times show a consistent decrease from 1.1 s in the NE to 0.2 s in the SW, with the lowest values being associated with the Minnesota River Valley Terrane (MRVT). From modeling and other geophysical constraints, we interpret the split time variations to represent variations in fabric strength within a thick lithosphere, rather than lithospheric thinning or a multi-layered effect, and propose that the weak fabric of the MRVT is associated with a different mechanism of formation than elsewhere in the Superior Province. The fast directions we measure range from NNE-SSW to E-W and vary on a shorter length scale than the split times, with a pattern of NE-SW splits that closely follows the axis of the MCR. We interpret this as a perturbation of the net fast direction due to anisotropy in an underplate along the rift.

中文翻译：

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中部大陆裂谷下的改造地幔织物

我们展示了来自明尼苏达州南部和邻近州的新的、密集采样的横波分裂结果，采样了太古代苏必利尔省的西南边界并跨越了元古代中大陆裂谷 (MCR)。新的测量包括来自地球镜可移动阵列 (TA) 以及苏必利尔省裂谷地球镜实验 (SPREE) 的数据，产生了 99 个新的站点平均测量。分割时间显示从东北的 1.1 秒到西南的 0.2 秒持续减少，最低值与明尼苏达河谷地体 (MRVT) 相关。从建模和其他地球物理约束，我们解释分裂时间变化以表示厚岩石圈内结构强度的变化，而不是岩石圈变薄或多层效应，并提出 MRVT 的薄弱结构与与苏必利尔省其他地方不同的形成机制有关。我们测量的快速方向范围从 NNE-SSW 到 EW，并且在比分裂时间更短的长度尺度上变化，NE-SW 分裂模式紧跟 MCR 的轴。我们将此解释为由于沿裂谷的底板中的各向异性而导致的净快速方向的扰动。