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 explained by a single layer of anisotropic fabric, which we interpret to be dominated by a lithospheric contribution. The fast S polarization directions are consistently ENE-WSW, but the split time varies greatly across the study area, showing strong anisotropy (up to 1.48 s) in the western Superior, moderate anisotropy in the eastern Superior, and moderate to low anisotropy in the terranes south of Lake Superior. We locate two localized zones of very low split time (< 0.6 s) adjacent to the MCR: one in the Nipigon Embayment, an MCR-related magmatic feature immediately north of Lake Superior, and the other adjacent to the eastern end of the lake, at the southern end of the Kapuskasing Structural Zone (KSZ). Both low-splitting zones are adjacent to sharp bends in the MCR axis. We interpret these two zones, along with a low-velocity linear feature imaged by a previous tomographic study beneath Minnesota and the Dakotas, as failed lithospheric branches of the MCR. Given that all three of these branches failed to propagate into the Superior Province lithosphere, we propose that the sharp bend of the MCR through Lake Superior is a consequence of the high mechanical strength of the Superior lithosphere ca. 1.1 Ga.
Bibliographical noteFunding Information:
Instrumentation for the Canadian portion of the SPREE project was provided by Earthscope and the IRIS PASSCAL program, and deployed by the kind permission of Ontario Parks and individual landowners. Algoma University and Lakehead University generously provided work space during deployment. Instrument deployers included Ghassan Aleqabi, Miguel Merino, Mulu Serzu, Allen Wywrot, and Taras Zaporozan. The SPREE field project was funded by NSF grant EAR-0952345 ; additional NSF grant PIs were Donna Jurdy and Doug Wiens. Additional field work and support for Oyekunle Ola were provided by NSERC grant RGPIN/261640-2008 . Figures were prepared using Generic Map Tools (GMT); absolute plate motion was calculated using an online calculator provided by the University of Tokyo.
- Mid-Continent Rift
- Nipigon Embayment
- Shear-wave splitting
- Superior Province