Title A clearer look at a giant scar that underlies the American Midwest

Award Number EAR- 0952154 (2010-2014)

Collaborators

Weisen Shen, Douglas A Wiens,

Ghassan Aleqabi, Michael Wysession, Suzan van der Lee, Hao Zhang, Seth Stein, Andrew Frederiksen, Fiona Darbyshire, Emily Wolin and Trevor Bollmann

Project Highlights

Buried beneath the fertile flat farmland of the midwest, a huge scar gives rise to the most prominent gravity and magnetic anomalies within the old and stable core of the North America continent. The Midcontinent Rift (MCR) was the location of an episode of extension between eastern and western North America, and is now filled by a large volume of ~ 1.1 Ga old igneous rocks. Recent deployments of the Earthscope/USArray and the Superior Province Rifting Earthscope Experiment (SPREE) flexible array experiment enabled us to perform a comprehensive seismic investigation of the crustal and uppermost mantle structure in the vicinity of the MCR, including the part in the southern Superior craton, and provide a better understanding of MCR geology. Receiver function images show a complex structure at the depth of the typically simple, sharp boundary between crust and mantle (see Shen et al., 2013, and Zhang et al., 2016), perhaps representing a distinct layer of magmatic underplating beneath the rift. Surface wave tomography reveals the spatial distribution of this anomalous crustal underplating structure, extending beneath the Lake Superior (Fig. 2). By incorporating these receiver function and surface wave data, a refined seismic model for the MCR area is being constructed. Combining these complementary seismic observables improves our shear velocity structure of the crust and mantle. A joint Bayesian Monte-Carlo inversion finds a distinct layer with shear wave speed between 4.0 and 4.4 km/sec associated with the Moho beneath the MCR, further confirming crustal underplating associated with rifting.

Location Keywords: Midwest, Central US, Lake Superior

Figure 1. (a) SPREE array stations (red triangles) run along and across the gravity anomaly (blue) that is the rift. These stations are much closer together than the USArray ones (blue triangles). Images of the receiver function show a gradient rather than sharp Moho beneath the rift (b), which may be due to magma trapped at the Moho or in the lowermost crust/uppermost mantle (c).

Figure 2. Phase speed map for a type of surface wave called a Rayleigh wave. The speed mapped is for a Rayleigh wave with a periodicity of 20 s (a wavelength of 72 km). This wave's speed is sensitive to the stiffness of the lower crust and the density of the upper crust. The map shows that the buried layers of dense volcanic rock have an impact on this Rayleigh wave's speed and that these layers extend along the entire rift, to Lake Superior.

Photo: co-PI Doug Wiens drilling holes for solar-panel frames for SPREE stations

References:

Shen, Weisen, Michael H. Ritzwoller, and Vera Schulte‐Pelkum. "Crustal and uppermost mantle structure in the central US encompassing the Midcontinent Rift." Journal of Geophysical Research: Solid Earth 118, no. 8 (2013): 4325-4344.

Zhang, H., et al. (2016), Distinct crustal structure of the North American Midcontinent Rift from P wave receiver functions, J. Geophys. Res. Solid Earth, 121, doi:10.1002/2016JB013244.