Title: One billion year-old Mid-continent Rift leaves no clues in the mantle

NSF award number: EAR-0952345 ("SPREE") 2010-2014

Collaborators:

Trevor A. Bollmann1, Emily Wolin1,

Suzan van der Lee1, Andrew W. Frederiksen4,

Justin Revenaugh2, Douglas A. Wiens3, Fiona A. Darbyshire5, Ghassan I. Aleqabi3, Michael E. Wysession3, Seth Stein1, and Donna M. Jurdy1

1. Northwestern University

2. University of Minnesota

3. Washington University

4. University of Manitoba

5. University of Quebec at Montreal

Summary:

Seismic stations of the Superior Province Rifting Earthscope Experiment (SPREE) recorded seismic waves from distant earthquakes for two and a half years. SPREE stations were installed in Minnesota, Wisconsin, and Ontario, around the Mid-continent Rift, a feature distinguished by voluminous one billion year-old lava flows. These lava flows along with deeper, "underplated" igneous rocks at the bottom of the crust (see nugget by Zhang et al.) slightly slow these seismic waves down compared to seismic waves recorded by stations away from the Mid-continent Rift.

We measured the differences in arrival times between P waves from distant earthquakes and mapped them into a 3-dimensional image of structures in the Earth's mantle (Bollmann et al., 2018), using a technique called seismic tomography. Bollmann's seismic-tomographic image extends more than 1000 km into the mantle, where it shows fascinating structures related to fossil ocean floors and and a previously little-recognized E-W contact within the mantle beneath Ontario, which is billions of years old.

Prior seismic-tomography research showed that some mantle structures broadly correlate with the geologic age of basement rocks in the crust, but that this correlation is not as strong within geologic domains older than 500 millon years (Van der Lee and Nolet, 1997). However, Wolin (2016) included full-waveform seismograms from SPREE into a new continental-scale seismic-tomographic image, which does show correlations between subcrustal mantle structure and geologic basement age within these older domains (Figure 1). The new image also shows that the mantle did not preserve structures related to the Mid-continent Rift.

Earthscope instrumentation and facilities: Portable broadband seismic instrumentation for 83 seismic stations from Earthscope's Flexible Array pool, as well as two weeks of a technician from IRIS' PASSCAL center.

Location tags: Minnesota, Wisconsin, Ontario

Figure 1: "Seismic-geologic" units mapped by Wolin (2016) via seismic tomography of full-waveform seismograms from SPREE and other Earthscope and pre-Earthscope data.

Figure 2: P wave seismic-tomography image by Bollmann et al. (2018). Blue structures speed upP waves, red structures slow them down. SPREE stations are represented by tiny squares.

Photos: Field experiment co-managers Emily Wolin (top) and Trevor Bollmann (bottom). Top photo taken in the spring in Wisconsin and bottom photo taken in the fall in Ontario, both while servicing SPREE stations.

References:

Bollmann, T., et al. (2018), P-wave Teleseismic Traveltime Tomography of the North American Midcontinent , submitted for publication to J. Geophys. Res. Solid Earth.

Wolin, E. L. G. "Structure, seismicity, and instrumentation of stable North American lithosphere." PhD diss., Northwestern University, 2016.

Van der Lee, S., and G. Nolet, "Upper mantle S velocity structure of North America." Journal of Geophysical Research: Solid Earth 102, no. B10 (1997): 22815-22838.

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.