North-Central Section - 50th Annual Meeting - 2016

Paper No. 15-10
Presentation Time: 4:45 PM


STEIN, Carol A.1, STEIN, Seth2, KLEY, Jonas3, KELLER, G. Randy4, WIENS, Douglas5, WYSESSION, Michael6, ALEQABI, Ghassan7, SHEN, Weisen5, MERINO, Miguel8, FREDRICKSEN, Andrew9, DARBYSHIRE, Fiona10, JURDY, Donna11, WAITE, Greg12, ROSE, William I.13, ROONEY, Tyrone14, MOUCHA, Robert15 and BROWN, Eric16, (1)Earth & Environmental Sciences, University of Illinois at Chicago, Chicago, IL 60607, (2)Earth & Planetary Sciences, Northwestern University, Evanston, IL 60208-3130, (3)Geoscience Center, University of Goettingen, Goldschmidtstr. 3, Goettingen, 37077, Germany, (4)School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd, Norman, OK 73019, (5)Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130-4899, (6)Earth and Planetary Sciences, Washington University, Campus Box 1169, 1 Brookings Dr, St. Louis, MO 63130, (7)St. Louis, MO 63130, (8)Chevron Corporation, 1400 Smith Street, Houston, TX 77002, (9)Geological Sciences, University of Manitoba, , Winnipeg, MB R3T 2N2, Canada, (10)Geotop, University of Quebec, Montreal, QC H3C3P8, Canada, (11)Earth and Planetary Sciences, Northwestern University, Locy Hall, 1850 Campus Drive, Evanston, IL 60208, (12)Gmes, Michigan Technological University, 1400 Townsend Dr, Houghton, MI 49931, (13)Michigan Technological University, Houghton, MI 49931, (14)Geological Sciences, Michigan State University, 206 Natural Sciences, East Lansing, MI 48824, (15)Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, (16)Department of Geoscience, Aarhus University, Aarhus, 8000, Denmark,

Rifts are segmented linear depressions, filled with sedimentary and igneous rocks, which form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces (LIPs), are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not filled with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. The Midcontinent Rift (MCR) is an unusual combination. Its 3000-km length formed as part of the 1.1 Ga rifting of Amazonia (Precambrian NE South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established, but it contains an enormous volume of igneous rocks. MCR volcanics are significantly thicker than other flood basalts, due to deposition in a narrow rift rather than a broad region, resulting in a rift geometry with a LIP's magma volume. Structural modeling of seismic reflection data shows an initial rift phase where flood basalts filled a fault-controlled extending basin, and a postrift phase where volcanics and sediments were deposited in a thermally subsiding basin without associated faulting. The crust thinned during rifting and rethickened during the postrift phase and later compression, yielding the present thicker crust. EarthScope data show two surprising results. Although the MCR's dense volcanic rocks appear clearly on gravity, seismic reflection, and receiver function data, which are sensitive to variations in density, surface wave tomography "sees" primarily low-velocity sediments, with no strong anomaly below. Presumably compared to the surrounding crust, the basalt rift fill is denser but has similar or slightly lower S-wave velocity. Another surprise is that mantle below the MCR shows no significant velocity anomaly or anisotropy, suggesting melt depletion had little effect on seismic velocities and likely reflecting the fact that much of the volcanism occurred after extension ended.