GLIMPCE SEISMIC REFLECTION EVIDENCE OF DEEP-CRUSTAL AND UPPER MANTLE INTRUSIONS AND MAGMATIC UNDERPLATING ASSOCIATED WITH THE MIDCONTINENT RIFT SYSTEM (MRS) OF NORTH AMERICA – ANOTHER LOOK

“Deep-crustal and Moho reflections recorded on vertical incidence and wide-angle ocean bottom seismometer (OBS) data in the 1986 (Great Lakes International Multidisciplinary Program on Crustal Evolution) experiment provide evidence for magmatic underplating and intrusions within the lower crust and upper mantle contemporaneous with crustal extension in the Midcontinent Rift system at 1100 Ma. The rift fill consists of 20-30 km (7-10s) of basalt flows, secondary synrift volcaniclastic, and post-basalt sedimentary rock. Moho reflections recorded in Lake Superior over the Midcontinent Rift system have times from 14-18 s (about 46 km to as great as 58 km) in contrast to times of about 11-13 s (about 36-42 km of crustal thickness) beneath the surrounding Great Lakes. The seismically complex deep-crust to mantle transition zone (30-60 km) in north-central Lake Superior, which is 100 km wider than the rift half graben, reflects the complicated products of tectonic and magmatic interaction of lower-crustal and mantle components during evolution or shutdown of the aborted Midcontinent Rift. In effect mantle was changed into crust by lowering seismic velocity (through intrusion of lower density magmatic rocks) and increasing Moho (about 8.1 km/s) depth.” From J.C. Behrendt, D.R. Hutchinson, M. Lee, C.R. Thornber, A. Trehu, W. Cannon and A. Green, 1990, Tectonophysics.

Hutchinson et al., 1990 interpreted the MRS as resulting from a mantle plume – the Keweenaw Hot Spot. Here I reexamine the MRS seismic data in comparison to other plume related rifts – specifically the West Antarctic Rift.