THE NORTH AMERICAN MIDCONTINENT RIFT SYSTEM-AN OVERVIEW OF A CENTURY OF STUDIES

The first comprehensive description of the flood basalts and red clastic rocks around Lake Superior was by R.D. Irving of the USGS in 1893. Research during the ensuing century has defined the Midcontinent Rift System (MRS), a 2,200 km-long feature extending from the Lake Superior region southwestward to Kansas and southeastward to southern Michigan. From the turn of the century through the 1950's studies focused on the Keweenaw Peninsula of Michigan because of economically important native copper deposits. Details of the geology and mines were published by Butler and Burbank of the USGS in 1929 with additional knowledge provided by surface and underground mapping led by Walter White of the USGS in the 1940's and 1950's. Concurrently, continental-scale gravity and magnetic surveys showed that strong positive anomalies extend from the Lake Superior region and track the basalts beneath Paleozoic strata to define the full extent of the MRS. Recent decades have seen a proliferation of studies of petrochemistry, radiometric ages, potential field and seismic surveys, and deep drilling, which together have produced our current understanding of the MRS as a rift that transects older terranes of the North American craton. The GLIMPCE program in the late 1980's, a consortium of USGS, GSC, and several university, state, and provincial research groups, produced deep seismic reflection and refraction images of the rift. These confirmed that basalt sequences are as much as 30 km thick and that older crust was nearly completely separated during synvolcanic extension. Seismic and age-dating studies showed that an enormous volume of flood basalt was erupted from1108-1094 Ma. This great volume of basalt erupted during a short period, and its primitive isotopic signature are consistent with onset of a new mantle plume beneath the area at about 1110 Ma. The plume head provided an anomalously hot and petrologically fertile source for production of basaltic magma by decompression melting. Deposition of more than 5 km of mostly fluvial sediments continued for a few tens of millions of years after volcanism and extension ceased. The extensional phase of the rift was followed by compression and partial inversion that culminated at about 1060 Ma, probably a result of concurrent compressive deformation in the Grenville Province to the east.