The Geoframe Midcontinent Transect: An Opportunity to Study the Effects of the ~1.4 Ga Magmatic Event on the Lithospheric Evolution of the Continent

A distinctive aspect of the Proterozoic history of the southern midcontinent is the extensive belt of felsic igneous rocks of Mesoproterozoic age commonly termed the Granite-Rhyolite Province. Granitoids of this belt are "anorogenic" or A-type granite and rhyolite and so dominate the subcrop that the earlier Paleoproterozoic rocks are rarely sampled over vast areas. Moreover, recent dating shows a much larger subcrop of ~1.1 Ga igneous rocks in the southwestern U.S. than previously thought. The probable association of 1.4 Ga granites, and the definite association of 1.4 Ga granites with smaller amounts of coeval mafic rocks has led a number of workers to conclude that these magmatic events occurred during a mildly extensional tectonic regimes. The 1.4 Ga magmatic event has been proposed to occur in response to the stabilization of a Proterozoic supercontinent. In fact, this event is often cited as a prime example of a continental stabilization process. It is presumed that the structure and composition of the lithosphere was profoundly modified by this event, yet we have little direct information on the nature of this modification or about tectonic activity such as faulting and basin formation that may have accompanied the magmatism. The Geoframe midcontinent transect offers an excellent opportunity to explore the nature and origin of this event. Existing COCORP and industry seismic reflection data show evidence for Precambrian basins, interpreted to contain volcaniclastics as well as intriguing reflections from within the upper mantle. The basin interpretation has never been corroborated with seismic velocity constraints. The origin and possible tectonic significance of the mantle reflections is not known. Geochronologic and geochemical analysis from wells that sample basement could shed new light nature of this event as it has recently in the Texas Panhandle. Scant refraction data point to the possible occurrence of a 20-km-thick-layer of material with velocities significantly greater than 7 km/s at the base of the crust. If proven to be present, this layer might represent the mafic underplate that triggered the felsic magmatism. Such a major magmatic event that involved fractionation of mafic material and/or melting of continental crust should also have had a profound signature in the mantle that might be detectable with transportable array data.