SEISMIC REFLECTION IMAGING OF THE MIDCONTINENT RIFT AND ITS ROLE IN MODERN STUDIES OF THE PROTEROZOIC CRUSTAL EVOLUTION OF THE NORTH-CENTRAL UNITED STATES

The Midcontinent rift was identified by potential field and geologic studies in the 1950’s. However, much of the detail of this largely buried structure was obscure except in a few well-exposed areas around Lake Superior. From 1978 to 1986 a series of seismic reflection profiles were obtained by a combination of academic (COCORP), petroleum company, and government (GLIMPCE) research. Those profiles revealed a wealth of new details of the lateral and vertical extent of the rift and its internal structure and stratigraphy. Particularly significant was the discovery that thick sequences of flood basalts produced exceptionally strong and coherent reflections so that the volume of basalts could be accurately estimated. Other important advances in rift studies were proceeding concurrently. Highly precise U/Pb zircon dating from volcanic and intrusive rocks of the rift was delineating a detailed geochronology which, together with the seismic data, documented an extraordinarily rapid and voluminous, but short-lived, period of basaltic volcanism. Also, petrogenetic studies of volcanic rocks using Sr, Nd, and Pb isotopes, and rare-earth elements were identifying a history of evolving magmatic sources varying from undepleted mantle to MORB. These three concurrent lines of investigation combined to suggest that the Midcontinent rift formed in response to the arrival of a new mantle plume at the base of the lithosphere. The short-lived outburst of a great volume of basaltic magma, much of which was derived from undepleted mantle, was a result of large scale melting of a plume head that spread and eventually cooled beneath the north-central United States.