GSA GEOPHYSICS DIVISION GEORGE P. WOOLLARD LECTURE: CRACKING OF A CONTINENT

George Prior Woollard discovered the Midcontinent Geophysical Anomaly nearly 80 years ago during a transcontinental gravity and magnetic anomaly traverse. He and his students at the University of Wisconsin subsequently mapped this intense gravity anomaly and identified it as originating from a rift that cracked the lithosphere of Laurentia 1100 m. y. ago. The rift, arguably the most interesting geological structure of the North American midcontinent, has been the locus of intense geophysical, geological, and geochemical studies since its discovery. Investigations were initially concentrated on the surface rocks in the Lake Superior Structural Basin where the rift crops out, but were rapidly expanded to the arms of the rift that extend to the south beneath the cover of Phanerozoic sedimentary rock. Seismic reflection profiling by COCORP, GLIMPCE, and the petroleum exploration industry, gravity and magnetic modeling, and deep drilling into the rift rocks have shown that the rift disturbed the entire crust and consists of segmented basins containing up to 30 km of volcanic and clastic sedimentary rocks in Lake Superior. The structurally and thermally weakened rift crust was subsequently subjected to compressional forces associated with the nearby Grenville Orogeny causing inversion of the movement on the rift faults and other compressional structures. The basins of the rift arms have an average width of the order of 55 km with adjacent largely clastic-rock sag basins and the entire rift extends for at least 2000 km over the midcontinent. Numerous questions remain regarding the extent of the rift, its origin, and evolution. However, we have entered a new era in the rift studies involving enhanced intensity and improved methodologies that are being used by academic science, the U.S. Geological Survey’s Characterization of the Midcontinent Rift and Related Mineral Deposits Initiative, and seismic and magneto-telluric studies associated with the national EarthScope program. The results of these investigations together with new and improved isotopic age, paleomagnetic, geochemistry, and modeling studies offer the promise of greatly increasing our understanding of this most interesting geological feature.