REFLECTIONS ON THE MIDCONTINENT RIFT SYSTEM (Invited Presentation)

The source of the Lake Superior structural basin and the intense geophysical anomalies extending from the basin into central Kansas and southeastern Michigan has been accepted for the past half-century as a Mesoproterozoic aborted rift, the Midcontinent Rift System. Despite the prominence of compressional features in the surface geology of the basin where the rift rocks crop out, deep seismic profiling and sparse deep drilling testify to the rift origin of the basalt- and clastics-filled basins that reach up to 30 km in thickness. The rift and the post-volcanic sag basins of clastic rocks that developed over it and its margins were subject to compressional forces resulting in inversion of the throw of rift faults and other compressional features. The volcanic rocks, perhaps reaching a volume of 2 x 10⁶ km³, together with intrusive mafic rocks are the source of the intense gravity and magnetic anomalies that mark the arms of the rift. The gravity highs commonly are bordered by negative anomalies derived from both thickening of the crust and marginal clastics-filled basins. Magmatic activity may have begun as early as 1150 Ma, but volcanism in the rift basins was primarily active only during a period of roughly 15 m.y. from 1109 to 1106 and 1100 to 1094 Ma, but was locally more intense in some segments of the rift at any particular time.

Although we have learned much about the rift, numerous questions remain about its origin, nature, and evolution and new interpretations challenge conventional concepts. These include the postulated extensions of the rift into Oklahoma and southeasterly into Ontario and perhaps south through Ohio and Kentucky and beyond, the extent and origin of intrusions along the rift, the age and duration of the extensional forces, the source, nature, age, and impact of the late-stage compressional forces, and the role of the mantle in the rifting process and the origin of volcanic rocks within and marginal to the rift. The journey to solve these and related questions is complicated by numerous pitfalls, but increasingly available geochemical and geological data, enhanced age dating, process and geochemical modeling, and geophysical results offer hope for answering them.