THE MYSTERIOUS US MIDCONTINENT: A GEOLOGIC HISTORY PRESERVED IN NEGATIVE TOPOGRAPHY (Invited Presentation)

Plate-interior geology of the US remains poorly understood, because there is relatively little direct access to bedrock, and the spatial resolution of existing geophysical studies is insufficient. Yet, hidden below the Midcontinent’s mostly flat and unassuming views, significant vertical topography characterizes subsurface geology that challenges the plate tectonics paradigm of plate rigidity. The region includes failed rifts that deeply fractured the lithosphere, topography of the Precambrian/Paleozoic unconformity, networks of fault-and-fold zones that have undergone repeated reactivation, intracratonic sedimentary basins with depocenter depths that rival those of foreland basins, and distinct seismic zones, some of which have hosted great earthquakes. All of these features emphasize that the interior of the plate, far from plate boundaries has remained tectonically active and that plate interiors, even when underlain by cratonic crust, are not rigid. EarthScope studies contributed significantly to understanding the deeper, sub-crustal structure of the eastern US, but, literally, did not scratch the surface. A scattered and often inaccessible record of industry seismic lines and drill core provides tantalizing hints to crustal complexity in the region, but the link between deep structure imaged by EarthScope and shallow structure of interest to geologists, can only be made by undertaking a campaign of high-resolution seismic reflection, magnetotellurics, gravity and magnetism that spatially resolves links between upper-crustal and deeper features. The results of such work would provide major new understanding of the region’s 1+ billion years of geology and fundamental insights into plate-interior processes.