Potential field studies of continental rifts—the Great Lakes region: Introduction

Abstract

The ongoing tectonic activity of modern continental rifts commonly results in striking physiographic forms, intense volcanism, and seismicity, all of which are of special interest to geoscientists. As a result, they have been investigated since the very inception of scientific geological studies. Continental and oceanic rifts are fundamental elements of modern plate tectonic theory. Plate tectonics, apparently, has operated in a similar fashion for at least 2500 Ma and ancient rifts date back to Early Proterozoic time. These paleorifts, or more specifically the remnants thereof, are generally no longer active under an extensional stress field. An outstanding example is the roughly 2000 km long North American Midcontintent Rift (MCR) System, dated at about 1100 Ma. For most of its length it remains buried beneath Phanerozoic sedimentary cover of the continental interior. The rift surfaces only in the Great Lakes region, mainly around the shores of Lake Superior, and developmental theories relating to the rift have been established primarily on the basis of geological and geophysical data obtained in this area. The series of papers presented in this volume focuses on the Lake Superior segment of the MCR. Paleorifts are of interest for both scientific and practical reasons. Ancient rift faults are potential zones of weakness that may be reactivated under ambient stress fields totally different from the original rift-causing extensional forces. As a result, paleorifts may be a seismic hazard in an otherwise aseismic craton. A corollary of this is that related structural deformation may be propagated upward, modifying the surface morphology, which in turn may control the lithofacies of sediments being deposited in overlying basins. These structures and lithologic variations may have an important role in the hydrologic regime of sedimentary basins and the entrapment of hydrocarbons. These basins are often the host for important petroleum deposits, as a significant proportion of the world's known petroleum reservoirs occurs in paleorifts. Furthermore, numerous mineral resources of sedimentary and hydrothermal origin occur within rift-fill assemblages, as well as within overlying, late-stage sedimentary basins. The study of modern rifts is important because of the oppor-