The Response to Rifting by the North Shore Volcanics in Minnesota

Abstract

The Midcontinent Rift is a 1.1 billion year old feature which has been defined primarily by geophysical studies. Many studies have examined the rift from a petrologic and geophysical view, but few have examined the structural geology in any detail. This study examines the structural aspects of the rift in Minnesota and elucidates the orientations of the principal stresses during the 100 Ma of igneous activity. In contrast to the geology in Michigan, the North Shore of Minnesota has quite a complex history. In Michigan, pulsations of volcanism alternated with periods of erosion and deposition. However, in Minnesota, these volcanic pulses were often cogenetic with emplacement of intrusive complexes, as well. With the midcontinent region initially in a tensional regime, these units were more easily emplaced, but not without some net stress imposed on the existing rock units. Since compression of the area is documented by the significant reverse motion along the Keweenaw, Isle Royale, Douglas, and Lake Owen faults (Cannon et al. 1989), it is likely this motion has been translated and modified by other activities along the North Shore of Minnesota.