DIVERSE FEEDER SOURCES FOR THE PORTAGE LAKE VOLCANICS, MIDCONTINENT RIFT SYSTEM (Invited Presentation)

The Portage Lake Volcanics (PLV), a sequence of subaerially erupted basalt flows as much as 5 km thick, are the culminating major volcanic phase of the Midcontinent Rift System. The PLV erupted between ~1096 and ~1094 Ma and are exposed on both sides of a central basin in western Lake Superior at Isle Royale on the north and the Keweenaw Peninsula on the south. ‘Named’ flows on both Isle Royale and the Keweenaw Peninsula can be thick and laterally extensive. Early workers theorized that PLV flows erupted from fissures along the rift axis and spread outward across a central subsiding basin. Legacy seismic reflection profiles in the western lake, however, have generally continuous reflections that offer no evidence of axial volcanic vents cutting older flows, suggesting that feeder dikes and vents must have been located along the margins of the volcanic basin, perhaps along areas of active extension. Lane (1911) first recognized broad stratigraphic similarities between Isle Royale and the Keweenaw and correlated several thick and persistent PLV flows across the lake, such as the voluminous Greenstone flow. Many flows, however, cannot be correlated between Isle Royale and the Keweenaw, indicating diverse sources for at least some of the PLV stratigraphic section. A series of rift-parallel dikes exposed on land in Ontario and imaged with aeromagnetic data may represent a zone of feeding fissures for flows unique to Isle Royale. A similar feeder zone is less certain for the Keweenaw Peninsula as few dikes are known. Reversed faulting during a late (beginning ~1060 Ma) compressional stage uplifted PLV basalt flows and overlying sedimentary rocks along the Keweenaw fault, carrying older basalt over younger sedimentary rocks. This reversed motion, with an estimated throw of more than 5 km, would have carried PLV rocks over a rift-parallel marginal volcanic feeder zones, consequently obscuring much of this proposed feeder vent system. The location and extent of increasingly younger thick named flows on the Keweenaw Peninsula suggest that the locus of eruptive activity may have shifted north along a linear vent system with time, from near the center of the peninsula to an area now offshore of its tip.

Lane, A.C., 1911, The Keweenaw series of Michigan: Michigan Geological and Biological Survey Publication 6, Geological Series 4, 983 p