AN INTERPRETATION OF GRAVITY-ANOMALIES OVER THE MIDCONTINENT RIFT, LAKE-SUPERIOR, CONSTRAINED BY GLIMPCE SEISMIC AND AEROMAGNETIC DATA

Cross sections of the Midcontinent Rift in Lake Superior, derived from GLIMPCE seismic reflection images, provide unprecedented structural d etails of the rift and a new constraint for modelling associated gravi ty anomalies. In turn, gravity modelling, constrained also by new high -resolution aeromagnetic data, has permitted critical examination of t he seismic models. The latter generate gravity anomalies having limite d agreement with observed anomalies when appropriate rock densities ar e assigned. Good agreement may be achieved, generally, by making compa ratively local changes to the models, while retaining their larger-sca le attributes. Gravity modelling thus enhances and supports GLIMPCE se ismic models. Modifications to seismic models include revisions of ini tial densities within the geometrical framework of the models, leading to a redefinition of lithologies. For example, in some segments of th e rift, mafic volcanics are substituted for Keweenawan sedimentary and sedimentary-volcanic sequences and for Lower Proterozoic sediments, a nd a felsic igneous body is modelled within a mafic volcanic unit. Pos itions of some unit boundaries and faults, or segments thereof, have a lso been modified. Gravity modelling traces the paths of the Keweenaw, Isle Royale, Thiel, Douglas, and Michipicoten Island faults deep into the crust, generally supporting the configurations outlined by seismi c images and, thereby, arguments for rift development controlled by gr owth faults. Modelling also indicates a requirement for large, buried masses of mafic (plutonic?) igneous rocks of presumed Keweenawan age a long the northern margin of the rift. This imparts an asymmetry to the rift, with northern and southern margins dominated by plutonic and vo lcanic igneous rocks, respectively.