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 details of the rift and a new constraint for modelling associated gravity anomalies. In turn, gravity modelling, constrained also by new high-resolution aeromagnetic data, has permitted critical examination of the seismic models. The latter generate gravity anomalies having limited agreement with observed anomalies when appropriate rock densities are assigned. Good agreement may be achieved, generally, by making comparatively local changes to the models, while retaining their larger-scale attributes. Gravity modelling thus enhances and supports GLIMPCE seismic models. Modifications to seismic models include revisions of initial densities within the geometrical framework of the models, leading to a redefinition of lithologies. For example, in some segments of the rift, mafic volcanics are substituted for Keweenawan sedimentary and sedimentary-volcanic sequences and for Lower Proterozoic sediments, and a felsic igneous body is modelled within a mafic volcanic unit. Positions of some unit boundaries and faults, or segments thereof, have also 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 seismic images and, thereby, arguments for rift development controlled by growth faults. Modelling also indicates a requirement for large, buried masses of mafic (plutonic?) igneous rocks of presumed Keweenawan age along the northern margin of the rift. This imparts an asymmetry to the rift, with northern and southern margins dominated by plutonic and volcanic igneous rocks, respectively.