INTEGRATED GEOPHYSICAL MODELING PROVIDES INSIGHTS INTO THE THREE-DIMENSIONAL GEOMETRY OF THE MIDCONTINENT RIFT IN WESTERN LAKE SUPERIOR

Reinterpretation and integration of legacy geophysical data provide new insights into the three-dimensional (3D) geometry of volcanic packages within the 1.1 Ga Midcontinent Rift System (MRS) beneath western Lake Superior. Modeling involves 11 seismic-reflection sections and aeromagnetic and gravity data, with constraints from seismic-refraction, magnetotelluric, heat-flow, and geologic studies. The integration of diverse data sets provides information on multiple aspects of the rift that cannot be achieved otherwise.

Reduced-to-pole aeromagnetic data (RTP data) provide guides for interpreting volcanic layers in between 2D seismic sections. The RTP transformation corrects for the offset of anomalies over their causative sources due to the variable orientation of the Earth’s magnetic field at different latitudes. Two end-member versions of RTP data were computed: one assuming no remanent magnetization, and the other assuming that remanence dominates. The strikes and dips of gradients of both RTP versions give good matches to strikes and dips of volcanic layers determined from crossing seismic lines. In contrast, the strikes and dips of gravity gradients have greater correspondence to the orientations of interpreted pre-rift structures. Moreover, despite high densities expected for the MRS volcanic layers, gravity highs are concentrated more commonly toward the sides of the seismically determined volcanic subbasins rather than over the greatest accumulations, suggesting that the gravity highs are caused by deeper sources. Aeromagnetic modeling can suggest where significant volumes of volcanic rock have strong, reversed-polarity remanence (R-polarity) when seismic interpretations are used as constraints. R-polarity for MRS rocks suggests they cooled prior to ~1100 Ma.

The overall view of 3D rift geometry of the MRS in the western lake shows an elongate, synformal volcanic subbasin that trends southwestward along the southern shore to where it expands toward the north shore into a bowl-like, deep subbasin. Volcanic layers dip into the subbasins from the shores and from two seismic highs that correspond to prominent gravity lows. Aeromagnetic modeling suggests that R-polarity rocks occur in the centers of the subbasins, with the greatest accumulation focused next to the south shore.