GRAVITY AND MAGNETIC MODELS OF THE MIDCONTINENT RIFT IN EASTERN LAKE-SUPERIOR

Gravity and magnetic models of the Midcontinent Rift (MCR) in eastern Lake Superior supplement recent structural and stratigraphic interpret ations based on the seismic reflection method. An algorithm developed to accommodate spatially varying direction and magnitude of magnetizat ion within a magnetic source is used in both forward and inverse model ing procedures. Structural attitudes of rift-filling basalts derived f rom seismic reflection sections are used to rotate the Keweenawan rema nent magnetization vectors in the direction of deformation. An iterati ve linear inversion routine calculates magnitudes of induced and reman ent magnetizations, as well as normal and reversed polarity basalt flo w distributions. The results indicate that the Koenigsberger ratios of these basalts generally range from 1 to 3, which is in agreement with values obtained from rock property measurements. The models also sugg est that the greater volume of the Keweenawan basalt section in easter n Lake Superior is reversely polarized and that remanent magnetization s persist to depths of up to 20 km. Our results, supplemented by isoto pic and paleomagnetic data, suggest that the vast majority of the basa lts predate 1097 +/- 1 Ma. A prominent positive magnetic anomaly and a corresponding gravity low strike west across the trend of the rift fr om the vicinity of Michipicoten Island. These anomalies may reflect a relatively strongly magnetized, felsic igneous body of late-middle to upper Keweenawan in age. Forward gravity models suggest clastic sedime ntary rocks up to several kilometers thick overlay the volcanic rocks in localized depressions. Deep crustal seismic data used to constrain gravity models provide evidence of anomalously dense lower crust benea th the MCR.