3-DIMENSIONAL MAGNETIC MODELING OF THE MIDCONTINENT RIFT BENEATH CENTRAL LAKE-SUPERIOR

In 1987, a high-resolution aeromagnetic survey was carried out over La ke Superior as a contribution to the Great Lakes International Multidi sciplinary Program on Crustal Evolution (GLIMPCE). The survey compleme nted a 20 s (two-way traveltime) shipborne seismic survey conducted ov er five profiles on the lake in 1986, the purpose of which was to stud y the 1.1 Ga Midcontinent Rift under the lake. Two and one-half dimens ional (2.5D) magnetic modelling has demonstrated that the model predic ted by the seismic results is compatible with the magnetic anomalies u nder the assumption that the magnetization is due essentially to a nor mally polarized upper layer and a reversely polarized lower layer. An approach has been developed and used to extend the model based on GLIM PCE seismic data westwards over the central portion of the lake by shi fting the 2.5D model laterally to the adjacent north-south profile, ad justing the node points, and repeating the process until the area is c overed. A surface integration was then carried out over the resulting three-dimensional (3D) structure. The modelling was constrained primar ily by 8.0 s (two-way traveltime) seismic lines provided by industry, which are capable of defining the upper surface of the normally magnet ized layer and the boundary between the normally and reversely magneti zed layer. Values for the magnetization strengths and directions of bo th layers were estimated from published values on the shore and on isl ands in the lake. The model was adjusted to match the measured field a s closely as possible, thus providing an estimate of the topography of die lower surface of the reversely magnetized layer. The final model was found to be compatible with the gravity anomaly pattern and seismi c refraction studies. Varying the magnetization of the upper and lower layer within reasonable limits was found to change the absolute value of the maximum depth (48 km) of the model by approximately 10% but no t to significantly effect the shape. The 3D modelling program was show n to be useful for studies of this type where continuity along strike is reasonable.