AEROMAGNETIC DATA ESTIMATE MAGNETIC SOURCE DEPTHS BENEATH LAKE SUPERIOR

Aeromagnetic anomalies map lateral variations of magnetic material. Anomaly shapes can be used to estimate depth to magnetic source. Magnetic anomalies over Lake Superior occur as both short (narrow) and long (broad) wavelength anomalies that are generally curvilinear or circular in form. Extended Euler depth analyses were applied to estimate the top of magnetic sources, such as volcanic and intrusive rocks related to the Midcontinent Rift System (MRS) and/or pre-rift granite-greenstone rocks, within the lake. Analyses were performed within a window moving across the aeromagnetic grid, resulting in solutions independent of the anomaly orientation. Window sizes ranged from 4,400 m to 20,000 m and structural indices of 0 and 1 were used to account for varying anomaly wavelength and source geometries. Results were compared to interpretations of legacy seismic reflection lines. Short wavelength anomalies reflect upturned rift-related volcanic rocks that crop out onshore near the lake shoreline. Depth estimates for such anomalies extend from the surface to shallower than -500 m mean sea level (MSL). Parallel to, and lakeward of, the short wavelength anomalies are long wavelength anomalies with gradients sloping towards the center of the lake. Depth estimate results exhibit basinward dipping magnetic sources that agree well with seismic interpretations of tops of volcanic rock packages; however, in some areas the source depths are not well resolved and spurious magnetic sources are present which may indicate magnetic sources in overlying sedimentary rocks or noise in the filtered aeromagnetic data. Long wavelength, circular and semi-circular magnetic features with diameters exceeding 30 km occur within the lake. Magnetic depth estimates suggest source locations between -1,000 and -3,000 m MSL and agree with interpreted seismic reflection horizons. Magnetic depth estimate results reveal a 3D rift geometry for the MRS that includes shallow, rift-margin volcanic rocks that dip abruptly basinward to depths exceeding -10,000 m MSL, a geometry that is most pronounced in western Lake Superior. The eastern lake shows similar, but less well-defined geometry. Additional magnetic sources of uncertain origin, which may include pre-rift rocks or rift-related volcanic flows or sills, have moderate depths.