SEISMIC STRATIGRAPHY OF THE 1.1 GA MIDCONTINENT RIFT BENEATH WESTERN LAKE SUPERIOR PART II: EVOLUTION OF RIFT-FILL THROUGH TIME

Isochron maps interpreted from 2D seismic lines over Lake Superior inform the development of the ca. 1.1 GA Midcontinent Rift. Grand Marais Ridge (GMR), identified by a gravity low and seismic high south of Grand Marais, MN, is previously recognized as pre-rift crust that impacted rift basin development. Seismic reflection geometry and aeromagnetic modeling constraints define three distinctive packages. Aeromagnetic modeling helps distinguish igneous rocks with strong, reversed- versus normal-polarity remanent magnetization, which constrains the cooling ages of syn-rift rocks to before or after about 1100 Ma, respectively. Lowest unit U1 has subparallel reflections with clinoform geometry that cover and dip away from GMR. U1 goes below the 8-second limit of most seismic lines inhibiting full characterization. The middle (U2) and upper (U3) units are mostly subparallel continuous reflections that dip away from the sides of GMR as synforms. U2 reflections extend east and south of GMR, dipping towards a basin axis southeast of GMR. Separation between reflections increases towards the basin axis, and reflections truncate locally south-central to GMR. U3 reflection extent is more continuous around GMR. U3 reflection spacing increases with dip towards the axis of a notably deep, symmetric basin west of GMR. Increased accommodation preserves reflections to the west that are truncated to the east. Aeromagnetic modeling indicates normal polarity for U1 and U3, but reversed polarity for much of U2, suggesting U2 is >1100 Ma. U1 may represent material sourced north of GMR that deposited into a pre-rift basin as a prograding shelf, perhaps containing sheet intrusions with normal-polarity magnetization emplaced into a Paleoproterozoic basin. Overlying U2 and U3 record laterally extensive syn-rift volcanics and sills deposited within subsiding sub-basins locally modified by faulting. Variable increases in spacing between reflections east and west of GMR indicate subsidence contemporaneous with deposition focused in separate sub-basins at different times. Subsidence during U2 deposition was greatest southeast of GMR, and U2 reflections truncate against a fault in this area. During U3 deposition, subsidence increased dramatically to form a strikingly symmetric, deep bowl-like sub-basin west of GMR.