The Animikie basin in Minnesota developed during early Proterozoic time over and approximately parallel to the Great Lakes tectonic zone—an Archean suture between an ancient gneiss terrane to the south and a younger greenstone-granite terrane to the north. The evolution of the basin can be divided into an extensional stage with attendant deposition of stratified rocks, and a subsequent compressional stage assigned to the Penokean orogeny.
The lower Proterozoic stratified rocks in the Animikie basin in Minnesota can be divided into two zones on the basis of pronounced differences in facies and thickness. These are (1) a relatively thin succession of predominantly sedimentary rocks north of the Great Lakes tectonic zone and (2) a much thicker succession of intercalated sedimentary and volcanic rocks to the south. The thicker succession is intensely deformed, metamorphosed, and intruded locally by igneous rocks. The close correspondence between the sedimentological and tectonic patterns implies that sedimentation and tectonism were both part of a tectonic continuum that began with the development of the depositional basin and culminated with the major tectono-thermal pulse of the Penokean orogeny. If a tectonic continuum is assumed, then any hypothesis to account for extension in the basin must also account for the compression.
A close correspondence between sedimentological and tectonic patterns exists between the lower Proterozoic strata in Minnesota and those of the southeastern segment of the Animikie basin in Wisconsin and northern Michigan. Therefore, it seems likely that sedimentation in both segments occurred within similar tectonic regimes and that any hypothesis to account for the evolution of either segment must account for the evolution of both.
The similarity of sedimentological attributes of lower Proterozoic strata in the Animikie basin and Phanerozoic strata deposited in geosynclines has prompted various plate-tectonic models to explain the formation of the basin. The structural features formed in the basin during the compressional stage, however, lack attributes typical of a consuming continental margin or colliding continental plates. The lack of these attributes does not negate the usefulness of plate-tectonic processes in interpreting the basin’s history, nor does it prove that such processes were not operative. We still do not know if the evolution of the Animikie basin followed the same “rules” of extension and compression that govern the evolution of a “typical” Phanerozoic geosyncline, or if the basin formed by a sequence of processes unique to its time and place.