The Early Proterozoic structural and tectonic history of the south central Lake Superior Region

Abstract

The early Proterozoic tectonic evolution of the south central Lake Superior region is complex, owing to the presence of four tectonostratigraphic terranes, which were affected by six phases of deformation. The four terranes are the passive margin of the Superior craton, two paraautochthonous passive margin terranes (Crystal Falls and Florence‐Niagara terranes), and a southern magmatic arc complex which is probably allochthonous with respect to the other terranes. Four of the six deformational episodes accompanied subhorizontal shortening, while two were caused by subvertical shortening. The first and the most penetrative phase of deformation is marked by subhorizontal shortening in a NNE‐SSW direction. The second and fourth deformations were characterized by subvertical shortening and did not significantly modify the structural orientations from previous events in the study area. The third, fifth, and sixth deformations mostly caused open folding, and shortening directions were NW, NE, and W, respectively. Because all the terranes in the south central Lake Superior region share parallel deformational histories, it is suggested that the accretion of these terranes occurred during the first deformational episode. After removal of younger deformational effects, including open folding of the suture zone, the tectonostratigraphic assemblages in this region show the following sequence from NNE to SSW: a platformal assemblage overlying sialic basement, a basinal assemblage of tholeiitic volcanic rocks overlain by deep‐water turbidites, an assemblage of basin floor deposits (Crystal Falls terrane) with apparently no demonstratably underlying crystalline basement, a fault‐bounded terrane with highly strained passive margin strata (Florence‐Niagara terrane), and a calc‐alkaline magmatic arc assemblage. Such an arrangement of tectonostratigraphic assemblages is comparable with cross sections through Phanerozoic accretionary continental margins and therefore supports an arc‐continent collision model.