Upper-crustal, basement-involved folding in the East Range of the Sudbury Basin, Ontario, inferred from paleomagnetic data and spatial analysis of mafic dykes

Abstract

Tilting of crystalline basement rocks associated with folding strain at uppermost crustal levels is difficult to recognize if basement rocks are devoid of traceable marker planes. Here we use the spatial variation in strike of Paleoproterozoic mafic dyke segments complemented by compiled paleomagnetic data to identify tilting in Archean basement rocks associated with kilometre-scale folds of the eastern Sudbury Basin, Ontario. Spatial analysis of the strike of dyke segments is consistent with generation of the NE lobe and a newly identified anticline, referred to as the West Bay Anticline, in the layered Sudbury Igneous Complex (SIC). This anticline accounts better for the structural characteristics of the eastern Sudbury Basin than a previously proposed anticline with west-plunging hinge line. The West Bay Anticline is characterized by abrupt plan-view thickness variations in the lower SIC and curved faults displaying significant strike separations of SIC contacts. These structural characteristics are consistent with folding strain imparted to the SIC and adjacent Archean rocks during formation of the West Bay Anticline. Sublayer embayments and associated quartz diorite dykes likely served as zones of mechanical weaknesses, at which the higher-order folds localized. Unfolding magnitudes of the NE lobe based on primary paleomagnetic remanence directions are significantly smaller than inferred magnitudes that are based on the assumption that the basal SIC contact was initially planar. Thus, the basal SIC contact in the NE lobe likely had a trough-like geometry at the time of remanence acquisition. We advocate a scenario for the formation of the NE lobe, in which the trough geometry of the SIC is primary rather than a consequence of tilting prior to solidification of, and remanence acquisition in, the SIC. Finally, we caution the interpretation of photo lineaments in eroded basement terranes purely as a consequence of faulting.