A field-based structural analysis was made in the southern footwall of the Sudbury Basin to facilitate a reassessment of the igneous emplacement mechanism of the 1.85 Ga Sudbury norite (i.e., synfold intrusion, impact-melt ponding). More specifically, we wanted to discern whether attitude and curvature of the norite shell were primary or produced mainly by noncylindrical folding.
The southern footwall of the Sudbury Basin is composed of Paleoproterozoic metavolcanic rocks of the lower Huronian Supergroup that host the 2.3 Ga Creighton and Murray plutons. The geometry of foliation trajectories and other structural evidence reveal that the plutons were emplaced into ductilely deforming host rocks during the Blezardian tectonic pulse, 2.4–2.2 Ga. This deformation led to a southward overturn of Huronian metavolcanic rocks, and was accompanied by amphibolite facies metamorphism. Static greenschist facies metamorphism in metavolcanic and granitoid rocks was prompted by Penokean northward thrusting, after subconcordant emplacement of the norite. The lack of evidence for Penokean folding of Huronian footwall rocks suggests that the curvilinear shape of the norite shell is primary. This in turn constitutes evidence against impact-melt ponding and lends indirect support to an intrusive origin.