Fold origin of the NE-lobe of the Sudbury Basin, Canada: Evidence from heterogeneous fabric development in the Onaping Formation and the Sudbury Igneous Complex

Structural analysis of the Onaping Formation, a heterolithic impact melt breccia, and the Granophyre in the NE-lobe of the 1.85 Ga Sudbury Igneous Complex (SIC) assist in understanding the formation of the Sudbury Basin. Previously, the lack of mesoscopic strain fabrics in the SIC, in contrast to pervasive fabrics in the Onaping Formation of the NE-lobe, led to interpretations of the shape of the NE-lobe as primary. We demonstrate that the structures of the Onaping Formation are consistent with deformation in a fold core controlled by the mechanically stronger Granophyre of the NE-lobe. Evidence for this interpretation includes the (1) presence of open structural domes and basins, (2) geometry of planar mineral shape fabrics, (3) variation in shape fabric intensity, and (4) kinematics of prominent faults. Folding of the Onaping Formation affected the SIC because fold geometry is based on dip data in the SIC and fold-related faults cut both the Onaping Formation and Granophyre. Microstructural observations point to different temperatures during deformation in both units that are consistent with initiation of folding during cooling of the SIC upon reaching middle greenschist-facies metamorphic conditions.