Interpreted either as a folded igneous sheet or a basin-shaped intrusion, the origin of the Palaeoproterozoic Sudbury Igneous Complex (SIC), Ontario, Canada, has been debated by geologists for over a century. Most recent proposals view the SIC to have been emplaced as a horizontal impact melt and subsequently folded after solidification. This requires the SIC to have folded from an originally horizontal sheet, to the present basinal geometry, with dips of 30 degrees in the North Range and 70 degrees in the East Range. The fold scenario would require the sharply curved juncture between the North and East Ranges (North Lobe) to be deformed in the solid-state due to folding. In order to test the fold model, the rock fabric was investigated in the East and North Ranges and the North Lobe of SIC at 256 sites with the aid of anisotropy of magnetic susceptibility (AMS). A sampling transect through an undeformed portion of the SIC in the western North Range reveals that the gabbro-norite exhibits SIC contact-parallel magnetic foliation and down-dip magnetic lineation, both of which, respectively, correlate to an igneous foliation and lineation. The granophyre, in contrast, features spatially consistent magnetic lineation that is oriented orthogonal to the SIC contact. This magnetic lineation can be correlated to a mineral fabric characterised by aligned acicular crystals of plagioclase, and is interpreted to be a wall-orthogonal crystallisation texture. The East and North Ranges in the eastern SIC reveal similar patterns of AMS fabrics. The magnetic lineation of the granophyre is orthogonal to the basal contact of the SIC in the East and North Ranges, and in the fold-like North Lobe between the two ranges. An axial-planar magnetic foliation is developed in the granophyre as the North Lobe is approached, but the magnetic Lineation is nevertheless contact-orthogonal and well-defined. This AMS fabric is interpreted to be a combination of an original igneous contact-orthogonal lineation and a foliation resulting from tectonic overprint. The preservation of the contact-orthogonal magnetic lineation implies low levels of tectonic overprint. Conservative shortening strains estimated from AMS numerical modelling suggest <15% strain in the North Lobe, but most likely <5% strain, as judged from the paucity of microstructural evidence for solid-state deformation. Such low levels of strain are more consistent with a basin-shaped intrusive origin for the SIG, rather than folding of an initially horizontal, and consolidated igneous sheet. This rules out models of initial horizontal sheet emplacement including the recently proposed impact-melt model for the SIC. (C) 1999 Elsevier Science B.V. All rights reserved.
