Low-temperature deformation mechanisms at a lithotectonic interface near the Sudbury Basin, Eastern Penokean Orogen, Canada

Abstract

Southwest of the Sudbury Basin, variations in macroscopic strain, microstructure and deformation mechanisms within Archaean granitoid basement and overlying Proterozoic rocks suggest that rock strength was strongly influenced by large amounts of fluids. Ductilely deformed granitoids with strong shape fabrics are concentrated in a 150–200-m-wide zone parallel to the subvertical basement-cover interface. High fluid activity in this zone is indicated by: (1) thickness and close spacing of foliation-parallel quartz veins, (2) abundant fluid-inclusion trails in quartz; (3) quartz c-axis fabrics: and (4) transformation of feldspar into phyllosilicates under middle greenschist-facies metamorphic conditions. Away from this zone, brittle-ductile deformation in Archaean granitoids caused grain size reduction of feldspar by fracturing and crystal-plastic deformation of quartz indicating largely ‘dry’ conditions. Strain was concentrated at the interface, in a region of bulk horizontal shortening and linked kinematically to large-scale shortening of the Sudbury Igneous Complex and thrusting in the South Range Shear Zone. After early open buckling of the Sudbury Igneous Complex, reverse shearing in this zone may have been controlled by ductile deformation at the basement-cover interface. Physical continuity of, and age constraints on, strain fabrics suggest that the South Range Shear Zone represents a segment of a large ductile deformation zone which is most likely linked to the Murray fault and on which higher-grade rocks ascended between 1.85 and 1.23 Ga ago.