Regional geophysical investigation of the Sudbury structure

Abstract

The ca. 1850 Ma Sudbury structure (SS) lies in the Canadian Shield at the paleocontinental margin between the Archean Superior province and the Proterozoic Southern Province. It comprises the Sudbury Igneous Complex (SIC), underlying brecciated footwall rocks, and the overlying sedimentary sequence of the Whitewater Group that occupy the central depression of the SIC. Having gone through a series of syn- and post-formation orogenies, the hypothesized initial circular shape of the SIC has been deformed into an elliptical shape of about 28 km by 60 km. Contacts qualitatively interpreted from the computed directional and tilt derivatives of the magnetic field are mostly coincident with the mapped geological contacts, especially in the North Range, but show some inconsistencies in the South Range probably due to extensive alteration and deformation processes. Interpreted magnetic lineaments further highlight the continuity of some previously known faults and dykes and also revealed some new lineaments probably buried or otherwise obscured. Two-and-a-half dimensional joint magnetic and gravity forward modelling consistent with the magnetic contacts and a new interpretation of the Lithoprobe seismic section incorporated a north-verging fold into the basal portion of the SIC in the South Range. The location of the basal deformation is coincident with and could be related to a linear gravity high observed within the Sudbury Basin. Geological interpretation along six profiles was used to develop a three-dimensional geological model of the Sudbury structure and forward gravity computation suggests the existence of a dense feature, interpreted to be mafic volcanic rocks of the Elliot Lake Group, at about 5-6 km depth under the Sudbury Basin. Sulfide-rich sublayer norite could lie above this deformed zone, suggesting it to be a deep prospective zone for future investigation. Northwest-directed closure and reversal of the Huronian rift basin during the Penokean orogeny is suggested to explain the development of iv structures at the base of the SIC. This interpretation is consistent with most reflectors of the Lithoprobe seismic section.