The role of gravity and magma in the structural evolution of a deformed Archean volcanic centre, Timmins, Ontario, Canada

A deformed bimodal subaqueous volcanic centre in the western Archean Abitibi Subprovince of the Canadian Shield contains structures comparable with those of relatively undisturbed igneous complexes. Similarities include an annular fold pattern initiated by structural doming and downsagging of basaltic flows prior to the terminal felsic volcanism. During this cycle an ancestral dome was ruptured by a northeasterly regional graben defined by fossil fault scarps preserved beneath the terminal volcanic deposits. Fissure vents developed along the northwest boundary of the graben, gradually drained the underlying magma chamber, and transformed the crest of the dome into a central collapse basin surrounded by an annular anticlinal uplift marking the inner periphery of the former dome. Basalts on the basinward side of the fissure vents became detached along interflow argillites and glided into the central collapse, forming secondary gravity folds within the uppermost basalts. Subaqueous deposition of felsic debris occurred in the graben and subsiding collapse basin, succeeded by postvolcanic turbidites. The annular folds were modified by two phases of regional deformation, separated by an episode of Archean molasse sedimentation along the reactivated south boundary of the paleograben.