Structural style and evolution of a segment of the Dunnage Zone from the Quebec Appalachians and its tectonic implications

Abstract

Southern Quebec Appalachians Cambro-Ordovician rocks, southeast of the Baie Verte-Brompton Line, belong to the Dunnage Zone and consist of the St. Daniel Melange, the Ascot Complex, and the Magog Group. The Ascot Complex and the Magog Group are well exposed in the Sherbrooke area where detailed structural mapping has been conducted. Structural analysis indicates that regional folding and faulting in rocks of the Dunnage Zone are related to the Acadian orogeny. Three phases of deformation are recognized; an earlier pre-Acadian phase (D 1 ) is confined to the Ascot Complex, whereas D 2 and D 3 are Acadian-related. The St. Victor and Gaspe-Connecticut Valley synclinoria are Acadian structures. The Acadian Orogeny culminated in the formation of high-angle reverse faults, of which the La Guadeloupe fault is the most important. A crenulation cleavage and gentle folding of earlier fabrics are associated to the third phase (D 3 ). The lack of major pre-Acadian deformation in the Magog Group is a regional structural feature which has important implications for the tectonic evolution of the Taconic orogeny in southern Quebec. Problems arising from lithologic and structural correlations with similar rocks from northern Vermont contribute to specify the evolution of deformation along the orogen. To reconcile lithological variations and structural discrepancies between the Quebec and Vermont Appalachians, we propose that the Taconic orogeny results solely from subduction movements along a nonlinear continental margin.