LATE DEVONIAN CARBONIFEROUS DETACHMENT FAULTING AND EXTENSIONAL TECTONICS IN WESTERN CAPE-BRETON ISLAND, NOVA-SCOTIA, CANADA

Shallow-dipping detachment faults have played a key role in the evolution of the Late Devonian to Carboniferous Maritimes Basin of western Cape Breton Island. The faults are responsible for large-scale post-orogenic crustal thinning and denudation of mid-crustal levels. Tournaisian and Westphalian elastic sedimentation can be related to detachment faulting along the Margaree Shear Zone (MSZ) and the Ainslie Detachment (AD), respectively. The MSZ consists of thick shallow-dipping retrogressive mylonite overprinted by cataclastic horizons, brittle detachment faults, and chloritic breccia. The shear zone crosscuts and transports low-grade Late Devonian bimodal volcanic rocks and redbeds of the Fisset Brook Formation in its hanging wall towards the west, and juxtaposes them against exhumed medium- to high-grade metamorphic rocks and basement in its footwall. Locally the mylonite is 200-700 m thick and outcrops along strike from north to south for approximately 60 km. Tournaisian coarse elastic units and debris now deposits of the Horton Group unconformably overlie ductile mylonite of the MSZ due to incision of brittle faults into the MSZ. Regional transgression, and marine flooding of the extensional complex and exhumed units are marked by the thick Visean carbonate and evaporite deposits of the Windsor Group. The carbonates reflect a relatively quiescent tectonic regime, and widespread subsidence was likely due to the cessation of extension-related thermally induced buoyancy in a tectonically thinned crust. The Ainslie Detachment nucleated as a regional flat-lying fault along the carbonate-evaporite contact at the top of the Macumber Formation, forming calc-mylonite near the base of the Windsor Group. Significant stratigraphic omissions in the Windsor Group above the detachment provide clear evidence of an extensional regime during faulting. Clastic deposits of the Westphalian Cumberland Group are bounded by the detachment, and sedimentation is related to listric normal faults which cut down-section towards the west-southwest. The documentation of detachment faults has widespread implications for the interpretation of the post-orogenic evolution of the northern Appalachians. An extensional regime can now be recognized. Deep reflection seismic data are interpreted in light of the new findings and can be related to Late Devonian-Carboniferous basin formation.