INFLUENCE OF THE AINSLIE DETACHMENT ON THE STRATIGRAPHY OF THE MARITIMES BASIN AND MINERALIZATION IN THE WINDSOR GROUP OF NORTHERN NOVA-SCOTIA, CANADA

The Late Devonian to Lower Permian Maritimes basin of eastern Canada d eveloped as a late to posttectonic extensional basin following the Ear ly Devonian Acadian orogenic event and is subdivided into five main li thostratigraphic groups. The principal host to base metal mineralizati on is the Visean Windsor Group, which is also the only marine-dominate d interval within the basin. The Windsor Group is a thick accumulation of evaporites, carbonates, and siltstone, deposited during passive re gional subsidence and intracontinental submersion of a tectonically th inned crust. Earlier rift deposits consist of the nonmarine coarse ela stic sediments of the underlying Tournaisian Horton Group, and the Hor ton to Windsor succession defines a typical rift and sag transgressive event. The Macumber and stratigraphically equivalent Gays River Forma tions are the marine basal carbonate units to the Windsor Group and co ntain numerous Pb-Zn-Ba occurrences in central and northern Nova Scoti a. These carbonate units are overlain by thick evaporite deposits of t he Lower Windsor Group, consisting of gypsum, anhydrite, and salt. In northern Nova Scotia the evaporite-carbonate contact at the top of the basal carbonates is the locus of intense shear and brecciation which developed in relation to the Ainslie detachment. This detachment is a stratigraphically controlled regional-scale flat-lying extensional fau lt that affected the hydrodynamic regime and mineralizing environment in the Carboniferous Maritimes basin. Movement on the detachment has s tripped away thick evaporitic units and excised the entire Windsor Gro up across wide areas, effectively breaching a regional aquiclude. With shearing, permeability was locally enhanced through brecciation, crea ting a favorable environment for mineralization. Significant thickness variations within underlying coarse elastic rocks and pinch outs of t he Horton Group aquifer also focused basin fluids. The detachment is r ooted in a master basement normal fault which occurs offshore along th e southeast margin of the Gulf of St. Lawrence, with the basement faul t displaying up to 10 to 12 km of offset. Structural evidence from ons hore exposures in the uplifted or horsted regions of mainland Nova Sco tia and Cape Breton Island indicate that much of this movement was tra nsferred horizontally along the Ainslie detachment. Thin-skinned gravi tational sliding produced large rafts of allochthonous strata which pr ogressively draped over the edge of basement normal faults during exte nsion, resulting in tectonic salt build-up in the main offshore graben and a salt massif. Buttressing at the front end of the raft within th e graben produced seismically imaged large amplitude buckle folds abov e the detachment. As well as a regionally extensive planar low-strengt h or low-viscosity layer, large-scale detachment faults require high f luid pressures to sustain motion of the allochthonous sheets. Abnormal fluid pressures are known to occur beneath evaporites, which is the s ite of nucleation for the Ainslie detachment. Fluid focusing along the detachment is suggested from the structural style, and evidence is pr ovided by widespread synkinematic calcite +/- fluorite-barite-pyrite v eins which occur in the calc-mylonite and breccia.