Influence of the Ainslie Detachment on the stratigraphy of the Maritimes Basin and mineralization in the Windsor Group of northern Nova Scotia, Canada

Abstract

The Late Devonian to Lower Permian Maritimes basin of eastern Canada developed as a late to post-tectonic extensional basin following the Early Devonian Acadian orogenic event and is subdivided into five main lithostratigraphic groups. The principal host to base metal mineralization is the Visean Windsor Group, which is also the only marine-dominated interval within the basin. The Windsor Group is a thick accumulation of evaporites, carbonates, and siltstone, deposited during passive regional subsidence and intracontinental submersion of a tectonically thinned crust. Earlier rift deposits consist of the nonmarine coarse clastic sediments of the underlying Tournaisian Horton Group, and the Horton to Windsor succession defines a typical rift and sag transgressive event. The Macumber and stratigraphically equivalent Gays River Formations are the marine basal carbonate units to the Windsor Group and contain numerous Pb-Zn-Ba occurrences in central and northern Nova Scotia. These carbonate units are overlain by thick evaporite deposits of the 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 fault that affected the hydrodynamic regime and mineralizing environment in the Carboniferous Maritimes basin. Movement on the detachment has stripped away thick evaporitic units and excised the entire Windsor Group across wide areas, effectively breaching a regional aquiclude. With shearing, permeability was locally enhanced through brecciation, creating a favorable environment for mineralization. Significant thickness variations within underlying coarse clastic rocks and pinch outs of the Horton Group aquifer also focused basin fluids.The detachment is rooted in a master basement normal fault which occurs offshore along the southeast margin of the Gulf of St. Lawrence, with the basement fault displaying up to 10 to 12 km of offset. Structural evidence from onshore exposures in the uplifted or horsted regions of mainland Nova Scotia and Cape Breton Island indicate that much of this movement was transferred horizontally along the Ainslie detachment. Thin-skinned gravitational sliding produced large rafts of allochthonous strata which progressively draped over the edge of basement normal faults during extension, resulting in tectonic salt build-up in the main offshore graben and a salt massif. Buttressing at the front end of the raft within the graben produced seismically imaged large amplitude buckle folds above the detachment.As well as a regionally extensive planar low-strength or low-viscosity layer, large-scale detachment faults require high fluid pressures to sustain motion of the allochthonous sheets. Abnormal fluid pressures are known to occur beneath evaporites, which is the site of nucleation for the Ainslie detachment. Fluid focusing along the detachment is suggested from the structural style, and evidence is provided by widespread synkinematic calcite + or - fluorite-barite-pyrite veins which occur in the calc-mylonite and breccia.