Structural denudation of Silurian-Devonian high-grade metamorphic rocks and postorogenic detachment faulting in the Maritimes Basin, northern Nova Scotia

Abstract

In northern Nova Scotia, the Canadian Appalachian belt underwent dynamic orogenic build-up and post-orogenic collapse, from Silurian to Carboniferous time. Early shortening and high grade metamorphism occurred during the Acadian event, followed by crustal extension and sedimentation within the Maritimes Basin. Three regional-scale fault structures which, in part, accomodated this orogenic cycle are described here, namely (1) the Highlands Shear Zone, (2) the Margaree Shear Zone, and (3) the Ainslie Detachment. Neoproterozoic basement rocks were imbricated with Ordovician-Silurian cover sequences during compression. High grade gneissic units of late Silurian age in the region indicate that significant tectonic burial and crustal thickening occurred as a result of the thrusting. Partial denudation of the high grade assemblages occurred during Early Devonian thrust emplacement of the Cabot nappe toward the northwest, along the Highlands Shear Zone. The nappe is characterized by an amphibolitic gneiss and high-grade schist complex defining a large folded klippe. Inverted metamorphic isograds occur along the margins of the nappe. Further unroofing of the metamorphic rocks took place during Late Devonian extension from beneath the low-angle Margaree Shear Zone, which was eventually incised by Tournaisian coarse clastic rocks of the Horton Group. Tectonic sag of the extensional complex is marked by a cover of marine carbonates and evaporites from the Visean Windsor Group. Late Carboniferous block faulting in the basement and uplift of Cape Breton Island as a horst, triggered a regional scale evaporite-controlled bedding-parallel gravity slide near the base of the Windsor Group, defining the Ainslie Detachment.