STRUCTURAL CHARACTERIZATION OF THE ST-JOSEPH FAULT AND BAIE VERTE-BROMPTON LINE, SOUTHERN QUEBEC APPALACHIANS

In the southern Quebec Appalachians, the Humber and Dunnage zones are remnants of aLaurentian continental margin and adjacent oceanic domain, which amalgamated during the Ordovician-Taconian orogeny. Penetrative Taconian deformation is restricted to the internal Humber zone and hasbeen mainly attributed to ophiolite obduction/accretion to Laurentia. To the East, the NDMA is bounded by the SE-dipping St-Joseph fault and Baie Verte-Brompton line (BBL), which represent Siluriannormal faults defining a polyphased «suture» between Laurentia and oceanic terranes. We present a detailed structural analysis and sampling of fault-rocks related to both the BBL and the St-Joseph fault at two different sites, (1) the Jeffrey mine of the Asbestos ophiolite where the BBL is exposed on the north wall of the open pit, and (2) an abandoned quarry NE of the Thetford-Minesophiolite, in which the St-Joseph fault is exposed and cuts across Laurentian metasedimentary rocks. The BBL and the St-Joseph fault zones trend NE and dips approximately 60°-70° SE. At both sites, the faults zones are marked by sheared/altered serpentinites at the contact between various types of metamorphic rocks of the footwall, and low-grade sedimentary rocks and/or mafic/ultramafic rocks(peridotites, basaltes) in the hanging wall. Metre-thick «granitic» dykes were observed along the BBL inthe Jeffrey mine, it remains to be defined if they represent dioritic intrusives or rodingite dykes ofmetasomatic origin. The occurrence of well-developed shear bands and slickenside fault steps clearlyindicate that the major fault increment has been related to normal-sense faulting. The metamorphic layering of the footwall rocks is commonly reoriented parallel to fault fabrics and structures, attestingfor the «late-stage» origin of the BBL-St-Joseph fault compared to regional metamorphism and deformation as preserved in the footwall metamorphic rocks. We locally observed two different generations of slickenside lineations, suggesting polyphased brittle faulting, the oldest generation plunging approximately 30° NE and the youngest being basically down-dip on hosting fault planes. At the Jeffrey mine, a serpentinite horizon occurring in the footwall of the BBL preserves an early phase off olding affecting both the ophiolite and its metamorphic sole.