SILURIAN EXTENSIONAL FAULTING IN THE QUEBEC APPALACHIANS. STRUCTURAL CHARACTERIZATION AND TECTONIC IMPLICATIONS

In the Québec Appalachians, a composite system of normal faults marked by the Baie Verte-Brompton line (BBL) and the St. Joseph fault (SJF) defines a polyphased «suture» between the Humber and Dunnage zones, which are remnants of the Laurentian margin and adjacent oceanic domain that amalgamated during the Ordovician Taconian orogeny. In Québec, both zones are unconformably overlain by Silurian-Devonian sedimentary rocks of the Gaspé Belt which have been deformed during the Devonian Acadian orogeny.

We present a detailed structural mapping and microanalysis of fault-rocks related to both the BBL and the SJF in three distinct areas of the Québec Appalachians, from south to north; (1) the Asbestos area where the BBL is exposed in the Jeffrey mine and the Norbestos quarry, (2) the Beauce area, where the SJF is exposed in an asbestos quarry, and (3) the lake Temiscouata area, exposing the northeastern tip of the SJF. The fault zones trend NE and dips approximately 70^o toward the SE, and are marked by lenses of sheared serpentinites at the contact between various metamorphic rocks in the footwall and low-grade sedimentary rocks and/or mafic/ultramafic ophiolitic rocks in the hanging wall. Shear bands and slickenside fault steps indicate a major increment of deformation related to normal-sense faulting. The metamorphic layering of the footwall rocks is commonly reoriented parallel to fault fabrics, attesting for the «late-stage» origin of the BBL-SJF compared to the regional structures preserved in the Humber zone. In the lake Temiscouata area, the Ludlovian Robitaille Formation overlies the SJF, suggesting that extensional faulting is pre-late Silurian, which is consistent with ⁴⁰Ar/³⁹Ar age constraints for post-Taconian normal faulting in southern Québec (ca. 430-410 Ma). In Vermont, this composite system merges into the Burgess Branch fault zone which separates domains of contrasting metamorphism. This suggests that Silurian extensional faults also occur in the New England Appalachians, although intense Acadian metamorphism overprint has probably obliterated most field evidence of these normal faults. This extensional faulting is genetically related to the formation of sedimentary basin(s) of the Gaspé Belt, and has been possibly triggered by plate delamination during late stages of the Taconian orogeny.