THE RIFT BETWEEN ONTARIO AND QUEBEC: STRUCTURAL ANALYSIS OF THE OTTAWA-BONNECHERE GRABEN FAULTS

The Ottawa-Bonnechere Graben is a late Neoproterozoic to Cambrian aulacogen associated with the breakup of the supercontinent Rodinia and formation of Iapetus Ocean. Graben-defining faults also offset Late Cambrian to Middle Ordovician units. The Late Cambrian Nepean Sandstone (Potsdam Formation) exhibits pervasive brecciation and bulging grain recrystallization of quartz, signifying moderate temperature deformation (<300°C). Higher in the stratigraphy, vein style and fault breccia calcites within Middle Ordovician Ottawa Group limestone possesses extensive lamellar twinning: vein calcite preserve thicker and less intensive twinning (~35 μm width, 8 twins/mm) than fault breccia calcite (~8 μm width, 40 twins/mm). This indicates the brecciated calcite experienced lower temperature deformation (170-200°C) than the vein calcite (>200°C). Below the Paleozoic strata, fault gouge within the Mesoproterozoic Grenville gneiss exhibits brittlely fractured quartz and feldspar, and fibrous calcite veins. Anastomosing pseudotachylite bands (0.3-1.0 mm thick) are present within the fault gouge. A previous study defined three extensional deformation events that have reactivated the graben, and the paleostress fields have rotated counter-clockwise from D1 (σ₃ northeast-southwest) to D3 (σ₃ north-south). The paleostress orientation of the pseudotachylite is consistent with D1 and ⁴⁰Ar/³⁹Ar geochronology conducted on the pseudotachylite has yielded a date of ca. 494 Ma. Tectonism during this time is supported by syn-depositional tectonic deformation ('sand volcanoes') within Late Cambrian Nepean Sandstone. The date provided can possibly be used to resolve the timing of D1. Based on paleostress directions, deformation of the Paleozoic strata appears to be associated with D2. Previously reported AFTA ages of carbonatite dykes within the graben yield Mesozoic dates (115-103 Ma). Emplacement of the dykes has been defined as pre- to syn-D2, based on relationships to D2 structures. Our attempts at U-Pb dating calcite within fractures of the Ottawa Group limestone provided a lower intercept age of ca. 390 Ma. Our study demonstrates that pre-existing weaknesses in the continental crust, in particular rift zones and deep-seated faults, could be reactivated by stresses generated at distant plate boundaries.