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Traditional structural interpretations of faults identified in the autochthonous, Ordovician platform rocks of the St. Lawrence Lowlands basin of Quebec, Canada, assert that the primary sense of faulting is normal, extensional, and dip-slip in motion. Interpretation of field observations such as the offset of facies boundaries in outcrop, fault surface features, fracture patterns, and fault volume considerations lead directly to difficulties with a mobile crustal bulge model for normal faulting. Newly reprocessed seismic data and well-log correlations support this interpretation and indicate a need for a different model for the deformation of the platform margin.
A left-lateral, basement-involved wrench system resolves stratigraphic discordances across fault zones, contradictions in the observed geometry of hanging-wall blocks, problems concerning timing of tectonic events, and the difficulty of using extensional deformation models in a compressional setting. This reinterpretation places the timing of the platform faulting just prior to the arrival of the Taconic thrust sheet. The lateral offset of subsurface and surface depositional facies, fault trace features, well correlations, and seismic expressions document approximately 60 km of left-lateral strike-slip motion in the St. Lawrence Lowlands basin.
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