STRIKE-SLIP INTERPRETATION OF BASIN-BOUNDING FAULTS OF THE ST-LAWRENCE LOWLANDS BASIN IN THE QUEBEC-CITY AREA, CANADA

Traditional structural interpretations of faults identified in the aut ochthonous, Ordovician platform rocks of the St. Lawrence Lowlands bas in of Quebec, Canada, assert that the primary sense of faulting is nor mal, extensional, and dip-slip in motion. Interpretation of field obse rvations such as the offset of facies boundaries in outcrop, fault sur face 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 su pport this interpretation and indicate a need for a different model fo r the deformation of the platform margin. A left-lateral, basement-inv olved wrench system resolves stratigraphic discordances across fault z ones, contradictions in the observed geometry of hanging-wall blocks, problems concerning timing of tectonic events, and the difficulty of u sing extensional deformation models in a compressional setting. This r einterpretation places the timing of the platform faulting just prior to the arrival of the Taconic thrust sheet. The lateral offset of subs urface and surface depositional facies, fault trace features, well cor relations, and seismic expressions document approximately 60 km of lef t-lateral strike-slip motion in the St. Lawrence Lowlands basin.