SUPRACRUSTAL FAULTS OF THE ST. LAWRENCE RIFT SYSTEM, QUEBEC: KINEMATICS AND GEOMETRY AS REVEALED BY FIELD MAPPING AND MARINE SEISMIC REFLECTION DATA

The St. Lawrence rift system (SLRS) is a seismically active zone where fault reactivation is believed to occur along late Proterozoic to early Paleozoic normal faults related to the opening of Iapetus ocean. The rift-related faults fringe the contact between the Grenvillian basement to the NW and Cambrian-Ordovician rocks of the St. Lawrence Lowlands to the SE, and occur also within the Grenvillian basement. The SLRS trends NE-SW and represents a half-graben that links the NW-SE-trending Ottawa-Bonnechère and Saguenay River grabens, both interpreted as Iapetan failed arms. Coastal sections of the St. Lawrence River that expose fault rocks related to the SLRS have been studied between Quebec city and the Saguenay River. Brittle faults marking the SLRS consist of NE- and NW-trending structures that show mutual crosscutting relationships. Fault rocks consist of fault breccias, cataclasites, and pseudotachylytes. Field relations suggest that the various types of fault rocks are associated to the same tectonic event. High-resolution, marine seismic reflection data acquired in the St. Lawrence River estuary, between Rimouski, the Saguenay River and Baie-Comeau, successfully identify submarine topographic reliefs attributed to the SLRS. NE-trending seismic profiles show a horst-and-graben geometry that well agrees with onland structural features. NW-trending seismic profiles suggest that normal faults that fringe the St. Lawrence River are associated to a major topographic depression in the estuary with up to 500 metres of basement topographic relief. The lack of isotopic age data and the absence of rock strata younger than the Ordovician along the onland section of the SLRS make difficult to constrain the timing of younger increments of faulting. However, field relationships suggest that faulting is younger than the Devonian impact crater of the Charlevoix area, whereas preliminary fission tracks data from rift faults of the Quebec city area are consistent with faulting during Late Jurassic-Early Cretaceous times, which would be consistent with far-field tectonism during the opening of the North Atlantic ocean. Moreover, seismic data from the St. Lawrence estuary are suggestive of even younger (Cenozoic?) faulting increments.