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

The St. Lawrence Rift System (SLRS) from the Laurentian craton core to the offshore St. Lawrence River is a seismically active zone in which 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 within the Grenvillian basement as well. Coastal sections of the St. Lawrence River of 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 with the same tectonic event. High-resolution, marine seismic reflection data, acquired in the St. Lawrence River estuary between Rimouski, the Saguenay River, and Forestville identify submarine topographic relief attributed to the SLRS. NE-trending seismic profiles show a basement geometry that agrees with onshore structural features. NW-trending seismic profiles suggest that normal faults fringing the St. Lawrence River are associated with a major topographic depression in the estuary - the Laurentian Channel trough - with up to 700 m of basement relief. A depth-to-bedrock map from the St. Lawrence estuary, and comparison with the onshore rift segment suggest that the Laurentian Channel trough varies from a half-graben to a graben structure from SW to NE. Preliminary apatite fission track (FT) data from Grenvillian rocks and Ordovician platform rocks of the Charlevoix area yield ages clustering at 200 Ma. FT data indicates that these rock units now at the surface cooled through ca. 100oC in the Triassic-Jurassic boundary, possibly during reactivation of the rift system, but remained within the apatite partial annealing zone (>60oC) until final exhumation in the Cenozoic. It is speculated that natural gas occurrences and gas hydrates within the onshore and offshore sequences of Quaternary deposits are related to degassing processes of Mesozoic(?), Paleozoic and/or older basement rocks, and that hydrocarbons trickled upward along the rift faults.