THE ST.LAWRENCE RIFT SYSTEM IN EASTERN CANADA – FIELD AND ISOTOPIC EVIDENCE FOR PALEOZOIC TO MESOZOIC(?) REACTIVATIONS OF IAPETUS FAULTS

The St. Lawrence rift system (SLRS) is an active fault zone where reactivation of Iapetus-related structures is believed to occur. The rift faults fringe the contact between the Grenvillian basement to the NW and the St. Lawrence Lowlands to the SE. The SLRS trends NE-SW and forms a half-graben that links the Ottawa-Bonnechère and the Saguenay River grabens. Rift faults are NE- and NW-trending and marked by breccias and cataclasites. The paucity of isotopic age data and the absence of rock strata younger than the Ordovician make it difficult to constrain the timing of the different faulting increments. Field relations suggest that faulting is younger than the Charlevoix impact crater of probable Devonian age, whereas the isotopic signature of fault-related quartz-calcite veins is consistent with faulting during the Late Jurassic-Early Cretaceous. Existing and new clay K-Ar and apatite fission tracks (FT) data are presented for the SLRS. K-Ar dating was performed on clay material <0,2µm-size fractions from fault gouges and breccias along the Montmorency Falls (MF) and St-Laurent (SL) faults. The results suggest a tectonic activity along the MF fault from ca. 465 to 445 Ma, whereas results from the SL fault are suggestive of Middle Ordovician (ca. 465 Ma) and Middle Devonian (ca. 390 Ma) faulting. Apatite FT ages are from transects in the Montréal, Québec city and Charlevoix areas. In the Montreal transect, the ages vary from 190 Ma to 175 Ma in the footwall of rift faults, and from 135 to 110 Ma in the Oka Intrusion and adjacent Grenvillian rocks, which is obviously related to Cretaceous magmatism. In the Québec city area, FT dating was performed on both Grenvillian and overlying Appalachian rocks. Except for an amphibolite of the MF fault yielding 120 Ma due to either local thermal heat-flow or fluid-assisted faulting, the FT ages vary from 155 and 135 Ma. Preliminary FT data from the footwall of the SL fault in the Charlevoix area yielded ages of ca. 200 Ma. FT age data need to be interpreted with caution until uncertainties due to paleo-heatflows or to unusual mineral compositions are resolved. They suggest, however, that rocks cooled through ca. 100oC in Jurassic to Early-Cretaceous times, possibly during reactivation of the SLRS, which would be consistent with far-field tectonic activity related to North-Atlantic rifting.