APATITE FISSION-TRACK EVIDENCE FOR LATE PALEOZOIC TO EARLY MESOZOIC UNROOFING AND POTENTIAL FAULT REACTIVATION ALONG THE SAGUENAY RIVER GRABEN, QUEBEC

The Saguenay River graben is a NW-trending structure which is part of the NE-SW-trending St. Lawrence Rift System (SLRS) in Québec. It is parallel to the Ottawa-Bonnechère graben to the south and both structures have been interpreted to be Iapetan aulacogens. The SLRS is composed of a series of normal faults that extend for hundreds of kilometers along the northern boundary of the St. Lawrence River valley and continue into a series of offshore structures, ca. 120 km downriver from Québec City, in the St. Lawrence River estuary. Reactivation of the rift faults has been suggested by field relations to be younger than the Devonian Charlevoix impact crater. Apatite fission-track (AFT) age discontinuites across the Montmorency and Saint-Laurent faults have been determined at four locations along the SLRS (Roden-Tice and Tremblay, 2008) and have been interpreted as consistent with Late Jurassic-Early Cretaceous fault reactivation and uplift.

Eleven samples of Grenville rocks collected along the Saguenay River graben yielded AFT ages ranging from 337 ± 43 Ma to 197 ± 18 Ma on the north side and from 281 ± 30 Ma to 175 ± 21 Ma on the south side. Two AFT ages from the north side of the Sainte-Marguerite fault on the north shore of the Saguenay River, 337 ± 43 Ma and 287 ± 36 Ma, showed a discontinuity with three AFT ages from the south side of the fault, 243 ± 26 Ma, 221 ± 27 Ma and 197 ± 18 Ma, suggesting Late Triassic to Early Jurassic reactivation of the Sainte-Marguerite fault.

A second AFT age discontinuity exists across the Lac Kénogami fault, on the south side of the Saguenay River graben, with the north side of the fault yielding an AFT age of 175 ± 21 Ma versus 235 ± 28 Ma on the south side. Compared to AFT age offset across the Sainte-Marguerite fault to the north, this AFT age discontinuity suggests a slightly younger, Middle Jurassic uplift and fault reactivation. The remaining four samples from the south side of the Saguenay graben yielded early Permian to Early Jurassic AFT ages ranging from 281 ± 30 Ma near Hébertville to 205 ± 21 Ma at Baie-Éternité.