POST-JURASSIC UPLIFT AND FAULTING ALONG THE ST. LAWRENCE RIFT SYSTEM, QUEBEC BASED ON APATITE FISSION-TRACK EVIDENCE

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 Grenville to the NW and the St. Lawrence Lowlands to the SE. Rift faults are NE- and NW-trending and marked by breccias and gouges. The paucity of isotopic age data and the absence of strata younger than the Ordovician make it difficult to constrain the timing of the different faulting increments, if any. Field relations suggest that faulting is younger than the Devonian Charlevoix astrobleme whereas the isotopic signature of fault-related quartz-calcite veins is interpreted as consistent with Late Jurassic-Early Cretaceous faulting.

Apatite fission-track (AFT) ages determined for six samples of Grenville rocks collected at four locations across the Montmorency and Saint-Laurent faults along the SLRS show significant age discontinuities between the hanging wall and footwall samples. At Montmorency Falls, Sault-au-Cochon, and Cap-aux-Oies, the footwall rocks yielded Early Jurassic AFT ages ranging from 200 ± 20 to 184 ± 19 Ma. Hanging wall samples yielded younger Late Jurassic AFT ages ranging from 152 ± 17 Ma to 149 ± 14 Ma, suggesting post-Jurassic uplift and fault displacement at < ~150 Ma.

The AFT age discontinuities suggest a reverse sense of motion in the Late Jurassic to Early Cretaceous at < 150 Ma whereas field relations clearly show normal-sense faulting. Evidence for the reactivation of pre-existing normal faults during or after Early Jurassic time is also found in the Mesozoic rift-related Fundy and Orpheus basins off the Atlantic coast (Withjack et al., 1998). The NW-SE trending compressive stress field currently documented along the St. Lawrence River (Du et al., 2003) is also consistent with reverse faulting documented in this study along the SLRS, thus providing chronological support for Atlantic rift-related compressive deformation in the interior of the Canadian Shield, more than 500 km west of the axis of the Mesozoic rift basins.