EVIDENCE FOR MESOZOIC FAULT REACTIVATION AND UNROOFING OF THE CANADIAN SHIELD IN SOUTHERN QUEBEC BASED ON APATITE FISSION-TRACK ANALYSIS

The St. Lawrence rift system (SLRS), the Saguenay River graben (SRG) in southern Québec and the Ottawa-Bonnechère graben (OBG) along the southern Québec – Ontario border are active fault zones along which reactivation of Iapetan-related structures is believed to occur. The SLRS forms a half-graben trending NE-SW and linking the NW-trending SRG and OBG, which are both interpreted as Iapetan aulacogens. The SLRS faults border the contact between the Precambrian Grenville to the NW and Paleozoic St. Lawrence Lowlands to the SE whereas the SRG and OBG are bounded by WNW-ESE trending faults. Based on field relations, faulting along the SLRS has been dated as younger than the Devonian Charlevoix impact crater and it may be as young as Mesozoic based on the isotopic signature of fault-related quartz-calcite veins. However, it is difficult to constrain the timing of the most recent faulting along these structures due to the lack of thermochronologic data and the absence of strata younger than Ordovician.

Apatite fission-track (AFT) ages from the SLRS and SRG determined from Grenville basement rocks have shown Mesozoic age discontinuities suggesting fault reactivation. Along the SLRS, AFT age offsets exist between the footwall (200-184 Ma) and hanging wall (152-149 Ma) samples at Sault-au-Cochon, Cap-aux-Oies and Montmorency Falls and suggest that Early Jurassic normal faulting was followed by Late Jurassic fault inversion. Five new AFT ages from the Shawinigan – Trois Rivières region along the southern part of the SLRS also show comparable AFT age offsets between the footwall (216-211 Ma) and hanging wall samples (165-162 Ma) of the St-Cuthbert fault, extending the area affected by Late Jurassic fault inversion ~130 km farther south.

Along the SRG, AFT age offsets have been determined between the hanging wall (430-280 Ma) and footwall (220-180 Ma) samples along the Lac Kénogami and Rivière Ste-Marguerite faults suggesting normal faulting at ~200 Ma. Twenty-four samples of Grenville basement rocks and Potsdam Sandstone were collected along the OBG last summer for AFT analysis to determine if Mesozoic reactivation of these faults has occurred. AFT age determinations for these samples are in process.