LATE PALEOZOIC TO EARLY MESOZOIC UNROOFING OF THE CANADIAN SHIELD IN SOUTHERN QUEBEC BASED ON APATITE FISSION-TRACK ANALYSIS

The St. Lawrence rift system (SLRS) and the Saguenay River graben (SRG) in southern Québec are active fault zones along which reactivation of Iapetan-related structures is believed to occur. The rift faults border the contact between the Precambrian Grenville to the NW and Paleozoic St. Lawrence Lowlands to the SE. It is difficult to constrain the timing of the most recent faulting because of a lack of isotopic data and the absence of strata younger than Ordovician. Field relations suggest that faulting is younger than the Devonian Charlevoix impact crater and may be as young as Mesozoic based on isotopic dating of fault-related quartz-calcite veins. Apatite fission-track (AFT) ages determined for Grenville basement rocks from the SLRS and SRG have suggested Mesozoic offsets have occurred along these fault systems. In order to determine the extent of Mesozoic unroofing away to these major fault zones, samples were collected along two transects between them. One transect, yielding AFT ages ranging from 260 ± 34 to 212 ± 28 Ma, extends for ~40 km to the NW from the St. Lawrence River. The second traverse, extending SE along Rt. 381 from the Saguenay graben near La Baie yields AFT ages ranging from 241 ± 32 Ma to as old as 380 ± 40 Ma in the interior of southern Québec. However, at Cap-aux-Corbeau and Ile-aux-Coudres along the SLRS, AFT ages are significantly younger, 188 and 186 Ma, respectively. Along the SLRS, AFT age discontinuities exist between the footwall (200-184 Ma) and hanging wall (152-149 Ma) samples at Sault-au-Cochon, Cap-aux-Oies and Montmorency Falls suggesting Early Jurassic normal faulting followed by Late Jurassic fault inversion. AFT age discontinuities also exist along the SRG where the hanging wall of the Lac Kénogami and Rivière Ste-Marguerite faults yields ages of ~430- 280 Ma, whereas, the footwall ages are ~220-180 Ma. AFT ages from the transects through the interior of the Canadian Shield in southern Québec indicate a comparable unroofing time to those determined for the SRG hanging wall samples, ~350-220 Ma, during the Early Carboniferous to Late Triassic. Moreover, this part of the Canadian Shield appears to be unaffected by Mesozoic fault reactivation.