MESOZOIC UNROOFING OF THE OTTAWA-BONNECHERE GRABEN, ONTARIO, BASED ON APATITE FISSION-TRACK THERMOCHRONOLOGY

The Ottawa-Bonnechère graben (OBG) along the southern Quebec-Ontario border, the Saguenay River graben (SBG), and the St. Lawrence Rift System (SLRS) are major brittle fault zones along which reactivation of Iapetan-related structures is believed to occur. Both the OBG and the SRG are bounded by WNW-ESE trending faults and are interpreted as Iapetan aulacogens. Their NW-trending structures are linked by the half-graben formed by the NE-SW trending SLRS. Recent regional tectonic activity along the OBG is indicated by the occurrence of a magnitude 5.5 earthquake in June 2010 about 60 km from Ottawa.

Twenty-four samples of Grenville basement rocks and Potsdam Sandstone were collected for apatite fission-track (AFT) analysis along a series of transects that crossed the OBG faults throughout the graben. Hanging wall and footwall sample pairs were analyzed in order to constrain the timing of potential fault reactivation in the graben. From these samples, 18 AFT ages ranging from 252 ± 38 to 162 ± 18 Ma were determined, indicating early Triassic to middle Jurassic unroofing. All of the hanging wall and footwall sample pairs yielded AFT ages that were within analytical error of each other with one exception, a hanging wall sample from near Shamrock, Ontario (252 ± 38 Ma) and a footwall sample from near Burnstown, Ontario (183 ± 19 Ma). Preliminary track length measurements for a hanging wall and a footwall sample yielded comparable mean track lengths and distributions of 12.9 ± 1.6 and 12.9 ± 1.3 mm, respectively. The absence of consistent AFT age discontinuities across the OBG faults indicates that there was no Mesozoic reactivation of these structures or, more likely, that the displacement was too small to be detected by AFT analysis.

In contrast, AFT ages from Grenville basement rocks from the SLRS and SRG have yielded Mesozoic age discontinuities that suggest fault reactivation. Along the SLRS, AFT age offsets exist between the footwall (200-184 Ma) and hanging wall (152-149 Ma) samples along the Montmorency and Saint-Laurent faults suggesting early Jurassic normal faulting followed by late Jurassic fault inversion. In the SRG, AFT age discontinuities 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 indicating normal fault reactivation at ~ 200 Ma.