Abstract

The St. Lawrence rift system is an active fault zone where reactivation of Late Proterozoic, Iapetus-related faults is believed to occur. It forms both a NE-SW–trending half graben and NW-SE–trending grabens that link and include the Ottawa-Bonnechère and Saguenay River grabens, both interpreted as aulacogens. The rift faults locally preserve kinematic evidence for normal-sense displacement. The lack of isotopic chronological constraints and the absence of strata younger than the Ordovician make it difficult to determine the timing of fault increments. Field relations in the Québec City, Charlevoix, and Saguenay River regions suggest faulting event(s) younger than the Devonian-age Charlevoix crater. We determined apatite fission-track (AFT) ages for Grenvillian basement rocks from the hanging wall and footwall of typical rift faults at different locations of the St. Lawrence and Saguenay River fault systems along the St. Lawrence rift system. The footwall rocks of major faults yielded Late Triassic–Early Jurassic AFT ages ranging from 235 ± 28 Ma to 184 ± 22 Ma. The hanging-wall samples yielded older ages, between 443 ± 44 Ma and 279 ± 28 Ma along the NW-trending Saguenay River fault system that are consistent with Triassic–Early Jurassic normal faulting at ca. 250–200 Ma based on HeFTy inverse models of fission-track length distributions. Younger Late Jurassic AFT ages, ranging from 152 ± 17 Ma to 149 ± 14 Ma in the hanging wall of the NE-SW–trending St. Lawrence fault system, suggest the occurrence of localized faulting increments during the Late Jurassic. The latter AFT age discontinuities are interpreted as the result of normal faulting at ca. 250–200 Ma, followed by tectonic inversion. Evidence for the reactivation and inversion of preexisting normal faults during or after Jurassic time is also found in the Mesozoic rift-related basins of the Atlantic coast. The NW-SE–trending compressive stress field currently documented along the St. Lawrence River is consistent with tectonic inversion along the St. Lawrence rift system. This study provides geochronological support for Atlantic rift-related extensional deformation followed by compressional deformation in the interior of the Canadian Shield, more than 500 km west of the axis of the Mesozoic Atlantic rift basins.

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