IAPETAN VERSUS ATLANTIC RIFTING HISTORY OF LAURENTIA – CONSTRAINTS FROM FIELD MAPPING AND AFT DATING OF PRECAMBRIAN BASEMENT ROCKS, CANADA

The St. Lawrence rift system (SLRS) of eastern North America is a crustal-scale fault zone where reactivation occurs along Proterozoic-Early Paleozoic faults attributed to Iapetus rifting. There is, however, no clear evidence for faulting along the SLRS during the Cambrian, and the temporal and spatial distribution of SLRS structures are still inadequately constrained, particularly the relationships with extension of the Canadian Shield during the opening of the Atlantic Ocean. The SLRS trends NE-SW and forms a half-graben that links the NW-trending Ottawa-Bonnechère (OBG) and Saguenay River grabens (SRG), both interpreted as Iapetan aulacogens. The SLRS faults fringe the contact between the Grenville to the NW and the St. Lawrence Lowlands to the SE whereas the OBG and the SRG are marked by WNW-ESE trending faults. All the faults consist of cohesive cataclastic rocks, major fault zones being marked by ³ 10-to-20 meters-thick of fault breccia, ultracataclasite and foliated fault gouge.

The lack of isotopic ages and absence of post-Ordovician strata along the structures make it difficult to determine the timing of faulting. Apatite fission-track (AFT) ages have been determined for Grenville basement rocks, from the hanging wall and footwall of several brittle faults along both the SLRS and the SRG. Along the SRG, the hanging wall of the Lac Kénogami and Rivière Ste-Marguerite faults yield ages older than ca. 300 Ma (locally as old as 443 ± 44 Ma), whereas the footwall ages cluster at 220-180 Ma. For three transects across the SLRS, the footwall yields Early Jurassic AFT ages, 200 ± 20 to 184 ± 19 Ma, consistent with ages from the SRG, whereas the hanging wall rocks yields younger Late Jurassic ages ranging from 152 ± 17 Ma to 149 ± 14 Ma. These AFT age discontinuities are interpreted as the result of normal faulting at ca. 200 Ma along both the SLRS and the SRG, followed by tectonic inversion at ca. 150 Ma (or younger) along the SLRS. Early Jurassic normal faulting and tectonic inversion are found in the Mesozoic Fundy and Orpheus basins of the Canadian Atlantic coast and have been attributed to rift-drift transition in the Central Atlantic Ocean. Our study provides support for Atlantic-related, extensional and compressive deformation within the interior of the Canadian Shield, > 500 km west of the axis of the Mesozoic rift basins.