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