A NEW MODEL FOR THE STRUCTURAL EVOLUTION OF THE LAURENTIAN MARGIN, SOUTHERN QUEBEC: TECTONIC IMPLICATIONS AND COMPARISON WITH NEW ENGLAND

In southern Québec, the Laurentian margin is divided into external and internal zones on the basis of constrasting metamorphism and structures. The external zone is made up of low-grade rocks forming NW-verging thrust slices. The internal zone consists of greenschist- and amphibolite-facies metamorphic rocks that mainly occur in the Sutton-Notre-Dame Mountains anticlinorium (SNDA). It is mostly characterized by SE-verging structures, yielding Late Silurian to Early Devonian 40Ar/39Ar ages, which are superimposed on NW-verging structures. West of the anticlinorium, the Bennett/Brome fault is a major backthrust that defines an upper plate-lower plate (UP-LP) boundary with the external zone in the hanging wall and the internal zone in the footwall. East of the anticlinorium, the St-Joseph fault and the Baie Verte-Brompton line (BBL) represent major normal faults crosscutting that UP-LP boundary. Metamorphic rocks, yielding Ordovician 40Ar/39Ar ages, and rock units of the external zone occur in the hanging wall of the BBL/St-Joseph fault. Isotopic ages suggest that NW-verging structures are related to ophiolite obduction and crustal thickening during the Taconian orogeny (~480 to 445 Ma). SE-verging structures are attributed to backthrusting along the Bennett/Brome fault and to normal faulting along the BBL/St-Joseph fault, from ~430 Ma to 410 Ma. This event involves the hinterland-directed transport of the Taconian crustal wedge, with Ordovician metamorphic ages preserved in the upper plate, followed by normal faulting and juxtaposition to metamorphic rocks of the lower plate along the BBL/St-Joseph fault. In northern New England, the Green Mountains anticlinorium (GMA) correlates with the SNDA. West of the GMA, the Brome thrust/Honey Hollow fault system corresponds to the Bennett/Brome fault. East of the GMA, the Burgess Branch fault separates domains of contrasting metamorphism and correlates with the BBL/St-Joseph fault. Due to Acadian metamorphism, the recognition of distinct Ordovician and Silurian/Early Devonian tectonic events is difficult in New England. However, the structural pattern compares well with southern Quebec, suggesting that both regions share the same structural evolution.