LITHOLOGICAL AND STRUCTURAL CORRELATIONS BETWEEN THE QUÉBEC AND WESTERN NEW ENGLAND APPALACHIANS – A REVIEW

In Québec and western New England, the Appalachian orogen is made up of three megasequences, (1) Neoproterozoic to Late Ordovician rocks of the Humber zone, (2) Ordovician rocks of the Dunnage zone, both being the remnants of the Laurentian margin and adjacent Iapetan oceanic domain, respectively, and (3) the unconformable sequence of Silurian-Devonian rocks of the Gaspé Belt, a sedimentary basin that extends from New England to Gaspé Peninsula. This talk presents a review of the progress of knowledge of these rocks in terms of cross-border correlations, with emphasis on the structural and metamorphic variations. The Humber zone is divided into external and internal zones. The external Humber zone forms a NW-verging nappes stacking of very low- to low-grade rocks formed during the Ordovician Taconian orogeny, whereas the internal Humber zone consists of greenschist- to upper amphibolite-facies metamorphic rocks preserving complex polyphased structures spanning Ordovician to Devonian metamorphic ages. The external-internal Humber zones contact is marked by SE-directed thrust faults, the Bennett-Brome-Honey Hollow faults, that define an upper plate-lower plate (UP-LP) boundary. Eastward, a series of major SE-dipping normal faults crosscut that UP-LP boundary, and separate domains of contrasting structures and metamorphic ages. The Dunnage zone occurs in the hanging wall of the normal faults. It consists of ophiolites and their metamorphic basement, both unconformably overlain by syn- to post-orogenic deposits that are, however, not preserved in western New England. Stratigraphic relations between the Humber-Dunnage zones and the Gaspé Belt suggest N-S trending differential exhumation during the Ordovician and the Silurian. Acadian deformation is more penetrative in New England than in Quebec, rendering difficult the distinction between Taconian and Salinian structures in the pre-Silurian sequences. Acadian peak metamorphism is significantly younger and of lower grade in southern Québec than in amphibolite- and higher-grade rocks of western New England. Along-strike comparison and correlation of tectonic fabrics suggest that variations in the timing and P-T conditions of deformation have been the result of the northward migration of metamorphic and structural fronts during orogenic collisions.