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UBC Theses and Dissertations

Late Devonian shear zone reactivation in the Canadian Appalachian orogen Piette-Lauzière, Nicolas


Accretionary orogens are characterized by sequential collisions of buoyant crustal material with a continental margin or an oceanic arc. Because the timing, kinematics, and intensity of deformation are controlled by the obliquity of the collisions, pre-accretionary shape, and rheology of the accreted crustal material, it is often difficult to determine what processes controlled the generation or reactivation of shear zones not located in the immediate vicinity of a suture marking a collision. In the Canadian Appalachians, this problem is compounded by the lack of tectonic interpretation for the regional deformation occurring during the Late Devonian oblique collision of Meguma to composite Laurentia during the Neoacadian Orogeny. To address these issues, this thesis targets the Eastern Highlands and Pocologan-Kennebecasis shear zones (Nova Scotia and New Brunswick, respectively) which were part of the composite margin of Laurentia at the time of the Neoacadian orogeny. Several characteristics of these shear zones are quantified including 1) the timing of deformation, through U-Pb and ⁴⁰Ar/³⁹Ar geochronology; 2) the kinematics based on field and thin section observations and quartz crystallographic preferred orientations; and 3) the potential impact of fluid-rock interaction on shear zone rheology. The new information from this work is combined with regional interpretation of aeromagnetic and gravimetric depth slices, and compilation of shear zone deformation ages and ⁴⁰Ar/³⁹Ar cooling ages to provide insight into the tectonic setting of the Neoacadian Orogeny. In the northern Appalachians, between the Late Devonian and Mississippian, regionally extensive NE-SW and ENE-WSW oriented shear zones were formed, or reactivated, with a dextral strike-slip shear sense consistent with a large-scale C-C’ system. Aeromagnetic and gravity geophysical data indicate that several of these structures are listric, and thus formed a lateral succession of transpressive and transtensive segments crosscut by antithetic sinistral shear zones. These structures appear to define a syntaxis in New England. ⁴⁰Ar/³⁹Ar age transects across the region outline Neoacadian cooling and exhumation that correlates spatially to an area of high paleo-elevation that was contemporaneous to the opening of the Maritimes and Catskills basins. Such a setting is similar to the present-day oblique collision of Yakutat terrane to Western North America.

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