GEOMETRY AND KINEMATICS OF A TRICLINIC TRANSPRESSIVE TERRANE BOUNDARY IN THE CANADIAN APPALACHIANS

The Minas Fault zone, a transpressive Palaeozoic terrane boundary, is a long-lived zone of intense deformation of very low metamorphic grade and significant displacement that was reactivated as a Mesozoic transtensional boundary. Current interpretations indicate dominantly dextral transpression between the Avalon and Meguma terranes during the Acadian orogeny.

In the Minas fault zone triclinic transpression symmetry is indicated by fault zone structural elements. Strain partitioning played a crucial role in the development of the geometries observed in this transpressive regime. There is a sharp demarcation between an internal high strain zone, in which transcurrent motion was concentrated, and the external portion of the fault zone which exhibits contractional structures and less intense deformation. The abruptness of this transition within a crustal scale shear zone is remarkable and defines distinct deformation domains within the same overall tectonic picture.

Within the internal zone intense deformation has accommodated large displacements and produced a wide range of fault rocks and tectonic fabrics including compositional layering, sheath folds, cleavages, cyclic-introduced veins, S-C fabrics, shear bands, fault gouge, breccia and discrete fractures. With the increase in strain from margin to core of the internal zone comes the progressive localization of deformation, culminating in the discrete S/C–shear band domain. The narrowing of these domains and the concomitant increase in strain can be explained as reflections of variation in strain rate across the fault zone.

There is a general progression from ductile through semi-brittle to brittle deformation as the fault zone evolves. Nevertheless, deviations in mechanical response from the deformation regime dominant at anyone time are legion. Localization, partitioning and distribution of deformation routinely reflect these deviations, and are in turn factors which influence the type and orientation of foliation or planar feature produced; for example, shear bands vs Riedel shears and compositional layering vs fault gouge. A high degree of correlation that corresponds to the regional tectonic movement picture is demonstrable amongst ductile, brittle and transitional deformation features.