MAGMATISM AND THE LOCALIZATION OF STRAIN ALONG VERTICAL BOUNDARIES IN MAGMATIC ARCS

The Coast plutonic complex of western British Columbia records a west to east age progression that has been interpreted to represent arc migration from west to east. However, there are several distinct breaks in the age progression that coincide with domains of vertical foliation, strong deformation gradients and changes in the peak metamorphic pressure. These structures have shallowly plunging lineations that are most consistent with large transcurrent displacement, in addition to the obvious contraction recorded by folds and intense foliations. The overall geometry of the structures indicates that pluton emplacement was in part facilitated by motion on the shear zones which acted as vertical corridors for melt transport. Thermal softening during plutonism focused deformation onto the side of the shear zones which were actively undergoing pluton emplacement. In contrast, the crustal panels with solidified plutons resist ductile deformation resulting in eastward migration of plutonism and deformation. Thus, the age progression is in part related to the eastward migration of transpressive deformation and cannot be solely attributed to subduction zone processes such as slab shallowing. Once established these vertical zones have been repeatedly reactivated, primarily as brittle faults that control much of the fjord topography in central British Columbia, high-lighting the persistence of these vertical structures in controlling deformation.