Pluton Emplacement Kinematics and Arc Collapse during Extensional Tectonics

The Chief Matthew's Bay pluton is a granodioritic sill complex that intruded in the early Eocene into the central gneiss complex, in the Coast Mountains of British Columbia. The pluton is part of the voluminous magmatic flare-up that occurred during the late Paleocene and early Eocene throughout much of the Canadian Cordillera. The pluton contains magmatic and solid-state foliations that are concordant with NW striking, gently SW dipping magmatic fabrics. Stretching lineations defined by aligned minerals define a girdle parallel to the foliation plane consistent with bulk flattening at the scale of the pluton. We analyzed the structural symmetry of shear bands in order to obtain the kinematics in distinct domains throughout pluton. Ductile shear bands were projected stereographically into a ductile flow reference frame (E-W vertical foliation and horizontal lineation). The results show portions of the pluton have shear band distributions with a monoclinic symmetry indicative of non-coaxial deformation, whereas other areas have shear band distributions characterized by orthorhombic symmetry, consistent with a flattening strain. L-tectonites occur in areas where shear bands have monoclinic symmetry and chocolate tablet boudinage occurs in areas where shear bands have an orthorhombic symmetry. The overall pattern of deformation also indicates partitioning of strain into distinct domains of orogen parallel and orogen perpendicular slip. The finite pattern of deformation at the scale of the pluton indicates subvertical flattening and radial stretching consistent with pluton emplacement during tectonic extension. The transposition of earlier steeply dipping foliation is consistent with collapse of the arc during emplacement of subhorizontal sills. Similar patterns of deformation have been documented in other parts of the Coast plutonic complex to the north, indicating that extensional deformation occurred at a regional scale, but is focused into the regions undergoing emplacement of sub-horizontal sills.