Kinematics and tectonic significance of transpressive structures within the Coast Plutonic Complex, British Columbia

Structural data from the Coast Plutonic Complex, near Prince Rupert, British Columbia, are consistent with a deformational history dominated by dextral transpression from Campanian to Paleocene time. Penetrative east-side-up, southwest-directed, ductile shearing produced moderately northeast-plunging overturned kilometer-scale isoclinal folds. These folds are dextrally sheared and refolded into kilometer-scale upright northwest-plunging folds and steeply dipping transposed foliations with moderate to shallow northwest-plunging lineations along their western side. An east-side-up component to the transcurrent shearing is kinematically compatible with east-side-up shearing found within the Great Tonalite Sill. Kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression. The location of these structures and overprinting relationships suggest the Great Tonalite Sill intruded late-kinematically into a crustal-scale dextral transpressive shear zone. These results indicate this shear zone could form part of the Baja-B.C. fault system that would have accommodated large northward displacements of the terranes making up western British Columbia and southeast Alaska. This conclusion is based on: (1) It is favorably located to accommodate the proposed displacements; (2) Deformation occurred during the time period of proposed large displacement (83–59 Ma); (3) The 15-km thickness of the shear zone indicates it records large displacements.