Kinematics and timing of shear zone deformation in the western Coast Belt: evidence for mid-Cretaceous orogen-parallel extension

Abstract

Northwest-striking sinistral shear zones characterize mid-Cretaceous deformation in the western Coast Belt south of Prince Rupert in north coastal British Columbia. Structurally focused mapping and geochronology have revealed a component of lateral extension to this deformation. General flow characteristics of the shear zones are identified by comparison of fabric patterns to published models for fabric development in shear zones. U–Pb ages from synkinematic dykes constrain motion on northwest-striking sinistral transpressional shear zones, including the Useless, Barrett and Salt Lagoon shear zones, to ca. 104–96 Ma, and dextral transpression on the north-striking Telegraph Passage shear zone to ca. 97 Ma. The geometry, kinematics, and coeval nature of these shear zones suggest that they formed in part as a ductile conjugate set. The orientation of these conjugate sets indicates ENE–WSW (orogen perpendicular) shortening and NNW–ESE (orogen parallel) extension. The conjugate shear zones merge together into the Grenville Channel shear zone, a sinistral transpressional shear zone with high strike-parallel stretch indicating the same strain field. This local strain regime is consistent with large-scale mid-Cretaceous tectonic escape as proposed for the northern Cordillera at that time, expressed in coeval sinistral faulting in the Coast Belt and dextral faulting in the northern Omineca belt.