Evidence for regionally continuous Early Cretaceous sinistral shear zones along the western flank of the Coast Mountains, coastal British Columbia, Canada

Abstract

The plate-boundary conditions of the Mesozoic North American Cordillera remain poorly constrained, but most studies support large (>800 km) southward motion of the Insular and Intermontane superterranes during Jurassic–Cretaceous time. An implicit feature in these models of large coastwise displacements is the presence of one or more continental-scale sinistral strike-slip faults that could have dismembered and displaced terrane fragments southward along the western margin of North America prior to the onset of mid-Cretaceous shortening and dextral strike-slip faulting. In this study, we documented a system of sinistral intra-arc shear zones within the Insular superterrane that may have accommodated large southward motion. Employment of a new large-n igneous zircon U-Pb method more than doubled the precision of measurements obtained by laser ablation–inductively coupled plasma–mass spectrometry (from ~1% to 0.5%) and allowed us to demonstrate the close temporal-spatial relationship between magmatism and deformation by dating comagmatic crosscutting phases. Crystallization ages of pre-, syn-, and postkinematic intrusions show that the intra-arc shear zones record an Early Cretaceous phase of sinistral oblique convergence that terminated between 107 and 101 Ma. Shear zone cessation coincided with: (1) collapse of the Gravina basin, (2) development of a single voluminous arc that stitched the Insular and Intermontane superterranes together, and (3) initiation of east-west contractional deformation throughout the Coast Mountains. We interpret these concurrent tectono-magmatic events to mark a shift in plate kinematics from a sinistral-oblique system involving separate terranes and intervening ocean basins to a strongly convergent two-plate margin involving a single oceanic plate and the newly assembled western margin of North America.