LARGE MID-CRETACEOUS SINISTRAL SHEAR ZONES LOCALIZED IN THE COAST MOUNTAINS BATHOLITH, BC

The Coast Mountains batholith (CMB) is a 1700 km-long magmatic arc province that extends from northern Washington to southeastern Alaska along the British Columbia coast. Plutonic rocks of the CMB straddle and intrude the boundary between the Insular and Intermontane superterranes and preserve key age constraints on the assemblage of suspect terranes in the northern North American Cordillera. The plate boundary conditions of NW North America during Jura-Cretaceous time remain poorly understood, but recent zircon U-Pb, paleomagnetic, and plate circuit reconstruction studies support large (>800 km) southward motion of the outboard Insular terrane relative to the inboard Intermontane terrane during Early Cretaceous time. In this study, we document large (>270 km long) sinistral shear zones along the western margin of the CMB that could have accommodated this motion. We also provide large-n igneous zircon U-Pb ages of pre-, syn-, and post-kinematic intrusions that bracket the ages of shear zone deformation at three locations between Kitkatla (~54ºN) and Klemtu (~52ºN). By producing ~100 analyses per sample, we are able to lower internal and external uncertainties associated with LA-ICPMS and increase igneous age precision to ~0.5% (2-sigma). Samples of ductilely deformed plutonic host rock consistently yield ages between 107 and 103 Ma. Cross-cutting pegmatites and aplitic dikes yield slightly younger ages between 106 and 101 Ma, and at each sampling location, cross-cutting ages are within 2 million years of the deformed host. These age relationships provide tight constraints on the timing of shear zone cessation and highlight the synchronicity of sinistral strike-slip deformation in the western CMB during mid-Cretaceous time. Deformed tonalites within the shear zone also show evidence of pre-full crystallization fabrics, suggesting that the shear zones were localized within the active magmatic arc during oblique convergence. This interpretation is supported by the coincidence of shear zone cessation with an eastward migration in CMB magmatism and a change in relative plate motions from sinistral-oblique to orthogonal convergence.