Deformation of the Pacific-North America plate boundary at Queen Charlotte Fault: The possible role of rheology
The Pacific-North America (PA-NA) plate boundary between Vancouver Island and Alaska is similar to the PA-NA boundary in California in its kinematic history and the rate and azimuth of current relative motion, yet their deformation styles are distinct. The California plate boundary shows a broad zone of parallel strike-slip and thrust faults and folds, whereas the 49 mm/y PA-NA relative plate motion in Canada and Alaska is centered on a single, narrow, continuous ~900-km-long fault, the Queen Charlotte Fault (QCF). Using gravity analysis, we propose that this plate boundary is centered on the ocean-continent boundary (OCB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the OCB is a boundary between materials of contrasting elastic properties, once a fault is established there, it will probably remain stable. We propose that deformation progressively shifted to the OCB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by normal faults reactivation on Pacific crust and by an eastward dipping QCF. Underthrusting of Pacific slab under Haida Gwaii occurs at convergence angles >14°-15° and may have been responsible for the emergence of the archipelago. The calculated slab entry dip (5°-8°) suggests that the slab probably does not extend into the asthenosphere. The PA-NA plate boundary at the QCF can serve as a structurally-simple site to investigate the impact of rheology and composition on crustal deformation and the initiation of slab underthrusting.