Deformation of the Pacific/North America Plate Boundary at Queen Charlotte Fault: The Possible Role of Rheology

• Published in: Journal of geophysical research. Solid earth 2018-05, Vol.123 (5), p.4223-4242
• Main Authors: Brink, U. S., Miller, N. C., Andrews, B. D., Brothers, D. S., Haeussler, P. J.
• Format: Article
• Language: English
• Subjects: Activation; Archipelagoes; Asthenosphere; Azimuth; Basins; bimaterial interface; Boundaries; Composition; Concretions; Convergence; Crustal deformation; Crusts; Deformation; Deformation mechanisms; Dipping; Elastic properties; Fault lines; Faults; Folds; Geological faults; Geophysics; Gravity; Haida Gwaii; Kinematics; Lithosphere; Oceanic crust; Oceans; ocean‐continent boundary; Offshore; Plate boundaries; Plate motion; Plate tectonics; Properties; Rheological properties; Rheology; Rock properties; Slip; strike‐slip fault; subduction initiation; Thrust; Thrust faults; Transform faults; Transform plate boundaries; Yakutat terrane
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2017JB014770

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/yr 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 continent/ocean boundary (COB), an unusual location for continental transform faults because plate boundaries typically localize within the continental lithosphere, which is weaker. Because the COB 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 COB in the wake of Yakutat terrane's northward motion along the margin. Minor convergence across the plate boundary is probably accommodated by fault 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. Plain Language Summary The Pacific‐North America plate boundary offshore British Columbia and Alaska is similar in its length, relative motion, and kinematic history to the plate boundary in California, yet the deformation styles along these plate boundaries are distinct. The California boundary shows a broad zone of parallel strike slip and thrust faults and folds, whereas deformation in Canada and Alaska is centered on a single ~900‐km‐long fault, the Queen Charlotte Fault (QCF). We propose that the plate boundary along the QCF follows the ocean‐continent boundary, an unusual location for continental transform plate boundaries. We propose a scenario for the transfer of plate boundary deformation to the ocean‐continent boundary following the passage of a thick oceanic crustal block along the plate boundary. We argue that the