Slip partitioning, crustal tectonics and deformation of the Queen Charlotte margin and northern Vancouver Island




Hippchen, Sabine

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Part I of this thesis investigates current deformation in western British Columbia from northern Vancouver Island in the south to Haida Gwaii in the north. The area is characterized by transition from the Cascadia subduction zone to the Queen Charlotte transform fault. The tectonic setting involves interactions between the Pacific, North America, Juan de Fuca, and Explorer plates, and the Winona block, involving a number of plate boundaries: the mainly strike-slip Queen Charlotte, Revere-Dellwood-Wilson and Nootka faults, the Explorer ridge, and the Cascadia subduction zone. Using GPS campaign data from 1993 to 2008 I derive a new crustal velocity field for Northern Vancouver Island and the adjacent mainland, and integrate it with previous velocity fields developed for Haida Gwaii, southern Vancouver Island and the adjacent mainland. The northern limit of the subduction zone is confirmed to be at Brooks Peninsula, where the direction of the crustal motion changes abruptly from ENE to NNE. I use viscoelastic models to explore what percentage of the observed deformation is transient, related to the earthquake cycle, and how much is permanent ongoing deformation, distributed off the continental margin. Previous authors have developed two competing end-member models that can each explain how the Pacific/North America plate convergence is accommodated off Haida Gwaii. These models assume either internal crustal shortening or underthrusting of the Pacific plate. These new GPS data allow me to conclude that underthrusting does occur, and that a small component (<15%) of the observed data reflects long-term deformation. South of Haida Gwaii the distinction between transient and long-term deformation is not as clear; however, I conclude that transient deformation alone cannot fully explain the observed velocities, and so long-term deformation likely must also occur. Part II of the thesis investigates the updip and downdip limits of the seismogenic zone of the Sumatra megathrust fault. Temperature and downdip changes in formation composition are controls proposed for these limits. To examine the thermal control I developed 2-D finite element models of the Sumatra subduction zone with smoothly varying subduction dip, variable thermal properties of the rock units, frictional heating along the rupture plane, and an appropriate thermal state for the incoming plate. The common updip thermal limit for seismic behaviour of 100-150°C occurs close to or at the trench in agreement with the rupture limit of the 2004 earthquake. Off central Sumatra the common downdip thermal limit range of 350-450°C occurs at 30-60 km depth. The 350°C isotherm location is in agreement with the earthquake limits but 450°C is deeper. North of Sumatra, 350°C occurs ~14 km deeper than the earthquake rupture limit. The proposed composition control for the downdip limit, the intersection of the subduction thrust with the forearc mantle, is at a depth of ~30 km, 140-200 km from the trench, in good agreement with the earthquake limits. These results support the conclusion that the Sumatra updip seismogenic limit is thermally controlled, but the downdip limit is governed by the intersection of the downgoing plate with the forearc Moho.



geophysics, geodesy, GPS, numerical modeling, viscoelastic, postseismic, interseismic