T273 Living on the Edge: Deformation along the Cascadia Margin

Wednesday, 21 October 2009
1:30 PM - 5:30 PM
B110/111/112

CURRENT TECTONICS AND DEFORMATION IN NORTHERN VANCOUVER ISLAND, SOUTHERN QUEEN CHARLOTTE ISLANDS, AND THE ADJACENT MAINLAND

Paper number: T273-9
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

The area south of the Queen Charlotte Islands and north of Vancouver Island is characterized by transition from the Cascadia subduction zone to the Queen Charlotte transform fault zone. The tectonic setting involves Pacific, North American, Juan de Fuca, Explorer plate, and Winona block; as well as Queen Charlotte and Revere-Dellwood-Wilson fault, Explorer ridge, Nootka fault, and the Cascadia subduction zone. Using GPS campaign data from 1993 to 2008 we derive a crustal velocity field for North Vancouver Island and the adjacent mainland. This velocity data is the basis for interpretation of the tectonics of the transition from the plate convergent to transform boundaries. Our GPS data show significant shear velocities in the Bella Coola region, ~ 250 km inland from the Queen Charlotte fault. At that distance interseismic and postseismic effects are expected to be small, and there is very little seismic activity in the area. We use geodynamic models to better understand the discrepancy between the GPS data and the seismic data, and to find out if deformation in the region is transient or permanent ongoing deformation.

To constrain the model, we use the rheology and structure of the region, with reasonable values for elastic thickness and viscosity. If the results suggest that visco-elastic deformation associated with the Q.C. fault is not the cause for the GPS results, other possible explanations (e.g., rigid rotation) will be considered.

Two end-member models to describe how the Pacific/North America plate convergence is accommodated off the Queen Charlotte Islands have been developed by others. They assume either internal crustal shortening or underthrusting of the Pacific plate. With the new GPS data we can determine which model explains the tectonic situation more appropriately. Earlier model results strongly suggest an underthrusting fault fully locked down to 14 km depth, followed by a transition zone to 20 km depth. We are also looking at the contemporary transform motion between the Pacific and North American plates, and how far inland this motion can be accommodated. It is possible that this deformation zone extends up to ~ 400 km inland and could include the Coast Mountains and the western part of the Central Cordillera. Alternatively, the shear represents elastic build-up to be released by large future earthquakes.