Transpression between two warm mafic plates: The Queen Charlotte fault revisited

The Queen Charlotte Fault is a transpressive transform plate boundary betwe en the Pacific and North American plates offshore western Canada. Previous models for the accommodation of transpression include internal deformation of both plates adjacent to the plate boundary or oblique subduction of the oceanic plate; the latter has been the preferred model. Both plates are war m and mafic and have similar mechanical structures. New multichannel seismi c reflection data show a near-vertical Queen Charlotte Fault down to the fi rst water bottom multiple, significant subsidence east of the Queen Charlot te Fault, a large melange where the fault is in a compressive left step, an d faulting which involves oceanic basement. Gravity modeling of profiles in dicates that Moho varies fairly smoothly across the plate boundary. Isostat ic anomalies indicate that the Pacific plate is flexed downward adjacent to the Queen Charlotte Fault. Upward flexure of North America along with crus t thickened relative to crust in the adjacent basin creates topography know n as the Queen Charlotte Islands. Combined with other regional studies, the se observations suggest that the plate boundary is a vertical strike-slip f ault and that transpression is taken up within each plate.