Deposition, deformation, and flexure in a transpressional trough, Queen Charlotte fault, offshore Haida Gwaii (British Columbia, Canada)

Transpressional deformation along the Pacific–North American plate boundary off British Columbia (Canada) generates interactions between tectonic and depositional processes; first-order deformation is creation of a bathymetric trough by flexure of the Pacific plate. Interpretation of a suite of single-channel seismic reflection data in the central and southern trough shows second-order internal structure and depositional systems within the trough. Much of these sediments originated from Quaternary slope fans, although the age of the underlying oceanic crust is 8–13 Ma. These turbidite deposits overlie hemipelagic deposits. Two distinct layers of turbidite deposits in the trough can be explained by sediment deposition systems rather than tectonic events. We infer that the most recent flat-lying deposits, which have been previously interpreted in the northern trough as indicating a lack of compressive deformation, consist, in fact, of along-axis flows based on the general geomorphology of the margin. Second-order deformation consists of small-offset extensional and compressive structures common within 15 km of the foot of the continental slope. A few normal faults are observed tens of kilometers further out on the flexural bulge. Recent faults that cut the seafloor are observed offshore from the rupture plane of the 2012 M7.8 earthquake and match an area of extensional aftershocks. These faults may not be from bending of the entire bulge, which is 30–40 km wide, but from the plate being pulled into the underthrust zone. Maximum depression of the Pacific plate is also offshore from the region of recent rupture, indicating that this rupture is representative of long-term geologic processes.