Speaker: Collin Brandl, Lamont-Doherty Earth Observatory

Host: Harm Van Avendonk

Title: A New Subduction Zone in the Northeast Pacific? Seismic Imaging of the Queen Charlotte plate boundary

Abstract: The formation of new subduction zones is critical for plate tectonics, but detailed records of subduction initiation are scarce. Obliquely convergent ocean-continent transform plate convergence may be one of the most favorable environments for subduction initiation due to their pre-existing weak zone and significant contrast in lithospheric properties, but there are few locales of this nature that can be observationally studied. The transform Queen Charlotte plate boundary (QCPB) separates the North American and Pacific offshore western British Columbia and southeastern Alaska, connecting the Cascadia and Alaska subduction zones. The QCPB accommodates up to 55 mm/yr of plate motion, mostly along the strike-slip Queen Charlotte Fault, but up to 15° of oblique convergence occurs in its southern segment, offshore Haida Gwaii, BC. A coincident sedimentary wedge (the Queen Charlotte Terrace), a 2012 M7.8tsunamigenic thrust earthquake, and seismological indications of a dipping slab has led many investigators to consider this southern segment a subduction zone, but the region lacks many of the other defining characteristics. A paucity of crustal scale seismic imaging along the plate boundary has left the structure and behavior of the system uncertain. In this talk I will use multichannel seismic reflection profiles acquired in 2021 to constrain crustal structure along the southern segment of the QCPB. This dataset reveals the accommodation mechanisms of oblique convergence and is used to classify this segment as an incipient subduction zone, settling a decades long-debate over the nature of the plate boundary. As an incipient subduction zone, the southern QCPB provides crucial observations of early deformation and the structural evolution that occurs during subduction initiation. Motivated by the 2012 M7.8 earthquake and the potential for future events, I will also use this seismic dataset to estimate the thermal structure of the plate boundary through an analysis of bottom simulating reflectors, better informing future hazard analysis of the region.