Strain and stress partitioning of the Pacific/North America interaction along the Queen Charlotte Islands-Alaska Panhandle

Along the western margin of central British Columbia and southern Alaska, the Queen Charlotte (QC) and Fairweather faults accommodate most of the predominantly transform motion (~50 mm/yr) between the Pacific and North American plates. This fault system is ~1000 km long and was the locus of M=7-8 earthquakes in the last century. Off the Queen Charlotte Islands, the relative Pacific/North America motion also has a 10-15 mm/yr convergence component. New GPS data acquired over the last 5 years allow us to estimate the distribution of transient and permanent strain along this plate boundary. We show that the Queen Charlotte-Alaska Panhandle margin is affected by both transient and long-term dextral shear strain, respectively in response to the interseismic locking of the QC-Fairweather Fault and a 6-7 mm/yr NNE migration of the continental margin. The impact of postseismic deformation following the 1949 M=8.1 QC earthquake is likely not significant. The stress pattern derived from earthquake focal mechanisms is consistent, to a first order, with the GPS kinematic model. Earthquake mechanisms indicate a partitioning between strike-slip and compression on the oceanic side (QC Fault and Terrace). The continental margin (northern QC Islands and Hecate Strait) is affected by a N to NE maximum compression associated with dextral shear of the crust. Long-term dextral shear and shortening of the continental margin is also supported the tectonic inversion of the Miocene normal faults that form the QC Basin. Several kilometers of shortening and strike-slip deformation have been identified on these structures that affect Upper Miocene and Pliostocene deposits.