Kinematics of the Fairweather-Queen Charlotte Transform System and Deformation of the Northeast Pacific plate

Abstract:

The Fairweather-Queen Charlotte system is a 1200-km long transform fault that stretches from the Cascadia subduction zone to the St. Elias collisional belt in Alaska and Canada. The fault system is very active, with 6 M7+ earthquakes over the past century. Previous geodetic models, which are based only on onshore data, predicted high amounts of obliquity along the offshore, southern portion of the fault system. Recent geomorphic studies utilizing high-resolution bathymetric data suggest that the offshore sections of the fault may have highly localized strike-slip motion and only limited convergence. In this presentation, we will evaluate how the geomorphic and geodetic data can be reconciled into an improved regional tectonic model.

Nominally the boundary between the Pacific and North American plates, the regime revealed by geodetic data is much more complex. The Yakutat block, a terrane that moved northward along the transform system and is currently colliding with and subducting beneath southern Alaska, is a major tectonic feature. GPS data from sites in coastal southeast Alaska require that the Yakutat block move distinctly from the Pacific plate. The northernmost Queen Charlotte fault serves as the eastern boundary of the Yakutat block. The Pacific plate is assumed to lie to the west of the central and southern Queen Charlotte fault, but published estimates of Pacific plate motion generate fault normal motion that disagrees with the geomorphic evidence. Adding obliquity and slip rate estimates derived from the geomorphic data to the geodetic model allow an estimation of the motion of a Northeast Pacific block, which moves at a rate different from either Yakutat or Pacific. The northwesterly moving Northeast Pacific block results in strike-slip rates along the Queen Charlotte fault consistent with previous geodetic and geomorphic rates and significantly lower convergence rates than those of previous geodetic models. The predicted motion of the Northeast Pacific block is not compatible with right-lateral slip along the Gulf of Alaska shear zone. Explaining motion along the shear zone may require a separate block north of the Northeast Pacific block or significant internal deformation of the block, suggesting that the Pacific plate may be severely strained due to far-field stresses such as the Yakutat collision.