CONSTRAINTS ON CURRENT TECTONICS OF THE NORTHERN CANADIAN CORDILLERA

The northern Canadian Cordillera is remarkably tectonically and seismically active, extending from a terrane collision zone on the continental margin to the fold and thrust belt at the eastern mountain front. We describe the current tectonics of the region, the origin of the deformation, and what controls the distribution of deformation. Constraints on the tectonic regime are provided by: (1) precision GPS data; (2) the seismicity distribution, mechanisms, and rates; (3) the thermal regime; (4) estimates of lithosphere thickness and strength; (5) and topography and gravity. The ongoing collision of the Yakutat block in the corner of the Gulf of Alaska has produced large deformation and uplift in the adjacent St. Elias - Chugach mountains. Surprisingly, this collision also appears to be responsible for the strong current deformation 600-800 km to the northeast near the Cordillera deformation front. We conclude that stress is transmitted across the whole northern Cordillera with little intervening deformation. The kinematics are constrained by GPS velocities of ~5 mm/yr northeasterly of the northern Cordillera relative to the stable Craton. The deformation rates from earthquake statistics are similar, ~4 mm/yr of thrust shortening across the MacKenzie mountains, and a similar rate of right-lateral strike-slip in the Richardson mountains. This large-scale motion of the northern Cordillera requires a quasi-rigid displacement of the upper crust over a detachment probably in the lowermost crust. Using a 2D Finite Element mechanical model, we show that the horizontal strain transfer over 600-800 km and the lower crust detachment are made possible by the high temperature of the northern Cordillera lithosphere. Temperatures at the Moho of 800-1000C are indicated by the high heat flow across the whole Cordillera, by several other indicators of deep temperature, and by the thin lithosphere, Te generally less than 20 km, determined from gravity-topography coherence modelling. This strain transfer also requires a weak eastern Cordillera deformation front where the lower crust shear zone rises to join the basal detachment of foreland thrusting. The model for the northern Cordillera may have application in a number of other areas, such as the earlier thrusting in the southern Canadian Rocky Mountains driven by terrane collision along the Cascadia Pacific margin.