Yakutat Block Collision in the Gulf of Alaska and Stress Transfer Across the NE Canadian Cordillera

The collision of the Yakutat block with the North American plate in the SE corner of the Gulf of Alaska produces large uplift and deformation in the adjacent St Elias mountains. Surprisingly, this collision also results in strong thrust faulting seismicity in the MacKenzie mountains near the eastern Canadian Cordillera deformation front, about 800 km to the northeast of the collision zone. We propose that stress is transmitted across the whole northern cordillera with little intervening deformation, despite the high heat flow and the inferred thin mechanical lithosphere. To support this hypothesis, we have developed a kinematic and mechanical model of stress transfer from the collision zone to the Cordillera deformation front. The kinematics is contrained by campaign GPS velocities recently acquired in the Yukon and Northwest Territory, and by the Whitehorse-Yellowknife permanent GPS baseline. GPS data indicate a northeast motion of the northern Cordillera block at about 5 mm/yr, relative to the stable craton to the east. We also show seismic moment release statistics that indicate 1-2 mm/yr of shortening across the MacKenzie range and about 3 mm/yr of right-lateral strike-slip in the Richardson mountains to the north, in agreement with the GPS velocities. This northeast motion of the northern Cordillera requires a horizontal quasi-rigid displacement of the upper crust accommodated by a detachment in the weak lowermost crust. Using a 2D Finite Element mechanical model, we show that the horizontal strain transfer over 800 km and the development of a lower crust detachment zone is made possible by the high temperature of the northern Cordillera lithosphere (up to 800-1000C at the Moho based on very high heat flow data). Our model also indicates that this strain transfer requires a weak eastern Cordillera deformation front where the lower crust shear zone rises to reach the basal detachment of thrusting in the foreland basin.