Abstract

The collision of the Yakutat block in the corner of the Gulf of Alaska has resulted in large deformation in the adjacent Chugach–Saint Elias Mountains. This collision is inferred to produce the strong seismicity in the Mackenzie and Richardson Mountains of the northern Canadian Cordillera foreland belt, 800 km to the northeast. Strain is transmitted from the Yakutat collision across the northern Cordillera with little intervening deformation, in spite of high heat flow and thin mechanical lithosphere. Global Positioning System results and seismic deformation rates indicate a northeastward motion of the northern Cordillera at ∼5 mm/yr, relative to the craton to the east, that is mostly accommodated in the foreland belt. This quasi-rigid displacement of the Cordilleran upper crust requires a decoupling level in the weak lower crust that rises to join the basal detachment of thrusting in the foreland. Based on a two-dimensional mechanical model, we show that the strain transfer and the lower crust detachment are made possible by the high temperature (∼900 °C) at the base of the Cordilleran crust. The northern Cordillera presents a type example of an orogenic float associated with large-scale décollement in weak lower crust.

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