NORTHERN CANADIAN CORDILLERA AND CENTRAL ANDES/ALTIPLANO: STAGES OF DEVELOPMENT OF CURRENTLY ACTIVE HIGH MOUNTAIN PLATEAUX

We review the tectonics in two regions of the North and South America Cordilleras that have current compressive deformation and active thrusting in their landward foreland belts. These examples may provide insights into earlier orogenic processes in other areas of the Cordilleras and in ancient orogenic belts elsewhere. The areas are the central Peru-Chile-Bolivia part of the South America Cordillera and the Yukon northern part of the North American Cordillera. Both areas are in current or recent backarcs with very high temperatures in the lower crust and lithosphere thicknesses of only ~60 km. In both areas there is thrust shortening in the eastern foreland. The convergence in the Yukon is driven by the collision of the small Yakutat Terrane in the corner of the Gulf of Alaska. There is very strong deformation and seismicity in the coastal collision zone of the St. Elias Mountains. Inland, the whole northern Cordillera is moving to the northeast at about 4 mm/yr, with rather little internal deformation as indicated by seismicity and GPS data. On its northeastern margin, the Cordillera upper crust is over-riding the adjacent craton in foreland thrusting at a similar rate. The convergence in the Peru-Chile margin may be driven either by flat slab subduction or by the increased subduction thrust traction associated with the low sediment input into the trench. Inland, the Cordillera upper crust appears to be overthrusting the adjacent craton at about 10 mm/yr as indicated by both GPS data and the rate of seismicity. As in the Yukon, there is relatively little current deformation in the intervening Altiplano/Puna. In the Yukon Cordillera, the crust is thin, similar to most other areas of the N. Am. Cordillera, averaging about 33 km. There has been little crustal thickening, perhaps because of the short duration of the present terrane collision, less than about 5 m.y., and because the effect of the small collision area is spread over a large 3D area. In contrast, the central S. Am. Cordillera has a thick crust probably produced in the past ~10 m.y. from a roughly 2D convergence. The over-ridden craton appears to act as a piston, shorting and thickening the lower crust of the hot and weak Cordillera and uplifting the Altiplano/Puna plateau.