Pacific Geoscience Centre, Geological Survey of Canada
Why is the North America Cordillera high? Why is it a “Mobile Belt”?
Global mountain belts are commonly concluded to be a consequence of crustal thickening resulting from continental collision, with high elevations supported by crustal roots. However, accumulating seismic structure data indicate that only a few mountain belts have a crustal root. Most of the North American Cordillera has 30-35 km crust in contrast to 40-45 km for the lower elevation craton and other stable areas, a violation of simple Airy isostasy. The explanation lies in the thermal regime. The Cordillera and most other current and recent subduction backarcs are concluded to be remarkably uniformly hot, 800-900C at the Moho, in contrast to cratons, 400-500C at the Moho. Constraints come from: heat flow-heat generation models; mantle tomography velocities; xenolith pressure (depth)-temperatures, Te (effective elastic lithosphere thickness); seismic and electrical thickness of lithosphere; maximum depth of seismicity; depth of Curie temperature; and thermal elevation. Based on these constraints, the thermal contribution to the Cordillera elevation relative to the the craton is ~1,600 m, very close to the elevation difference observed for similar crustal thicknesses. The thermal expansion contribution results in the Cordillera having a high elevation in spite of having an average thin crust. Exceptionally high elevations in Tibet-Himalayas and central Andes have elevation contributions from thick crust as well as thermal expansion.
Other consequences of high temperatures in backarc mountain belts are: (1) Weak mobile belts like the Cordillera are a consequence of their high temperatures. Plate boundary forces and the elevation gravitational potential are sufficient for ongoing lithosphere deformation. Cratons usually are too cold and strong. (2) In the Cordillera and most other hot back arcs, the lower crust is very weak and commonly acts as a detachment over long distances. Lower crust horizontal reflectors may mark the detachment. Also, foreland basal thrusting often detaches into the backarc lower crust. (3) In hot backarcs, high metamorphic temperature gradients (e.g., Barrovian metamorphism) predate collision orogeny. No "heat of orogeny" is required to explain high grade metamorphism in ancient orogenic belts. (4) Cordillera earthquakes occur only in the brittle upper crust, 10-15 km. (5) When subduction terminates, backarcs cool with a time constant of about 300 m.y. and approach a cold craton thermal regime by ~1 Ga. As an example, the former Appalachian backarc has now substantially but not completely cooled.