Geoscience Colloquium: Why is the North America Cordillera high? Hot backarcs, thermal isostasy, and mountain belts

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 Airy isostasy. It has been shown previously that most such mountain belts are in present or recent backarcs that are uniformly hot. From thermal constraints we predict a uniform ~1,600 m elevation support of the Cordillera by thermal expansion compared to stable areas. Over most of the Cordillera the actual elevation difference after correction for variable crustal thickness and density is in excellent agreement. When subduction and shallow backarc convection stop, the lithosphere may cool and the elevation of mountain belts subside over ~300 Ma.

Some other consequences of uniformly hot backarcs like the Cordillera are: (1) thin, hot, and weak lithospheres, i.e., “mobile belts” that deform readily under changing plate tectonic forces, (2) Regional horizontal detachments in the ductile lower crust that rise in the basal detachments of foreland belts, (3) Regional Barrovian metamorphism gradients with temperatures at the Moho of 800-900C, (4) Earthquakes that only occur in the upper 10-15 km “brittle” part of the crust.