THE ORIGIN OF GLOBAL MOUNTAIN BELTS: HOT SUBDUCTION ZONE BACKARCS AND HIGH PLATEAUX

Our understanding of the origin of current mountain belts and ancient orogenic belts was revolutionized by the simple and elegant plate tectonic model of continental collision and crustal thickening. However, the significant discrepancies between some of the predictions of the simple collision model and observations are not widely appreciated. Examples are: (1) many mountain belts show rather little associated shortening deformation of the exposed crust; they are high plateaux, e.g., Tibet, Altiplano/Puna, (2) a number of major mountain belts have normal or thin crust, e.g., most of N. American Cordillera, (3) major mountain belts occur where there is no current or recent continental or terrane collision, e.g., S. Am. Cordillera, most of N. Am. Cordillera. These discrepancies can be understood by recognizing that most major mountain belts are in subduction zone backarcs, and that most backarcs, not just extensional, are very hot and have uniformly thin weak lithospheres over considerable widths. Moho temperatures are 800-900C and lithosphere thicknesses are 50-60 km, compared to 400-500C and 200-300 km for cratons. The temperature differences result in backarc lithospheres being more than a factor of 10 weaker than cratons. They are sufficiently weak to be deformed by plate boundary forces whereas cratons are not. The lower crust viscosity in continental backarcs is sufficiently low for channel flow and to result in very flat Mohos. Backarcs may be hot because shallow asthenosphere convection results from viscosity reduction by water rising from the underlying subducting plate. Thermal expansion due to the high lithosphere temperatures, in contrast to the cold stable cratons and platforms, accounts for about 2500 m of backarc mountain belt elevations with no crustal thickening. Crustal thickening does occur in some mountain belts with and without collision (e.g., Tibet and central S. America Cordillera), but the shortening and thickening appear to be primarily in the weak lower crust. The upper crust is uplifted as a plateau and overthrusts adjacent stable areas, but remains largely undeformed.