HR: 0800h
AN: T11B-0367    [Abstracts]
TI: The Origin of Global Mountain Belts: Hot Subduction Zone Backarcs
AU: * Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada 9860 W. Saanich Road, Saanichton, BC V8L4B2 Canada
AU: * Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria , Victoria, BC V8L4B2 Canada
AU: Currie, C
EM: claire.currie@dal.ca
AF: Department of Oceanography, Dalhousie University, Halifax, NS B3H4R2 Canada
AU: Mazzotti, S
EM: smazzotti@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada 9860 W. Saanich Road, Saanichton, BC V8L4B2 Canada
AB: 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, it is not widely appreciated that there are significant discrepancies between some of the predictions of the simple collision model and observations. Examples are: (1) many mountain belts show little shortening deformation of the exposed crust; they are eroded high plateaux, e.g., Tibet, (2) a number of major mountain belts have normal or thin crust, e.g., most of N. American Cordillera, (3) several major mountain belts occur where there is no current or recent continental or terrane collision, e.g., S. American Cordillera. We deal with the first two; the last may be related to shallow angle flat slab subduction. 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. 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. Most mountain belts are also broad mobile belts with a long history of distributed deformation. They are mobile because they are sufficiently weak to be deformed by the forces developed at plate boundaries, and usually they are in backarcs. 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. Consequences include the ongoing complex histories of deformation in response to changing plate boundary forces, and that hot weak former backarcs are the locus of most deformation during continent or terrane collision orogeny, i.e., the vice or inherited weakness model.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8108 Continental tectonics: compressional
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005