HR: 0800h
AN: T31A-1271    [Abstracts]
TI: Continental mountain belts and subduction zone backarcs
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6 Canada
AU: * Currie, C A
EM: ccurrie@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: * Currie, C A
EM: ccurrie@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6 Canada
AU: Mazzotti, S
EM: smazzott@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AB: A critical problem of continental tectonics is the existence of long-lived active mobile belts compared with the long-term stability of cratons and platforms. At many continental margin plate boundaries, there are broad mobile mountain belts with a long history of distributed deformation, such as the Cordillera along the western margin of North and South America. Mobile belts cover nearly one quarter of the continental area on Earth, and are key regions of lithosphere deformation and mountain building. However, the origin and longivity of mobile belts are not well-understood. Mobile belts are mobile and deform readily because they are sufficiently weak to be deformed by plate boundary forces, whereas cratons and stable platforms are too strong. We conclude that they are weak because they are hot, and they are hot because they are in present or recent subduction zone backarcs. Nearly all backarcs are very hot, not just those with extensional or rift zones. For present-day continental backarcs (e.g., Cascadia), surface heat flow is commonly ~75 mW/m$^{2}$ over backarc widths of as much as 1000 km, with Moho temperatures of 800-900\deg C, and lithosphere thicknesses of 50-60 km. In contrast, cratons exhibit surface heat flow of 40-50 mW/m$^{2}$, Moho temperatures of 400-500\deg C and a 200-300 km thick lithosphere. The difference in thermal regime results in backarc lithosphere being more than a factor of ten weaker than cratons. Backarcs may be hot because of vigourous thermal convection in the shallow asthenosphere due to the viscosity reduction by water released from the subducting plate. The backarc regions appear to remain hot for approximately 300 my after subduction has terminated. This model provides a solution to another long-standing question in continental tectonics: the origin of the heat for orogeny. Hot and weak former backarcs are the locus of most deformation during continent or terrane collision orogeny, i.e., the vice or inherited weakness model. The orogenic heat required for weakening the crust, as indicated by the observed widespread orogenic granitic plutonism, high temperature-moderate pressure regional metamorphism, and ductile deformation at mid-crustal depths, comes from pre-existing high temperatures in the former backarc, and not from the orogenic deformation process itself.
DE: 8102 Continental contractional orogenic belts
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting