HR: 0800h
AN: T11B-0366    [Abstracts]
TI: The thermal structure of subduction zones: The case for a hot backarc
AU: Currie, C A
EM: claire.currie@dal.ca
AF: Dept. Oceanography, Dalhousie University, 1355 Oxford St, Halifax, NS B3H 4J1 Canada
AU: * Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, 9860 West Saanich Rd, Sidney, BC V8L 4B2 Canada
AU: * Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, Univ. Victoria, PO Box 3055, Victoria, BC V8W 3P6 Canada
AB: Arc volcanism at subduction zones requires sub-arc mantle temperatures >1200C, despite the underthrusting cool subducting slab. It has not been widely appreciated that the high mantle temperatures usually extend several 100 km across the backarc, even without backarc extension. We have reviewed constraints on backarc mantle temperatures using several independent indicators, focussing on backarcs that have not undergone significant recent extension. Nearly all backarcs in our compilation share similar characteristics: 1) surface heat flow >70 mW/m² for continental crust with average radiogenic heat production (>60 mW/m² for oceanic crust); 2) Pn velocities <8.0 km/s for crustal thicknesses of 30-40 km; 3) slow upper mantle seismic velocities at 50-150 km depth (P-waves 1-3% slow, S-waves at least 3% slow relative to average mantle); 4) effective elastic thickness Te <30 km; 5) high in-situ shallow mantle temperatures from xenoliths; 6) widespread sporadic basaltic volcanism, 7) high elevations (1-2 km) for continental crustal thicknesses of 30-40 km; and 8) shallow upper mantle viscosities <10²⁰ Pa s. These observations indicate nearly uniform high temperatures in the mantle (1200C at 50-60 km depth) for 100's of km behind the volcanic arc. Similar high temperatures are inferred for other backarcs in the western Pacific and southern Europe/Asia, but the thermal structure is complicated by extension and spreading. In addition, regions in the backarc of former subduction zones exhibit high mantle temperatures that appear to decay over 300 myr after subduction termination. We argue that a broad hot backarc is a fundamental characteristic of a subduction zone that places important constraints on backarc mantle dynamics. The thermal structure predicted for slab-driven corner flow at plate rates is inconsistent with the observations. We favor the alternate model that heat is rapidly carried from depth into the subduction zone by vigorous small-scale convection in the backarc mantle. Such convection may be promoted by low mantle viscosities, due to high temperatures and hydration by fluids from the subducting plate.
DE: 8108 Continental tectonics: compressional
DE: 8130 Heat generation and transport
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005