HR: 1340h
AN: T43B-1320    [Abstracts]
TI: The Thermal Structure of Subduction Zone Backarcs
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: * 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: 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: 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: Wang, K
EM: kwang@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6 Canada
AU: Wang, K
EM: kwang@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AB: A remarkable feature of subduction zones is that the mantle beneath the volcanic arc and well into the backarc is hot, even in areas that have not undergone recent extension. The occurrence of active volcanism at subduction zones suggests that temperatures in the mantle wedge below the arc are $>$ 1200 \deg C, despite the cooling effects of the subducting slab. Such high temperatures are inferred to result from mantle flow above the slab, which carries heat into the subduction zone from below. A key constraint on backarc mantle dynamics is the thermal structure of the mantle wedge, especially well behind the arc. We determine the detailed thermal structure of several backarcs using a number of independent indicators of temperature (e.g., surface heat flow, mantle seismic velocity, xenoliths, effective elastic thickness, lithosphere thickness, thermal isostasy studies, mantle viscosity). We focus our analysis on six backarcs that have not undergone recent ($<$ 50 my) extension: 1) northern Cascadia, 2) Mexico/Central America, 3) South America, 4) Alaska/eastern Aleutians, 5) Kamchatka, and 6) Sunda. For each region, the observations show that high temperatures in the mantle wedge (1200\deg C at 60 km depth) extend well into the backarc regions, with little variation for 100's to 1000's of km behind the arc. Similar high temperatures are inferred for extensional backarcs in the western Pacific and southern Europe/Asia, but their thermal structure is complicated by extension and spreading. Although there are site-specific explanations for the high temperatures in each backarc, we propose that a hot mantle wedge and backarc is a fundamental feature of subduction zones. The only evidence for a cool backarc is the Peru flat slab region of South America, where the flat subducting slab may exclude a hot mantle wedge. Observations of a hot and nearly isothermal backarc mantle place a very restrictive constraint on backarc mantle dynamics that has not been widely recognized. In particular, such high temperatures are inconsistent with the thermal structure predicted by numerical models of slab-driven corner flow, which can produce high temperatures beneath the arc but low temperatures in the shallow backarc mantle further landward. We conclude that vigourous small-scale convection is required in the backarc upper mantle. High temperatures and hydration of the mantle wedge by the subducting slab may reduce the backarc mantle viscosity, allowing convection, which rapidly carries heat upwards from depth.
DE: 8102 Continental contractional orogenic belts
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8130 Heat generation and transport
DE: 5418 Heat flow
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting