HR: 14:35h
AN: T23D-04 INVITED     [Abstracts]
TI: Mantle wedge flow at the northern Cascadia subduction zone: Observational constraints and numerical models
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
AU: Cassidy, J F
EM: jcassidy@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6 Canada
AU: Cassidy, J F
EM: jcassidy@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AU: He, J
EM: jhe@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC V8L 4B2 Canada
AB: We investigate mantle flow in the backarc of the northern Cascadia subduction zone (48-51\deg N). Surface heat flow, seismic velocity, effective elastic thickness, thermal isostasy, and xenolith studies indicate that the backarc is extremely hot, with estimated temperatures of 800-1000\deg C at the Moho (35 km depth) and a lithosphere thickness of only 50-60 km. The uniformly high temperatures are inferred across the entire 500 km width of the northern Cascadia backarc, despite the presence of the cool subducting Juan de Fuca plate on the west and the cool North America craton on the east. Local sources of heat are concluded to be negligible, and thus mantle flow is required to carry heat from depth into the backarc upper mantle. One constraint on backarc mantle flow directions comes from observations of seismic anisotropy, assuming that anisotropy is produced by the flow-induced lattice-preferred orientation anisotropic mantle minerals. Shear wave splitting from SKS arrivals at seismic stations in the Cascadia backarc shows significant anisotropy, with delay times of 1-1.5 s and fast directions parallel to the Juan de Fuca-North America convergence direction (~N70\deg E). These observations are consistent with the model of subduction-induced corner flow in the backarc mantle, generated by the entrainment of the mantle wedge material by the subducting plate. Numerical models are used to assess the thermal effects of corner flow. Using realistic model geometry and boundary conditions, we find that slab-driven corner flow at plate rates is too inefficient to transport the required amount of heat into the backarc to satisfy the thermal observations; an additional component of more vigourous flow is required. We propose that backarc mantle flow is dominated by vigourous small-scale thermal convection in a low viscosity backarc upper mantle. This is superimposed on a slower, regional corner flow type flow pattern, induced either by eastward subduction or westward motion of the over-riding plate. Although our study focusses on the Cascadia subduction zone, a compilation of thermal constraints and anisotropy observations suggests a similar backarc mantle flow regime at many subduction zones, including South America, Mexico, and NE Japan.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8130 Heat generation and transport
DE: 8160 Rheology--general
DE: 7203 Body wave propagation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting