HR: 14:15h
AN: T23D-03 INVITED     [Abstracts]
TI: Rheologic Controls on the Dynamic Evolution of Slabs in the Upper Mantle
AU: * Billen, M
EM: billen@geology.ucdavis.edu
AF: U. C. Davis, Department of Geology , Davis, CA 95616 United States
AU: Hirth, G
EM: ghirth@whoil.edu
AF: Woods Hole Oceanographic Inst., Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Subduction of tectonic plates is characterized by long-lived subduction zones, asymmetric subduction and slab dip angles of 25--80$^\circ$ in the upper mantle. Several mechanisms proposed to explain the variation in observed dip include large-scale mantle flow, trench roll-back, and interaction of the slab with the transition zone. Previous dynamic models of subduction that include only Newtonian viscosity and moderately strong slabs generally fail to predict subduction angles less than 60--90$^\circ$ at shallow depths (100--300 km). We find that the observed characteristics of subduction are reproduced by viscous flow models, in which the rheologic structure is consistent with experimentally determined flow laws for Newtonian and non-Newtonian visco-plastic deformation of olivine. The properties of the models required to match the observed characteristics of slabs are: non-Newtonian viscosity in the mantle producing a weak mantle wedge ($10^{18}$--$10^{19}$~Pa s), a stiff slab interior ($10^{25}$~Pa s) limited by a plastic yield criterion and a weak plate boundary shear zone ($10^{20}$--$10^{21}$~Pa s). The shallow slab dip reaches a minimum of 25--30$^\circ$ for high convergence rates and a stiff slab, without trench roll-back or relative motion of the entire lithosphere with respect to the mantle, suggesting these other mechanisms are not the primary controls on slab geometry. The deep slab dip (350--650 km) decreases as the slab penetrates the stiffer (x10), Newtonian viscosity lower mantle, eventually stabilizing the upper mantle slab geometry.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting