HR: 0830h
AN: T41C-0244    [PDF]
TI: Slab Dynamics and Non-Newtonian Rheology in the Upper Mantle
AU: * Billen, M I
EM: billen@geology.ucdavis.edu
AF: U. C. Davis, Geology Department, Geology/Physics Building, One Shields Avenue, Davis, CA 95616 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, Geology and Geophysics, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Kelemen, P B
EM: pkelemen@whoi.edu
AF: Woods Hole Oceanographic Institution, Geology and Geophysics, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AB: One of the fundamental aspects of plate tectonics and mantle convection in the Earth is one-sided subduction of plates at trenches with apparent slab dips ranging from 30$^\circ$ to 90$^\circ$. While this aspect of plate tectonics is not often reproduced in large scale mantle convection simulations, the importance of slab dip to the thermal structure of the slab-wedge system has lead to numerical models of slab thermal structure, which fix the slab dip at a specified value and impose one-sided subduction.Understanding the balance of forces that lead to the range of observed slab dips can provide constraints on the viscosity structure of the shallow mantle and the importance of phase changes in modifying the buoyancy forces driving subduction. We present 2-D, time-dependent, finite element models of thermal convection exploring the dependence of slab dynamics on the viscosity structure and phase changes in the upper mantle. The viscosity structure evolves in time and is defined by a composite rheology which is temperature, pressure and strain-rate dependent, including both diffusion (Newtonian) and dislocation (non-Newtonian) flow laws and a yield criterion at low temperatures. The plate boundary within the lithosphere is included as a narrow shear zone with low viscosity. We find that slab dynamics are strongly dependent on the assumed boundary conditions and proximity of the subduction zone to the side boundaries.Flat slab subduction and/or two-sided subduction often occurs for models in which the viscosity is not strain-rate dependent. The strain-rate dependence of viscosity can lead to weak regions that cut through the full thickness of the subducting lithosphere within the subduction zone, producing steeply dipping slabs.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 8159 Rheology--crust and lithosphere
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2003 Fall Meeting