HR: 17:45h
AN: T24B-08    [Abstracts]
TI: Factors Controlling Slab Roll-Back and Back-Arc Extension: Insights From Numerical Models
AU: * Huismans, R S
EM: ritske.huismans@dal.ca
AF: Department Oceanography, Dalhousie University, Halifax, NS B3H4J1 Canada
AU: Beaumont, C
EM: chris.beaumont@dal.ca
AF: Department Oceanography, Dalhousie University, Halifax, NS B3H4J1 Canada
AB: Although subduction is a first order plate tectonic process, the factors controlling the dynamics of slab roll-back and back-arc formation are still not very well understood. The major driving forces for subduction and slab roll-back are well established as the slab pull and ridge push forces, their relative importance and the relative importance of forces modifying and interacting with these driving forces is, however, not very clear. A number of forces may resist subduction and roll-back of the slab. 1) Normal and tangential forces resisting downwelling of the slab, 2) Bending resistance in the slab at the trench and at the 660 km discontinuity, 3) Resistance to lateral flow of the upper mantle below and above the subducting slab. To investigate the relative contribution of these resisting forces we use 2D plane strain thermo-mechanical finite element models. The model evolution is calculated using an Arbitrary-Lagrangian-Eulerian (ALE) method for the finite element solution of incompressible viscous-plastic creeping flows (Fullsack, 1995). In a first set of models we test the relative roles of bending resistance and upper mantle viscosity with a subducting plate where an overlying plate is not included. The models extend from the surface to 660 km depth. The upper surface of the model is free to move. Upper mantle rheology is linear viscous, whereas the rheology of the subducting slab is either linear viscous or combined linear viscous and von Mises plastic. Reflective and periodic boundary conditions are used. The slab is allowed to sink in the underlying mantle under its own weight. The models indicate that the resisting forces form the primary control on the rate of subduction and roll-back where the velocity of trench retreat depends linear on the viscosity of the upper mantle. Variation of roll-back with viscosity of the subducting slab is minor. Interaction of the slab with the 660 km discontinuity results in a small, $<$ 5 %, decrease in roll-back velocity. This suggests a subordinate role of bending forces. The results are compared with a scaling analysis of relative importance of contribution forces. In a second set of models we investigate interaction of the subducting slab with the overlying plate and specifically focus on factors that may control the opening of a back-arc basin. The down going plate is driven by a kinematic boundary condition, far from the zone of subduction. After an initial stage of far-field driven contraction, the negative buoyant down welling of the mantle lithosphere may drive continued formation of the subduction zone leading to mature subduction. The models suggest that two major factors control whether the subducting system develops an extensional back-arc system: 1) The relative contribution of the imposed far-field velocity and roll-back velocity, 2) The strength of the overlying plate. We investigate the roles of small scale convection, enhanced by corner flow above the subducting plate, and far field plate velocity on the opening of the back arc basin.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3210 Modeling
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting