HR: 11:20h
AN: T12A-05    [Abstracts]
TI: Testing Corner Flow Models at a Retreating Plate Boundary
AU: * Okaya, D
EM: okaya@usc.edu
AF: Univ. Southern California, Dept Earth Sciences, USC, Los Angeles, CA 90089-0740 United States
AU: Brandon, M
EM: mark.brandon@yale.edu
AF: Yale University, Dept. Geology and Geophysics, New Haven, CT 06520-8109 United States
AB: and the RETREAT geodynamical and seismological working groups. The Apennine Mountains in northern Italy arguably represents the most accessible "type locality" of syn-convergent orogen-normal extension. Rollback -- which describes the tendency of a subducting plate to retreat from the orogenic front -- is commonly invoked as an explanation for syn-convergent extension whereby the subducting plate pulls away (rolls back) from the overriding plate inducing a stretching flow in the asthenosphere and extension of the upper plate. An important goal of the NSF Continental Dynamics-sponsored RETREAT (Retreating-trench, extension, and accretion tectonics) multi-disciplinary project is to use seismological observations of ray-paths that pass through the supra-slab mantle wedge to resolve the mantle flow field induced by the Adriatic plate as it subducts beneath the Apennines. There are three steps to this analysis: 1) we use a generic model to represent the flow field within the mantle wedge, 2) the flow field is used to generate a lattice-preferred orientation (LPO) field created by viscous flow of olivine and pyroxene, and 3) 3D wave propagation is used to determine the resulting seismic waveforms for raypaths that propagation upward through the wedge to surface stations. The forward calculation links the parameters for the mantle flow field to predicted seismic waveforms. This approach can be inverted so that observed waveforms are used to estimate parameters for the mantle flow field. As a first step we have designed a generic 2D model for steady flow within the supra-slab mantle wedge. The model and associated velocities are all referenced relative to the corner of the mantle wedge (S point), where Adriatic plate first starts to subduct beneath the supra-slab asthenosphere. The velocity field in the mantle wedge are most strongly dependent on the following boundary parameters: (a) the dip and subduction velocity of the Adriatic plate, (b) the degree of coupling between the Adriatic plate and the supra-slab asthenospheric wedge, (c) the far-field velocity of the overriding plate, and (d) the divergent point in the overlying plate. The internal supra-slab flowfield will vary if any of these four parameters are modified. Due to anisotropic physical properties associated with supra-slab flow, seismological observations such as mantle shear wave splitting and P-wave traveltime delays may be diagnostic of these differing wedge patterns. Within RETREAT, we define a multi-disciplinary analysis workflow involving the following procedures in order to obtain a conceptual understanding of the relationship between wedge flow geometries and predicted observations: (A) tectonic model and kinematic geometry of lithospheric structure and plate motions; (B) supra-slab flowfield patterns via analytical and geodynamic modeling; (C) determination of mantle lattice preferred orientation (LPO) associated with the mantle wedge flow; (D) anisotropic synthetic modeling of the wedge using this LPO; (E) analysis of the seismological "predicted observations" such as traveltime delays and mantle shear wave splitting. Analysis of these predictions allows for an improved tie between geodynamical models of Apennine slab retreat and actual ongoing seismological observations made within the Apennines by the RETREAT project.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3909 Elasticity and anelasticity
DE: 7203 Body waves
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Tectonophysics [T]
MN: Fall Meeting 2005