HR: 16:00h
AN: U14A-01    [Abstracts]
TI: Mechanisms for 3-D Flow in the Mantle Wedge at Subduction Zones
AU: * Behn, M D
EM: mbehn@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. Geology and Geophysics 360 Woods Hole Road - MS 22, Woods Hole, MA 02543, United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Brown University, Dept Geological Sciences, Providence, RI 02912, United States
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AB: Shear-wave splitting measurements from local events at volcanic arcs show strong variability in the orientation of seismic anisotropy, with certain regions characterized by trench-parallel anisotropy and other regions displaying more variable fast polarization directions. This pattern of anisotropy is inconsistent with simple models for corner flow and suggests a more three-dimensional (3-D) style of convection in the mantle wedge. We explore the origin of 3-D flow driven by gravitational instabilities arising in both lower arc crust and the subducting plate. At conditions appropriate for arc lower crust (800°C-1000°C; 1 GPa) many crustal assemblages are denser than the underlying mantle. Such high-density layers can become gravitationally unstable and sink into the mantle with a characteristic along-arc spacing of 30-50 km on timescales of ~10⁶ years. 3-D finite element calculations show that "foundering" of this material can produce regions of trench-parallel flow in the mantle wedge, consistent with seismic observations and fabrics in residual peridotites at the base of the Jurassic Talkeetna arc section in south central Alaska. The initiation of instabilities will likely occur between active volcanoes where igneous crust cools to <850°C near the Moho. Instability growth in these locations will in turn drive upwelling and melting in the intervening regions. This process could persist through time, resulting in alternating periods of magmatic activity and quiescence at arc volcanoes. An analogous process that can produce similar flow patterns in the mantle wedge and localization of arc volcanism, is upward transport of low density diapirs composed of subducted sediment, serpentinite, and/or partial melt rising from the slab. In summary, the observation of trench parallel anisotropy at volcanic arcs, suggests that the composition of the mantle wedge may be modified by a combination of foundering of arc lower crust and hydrous diapirs rising from the subducting slab.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8411 Thermodynamics (0766, 1011, 3611)
SC: Union [U]
MN: 2007 Fall Meeting