| 英文摘要: | The theory of plate tectonics predicts the outer layer of the Earth is composed of lithospheric plates that are in motion. Subduction zones occur where tectonic plates converge and one plate subducts beneath the other, descending into the underlying mantle. The majority of the deformation observed at the Earth?s surface, in the form of earthquakes, volcanism, and mountain building, occurs at subduction zone plate boundaries. However, how this deformation is manifested in the subsurface, in the mantle and mantle wedge region between the subducting plate and overriding plate, is less understood. The proposed research will use 3D multi-plate geodynamic models to address the controls on the rheology of the mantle wedge, the coupling of the mantle to the overriding plate lithosphere, and associated dimensions of complex seismic anisotropy near subduction zone plate boundaries. The models will simulate the Earth as a highly viscous fluid body deforming on a timescale of millions of years, with each model run on hundreds to thousands of processors. Results from this research will have broad scientific implications, including the transport of chemical signatures in subduction zones which are then erupted in volcanoes, the magnitude and scale of circulation in the mantle wedge, how the mantle couples into overriding plate deformation and mountain building, and the viscous support of the slab and/or viscous resistance to slab sinking and thereby the velocity of the tectonic plates.
Recent three-dimensional models of subduction ranging from laboratory experiments, to semi-analytic methods, to fully numerical simulations indicate the mantle flow field in subduction zones is spatially variable, containing poloidal, toroidal, and trench parallel flow. However, previous models do not address what controls the transition from a locally decoupled mantle-overriding plate near the subduction zone to a coupled mantle-overriding plate far from the subduction zone. Such a transition must occur if the mantle motion is non-parallel to the surface plates near the subduction zone but becomes parallel farther from the subduction zone as indicated by global models. A series of 3D multi-plate geodynamic models will be constructed to systematically test the length-scales of the emergent low viscosity in the mantle wedge and quantify the vertical and lateral gradients in mantle-overriding plate viscosity, velocity, and tractions. The numerical models will systematically increase in complexity from generalized 3D models to geographically referenced 3D models for two target sites. The relative effects of rheology formulation and variations in the olivine hydration and melt fraction in the mantle wedge will be tested. In order to properly span the transition from the decoupled mantle wedge-overriding plate to coupled mantle-plate interior, the model domain will extend farther into the plate interior than is typically done for 3D regional models of subduction. A C/C++ code, SlabGenerator, will be used to construct the 3D finite-element mesh, plate boundary interface, and initial temperature. A thermo-chemical finite-element mantle convection code, CitcomCU, will be used to run the 3D viscous flow simulations. The lattice preferred orientations of minerals will be calculated from the predicted flow field and used to calculate synthetic elastic constants over the model domain. The synthetic shear wave splitting will be compared with actual observations of shear wave splitting from the target study sites. For the generalized 3D models, the length-scales of the complex mantle flow field will be compared to the length-scales observed from a global analysis of shear wave splitting at plate boundaries. Understanding the mechanisms and length-scales of the decoupling zone in subduction zones can place constraints on what controls the lateral variability in tractions along the base of the lithosphere, the length-scales of complex seismic anisotropy observed near subduction zones, and the strength of overriding plate-mantle coupling in the vertical direction and, thus, the rheological signature of the lithosphere-asthenosphere boundary. |