Modeling The Dynamics Of Slab Subduction And Characteristic Analysis

The simulation of subduction dynamics is an essential research area of geoscience, providing better understanding of the impacts of slab subduction on vital geologic activities, such as earthquakes and volcanoes. The present study refers to the dynamic coupling of slab subduction and mantle convection using two-dimensional numerical simulation, to simulate the dynamic processes of subducting slabs based on multiple subduction models. Several key factors are highlighted for their influences on the evolution of slab subduction, back-arc deformation, and intermediate-deep focus earthquakes. This study focuses on the characterizations of major subduction zones in western Pacific region, interpretations of the relationship between the subduction geometries and geological phenomena. The main works and achievements during my PhD study are summarized as follows:In this thesis, I simulate the thermal structure, the distribution of buoyancy, and P-wave velocity anomalies in Japan Trench subduction zone using finite-difference method, and predict the range of metastable olivine referring to whole and layered mantle convection models. Interesting results are obtained for the layered mantle convection model of Japan Trench subduction zone that the modeled P-wave velocity anomalies not only match seismic tomography results much better, but also highly correlate to the distribution of deep-focus earthquakes. The maximum value of negative buoyancy reaches the depth of about 400 km. The metastable olivine results in the decrease of negative buoyancy, or even positive buoyancy within the subduction zone against the penetration of the subduction zone into the 660 km discontinuity.The evolution of thermal and viscosity fields during subduction, as well as stress distributions and mantle convection characteristics are discussed on the basis of full dynamic model and kinematic-dynamic model with Underworld 1.7 software. The results show that the full dynamic model with density contrast between the upper and lower mantle leads to non-steady state evolution of subduction, which matches the Cenozoic plate motions better. The stress state exhibits various behaviors in fore-arc and back-arc, relating to the interaction between the overriding and subducting plates and also the mantle flow beneath the overriding plate. Accompanied by slab subduction, the mantle convection demonstrates complex patterns, which have a strong effect on the slab geometry and the stress regime.Factors that may affect the dynamics of subduction are discussed based upon full dynamic models. The modeling results indicate that when the density or viscosity contrast between the upper and lower mantle differs greatly, the slab can lay stagnant in transition zone with the role of trench retreat, however, the viscosity contrast introduces rather stationary subductions. Before the slab reaches the upper-lower mantle discontinuity, the slab dip is relevant to slab strength and overriding-plate age. As the slab contacts with the lower mantle, the slab dip then depends mainly on the trench velocity during retreat. The subducting-plate age has weak effect on the dynamics of slab subduction. Compression occurs in fore-arc, while the overriding plate is either under sustained back-arc extension, or undergoes a switch from extension to compression, as a joint result of overriding-plate ages, trench motions and mantle flow beneath the overriding plate.The characteristics of subduction dynamics are summarized and the slab geometries, back-arc deformations and causes of intermediate-deep focus earthquakes are interpreted, combined with typical subduction zones in western Pacific region. The results show that the depth of isotherm at 700? has a positive-logarithm relationship with the subducting or converging rate, therefore the deep-focus earthquakes in subduction zones may be triggered by olivine metastability due to the low temperature within slabs. Stress state under slab unbending is in good agreement with the range and focal mechanism of double seismic zones observed in most subducting plates, thus can be a rational explanation for the formation of intermediate-focus earthquakes, especially for double seismic zones. The trench retreat or advancing exerts a strong control on slab geometries in western-Pacific subduction zones. The episode of back-arc deformation relates to the combined action of trench motion, overriding-plate age and tectonic activity.