A Tomographic Study Of The Interior Structure And Dynamics Of Subduction Zones And The Continental Lithosphere

The plate tectonics theory was established in mid 1960s.Since then it has provided many new insights into geodynamics and geological revolution.According to this theory,subduction zones,one of the three types of plate boundaries,play an important role.At subduction zones,the cold and dense oceanic lithosphere sinks into the underlying asthenosphere and even the deeper mantle,which brings raw materials and energy to reequilibrate the Earth's interior,and provides most of the main force driving the plates and mantle convections simultaneously.In contrast,the continental lithosphere has experienced a long history of geological evolution,and so contains multiple geological structures,and can provide important information on the geological evolution.A better knowledge of the rheological structure of the continental lithosphere and its anisotropy patterns will be very helpful to understand the Earth dynamics.In this doctoral dissertation,we have chosen the Japan subduction zone and the North American continental region as our study targets to investigate the following scientific issues:1)the detailed structure of the Nankai subduction zone in southwest Japan and its implications for the generation mechanism of slow earthquakes,2)the deep structure and mantle dynamics of the Taiwan-Philippines arc,3)The causal mechanism and its tectonic implications of the 2013 Wyoming upper-mantle earthquake(M 4.8)at 75 km depth,and 4)seismic anisotropy tomography of the western United States.The updated seismic tomography methods are applied,including local earthquake tomography,teleseismic tomography,and P-wave anisotropic tomography.We used abundant,high-quality seismic data recorded by the Japanese seismic networks,the USarray and many other stations compiled by the International Seismmological Centre.1.Our results clearly reveal the spatial extent and variation of a low-velocity(low-V)and high Poisson's ratio layer which is interpreted as the remnant of the subducted oceanic crust beneath SW Japan.The low-V layer disappears at depths>50 km,which is attributed to crustal eclogitization and consumption of fluids.Considering the change of the subducting oceanic crust and locations of slow earthquakes,we suggest that the crustal eclogitization and destruction of the impermeable seal of the upper boundary of the subducting Philippine Sea slab play a key role in the generation of slow earthquakes.The Moho depth in the overlying Eurasian plate is an important factor affecting the depth range of slow earthquakes in warm subduction zones due to the transition of interface permeability from low to high there.The possible mechanism of the slow earthquakes in SW Japan is the dehydrated oceanic crustal rupture and shear slip at the transition zone in response to the crustal eclogitization and the temporal stress/strain field.2.The subducting Eurasian plate is revealed from the north of Taiwan to the southern Philippine islands.It is subducting with a steep angle.The young South China Sea oceanic lithosphere is subducting with the Eurasian plate simultaneously.There is a large magma chamber below-400 km depth beneath the Berham Rise.The subducted Eurasian slab was affected by the magma chamber to thin or fracture or rollback.The westward subducting Philippine Sea plate at the Philippine trench gradually steepens from the Berham Rise to the south,and its bottom depth gradually increases.The subducted Eurasian and Philippine Sea slabs have encountered and collided with each other at depth,causing the slab rollbacks.The anisotropic structure of the Taiwan-Philippine arc is complicated.North of N20°,in the area down to 70 km depth,P-wave anisotropy is characterized by three distribution patterns.However,in the upper mantle deeper than 70 km,P-wave anisotropy is characterized by two main distribution patterns.These anisotropic features are mainly caused by interactions of the subducted Philippine Sea slab and the Eurasian slab.South of N20°,the fast axis of P-wave anisotropy is perpendicular to the subducting direction of the slabs.3.Our P-wave tomography shows that the 2013 Wyoming continental lithospheric mantle earthquake occurred in an isolated high-velocity(high-V)anomaly which extends down to a depth of?160 km.We propose two possible scenarios about the cause of the 2013 Wyoming earthquake.One is that the high-V anomaly reflects a remnant of the subducted Farallon slab which fractured under long-term negative buoyancy and heating by the infilling asthenosphere.The other is removing of a dense mantle lithospheric root.4.P-wave anisotropy tomography shows that the fast velocity directions(FVD)in the lithosphere exhibit a good correlation with the geological trends,which has very important implications for ancient orogenic collisional and magmatic activities.The FVD in the Wyoming cratonic lithosphere is NE-SW,which may be caused by the compressional stress from the slab or the fossil anisotropy preserved in the old craton.The FVD in the deeper part of the upper mantle aligns roughly along the direction of the North American plate motion.Meanwhile,there are contributions from the subducting slabs.Our anisotropy results suggest that a two-layer model is more reasonable to explain the anisotropy patterns in the western US tectonic region.However,a one-layer model seems more appropriate in the eastern US cratonic region.