Interactions Between Mantle Plume,Subduction Zone And The Earth's Layer Structures

Mantle plume possibly originates from the thermal boundary layer of the core mantle boundary or the margin of the Large Low Shear Wave Velocity Provinces(LLSVPs)located below the Africa and Pacific Ocean.It will interact with the various inner layer structures of the Earth during the rising process,including mantle transition zone,lithosphere and crust,thus generating a series of geological responses on the surface,such as Large Igneous Provinces(LIPs)and hotspot tracks.Meanwhile,subduction zone brings the surface material into the deep mantle,and breeds the global seismic zone.Mantle plume and subduction zone together constitute two indispensable units in global tectonics.Therefore,the studies of mantle plume and subduction zone dynamics can not only make us further understand the material circulation inside the Earth,but also provide new constraints for global plate tectonics,deposit development and ecological environment.Geological,geochemical and sedimentological studies have proved that there is a typical LIP in Northwest China at 300-280 million years ago(Ma),namely Tarim LIP.However,the main factors controlling the formation of the LIP and the characteristics of the Tarim mantle plume are still unclear.Therefore,we have established a series of three-dimensional geodynamic models,and combined with geological observations to constrain the formation and evolution of the Tarim LIP.Our results show that continuous decompression melting,excess temperature and large radius of the mantle plume,and a high water content in the mantle source control the formation of the Tarim LIP.Specifically,the uplift range of pre-eruption topography is sensitive only to the plume radius,and enables us to constrain the ancient mantle plume size well.Our model not only provides new constraints for the formation of the Tarim LIP,but also first demonstrates that water in the mantle plays a key role in forming continental flood basalt.Meanwhile,global and regional seismic tomographic models confirm that a mantle plume exists below Hainan Island,but there is little evidence for a hotspot track related to the Hainan mantle plume.By performing a joint inversion of satellite gravity and seismic surface wave dispersion data,we found that there is an obvious high seismic wave velocity zone in the northeast of Hainan Island.Besides,geodynamic modelling and volcanic distribution further demonstrate that this is attributed to the interactions between the Hainan mantle plume and South China block.Thus,we can constrain the movement of South China block to the northeast at a velocity of?1.8 cm/yr relative to the Hainan mantle plume since the late Cretaceous.This result provides an independent way to constrain the plate motion history of continental lithosphere.In addition,the interaction between a mantle plume and mantle transition zone will strongly affect its own shape and dynamic properties,thus further affect the responses of the surface,and provide new enlightenment for seismological observations.Mineralogical studies show that there are two major phase changes in the Earth's pyrolitic mantle near the 660 km depth:the ringwoodite to perovskite+magnesiowustite(post-spinel phase transition)and the majorite to perovskite(post-garnet phase transition).However,the previous geodynamic models only consider the influence of post-spinel phase transformation on mantle plume dynamics,but ignore the influence of post-garnet phase transition.Thus,we have established a three-dimensional geodynamic model in spherical geometry to study the combined effects of these two phase transitions on mantle plume dynamics.The results show that double phase boundaries occurring in the plume center area correspond to the double reflections in seismic observations near the 660 km discontinuity.Other plume regions with relative low temperature feature a single and flat uplifted phase boundary.Besides,large amounts of plume material with relatively lower temperature is trapped in the mantle transition zone,forming a complex truncated cone structure due to the joint influence of the two phase changes.Meanwhile,the post-garnet phase transition greatly improves the ability of mantle plume to pass through the 660 km discontinuity,and significantly increases its volume flux in the upper mantle.Thus,our results provide new constraints for further understanding the fluctuation of the 660 km discontinuity,seismic wave velocity structure and mantle plume dynamics.The previous seismological results and numerical simulations show that mantle plume has a thin tail.However,the latest geophysical observations reveal that the presence of broad and bifurcate plumes in the lower and upper mantle.This provides a new challenge for us to further understand the evolution of mantle plume.Therefore,we developed a three-dimensional geodynamic model to study the formation of plume-tree structure.The results show that the weak layers in the asthenosphere and at the bottom of the mantle transition zone control the bifurcation structure of a mantle plume in the upper mantle.The plume-tree structure possibly explains the subparallel hotspot tracks with distinct isotopic anomalies in the Pacific and Atlantic Oceans.Meanwhile,mantle plume can drift in the upper mantle at a rate of?1.5 cm/yr without strong mantle wind,which provides a new mechanism for the movement of hotspots in the mantle.Therefore,our model builds a new bond between seismology,geochemistry and plate tectonics.Furthermore,subduction zone will also interact with mantle transition zone and produce a large number of deep earthquakes.The deep seismicity located in the Tonga-Fiji subduction zone accounts for about two thirds of the global total,which provides important constraints on the deformation of subduction zone.However,the factors controlling the intense deformation of the Tonga-Fiji subduction zone are not clear.Therefore,a series of two-dimensional,kinematic numerical models are established to study the morphology,stress state and thermal structure of the Tonga-Fiji subduction zone.The results show that the collision between the Tonga slab and a relic slab derived from the Vanuatu trench may control the steep dip of the Tonga subduction zone and the occurrences of a large number of earthquakes in the mantle transition zone.Moreover,we suggest that the magnitude 8.2 and 7.9 earthquakes in 2018 occurred in the warm rim of the Tonga subduction zone and beneath a strongly folded relic slab.The surrounding temperature of the two deep earthquakes is?900? and?1100?,respectively.The findings further support the latest inference in seismology:the local temperature of a slab controls the rupture details of deep focus earthquakes.Moreover,the radial viscosity structure of Earth's mantle plays a key role in the transport of heat and mass by convection.It is generally thought to increase by?10-100 times from the upper to lower mantle with a putative,abrupt increase at 660 km depth.Recently,a low viscosity channel(LVC)between 660 and 1000 km has been suggested.We conduct a series of time-dependent flow models with viscosity either increasing or decreasing at 660 km depth while tracking slab structure,state-of-stress and geoid.We find that a LVC will lower the amplitude of long wavelength(>5000 km)geoid highs over slabs,with amplitudes<10 m in height,while increasing the slab dip angle and downdip tension in the upper 300 km of slabs.A viscosity increase at 660 km gives rise to strong downdip compression throughout a slab and this pattern will largely go away with the introduction of the LVC.In addition,the endothermic phase change at 660 km depth can substantially affect the stress distribution within slabs but has a minor influence on the geoid.Models that fit the observed long wavelength geoid and observed focal mechanism in the western Pacific favor models without the presence of the LVC between 660 km and 1000 km depths.