地震学报2025，Vol.47Issue(4)：455-478,24.DOI:10.11939/jass.20240022
410-km和660-km间断面对太平洋板块向长白山火山区下俯冲过程和板片结构的影响
Effects of 410-km and 660-km seismic discontinuities on the Pacific Plate subduction and slab geometry beneath the Changbaishan volcanic province
摘要
Abstract
The origin of the Changbaishan volcanic province(CBSVP)has been considered to mainly relate to the Pacific Plate subduction.Therefore,it is necessary to dynamically recon-struct the Pacific Plate subduction processes towards and beneath the CBSVP since the Cenozoic in order to investigate its origin.This reconstruction requires consideration of the seismic dis-continuities in the upper mantle.There are two main global seismic discontinuities in the upper mantle:the 410-km and 660-km discontinuities,which have been considered as phase trans-ition boundaries.The 410-km discontinuity represents the exothermic boundary of the α-olivine to β-spinel transition,featuring a positive Clapeyron slope.Consequently,negative buoyancy is generated and attaches to a subducting slab near this discontinuity,accelerating the slab's subduction into the the mantle transition zone(MTZ)and slightly enhancing the transportation of upper mantle materials and the penetration of the slab through the 660-km discontinuity.However,some investigators hold opposing views regarding the effects of the olivine-spinel transition on the penetration of the 660-km discontinuity.The Clapeyron slope(γ410)and thick-ness(δh410)of the 410-km discontinuity range from 1.0 to 3.8 MPa/K and from 2 to 40 km.The 660-km discontinuity is the endothermic boundary of the transition from ringwoodite to bridgmanite and magnesiowüstite.This boundary,generally speaking,exhibits a negative Clapeyron slope.Consequently,positive buoyancy is generated and attaches to a subducting slab near the discontinuity,hindering the slab's sinking into the lower mantle.Some investi-gators have considered this to be the primary cause of stagnant slabs in the MTZ,while others do not believe that the buoyancy resulting from the phase transition at this boundary is sufficient to compensate for the density of a cold subducting slab.The Clapeyron slope(γ660)and thick-ness(δh660)of the 660-km discontinuity range from-0.4 to-6 MPa/K and from approximately 2 km to 70 km.The δh660 is between 35 km and 70 km in Northeast China.These results indi-cate that the two discontinuities have the wide ranges of the Clapeyron slopes and thicknesses.It favors predicting a stagnant slab in the MTZ mapped by seismic tomography,using a geody-namic model that incorporates a γ660 ranging from-1.5 to-8 MPa/K,without considering the 410-km discontinuity,while when considering the 410-km discontinuity,the γ410 generally takes 3 to 4 MPa/K and the γ660-2 to-6 MPa/K.It may well reproduce the stagnant Pacific slab in the MTZ beneath East Asia using geodynamic models that consider either γ660=-1.5—-3 MPa/K and δh660=40 km or γ660=-2 MPa/K and δh660=40 km in the absence of the 410-km discontinuity.Alternatively,when considering the 410-km discontinuity,it may use models with either γ410=4 MPa/K,γ660=-2 MPa/K and δh410=δh660=64 km or γ410=3 MPa/K and γ660=-3 MPa/K in the presence of the 410-km discontinuity.These studies have provided us with basic knowledge of the effects of the two discontinuities on slab subduction dynamics,but in the meantime,we find that previous researches have presented a wider range of Clapeyron slopes and thicknesses for the 410-km and 660-km discontinuities,aiming to rep-licate the stagnant slab structure in the MTZ.Consequently,it is essential to clarify the impacts of these two discontinuities on the dynamic process of Pacific Plate subduction,as well as the slab structure,westward movement distance,and sinking depth within the mantle.Therefore,in this study,we set up a series of data-assimilation three-dimensional geodynamic models that incorporate varying values of γ410,γ660,δh410 and δh660 to predict the westernmost positions,bottom depths and subduction processes of the Pacific Plate since the Cenozoic.It is found that:① The γ410,to a certain degree,affects the westward movement distance of the Pacific slab in the mantle,while it slightly influences the sinking depth of the slab.As the γ410 de-creases,the average movement distance increases gradually,and the sinking depth becomes slightly shallower.The maximum differences are about 80 km and 50 km for the distance and sinking depth,respectively,over a range of γ410 from 0 to 6 MPa/K;② With the increase of|γ660|,the effect of γ410 gradually weakens.When|γ660|≥4 MPa/K,the effect of γ410 can be almost ignored;③ The γ660 has significant effects on both the westward movement distance and sinking depth of the Pacific slab in the mantle.When|γ660|≤5 MPa/K,the average slab westward movement distance increases as|γ660|increases.However,when|γ660|＞5 MPa/K,the situation reverses,that is,the larger the|γ660|,the smaller the average slab westward move-ment distance.The sinking depth consistently becomes shallower with the increase of|γ660|.The maximum differences exceed 300 km and 600 km for the distance and sinking depth,respec-tively,over a range of γ660 from-1 MPa/K to-6 MPa/K;④ Both δh410 and δh660 have slight effects on the dynamic processes of the Pacific slab and the slab structure within the mantle;⑤ A geodynamic model that merely considers the effects of both 410-km and 660-km discon-tinuities may predict the westernmost position of the Pacific slab in the modern mantle reason-ably,but it struggles to reproduce the slab's sinking depth reasonably.关键词
板块俯冲/地震间断面/太平洋板块/地球动力学模拟Key words
plate subduction/seismic discontinuity/pacific slab/geodynamic modeling分类
天文与地球科学引用本文复制引用
朱涛,彭典典,刘丽军..410-km和660-km间断面对太平洋板块向长白山火山区下俯冲过程和板片结构的影响[J].地震学报,2025,47(4):455-478,24.基金项目
国家自然科学基金(41974103)资助. （41974103）
