Seismic Evidence for Plume and Subducting Slab in West Yunnan,Southwestern China

Western Yunnan is located within a gigantic Tethys orogenic zone between Gondwana and Laurasia.Magmatic activity records of the associated Wilson orogenic cycle show that the causes of plate spreading in the region might have been related to sub-mantle plume.Tectonics,geophysics, sedimentary strata,tectonic evolution of the lithosphere and other research results indicate that there is geological evidence for mantle plume magmatic activities in West Yunnan.Tomography also supports the idea that there is a subducting slab near the Red River fault and an upwelling mantle plume in West Yunnan.Here our research presents seismic evidence for upwelling mantle plume,which is a main cause leading to subducting slab in West Yunnan.The analysis is based on compressional-to-shear（P-to -S）converted seismic phases,recorded on seismograph stations in the Sichuan-Yunnan seismic network,and made a study on 410-km and 660-km discontinuities,as well as on three sections of CCP stacking.

Phase Stability and Reactions of Subducting CaCO_(3)under Upper Mantle Conditions

CaCO_(3)is an important component of marine sediments and one of the major deep-carbon carriers at subduction zones.Some subducted CaCO_(3)can be dissolved in subduction fluids and recycled back to the surface via arc volcanoes degassing.At the same time,there still remain large amounts of CaCO_(3)and its reaction products,which could be further transported into Earth's deep interior.These internal processes link atmosphere,hydrosphere and biosphere with the deep solid Earth,modifying the environments of our planet.In this review,we summarize current understanding from high pressure-temperature experiments and field petrological observations on the physical and chemical properties of CaCO_(3).In particular,the phase stability and reactions of CaCO_(3)largely control the migration and reservation of oxidized carbon in subducting slabs.Finally,we present several critical but unsolved questions on CaCO_(3)subducting in the deep mantle.

Petrogenesis and Geodynamic Implications of Early Cretaceous(~130 Ma)Magmatism in the Baingoin Batholith,Central Tibet:Products of Subducting Slab Rollback

The Baingoin batholith is one of the largest granitic plutons in the North Lhasa terrane.Its petrogenesis and tectonic setting have been studied for decades,but remain controversial.Here we report data on geochronology,geochemistry and isotopes of Early Cretaceous granitoids within the Baingoin batholith,which provide more evidence to uncover its petrogenesis and regional geodynamic processes.The Early Cretaceous magmatism yields ages of 134.4–132.0 Ma and can be divided into I-type,S-type and highly fractionated granites.The I-and S-type granites exhibit medium SiO2,high K_(2)O/Na_(2)O with negativeεNd(t)andεHf(t)values,whereas,the albite granites have very high SiO_(2)(79.04%–80.40%),very low K_(2)O/N_(2)O,negativeεNd(t)and a large variation inεHf(t).Our new data indicate that these granitoids are derived from unbalanced melting in a heterogeneous source area.The granodiorites involved had a hybrid origin from partial melting of basalt-derived and Al-rich rocks in the crust,the porphyritic monzogranites being derived from partial melting of pelitic rocks.The albite granites crystallized from residual melt separated from K-rich magma within the‘mush’process and underwent fractionation of K-feldspar.We believe that the Early Cretaceous magmatism formed in an extensional setting produced by the initial and continuous rollback of a northward-subducting slab of the NTO.

Chemical composition of sediments from the subducting Cocos Ridge segment at the Southern Central American subduction zone

Subducted sediments play an important role in the magmatism at subduction zones and the formation of mantle heterogeneity,making them an important tracer for shallow crustal processes and deep mantle processes.Therefore,ascertaining the chemical compositions of different subduction end-members is a prerequisite for using subducted sediments to trace key geological processes.We reports here the comprehensive major and trace element analyses of 52 samples from two holes(U1414 A and U1381 C)drilled on the subducting Cocos Ridge segment at the Southern Central American(SCA)subduction zone during Integrated Ocean Drilling Program(IODP)Expedition 344.The results show that the SCA subducting sediments contain 51%(wt%)Ca CO_(3),27%(wt%)terrigenous material,16%(wt%)opal,and 6%(wt%)mineral-bound H2 O+.Compared to the global trenches subducting sediment,the SCA subducting sediments are enriched in biogenic elements(Ba,Sr,and Ca),and depleted in high field strength elements(Nb,Ta,Zr,Hf,and Ti)and alkali elements(K,Rb,and Cs).Meanwhile,the sediments in this area were affected by the carbonate crash event,which could have been caused by a~800 m rise in the carbonate compensation depth at 11 Ma in the Guatemala Basin.The reason for the sedimentary hiatus at Hole U1381 C may be the closure of the Panama Isthmus and the collision between the Cocos Ridge and the Middle America Trench.In addition,the sediments from the subducting Cocos Ridge segment have influenced the petrogenesis of volcanic lavas erupted in the SCA.

Geochemistry and Petrogenesis of Late Cretaceous–Paleocene Granites from the Tengchong Block, Western Yunnan: Implications for Angle-switching of Subducting Slab

The Cenozoic geological hallmark of Western Yunnan is the characteristic voluminous Late Cretaceous-Eocene granites;however, their geological background and petrogenesis have not been well constrained and elucidated. In this study, we present new zircon U-Pb dating, along with geochemical and Sr-Nd-Hf isotopic data for granites from the Tengchong–Lianghe granitoid belt(as abbreviated to Tengliang belt) and West Yingjiang batholiths from the Tengchong block. The mineralogical and geochemical features of the Tengliang granites and the West Yingjiang batholiths are ascribed to aluminous S-type granites and weak peraluminous I-type, respectively. Zircon U-Pb analyses yielded consistent ages ranging from 67.5 Ma, 68.4 Ma and 66.2 Ma from the Tengliang granitoid belt and 50.4 Ma to 60.8 Ma for three samples from the west Yingjiang batholiths. The Tengliang granites were emplaced during the Late Cretaceous(68-66 Ma) and demonstrate negative ε_(Hf)(t) values(-24 to-4) and initial ~(87)Sr/~(86)Sr ratios of 0.7101–0.7139 and significant negative ε_(Nd)(t) values from-8.91 to-13.2, indicating a Proterozoic sedimentary source or enriched components. The hornblende-bearing I-type granites from West Yingjiang are characterized by lower initial ~(87)Sr/~(86)Sr ratios of 0.7076–0.7106, compared to Tengliang granite and negative whole-rock ε_(Nd)(t) values from-4.0 to-11.9. The early Eocene west Yingjiang gneissic granites show wide ranges of ε_(Hf)(t) values from +7.4 to-8.5 and T_(2DM) of 1.30–0.65 Ga, indicating partial melting of ancient crust with contributions of depleted mantle materials. In combination with the regional background and previous studies, we propose that such a spatio-temporal distribution of the Tengchong granitoid belt might be related to the rollback or angleswitching of the Neo-Tethyan subducting slab. This study sheds new light on the evolutionary history of the Tengchong block.

A Kinematic Thermal Model for Descending Slabs with Velocity Boundary Layers:A Case Study for the Tonga Subducting Slab

For the purpose of investigating the influence of metastable olivine（MO） phase transformations on both deep seismicity and stagnation of slabs,we constructed a 2-dimensional finite element thermal model for a 120 Ma-old 50°dipping oceanic lithosphere descending at 10 cm/yr with velocity boundary layers,which would mitigate the interference of constant velocity field for the slab. The resulting temperatures show that most of intermediate and deep earthquakes occurring within the Tonga slab are occurring inside the 800℃and 1200℃isotherm,respectively.The elevation of olivine transformation near～410 km and respective persistence of metastable olivine and spinel within the transition zone and beneath 660 km would thus result in bimodal positive,zonal,negative density anomalies,respectively.These results together with the resulting pressure anomalies may reflect the stress pattern of the Tonga slab：（i） slab pull force exerts above a depth of～230 km;（ii） MO existence changes the buoyancy force within the transition zone and facilitates slab stagnation at a depth of 660 km;（iii） as the subducting materials accumulated over 660 km,deepest earthquakes occur due to MO transformation;（iv） a flattened‘slab’ may penetrate into the lower mantle due to the density increment of Sp transformation.

Conjecture with water and rheological control for subducting slab in the mantle transition zone

Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the ＂660 kin＂ discontinuity depths and low viscosity layers （LVLs） beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a sepa- ration of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20 30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ~660 kin. The variation of the disconti- nuity depths and highly localized low seismic speed anomaly （LSSA） zones observed from seismic P waveforms in a relatively high frequency band （~ 1 Hz） support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen （H＋） diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies. The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously.

Seismic imaging of the double seismic zone in the subducting slab in Northern Chile

Double seismic zones are commonly observed in the subducting slabs in a global scale,serving as ideal examples for studying the seismogenetic mechanism of the intermediate-depth earthquakes.In this study,we relocate earthquakes and determine seismic velocity models using the double-difference seismic tomography method in the northern Chile subduction zone where a double seismic zone exists.The results suggest that the double seismic zone in northern Chile is located at about 50-140 km depth,with an interval of approximately 20 km between the two zones.The upper seismic zone is characterized by relatively low Vp(~7.8-8.0 km/s),low Vs(~4.4-4.5 km/s)and high Vp/Vs(~1.85)above the depth of~90 km,while the region below~90 km is distinguished by relatively high Vp(~8.2 km/s),high Vs(~4.8 km/s)and slightly high Vp/Vs(~1.75),which may be related to a series of dehydration reactions of hydrous minerals in the subducted oceanic crust.In comparison,the lower seismic zone is featured by the anomaly of low Vp/Vs(~1.7),although some local areas may consist of relatively high Vp/Vs values(~1.8),possibly due to the dehydration reaction of serpentine.Based on the Vp,Vs,Vp/Vs anomalies combined with previous petrological experiments and thermodynamic models,it can be derived that intermediate-depth earthquakes are mainly related to the dehydration of various hydrous minerals in the subducting slab.The dehydration process of hydrous minerals releases water into the subducting slab and subsequently leads to the increase of pore fluid pressure and the decrease of effective normal stress,thus causing the occurrence of brittle failure and intermediate-depth earthquakes in subduction zones.The imaging results of the northern Chile subduction zone further indicate that the existence of the double seismic zone is related to the dehydration process of different hydrous minerals.

Topographies of seismic velocity discontinuities and penetrations of subducting slabs beneath the Sea of Okhotsk

The existence of discontinuities, the topographies of the 410 km and 660 km discontinuities, and the penetrations of subducting slabs near the 660 km discontinuities beneath the Sea of Okhotsk were studied using Nth root slant stack and digital records from networks in Germany and the western United States. Results show the obvious evidence for reflected and refractive phases associated with the 410 km and 660 km discontinuities. There may be discontinuities at other depths such as 150 km, 220 km and 520 km. The 410 km discontinuity is elevated and the 660 km discontinuity is depressed respectively, consistent with the expected thermal signature of the phase transitions. The subducting slab has penetrated into the lower mantle in the northern part of the Sea of Okhotsk, while it is stagnant on the 660 km discontinuity in the southern part.

Progress in numerical modeling of subducting plate dynamics

The core concerns of plate tectonics theory are the dynamics of subducting plates, which can be studied by integrating multidisciplinary fields such as seismology, mineral physics, rock geochemistry, geological formation studies, sedimentology,and numerical simulations. By establishing a theoretical model and solving it with numerical methods, one can replicate the dynamic effects of a subducting plate, quantifying its evolution and the surface response. Simulations can also explain the observations and experimental results of other disciplines. Therefore, numerical models are among the most important tools for studying the dynamics of subducting plates. This paper provides a review on recent advances in the numerical modeling of subducting plate dynamics. It covers various aspects, namely, the origin of plate tectonics, the initiation process and thermal structure of subducting slab, and the main subduction slab dynamics in the upper mantle, mantle transition zone, and lower mantle. The results of numerical models are based on the theoretical equations of mass, momentum, and energy conservation. To better understand the dynamic progress of subducting plates, the simulation results must be verified in comparisons with the results from natural observations by geology, geophysics and geochemistry. With the substantial increase in computing power and continuous improvement of simulation methods, numerical models will become a more accurate and efficient means of studying the frontier issues of Earth sciences, including subducting plate dynamics.

Melt Evolution above a Spontaneously Retreating Subducting Slab in a Three-Dimensional Model

Dehydration of the subducting slab favors the melting of the surrounding mantle. Water content and melt evolution atop a spontaneously retreating subducting slab are reported in a three- dimensional （3-D） model. We find that fluids, including water and melts in the rocks, vary substantially along the trench, which cannot be found in two-dimensional （2-D） models. Their maxima along the subducting slab are mainly located at about 50 to 70 and 120 to 140 kin. Volumetric melt production rate changes spatially and episodically atop the slab, which may reflect the intensity and variations of volcanoes.

Study on the rheology of subducting slabs

We calculate thermal and phase structures of subducting slabs for different subducting velocities by a modified coupling code of the kinetic phase-transformation equations and the heat-diffusion equation with latent-heat release. Whereafter, we estimate their rheology structures based on the thermal and phase structures from the mineral physical point of view. At shallow depth, the upper layer has a high effective viscosity greater than 1034Pa · s; while the lower layer has a relatively low effective viscosity, which is greater than 1026Pa · s nevertheless. The effective viscosities below the kinetic phase boundary of olivine to wadsleyite decrease obviously, and reach a minimum of 1022Pa · s. Small areas with higher effective viscosities exist above the depth of about 700 km in subducting slabs, which are produced by lower temperatures that are related with endothermic phase transformation of spinel to perovskite and magnesiowustite. The 1% and 99% isograds of spinel proportion delineate tortuous belts with low effective viscosities, which would affect the geodynamic behavior of subducting slabs.

The Origin of Mesozoic A-type Granitoids,Fujian Province,Southeast China:Insights from Geochronology,Mineralogy and Geochemistry

The magma sources,origins and precise forming ages of the miarolite from Qishan and Kuiqi intrusions are still uncertain.New results reveal that,miarolites from the Qishan and Kuiqi intrusions yield crystallization ages of~101 and~98 Ma,and they have a high formation temperature(~910℃)and low oxygen fugacity value,indicating crystallization condition at low pressure in the upper crust with temperature of 678℃.The Qishan and Kuiqi miarolites are characterized by enrichment in SiO_(2) and high-K alkali,depletion in Ca and Mg,and belong to the high-K weak peraluminous rock series.The samples are enriched in HFSEs(i.e.,Ta,Zr and Hf)and LILEs(i.e.,Ba,P and Sr),depleted in Ba and Sr with the negative anomaly of Eu.In the primitive mantle normalized trace element spider diagram,the samples show a right-inclined‘seagull-type’pattern,combined the ratios of(La/Yb)_(N),10000×Al/Ga,Rb/Nb and Nb/Ta etc.,they were proved to be alkaline A-type granite.Combined the characterize of the trace elements,they were derived from clay-rich source accompanied pelite melting,and subjected to K-feldspar crystallization fractional.The values of ε_(Hf)(t)and tDM2 are distributed in the range of-2.8 to 3.3 with~1.2 Ga,and-6.0 to 4.0 with~1.2 Ga,revealing that they were generated from the Mesoproterozoic Cathaysia basement rocks.The comprehensive research reveals the Kuiqi and Qishan intrusions derived from crust-mantle mixing and partial melting of the crust,respectively,resulting from lithospheric extension generated by the Paleo-Pacific Plate subducted into the European-Asian Plate.

Geochemistry and mineral chemistry of the armoor granitoids,eastern dharwar craton: implications for the redox conditions and tectono-magmatic environment

The mineralogical and geochemical characteristics of the K-rich granites from the Armoor granitic rocks in the northeastern portion of the Eastern Dharwar Craton(EDC) are presented.In order to understand its physicochemical conditions,the petrogenesis of the granitoid was explained from biotite chemistry and geochemical systematics.Studies of mineral chemistry expose that compositionally,K-feldspar and plagioclase in Armoor granite rocks range from An0,Ab_(3-5.9),Or_(94-96.9) and An_(5-29,-Ab71.9-94.9),Or_(0-1.5),respectively.The mineral chemistry of biotite crystals exhibits composition that varies from primary to re-equilibrated primary biotites.Although biotites from the Armoor granites generally exhibit an I-type trend,with calc-alkaline parental magma in a subduction setting.Biotite chemistry of granites displays magnetite(oxidized)series nature,which has oxygen fugacity(fO_(2))=-15.1 to-16.7(log_(10) bar),under high oxidizing conditions.Temperature and pressure estimates for the crystallization of Armoor granites based on biotite composition are T=612-716 ℃ and 1.0-0.4 kbar,respectively.Geochemically,these rocks are metaluminous to slightly peraluminous and magnesian,with calc-alkaline potassiumrich granite.On the chondrite normalized REE diagram,the granites have positive europium anomalies;rich Sr/Y,(Dy/Yb)_(N) ratios and reduced Mg#,Rb/Sr,Rb,Sr indicate that the melting of earlier rocks,crystal accumulation and residual garnet source formed at high pressures.The examined granites show that they are produced from the melting of crustal sources.Thus,the extensive analyses of the described Armoor granite suggest that they were produced by crust sources and developed under oxidizing conditions in subduction setting.

Geochronology and Petrogenesis of Late Carboniferous to Early Permian Basalts in the Central Lhasa Subterrane, Southern Tibet: Implications for the Evolution of the Sumdo Paleo-Tethys Ocean

Basalts from the Late Carboniferous to Early Permian are extensively developed in the central Lhasa subterrane, southern Tibet. Studying the petrogenesis of these rocks may have implications for the late Paleozoic arc magmatism along the central Lhasa subterrane uncovering more of the evolution of the Sumdo Paleo-Tethys Ocean and its dynamic mechanism. Basalt samples from the Luobadui Formation in the Leqingla area, NW of Linzhou City in the central Lhasa subterrane, southern Tibet exhibit arc-like geochemical signatures in a subduction-zone tectonic setting characterized by high Al_(2)O_(3) and low TiO_(2) contents, fractionated REE patterns with low Nb/La ratios and high LREE concentrations, and negative HFSE anomalies. Based on their higher Th/Ce, Nb/Zr, and lower Ba/Th, Pb/Nd ratios, slightly negative to positive ε_(Nd)(t) values, and the relatively high Sr-Pb isotopic compositions, these samples were probably derived from partial melting of a depleted mantle source of garnet + spinel lherzolite, metasomatized by subducted sediments around 297 Ma. Modeling of the trace elements indicates that these basalts experienced fractional crystallization of olivine, clinopyroxene and minor plagioclase during magma ascent and eruption. It is proposed that these Late Carboniferous–Early Permian basalts are associated with the northward subduction of the Sumdo Paleo-Tethys Ocean seafloor along the southern margin of the central Lhasa subterrane.

Early Triassic Silicic Volcanics of South China:Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province.LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8±1.6 and 246.5±1.3 Ma,coeval with peraluminous granites of the Qinzhou Bay Granitic Complex.The studied rhyolites and dacites are characterized by high SiO_(2),K_(2)O,and Al_(2)O_(3),and low MgO,CaO,and P_(2)O_(5) contents and are classified as high-K calc-alkaline S-type rocks,with A/CNK=0.98-1.19.The volcanic rocks are depleted in high field strength elements,e.g.,Nb,Ta,Ti,and P,and enriched in large ion lithophile elements,e.g.,Rb,K,Sr,and Ba.Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions(ε_(Hf)(t)=-29.1 to-6.9),source discrimination indicators and high calculated Ti-in-zircon temperatures(798-835℃)reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma.The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region,as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.

Mantle Driven Early Eocene Magmatic Flare-up of the Gangdese Arc, Tibet: A Case Study on the Nymo Intrusive Complex

Magmatic periodicity is recognized in continental arcs worldwide, but the mechanism responsible for punctuated arc magmatism is controversial. Continental arcs in the Trans-Himalayan orogenic system display episodic magmatism and the most voluminous flare-up in this system was in early Eocene during the transition from subduction to collision. The close association of the flare-up with collision is intriguing. Our study employs zircon Lu-Hf and bulk rock Sr-Nd isotopes, along with mineral geochemistry, to track the melt sources of the Nymo intrusive complex and the role of mantle magma during the early Eocene flare-up of the Gangdese arc, Tibet. The Nymo intrusive complex is composed of gabbronorite, diorite, quartz diorite, and granodiorite which define an arc-related calc-alkaline suite. Zircon U-Pb ages reveal that the complex was emplaced between ~50–47 Ma. Zircon Hf isotopes yield εHf(t) values of 8.2–13.1, while whole-rock Sr and Nd isotopes yield εNd(t) values of 2.7–6.5 indicative of magmatism dominated by melting of a juvenile mantle source with only minor crustal assimilation(~15%–25%) as indicated by assimilation and fractional crystallization modeling. Together with published data, the early Eocene magmatic flare-up was likely triggered by slab breakoff of subducted oceanic lithosphere at depths shallower than the overriding plate. The early Eocene magmatic flare-up may have contributed to crustal thickening of the Gangdese arc. This study provides important insights into the magmatic flare-up and its significant role in the generation of large batholiths during the transition from subduction to collision.

Remnant Paleoproterozoic Subduction or Lithospheric Drip Initiation at the Yilgarn Craton Margin:Constraints from P-wave Tomography

The Paleoproterozoic was a critical time in whether modern-style plate tectonics had become globally dominant(e.g.,Wan et al.,2020).The Capricorn Orogen witnessed the assembly of the Pilbara and Yilgarn Cratons and an exotic microcontinent,the Glenburgh Terrane,to form the West Australia Craton(WAC)through two collisional orogenic events,the 2215–2145 Ma Ophthalmian and 2005–1950 Ma Glenburgh Orogenies(Johnson et al.,2013;Fig.1).Compared to other Proterozoic orogenic belts in Australia,the Capricorn Orogen preserves‘complete'opposing continental margin successions,together with intervening arc fragments associated with oceanic closure and foreland basins associated with collisional loading(Cawood et al.,2009).

Deep structure of the Southeast Asian curved subduction system and its dynamic process

The deep structure,material circulation,and dynamic processes in the Southeast Asia have long been an elusive scientific puzzle due to the lack of systematic scientific observations and recognized theoretical models.Based on the deep seismic tomography using long-period natural earthquake data,in this study,the deep structure and material circulation of the curved subduction system in Southeast Asia was studied,and the dynamic processes since 100 million years ago was reconstructed.It is pointed out that challenges still exist in the precise reconstruction of deep mantle structures of the study area,the influence of multi-stage subduction on deep material exchange and shallow magma activity,as well as the spatiotemporal evolution and coupling mechanism of multi-plate convergence.Future work should focus on high-resolution land-sea joint 3-D seismic tomography imaging of the curved subduction system in the Southeast Asia,combined with geochemical analysis and geodynamic modelling works.

No basalt accumulation and segregation atop the 660-km discontinuity beneath the Sea of Okhotsk

Recent seismic evidence shows that basalt accumulation is widespread in the mantle transition zone(MTZ),yet its ubiquity or sporadic nature remains uncertain.To investigate this phenomenon further,we characterized the velocity structure across the 660-km discontinuity that separates the upper mantle from the lower mantle beneath the Sea of Okhotsk by modeling the waveform of the S660P phase,a downgoing S wave converting into a P wave at the 660-km interface.These waves were excited by two regional>410-km-deep events and were recorded by stations in central Asia.Our findings showed no need to introduce velocity anomalies at the base of the MTZ to explain the S660P waveforms because the IASP91 model adequately reproduced the waveforms.This finding indicates that the basalt accumulation has not affected the bottom of the MTZ in the study area.Instead,this discontinuity is primarily controlled by temperature or water content variations,or both.Thus,we argue that the basalt accumulation at the base of the MTZ is sporadic,not ubiquitous,reflecting its heterogeneous distribution.