Reviewing subduction initiation and the origin of plate tectonics:What do we learn from present-day Earth?

The theory of plate tectonics came together in the 1960s,achieving wide acceptance after 1968.Since then it has been the most successful framework for investigations of Earth’s evolution.Subduction of the oceanic lithosphere,as the engine that drives plate tectonics,has played a key role in the theory.However,one of the biggest unanswered questions in Earth science is how the first subduction was initiated,and hence how plate tectonics began.The main challenge is how the strong lithosphere could break and bend if plate tectonics-related weakness and slab-pull force were both absent.In this work we review state-of-the-art subduction initiation(SI)models with a focus on their prerequisites and related driving mechanisms.We note that the plume-lithosphere-interaction and mantleconvection models do not rely on the operation of existing plate tectonics and thus may be capable of explaining the first SI.Reinvestigation of plate-driving mechanisms reveals that mantle drag may be the missing driving force for surface plates,capable of triggering initiation of the first subduction.We propose a composite driving mechanism,suggesting that plate tectonics may be driven by both subducting slabs and convection currents in the mantle.We also discuss and try to answer the following question:Why has plate tectonics been observed only on Earth?

Ripple Tectonics—When Subduction Is Interrupted

Subduction plays a fundamental role in plate tectonics and is a significant factor in modifying the structure and topography of the Earth. It is driven by convection forces that change over a >100 Myr time scale. However, when an oceanic plateau approaches, it plugs the subduction, and causes slab necking and tearing. This abrupt change may trigger a series of geodynamic (tectonic, volcanic) and sedimentary responses recorded across the convergence boundary and its surrounding regions by synchronous structural modifications. We suggest that a large enough triggering event may lead to a ripple tectonic effect that propagates outwards while speeding up the yielding of localized stress states that otherwise would not reach their threshold. The ripple effect facilitates tectonic, volcanic, and structural events worldwide that are seemingly unrelated. When the world’s largest oceanic plateau, Ontong Java Plateau (OJP), choked the Pacific-Australian convergence zone at ~6 Myr ago, it induced kinematic modifications throughout the Pacific region and along its plate margins. Other, seemingly unrelated, short-lived modifications were recorded worldwide during that time window. These modifications changed the rotation of the entire Pacific plate, which occupies ~20% of the Earth’s surface. In addition, the Scotia Sea spreading stopped, global volcanism increased, the Strait of Gibraltar closed, and the Mediterranean Sea dried up and induced the Messinian salinity crisis. In this paper, we attribute these and many other synchronous events to a new “ripple tectonics” mechanism. We suggest that the OJPincipient collision triggered the Miocene-Pliocene transition. Similarly, we suggest that innovative GPS-based studies conducted today may seek the connectivity between tectonic, seismic, and volcanic events worldwide.

The progressive onset and evolution of Precambrian subduction and plate tectonics

The regime of plate tectonics on early Earth is one of the fundamental problems in Earth sciences.Precambrian era takes the majority(ca.88%)of Earth’s history and thus plays a key role in understanding the onset of plate tectonics and the mechanism,distribution and process of Precambrian subduction zones.This paper presents a review on the progresses of subduction and subduction zones in different stages of Precambrian era,and sorts out some key issues and fields that merits further attention.We suggest that there was progressive onset and evolution of subduction and plate tectonics from Archean to Proterozoic eras.We emphasize the importance of comprehensive studies on subduction mechanism,metamorphic type,plate tectonics regime,the compositional evolution of continental crust,and petrogenesis of diverse granitoids formed in the Archean.It is proposed that innovative analytical techniques,big data,experimental petrology and numerical geodynamic modeling will facilitate future studies of Precambrian subduction zones.

Early Cenozoic Tectonics of the Tibetan Plateau

Geological mapping at a scale of 1：250000 coupled with related researches in recent years reveal well Early Cenozoic paleo-tectonic evolution of the Tibetan Plateau. Marine deposits and foraminifera assemblages indicate that the Tethys-Himalaya Ocean and the Southwest Tarim Sea existed in the south and north of the Tibetan Plateau, respectively, in Paleocene-Eocene. The paleo- oceanic plate between the Indian continental plate and the Lhasa block had been as wide as 900km at beginning of the Cenozoic Era. Late Paleocene transgressions of the paleo-sea led to the formation of paleo-bays in the southern Lhasa block. Northward subduction of the Tethys-Himalaya Oceanic Plate caused magma emplacement and volcanic eruptions of the Linzizong Group in 64.5-44.3 Ma, which formed the Paleocene-Eocene Gangdise Magmatic Arc in the north of Yalung-Zangbu Suture （YZS）, accompanied by intensive thrust in the Lhasa, Qiangtang, Hoh Xil and Kunlun blocks. The Paleocene- Eocene depression of basins reached to a depth of 3500-4800 m along major thrust faults and 680-850 m along the boundary normal faults in central Tibetan Plateau, and the Paleocene-Eocene depression of the Tarim and Qaidam basins without evident contractions were only as deep as 300-580 m and 600-830 m, respectively, far away from central Tibetan Plateau. Low elevation plains formed in the southern continental margin of the Tethy-Himalaya Ocean, the central Tibet and the Tarim basin in Paleocene-Early Eocene. The Tibetan Plateau and Himalaya Mts. mainly uplifted after the Indian- Eurasian continental collision in Early-Middle Eocene.

Early Cretaceous Tectonics and Evolution of the Tibetan Plateau

Selected geological data on Early Cretaceous strata, structures, magmatic plutons and volcanic rocks from the Kunlun to Himalaya Mountains reveal a new view of the Early Cretaceous paleo-tectonics and the related geodynamic movement of the Tibetan Plateau. Two major paleo- oceans, the Mid-Tethys Ocean between the Qiangtang and Lhasa blocks, and the Neo-Tethys Ocean between the Lhasa and Himalayan blocks, existed in the Tibetan region in the Early Cretaceous. The Himalayan Marginal and South Lhasa Seas formed in the southern and northern margins of the Neo- Tethys Ocean, the Central Tibet Sea and the Qiangtang Marginal Sea formed in the southern and northern margins of the Mid-Tethys Ocean, respectively. An arm of the sea extended into the southwestern Tarim basin in the Early Cretaceous. Early Cretaceous intensive thrusting, magmatic emplacement and volcanic eruptions occurred in the central and northern Lhasa Block, while strike- slip formed along the Hoh-Xil and South Kunlun Faults in the northern Tibetan region. Early Cretaceous tectonics together with magmatic K20 geochemistry indicate an Early Cretaceous southward subduction of the Mid-Tethys Oceanic Plate along the Bangoin-Nujiang Suture which was thrust ~87 km southward during the Late Cretaceous-Early Cenozoic. No intensive thrust and magmatic emplacement occurred in the Early Cretaceous in the Himalayan and southern Lhasa Blocks, indicating that the spreading Neo-Tethys Oceanic Plate had not been subducted in the Early Cretaceous. To the north, terrestrial basins of red-beds formed in the Hoh-Xil, Kunlun, Qilian and the northeastern Tarim blocks in Early Cretaceous, and the Qiangtang Marginal Sea disappeared after the Qiangtang Block uplifted in the late Early Cretaceous.

Dynamic subduction process of local plate revealed by Ibaraki earthquake sequence of 1982 in Japan

The kinematics and dynamics of plate tectonics are frontal subjects in geosciences and the strong earthquake occurred along the plate boundary result directly from plate movement. By analyzing Ibaraki earthquake sequence, it has been found that the focal fault plane shows a special image of grading expansion along the direction of strike and adjustment along the dip direction respectively. With the consideration of strike, dip and slip directions of focal mechanism, we have confirmed that Ibaraki earthquake belongs to a thrust fault earthquake occurred under the Japan Trench. The cause of the earthquake sequence is discussed in the paper. The study on the temporal-spatial distribution of the earthquake sequence with a time-scale between the year-scale spatial geodetic data and the second-scale moment tensor of the strong earthquake has indicated the dynamic process of Pacific Plate subduction under the Eurasia Plate. According to the average slip distance of earthquake and the velocity of plate movement, it is predicted that a strong earthquake might occur in recent years.

Characterization of subduction initiation

Compression is required for all kinds of subduction initiations,which may cause either subsidence or uplift,depending on the ages of the oceanic plates.Subduction initiations associated with the old oceanic crust tend to amplify preexisting subsidence by compression,whereas those associated with young oceanic plates may result in uplift.

Two styles of plate tectonics in Earth’s history

When plate tectonics started to occur on Earth and how it has evolved through time are two of the most fundamental questions in earth sciences. While gravity-driven subducting has been accepted as a critical condition for the operation of plate tectonics on Earth, it is intriguing how the dynamic regime and thermal state of subduction zones have affected the style of plate tectonics in Earth’s history. The metamorphic rocks of regional distribution along convergent plate boundaries record reworking of crustal rocks through dehydration and melting at lithospheric depths. The property of regional metamorphism is determined by both dynamic regime and thermal state of plate margins. The two variables have secularly evolved in Earth’s history, which is recorded by changes in the global distribution of metamorphic facies series through time. This results in two styles of plate tectonics. Modern-style plate tectonics has developed since the Neoproterozoic when plate margins were rigid enough for cold subducting, whereas ancient-style plate tectonics has developed since the Archean when plate margins were ductile enough for warm subducting. Such a difference is primarily dictated by higher mantle temperatures in the Archean than in the Phanerozoic. The development of plate subduction in both cold and warm realms is primarily dictated by the rheology of plate margins. This leads to a holistic model for the style of plate tectonics during different periods in Earth’s history.

Proto-South China Sea Plate Tectonics Using Subducted Slab Constraints from Tomography

The past size and location of the hypothesized proto-South China Sea vanished ocean basin has important plate-tectonic implications for Southeast Asia since the Mesozoic. Here we present new details on proto-South China Sea paleogeography using mapped and unfolded slabs from tomography. Mapped slabs included： the Eurasia-South China Sea slab subducting at the Manila trench; the northern Philippine Sea Plate slab subducting at the Ryukyu trench; and, a swath of detached, subhorizontal, slab-like tomographic anomalies directly under the South China Sea at 450 to 700 km depths that we show is subducted ‘northern proto-South China Sea’ lithosphere. Slab unfolding revealed that the South China Sea lay directly above the ‘northern Proto-South China Sea’ with both extending 400 to 500 km to the east of the present Manila trench prior to subduction. Our slab-based plate reconstruction indicated the proto-South China Sea was consumed by double-sided subduction, as follows：（1） The ‘northern proto-South China Sea’ subducted in the Oligo–Miocene under the Dangerous Grounds and southward expanding South China Sea by in-place ‘self subduction’ similar to the western Mediterranean basins;（2） limited southward subduction of the proto-South China Sea under Borneo occurred pre-Oligocene, represented by the 800–900 km deep ‘southern proto-South China Sea’ slab.

Global kinematics of tectonic plates and subduction zones since the late Paleozoic Era

Detailed global plate motion models that provide a continuous description of plate boundaries through time are an effective tool for exploring processes both at and below the Earth's surface. A new generation of numerical models of mantle dynamics pre-and post-Pangea timeframes requires global kinematic descriptions with full plate reconstructions extending into the Paleozoic(410 Ma). Current plate models that cover Paleozoic times are characterised by large plate speeds and trench migration rates because they assume that lowermost mantle structures are rigid and fixed through time. When used as a surface boundary constraint in geodynamic models, these plate reconstructions do not accurately reproduce the present-day structure of the lowermost mantle. Building upon previous work, we present a global plate motion model with continuously closing plate boundaries ranging from the early Devonian at 410 Ma to present day.We analyse the model in terms of surface kinematics and predicted lower mantle structure. The magnitude of global plate speeds has been greatly reduced in our reconstruction by modifying the evolution of the synthetic Panthalassa oceanic plates, implementing a Paleozoic reference frame independent of any geodynamic assumptions, and implementing revised models for the Paleozoic evolution of North and South China and the closure of the Rheic Ocean. Paleozoic(410-250 Ma) RMS plate speeds are on average ～8 cm/yr, which is comparable to Mesozoic-Cenozoic rates of ～6 cm/yr on average.Paleozoic global median values of trench migration trend from higher speeds(～2.5 cm/yr) in the late Devonian to rates closer to 0 cm/yr at the end of the Permian(～250 Ma), and during the Mesozoic-Cenozoic(250-0 Ma) generally cluster tightly around ～1.1 cm/yr. Plate motions are best constrained over the past 130 Myr and calculations of global trench convergence rates over this period indicate median rates range between 3.2 cm/yr and 12.4 cm/yr with a present day median rate estimated at～5 cm/yr. For Paleozoic times(410-251 Ma) our model results in median convergence rates largely～5 cm/yr. Globally,～90% of subduction zones modelled in our reconstruction are determined to be in a convergent regime for the period of 120-0 Ma. Over the full span of the model, from 410 Ma to 0 Ma,～93% of subduction zones are calculated to be convergent, and at least 85% of subduction zones are converging for 97% of modelled times. Our changes improve global plate and trench kinematics since the late Paleozoic and our reconstructions of the lowermost mantle structure challenge the proposed fixity of lower mantle structures, suggesting that the eastern margin of the African LLSVP margin has moved by as much as ～1450 km since late Permian times(260 Ma). The model of the plate-mantle system we present suggests that during the Permian Period, South China was proximal to the eastern margin of the African LLSVP and not the western margin of the Pacific LLSVP as previous thought.

When and how did plate tectonics begin?Theoretical and empirical considerations

板 tectonics 在对流传热的披风(岩流圈) 上是地球的热界面层(岩石圈) 的水平运动并且被岩石圈 inking 主要在 subduction 地区驾驶。板 tectonics 是组织的自我的一个突出的例子，远离平衡建筑群系统(SOFFECS ) ，由热界面层的否定快活开车并且由在弯曲岩石圈的驱散控制并且粘滞披风。板 tectonics 是一个不平常的方法让一个硅酸盐行星失去热，当它存在在上仅仅之一大在内部太阳系的五硅酸盐身体。构造活动和热损失的这个模式什么时候开始了，不被知道在地球上。所有硅酸盐行星可能经历了一个短命岩浆海洋阶段。在这团结的、停滞的盖行为是行星的热损失的普通模式以后，与内部，热被融化在更热的早地球中的火山作用和浅 intrusions.Decompression 产生了的分层和“热点”正在失去一个不同岩石圈比今天，与更厚的海洋的外壳和更薄的披风岩石圈；如此的岩石圈将比花更长时间目前变得否定地快活，在早地球上建议那板 tectonics 偶发地发生了如果根本。板 tectonics 变得持续(现代风格) 当土足够地冷却了时，融化在散布山脉下面的那解压缩做了薄海洋的外壳，允许海洋的岩石圈在百万年中的一些十个以后变得否定地快活。板 tectonics 什么时候开始了的 Ultimatelythe 问题必须被从地质的记录检索的信息回答。标准因为板 tectonics 的操作包括 ophiolites，蓝片岩 andultra 高的压力变形的带， eclogites，被动边缘，变换差错，不同 cratons 的不同运动的 paleomagneticdemonstration，和在火的岩石中的诊断 geochemicaland 同位素示踪剂的存在。这个记录必须个别地被解释；我解释记录在锝? 从活跃太古代的 tectonics 和类似于板 tectonics 的 magmatismto 一些东西显示构造式样的前进 9 Ga 到有深 subduction 的持续、现代风格板 tectonics—并且强大的平板拉—在 Neoproterozoic 时间开始。

Subduction tectonics vs.plume tectonics——Discussion on driving forces for plate motion

Plate tectonics describes the horizontal motions of lithospheric plates,the Earths outer shell,and interactions among them across the Earths surface.Since the establishment of the theory of plate tectonics about half a century ago,considerable debates have remained regarding the driving forces for plate motion.The early"Bottom up"view,i.e.,the convecting mantledriven mechanism,states that mantle plumes originating from the core-mantle boundary act at the base of plates,accelerating continental breakup and driving plate motion.Toward the present,however,the"Top down"idea is more widely accepted,according to which the negative buoyancy of oceanic plates is the dominant driving force for plate motion,and the subducting slabs control surface tectonics and mantle convection.In this regard,plate tectonics is also known as subduction tectonics."Top down"tectonics has received wide supports from numerous geological and geophysical observations.On the other hand,recent studies indicate that the acceleration/deceleration of individual plates over the million-year timescale may reflect the effects of mantle plumes.It is also suggested that surface uplift and subsidence within stable cratonic areas are correlated with plumerelated magmatic activities over the hundred-million-year timescale.On the global scale,the cyclical supercontinent assembly and breakup seem to be coupled with superplume activities during the past two billion years.These correlations over various spatial and temporal scales indicate the close relationship and intensive interactions between plate tectonics and plume tectonics throughout the history of the Earth and the considerable influence of plumes on plate motion.Indeed,we can acquire a comprehensive understanding of the driving forces for plate motion and operation mechanism of the Earth's dynamic system only through joint analyses and integrated studies on plate tectonics and plume tectonics.

Two-dimensional Numerical Modeling Research on Continent Subduction Dynamics

Continent subduction is one of the hot research problems in geoscience. New models presented here have been set up and two-dimensional numerical modeling research on the possibility of continental subduction has been made with the finite element software, ANSYS, based on documentary evidence and reasonable assumptions that the subduction of oceanic crust has occurred, the subduction of continental crust can take place and the process can be simplified to a discontinuous plane strain theory model. The modeling results show that it is completely possible for continental crust to be subducted to a depth of 120 km under certain circumstances and conditions. At the same time, the simulations of continental subduction under a single dynamical factor have also been made, including the pull force of the subducted oceanic lithosphere, the drag force connected with mantle convection and the push force of the mid-ocean ridge. These experiments show that the drag force connected with mantle convection is critical for continent subduction.

Why primordial continents were recycled to the deep:Role of subduction erosion

Geological observations indicate that there are only a few rocks of Archean Earth and no Hadean rocks on the surface of the present-day Earth.From these facts,many scientists believe that the primordial continents never existed during Hadean Earth,and the continental volume has kept increasing.On the other hand,recent studies reported the importance of the primordial continents on the origin of life,implying their existence.In this paper,we discussed the possible process that could explain the loss of the primordial continents with the assumption that they existed in the Hadean.Although depending on the timing of the initiation of plate tectonics and its convection style,subduction erosion,which is observed on the present-day Earth,might have carried the primordial continents into the deep mantle.

天然氢气规模生成的成因类型与成藏特点

目前对全球天然氢气资源的估量十分巨大,寻找天然氢气的规模聚集区有赖于对其形成机理和富集规律的不断认识。通过对国内外典型天然氢气显示的数据统计,系统总结了全球天然氢气规模聚集的成因类型、并分析了天然氢气藏的分布和成藏特征。研究结果表明:①天然氢气的成因复杂多样,主要包括水岩反应、地幔脱氢、水的辐解、岩石破碎、有机质热解以及微生物作用等,其中,水岩反应生氢和地幔脱气生氢在自然界中普遍发生,在各种地质环境中广泛存在,且其生氢速率高、生氢量大,因此是天然氢气规模生成最重要的2种成因类型。②天然氢气藏的赋存环境集中体现于三大地质背景中:板块俯冲带、前寒武纪富铁地层发育区以及裂谷构造系统。③天然氢气藏的盖层条件受多个因素的影响,不仅要考虑盖层本身的封盖能力,还要考虑由于氢活跃的物理化学性质导致盖层机械性能发生的变化,影响其脆性-韧性行为形成新的裂缝而产生氢气的逃逸。④地下微生物利用氢气进行代谢活动、中深层的加氢生烃作用等耗烃作用不利于氢气规模聚集,因此在寻找天然氢气生成有利区时,应该避开氢被大量消耗的区域。⑤天然氢气的生成时间尺度短和易扩散性等因素,使得天然氢气成藏表现出动态成藏的特征,只要氢生成与散失始终处于一种动态平衡,就能够富集成藏。地下水是水岩反应生氢的必要条件,国外发现的很多天然氢气藏都分布在地下水循环较好的地区。

上覆板块对大陆深俯冲构造响应:以胶辽地区为例

在大陆深俯冲形成的超高压造山带中,由于超高压岩石折返构造的强烈改造,板块汇聚过程中的收缩构造难以在造山带内部识别,进而影响了对造山过程的完整认识。上覆板块,由于其构造部位的特殊性,较少受折返构造的影响,较大可能地记录了板块汇聚过程的收缩构造,成为理解板块汇聚过程和演化的关键地区。本文以华南板块大陆深俯冲过程中苏鲁超高压造山带上覆华北大陆岩石圈的构造响应为切入点,以胶辽地区为研究靶区,运用多尺度构造解析的方法,综合前人研究成果开展三叠纪收缩构造的流变学特点研究,结合造山带内部变形特征,探讨苏鲁超高压造山带及其上覆板块构造演化过程和动力学机制。研究结果表明,上覆板块在华南-华北汇聚过程中经历了早期上部向NE剪切的逆冲构造和晚期上部向SW剪切的反冲构造。早期收缩构造表现为胶北地体莱西单元早-中三叠世上部指向NE的韧性变形,辽东地体中-晚三叠世上部指向NE的韧性变形和极性向NE的褶皱-逆冲构造;从南向北,变形层次渐浅,时间渐新。晚期收缩构造主要表现为胶北地体林寺山单元晚三叠世早期上部指向SW的韧性变形和辽东地体层次较浅的上部向SW剪切的反冲构造。综合苏鲁超高压造山带晚三叠世晚期造成超高压变质岩石折返的伸展构造,我们建立了华南-华北板块汇聚的三阶段演化模型。我们认为华南板块的低角度-平板俯冲可能是造成上覆华北板块强烈收缩构造发育的重要原因。此外,华南-华北NE-SW向汇聚,但俯冲板块造山后折返方向为NW-SE向,造就了现今苏鲁超高压造山带的构造格局。

基于震源机制分析中国东北地区现今构造动力学背景

通过收集1957—2022年以来日本东北至中国东北地区的不同深度的地震震源机制解,采用联合迭代应力反演的方法,计算俯冲带构造应力场及中国东北地区构造应力场状态。研究结果显示:日本海沟浅部区域不仅受到太平洋板块的俯冲挤压,也与北美板块的推挤作用有关;中国东北区域的深源地震与海沟俯冲带的长期作用存在密切的联系。长白山火山区的形成与俯冲带的逆冲存在巨大的联系,俯冲带的地震活动间接控制着东北亚火山区的形成与活动。辽宁营口地区主压应力轴分布于NEE-SWW方向,主张应力轴分布于NNW-SSE方向。结合火山区及周边浅源地震震源机制的结果,认为东北火山区现今构造应力场延续了东北区域应力的整体结构,主压应力轴呈NEE-SWW向,主张应力轴呈NNWSSE向。

Plate convergence in the Indo-Pacific region

The Indo-Pacific convergence region is the best target to solve the teo remaining challenges of the plate tectonics theory,i.e.,subduction initiation and the driving force of plate tectonics.Recent studies proposed that the Izu-Bonin subduction initiation belongs to spontaneous initiation,which implies that it started from extension,followed by low angle subduction.Numerical geodynamic modeling suggests that the initiation of plate subduction likely occurred along a transform fault,which put the young spreading ridge in direct contact with old oceanic crust.This,however,does not explain the simultaneous subduction initiation in the west Pacific region in the Cenozoic.Namely,the subduction initiations in the Izu-BoninMariana,the Aleutian,and the Tonga-Kermadec trenches are associated with oceanic crusts of different ages,yet they occurred at roughly the same time,suggesting that they were all triggered by a maj or change in the Pacific plate.Moreover,low angle subduction induces compression rather than extension,which requires external compression forces.Given that the famous Hawaiian-Emperor bending occurred roughly at the same time with the onset of westward subductions in the west Pacific,we propose that these Cenozoic subductions were initiated by the steering of the Pacific plate,which are classified as induced initiation.Induced subduction initiation usually occurs in young ocean basins,forming single-track subduction.The closure s of Neo-Tethys Oceans were likely triggered by plume s in the south,forming northward subductions.Interestingly,the Indian plate kept on moving northward more than 50 Ma after the collision between the Indian and Eurasian continents and the break-off of the subducted oceanic slab attached to it.This strongly suggests that slab pull is not the main driving force of plate tectonics,whereas slab sliding is.

Blueschist:A window into high-pressure/low-temperature metamorphism and subduction zone dynamics

Blueschist is a regional metamorphic rock formed under high-pressure(HP)low-temperature(LT)conditions.It is formed in the subduction zone environment with low geothermal gradients(4-14℃km^(−1)),and is characterized by the presence of HP/LT index minerals like glaucophane,lawsonite,aragonite,jadeite,and deerite.In general,blueschist-facies rocks are stable in subduction zones at depths of 30-60 km,and transform to eclogite-facies rocks at greater depths.The preservation of blueschists requires a fast exhumation rate.Based on protolith and tectonic setting,blueschists can be grouped into Type-A and Type-B,but some metasomatic blueschists also occur.Blueschist belts distribute mainly as bands along the margins of orogenic belts,and their occurrences within cratons are very limited.Precambrian blueschists,of which the oldest ones are about 800-700 Ma in age,are rare;most of the exposed blueschist terranes are of post-Paleozoic metamorphic age.As diagnostic evidence of ancient subduction zones,blueschist plays an important role in understanding plate tectonics.Blueschist-eclogite transition at cold subduction zones involves dehydration reactions and fluid release,which are of great importance in facilitating slab-mantle wedge water and element recycling,generating arc magmatism,and triggering intermediate-depth earthquakes in the subducting slab.Metamorphic P-T paths of blueschists and associated rocks provide key information on constraining the onset of the subduction initiation and subsequent geodynamic evolution.As a cold geothermal indicator,the emergence of blueschist offers robust evidence for the start of modern plate tectonics on the Earth.Blueschist-facies metamorphism still represents an important research direction in metamorphic geology,which requires further investigations on determining the beginning of plate tectonics,constraining phase-transition processes,and constructing a global blueschist database.

The Active Yakutat （Kula？） Plate and Its Southcentral Alaska Megathrust and Intraplate Earthquakes

Alaska geology and plate tectonics have not been well understood due to an active Yakutat plate, believed to be part of the remains of an ancient Kula plate, not being acknowledged to exist in Alaska. It is positioned throughout most of southcentral Alaska beneath the North American plate and above the NNW subducting Pacific plate. The Kula？ plate and its eastern spreading ridge were partially ＂captured＂ by the North American plate in the Paleocene. Between 63 Ma and 32 Ma, large volumes of volcanics erupted from its subducted N-S striking spreading ridge through a slab window. The eruptions stopped at 32 Ma, likely due to the Pacific plate fiat-slab subducting from the south beneath this spreading ridge. At 28 Ma, magmatism started again to the east; indicating a major shift to the east of this ＂refusing to die＂ spreading ridge. The captured Yakutat plate has also been subducting since 63 Ma to the WSW. It started to change to WSW fiat-slab subduction at 32 Ma, which stopped all subduction magmatism in W and SW Alaska by 22 Ma. The Yakutat plate subduction has again increased with the impact/joining of the coastal Yakutat terrane from the ESE about 5 Ma, resulting in the Cook Inlet Quaternary volcanism of southcentral Alaska. During the 1964 Alaska earthquake, sudden movements along the southcentral Alaska thrust faults between the Yakutat plate and the Pacific plate occurred. Specifically, the movements consisted of the Pacific plate moving NNW under the buried Yakutat plate and of the coastal Yakutat terrane, which is considered part of the Yakutat plate, thrusting WSW onto the Pacific plate. These were the two main sources of energy release for the E part of this earthquake. Only limited movement between the Yakutat plate and the North American plate occurred during this 1964 earthquake event. Buried paleopeat age dates indicate the thrust boundary between the Yakutat plate and North American plate will move in about 230 years, resulting in a more ＂continental＂ type megathrust earthquake for southcentral Alaska. There are, therefore, at least two different types ofmegathrust earthquakes occurring in southcentral Alaska： the more oceanic 1964 type and the more continental type. In addition, large ＂active＂ WSW oriented strike-slip faults are recognized in the Yakutat plate, called slice faults, which represent another earthquake hazard for the region. These slice faults also indicate important oil/gas and mineral resource locations.