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1.
Continental subduction and collision normally follows oceanic subduction,with the remarkable event of formation and exhumation of high-to ultra-high-pressure(HP-UHP)metamorphic rocks.Based on the summary of numerical geodynamic models,six modes of continental convergence have been identified:pure shear thickening,folding and buckling,one-sided steep subduction,flat subduction,two-sided subduction,and subducting slab break-off.In addition,the exhumation of HP-UHP rocks can be formulated into eight modes:thrust fault exhumation,buckling exhumation,material circulation,overpressure model,exhumation of a coherent crustal slice,episodic ductile extrusion,slab break-off induced eduction,and exhumation through fractured overriding lithosphere.During the transition from subduction to exhumation,the weakening and detachment of subducted continental crust are prerequisites.However,the dominant weakening mechanisms and their roles in the subduction channel are poorly constrained.To a first degree approximation,the mechanism of continental subduction and exhumation can be treated as a subduction channel flow model,which incorporates the competing effects of downward Couette(subduction)flow and upward Poiseuille(exhumation)flow in the subduction channel.However,the(de-)hydration effect plays significant roles in the deformation of subduction channel and overriding lithosphere,which thereby result in very different modes from the simple subduction channel flow.Three-dimensionality is another important issue with highlighting the along-strike differential modes of continental subduction,collision and exhumation in the same continental convergence belt.  相似文献   

2.
Subduction-zone peridotites and their records of crust-mantle interaction   总被引：1，自引：0，他引：1  
Subduction is the core process of plate tectonics. The mantle wedge in subduction-zone systems represents a key tectonic unit, playing a significant role in material cycling and energy exchange between Earth's layers. This study summarizes research progresses in terms of subduction-related peridotite massifs, including supra-subduction zone(SSZ) ophiolites and mantle-wedge-type(MWT) orogenic peridotites. We also provide the relevant key scientific questions that need be solved in the future. The mantle sections of SSZ ophiolites and MWT orogenic peridotites represent the mantle fragments from oceanic and continental lithosphere in subduction zones, respectively. They are essential targets to study the crust-mantle interaction in subduction zones. The nature of this interaction is the complex chemical exchanges between the subducting slab and the mantle wedge under the major control of physical processes. The SSZ ophiolites can record melt/fluid-rock interaction, metamorphism,deformation, concentration of metallogenic elements and material exchange between crust and mantle, during the stages from the generation of oceanic lithosphere at spreading centers to the initiation, development, maturation and ending of oceanic subduction at continental margins. The MWT orogenic peridotites reveal the history of strong metamorphism and deformation during subduction, the multiple melt/fluid metasomatism(including silicatic melts, carbonatitic melts and silicate-bearing C-HO fluids/supercritical fluids), and the complex cycling of crust-mantle materials, during the subduction/collision and exhumation of continental plates. In order to further reveal the crust-mantle interaction using subduction-zone peridotites, it is necessary to utilize high-spatial-resolution and high-precision techniques to constrain the complex chemical metasomatism, metamorphism,deformation at micro scales, and to reveal their connections with spatial-temporal evolution in macro-scale tectonics.  相似文献   

3.
几乎所有大陆岩石层的减薄现象,可能都与海洋板块的俯冲作用相关,但是两者之间的内在联系迄今仍不十分明确,为此,我们设计了一系列包含洋-陆俯冲系统的二维数值模型,来探讨海洋板块的俯冲作用对上覆大陆岩石层变形行为的影响,尤其对大陆岩石层减薄效应的制约.模型结果表明,海洋板块俯冲过程中的地幔楔熔体对大陆岩石层地幔的热侵蚀以及由熔体上升所诱发的地幔局部对流的强烈扰动会导致上覆大陆岩石层的减薄效应.这种效应不仅表现在横向上的向陆内蔓延,还表现在垂向上的向浅部发展.且多类动力学参数都能制约大陆岩石层的减薄效应.具体地,随着汇聚速率和洋壳厚度的增加,上覆大陆岩石层在横向上的减薄范围越大,在垂向上的减薄程度也越深;而随着俯冲海洋板块年龄的增加,上覆大陆岩石层在横向上的减薄范围增大,但在垂向上的减薄程度会减小;随着上覆大陆岩石层厚度的增加,其横向减薄范围会减小,但在垂向上的减薄程度会加深.本文研究成果能为揭示华北克拉通减薄/破坏的动力学过程提供一定的理论参考依据.  相似文献   

4.
We have developed a two-dimensional dynamical model of asymmetric subduction integrated into the mantle convection without imposed plate velocities. In this model we consider that weak oceanic crust behaves as a lubricator on the thrust fault at the plate boundary. We introduce a rheological layer that depends on the history of the past fracture to simulate the effect of the oceanic crust. The thickness of this layer is set to be as thin as the Earth's oceanic crust. To treat 1-kilometer scale structure at the plate boundary in the 1000-kilometer scale mantle convection calculation, we introduce a new numerical method to solve the hydrodynamic equations using a couple of uniform and nonuniform grids of control volumes. Using our developed models, we have systematically investigated effects of basic rheological parameters that determine the deformation strength of the lithosphere and the oceanic crust on the development of the subducted slab, with a focus on the plate motion controlling mechanism. In our model the plate subduction is produced when the friction coefficient (0.004–0.008) of the modeled oceanic crust and the maximum strength (400 MPa) of the lithosphere are in plausible range inferred from the observations on the plate driving forces and the plate deformation, and the rheology experiments. In this range of the plate strength, yielding induces the plate bending. In this case the speed of plate motion is controlled more by viscosity layering of the underlying mantle than by the plate strength. To examine the setting of the overriding plate, we also consider the two end-member cases in which the overriding plate is fixed or freely-movable. In the case of the freely-movable overriding plate, the trench motion considerably changes the dip angle of the deep slab. Especially in the case with a shallow-angle plate boundary, retrograde slab motion occurs to generate a shallow-angle deep slab.  相似文献   

5.
Thinning of the cratonic lithosphere is common in nature, but its destruction is not. In either case, the mechanisms for both thinning and destruction are still widely under debate. In this study, we have made a review on the processes and mechanisms of thinning and destruction of cratonic lithosphere according to previous studies of geological/geophysical observations and numerical simulations, with specific application to the North China Craton (NCC). Two main models are suggested for the thinning and destruction of the NCC, both of which are related to subduction of the oceanic lithosphere. One is the “bottom-up” model, in which the deeply subducting slab perturbs and induces upwelling from the hydrous mantle transition zone (MTZ). The upwelling produces mantle convection and erodes the bottom of the overriding lithosphere by the fluid-melt-peridotite reaction. Mineral compositions and rheological properties of the overriding lithospheric mantle are changed, allowing downward dripping of lithospheric components into the asthenosphere. Consequently, lithospheric thinning or even destruction occurs. The other is the “top-down” model, characterized by the flat subduction of oceanic slab beneath the overriding cratonic lithosphere. Dehydration reactions from the subducting slab would significantly hydrate the lithospheric mantle and decrease its rheological strength. Then the subduction angle may be changed from shallow to steep, inducing lateral upwelling of the asthenosphere. This upwelling would heat and weaken the overriding lithospheric mantle, which led to the weakened lithospheric mantle dripping into the asthenosphere. These two models have some similarities, in that both take the subducting oceanic slab and relevant fluid migration as the major driving mechanism for thinning or destruction of the overriding cratonic lithosphere. The key difference between the two models is the effective depth of the subducting oceanic slab. One is stagnation and flattening in the MTZ, whereas the other is flat subduction at the bottom of the cratonic lithosphere. In the NCC, the eastern lithosphere was likely affected by subduction of the Izanagi slab during the Mesozoic, which would have perturbed the asthenosphere and the MTZ, and induced fluid migration beneath the NCC lithosphere. The upwelling fluid may largely have controlled the reworking of the NCC lithosphere. In order to discuss and analyze these two models further, it is crucial to understand the role of fluids in the subduction zone and the MTZ. Here, we systematically discuss phase transformations of hydrous minerals and the transport processes of water in the subduction system. Furthermore, we analyze possible modes of fluid activity and the problems to explore the applied feasibility of each model. In order to achieve a comprehensive understanding of the mechanisms for thinning and destruction of cratonic lithosphere, we also consider four additional possible dynamic models: extension-induced lithospheric thinning, compression-induced lithospheric thickening and delamination, large-scale mantle convection and thermal erosion, and mantle plume erosion. Compared to the subduction-related models presented here, these four models are primarily controlled by the relatively simple and single process and mechanism (extension, compression, convection, and mantle plume, respectively), which could be the secondary driving mechanisms for the thinning and destruction of lithosphere.  相似文献   

6.
三维板块几何形态对大陆深俯冲动力学的制约   总被引：1，自引：1，他引：0       下载免费PDF全文
大陆深俯冲及超高压变质作用是大陆动力学的重要研究内容,前人进行了系统的地质、地球物理观测以及数值模拟研究.然而,自然界中大陆板块的俯冲、碰撞及造山过程大部分具有明显的沿走向的差异性,这种典型的三维特征可能很大程度上依赖于会聚大陆板块的初始几何学和运动学特征.本文采用三维高分辨率的动力学数值模拟方法,建立了方形大陆板块和楔形大陆板块两种不同的俯冲-碰撞模型,并且俯冲大陆板块侧面与大洋俯冲带相邻.数值模拟结果揭示大洋板块可以持续地俯冲到地幔之中,而大陆板块俯冲到一定深度处,其前端的俯冲板块将发生断离,并进而造成残余的大陆板块俯冲角度的减小.方形大陆俯冲板块的断离深度约为150km,而楔形大陆俯冲板块的断离深度较大,约250~300km,这很大程度上取决于俯冲带中大洋板块的牵引力和大陆板块的负浮力之间的竞争关系.同时,无论方形还是楔形大陆板块俯冲模型中,板块断离后,侧向的大洋俯冲板块仍可以拖曳约60~70km宽的大陆边缘岩石圈持续向下俯冲,揭示了新西兰东部的洋-陆空间转换俯冲带的动力学机制.并且,数值模型与喜马拉雅造山带和秦岭—大别—苏鲁造山带进行了对比,进而对其高压-超高压岩石空间展布沿走向的差异性特征和机制提供了一定的启示.  相似文献   

7.
智利三联点(CTJ)位于纳兹卡板块、南极洲板块与南美板块的交界处,由南极洲—纳兹卡板块之间的智利洋脊俯冲到智利海沟而形成.巴塔哥尼亚板片窗的发展是智利洋脊长期扩张俯冲的结果之一.随着纳兹卡板块的不断东向俯冲,纳兹卡板块范围逐渐变小,CTJ同时向北移动.本文采用数值模拟方法,建立了关于洋脊海沟碰撞的简单二维模型,来研究智利三联点南部扩张洋脊俯冲区域岩石层的热结构.模拟结果表明,洋脊的位置、板块相对汇聚速度及上覆大陆板块的存在均对俯冲区域海洋板块的温度结构有着很大影响,并且大陆板块下方海洋板块温度变化最大的位置距洋脊的水平距离与洋脊到板片窗范围的水平距离两者之间具有较好的一致性.同时,当存在两两板块间的相对汇聚时,洋脊右侧大陆板块下表面的温度升高,俯冲带内海洋板块温度接近于地幔温度.纳兹卡板块以7.8 cm·a~(-1)的速度急速俯冲于南美板块之下的过程中,同时伴随着智利洋脊的持续扩张俯冲,在智利三联点南部,南美板块之下纳兹卡板块的温度因而可以更快地达到地幔软流层的约1300℃温度,并最终消亡于地幔之中.  相似文献   

8.
We report a new model of the upper mantle structure beneath Italy obtained by means of P-wave teleseismic tomography. Besides the recent and remarkable development of the Italian Seismic Network, a high model resolution has been achieved improving the inversion method upon the ACH method used in previous investigations and picking high quality arrival times with the Multi-Channel Cross-Correlation technique. The finer details of our Vp model yield new insights into the heterogeneous structure of the Adria continental lithosphere involved in the collision between the Africa and Europe plates. A wide low Vp anomaly located in the northern Adria mantle, facing the Alpine high Vp slab, supports the idea that the Adria lithosphere has been hydrated and thinned during the Alpine subduction. We argue that this mantle softening may have played a key role in favoring the subsequent delamination of the Adria lithosphere in the northern Apennines. We hypothesize that delamination of continental lithosphere previously thinned in a back-arc setting may be considered a key process to favor subduction polarity reversal and recycling of continental material into the mantle circulation. Conversely, in the central-southern Apennines, the velocity structure is consistent with the existence of a deeper oceanic slab that flattens at the base of the upper mantle, in agreement with the widely accepted geodynamic evolution of the central Mediterranean by slab retreat and back-arc spreading. The oceanic slab is discontinuously detached from the surface plate, suggesting a different structure of the Adria lithosphere, which resists subduction instead of favoring delamination.  相似文献   

9.
Abstract   The development of voluminous granitic magmatism and widespread high-grade metamorphism in Mid-Cretaceous southwest Japan have been explained by the subduction of a spreading ridge (Kula–Pacific or Farallon–Izanagi plate boundaries) beneath the Eurasian continent and the formation of a slab window. In the present study, the thermal consequences of the formation of a slab window beneath a continental margin are evaluated through a 2-D numerical simulation. The model results are evaluated by comparison with the Mid-Cretaceous geology of southwest Japan. Of particular interest are the absence of an amphibolite- to granulite-facies metamorphic belt near the Wadati–Benioff plane, and significant melting of the lower crustal-mafic rocks sufficient to form a large amount of granitic magma. Because none of the model results simultaneously satisfied these two geological interpretations, it is suggested that subduction of plate boundaries in Mid-Cretaceous southwest Japan was not associated with the opening of a slab window. According to previous studies, and the results of the present study, two different tectonic scenarios could reasonably explain the geological interpretations for Mid-Cretaceous southwest Japan: (i) The spreading ridge did not subduct beneath the Eurasian continent, but was located off the continental margin, implying the continuous subduction of very young oceanic lithosphere; (ii) ridge subduction beneath the continental margin occurred after active spreading had ceased. Consequently, in both tectonic scenarios, the subduction of plate boundaries at the Mid-Cretaceous southwest Japan was not associated with a slab window, but very young (hot) oceanic lithosphere.  相似文献   

10.
The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.  相似文献   

11.
Numerical experiments are used in this study to systematically investigate the effects of convergence rate, crustal rheological strength, and lithospheric thermal structure on the dynamics of continental collision. The study focuses on the types, conditions and processes of unstable continental subduction. Modelling results suggest that the development of unstable continental subduction can be promoted by conditions that tend to decrease rheological strength of the lithosphere, such as low crustal rheological strength, “hot” thermal structure of the lithosphere, or low convergence rate. Unstable subduction mode can be further categorized into three types: (1) multi-stage slab breakoff, (2) continuously “flowing” of fluid-like slab into the upper mantle, and (3) large-scale detachment of the thickened orogenic root. These three types of unstable continental subduction are respectively associated with (1) a low convergence rate, (2) “hot” thermal structure of the lithosphere with a high convergence rate, and (3) moderate-high crustal rheological strength with a low convergence rate. It is also revealed that the evolution of crustal melting is dominated by the deformation pattern of continental collision, which is mainly controlled by crustal rheological strength. The modelling results have important implications for understanding of continental subduction mode selection under specific geodynamic conditions.  相似文献   

12.
Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products(i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.  相似文献   

13.
论青藏高原及邻区板片构造的一个新模式   总被引：1，自引：1，他引：0       下载免费PDF全文
本文首先论述了板块学说提出的过程和存在的一些不足与疑问,特别是该学说将Holmes（1948）的地幔热对流说作为驱使岩石圈板块运动的动力机制.而后又以青藏高原及邻区为例,根据区域地质、蛇绿岩和地质构造研究的成果,特别是地震测深研究的成果,详细地论证了本区不存在有大洋中脊扩张成为大洋盆地的新大洋和大洋板块简单的B型俯冲模式,但存在有海底扩张的陆间海和海洋地壳板片（蛇绿岩构造岩片）的仰冲以及大陆岩石圈板片复杂的A型俯冲新模式.新模式不是以地幔对流运动,而是以扩张分离A型俯冲的大陆岩石圈板片与软流圈之间的水平剪切相对运动机制作为它的躯动力.  相似文献   

14.
The continental plate collision across the South Island of New Zealand is highly oblique (dextral) and bounded by oppositely verging ocean plate subduction zones. As such, the region can be considered as a type of ‘subduction scissor’. Within this tectonic context, we use three-dimensional computational geodynamic models to consider how convergent mantle lithosphere can be modified by scissor and strike–slip effects. Bounding subduction at both ends of the continental collision causes flow of the descending mantle lithosphere in the direction along strike of the model plate boundary, with thinning in the centre and thickening towards the subduction zones that bifurcates the continental mantle lithosphere root. With dipping bounding subduction, the mantle lithosphere root takes on a more complex morphology that folds over from one subduction polarity to the other, but remains as a continuous feature as it folds under the collision zone. In the absence of bounding subduction, the plate convergence causes a linear (along strike) mantle lithosphere root to develop. A rapid strike–slip motion between the converging plates transfers material in the plate boundary-parallel direction and tends to blur out features that develop in this direction—such as descending viscous instabilities. The along-strike variations in the morphology of the mantle lithosphere root that develop in the models—viz., thickening of the root towards the subduction edges, thinning in the centre—are consistent with recent, albeit poorly constrained, geophysical interpretations of the large-scale lithospheric structure of the South Island. We speculate that this reflects the nature of the evolution of the South Island collision as a limited continental segment of the plate boundary that it is dominated and guided by adjacent well-developed/developing ocean plate subduction.  相似文献   

15.
Deviations of slip vector azimuths of interplate thrust earthquakes from expected plate convergence directions at oblique subduction zones provide kinematic information about the deformation of forearcs and indirect evidence on the dynamics of the plate boundary. A global survey of slip vectors at major trenches of the world reveals a large variability in the kinematic response of forearcs to shear produced by oblique convergence. The variability in forearc deformation inferred from slip vector deflections is suggested to be caused by variations in forearc rheology rather than in the stresses acting on subduction zone thrust faults. Estimated apparent macroscopic rheologies range from elastic to perfectly plastic (or viscous). Forearc rheologies inferred from slip vectors do not correlate with age of the subducting lithosphere, but continental forearcs or old arcs appear to deform less than oceanic or young arcs. The inferred absence of forearc deformation at continental arcs from this study is counter to inferences drawn from compiled geologic information on forearc faults. Correlations of the apparent forearc rheology with backarc spreading, convergence rate, slab dip, arc curvature, and downdip length of the thrust contact are poor. However, great subduction zone earthquakes occur where forearcs are apparently more elastic (i.e., less deformed by oblique convergence), which suggests that the mechanical properties of forearcs rather than stress magnitude on thrust faults control both the kinematic behavior of forearcs and where great subduction zone earthquakes occur.  相似文献   

16.
Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.  相似文献   

17.
俯冲带的后撤与弧后扩张   总被引：12，自引：1，他引：12       下载免费PDF全文
西太平洋地壳年龄较老,因而岩石层较冷和比重较大,俯冲带的角度也较大,活动和成熟的弧后盆地则较多;条件与之相反的东太平洋弧后盆地则较少.本文探讨这种相关关系的力学成因,计算了俯冲板块诱生的弧后上涌地幔流动.计算表明,俯冲角度大及存在后撤俯冲时,有利于在弧后地区产生明显的上涌地幔流,这种深部热物质的上涌会导致弧后扩张.反之,年龄较轻的海洋地块较热和较轻,俯冲角度一般也较小,不易诱生上涌地幔物质流动和弧后扩张.大陆地壳密度小于地幔物质,大陆碰撞区就更不具备弧后扩张的条件.  相似文献   

18.
According to the experimental studies on the rheology of two important mantle rocks (eclogite and harzburgite), the rheological properties of the deep subducted oceanic lithosphere are investigated by assuming a simplified harzburgite type slab model with moderate thickness of basaltic layer. When the mantle convergence rate is small or the subducting slab has been trapped in the mantle for an enough long time, the strength profile of the slab is characterized by a strong subducting crustal component lying on a weak subducting upper mantle. However, if the convergence rate is large enough, the subducting slab will be featured only by a rigid cold center. Our study suggests that the detachment of the subducting crust component from the underlying upper mantle is only likely to happen in hot slow subducting slabs, but not the cold fast subducting lithosphere. Rheological properties of the harzburgitic and the eclogitic upper mantle vary with depths. The eclogitic upper mantle is stronger than the peridotitic upper mantle across the upper mantle. Transition zone is the high strength and high viscosity layer in the upper mantle except the lithosphere.  相似文献   

19.
汕头－吕宋岛岩石圈速度结构剖面，划分出华南陆缘古生代陆壳、陆架区晚古生代－中生代陆壳、陆坡带中生代－早第三纪过渡壳、新生代南海海盆洋壳及吕宋岛中生代－新生代岛弧陆壳与东吕宋海槽洋壳等地壳构造组分，并确定了上述地壳构造之间的边界断裂构造及其性质。结合地震震源分布及机制，初步确定了华南陆架盆岭构造带北、南两侧地震构造的控震构造与发震构造性质及其震源力学特征；１）指出１９９４年９月１６日台湾浅滩７．３级地震属于板缘壳幔地震及造成一千公里有感范围的原因；２）马尼拉海沟的海底地堑构造与南海海盆岩石圈地幔上隆是马尼拉海沟俯冲带震源显示正断层性质的原因，且为被动的或转换俯冲带；３）东吕宋海槽仍属于菲律宾海俯冲带性质；吕宋岛东西两侧俯冲带岩石圈板片震源深度的准三层分布，可能表明俯冲带岩石圈板片存在相应的低速滑移层。  相似文献   

20.
Late Cenozoic volcanism in Baja California records the effects of cessation of subduction at a previously convergent, plate margin. Prior to 12.5 m.y., when subduction along the margin of Baja ceased, the predominant volcanic activity had a calc-alkaline signature, ranging in composition from basalt to rhyolite. Acidic pyroclastic activity was common, and possibly represented the westermost, distal edge of the Sierra Madre Occidental province. After 12.5 m.y., however, the style and composition of the magmatic products changed dramatically. The dominant rock type within the Jaraguay and San Borja volcanic fields is a magnesian andesite, with up to 8% MgO at 57% SiO2, low Fe/Mg ratios, and high Na/K ratios. These rocks have unusual trace-element characteristics, with high abundances of Sr (up to 3000 ppm), low contents of Rb; K/Rb ratios are very high (usually over 1000, and up to 2500), and Rb/Sr ratios are low (less than 0.01). Furthermore, Lan/Ybn ratios are high, consistent with derivation from a mantle source with fractionated REE patterns. 87Sr/86Sr ratios are less than 0.7048, and usually less than 0.7040, whereas the pre-12.5 m.y. lavas have 87Sr/86Sr ratios between 0.7038 and 0.7063. We have previously termed these rocks bajaites, in order to distinguish them from other magnesian andesites. Bajaites also occur in southernmost Chile and the Aleutian Islands, areas which also have histories of attempted or successful ridge subduction.It is proposed that the bajaite series is produced during the unusual physico-chemical conditions operating during the subduction of young oceanic lithosphere, or subduction of a spreading centre. During normal subduction, the oceanic crust dehydrates, releasing volatiles (water, Rb and other large-ion lithophile elements) into the overlying wedge. Subduction of younger crust will result in a progressive decrease, and eventual cessation of the transfer of volatiles when subduction stops. Thermal rebound of the mantle may cause the slab to melt, perhaps under eclogitestable conditions. The resulting melt will be heavy-REE-depleted, perhaps dacitic, but will otherwise inherit MORB-like Rb/Sr and K/Rb ratios. The ascending melt will react with the mantle to form the source of the bajaitic rocks. Furthermore, any amphibole in the mantle, stabilised during the higher PH2O conditions of earlier subduction, will break down and contribute a high-K/Rb ratio component.The implications of this study are that firstly, the subducted slab does not contribute a highly fractionated REE component in most modern arcs (i.e. the slab does not melt); secondly, Rb has a very short residence time in the mantle, and its abundance in arc rocks is a direct reflection of the input from the dehydrating slab; and thirdly, bajaitelike rocks may provide recognition of attempted or successful ridge subduction in the geologic past.  相似文献   

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