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1.
《Gondwana Research》2014,26(4):1690-1699
The continental collision between the Indian and Asian plates plays a key role in the geologic and tectonic evolution of the Tibetan plateau. In this article we present high-resolution tomographic images of the crust and upper mantle derived from a large number of high-quality seismic data from the ANTILOPE project in western Tibet. Both local and distant earthquakes were used in this study and 35,115 P-wave arrival times were manually picked from the original seismograms. Geological and geochemical results suggested that the subducting Indian plate has reached northward to the Lhasa terrane, whereas our new tomography shows that the Indian plate is currently sub-horizontal and underthrusting to the Jinsha river suture at depths of ~ 100 to ~ 250 km, suggesting that the subduction process has evolved over time. The Asian plate is also imaged clearly from the surface to a depth of ~ 100 km by our tomography, and it is located under the Tarim Basin north of the Altyn Tagh Fault. There is no obvious evidence to show that the Asian plate has subducted beneath western Tibet. The Indian and Asian plates are separated by a prominent low-velocity zone under northern Tibet. We attribute the low-velocity zone to mantle upwelling, which may account for the warm crust and upper mantle beneath that region, and thus explain the different features of magmatism between southern and northern Tibet. But the upwelling may not penetrate through the whole crust. We propose a revised geodynamic model and suggest that the high-velocity zones under Lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost Tibetan plateau. 相似文献
2.
Convergent margins, being the boundaries between colliding lithospheric plates, form the most disastrous areas in the world due to intensive, strong seismicity and volcanism. We review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure, seismicity, and geometry of subducting slab. We present global maps of free-air and Bouguer gravity anomalies, heat flow, seismicity, seismic Vs anomalies in the upper mantle, and plate convergence rate, as well as 20 profiles across different convergent margins. A global analysis of these data for three types of convergent margins, formed by ocean–ocean, ocean–continent, and continent–continent collisions, allows us to recognize the following patterns. (1) Plate convergence rate depends on the type of convergent margins and it is significantly larger when, at least, one of the plates is oceanic. However, the oldest oceanic plate in the Pacific ocean has the smallest convergence rate. (2) The presence of an oceanic plate is, in general, required for generation of high-magnitude (M > 8.0) earthquakes and for generating intermediate and deep seismicity along the convergent margins. When oceanic slabs subduct beneath a continent, a gap in the seismogenic zone exists at depths between ca. 250 km and 500 km. Given that the seismogenic zone terminates at ca. 200 km depth in case of continent–continent collision, we propose oceanic origin of subducting slabs beneath the Zagros, the Pamir, and the Vrancea zone. (3) Dip angle of the subducting slab in continent–ocean collision does not correlate neither with the age of subducting oceanic slab, nor with the convergence rate. For ocean–ocean subduction, clear trends are recognized: steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate, with slab rotation towards a near-vertical dip angle at depths below ca. 500 km at very high convergence rate. (4) Local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs. However, near-isostatic equilibrium may exist in broad zones of distributed deformation such as Tibet. (5) No systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation, magmatic activity, crustal faulting, horizontal heat transfer, and also due to low number of heat flow measurements across many margins. (6) Low upper mantle Vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels. 相似文献
3.
Geophysical data illustrate that the Indian continental lithosphere has northward subducted beneath the Tibet Plateau, reaching the Bangong–Nujiang suture in central Tibet. However, when the Indian continental lithosphere started to subduct, and whether the Indian continental crust has injected into the mantle beneath southern Lhasa block, are not clear. Here we report new results from the Quguosha gabbros of southern Lhasa block, southern Tibet. LA-ICP-MS zircon U–Pb dating of two samples gives a ca. 35 Ma formation age (i.e., the latest Eocene) for the Quguosha gabbros. The Quguosha gabbro samples are geochemically characterized by variable SiO2 and MgO contents, strongly negative Nb–Ta–Ti and slightly negative Eu anomalies, and uniform initial 87Sr/86Sr (0.7056–0.7058) and εNd(t) (− 2.2 to − 3.6). They exhibit Sr–Nd isotopic compositions different from those of the Jurassic–Eocene magmatic rocks with depleted Sr–Nd isotopic characteristics, but somewhat similar to those of Oligocene–Miocene K-rich magmatic rocks with enriched Sr–Nd isotopic characteristics. We therefore propose that an enriched Indian crustal component was added into the lithospheric mantle beneath southern Lhasa by continental subduction at least prior to the latest Eocene (ca. 35 Ma). We interpret the Quguosha mafic magmas to have been generated by partial melting of lithospheric mantle metasomatized by subducted continental sediments, which entered continental subduction channel(s) and then probably accreted or underplated into the overlying mantle during the northward subduction of the Indian continent. Continental subduction likely played a key role in the formation of the Tibetan plateau at an earlier date than previously thought. 相似文献
4.
西藏及其周围地区地壳、地幔地震层析成像——印度板块大规模俯冲于西藏高原之下的证据 总被引：20，自引：1，他引：20
文中介绍有关西藏—喜马拉雅碰撞带的一项地震层析成像研究。根据一个用天然地震数据产生的全球波速模型 ,印度板块有可能以近水平状俯冲于整个西藏高原之下至 16 5～ 2 6 0km深度。西藏岩石圈具有低波速地壳和高波速下岩石圈 (75～ 12 0km深 )。在 12 0～ 16 5km深度范围 ,西藏岩石圈与俯冲的印度板块之间有一层低速软流圈物质。高原中部从地表到 310km深处有一低速体 ,说明地幔物质有可能穿过俯冲板块的脆弱部位上隆。这些结果以及野外实测的地壳缩短值说明高原的抬升得助于印度板块的近水平俯冲。我们推论俯冲印度板块的升温上浮以及上覆软流层的存在是造成西藏高原高海拔抬升以及内部地表仍相对平坦的主要原因。2 0 0 1年 1月 2 6日在印度西部发生的毁灭性大地震有可能是俯冲应力在印度板块后缘薄弱处引发的岩石圈大断裂。 相似文献
5.
We present a 3D thermochemical model of the North China Craton (NCC) from the surface down to 350 km by jointly inverting surface wave phase velocity data, geoid height, surface heat flow and absolute elevation with a multi-observable probabilistic inversion method. Our model reveals a thin (~ 65–100 km) and chemically fertile lithosphere (87 < Mg# < 90) beneath the Eastern NCC, consistent with independent results from mantle xenoliths, and supports the idea that the Eastern NCC experienced significant lithospheric destruction and refertilization during the Phanerozoic. In contrast, beneath the Trans-North China Orogen, Inner Mongolia Suture Zone and Yinshan belt, we observe a more heterogeneous (chemically and thermally) lithosphere, indicating that these areas have been partly involved in lithospheric modification and mechanical erosion at multiple scales. A cold and chemically refractory (Mg# > 90) lithospheric mantle is imaged beneath the central TNCO and Ordos Block, reaching depths > 260 km. This lithospheric “keel” is surrounded to the east by a high-temperature sublithospheric anomaly that originates at depths > 280 km. The spatial distribution of this anomaly and its correlation with the location of recent volcanism in the region suggest that the anomaly represents a deep mantle upwelling being diverted by the cratonic keel and spreading onto regions of shallow lithosphere. Our results indicate that the present-day thermochemical structure beneath the NCC is the result of a complex interaction between a large-scale return flow associated with the subduction of the Pacific slab and the shallow lithospheric structure. 相似文献
6.
The tectonic evolution of the Indian plate, which started in Late Jurassic about 167 million years ago (~ 167 Ma) with the breakup of Gondwana, presents an exceptional and intricate case history against which a variety of plate tectonic events such as: continental breakup, sea-floor spreading, birth of new oceans, flood basalt volcanism, hotspot tracks, transform faults, subduction, obduction, continental collision, accretion, and mountain building can be investigated. Plate tectonic maps are presented here illustrating the repeated rifting of the Indian plate from surrounding Gondwana continents, its northward migration, and its collision first with the Kohistan–Ladakh Arc at the Indus Suture Zone, and then with Tibet at the Shyok–Tsangpo Suture. The associations between flood basalts and the recurrent separation of the Indian plate from Gondwana are assessed. The breakup of India from Gondwana and the opening of the Indian Ocean is thought to have been caused by plate tectonic forces (i.e., slab pull emanating from the subduction of the Tethyan ocean floor beneath Eurasia) which were localized along zones of weakness caused by mantle plumes (Bouvet, Marion, Kerguelen, and Reunion plumes). The sequential spreading of the Southwest Indian Ridge/Davie Ridge, Southeast Indian Ridge, Central Indian Ridge, Palitana Ridge, and Carlsberg Ridge in the Indian Ocean were responsible for the fragmentation of the Indian plate during the Late Jurassic and Cretaceous times. The Réunion and the Kerguelen plumes left two spectacular hotspot tracks on either side of the Indian plate. With the breakup of Gondwana, India remained isolated as an island continent, but reestablished its biotic links with Africa during the Late Cretaceous during its collision with the Kohistan–Ladakh Arc (~ 85 Ma) along the Indus Suture. Soon after the Deccan eruption, India drifted northward as an island continent by rapid motion carrying Gondwana biota, about 20 cm/year, between 67 Ma to 50 Ma; it slowed down dramatically to 5 cm/year during its collision with Asia in Early Eocene (~ 50 Ma). A northern corridor was established between India and Asia soon after the collision allowing faunal interchange. This is reflected by mixed Gondwana and Eurasian elements in the fossil record preserved in several continental Eocene formations of India. A revised India–Asia collision model suggests that the Indus Suture represents the obduction zone between India and the Kohistan–Ladakh Arc, whereas the Shyok-Suture represents the collision between the Kohistan–Ladakh arc and Tibet. Eventually, the Indus–Tsangpo Zone became the locus of the final India–Asia collision, which probably began in Early Eocene (~ 50 Ma) with the closure of Neotethys Ocean. The post-collisional tectonics for the last 50 million years is best expressed in the evolution of the Himalaya–Tibetan orogen. The great thickness of crust beneath Tibet and Himalaya and a series of north vergent thrust zones in the Himalaya and the south-vergent subduction zones in Tibetan Plateau suggest the progressive convergence between India and Asia of about 2500 km since the time of collision. In the early Eohimalayan phase (~ 50 to 25 Ma) of Himalayan orogeny (Middle Eocene–Late Oligocene), thick sediments on the leading edge of the Indian plate were squeezed, folded, and faulted to form the Tethyan Himalaya. With continuing convergence of India, the architecture of the Himalayan–Tibetan orogen is dominated by deformational structures developed in the Neogene Period during the Neohimalayan phase (~ 21 Ma to present), creating a series of north-vergent thrust belt systems such as the Main Central Thrust, the Main Boundary Thrust, and the Main Frontal Thrust to accommodate crustal shortening. Neogene molassic sediment shed from the rise of the Himalaya was deposited in a nearly continuous foreland trough in the Siwalik Group containing rich vertebrate assemblages. Tomographic imaging of the India–Asia orogen reveals that Indian lithospheric slab has been subducted subhorizontally beneath the entire Tibetan Plateau that has played a key role in the uplift of the Tibetan Plateau. The low-viscosity channel flow in response to topographic loading of Tibet provides a mechanism to explain the Himalayan–Tibetan orogen. From the start of its voyage in Southern Hemisphere, to its final impact with the Asia, the Indian plate has experienced changes in climatic conditions both short-term and long-term. We present a series of paleoclimatic maps illustrating the temperature and precipitation conditions based on estimates of Fast Ocean Atmospheric Model (FOAM), a coupled global climate model. The uplift of the Himalaya–Tibetan Plateau above the snow line created two most important global climate phenomena—the birth of the Asian monsoon and the onset of Pleistocene glaciation. As the mountains rose, and the monsoon rains intensified, increasing erosional sediments from the Himalaya were carried down by the Ganga River in the east and the Indus River in the west, and were deposited in two great deep-sea fans, the Bengal and the Indus. Vertebrate fossils provide additional resolution for the timing of three crucial tectonic events: India–KL Arc collision during the Late Cretaceous, India–Asia collision during the Early Eocene, and the rise of the Himalaya during the Early Miocene. 相似文献
7.
《Gondwana Research》2014,25(2):464-493
We discuss possible scenarios of continental collision, and their relation to mechanisms of exhumation of HP and UHP rocks, inferred from thermo-mechanical numerical models accounting for thermo-rheological complexity of the continental lithosphere. Due to this complexity, mechanisms of continental convergence are versatile and different, in many aspects from those that control oceanic subduction. Elucidating these mechanisms from conventional observations is difficult, and requires additional constraints such as those derived from petrological data. Indeed, exhumation of HP/UHP rocks is an integral part of convergent processes, and burial/exhumation dynamics inferred from metamorphic P–T–t paths provides strong constraints on the collision scenarios. Metamorphic rocks also play an active role due to their contrasting physical properties (rheology, density, fluid transport capacity). Numerical thermo-mechanical experiments suggest that HP/UHP exhumation can only be produced in subduction contexts, as well as that long-lasting (> 10 Myr) continental subduction can only occur in case of cold strong lithospheres (TMoho < 550 °C, the equivalent elastic thickness Te > 50 km) and of relatively high convergence rates (> 3–5 cm yr− 1 ). In this case, high density UHP material in the crustal part of subduction interface provides additional pull on the slab and is not always exhumed to the surface. In case of slower convergence and/or weaker lithosphere (Te < 40 km), continental subduction is a transient process that takes a limited time span in the evolution of collision zone. Under these conditions, hot mechanically weak UHP rocks enhance decoupling between the upper and lower plate while their exhumation may be rapid (faster than convergence rate) and abundant. Therefore, the UHP exhumation paths can be regarded as sensitive indicators of subduction. Rheological changes and fluid exchanges associated with low-to-middle pressure phase transitions along the subduction interface, such as serpentinization during the oceanic phase and schisting, play a major role producing necessarily mechanical softening of the subduction interface and of the hydrated mantle wedge. The oceanic UHP rocks are exhumed thanks to mixing with low-density continental crustal units during transition from oceanic to continental subduction. At the continental phase, the UHP exhumation occurs as a result of a multi-stage process: at the deep stage (< 40 km depth) the exhumation is rapid and is driven by buoyancy of partly metamorphosed (or partly molten) UHP material often mixed with non-metamorphosed crustal volumes. At final stages, exhumation takes common slow path through the accretion prism mechanism and the erosional denudation. The experiments suggest that formation of UHP rocks requires that continental subduction starts at higher oceanic subduction rate. It then may progressively slow down until the lockup of the subduction interface and/or slab-break-off. A rate of ~ 1–2 cm yr− 1 is generally sufficient to drive continental subduction during the first several Myr of convergence, but pertinent subduction requires faster convergence rates (> 3–5 cm yr− 1). We suggest that most continental orogenic belts could have started their formation from continental subduction but this process has been generally limited in time. 相似文献
8.
Kara J. Matthews Alina J. Hale Michael Gurnis R. Dietmar Müller Lydia DiCaprio 《Gondwana Research》2011,19(2):372-383
During the Early Cretaceous Australia's eastward passage over sinking subducted slabs induced widespread dynamic subsidence and formation of a large epeiric sea in the eastern interior. Despite evidence for convergence between Australia and the paleo-Pacific, the subduction zone location has been poorly constrained. Using coupled plate tectonic–mantle convection models, we test two end-member scenarios, one with subduction directly east of Australia's reconstructed continental margin, and a second with subduction translated ~ 1000 km east, implying the existence of a back-arc basin. Our models incorporate a rheological model for the mantle and lithosphere, plate motions since 140 Ma and evolving plate boundaries. While mantle rheology affects the magnitude of surface vertical motions, timing of uplift and subsidence depends on plate boundary geometries and kinematics. Computations with a proximal subduction zone result in accelerated basin subsidence occurring 20 Myr too early compared with tectonic subsidence calculated from well data. This timing offset is reconciled when subduction is shifted eastward. Comparisons between seismic tomography and model temperature cross-sections, and an absence of subduction zone volcanism in eastern Australia in the Early Cretaceous provide support for the back-arc basin scenario. 相似文献
9.
中国大陆构造及动力学若干问题的认识 总被引：17，自引：2，他引：15
中国(东亚)大陆受特提斯、古亚洲和太平洋构造体系的制约,具有复杂的地体构架和特殊的岩石圈结构。本文从地学前沿——大陆动力学的视野出发,围绕中国大陆构造及动力学四个方面的研究,总结已有的进展并提出新的思考:①中国大陆板块下的构造和整个地幔运动的构架:地震层析资料揭示西太平洋板片向西俯冲到东亚大陆之下,其倾角逐渐减小,最后近水平地插进400～600km深度的地幔过渡带中,成为箕状几何形态的超深俯冲板片。印度岩石圈板片超深俯冲至青藏高原之下～800km的深度,在喜马拉雅西构造结部位发生双向不对称深俯冲,印度岩石圈板片向东俯冲至东构造结东侧之下300～500km的深度。②中国大陆变质基底的再活化:中国大陆的大部分陆块未受显生宙以来构造、变质和岩浆事件的改造与激活,在冈瓦纳大陆北缘的印度陆块和阿拉伯陆块北缘还发育有形成于泛非期(530～470Ma)的造山带,其影响范围至高喜马拉雅、拉萨地体和三江地区。新生代的变质活化普遍出现在喜马拉雅、南迦巴瓦、拉萨地体和三江-缅甸地区,最新的变质年龄仅2～1Ma(南迦巴瓦)。③中国主要高压-超高压变质带的大地构造背景及深俯冲-折返机制:中国及邻区含榴辉岩的高压-超高压(HP/UHP)变质带有洋壳(深)俯冲和陆壳(深)俯冲之分。青藏高原中,大部分洋壳俯冲形成的高压/超高压变质带与原-古特提斯洋盆中诸多微陆块之间的小洋盆的汇聚碰撞有关,陆壳深俯冲作用有两种机制,它们分别是大陆块之间剪式碰撞和撕裂式岩石圈舌形板片的深俯冲。④中国大陆造山带的深部物质可经3类机制挤出,即深部地壳物质"牙膏式"挤出、侧向挤出和"挤压转换式"挤出。 相似文献
10.
《Gondwana Research》2014,25(3-4):1080-1090
Geological studies have suggested that a significant amount of crustal material has been lost from the surface due to delamination, continental collision, and subduction at oceanic–continental convergent margins. If so, then the subducted crustal materials are expected to be trapped in the mid-mantle due to the density difference from peridotitic materials induced by the phase transition from coesite to stishovite. In order to study the effect of the subducted granitic materials floating around the mantle transition zone, we conducted two-dimensional numerical experiments of mantle convection incorporating a continental drift with a heat source placed around the bottom of the mantle transition zone. The simulations deal with a time-dependent convection of fluid under the extended Boussinesq approximation in a model of a two-dimensional rectangular box with a height of 2900 km and a width of 11,600 km, where a continent with a length of 2900 km and heat source below the continent are imposed. We found that the addition of heat source in the mantle transition zone considerably enhances the onset of upwelling plumes in the upper mantle, which further reduces the time scale of continental drift. The heat source also causes massive mechanical mixing, especially in the upper mantle. The results suggest that the heat source floating around the mantle transition zone can be a possible candidate for inducing the supercontinent cycle. 相似文献
11.
《Gondwana Research》2013,24(4):1455-1483
The crust and upper mantle in mainland China were relatively densely probed with wide-angle seismic profiling since 1958, and the data have provided constraints on the amalgamation and lithosphere deformation of the continent. Based on the collection and digitization of crustal P-wave velocity models along related wide-angle seismic profiles, we construct several crustal transects across major tectonic units in mainland China. In our study, we analyzed the seismic activity, and seismic energy releases during 1970 and 2010 along them. We present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects, yielding a better understanding of the lithosphere rheology strength beneath mainland China. Our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland China. The average crustal thickness is greater than 65 km beneath the Tibetan Plateau, about 35 km beneath South China, and about 36–38 km beneath North China and Northeastern China. For the basins, the thickness is ~ 55 km beneath Qaidam, ~ 50 km beneath Tarim, ~ 40 km beneath Sichuan and ~ 35 km beneath Songliao. Our study also shows that the average seismic P-wave velocity is usually slower than the global average, equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland China resulting from the complex process of lithospheric evolution during Triassic and Cenozoic continent–continent and Mesozoic ocean–continent collisions. We identify characteristically different patterns of seismic activity distribution in different tectonic blocks, with bi-, or even tri-peak distribution of seismic concentration in South Tibet, which may suggest that crustal architecture and composition exert important control role in lithosphere deformation. The calculated yield stress envelopes of lithosphere in mainland China can be divided into three groups. The results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern China, and crème brulee models with weak and strong lower crust corresponding to lithosphere beneath the western China and Kunlun orogenic belts, respectively. The spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra- or inter-plate deformation mechanism, and more studies are needed to further understand the tectonic process(es) accompanying different lithosphere rheology structures. 相似文献
12.
《Gondwana Research》2013,24(4):1429-1454
Different hypotheses have been proposed for the origin and pre-Cenozoic evolution of the Tibetan Plateau as a result of several collision events between a series of Gondwana-derived terranes (e.g., Qiangtang, Lhasa and India) and Asian continent since the early Paleozoic. This paper reviews and reevaluates these hypotheses in light of new data from Tibet including (1) the distribution of major tectonic boundaries and suture zones, (2) basement rocks and their sedimentary covers, (3) magmatic suites, and (4) detrital zircon constraints from Paleozoic metasedimentary rocks. The Western Qiangtang, Amdo, and Tethyan Himalaya terranes have the Indian Gondwana origin, whereas the Lhasa Terrane shows an Australian Gondwana affinity. The Cambrian magmatic record in the Lhasa Terrane resulted from the subduction of the proto-Tethyan Ocean lithosphere beneath the Australian Gondwana. The newly identified late Devonian granitoids in the southern margin of the Lhasa Terrane may represent an extensional magmatic event associated with its rifting, which ultimately resulted in the opening of the Songdo Tethyan Ocean. The Lhasa−northern Australia collision at ~ 263 Ma was likely responsible for the initiation of a southward-dipping subduction of the Bangong-Nujiang Tethyan Oceanic lithosphere. The Yarlung-Zangbo Tethyan Ocean opened as a back-arc basin in the late Triassic, leading to the separation of the Lhasa Terrane from northern Australia. The subsequent northward subduction of the Yarlung-Zangbo Tethyan Ocean lithosphere beneath the Lhasa Terrane may have been triggered by the Qiangtang–Lhasa collision in the earliest Cretaceous. The mafic dike swarms (ca. 284 Ma) in the Western Qiangtang originated from the Panjal plume activity that resulted in continental rifting and its separation from the northern Indian continent. The subsequent collision of the Western Qiangtang with the Eastern Qiangtang in the middle Triassic was followed by slab breakoff that led to the exhumation of the Qiangtang metamorphic rocks. This collision may have caused the northward subduction initiation of the Bangong-Nujiang Ocean lithosphere beneath the Western Qiangtang. Collision-related coeval igneous rocks occurring on both sides of the suture zone and the within-plate basalt affinity of associated mafic lithologies suggest slab breakoff-induced magmatism in a continent−continent collision zone. This zone may be the site of net continental crust growth, as exemplified by the Tibetan Plateau. 相似文献
13.
An overview of the crustal structure of the Tibetan plateau after 35 years of deep seismic soundings
Zhongjie Zhang Yangfan Deng Jiwen Teng Chunyong Wang Rui Gao Yun Chen Weiming Fan 《Journal of Asian Earth Sciences》2011,40(4):977-989
Since the pioneer wide-angle seismic profile along the Yadong–Gulu rift acquired in 1974 by the ex-Institute of Geophysics, Chinese Academy of Sciences (CAS), several research programs aimed to deep geophysics, performed thanks to the participation of Chinese national and international institutions, have been developed during last 35 years, including 23 wide-angle seismic profiles with total length of about 6000 km. These profiles are unevenly distributed, most of them in eastern Tibet and few profiles in western Tibet. In this paper, we make a summarized presentation of all these wide-angle seismic profiles and provide an overall view of the seismic velocity structure of the crust beneath the broad Tibetan plateau, which is the product of the continuous convergence and collision of the Indian and Eurasian plates since about 50 Ma ago. Different patterns of crustal thickness variation related to the tectonic blocks and along suture zones of the region are displayed. The crust thickness is confirmed to be about 70–75 km under southern Tibet, and 60–65 km under northern, northeastern and southeastern Tibet. The leading edge of the subducted lithosphere reaches the northern margin of the plateau and directly contacts with Tarim Basin. Westward of the 90°E boundary, the Indian crust is moving towards the northern edge of the plateau and collides with Tarim Basin at 80°E while reach the Bangong–Nujiang suture belt at 88°E; eastward of the 90°E boundary, the northern edge of the crust should be at 50–100 km south of Bangong–Nujiang suture. The results supply helpful constrains to understand the mechanism of the continent–continent collision and its consequences in the plateau and neighbouring areas. 相似文献
14.
The lithospheric structure of ancient cratons provides important constraints on models relating to tectonic evolution and mantle dynamics. Here we present the 3D lithospheric structure of the North China Craton (NCC) from a joint inversion of gravity, geoid and topography data. The NCC records a prolonged history of Archean and Paleoproterozoic accretion of crustal blocks through subduction and collision building the cratonic architecture, which was subsequently differentially destroyed during Mesozoic through extensive magmatism. The thermal structure obtained in our study is considered to define the lithosphere-asthenosphere boundary (LAB) of the NCC, and reflects the density variations within the mantle lithosphere. Employing the Moho depths from deep seismic sounding profiles for the inversion, and based on repeated computations using different parameters, we estimate the Moho depth, LAB depth and average crustal density of the craton. The Moho depth varies from 28 to 50 km and the LAB depth varies from 105 to 205 km. The LAB and Moho show concordant thinning from West to East of the NCC. The average crustal density is 2870 kg m− 3 in the western part of the NCC, higher than that in the eastern part (2750 kg m− 3). The results of joint inversion in our study yielded LAB depth and lithospheric thinning features similar to those estimated from thermal and seismic studies, although our results show different depth and variations in the thickness. The lithosphere gently thins from 145 to 105 km in the eastern NCC, where as the thinning is much less pronounced in the western NCC with average depth of about 175 km. The joint inversion results in this study provide another perspective on the lithospheric structure from the density properties and corresponding geophysical responses in an ancient craton. 相似文献
15.
Five domains (microplates) have been recognized by seismic anisotropy in the mantle lithosphere of the Bohemian Massif. The mantle domains correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed before their Variscan assembly. The present-day mantle fabric indicates that this process consisted of at least three oceanic subductions, each followed by an underthrusting of the continental lithosphere. The seismic anisotropy does not detect remnants of the oceanic subductions, but it can trace boundaries of the preserved continental domains subsequently underthrust along the paths of previous oceanic subductions. The most robust continent–continent collision was followed by westward underthrusting of the Brunovistulian mantle lithosphere, still detectable by seismic anisotropy more than 100 km beneath the Moldanubian mantle lithosphere. Major occurrences of the high-pressure/ultra high-pressure (HP–UHP) rocks follow the ENE and NNE oriented sutures and boundaries of the mantle–lithosphere domains mapped from three-dimensional modeling of body-wave anisotropy. The HP–UHP rocks are products of oceanic subductions and the following underthrusting of the continental crust and mantle lithosphere exhumed along the mantle boundaries. The close relation of the mantle sutures and occurrences of the HP–UHP rocks near the paleosubductions testifies for models interpreting the granulite–garnet peridotite association by oceanic/continental subduction/underthrusting followed by the exhumation of deep-seated rocks. Our findings support the bivergent subduction model of tectonic development of the central part of the Bohemian Massif. The inferences from seismic anisotropy image the Bohemian Massif as a mosaic of microplates with a rigid mantle lithosphere preserving a fossil olivine fabric. The collisional mantle boundaries, blurred by tectonometamorphic processes in easily deformed overlying crust, served as major exhumation channels of the HP–UHP rocks. 相似文献
16.
Cenozoic volcanism on the Tibetan plateau, which shows systematic variations in space and time, is the volcanic response to the India–Asia continental collision. The volcanism gradually changed from Na-rich + K-rich to potassic–ultrapotassic + adakitic compositions along with the India–Asia collision shifting from contact-collision (i.e. “soft collision” or “syn-collision”) to all-sided collision (i.e. “hard collision”). The sodium-rich and potasium-rich lavas with ages of 65–40 Ma distribute mainly in the Lhasa terrane of southern Tibet and subordinately in the Qiangtang terrane of central Tibet. The widespread potassic–ultrapotassic lavas and subordinate adakites were generated from ~ 45 to 26 Ma in the Qiangtang terrane of central Tibet. Subsequent post-collisional volcanism migrated southwards, producing ultrapotassic and adakitic lavas coevally between ~ 26 and 8 Ma in the Lhasa terrane. Then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since ~ 20 Ma in the western Qiangtang and Songpan–Ganze terranes. Such spatial–temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the Tibetan plateau. These processes involve roll-back and break-off of the subducted Neo-Tethyan slab followed by removal of the thickened Lhasa lithospheric root, and consequently northward underthrusting of the Indian lithosphere. The Tibetan plateau is suggested to have risen diachronously from south to north. Whereas the southern part of the plateau may have been created and maintained since the late-Oligocene, the northern plateau would have not attained its present-day elevation and size until the mid-Miocene when the lower part of the western Qiangtang and Songpan–Ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust Indian lithosphere. 相似文献
17.
Hudson Bay conceals several fundamental tectonic elements of the North American continent, including most of the ca. 1.9–1.8 Ga Trans-Hudson orogen (THO) and the Paleozoic Hudson Bay basin. Formed due to a collision between two cratons, the THO is similar in scale and tectonic style to the modern Himalayan–Karakorum orogen. During collision, the lobate shape of the indentor (Superior craton) formed an orogenic template that, along with the smaller Sask craton, exerted a persistent influence on the tectonic evolution of the region resulting in anomalous preservation of juvenile Proterozoic crust. Extensive products of 2.72–2.68 Ga and 1.9–1.8 Ga episodes of subduction are preserved, but the spatial scale of corresponding domains increases by roughly an order-of-magnitude (to 1000 km, comparable to modern subduction environments) from the Archean to the Proterozoic. Based on analysis of gravity and magnetic data and published field evidence, we propose a new tectonic model in which Proterozoic crust in the southeastern third of Hudson Bay formed within an oceanic or marginal-basin setting proximal to the Superior craton, whereas the northwestern third is underlain by Archean crust. An intervening central belt truncates the southeastern domains and is interpreted to be a continental magmatic arc.Thick, cold and refractory lithosphere that underlies the Bay is well imaged by surface-wave studies and comprises a large component of the cratonic mantle keel beneath North America. The existence of an unusually thick mantle root indicates that subduction and plate collision during the Trans-Hudson orogeny were ‘root-preserving’ (if not ‘root-forming’) processes. Although the Hudson Bay basin is the largest by surface area of four major intracratonic basins in North America, it is also the shallowest. Available evidence suggests that basin subsidence may have been triggered by eclogitization of lower-crustal material. Compared to other basins of similar age in North America, the relatively stiff lithospheric root may have inhibited subsidence of the Hudson Bay basin. 相似文献
18.
《Gondwana Research》2014,25(1):170-189
The Lhasa terrane in southern Tibet is composed of Precambrian crystalline basement, Paleozoic to Mesozoic sedimentary strata and Paleozoic to Cenozoic magmatic rocks. This terrane has long been accepted as the last crustal block to be accreted with Eurasia prior to its collision with the northward drifting Indian continent in the Cenozoic. Thus, the Lhasa terrane is the key for revealing the origin and evolutionary history of the Himalayan–Tibetan orogen. Although previous models on the tectonic development of the orogen have much evidence from the Lhasa terrane, the metamorphic history of this terrane was rarely considered. This paper provides an overview of the temporal and spatial characteristics of metamorphism in the Lhasa terrane based mostly on the recent results from our group, and evaluates the geodynamic settings and tectonic significance. The Lhasa terrane experienced multistage metamorphism, including the Neoproterozoic and Late Paleozoic HP metamorphism in the oceanic subduction realm, the Early Paleozoic and Early Mesozoic MP metamorphism in the continent–continent collisional zone, the Late Cretaceous HT/MP metamorphism in the mid-oceanic ridge subduction zone, and two stages of Cenozoic MP metamorphism in the thickened crust above the continental subduction zone. These metamorphic and associated magmatic events reveal that the Lhasa terrane experienced a complex tectonic evolution from the Neoproterozoic to Cenozoic. The main conclusions arising from our synthesis are as follows: (1) The Lhasa block consists of the North and South Lhasa terranes, separated by the Paleo-Tethys Ocean and the subsequent Late Paleozoic suture zone. (2) The crystalline basement of the North Lhasa terrane includes Neoproterozoic oceanic crustal rocks, representing probably the remnants of the Mozambique Ocean derived from the break-up of the Rodinia supercontinent. (3) The oceanic crustal basement of North Lhasa witnessed a Late Cryogenian (~ 650 Ma) HP metamorphism and an Early Paleozoic (~ 485 Ma) MP metamorphism in the subduction realm associated with the closure of the Mozambique Ocean and the final amalgamation of Eastern and Western Gondwana, suggesting that the North Lhasa terrane might have been partly derived from the northern segment of the East African Orogen. (4) The northern margin of Indian continent, including the North and South Lhasa, and Qiangtang terranes, experienced Early Paleozoic magmatism, indicating an Andean-type orogeny that resulted from the subduction of the Proto-Tethys Ocean after the final amalgamation of Gondwana. (5) The Lhasa and Qiangtang terranes witnessed Middle Paleozoic (~ 360 Ma) magmatism, suggesting an Andean-type orogeny derived from the subduction of the Paleo-Tethys Ocean. (6) The closure of Paleo-Tethys Ocean between the North and South Lhasa terranes and subsequent terrane collision resulted in the formation of Late Permian (~ 260 Ma) HP metamorphic belt and Triassic (220 Ma) MP metamorphic belt. (7) The South Lhasa terrane experienced Late Cretaceous (~ 90 Ma) Andean-type orogeny, characterized by the regional HT/MP metamorphism and coeval intrusion of the voluminous Gangdese batholith during the northward subduction of the Neo-Tethyan Ocean. (8) During the Early Cenozoic (55–45 Ma), the continent–continent collisional orogeny has led to the thickened crust of the South Lhasa terrane experiencing MP amphibolite-facies metamorphism and syn-collisional magmatism. (9) Following the continuous continent convergence, the South Lhasa terrane also experienced MP metamorphism during Late Eocene (40–30 Ma). (10) During Mesozoic and Cenozoic, two different stages of paired metamorphic belts were formed in the oceanic or continental subduction zones and the middle and lower crust of the hanging wall of the subduction zone. The tectonic imprints from the Lhasa terrane provide excellent examples for understanding metamorphic processes and geodynamics at convergent plate boundaries. 相似文献
19.
During convergence of Gondwana-derived microplates and Laurussia in the Palaeozoic, subduction of oceanic and continental crusts and their sedimentary cover introduced material of regionally contrasting chemical and isotopic compositions into the mantle. This slab material metasomatised the local mantle, producing a highly heterogeneous lithospheric mantle beneath the European Variscides. The eastern termination of the European Variscides (Moldanubian and Saxo-Thuringian zones of Austria, Czech Republic, Germany and Poland) is unusual in that the mantle was modified by material from several subduction zones within a small area. Orogenic lamproites sampled this lithospheric mantle, which has a chemical signature reflecting extreme depletion (low CaO and Al2O3 contents and high Mg-number) followed by strong metasomatic enrichment, giving rise to crust-like trace element patterns, variable radiogenic 87Sr/86Sr(330) (0.7062–0.7127) and non-radiogenic Nd isotopic compositions (εNd(330) = − 2.8 to − 7.8), crustal Pb isotopic compositions, and a wide range of δ7Li values (− 5.1 to + 5.1). This metasomatic signature is variably expressed in the lamproites, depending on the extent of melting and the nature of the source of the metasomatic component. Preferential melting of the metasomatically enriched (veined) lithospheric mantle with K-rich amphibole resulted in lamproitic melts with very negative, crust-like δ7Li values, which correlate positively with peralkalinity, HFSE contents and lower εNd. Both the higher degree of melting and progressive consumption of the metasomatic component reduce the chemical and isotopic imprints of the metasomatic end member. The very positive δ7Li values of some lamproites indicate that the source of these lamproites may have been modified by subducted oceanic lithosphere. Fresh olivine from the Brloh (Moldanubian) lamproitic dyke shows very high Fo (up to 94%) and very high Li contents (up to 25 ppm), demonstrating that the extremely depleted and later enriched lithospheric mantle may have contributed significantly to the Li budget of the lamproites. The regional distribution of lamproites with contrasting chemical and isotopic fingerprints mimics the distribution of the different Variscan subduction zones. 相似文献
20.
A dense nationwide seismic network recently constructed in Japan has resulted in the production of a large amount of high-quality data that have enabled the high-resolution imaging of deep seismic structures in the Japanese subduction zone. Seismic tomography, precise locations of earthquakes, and focal mechanism research have allowed the identification of the complex structure of subducting slabs beneath Japan, revealing that the subducting Philippine Sea slab underneath southwestern Japan has an undulatory configuration down to a depth of 60–200 km, and is continuous from Kanto to Kyushu without disruption or splitting, even within areas north of the Izu Peninsula. Analysis of the geometry of the Pacific and Philippine Sea slabs identified a broad contact zone beneath the Kanto Plain that causes anomalously deep interplate and intraslab earthquake activity. Seismic tomographic inversions using both teleseismic and local events provide a clear image of the deep aseismic portion of the Philippine Sea slab beneath the Japan Sea north of Chugoku and Kyushu, and beneath the East China Sea west of Kyushu down to a depth of ∼450 km. Seismic tomography also allowed the identification of an inclined sheet-like seismic low-velocity zone in the mantle wedge beneath Tohoku. A recent seismic tomography work further revealed clear images of similar inclined low-velocity zones in the mantle wedge for almost all other areas of Japan. The presence of the inclined low-velocity zones in the mantle wedge across the entirety of Japan suggests that it is a common feature to all subduction zones. These low-velocity zones may correspond to the upwelling flow portion of subduction-induced convection systems. These upwelling flows reach the Moho directly beneath active volcanic areas, suggesting a link between volcanism and upwelling. 相似文献