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1.
Qunshu Tang  Ling Chen   《Tectonophysics》2008,455(1-4):43-52
We have used Rayleigh wave dispersion analysis and inversion to produce a high resolution S-wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the North China Craton (NCC). Using waveform data from 45 broadband NCISP stations, Rayleigh wave phase velocities were measured at periods from 10 to 48 s and utilized in subsequent inversions to solve for the S-wave velocity structure from 15 km down to 120 km depth. The inverted lower crust and uppermost mantle velocities, about 3.75 km/s and 4.3 km/s on average, are low compared with the global average. The Moho was constrained in the depth range of 30–40 km, indicating a typical crustal thickness along the profile. However, a thin lithosphere of no more than 100 km was imaged under a large part of the profile, decreasing to only ~ 60 km under the Inner Mongolian Axis (IMA) where an abnormally slow anomaly was observed below 60 km depth. The overall structural features of the study region resemble those of typical continental rift zones and are probably associated with the lithospheric reactivation and tectonic extension widespread in the eastern NCC during Mesozoic–Cenozoic time. Distinctly high velocities, up to ~ 4.6 km/s, were found immediately to the south of the IMA beneath the northern Yanshan Belt (YSB), extending down to > 100-km depth. The anomalous velocities are interpreted as the cratonic lithospheric lid of the region, which may have not been affected by the Mesozoic–Cenozoic deformation process as strongly as other regions in the eastern NCC. Based on our S-wave velocity structural image and other geophysical observations, we propose a possible lithosphere–asthenosphere interaction scenario at the northeastern boundary of the NCC. We speculate that significant undulations of the base of the lithosphere, which might have resulted from the uneven Mesozoic–Cenozoic lithospheric thinning, may induce mantle flows concentrating beneath the weak IMA zone. The relatively thick lithospheric lid in the northern YSB may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection systems at the present time.  相似文献   

2.
Horizontal extension of a previously thickened crust could be the principal mechanism that caused the development of widespread extensional basins throughout the North China block (Hua-Bei region) during the Mesozoic. We develop here a regional tectonic model for the evolution of the lithosphere in the North China block, based on thin sheet models of lithospheric deformation, with numerical solutions obtained using the finite element method. The tectonic evolution of this region is defined conceptually by two stages in our simplified tectonic model: the first stage is dominated by N–S shortening, and the second by E–W extension. We associate the N–S shortening with the Triassic continental collision between the North and South China blocks, assuming that the Tan-Lu Fault system defines the eastern boundary of the North China block. The late Mesozoic E–W extension that created the Mesozoic basin systems requires a change in the regional stress state that could have been triggered by either or both of the following factors: First, gravitational instability of the lithosphere triggered by crustal convergence might have removed the lower layers of the thickened mantle lithosphere and thus caused a rapid increase in the local gravitational potential energy of the lithosphere. Secondly, a change to the constraining stress on the eastern boundary of the North China block, that might have been caused by roll-back of the subducting Pacific slab, could have reduced the E–W horizontal stress enough to activate extension. Our simulations show that widespread thickening of the North China block by as much as 50% can be explained by the collision with South China in the Triassic and Jurassic. If convergence then ceases, E–W extension can occur in the model if the eastern boundary of the region can move outwards. We find that such extension may occur, restoring crustal thickness of order 30 km within a period of 50 Myr or less, if the depth-averaged constitutive relation of the lithosphere is Newtonian, and if the Argand number (the ratio of buoyancy-derived stress to viscous stress) is greater than about 4. Widespread convective thinning of the lithosphere is not required in order to drive the extension with these parameters. If, however, the lithospheric viscosity is non-Newtonian (with strain-rate proportional to the third power of stress) the extensional phase would not occur in a geologically plausible time unless the Argand number were significantly increased by a lithospheric thinning event that was triggered by crustal thickening ratios as low as 1.5.  相似文献   

3.
Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160–245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4–13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability field, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability field. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted stratified lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/fluid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800–900 °C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean–Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.  相似文献   

4.
We use two suites of lithospheric-scale physical experiments to investigate the manner in which deformation of the continental lithosphere is affected by both (1) variations of lithospheric density (quantified by the net buoyant mass per area in the lithospheric mantle layer, MB), and (2) the degree of coupling between the crust and lithospheric mantle (characterized by a modified Ampferer ratio, Am). The dynamics of the experiments can be characterized with a Rayleigh–Taylor type ratio, CLM. Models with a positively buoyant lithospheric mantle layer (MB > 0 and CLM > 0) result in distributed root formation and a wide deformation belt. In contrast, models with a negatively buoyant lithospheric mantle layer strongly coupled to the crust (MB < 0, 0 > CLM > ≈ − 0.2, and Am > ≈ 10− 3) exhibit localized roots and narrow deformation belts. Syncollisional delamination of the model lithospheric mantle layer and a wide deformation belt is exhibited in models with negatively buoyant lithospheric mantle layers weakly coupled to the crust (MB < 0, CLM < 0, and Am < ≈ 10− 3). Syncollisional delamination of the continental lithosphere may initiate due to buoyancy contrasts within the continental plate, instead of resulting from wedging by the opposing plate. Rayleigh–Taylor instabilities dominate the style of deformation in models with a negatively buoyant lithospheric mantle layer strongly coupled to the crust and a slow convergence rate (MB < 0 and CLM > ≈ − 0.2). The degree of coupling (Am) between the model crust and lithospheric mantle plays a lesser role in both the style of lower-lithospheric deformation and the width of the crustal deformed zone with increasing density of the lithospheric mantle layer.  相似文献   

5.
Dapeng Zhao  Eiji Ohtani   《Gondwana Research》2009,16(3-4):401-413
We present new pieces of evidence from seismology and mineral physics for the existence of low-velocity zones in the deep part of the upper mantle wedge and the mantle transition zone that are caused by fluids from the deep subduction and deep dehydration of the Pacific and Philippine Sea slabs under western Pacific and East Asia. The Pacific slab is subducting beneath the Japan Islands and Japan Sea with intermediate-depth and deep earthquakes down to 600 km depth under the East Asia margin, and the slab becomes stagnant in the mantle transition zone under East China. The western edge of the stagnant Pacific slab is roughly coincident with the NE–SW Daxing'Anling-Taihangshan gravity lineament located west of Beijing, approximately 2000 km away from the Japan Trench. The upper mantle above the stagnant slab under East Asia forms a big mantle wedge (BMW). Corner flow in the BMW and deep slab dehydration may have caused asthenospheric upwelling, lithospheric thinning, continental rift systems, and intraplate volcanism in Northeast Asia. The Philippine Sea slab has subducted down to the mantle transition zone depth under Western Japan and Ryukyu back-arc, though the seismicity within the slab occurs only down to 200–300 km depths. Combining with the corner flow in the mantle wedge, deep dehydration of the subducting Pacific slab has affected the morphology of the subducting Philippine Sea slab and its seismicity under Southwest Japan. Slow anomalies are also found in the mantle under the subducting Pacific slab, which may represent small mantle plumes, or hot upwelling associated with the deep slab subduction. Slab dehydration may also take place after a continental plate subducts into the mantle.  相似文献   

6.
Eastern Anatolia consisting of an amalgamation of fragments of oceanic and continental lithosphere is a current active intercontinental contractional zone that is still being squeezed and shortened between the Arabian and Eurasian plates. This collisional and contractional zone is being accompanied by the tectonic escape of most of the Anatolian plate to the west by major strike-slip faulting on the right-lateral North Anatolian Transform Fault Zone (NATFZ) and left-lateral East Anatolian Transform Fault Zone (EATFZ) which meet at Karlıova forming an east-pointing cusp. The present-day crust in the area between the easternmost part of the Anatolian plate and the Arabian Foreland gets thinner from north (ca 44 km) to south (ca 36 km) relative to its eastern (EAHP) and western sides (central Anatolian region). This thinner crustal area is characterized by shallow CPD (12–16 km), very low Pn velocities (< 7.8 km/s) and high Sn attenuation which indicate partially molten to eroded mantle lid or occurrence of asthenospheric mantle beneath the crust. Northernmost margin of the Arabian Foreland in the south of the Bitlis–Pötürge metamorphic gap area is represented by moderate CPD (16–18 km) relative to its eastern and western sides, and low Pn velocities (8 km/s). We infer from the geophysical data that the lithospheric mantle gets thinner towards the Bitlis–Pötürge metamorphic gap area in the northern margin of the Arabian Foreland which has been most probably caused by mechanical removal of the lithospheric mantle during mantle invasion to the north following the slab breakoff beneath the Bitlis–Pötürge Suture Zone. Mantle flow-driven rapid extrusion and counterclockwise rotation of the Anatolian plate gave rise to stretching and hence crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland which is currently dominated by wrench tectonics.  相似文献   

7.
This contribution emphasizes first-order structural and metamorphic characters of Precambrian accretionary orogens to understand the kinematics and thermomechanical state of the continental lithosphere in convergent settings involving massive juvenile magmatism. We define a new class of orogens, called ultra-hot orogens (UHO), in which the weakest type of lithosphere on Earth is deformed. UHO are characterized by (1) distributed shortening and orogen-scale flow combining vertical and horizontal longitudinal advection, under long-lasting convergence, (2) homogeneous thickening by combined downward movements of supracrustal units and three-dimensional mass redistribution in the viscous lower crust, and (3) steady-state, negligible topography and relief leveled by syn-shortening erosion and near-field sedimentation. The flow analysis of UHO provides clues to understanding crustal kinematics beneath high plateaus and suggests that the seismic reflectivity pattern of hot orogens is an image of the layering produced by lateral flow of the lower crust and associated syn-kinematic plutonism.In between the UHO and the modern cold orogens (CO), developed by shortening of lithosphere bearing a stiff upper mantle, two classes of orogens are defined. Hot orogens (HO, representative of Cordilleran and wide mature collisional belts) share flow pattern characteristics with UHO, but involve a less intense magmatic activity and develop high topographies driving their collapse. Mixed-hot orogens (MHO, representative of magmatic arcs and Proterozoic collisional belts) are orogens made of UHO-type juvenile crust and display CO-like structure and kinematics. This classification points to the fundamental link between the presence of a stiff lithospheric mantle and strain localization along major thrusts in convergent settings. A high Moho temperature (> 900 °C), implying thinning of the lithospheric mantle, enhances three-dimensional flow of the lithosphere in response to convergence. Overall, this classification of orogens emphasizes the space and time variability of uppermost mantle temperature in controlling plate interactions and continental growth.  相似文献   

8.
Mantle peridotites from the Western Pacific   总被引：1，自引：0，他引：1  
We review petrographical and petrological characteristics of mantle peridotite xenoliths from the Western Pacific to construct a petrologic model of the lithospheric mantle beneath the convergent plate boundary. The peridotite varies from highly depleted spinel harzburgite of low-pressure origin at the volcanic front of active arcs (Avacha of Kamchatka arc and Iraya of Luzon–Taiwan arc) to fertile spinel lherzolite of high-pressure origin at the Eurasian continental margin (from Sikhote-Alin through Korea to eastern China) through intermediate lherzolite–harzburgite at backarc side of Japan island arcs. Oxygen fugacity recorded by the peridotite xenoliths decreases from the frontal side of arc to the continental margin. The sub-arc type peridotite is expected to exist beneath the continental margin if accretion of island arc is one of the important processes for continental growth. Its absence suggests replacement by the continental lherzolite at the region of backarc to continental margin. Asthenospheric upwelling beneath the continental region, which has frequently occurred at the Western Pacific, has replaced depleted sub-cratonic peridotite with the fertile spinel lherzolite. Some of these mantle diapirs had opened backarc basins and strongly modified the lithospheric upper mantle by metasomatism and formation of Group II pyroxenites.  相似文献   

9.
Speculations on the nature and cause of mantle heterogeneity   总被引：8，自引：0，他引：8  
Hotspots and hotspot tracks are on, or start on, preexisting lithospheric features such as fracture zones, transform faults, continental sutures, ridges and former plate boundaries. Volcanism is often associated with these features and with regions of lithospheric extension, thinning, and preexisting thin spots. The lithosphere clearly controls the location of volcanism. The nature of the volcanism and the presence of ‘melting anomalies’ or ‘hotspots’, however, reflect the intrinsic chemical and lithologic heterogeneity of the upper mantle. Melting anomalies—shallow regions of ridges, volcanic chains, flood basalts, radial dike swarms—and continental breakup are frequently attributed to the impingement of deep mantle thermal plumes on the base of the lithosphere. The heat required for volcanism in the plume hypothesis is from the core. Alternatively, mantle fertility and melting point, ponding and focusing, and edge effects, i.e., plate tectonic and near-surface phenomena, may control the volumes and rates of magmatism. The heat required is from the mantle, mainly from internal heating and conduction into recycled fragments. The magnitude of magmatism appears to reflect the fertility, not the absolute temperature, of the asthenosphere. I attribute the chemical heterogeneity of the upper mantle to subduction of young plates, aseismic ridges and seamount chains, and to delamination of the lower continental crust. These heterogeneities eventually warm up past the melting point of eclogite and become buoyant low-velocity diapirs that undergo further adiabatic decompression melting as they encounter thin or spreading regions of the lithosphere. The heat required for the melting of cold subducted and delaminated material is extracted from the essentially infinite heat reservoir of the mantle, not the core. Melting in the upper mantle does not requires the instability of a deep thermal boundary layer or high absolute temperatures. Melts from recycled oceanic crust, and seamounts—and possibly even plateaus—pond beneath the lithosphere, particularly beneath basins and suture zones, with locally thin, weak or young lithosphere. The characteristic scale lengths—150 to 600 km—of variations in bathymetry and magma chemistry, and the variable productivity of volcanic chains, may reflect compositional heterogeneity of the asthenosphere, not the scales of mantle convection or the spacing of hot plumes. High-frequency seismic waves, scattering, coda studies and deep reflection profiles are needed to detect the kind of chemical heterogeneity and small-scale layering predicted from the recycling hypothesis.  相似文献   

10.
论燕山运动的深部地球动力学本质   总被引：109，自引：0，他引：109  
对中国东部新生代玄武岩及其包体的矿物学、岩 石学和地球化学研究的总结发现,中国东部在燕山期主要表现为岩石圈的减薄,并在其东部 出现软流圈地幔与地壳直接接触的独特地质现象。早先应该存在的古老岩石圈地幔大多由于 拆沉作用而不复存在,现今岩石圈 地幔主体是在燕山晚期及其以后形成的。因此,中国东部燕山运动的本质就是岩石圈的减薄 乃至岩石圈地幔的消失。研究认为,这种岩石圈减薄的触发因素可能与当时东侧大洋板块的 俯冲有关。软流圈地幔与地壳直接接触的动力学效应是产生强烈的岩浆板底垫托作用及相伴 随的深部地壳的高温变质作用和部分熔融作用,形成巨量岩浆的侵位与喷发,并造成新生地 壳的显著增生和原有地壳的重新调整。同时,这种地球动力学过程将携带大量地幔物质（包 括成矿物质）进入地壳,并形成地壳尺度的大规模流体循环,从而产生大面积、突发性的巨 量成矿作用。  相似文献   

11.
华北东部大陆地幔橄榄岩组成、年龄与岩石圈减薄   总被引：16，自引：1，他引：15  
对比分析了华北东部地块陆下岩石圈地幔橄榄石Mg#值和单斜辉石的REE配分形式。报道了汉诺坝和鹤壁橄榄岩中不同产状硫化物的激光MC-ICPMS原位Re-Os年龄和信阳橄榄岩中锆石的U-Pb年龄和信阳橄榄岩锆石的U-Pb年龄。在这些资料基础上,进一步讨论了华北东部岩石圈中、新生代时的减薄机制。原位分析在揭示岩石圈深部过程的细节上,有比全岩分析更大的优越性,并揭示出了在华北深部有中元古代(14亿年)和新元古代(7～8亿年)热活动的记录。岩石圈拆沉作用不能很好地解释古老难熔地幔、过渡型地幔和新生饱满地幔并存的事实;同时,单纯的熔体-橄榄岩相互作用也难以解释中、新生代岩石圈的减薄过程和新生地幔单斜辉石中出现强烈LREE亏损现象,即历史复杂的克拉通岩石圈向历史明显简单的“大洋型”地幔的转换。因此,华北东部岩石圈减薄包括地幔伸展、熔-岩作用、侵蚀置换等复杂过程。这些过程可能包括:(1)早中生代时,扬子地块向北俯冲碰撞所引起华北岩石圈的熔/流体交代富集作用、地幔伸展和受扰动软流圈物质上涌并侵蚀被改造了的岩石圈;(2)晚中生代—古近纪,因太平洋俯冲的热扰动致使软流圈物质进一步的强烈侵蚀作用引起岩石圈的巨大减薄;(3)晚第三纪以来的软流圈热沉降作用所带来的小幅度岩石圈增厚过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体的减薄过程。喷发时代为100Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前曾有过地幔置换作用。  相似文献   

12.
华北克拉通破坏的物理、化学过程:地幔橄榄岩证据   总被引：1，自引：0，他引：1  
本文对比了华北东部不同时代、不同位置捕虏体/地质体橄榄岩的地幔属性,讨论了华北克拉通破坏的物理、化学过程。结果表明,拆沉作用不能很好地解释古老难熔、过渡型和新生饱满地幔并存的事实;单纯的熔体-橄榄岩相互作用也不易解释中、新生代岩石圈巨大减薄和新生饱满地幔Cpx中LREE亏损现象,即具复杂演化历史的克拉通地幔向演化历史简单的"大洋型"岩石圈的转换。华北东部克拉通破坏作用包括地幔伸展、熔-岩作用、侵蚀置换等复杂的物理、化学过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体减薄过程。喷发时代为100 Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前有过地幔置换作用。  相似文献   

13.
Yu J. Gu   《Tectonophysics》2006,424(1-2):41-51
This paper investigates the shear velocity structure under the northern East Pacific Rise at the latitude range of 9–18°N, using intermediate-period Rayleigh and Love waves. The selected ocean-bottom seismic records provide source–receiver paths that ideally constrain the lithospheric mantle structure beneath the southern Rivera plate and the Mathematician paleoplate. The Rayleigh wave data infer a relatively thin ( 30 km) lithosphere under the eastern side of the present-day East Pacific Rise. The associated shear velocities are consistent with existing models of oceanic mantle beneath this region, and the estimated plate age of 2–3 million years agrees with results from magnetic dating. The west of the rise axis is characterized by a thicker and faster lithosphere than the eastern flank, and such structural differences suggest the presence of a relatively old Mathematician paleoplate. The discontinuous change in mantle structure across the East Pacific Rise spreading center are observed in both isotropic and anisotropic velocities. The young oceanic lithosphere east of the rise axis shows strong polarization anisotropy, where the dominant orientation of crystallographic axes roughly parallels the spreading direction. However, the western flank of the rise axis is approximately isotropic, and the lack of anisotropy suggests complex deformation mechanisms associated with earlier episodes of ridge segmentation, propagation and dual-spreading on and around the Mathematician paleoplate.  相似文献   

14.
Active or passive continental rifting is associated with thinning of the lithosphere, ascent of the asthenosphere, and decompressional melting. This melt may percolate within the partially molten source region, accumulate and be extracted. Two-dimensional numerical models of extension of the continental lithosphere–asthenosphere system are carried out using an Eulerian visco-plastic formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component (crust–mantle) and two-phase (solid–melt) system. Temperature-, pressure-, and stress-dependent rheologies based on laboratory data for granite, pyroxenite and olivine are used for the upper and lower crust, and mantle, respectively. Rifting is modelled by externally prescribing a constant rate of widening with velocities between 2.5 and 40 mm/yr. A typical extension experiment is characterized by 3 phases: 1) distributed extension, with superimposed pinch and swell instability, 2) lithospheric necking, 3) continental break up, followed by oceanization. The timing of the transition from stages 1) to 2) depends on the presence and magnitude of a localized perturbation, and occurs typically after 100–150 km of total extension for the lithospheric system studied here. This necking phase is associated with a pronounced negative topography (“rift valley”) and a few 100 m of rift flanks. The dynamic part of this topography amounts to about 1 km positive topography. This means, if rifting stops (e.g. due to a drop of external forces), immediate additional subsidence by this amount is predicted. Solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks, often observed at volcanic margins. After continental break up, a second time-dependent upwelling event off the rift axis beneath the continental margins is found, producing further volcanics. Melting has almost no or only a small accelerating effect on the local extension value (β-value) for a constant external extension rate. Melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift. This upwelling velocity is only weakly dependent on the rifting velocity. The melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks. Melting increases the topography of the flanks by 100–200 m due to depletion buoyancy. Another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension/subsidence. Modelled magma amounts are smaller than observed for East African Rift System. Increasing the mantle temperature, as would be the case for a large scale plume head, better fits the observed magma volumes. If extension stops before a new ocean is formed, melt remains present, and convection remains active for 50–100 Myr, and further subsidence is significant.  相似文献   

15.
In contrast to the normal ‘Wilson cycle’ sequence of subduction leading to continental collision and associated mountain building, the evolution of the New Zealand plate boundary in the Neogene reflects the converse—initially a period of continental convergence that is followed by the emplacement of subduction. Plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes. Relative plate motions and reconstructions since the Early to Mid-Miocene require significant continental convergence in advance of the emplacement of the southward migrating Hikurangi subduction—a sequence of tectonism seen in the present plate boundary geography of Hikurangi subduction beneath North Island and convergence in the Southern Alps along the Alpine Fault. In contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable, the transition from a continental convergent regime, with its associated crustal and lithospheric thickening, to subduction of oceanic lithosphere requires substantial thinning (removal) of upper plate continental lithosphere to make room for the slab. The simple structure of the Wadati–Benioff zone seen in the present-day geometry of the subducting Pacific plate beneath North Island indicates that this lithospheric adjustment occurs quickly. Associated with this rapid lithospheric thinning is the development of a series of ephemeral basins, younging to the south, that straddle the migrating slab edge. Based on this association between localized vertical tectonics and slab emplacement, the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting Pacific slab. Although the New Zealand plate boundary is often described as simply two subduction zones linked by the transpressive Alpine Fault, in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction.  相似文献   

16.
Plume–lithosphere interactions (PLI) have important consequences both for tectonic and mineralogical evolution of the lithosphere: for example, Archean metallogenic crises at the boundaries of the West African and Australian cratons coincide with postulated plume events. In continents, PLI are often located near boundaries between younger plates (e.g., orogenic) and older stable plates (e.g., cratons), which represent important geometrical, thermal and rheological barriers that interact with the emplacement of the plume head (e.g., Archean West Africa, East Africa, Pannonian–Carpathian system). The observable PLI signatures are conditioned by plume dynamics but also by lithosphere rheology and structure. We address the latter problem by considering a free-surface numerical model of PLI with two stratified elasto-viscous–plastic (EVP) lithospheric plates, one of which is older and thicker than another. The results show that: (1) plume head flattening is asymmetric, it is blocked from one side by the cold vertical boundary of the older plate, which leads to the mechanical decoupling of the crust from the mantle lithosphere, and to localized faulting at the cratonic margin; (2) the return flow from the plume head results in sub-vertical down-thrusting (delamination) of the lithosphere at the margin, producing sharp vertical cold boundary down to the 400 km depth; (3) plume head flattening and migration towards the younger plate results in concurrent surface extension above the centre of the plume and in compression (pushing), down-thrusting and magmatic events at the cratonic margin (down-thrusting is also produced at the opposite border of the younger plate); these processes may result in continental growth at the “craton side”; (4) topographic signatures of PLI show basin-scale uplifts and subsidences preferentially located at cratonic margins. Negative Rayleigh–Taylor instabilities in the lithosphere above the plume head provide a mechanism for crustal delamination. Inferred consequences of PLI near intra-continental plate boundaries, such as faulting at cratonic edges and enhanced magmatic activity, could explain plume-related metallogenic crises, as suggested for West Africa and Australia.  相似文献   

17.
闫峻  陈江峰 《地质论评》2005,51(1):16-26
本文综合运用不同时代幔源包体平衡温压对比、玄武岩地球化学性质对岩石圈厚度的反演以及不同时代岩石圈地幔地球化学性质的对比的方法，把华北地块东部岩石圈的减薄时间限定在晚中生代至新生代之间。减薄的机制可能是华北东部地区晚白垩世以来大陆岩石圈的拉张作用。由于机械性拉薄和热、机械和化学侵蚀作用，岩石圈厚度最终减薄到70km以下。但古老的岩石圈地幔并没有完全因减薄而消失，残留部分受到了来自软流圈物质的强烈改造，使其Sr、Nd同位素组成类似于软流圈，但Os同位素没有受到明显的改变。改造后的岩石圈地幔成为华北地块东部新生代岩石圈地幔的主体。在时空上，岩石圈的减薄具有不均一的性质。  相似文献   

18.
中国边缘海域及其邻区的岩石层结构与构造分析   总被引：3，自引：0，他引：3  
利用中国边缘海域近年的地震层析成像结果,根据速度异常和各向异性分析东海、黄海和南海北部的岩石层结构和构造,讨论中朝块体和扬子块体在黄海内部的拼合边界(黄海东部断裂带)、东海陆架盆地上地幔异常与岩石层形成演化、南海北部地壳底部高速层的成因及地幔活动等问题。分析表明,黄海东部与朝鲜半岛之间存在一个深部构造界限(大致对应于黄海东部断裂带),分界两侧Pn波速度各向异性存在明显差异,反映不同构造应力和断裂剪切运动作用下的岩石层地幔变形特征。东海陆架下方的低速异常揭示了张裂盆地形成时期的地幔活动痕迹,表明中、新生代期间发生过地幔上涌并造成岩石层减薄,菲律宾海板块向西俯冲引发的地幔活动对东海陆架岩石层的形成、演化产生明显的影响。南海北部岩石层厚度较大并且温度相对偏低,地幔异常仅限于局部地区,估计南海北部大陆边缘的地壳底部高速层形成于张裂发生之前,或者是地壳形成时期壳幔分异时的产物。南海中央海盆的扩张不仅导致地壳拉张,软流层物质上涌,而且也造成岩石层地幔减薄甚至缺失。  相似文献   

19.
中国东部岩石圈热状态与流变学强度特征   总被引：10，自引：1，他引：9  
根据均衡原理制约的地热计算得到中国东部岩石圈的温度分布状态,以40、70、100km和莫霍面深度等温线图以及600°C、1100°C等温面深度的形式表示.同时计算了以1350°C等温面深度表示的中国东部的热岩石圈厚度.结果显示:在扬子克拉通西部四川盆地之下存在160～200km厚的岩石圈根,但在整个华北克拉通之下缺失岩...  相似文献   

20.
新生代火山岩中的深源捕虏体资料反映,南海北部及其沿岸地区岩石圈地幔的主体由主量元素易熔组分相对饱满的、同位素组成类似MORB-OIB型的、高温型的二辉橄榄岩所组成;但在其顶部残留有古老的岩石圈地幔,它由主量元素易熔组分相对贫瘠的、同位素组成类似EM型的、较低温的方辉橄榄岩组成。在下地壳底部,分布着由晚中生代幔源岩浆分离结晶和堆晶的基性麻粒岩。由此提出了该区中、新生代壳 -幔或岩石圈 -软流圈相互作用与构造演化的简略模式: (1)印支期 -燕山早期为地壳岩石圈厚度增大的华夏型后地台活化造山带环境;(2)燕山晚期岩石圈快速减薄(如拆沉作用),造山带拉伸塌陷,地壳深处并发生广泛的底侵作用; (3)始新世 -渐新世软流圈再次上涌(如地幔柱的影响),岩石圈地幔发生底蚀减薄,地壳也因为下部层的塑性流展和上部层的张裂拉伸而减薄; (4)中新世以来,由于地幔热源在拉伸环境中被释放,壳幔发生冷却,部分软流圈地幔转化为“新生的”岩石圈地幔。研究进一步说明,南海北部陆缘扩张是该区大陆构造演化到大陆活化造山带后期,在深部壳 -幔的相互作用下,岩石圈所发生的垂向减薄和侧向伸展,既不同于弧后扩张,也不是受控于大西洋式的海底扩张。  相似文献   

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