Heterogeneous stretching, simple shear and basin development

The models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric-scale shear zone, are but two end members of a range of possible extensional models. The homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin. The shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence. It is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy, but will often overlap in plan view. This will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural, stratigraphic and geophysical techniques. The model which combines heterogeneous lithospheric stretching and associated simple shear may explain: (1) variations in dip of the major detachment zones in the large basins, (2) variations in types of strain on or beneath the detachment zones, (3) regional uplift of part of a basin, to erode the earlier fault blocks, and (4) the development of volcanism in basins with only low values of upper crustal extension. The zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching, or to one side. It may be linked to the upper crustal zone by faults which dip consistently in one direction, or by extensional systems which change their dominant dip direction with depth, pulling out the mid-crust as one or more wedges. Possible examples are discussed from the Basin and Range province and northwest European continental shelf.     

大别造山带地壳结构反演及其动力学意义

大别造山带是全球最大的碰撞造山带之一,三叠纪时期,扬子板块深俯冲至地幔的200km处,经历了超高压变质作用。白垩纪早期,该造山带发生了强烈的伸展和垮塌,以及大规模的后造山地幔源岩浆侵入和火山活动。本研究收集了大别造山带及其邻区(29°~34°N、114°~119°E)的震相资料,采用双差层析成像技术,对大别造山带地壳结构进行反演,研究地壳结构与后造山地幔源岩浆侵入和火山活动之间的关系。结果显示,大别造山带中上地壳存在低速结构,该低速结构可能是熔融的幔源侵入物质,由于俯冲板片断裂,或下地壳/岩石圈发生拆沉,导致软流圈物质上涌至地壳底部、侵入地壳中,形成大别造山带地壳中的低速结构;同时,合肥盆地显示为低速区,可能是受浅部沉积层影响。研究中横切大别山的4条剖面显示,该地区下方存在北向倾斜高速结构,该高速结构可能是襄樊-广济断层,或者是扬子板块向华北板块下方俯冲的遗迹。     

The Cenozoic mantle magmatism and motion of lithosphere on the north margin of the Tibetan Plateau

Geodynamic properties and evolution of the lithosphere on the north margin of the Tibetan Plateau are recently hot topics to geoscientists in the world. Have the northern plates been subducting underneath the Plateau? It is still an unsolved problem. One of the keys to solving this problem is to understand the genetic processes of Cenozoic magmas on the north margin of the Tibetan Plateau. However, there is no enough evidence supporting the subduction model. In contrast, a series of evidence indicates that collision-induced huge shearing faults and large-scale crust shortening played a main role in lithosphere motion on the north margin of the Tibetan Plateau. The mantle-derived igneous rocks strictly distribute at the intersections of large strike-slip faults on the north margin of the Plateau. Generation of magmas may be related to local extensional condition induced by strike-slipping faults, which lead to lithosphere gravitational instability and collapse, as well as upwelling of the deep hot material. Heat induced by shearing and carried by upwelling hot material may cause partial melting on H₂O-bearing mantle.     

Continental subduction and exhumation of high-pressure rocks: insights from thermo-mechanical laboratory modelling

Thermo-mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust. The model consists of two lithospheric plates made of new temperature sensitive analogue materials. The lithosphere is underlain by liquid asthenosphere. The continental lithosphere contains three layers: the weak sedimentary layer, the crust made of a stronger material, and of a still stronger lithospheric mantle. The whole model is subjected to a constant vertical thermal gradient, causing the strength reduction with depth in each lithospheric layer. Subduction is driven by both push force and pull force. During subduction, the subducting lithosphere is heating and the strength of its layers reduces. The weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up. The subducted crust undergoes complex deformation, including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50-km depth. This failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force. This process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere. The delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates. Average equivalent exhumation rate of the crustal material during delamination is about 1 cm/year. The crust-asthenosphere boundary near the interplate zone is uplifted. The subducted mantle layer then breaks off, removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere. The latter produces shortening of the formed orogen and the growth of relief. The modelling reveals an interesting burial/exhumation evolution of the sedimentary cover. During initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone (at 60-70 km-depth). These sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases. This results in squeezing the underplated sediments out. Part of them is extruded upwards along the interplate zone to about 30-km depth at an equivalent rate of 5-10 cm/year.     

上通流对大陆岩石圈地幔-地壳热结构模式的潜在影响

本文以多孔介质中大尺度传热问题为基础,结合热平衡理论分析与数值计算,探讨了上通流对大陆岩石圈地幔-地壳热结构模式的潜在影响.根据大陆岩石圈中孔隙波传热概念模型的初步理论分析结果,指出了采用理论分析和数值模拟相结合的方法在研究大陆岩石圈地幔-地壳热结构模式时的重要性.理论分析方法可用来确定岩石圈尺度范围内大陆岩石圈的厚度和大陆地壳相关的边界条件,从而为地壳范围内数值模型的建立提供一些重要信息.数值模拟方法可以用来模拟地壳尺度范围内地壳的详细结构和复杂几何形状.如果地壳内的热分布是所考虑的主要因素,采用具有地壳尺度的合理数值模型可以有效减少计算机工作量.利用理论分析方法求出的岩石圈尺度范围内大陆岩石圈厚度与地幔传导热流之间关系的理论解,不仅可以用来验证模拟大陆岩石圈内传热问题所采用的数值方法, 而且可以用来初步研究大陆岩石圈内热分布的基本规律,为研究岩石圈地幔热事件中大陆岩石圈热减薄过程提供相应的边界条件.本文从理论分析的观点初步探讨了中国大陆不同构造背景下大陆岩石圈的热结构模式,其结果与从地球物理和地质资料中获得的大陆岩石圈热结构模式十分吻合.研究结果表明由大陆岩石圈中孔隙波传播所导致的上通流是影响大陆岩石圈地幔-地壳热结构模式及大陆岩石圈地幔与地壳之间物质和能量交换的可能机制之一.     

U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains

The U-Pb isotope geochemical study of the pyroxenite-gabbro intrusion in the Dabie Mountains shows that the post-collisional mafic-ultramafic rocks of the Dabie Mountains are characterized by relative high Pb contents, low U contents and low U/Pb ratios. These characters may be results of interaction between lithosphere or depleted asthenospheric mantle (DMM) and lower crust, but have nothing to do with mantle plume and subducted continental crust. It was first observed that some samples with lower ²⁰⁶Pb/²⁰⁴Pb and higher ²⁰⁷Pb/²⁰⁴Pb ratios show typical characters of the LOMU component. The Pb, Sr, and Nd isotopic tracing shows that three components are needed in the source of the Zhujiapu pyroxenite-gabbro intrusion. They could be old enriched sub-continental lithospheric mantle (LOMU component), lower crust and depleted asthenospheric mantle. The crust-mantle interaction process producing primitive magma of post-collisional mafic-ultramafic rocks in the Dabie Mountains could be described by a lithospheric delamination and magma underplating model. After continent-continent collision, delamination of the thickened lithosphere induced the upwelling of depleted asthenospheric mantle, which caused partial melting of asthenospheric mantle and residual sub-continental lithospheric mantle. The basaltic magma produced in this process underplated in the boundary between the crust and mantle and interacted with lower crust resulting in the geochemical characters of both enriched lithospheric mantle and lower crust.     

郯庐断裂带中南段的岩石圈精细结构

郯庐断裂带是中国东部规模最大的构造活动带,有着复杂的形成演化历史,对中国东部的区域构造、岩浆活动、矿产资源的形成和分布以及现代地震活动都有重要控制作用.2010年在郯庐断裂带中南段的江苏宿迁市附近,采用深地震反射探测方法对郯庐断裂带及其两侧地块的岩石圈结构进行了解剖.结果表明,该区莫霍面和岩石圈底界均向西倾,其中,地壳厚度约为31~36km,岩石圈厚度约为75~86km,且岩石圈厚度在郯庐断裂带下方出现突变.郯庐断裂带在剖面上表现为由多条主干断裂组成的花状构造,其内部发育有断陷盆地和挤压褶皱,具有伸展、挤压和走滑并存的构造形迹,暗示郯庐断裂带的形成和演化经历了多期复杂的构造活动.这一断裂带错断了近地表沉积层,向下切割莫霍面和岩石圈地幔,属岩石圈尺度的深大断裂构造系统.软流圈高温高压热物质沿断裂带的上涌、岩浆底侵或热侵蚀作用造成岩石圈出现拉张伸展和岩石圈减薄,并可能使岩石圈组构及其物质成分发生改变.本项研究结果不但可进一步加深对郯庐断裂带深、浅部结构的认识,而且还可为分析研究华北克拉通东部的深部过程和浅部构造响应提供资料约束.     

U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains——Crust-mantle interaction and LOMU component

Geochronological studies of mafic-ultramafic intrusions occurrence in the northern Dabie zone (NDZ) suggest that these pyroxenite-gabbro intrusions formed 120—130 Ma ago should be post-collisional magmatic rocks[1—4]. These mafic-ultramafic rocks provid…     

The characteristics,origins, and geodynamic settings of supergiant gold metallogenic provinces

There are six distinct classes of gold deposits, each represented by metallogenic provinces, having 100's to >1000 tonne gold production. The deposit classes are: (1) orogenic gold; (2) Carlin and Carlin-like gold deposits; (3) epithermal gold-silver deposits; (4) copper-gold porphyry deposits; (5) iron-oxide copper-gold deposits; and (6) gold-rich volcanic hosted massive sulfide (VMS) to sedimentary exhalative (SEDEX) deposits. This classification is based on ore and alteration mineral assemblages; ore and alteration metal budgets; ore fluid pressure(s) and compositions; crustal depth or depth ranges of formation; relationship to structures and/or magmatic intrusions at a variety of scales; and relationship to the P-T-t evolution of the host terrane. These classes reflect distinct geodynamic settings. Orogenic gold deposits are generated at mid-crustal (4–16 km) levels proximal to terrane boundaries, in transpressional subduction-accretion complexes of Cordilleran style orogenic belts; other orogenic gold provinces form inboard, by delamination of mantle lithosphere, or plume impingement. Carlin and Carlin-like gold deposits develop at shallow crustal levels (<4 km) in extensional convergent margin continental arcs or back arcs; some provinces may involve asthenosphere plume impingement on the base of the lithosphere. Epithermal gold and copper-gold porphyry deposits are sited at shallow crustal levels in continental margin or intraoceanic arcs. Iron oxide copper-gold deposits form at mid to shallow crustal levels; they are associated with extensional intracratonic anorogenic magmatism. Proterozoic examples are sited at the transition from thick refractory Archean mantle lithosphere to thinner Proterozoic mantle lithosphere. Gold-rich VMS deposits are hydrothermal accumulations on or near the seafloor in continental or intraoceanic back arcs.

The compressional tectonics of orogenic gold deposits is generated by terrane accretion; high heat flow stems from crustal thickening, delamination of overthickened mantle lithosphere inducing advection of hot asthenosphere, or asthenosphere plume impingement. Ore fluids advect at lithostatic pressures. The extensional settings of Carlin, epithermal, and copper-gold porphyry deposits result from slab rollback driven by negative buoyancy of the subducting plate, and associated induced convection in asthenosphere below the over-riding lithospheric plate. Extension thins the lithosphere, advecting asthenosphere heat, promotes advection of mantle lithosphere and crustal magmas to shallow crustal levels, and enhances hydraulic conductivity. Siting of some copper-gold porphyry deposits is controlled by arc parallel or orthogonal structures that in turn reflect deflections or windows in the slab. Ore fluids in Carlin and epithermal deposits were at near hydrostatic pressures, with unconstrained magmatic fluid input, whereas ore fluids generating porphyry copper-gold deposits were initially magmatic and lithostatic, evolving to hydrostatic pressures. Fertilization of previously depleted sub-arc mantle lithosphere by fluids or melts from the subducting plate, or incompatible element enriched asthenosphere plumes, is likely a factor in generation of these gold deposits. Iron oxide copper-gold deposits involve prior fertilization of Archean mantle lithosphere by incompatible element enriched asthenospheric plume liquids, and subsequent intracontinental anorogenic magmatism driven by decompressional extension from far-field plate forces. Halogen rich mantle lithosphere and crustal magmas likely are the causative intrusions for the deposits, with a deep crustal proximal to shallow crustal distal association. Gold-rich VMS deposits develop in extensional geodynamic settings, where thinned lithosphere extension drives high heat flow and enhanced hydraulic conductivity, as for epithermal deposits. Ore fluids induced hydrostatic convection of modified seawater, with unconstrained magmatic input. Some gold-rich VMS deposits with an epithermal metal budget may be submarine counterparts of terrestrial epithermal gold deposits. Real time analogs for all of these gold deposit classes are known in the geodynamic settings described, excepting iron oxide copper-gold deposits.

     

基于背景噪声和地震面波联合反演华北克拉通中部岩石圈结构

基于ChinArray三期项目布设于华北克拉通中部的流动台阵观测数据,利用背景噪声互相关和地震面波层析成像获取了研究区内6—140 s周期的瑞雷面波频散,使用蒙特卡罗非线性反演方法获得了华北克拉通中部岩石圈的高分辨率三维S波速度结构。结果显示华北克拉通不同地块的岩石圈速度结构存在显著的横向差异：其中鄂尔多斯盆地腹地整体表现为高速特征,延伸至200 km以下,但其东南缘存在小范围的低速异常;东部的华北盆地整体表现为低速特征,具有较薄的地壳和岩石圈厚度;中部造山带南北两端以及南北重力梯度线下方存在相连接的低速区域,在深处延伸至华北盆地下方;在下地壳和上地幔顶部,大同火山群区域的低速体逐渐向西偏移至鄂尔多斯盆地东北角下方;而在上地幔中,该区域的低速异常随深度增加而逐渐减弱,低速体延伸至东南方向的华北盆地下方。基于本研究获得的S波速度模型,我们认为：鄂尔多斯盆地腹地保持了克拉通特性,但其东南缘存在局部的岩石圈改造作用;华北盆地发生了强烈的岩石圈破坏减薄和地壳伸展变形;中部造山带南北端以及南北重力梯度线下方的岩石圈发生了局部的改造减薄,其机制可能都来源于华北盆地下方地幔热物质的上涌;大同火山群下...     

New constraints on lithospheric thickness of the Iranian plateau using converted waves

The study of mantle lithosphere plays a key role to reveal predominant tectonic setting process of a region. The current geological and tectonic setting of Iran is due to the ongoing continental–continental collision of the Arabian and Eurasian plates. We applied a combined P and S receiver function analysis to the teleseismic data of nine permanent broadband seismic stations of the International Institute of Earthquake Engineering and Seismology located in different tectonic zones of Iranian plateau. More than 4 years of data were used to estimate the thickness of the crust and mantle lithosphere. According to our results, the crust is 50 km thick beneath the Zagros fold and thrust belt (ZFTB). We found the maximum Moho depth of approximately 70 km under the Sanandaj-Sirjan zone (SSZ) indicating the overthrusting of the crust of Central Iran onto the Zagros crust along the main Zagros thrust (MZT). Below the northeasternmost part of the Urumieh–Dokhtar Magmatic Arc (UDMA) and Central Iran, the Moho becomes shallower and lies at 40 km depth. Towards northeast, beneath the Alborz zone, the crust is 55 km thick. Based on S receiver functions, we provided new insights into the thickness of the Arabian and Eurasian lithospheres. The location of the boundary between these plates was estimated to be beneath the SSZ, which is slightly shifted northeastward relative to the surficial expression of the MZT. Furthermore, the Arabian plate is characterized by the relatively thick lithosphere of about 130 km beneath the ZFTB reaching 150 km beneath the SSZ, where the thickest crust was also observed. This may imply that the shortening across the Zagros is accommodated by lithospheric thickening. In contrast, UDMA and Central Iran are recognized by the thin lithosphere of about 80–85 km. This thin lithosphere may be associated with the asthenospheric upwelling caused by either lithospheric delamination or Neo-Tethys slab detachment beneath the Zagros collision zone.     

Subduction and retreating of the western Pacific plate resulted in lithospheric mantle replacement and coupled basin-mountain respond in the North China Craton

The North China Craton (NCC) witnessed Mesozoic vigorous tectono-thermal activities and transition in the nature of deep lithosphere. These processes took place in three periods: (1) Late Paleozoic to Early Jurassic (～170 Ma); (2) Middle Jurassic to Early Cretaceous (160–140 Ma); (3) Early Cretaceous to Cenozoic (140 Ma to present). The last two stages saw the lithospheric mantle replacement and coupled basin-mountain response within the North China Craton due to subduction and retreating of the Paleo-Pacific plate, and is the emphasis in this paper. In the first period, the subduction and closure of the Paleo- Asian Ocean triggered the back-arc extension, syn-collisional compression and then post-collisional extension accompanied by ubiquitous magmatism along the northern margin of the NCC. Similar processes happened in the southern margin of the craton as the subduction of the Paleo-Tethys ocean and collision with the South China Block. These processes had caused the chemical modification and mechanical destruction of the cratonic margins. The margins could serve as conduits for the asthenosphere upwelling and had the priority for magmatism and deformation. The second period saw the closure of the Mongol-Okhotsk ocean and the shear deformation and magmatism induced by the drifting of the Paleo-Pacific slab. The former led to two pulse of N-S trending compression (Episodes A and B of the Yanshan Movement) and thus the pre-existing continental marginal basins were disintegrated into sporadically basin and range province by the Mesozoic magmatic plutons and NE-SW trending faults. With the anticlockwise rotation of the Paleo-Pacific moving direction, the subduction-related magmatism migrated into the inner part of the craton and the Tanlu fault became normal fault from a sinistral one. The NCC thus turned into a back-arc extension setting at the end of this period. In the third period, the refractory subcontinental lithospheric mantle (SCLM) was firstly remarkably eroded and thinned by the subduction-induced asthenospheric upwelling, especially those beneath the weak zones (i.e., cratonic margins and the lithospheric Tanlu fault zone). Then a slightly lithospheric thickening occurred when the upwelled asthenosphere got cool and transformed to be lithospheric mantle accreted (～125 Ma) beneath the thinned SCLM. Besides, the magmatism continuously moved southeastward and the extensional deformations preferentially developed in weak zones, which include the Early Cenozoic normal fault transformed from the Jurassic thrust in the Trans-North Orogenic Belt, the crustal detachment and the subsidence of Bohai basin caused by the continuous normal strike slip of the Tanlu fault, the Cenozoic graben basins originated from the fault depression in the Trans-North Orogenic Belt, the Bohai Basin and the Sulu Orogenic belt. With small block size, inner lithospheric weak zones and the surrounding subductions/collisions, the Mesozoic NCC was characterized by (1) lithospheric thinning and crustal detachment triggered by the subduction-induced asthenospheric upwelling. Local crustal contraction and orogenesis appeared in the Trans-North Orogenic Belt coupled with the crustal detachment; (2) then upwelled asthenosphere got cool to be newly-accreted lithospheric mantle and crustal grabens and basin subsidence happened, as a result of the subduction zone retreating. Therefore, the subduction and retreating of the western Pacific plate is the outside dynamics which resulted in mantle replacement and coupled basin-mountain respond within the North China Craton. We consider that the Mesozoic decratonization of the North China Craton, or the Yanshan Movement, is a comprehensive consequence of complex geological processes proceeding surrounding and within craton, involving both the deep lithospheric mantle and shallow continental crust.     

Thermal-rheological structure of the lithosphere beneath Jiyang Depression: Its implications for geodynamics

Thermal regime of the lithosphere is the scenario of the lithospheric thermal evolution, and the thermo-mechanical state of lithosphere definitively controls its deformation style and mechanism. Better understanding of the lithospheric deep thermal-rheo- logical structure of sedimentary basin will shed light on the formation and evolution dynamic process of the basin. Surface tectonics is the response of the deep structure, and is controlled by the lithospheric ther-mal-rheological properties.…     

Dynamics of thinning and destruction of the continental cratonic lithosphere: Numerical modeling

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.     

Vertical tectonism in oceanic fracture zones

Anomalous topographic highs are found along many large oceanic fracture zones, frequently in the form of transverse ridges elongated parallel to the fractures both within and outside of the transform zone. These crustal highs are one or more kilometers shallower than the adjacent “normal” crust of equivalent age. Their elevation is frequently higher than the axial zone of lithospheric accretion. Geophysical and petrological data from the Vema, St. Paul, Romanche (Atlantic Ocean), Owen (Indian Ocean), Alula (Gulf of Aden), and other fracture zones suggest that the anomalous transverse ridges are not the result of excess volcanism, but rather of tectonic uplift of upper mantle and crustal blocks.Factors which may determine vertical tectonism along fracture zones include: (1) horizontal thermal conduction across a fracture zone from a lithospheric accreting segment; (2) viscodynamic forces operating in a fracture zone close to its intersection with an accreting segment; and (3) compressional and tensional horizontal stresses operating along a fracture zone, and created by several causes, the major one being small changes in the direction of spreading. Among these various factors, compressional and tensional horizontal stresses are probably the main cause of vertical tectonism in fracture zones. During the vertical motion of upper mantle/crustal blocks, ultramafic rocks, due to their physical properties, are uplifted preferentially.One of the implications of fracture zone vertical tectonism is that the age/depth of the crust relationship may not apply in regions with large fracture zones. In one well-documented case, a transverse crustal block at the Romanche fracture zone subsided during the last 5 m.y. at an average rate more than one order of magnitude faster than crust of equivalent age.     

青藏高原P波速度层析成像与岩石圈结构

利用中国西部地震台网的数据,通过体波层析成像反演了青藏高原及邻域的三维P波速度结构.根据地壳和上地幔的速度变化和构造特征,重点讨论了下地壳流动、地幔上涌、岩石圈减薄以及与藏北新生代火山岩和藏南裂谷系的关系等问题.分析表明,青藏高原中、下地壳平均速度偏低,低速区主要分布在拉萨和羌塘块体内部,随着深度的增加逐渐扩大到松潘—甘孜块体.上述低速区之间多被高速带分隔,暗示地壳中、下部的韧性变形被限制在特定的区域,不太适于产生贯穿整个青藏高原的大规模横向流动.此外,地幔上涌也并非普遍发生于整个青藏高原,而是集中在羌塘、松潘—甘孜以及喜马拉雅东构造结附近,导致上述区域的岩石圈地幔较薄,并且伴生火山活动和岩浆作用.此外,由于印度大陆岩石圈在向北俯冲,板片下沉过程中引起地幔上涌,热流物质有可能上升进入地壳,这一作用对藏北新生代火山岩和藏南裂谷系的形成以及中、下地壳的韧性变形产生了明显的影响.     

造山带下岩石圈地幔小尺度减薄的动力学成因

地质与地球物理观测数据表明青藏高原、安第斯山、以及帕米尔等典型造山高原之下均有明显的岩石圈地幔小尺度/分段式减薄现象.这些小尺度岩石圈减薄难以用经典的拆沉或对流减薄理论来解释,一方面,拆沉预示大尺度岩石圈地幔的剥离过程,而对流减薄则在黏度相对低的地幔岩石圈中发生,其主要以小尺度的局部增厚触发并仅减薄地幔岩石圈的底部区域.另一方面,拆沉或对流减薄模型都预测造山带尺度的地幔岩石圈拆离,都假设造山带岩石圈横向均一,然而实际的造山带岩石圈往往由多个不同的地块构成,块体之间岩性、物性、流变结构可能大有差别,即横向不均一性.这些造山带岩石圈地幔的横向不均一性,能否有效解释观测到的局部小尺度减薄现象？为此,我们构建了一系列高精度动力学数值模型,系统模拟了碰撞造山过程中岩石圈地幔的形变和不稳定性.结果表明,在塑性屈服强度很低的情况下,横向不均一的造山带岩石圈有发生分段式/小尺度减薄的可能性;其主要机理是由位错蠕变与强塑性作用所导致的应变集中使得地块间及壳幔间耦合弱化,从而使得较弱地块的岩石圈地幔在增厚时由于重力不稳定性而产生局部剥离,进而诱发小尺度软流圈上涌.模拟结果可以良好地解释发生在青藏高原东北缘、安第斯中部高原、以及帕米尔高原之下岩石圈的局部小尺度/分段式减薄现象.     

Three-dimensional numerical simulations of crustal deformation and subcontinental mantle convection

3-D simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large-scale mantle flow. Simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 × 1 × 1 Cartesian domain. Convective stresses cause crust to thicken above a sheet-like mantle downwelling. For mild convective vigor an initial crustal thickness variation is required to induce 3-D lithospheric instability below the zone of crustal convergence. The amplitude of the required variation decreases with increasing convective vigor. Morphologically, instability is manifest in formation of drip-like thermals that exist within the large-scale roll associated with initial crustal thickening. A strong surface signature of the drips is their ability to cause deviations from local Airy compensation of topography. After the initial thickening phase, the crustal accumulation that forms serves as a model analog to a continent. Its presence leads to mantle flow patterns distinctly different from the steady-state roll that results in its absence. Large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small-scale thermal instabilities. Eventually these instabilities induce a restructuring of large-scale mantle flow, with the roll pattern being replaced by a square cell. Although preliminary and idealized, the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2-D mountain chain by the formation of the chain itself. The ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low-order despite the seemingly trivial volume of crust relative to mantle.     

深反射大炮揭示的青藏高原侧向碰撞带地壳骨架结构

青藏高原东南缘处于印度板块与欧亚板块碰撞的侧翼,揭示该地区的岩石圈结构有助于完整理解青藏高原碰撞造山的动力学过程,对构建大陆碰撞成矿理论框架至为关键.本研究对横过青藏高原侧向碰撞带的一条深反射地震剖面的15个大炮资料,进行了针对性静校正、去噪等处理和单次叠加成像,结果剖面显示了侧向碰撞带岩石圈结构的骨架特征:(1)双程走时(TWT)8～10s的强反射(Tc)将地壳分为上、下两层;Tc可能是大型滑脱构造的拆离面,其存在使上地壳的变形与下地壳解耦;(2)Moho间断面反射(Tm)为3～4个同相轴的窄带反射波组,横向不连续,与深大断裂交汇处被错断,但断距不大;(3)在兰坪—思茅地块下方TWT21s和扬子克拉通西缘下方TWT22～24s存在相向倾斜的反射波组(TL);以Tc、Tm和TL构成的骨架结构,定性地描绘出剖面下方岩石圈地幔以汇聚为主、地壳块体以侧向滑移为主和上地壳为薄皮逆冲或滑脱的分层动力学模式.该岩石圈变形样式明显不同于以正向碰撞挤压、地壳缩短垂向增厚为主的"冈底斯模式".     

Peculiarity of the Relationship between the Seismicity and Tectonic Structure of the Pyrenees

The geotectonic position of the Pyrenees mountain massif in the Alpine–Indonesian mobile belt is considered. The geological data testify to the formation of the structure of the Pyrenees in the setting of a subhorizontal compression perpendicular to the ridge. The commonly accepted interpretation considers this compression in the context of plate tectonic notions related to the collision between the Iberian and Eurasian lithospheric plates resulting from the convergence of the Eurasian and African plates. However, this interpretation is challenged by the the geodetic and seismological measurements. The GPS measurements suggest a certain cross-strike spreading rather than shortening of the Earth’s crust; the focal mechanisms of the earthquakes indicate the predominance of a subhorizontal extension perpendicular to the strike of the Pyrenees mountain range. The processes of the gravitational collapse of the mountain chain during the isostatic upwelling of the orogenic crust are considered as the most probable cause of this spreading by a number of the authors.