Mesozoic lithospheric deformation in the North China block: Numerical simulation of evolution from orogenic belt to extensional basin system

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.     

GPS results for Macedonia and its importance for the tectonics of the Southern Balkan extensional regime

GPS results from 25 stations in Macedonia measured in 1996 and 2000 show that Macedonia moves SSE relative to Eurasia essentially as a single crustal piece along with parts of westernmost Bulgaria. Geological studies show active N–S normal faults and two NNW-striking right-lateral faults in western Macedonia, and NW-trending left-lateral faults SE Macedonia, with a region in central Macedonia essentially devoid of active faults. Distribution of seismic activity supports the geological studies. However, the GPS results cannot discriminate the active faulting, except perhaps in the northern part of Macedonia in the Skopje and adjacent areas, where active ~NS extension occurs. Slip-rates on the strike-slip faults must be low, in the range of 0–2 mm/year. There is a progressive increase in GPS velocities southward in northern Greece toward the North Anatolian fault zone, across which the velocities increase and change direction dramatically.     

Geophysical profiling across the Sulu ultra-high-pressure metamorphic belt, eastern China

The largest ultra-high pressure metamorphic (UHPM) belt in the world is located along the Dabie–Sulu region, which tectonically belongs to the east part of the central orogenic belt of China. Integrated geophysical investigations of using deep seismic reflection, MT, and geothermal observations have been carried out in the Sulu area since 1997. The results of integrated interpretation suggest the existence of three features: (1) a rift beneath the Lianshui basin by the Jiashan–Xionshui fault; (2) a special crustal pattern, called the magmatic multi-arch structure occurs beneath the northern Sulu UHPM zone; and (3) a northwest-dipping regional thrust crosses the Sulu crust, representing the intracontinental subduction of the Yangtze craton beneath the Sulu metamorphic belts after collision between the Yangtze and Sino-Korean cratons. A magmatic multi-arch structure consists of some arched reflectors that occur in both the lower and the upper crust where arched reflectors coincide with granitoid plutons. The multi-arch structures are common in eastern China where many Mesozoic granitoid plutons of different scales occur. The crustal structures in the Sulu metamorphic belts resulted from intensive dynamic processes following the Triassic collision between the Yangtze and Sino-Korean cratons. The formation and exhumation of UHPM rocks followed the collision, and then intracontinental subduction of the Yangtze craton beneath the Dabie–Sulu terranes took place in the early and middle Jurassic. In the late Jurassic, the Sulu lithosphere turned to an extensional regime, large-scale granitic intrusions occurred in eastern China; these likely resulted from lithospheric thinning and asthenospheric uplifting. The granitic intrusions came to a climax during the Cretaceous and were followed by rifting along existing faults in the early Eogene, resulting in many petroleum basins. The granitoid emplacement that generated the magmatic multi-arch structure and the rift were consequences of the lithospheric thinning process, and deep intracontinental subduction of the Yangtze craton beneath the Sulu metamorphic belt might partially contribute to the lithospheric thinning.     

Mesozoic transtensional basin history of the Eastern Cordillera, Colombian Andes: Inferences from tectonic models

Backstripping analysis and forward modeling of 162 stratigraphic columns and wells of the Eastern Cordillera (EC), Llanos, and Magdalena Valley shows the Mesozoic Colombian Basin is marked by five lithosphere stretching pulses. Three stretching events are suggested during the Triassic–Jurassic, but additional biostratigraphical data are needed to identify them precisely. The spatial distribution of lithosphere stretching values suggests that small, narrow (<150 km), asymmetric graben basins were located on opposite sides of the paleo-Magdalena–La Salina fault system, which probably was active as a master transtensional or strike-slip fault system. Paleomagnetic data suggesting a significant (at least 10°) northward translation of terranes west of the Bucaramanga fault during the Early Jurassic, and the similarity between the early Mesozoic stratigraphy and tectonic setting of the Payandé terrane with the Late Permian transtensional rift of the Eastern Cordillera of Peru and Bolivia indicate that the areas were adjacent in early Mesozoic times. New geochronological, petrological, stratigraphic, and structural research is necessary to test this hypothesis, including additional paleomagnetic investigations to determine the paleolatitudinal position of the Central Cordillera and adjacent tectonic terranes during the Triassic–Jurassic. Two stretching events are suggested for the Cretaceous: Berriasian–Hauterivian (144–127 Ma) and Aptian–Albian (121–102 Ma). During the Early Cretaceous, marine facies accumulated on an extensional basin system. Shallow-marine sedimentation ended at the end of the Cretaceous due to the accretion of oceanic terranes of the Western Cordillera. In Berriasian–Hauterivian subsidence curves, isopach maps and paleomagnetic data imply a (>180 km) wide, asymmetrical, transtensional half-rift basin existed, divided by the Santander Floresta horst or high. The location of small mafic intrusions coincides with areas of thin crust (crustal stretching factors >1.4) and maximum stretching of the subcrustal lithosphere. During the Aptian–early Albian, the basin extended toward the south in the Upper Magdalena Valley. Differences between crustal and subcrustal stretching values suggest some lowermost crustal decoupling between the crust and subcrustal lithosphere or that increased thermal thinning affected the mantle lithosphere. Late Cretaceous subsidence was mainly driven by lithospheric cooling, water loading, and horizontal compressional stresses generated by collision of oceanic terranes in western Colombia. Triassic transtensional basins were narrow and increased in width during the Triassic and Jurassic. Cretaceous transtensional basins were wider than Triassic–Jurassic basins. During the Mesozoic, the strike-slip component gradually decreased at the expense of the increase of the extensional component, as suggested by paleomagnetic data and lithosphere stretching values. During the Berriasian–Hauterivian, the eastern side of the extensional basin may have developed by reactivation of an older Paleozoic rift system associated with the Guaicáramo fault system. The western side probably developed through reactivation of an earlier normal fault system developed during Triassic–Jurassic transtension. Alternatively, the eastern and western margins of the graben may have developed along older strike-slip faults, which were the boundaries of the accretion of terranes west of the Guaicáramo fault during the Late Triassic and Jurassic. The increasing width of the graben system likely was the result of progressive tensional reactivation of preexisting upper crustal weakness zones. Lateral changes in Mesozoic sediment thickness suggest the reverse or thrust faults that now define the eastern and western borders of the EC were originally normal faults with a strike-slip component that inverted during the Cenozoic Andean orogeny. Thus, the Guaicáramo, La Salina, Bitúima, Magdalena, and Boyacá originally were transtensional faults. Their oblique orientation relative to the Mesozoic magmatic arc of the Central Cordillera may be the result of oblique slip extension during the Cretaceous or inherited from the pre-Mesozoic structural grains. However, not all Mesozoic transtensional faults were inverted.     

Thin-sheet modelling of lithospheric deformation and surface mass transport

We study the effects of incorporating surface mass transport and the gravitational potential energy of both crust and lithospheric mantle to the viscous thin sheet approach. Recent 2D (cross-section) numerical models show that surface erosion and sediment transport can play a major role in shaping the large-scale deformation of the crust. In order to study these effects in 3D (planform view), we develop a numerical model in which both the dynamics of lithospheric deformation and surface processes are fully coupled. Deformation is calculated as a thin viscous layer with a vertically-averaged rheology and subjected to plane stresses. The coupled system of equations for momentum and energy conservation is solved numerically. This model accounts for the isostatic and potential-energy effects due to crustal and lithospheric thickness variations. The results show that the variations of gravitational potential energy due to the lateral changes of the lithosphere–asthenosphere boundary can modify the mode of deformation of the lithosphere. Surface processes, incorporated to the model via a diffusive transport equation, rather than just passively reacting to changes in topography, play an active role in controlling the lateral variations of the effective viscosity and hence of the deformation of the lithosphere.     

Thermal history, geodynamics, and current thermal activity of lithosphere in China

The tectonic framework of China includes major and smaller-scale units that differ in age and in style of tectonomagmatic activity, the latter being related to the thermal history of the lithosphere. Heat flow in the area varies from 25 to 150 mW/m² or higher, with an average of 58±11 mW/m². It is high in active faults, rifts, and other structures of extension (or sometimes compression) subject to heating from rising lithospheric and mantle plumes. The current thermal activity in the region is controlled by the Pacific subduction beneath Eurasia in eastern China and mainly by the lateral strain and rotation of the Ordos block associated with the India–Eurasia interaction in central and western China.     

Mantle flow-induced crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland (E Turkey) deduced from the geological and geophysical data

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.     

Late Mesozoic and Cenozoic rifting and its dynamic setting in Eastern China and adjacent areas

During the Late Mesozoic and Cenozoic, extension was widespread in Eastern China and adjacent areas. The first rifting stage spanned in the Late Jurassic–Early Cretaceous times and covered an area of more than 2 million km² of NE Asia from the Lake Baikal to the Sikhot-Alin in EW direction and from the Mongol–Okhotsk fold belt to North China in NS direction. This rifting was characterized by intracontinental rifts, volcanic eruptions and transform extension along large-scale strike–slip faults. Based on the magmatic activity, filling sequence of basins, tectonic framework and subsidence analysis of basins, the evolution of this area can be divided into three main developmental phases. The first phase, calc-alkaline volcanics erupted intensely along NNE-trending faults, forming Daxing'anling volcanic belt, NE China. The second phase, Basin and Range type fault basin system bearing coal and oil developed in NE Asia. During the third phase, which was marked by the change from synrifting to thermal subsidence, very thick postrift deposits developed in the Songliao basin (the largest oil basin in NE China).Following uplift and denudation, caused by compressional tectonism in the near end of Cretaceous, a Paleogene rifting stage produced widespread continental rift systems and continental margin basins in Eastern China. These rifted basins were usually filled with several kilometers of alluvial and lacustrine deposits and contain a large amount of fossil fuel resources. Integrated research in most of these rifting basins has shown that the basins are characterized by rapid subsidence, relative high paleo-geothermal history and thinned crust. It is now accepted that the formation of most of these basins was related to a lithospheric extensional regime or dextral transtensional regime. During Neogene time, early Tertiary basins in Eastern China entered a postrifting phase, forming regional downwarping. Basin fills formed in a thermal subsidence period onlapped the fault basin margins and were deposited in a broad downwarped lacustrine depression. At the same time, within plate rifting of the Lake Baikal and Shanxi graben climaxed and spreading of the Japan Sea and South China Sea occurred. Quaternary rifting was marked by basalt eruption and accelerated subsidence in the area of Tertiary rifting. The Okinawa Trough is an active rift involving back-arc extension.Continental rifting and marginal sea opening were clearly developed in various kind of tectonic settings. Three rifting styles, intracontinental rifting within fold belt, intracontinental rifting within craton and continental marginal rifting and spreading, are distinguished on the basis of nature of the basin basement, tectonic location of rifting and relations to large strike–slip faults.Changes of convergence rates of India–Eurasia and Pacific–Eurasia may have caused NW–SE-trending extensional stress field dominating the rifting. Asthenospheric upwelling may have well assisted the rifting process. In this paper, a combination model of interactions between plates and deep process of lithosphere has been proposed to explain the rifting process in East China and adjacent areas.The research on the Late Mesozoic and Cenozoic extensional tectonics of East China and adjacent areas is important because of its utility as an indicator of the dynamic setting and deformational mechanisms involved in stretching Lithosphere. The research also benefits the exploration and development of mineral and energy resources in this area.     

应用有限单元法恢复冲绳海槽和付加体古构造应力场及其意义

冲绳海槽是欧亚板块内大陆地壳的扩张作用所形成的一个弧后盆地。以弹性学原理和库仑—莫尔准则为基础,应用二维(2D)有限单元法模拟大陆岩石圈上部的构造应力场和断层发育状况。模型中使用两种位移边界条件,并采用平面应变条件进行计算。扩张位移边界条件加载于冲绳海槽内大陆地壳的底部,俯冲位移边界条件加载于菲律宾海板块的俯冲带。数学模拟的结果表明,大陆地壳底部的扩张作用使得冲绳海槽内发育正断层,最后导致冲绳海槽这一弧后盆地的形成。菲律宾海板块的俯冲作用引起欧亚板块付加体中形成逆掩断层。     

Lower crust indentation or horizontal ductile flow during continental collision?

Conditions for indentation and channelised flow are investigated with two-dimensional thermomechanical models of Alpine-type continental collision. The models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide, coherent with several geological reconstructions. We study in particular the role of lower crustal strength in developing peculiar geometries after 20 Ma of shortening at 1 cm/year. Crustal layers produce geometries of imbricate layers, which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid-like indentation:

– Channelised lateral flow develops when the lateral lower crust has a viscosity less than 10²¹ Pa s, exhibiting velocities opposite to the direction of convergence. This mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle. It is also associated with a topographic plateau that equilibrates with a wide (about 200 km) but quasi-constant crustal root about 50 km deep.

– In contrast, indentation occurs with lateral lower crust layers that have a viscosity greater than about 10²³ Pa s, producing significant shortening and thickening of the central crust. In this case topography develops steep and narrow (around 100 km wide), associated with a thickened crust exceeding 60 km depth. A crustal-scale pop-up forms bounded by subvertical shear zones that root into the mantle lithosphere.

Crustal Motion Characteristics in the Eastern Margin of the Tibetan Plateau and Adjacent Regions after the Wenchuan Earthquake

The Wenchuan earthquake has altered the crustal motion characteristics in the eastern margin of the Tibetan Plateau and adjacent regions.Using discontinuous GPS survey data for 2008–2012, the velocity field for the Eurasia reference framework has been obtained, and the general trend of contemporary crustal motion after the occurrence of the Wenchuan earthquake has been studied.In addition, using the velocity field, the block movement velocity has been estimated by least-squares fitting.Furthermore, the properties and displacement rates of main faults have been obtained from the differences in velocity vectors of the blocks on both sides of the faults.The results reveal that there are no obvious changes in the general characteristics of crustal motion in this area after the Wenchuan earthquake.The earthquake mainly changed the rate of the movement of the Chuan-Qing block and caused variation in the movement direction of the South China block.The effect of the earthquake on faults is mainly reflected in variations in fault displacement velocity; there is no fundamental change in the properties of fault activity.The displacement rates of the Xianshuihe fault decreased by 3–4 mm/a, the Longmenshan fault increased by 9–10 mm/a, and the northern segment of the Anninghe fault increased by approximately 9 mm/a.Furthermore, the displacement rates of the Minjiang, Xueshan, Huya, Longquanshan, and Xinjin faults increased by 2–3 mm/a.This implies that the effects of the Wenchuan earthquake on crustal movement can mainly be observed in the Chuan-Qing, South China, and N-Chuan-Dian blocks and their internal faults, as well as the Xianshuihe and Longmenshan faults and the northern section of the Anninghe fault.The reason for this is that the Wenchuan earthquake disturbed the kinematic and dynamic balance in the region.     

Plume head–lithosphere interactions near intra-continental plate boundaries

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.     

Neotectonic deformation in the transition zone between the Dead Sea Transform and the East Anatolian Fault Zone, Southern Turkey: a palaeomagnetic study of the Karasu Rift Volcanism

In southern Turkey ongoing differential impingement of Arabia into the weak Anatolian collisional collage resulting from subduction of the Neotethyan Ocean has produced one of the most complex crustal interactions along the Alpine–Himalayan Orogen. Several major transforms with disputed motions, including the northward extension of the Dead Sea Fault Zone (DSFZ), meet in this region. To evaluate neotectonic motion on the Amanos and East Hatay fault zones considered to be northward extensions of the DSFZ, the palaeomagnetism of volcanic fields in the Karasu Rift between these faults has been studied. Remanence carriers are low-Ti magnetites and all except 5 of 51 basalt lavas have normal polarity. Morphological, polarity and K–Ar evidence show that rift formation occurred largely during the Brunhes chron with volcanism concentrated at 0.66–0.35 Ma and a subsidiary episode at 0.25–0.05. Forty-four units of normal polarity yield a mean of D/I=8.8°/54.7° with inclination identical to the present-day field and declination rotated clockwise by 8.8±4.0°. Within the 15-km-wide Hassa sector of the Karasu Rift, the volcanic activity is concentrated between the Amanos and East Hatay faults, both with left lateral motions, which have rotated blocks bounded by NW–SE cross faults in a clockwise sense as the Arabian Block has moved northwestwards. An average lava age of 0.5 Ma yields a minimum cumulative slip rate on the system bounding faults of 0.46 cm/year according with the rate deduced from the Africa–Arabia Euler vector and reduced rates of slip on the southern extension of the DSFZ during Plio-Quaternary times. Estimates deduced from offsets of dated lavas flows and morphological features on the Amanos Fault Zone [Tectonophysics 344 (2002) 207] are lower (0.09–0.18 cm/year) probably because they are limited to surface fault breaks and do not embrace the seismogenic crust.Results of this study suggest that most strike slip on the DSFZ is taken up by the Amanos–East Hatay–Afrin fault array in southern Turkey. Comparable estimates of Quaternary slip rate are identified on other faults meeting at an unstable FFF junction (DSFZ, East Anatolian Fault Zone, Karatas Fault Zone). A deceleration in slip rate across the DSFZ and its northward continuation during Plio-Quaternary times correlates with reorganization of the tectonic regime during the last 1–3 Ma including tectonic escape within Anatolia, establishment of the North and East Anatolian Fault Zones bounding the Anatolian collage in mid–late Pliocene times, a contemporaneous transition from transpression to transtension and concentration of all basaltic magmatism in this region within the last 1 Ma.     

中国大陆岩石圈等效粘滞系数的计算和讨论

大陆岩石圈的流变结构对岩石圈动力学过程有很大的影响,因此对岩石圈等效粘度的估计是大陆动力学研究中基础和重要的问题。文中对利用实验室流变实验结果估算岩石圈流变结构的计算方法中包含的多种不确定性进行了讨论,包括岩性、温度、应变速率、实验室速率数据外推到地质构造运动速率等因素对等效粘滞系数估算的影响,并以温度和应变速率的新研究成果为基础,对中国大陆地壳和上地幔等效粘滞系数做出了估计。中国中地壳等效粘滞系数一般在1021~1024Pa.s,下地壳等效粘滞系数在1021~1022Pa.s,其中青藏高原下地壳等效粘滞系数较低,约为1019~1020Pa.s;与前人研究认为青藏高原存在柔性下地壳流动的结论吻合。     

Stress and strain during fault-controlled lithospheric extension—insights from numerical experiments

Two-dimensional finite element techniques are used to study the temporal evolution and spatial distribution of stress and strain during lithospheric extension. The thermomechanical model includes a pre-existing fault in the upper crust to account for the reactivation of older tectonic elements. The fault is described using contact elements which allow for independent meshing of hanging wall and foot wall as well as simulation of large differential displacements between the fault blocks. Numerical models are run for three different initial temperature distributions representing extension of weak, moderately strong and strong lithosphere and three different extension velocities. In spite of the simple geodynamic boundary conditions selected, i.e., wholesale extension at a constant rate, stress and strain vary substantially throughout the lithosphere. In particular, in case of the weak lithosphere model, lower crustal flow towards the locus of maximum upper crustal extension results in the formation of a lower crustal dome while maintaining a subhorizontal Moho relief. The core of the dome experiences hardly any internal deformation, although it is the part of the lower crust which is exhumed the most. Stress fields in the lower crustal dome vary significantly from the regional trend underlining mechanical decoupling of the lower crust from the rest of the lithosphere. These differences diminish if cooler temperatures and, hence, stronger rheologies are considered. Lithospheric strength also exerts a profound control on the basin architecture and the surface expressions of extension, i.e., rift flank uplift and basin subsidence. If the lower crust is sufficiently weak, its flow towards the region of extended upper crust can provide a threshold value for the maximum subsidence which can be achieved during the syn-rift stage. In spite of continuous regional extension, corresponding burial history plots show exponentially decreasing subsidence rates which would traditionally be interpreted in terms of lithospheric cooling during the post-rift stage. The models provide templates to genetically link the surface and sub-surface expressions of lithospheric extension, for which usually no contemporaneous observations are possible. In particular, they help to decipher the information on the physical state of the lithosphere at the time of extension which is stored in the architecture and subsidence record of sedimentary basins.     

郯庐断裂带的岩石圈结构及其成因分析

横穿郯庐断裂带的五条地学断面揭示,断裂带两侧地壳结构明显不同,这是平移运动造成不同块体拼合的结果。早白垩世走滑期的岩浆活动,指示当时断裂带切入了壳－幔边界。这表明断裂带在走滑中切穿了整个地壳,莫霍面当时应为平缓的大型拆离面,壳－幔之间发生了显著的失耦。断裂带在晚白垩世－早第三纪的伸展活动中,软流圈进行了强烈的上隆,岩石圈出现了显著的细颈化,属于纯剪切伸展模式。在晚第三纪以来的挤压活动中,浅埋软流圈背景上较高的上地幔温度,使郯庐断裂带成为岩石圈薄弱带,从而发生了较强的逆冲活动和大规模幔源玄武岩浆的喷发。     

A tectonic model for the Tertiary evolution of strike–slip faults and rift basins in SE Asia

Models for the Tertiary evolution of SE Asia fall into two main types: a pure escape tectonics model with no proto-South China Sea, and subduction of proto-South China Sea oceanic crust beneath Borneo. A related problem is which, if any, of the main strike–slip faults (Mae Ping, Three Pagodas and Aliao Shan–Red River (ASRR)) cross Sundaland to the NW Borneo margin to facilitate continental extrusion? Recent results investigating strike–slip faults, rift basins, and metamorphic core complexes are reviewed and a revised tectonic model for SE Asia proposed. Key points of the new model include: (1) The ASRR shear zone was mainly active in the Eocene–Oligocene in order to link with extension in the South China Sea. The ASRR was less active during the Miocene (tens of kilometres of sinistral displacement), with minor amounts of South China Sea spreading centre extension transferred to the ASRR shear zone. (2) At least three important regions of metamorphic core complex development affected Indochina from the Oligocene–Miocene (Mogok gneiss belt; Doi Inthanon and Doi Suthep; around the ASRR shear zone). Hence, Paleogene crustal thickening, buoyancy-driven crustal collapse, and lower crustal flow are important elements of the Tertiary evolution of Indochina. (3) Subduction of a proto-South China Sea oceanic crust during the Eocene–Early Miocene is necessary to explain the geological evolution of NW Borneo and must be built into any model for the region. (4) The Eocene–Oligocene collision of NE India with Burma activated extrusion tectonics along the Three Pagodas, Mae Ping, Ranong and Klong Marui faults and right lateral motion along the Sumatran subduction zone. (5) The only strike–slip fault link to the NW Borneo margin occurred along the trend of the ASRR fault system, which passes along strike into a right lateral transform system including the Baram line.     

中国典型大型走滑断裂及相关盆地成因研究

扭动走滑构造是最常见的构造样式之一,走滑拉分盆地也是重要的含油气盆地类型。大地构造运动本质是岩石圈在区域应力场作用下的变形过程,然而真正将走滑断裂和走滑拉分盆地的成因以及它们的特征和岩石圈性质联系起来研究的文献却很少。显然,区域应力场是形成构造运动的前提条件,是外因,决定了运动的基本方式,如走滑、拉张或挤压等(构造类型);而岩石圈是构造运动的主体,是内因,其性质决定了形成构造的规模和具体形态(样式)。中国境内发育的大量走滑断裂体系和走滑盆地展示:古老克拉通上发育大面积分布的多条小位移走滑断裂体系,如塔里木盆地古生代走滑断裂体系;被后期热活动破坏了的克拉通发育多条扭动断裂系,例如华北克拉通东部的郯庐断裂系、兰聊—盐山断裂系、太行山东麓断裂系,并和区域拉张应力场耦合形成雁列式断陷群(如渤海湾盆地);相对较弱的古生代基底岩石圈发育大型单一走滑断层,如郯庐断裂东北段,并沿断裂发育一些相互独立的走滑拉分盆地;而在固结较差的中新生代造山带往往形成一条平直的大型走滑断层,例如阿尔金走滑断裂、海原断裂等。本文内因外因相结合,从扭动应力场和岩石圈强度以及流变学特征,建立了不同岩石圈性质下下部韧性层和顶部(上地壳或上地壳上部)刚性层之间的耦合作用机制以及扭动构造形成和演化模式,较好地解释了中国陆内发育的典型走滑断裂和走滑拉分盆地的成因机制。     

Structure of the crust and uppermost mantle of the Yanshan Belt and adjacent regions at the northeastern boundary of the North China Craton from Rayleigh Wave Dispersion Analysis

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.     

Palaeozoic and Mesozoic tectonic evolution and palaeogeography of East Asian crustal fragments: The Korean Peninsula in context

East and Southeast Asia comprises a complex assembly of allochthonous continental lithospheric crustal fragments (terranes) together with volcanic arcs, and other terranes of oceanic and accretionary complex origins located at the zone of convergence between the Eurasian, Indo-Australian and Pacific Plates. The former wide separation of Asian terranes is indicated by contrasting faunas and floras developed on adjacent terranes due to their prior geographic separation, different palaeoclimates, and biogeographic isolation. The boundaries between Asian terranes are marked by major geological discontinuities (suture zones) that represent former ocean basins that once separated them. In some cases, the ocean basins have been completely destroyed, and terrane boundaries are marked by major fault zones. In other cases, remnants of the ocean basins and of subduction/accretion complexes remain and provide valuable information on the tectonic history of the terranes, the oceans that once separated them, and timings of amalgamation and accretion. The various allochthonous crustal fragments of East Asia have been brought into close juxtaposition by geological convergent plate tectonic processes. The Gondwana-derived East Asia crustal fragments successively rifted and separated from the margin of eastern Gondwana as three elongate continental slivers in the Devonian, Early Permian and Late Triassic–Late Jurassic. As these three continental slivers separated from Gondwana, three successive ocean basins, the Palaeo-Tethys,. Meso-Tethys and Ceno-Tethys, opened between these and Gondwana. Asian terranes progressively sutured to one another during the Palaeozoic to Cenozoic. South China and Indochina probably amalgamated in the Early Carboniferous but alternative scenarios with collision in the Permo–Triassic have been suggested. The Tarim terrane accreted to Eurasia in the Early Permian. The Sibumasu and Qiangtang terranes collided and sutured with Simao/Indochina/East Malaya in the Early–Middle Triassic and the West Sumatra terrane was transported westwards to a position outboard of Sibumasu during this collisional process. The Permo–Triassic also saw the progressive collision between South and North China (with possible extension of this collision being recognised in the Korean Peninsula) culminating in the Late Triassic. North China did not finally weld to Asia until the Late Jurassic. The Lhasa and West Burma terranes accreted to Eurasia in the Late Jurassic–Early Cretaceous and proto East and Southeast Asia had formed. Palaeogeographic reconstructions illustrating the evolution and assembly of Asian crustal fragments during the Phanerozoic are presented.