The Siwaliks of western Nepal: I. Geometry and kinematics

The Siwalik Group which forms the southern zone of the Himalayan orogen, constitutes the deformed part of the Neogene foreland basin situated above the downflexed Indian lithosphere. It forms the outer part of the thin-skinned thrust belt of the Himalaya, a belt where the faults branch off a major décollement (MD) that is the external part of the basal detachment of Himalayan thrust belt. This décollement is located beneath 13 Ma sediments in far-western Nepal, and beneath 14.6 Ma sediments in mid-western Nepal, i.e., above the base of the Siwalik Group. Unconformities have been observed in the upper Siwalik member of western Nepal both on satellite images and in the field, and suggest that tectonics has affected the frontal part of the outer belt since more than 1.8 Ma. Several north dipping thrusts delineate tectonic boundaries in the Siwalik Group of western Nepal. The Main Dun Thrust (MDT) is formed by a succession of 4 laterally relayed thrusts, and the Main Frontal Thrust (MFT) is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones. One of the major transfer zones is the West Dang Transfer Zone (WDTZ), which has a north-northeast strike and is formed by strike-slip faults, sigmoid folds and sigmoid reverse faults. The width of the outer belt of the Himalaya varies from 25 km west of the WDTZ to 40 km east of the WDTZ. The WDTZ is probably related to an underlying fault that induces: (a) a change of the stratigraphic thickness of the Siwalik members involved in the thin-skinned thrust belt, and particularly of the middle Siwalik member; (b) an increase, from west to east, of the depth of the décollement level; and (c) a lateral ramp that transfers displacement from one thrust to another. Large wedge-top basins (Duns) of western Nepal have developed east of the WDTZ. The superposition of two décollement levels in the lower Siwalik member is clear in a large portion of the Siwalik group of western Nepal where it induces duplexes development. The duplexes are formed either by far-travelled horses that crop out at the hangingwall of the Internal Décollement Thrust (ID) to the south of the Main Boundary Thrust, or by horses that remain hidden below the middle Siwaliks or Lesser Himalayan rocks. Most of the thrusts sheets of the outer belt of western Nepal have moved toward the S–SW and balanced cross-sections show at least 40 km shortening through the outer belt. This value probably under-estimates the shortening because erosion has removed the hangingwall cut-off of the Siwalik series. The mean shortening rate has been 17 mm/yr in the outer belt for the last 2.3 Ma.     

Depositional environment and provenance of Middle Siwalik sediments in Tista valley,Darjiling District,Eastern Himalaya,India

The frontal part of the active, wedge-shaped Indo-Eurasian collision boundary is defined by the Himalayan fold-and-thrust belt whose foreland basin accumulated sediments that eventually became part of the thrust belt and is presently exposed as the sedimentary rocks of the Siwalik Group. The rocks of the Siwalik Group have been extensively studied in the western and Nepal Himalaya and have been divided into the Lower, Middle and Upper Subgroups. In the Darjiling–Sikkim Himalaya, the Upper Siwalik sequence is not exposed and the Middle Siwalik Subgroup exposed in the Tista river valley of Darjiling Himalaya preserves a ~325 m thick sequence of sandstone, conglomerate and shale. The Middle Siwalik section has been repeated by a number of north dipping thrusts. The sedimentary facies and facies associations within the lithostratigraphic column of the Middle Siwalik rocks show temporal repetition of sedimentary facies associations suggesting oscillation between proximal-, mid- and distal fan setups within a palaeo-alluvial fan depositional environment similar to the depositional setup of the Siwalik sediments in other parts of the Himalaya. These oscillations are probably due to a combination of foreland-ward movement of Himalayan thrusts, climatic variations and mountain-ward shift of fan-apex due to erosion. The Middle Siwalik sediments were derived from Higher- and Lesser Himalayan rocks. Mineral characteristics and modal analysis suggest that sedimentation occurred in humid climatic conditions similar to the moist humid climate of the present day Eastern Himalaya.     

冈底斯-喜马拉雅碰撞造山带前陆盆地系统及构造演化

碰撞带前陆盆地的建立是大陆碰撞的直接标志和随后造山带构造变形的忠实记录。本文对欧亚板块与印度板块碰撞前后发育在拉萨地块上的冈底斯弧背前陆盆地,同碰撞产生的雅鲁藏布江周缘前陆盆地,以及碰撞后陆内变形产生的喜马拉雅前陆盆地的沉积地层演化以及碎屑锆石物源特征等进行了系统分析,结合前人及我们近些年的研究成果,认为冈底斯岛弧北侧发育一个典型的弧背前陆盆地系统而不是以前普遍接受的伸展盆地。除传统认为的喜马拉雅前陆盆地系统外,在碰撞造山带中还发育一个雅鲁藏布江前陆盆地系统,它是欧亚板块与印度板块碰撞以后,欧亚板块加载到印度被动大陆边缘产生的典型周缘前陆盆地。上述2个造山带前陆盆地系统的识别,大大提高了对新特提斯洋俯冲、碰撞过程的认识。造山带前陆盆地证据指示,新特提斯洋至少于140 Ma以前就已开始俯冲, 110 Ma俯冲速度开始提高,在65 Ma前后印度大陆与欧亚大陆发生碰撞,喜马拉雅山于40 Ma开始隆升,其剥蚀物质大量堆积在喜马拉雅前陆盆地中。     

中国中西部前陆盆地多期成藏、晚期聚气的成藏特征

中国中西部前陆盆地具有优越的天然气成藏条件,同时具有晚期成藏的特征。文中在典型气藏解剖的基础上重点讨论了川西、柴北缘和准南缘前陆盆地的成藏过程,从而指出中国中西部前陆盆地具有多期聚集、晚期聚气的成藏特征,明确指出中国中西部前陆盆地具有最主要的两大成藏期,一是燕山晚期,主要是被动陆缘或中部前陆盆地三叠系烃源岩的油气聚集期;二是喜山晚期,受新构造运动影响,主要是西部陆内前陆盆地的中、新生界烃源岩的天然气成藏期和中部周缘前陆盆地的天然气的调整期。新近纪前陆盆地的发育期控制了中国中西部前陆盆地以中生界煤系烃源岩为主的晚期天然气的聚集。     

Tectonically-driven underfilled–overfilled cycles,the middle Cretaceous in the northern Cordilleran foreland basin

It is shown that the middle Cretaceous succession in the northern Cordilleran foreland basin consists of several-million-year tectonically-driven cycles comprising two components: strata deposited in an underfilled basin with a prominent forebulge zone and strata deposited in an overfilled basin lacking evidence of a forebulge. The episodic thrusting of the Cordilleran orogenic wedge and its rich sediment supply to the basin are two main controlling factors for the formation of these cycles. A qualitative model of several-million-year tectonically-driven underfilled–overfilled cycle for migration and stratigraphic fill in this basin is proposed. During the early underfilled period (orogenic loading period), due to orogenic loading of emplaced thrust sheets, flexural subsidence is created in the region proximal to the mountain belt and a prominent forebulge is developed. During the late underfilled period (early orogenic unloading period), as the cratonward migration of the subsidence center of sediment loading in the foredeep zone, forebulge zones and backbulge zones migrate cratonwards, forming a diachronic erosion surface in the central basin. During the overfilled period (late orogenic unloading period), a prominent erosion forms in the proximal basin and a peripheral sag develops above the forebulge area of the previous underfilled period. This model may provide a pattern to subdivide sedimentary successions in the Cordilleran foreland basin. Using this model, alternative interpretations are suggested for some important, but controversial stratigraphic phenomena in the Cretaceous Cordilleran foreland basin: traditionally defined eustatic highstands, wide sedimentation area of the basin, erosion surfaces and widespread subtle topographic uplifts in the central basin, high-frequency coarsening-up cycles, extensively distributed erosive-based sandstones and conglomerates enclosed in marine mudstones.     

Sedimentary facies and soft-sediment deformation structures in the late miocene-pliocene Middle Siwalik subgroup,eastern Himalaya,Darjiling District,India

The Himalayan fold-and-thrust belt has propagated from its Tibetan hinterland to the southern foreland since ∼55 Ma. The Siwalik sediments (∼20 - 2 Ma) were deposited in the frontal Himalayan foreland basin and subsequently became part of the thrust belt since ∼ 12 Ma. Restoration of the deformed section of the Middle Siwalik sequence reveals that the sequence is ∼325 m thick. Sedimentary facies analysis of the Middle Siwalik rocks points to the deposition of the Middle Siwalik sediments in an alluvial fan setup that was affected by uplift and foreland-ward propagation of Greater and Lesser Himalayan thrusts. Soft-sediment deformation structures preserved in the Middle Siwalik sequence in the Darjiling Himalaya are interpreted to have formed by sediment liquefaction resulting from increased pore-water pressure probably due to strong seismic shaking. Soft-sediment structures such as convolute lamination, flame structures, and various kinds of deformed cross-stratification are thus recognized as palaeoseismic in origin. This is the first report of seismites from the Siwalik succession of Darjiling Himalaya which indicates just like other sectors of Siwalik foreland basin and the present-day Gangetic foreland basin that the Siwalik sediments of this sector responded to seismicity.     

Structures and morphotectonic evolution of the frontal fold–thrust belt,Kameng river section,Arunachal Himalaya,India

The Neogene–Quaternary Siwalik foreland fold and thrust belt is studied for better understanding of tectonics along the Kameng river section of Arunachal Pradesh, India. The Kimi, Dafla, Subansiri, and the Kimin Formation correspond to Lower, Middle and Upper Siwaliks, respectively. The lithology in the foreland basin is dominantly sandstones, siltstones, claystones, carbonaceous shales, and boulder beds in the upper part. The structural style of the sedimentary sequence from the Main Boundary Thrust southward shows first order ramp-flat geometry. The brittle shear transfers slip across glide horizons to shallower depth. Repeated splay generations from a major regional-scale floor transfers slip from one glide horizon to another that shortens and thickens the crust. In the micro-scale, the lithological response in the structural development is well documented as pressure solution seams and other diagenetic deformation signatures. The basement asperity plays a significant role as the moving thrust front produced a major lateral ramp. The differential movement of the mountain front on both sides of the ramp is decipherable. This is especially true at the western part of the SE flowing Kameng river. The tectonic evolution of the area initiated with slip along the MBT \(\sim \)11 Ma ago along with the deposition of the Siwalik sediments. With southward propagation of the mountain front, the foreland basin shifted towards S, produced splay thrusts from the Himalayan Frontal Thrust-1 (HFT-1), which has been uplifting the Kimin and the older terraces.     

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS

THRUST PACKAGES OF 1.68 Ga INDIAN SUPRA-CRUSTAL ROCKS IN THE MIOCENE SIWALIK BELT,CENTRAL NEPAL HIMALAYAS     

Oil and Gas Accumulation in the Foreland Basins, Central and Western China

<正>Foreland basin represents one of the most important hydrocarbon habitats in central and western China.To distinguish these foreland basins regionally,and according to the need of petroleum exploration and favorable exploration areas,the foreland basins in central and western China can be divided into three structural types:superimposed,retrogressive and reformative foreland basin(or thrust belt),each with distinctive petroleum system characteristics in their petroleum system components(such as the source rock,reservoir rock,caprock,time of oil and gas accumulation,the remolding of oil/gas reservoir after accumulation,and the favorable exploration area,etc.).The superimposed type foreland basins,as exemplified by the Kuqa Depression of the Tarim Basin, characterized by two stages of early and late foreland basin development,typically contain at least two hydrocarbon source beds,one deposited in the early foreland development and another in the later fault-trough lake stage.Hydrocarbon accumulations in this type of foreland basin often occur in multiple stages of the basin development,though most of the highly productive pools were formed during the late stage of hydrocarbon migration and entrapment(Himalayan period).This is in sharp contrast to the retrogressive foreland basins(only developing foreland basin during the Permian to Triassic) such as the western Sichuan Basin,where prolific hydrocarbon source rocks are associated with sediments deposited during the early stages of the foreland basin development.As a result, hydrocarbon accumulations in retrogressive foreland basins occur mainly in the early stage of basin evolution.The reformative foreland basins(only developing foreland basin during the Himalayan period) such as the northern Qaidam Basin,in contrast,contain organic-rich,lacustrine so urce rocks deposited only in fault-trough lake basins occurring prior to the reformative foreland development during the late Cenozoic,with hydrocarbon accumulations taking place relatively late(Himalayan period).Therefore,the ultimate hydrocarbon potentials in the three types of foreland basins are largely determined by the extent of spatial and temporal matching among the thrust belts,hydrocarbon source kitchens,and regional and local caprocks.     

楚雄前陆盆地的构造特征与沉积演化

对楚雄前陆盆地西部及相邻构造格架进行了分析 ,认为原形盆地的西南部及相邻造山带受到了新生代红河断裂的强烈改造 ,而盆地西北部的盆 -山体系保留了盆地演化时期的基本格架。利用当代前陆盆地演化模式理论 ,对原形盆地的沉积特征及其与西部推覆构造的耦合关系进行了综合分析 ,认为扬子西部被动陆缘向楚雄前陆盆地转化的时期发生在晚三叠世卡尼早期 ,前陆盆地经历了 3个发展阶段 :卡尼期深水复理石沉积、诺利期浅水复理石沉积和瑞替期磨拉石沉积 ,并提出了相应的沉积 -构造演化模式。     

中国前陆盆地构造地质特征综述与油气勘探

中国特定环境下发育的前陆盆地和冲断带在地质构造上具有许多的特殊性。在综述前人认识的基础上,笔者通过对中国前陆盆地的构造演化历程、沉积充填特征、构造成因及其空间分布规律、构造变形特征等的研究,提出了中国前陆盆地构造地质发育的5个主要特征:(1)两种不同性质的原型盆地发生正反转的叠合性,即挤压构造下作为“本体”的前陆层序与拉张构造下作为“基础”的裂谷、断陷盆地之间的叠置;(2)显生宙以来中国大陆先后发生了4期前陆冲断构造演化的多期性,它们分别是加里东晚期、海西晚期、印支期和喜马拉雅晚期;(3)基于小克拉通基底拼贴后在造山带前缘复活再生的继承性,即统一拼合大陆内部的构造变形导致古造山带的复活,在古造山带边缘发育新生代前陆盆地或前陆冲断带;(4)在空间分布上受环青藏高原巨型盆山体系控制发生陆内变形的系统性,在环青藏高原巨型盆山体系内构造变形强度、盆地沉降幅度、盆山耦合程度等从内环向外环依次降低,从古造山带向克拉通方向构造变形强度依次降低,构造变形样式逐渐简单、构造变形时间依次变新;(5)前陆冲断带的构造样式由于受边界力学条件和沉积地层介质作用而具有多变性,存在沉积盖层内脆性变形的断层相关褶皱、造山带前缘韧性变形的基底卷入构造、与走滑构造相伴生的基底卷入的断层相关褶皱、盆地内部塑性变形的盐构造。正是因为上述地质构造的特殊性,决定了油气聚集与分布特征的规律性和复杂性,由此提出了相应的一些构造地质研究与油气勘探工作的建议。     

Simulation of the Subsidence and Deposition Processes of the Foreland Basin in the Southwestern Margin of Ordos

Characteristics of deformation in the thrust belt and sequence stratigraphic framework in the foreland basin, structural features of the basin margin, and the episodic thrusting are studied in this paper by combining the eastern Qilian thrust belt and the Late Triassic foreland basin on the southwestern margin of Ordos. On this basis, a geological model and a mechanical model of coupling mechanism were established for the pair of thrust belt and foreland basin, and the subsidence and deposition process of the foreland basin were simulated on given parameters.     

前陆盆地沉降机理和地层模型

前陆盆地形成的主要原因是造山带负载导致的岩石圈挠曲。逆冲作用造成地壳增厚,造山带的巨大质量又导致下部岩石圈的区域均衡沉降,从而临近和平行于造山带发育了凹陷。另外,前陆盆地的演化也受到沉积物供应、盆地内沉积物扩散能力、岩石圈强度、造山带逆冲速率、全球海平面变化、和俯冲有关的动力沉降及俯冲负载等众多其它因素的影响。本文阐述了这些因素与前陆盆地沉降的关系,介绍了与幕式逆冲有关的地层模型和欠补偿-过补偿地层模型。希望本文能够对中国西北地区前陆盆地的研究起到一定的借鉴意义。     

Active tectonics of Himalayan Frontal Fault system

In the Sub-Himalayan zone, the frontal Siwalik range abuts against the alluvial plain with an abrupt physiographic break along the Himalayan Frontal Thrust (HFT), defining the present-day tectonic boundary between the Indian plate and the Himalayan orogenic prism. The frontal Siwalik range is characterized by large active anticline structures, which were developed as fault propagation and fault-bend folds in the hanging wall of the HFT. Fault scarps showing surface ruptures and offsets observed in excavated trenches indicate that the HFT is active. South of the HFT, the piedmont zone shows incipient growth of structures, drainage modification, and 2–3 geomorphic depositional surfaces. In the hinterland between the HFT and the MBT, reactivation and out-of-sequence faulting displace Late Quaternary–Holocene sediments. Geodetic measurements across the Himalaya indicate a ~100-km-wide zone, underlain by the Main Himalayan Thrust (MHT), between the HFT and the main microseismicity belt to north is locked. The bulk of shortening, 15–20 mm/year, is consumed aseismically at mid-crustal depth through ductile by creep. Assuming the wedge model, reactivation of the hinterland faults may represent deformation prior to wedge attaining critical taper. The earthquake surface ruptures, ≥240 km in length, interpreted on the Himalayan mountain front through paleoseismology imply reactivation of the HFT and may suggest foreland propagation of the thrust belt.     

尼泊尔及南侧邻区元古宙以来的构造-沉积演化

迄今,尼泊尔及其南侧邻区元古宙以来的构造-沉积演化尚缺乏系统性研究.为了促进区域地质认识,结合前人研究成果及新的研究发现,对尼泊尔低喜马拉雅带及以南的构造-沉积演化首次进行系统性总结与讨论.结果表明:尼泊尔低喜马拉雅带及以南与印度地盾北缘在地质历史中的构造-沉积演化息息相关,且自元古宙以来,发育了被动大陆边缘→陆内裂谷→被动大陆边缘→前陆盆地等不同构造演化阶段的沉积响应;尼泊尔西部的Dailekh群属于~1.8 Ga以前或前哥伦比亚超大陆之前的被动大陆边缘沉积;Vindhyan超群为下断上坳的陆内裂谷沉积,尼泊尔境内的Lakharpata群相当于下Vindhyan群;Gondwana超大陆裂解导致由北往南形成一系列初始发育时间越来越晚的裂谷盆地;Surkhet群至Siwalik群为被动大陆边缘至前陆盆地沉积,其中,Surkhet群Swat/Subathu组是喜马拉雅南侧地质历史上最后一套海相沉积地层,也是被动大陆边缘向前陆盆地转换期的沉积响应;Siwalik群大规模的磨拉石建造标志着喜马拉雅快速和大幅度隆升,该群沉积成岩后,印度-欧亚板块进一步的挤压作用导致了地质历史上迄今为止最后一次强烈的构造运动,形成MFT与Siwalik褶皱带,并奠定了喜马拉雅带现今构造格局.      

楚雄中生代前陆盆地的构造沉降史研究

云南楚雄盆地位于场子陆块的西南边缘,为一典型的中生代周缘前陆盆地,盆地演化阶段明显,晚三叠世为前陆早期复理石沉积,侏罗纪则为前陆晚期磨拉石沉积。对盆地构造沉降史研究后笔者认为：①晚三叠世复理石沉积盆地构造沉降幅度巨大,沉降与沉积中心位于盆地最西部,紧邻古哀牢山造山带,沉积体呈形楔形展布;③侏罗纪磨拉石沉积盆地构造沉降和沉积中心以及前缘隆起向内陆方向迁移明显;③中生代构造快速沉降的沉积体的楔形展布表     

环青藏高原巨型盆山体系构造与塔里木盆地油气分布规律

中国中西部受控于喜山期青藏高原的隆升和向北、向东的推挤,在其外围形成一个巨型的盆山构造体系,环青藏高原巨型盆山体系主要由复活后的古造山带、前陆冲断带和小型克拉通盆地三个基本的构造单元组成,其中古生界小型克拉通与中新生界前陆冲断带是重要的含油气单元,它决定了中国中西部油气分布主要受古生界克拉通古隆起和中新生界前陆冲断带的控制。塔里木盆地在纵向上由发育齐全的下古生代碳酸盐岩、上古生代海相-海陆交互相碎屑岩沉积和中新生代陆相碎屑岩等构造层序叠置而成,在平面上以较稳定的小型克拉通为核心,边缘环绕库车、喀什、塔西南、塔东南等褶皱或冲断变形的前陆冲断带。塔里木盆地古生界小型克拉通盆地与中新生界前陆逆冲带叠合-复合的构造特征,以及演化的多阶段性,决定了这类盆地具有"多套烃源岩、多储盖组合、多含油气系统"的叠合-复合含油气系统的特点;油气分布受小型克拉通盆地中的古隆起控制,形成大面积岩性地层油气藏,前陆盆地中的冲断带构造控制形成背斜油气藏,具有多期成藏并存与晚期成藏为主的特点。     

青藏高原东缘龙门山前陆逆冲带复合结构与生长

位于青藏高原东缘的北东向龙门山逆冲带,研究已经证明是中生代与新生代前陆复合扩展和生长的结果。然而,2008年5·12汶川地震地表破裂、余震和滑坡等的单向和分段迁移现象,对龙门山复合逆冲带的结构认识提出了挑战。文章在已有研究成果基础上,针对龙门山复合生长下构建的特殊结构进行了野外调查和构造解析。结果表明,以中生代与新生代两期前陆逆冲带复合生长为基础,龙门山复合逆冲带具有特殊的、主要由前陆逆冲楔叠加后形成的复合结构,而且这种复合逆冲楔具有分级和时序特征;中生代前陆逆冲楔是以逆冲断层-褶皱为特征,并分别组合形成碧口厚皮逆冲推覆体、唐王寨薄皮逆冲推覆体和龙王庙逆冲推覆体,总体从晚三叠世以前开始,至~160 Ma向南递进扩展生长;新生代前陆逆冲楔由逆冲断层和逆冲岩片组成,分为约35~10 Ma和10 Ma以来两个阶段,向南东向递进扩展生长,并可能与川西盆地东侧龙泉山构造相连通。因此,龙门山逆冲带具有前陆逆冲带和生长过程的双重复合结构。      

Apatite fission-track analysis of Neogene exhumation in northern Corsica (France)

Apatite fission-track analyses along a W–E-orientated transect across northern Corsica indicate an important episode of crustal exhumation in late early Miocene time. Samples taken from the Alpine orogenic wedge, from the adjacent foreland basin and from the crystalline basement complex flooring the basin are completely reset. This implies that a ≥ 2.0–2.3-km-thick crustal section made of thrust sheets and/or autochthonous foreland deposits has been removed by erosion since early Miocene time. A geometric projection of this lost cover towards the west indicates that all of northern Corsica was covered either by Alpine nappes or middle Eocene foreland deposits. Fission-track ages are the same across the main boundary fault system separating the Alpine orogenic wedge and the foreland, indicating the absence of significant differential vertical displacement between upper and lower plates during Neogene unroofing.     

环青藏高原盆山体系东段新构造变形特征——以川西为例

介于扬子板块与青藏高原之间的川西前陆冲断带是环青藏高原盆山体系东段的重要组成部分,它是研究喜马拉雅构造运动对青藏高原东缘沉积盆地构造作用的重要场所。本文分别选取川西南段、川西北段和川北西段米仓山前的区域构造地质剖面来研究沉积地层在喜马拉雅运动中发生的构造变形特征。通过前陆冲断构造变形带的宽度、水平缩短量,山体隆升、盆地沉降,新构造对早期古构造的叠加与改造关系的研究,揭示出在环青藏高原盆山体系内,造山带与盆地边缘的冲断构造变形从造山带向克拉通盆地内扩展的同时受欧亚大陆与印度板块碰撞及其远程效应的空间位置限制,靠近青藏高原的川西南段到远离它的川北西段,新构造变形强度、新构造变形范围、盆山耦合程度具有依次降低等特征。这种受环青藏高原盆山体系控制的前陆冲断带构造变形具有明显的资环效应,特别是对油气资源的聚集与分布有重要的影响,控制了川西南段晚期次生气藏发育,川西北段和川北西段的早期原生气藏的发育。