A Large-scale Tertiary Salt Nappe Complex in the Leading Edge of the Kuqa Foreland Fold-Thrust Belt, the Tarim Basin, Northwest China

The tectono-stratigraphic sequences of the Kuqa foreland fold-thrust belt in the northern Tarim basin, northwest China, can be divided into the Mesozoic sub-salt sequence, the Paleocene-Eocene salt sequence and the Oligocene-Quaternary supra-salt sequence. The salt sequence is composed mainly of light grey halite, gypsum, marl and brown elastics. A variety of salt-related structures have developed in the Kuqa foreland fold belt, in which the most fascinating structures are salt nappe complex. Based on field observation, seismic interpretation and drilling data, a large-scale salt nappe complex has been identified. It trends approximately east-west for over 200 km and occurs along the west Qiulitag Mountains. Its thrusting displacement is over 30 km. The salt nappe complex appears as an arcuate zone projecting southwestwards along the leading edge of the Kuqa foreland fold belt. The major thrust fault is developed along the Paleocene-Eocene salt beds. The allochthonous nappes comprise large north-dipping     

福建长汀童坊地区推覆构造

福建长汀童坊地区推覆构造具有变形强烈，走滑剪切作用明显，延续性好，岩片叠置清晰，同构造花岗岩发育等特点，具有冲断推覆而非褶皱推覆特征。其形成不仅与水平挤压作用有关，而且还与大型走滑作用有联系。     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.     

苏州西南部的推覆和伸展构造

根据大量的地质、钻孔资料,结合地球物理资料和岩组分析研究,发现苏州西南部中生代以来至少经历了两期重要的构造变动:先期的推覆、后期的伸展。推覆时代确定在早、中三叠世之后,晚侏罗世之前。其后该区便转入伸展构造发育期。推覆构造具有台阶状逆断层及其相关褶皱组合的几何学特征。推覆动向大体从北西(340°±)向南东,并有左旋逆冲性质。根据平衡剖面恢复,确定最小推覆缩短量为18—20公里,缩短率为45.6—47.6％。伸展拉张为表层的重力滑动,浅层形成NNE走向为主的“堑—垒”式对称正断层体系。区域盆地构造走向,野外地质和岩组的研究表明拉张主方向大体为NW305°—SE115°,并具右旋运动性质。挤压和拉张、推覆和伸展的叠加改造,使得苏州西南部的地层,包括大部分岩体在内,均成为无根构造岩片的叠置。这对研究该区高岭土和多金属矿床的成矿机理和指导找矿有重要意义。     

略论华北地块北缘显生宙三类不同的造山作用

华北地块北缘显生宙发育3种不同类型的造山作用。古生代,华北地块北缘处于古亚洲洋构造域,造山作用属陆缘俯冲-碰撞型,形成以EW向至NEE向为主的褶皱、逆冲推覆构造及韧性剪切带等构造类型。造山机制与古亚洲洋板块向南俯冲-碰撞导致近SN向构造动力的挤压作用密切相关。中生代,华北地块北缘处于西滨太平洋构造域,造山作用以陆内挤压型为主,形成以NE-NNE向与近EW向为主的多期不同方向的褶皱、逆冲断裂、推覆构造、韧性剪切带及局部地区的固态塑性流变构造等构造类型;造山动力以古太平洋(或Izanagi)板块西向俯冲导致NW-NWW向强烈挤压力为主。新生代,华北地块北缘虽仍属西滨太平洋构造域,但造山作用以陆内伸展型为主,裂谷作用与陆内伸展构造居主导地位,褶皱变形微弱,张性-张扭性断裂活动显著,形成现今盆-山构造地貌格局;造山动力以NW-NWW向主张应力为主。造山类型的两次重大转换分别发生于早、中三叠世与晚白垩世。      

造山带逆冲推覆构造研究的主要新进展

造山带逆冲推覆构造研究是造山带研究中最为重要的课题之一。造山带外带即前陆褶皱冲断带(主要发育盖层冲断推覆体,一般遵循薄皮构造变形规则)与造山带内带(主要是基底褶皱推覆体,呈现厚皮构造变形规律)结晶逆冲推覆构造的几何学、运动学特征存在较大差异,二者形成机制也不相同,但其间仍有紧密的联系。近20年来造山带逆冲推覆构造研究的主要新进展为:①前陆褶皱冲断带逆冲断层及其相关褶皱的几何学特征分析已趋定量化,对其组合类型与演化时序有了更全面的认识,且对前陆褶皱冲断带的发展演化模式取得了新的共识,即遵循临界库仑楔模式;②平衡剖面技术在前陆褶皱冲断带的应用已从二维平衡与复原演进到三维平衡与复原,且日渐计算机化;③对造山带内带结晶基底逆冲推覆构造的主要类型(C型与F型逆冲岩席)及其特征已有较深的理解;④对前陆褶皱冲断带与结晶基底逆冲构造的相互关系及其形成演化模式有了新认识。目前造山带逆冲推覆构造研究过程中存在的主要问题为:①造山带内带结晶逆冲推覆构造的研究比较薄弱;②造山带晚期走滑构造及伸展构造的叠加与改造使得造山带内结晶逆冲推覆构造更为复杂化,致使其研究难度加大;③全面、精细的造山带深部地球物理资料较缺乏;④造山带内结晶逆冲岩席变形变质历史与超高压变质岩的形成机制及折返过程之间的关系尚未揭示清楚。在今后研究过程中应加强对上述问题的深入研究。     

Structures and tectonic evolution of the external zone of Alpine Corsica

This paper presents a structural analysis of the external zone of Alpine Corsica, including the autochthonous domain and overlying external nappes (Santa Lucia and Balagne nappes). Two stages of nappe emplacement are identified occurring prior to and after the deposition of the Eocene sediments which were laid down upon first generation thrust contacts but are imbricated with their composite (continental and ophiolitic) basement by second generation thrusts. Five generations of structures with regional extent have been distinguished. However, the first generation has not been recognized within the visible part of the autochthon domain.Eoalpine first generation structures, restricted to allochthonous units, and Late Eocene to Early Oligocene second generation structures were nearly contemporaneous with the two stages of thrusting. The precise significance of E-W third generation structures is poorly understood. Broadly N-S fourth generation structures resulted from Oligocene compressive tectonics (folding and local backthrusting). Finally, fifth generation structures were generated during a Miocene extensional stage.These results are partly consistent with structural features previously reported in the southern and the northern outcrops of the Schistes lustrés, i.e. the main part of the allochthonous domain. A summary of a regional tectonic evolution is thus proposed for Alpine Corsica from Eoalpine obduction to Miocene extension.     

The restoration of thrust systems and displacement continuity around the Mont Blanc massif,NW external Alpine thrust belt

Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.     

Fold-axis parallel extension in an arcuate fold- and thrust belt: the case of the Helvetic nappes

The Helvetic nappes of western Switzerland are discussed as an example of an arcuate foreland fold- and thrust belt in which active fold-axis parallel stretching occurred. Fold-axis parallel extension is recorded by:

1. (1) Incremental strain data from pressure shadow fibres. The significance of pressure shadow fibres for the determination of the deformation history of a region is discussed. Pressure shadows are used to quantify the amount of, and to describe the distribution of fold-axis parallel extension occurring in the Helvetic nappes.

2. (2) The extension directions of conjugate systems of en échelon veins. It is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern. The relative age of the conjugate en échelon vein systems in the Helvetic deformation history can be assessed, the geometry of the conjugate sets relative to the local anisotropy plane is described, and the significance of the preferred orientation of their extension directions is discussed.

3. (3) Fold-axis parallel sections. A comparison of the regional distribution of the fold-axis parallel strain with the shape of the Helvetic nappes in fold-axis parallel sections shows that the fold-axis parallel strain cannot be related to the footwall topography of the nappes.

It is concluded that the fold-axis parallel extension in the Helvetic nappes was induced by a change of direction of overthrust shear. This change occurred late in the deformation history and was superposed on the already formed nappes. The changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting Alpine nappe pile, relative to the European plate, a rotation which lead to the arcuate shape of the Western Alps.     

四川龙门山地区反转构造样式分析及其成因机制探讨

反转构造是当今构造地质学研究的新兴热点领域,本文尝试以反转构造和断层相关褶皱理论来探讨龙门山褶皱冲断带及川西前陆盆地中的反转构造样式及其成因。著者在综合前人研究成果的基础上,通过野外地质调查,室内构造分析与建模系统研究了龙门山地区典型的反转构造样式,讨论了龙门山带的反转性质,主干断裂的成因以及反转动力学机制。研究表明,龙门山的发育机制为一斜向正反转过程,区内发育有反转断层转折褶皱、被动陆缘型反转滑脱褶皱、反转断层传播褶皱以及受古生代裂谷控制的反转构造等反转构造类型;反转时期主要为印支期,本区在印支运动之前同时属被动陆缘和裂谷的构造背景;进入印支期后,受扬子陆块、华北陆块、羌塘陆块之间相互碰撞的影响而造山。该过程在本区不同地段表现存在差异,这种差异受控于前期的构造格局以及后期不同方向挤压应力的叠加。四川前陆盆地的发育和该过程有密切的联系,盆地内部具有裂谷构造反转的证据。     

A geometrical and kinematical approach to the nappe structure in an arcuate fold belt: the Cantabrian nappes (Hercynian chain,NW Spain)

In northwest Spain thrust sheets occur in an arcuate fold belt. The fault style consists of an array of thrusts, merging downdip into a single décollement surface. Most of the thrust sheets were initiated as thrusts cutting across flat lying beds. Folds above the hanging-wall ramps and some minor structures indicate that the body of the nappes has been subjected to an inhomogeneous simple shear parallel to bedding (y = 1.15), with slip concentrated along bedding planes. This allows the rocks forming the nappe to remain unstrained. At the base of the nappes a thin zone of deformed rock exists. The thrust sheets die out laterally against an anticline-syncline couple, oblique to the thrust direction. A geometrical analysis shows that if anticline and syncline axes are oblique, the thrust sheet was emplaced with a rotational movement, which can be evaluated. As deformation progressed two sets of folds were formed: a circumferential set, following the arc, and a radial set. An arcuate trace of the thrust structures remains after unfolding the radial folds. With a rotational emplacement, the displacement vector for successive points has a progressively greater length, and forms a progressively lower angle with the thrust. The main thrust units are broken into several slices with rotational movements, so that each unit was curved as it was being emplaced, producing a first tightening of the arc. Later folding increased the arc curvature to its present shape. The palaeomagnetic data available support the above conclusions.     

The Upper Paleozoic preflysch and overthrusting in the Türkstan-Alay ranges,southern Fergana

The Upper Paleozoic preflysch sedimentary rocks in the Türkstan-Alay ranges are combined in a common section with limestone of autochthon and synsedimentation carbonate nappes, thus forming the upper-most layers of the stratigraphic section of the latter. By their lithology, relatively small thickness, facies variability, and position at the boundary between carbonate and flysch-olistostrome sequences (in the autochthon), these rocks correspond to a certain extent to preflysch of the Urals and the Mediterranean Alpine Belt. This association of clayey, carbonate, and terrigenous rocks is strictly constrained in stratigraphy (the upper portion of the lower Moscovian substage and the lower portion of the upper Moscovian substage) and localization (the southern slopes of carbonate platforms). The formation of this rock association immediately predated the Late Paleozoic overthrusting and deposition of terrigenous flysch. In paleotectonic terms, preflysch is an indicator of the initial stage of tectonic and magmatic activation that led to the replacement of carbonate sedimentation with deposition of terrigenous and clayey sediments, coeval volcanism, and stratiform ore formation. The following sequence of events has been outlined in the Early and Middle Carboniferous: (1) thrusting of volcanic-sedimentary rocks filling troughs over the northern margins of carbonate platforms, (2) lateritic weathering and deposition of marine bauxite in the Bashkirian and early Moscovian, (3) repeated overthrusting and deposition of preflysch on southern slopes of platforms, (4) invasion of the frontal flysch trough from the south, (5) scouring of preflysch and the underlying limestone, and (6) formation of flysch-olistostrome sequences and tectonic and gravity nappes in the late Moscovian time. This interpretation takes into account the southward vergence of thrust sheets and nappes, the structure and localization of allochthonous fragments of marginal zones of carbonate platforms, and the pre-Bashkirian thrusting of volcanic and sedimentary rocks over the condensed pelagic deposits of the Shalan Group. It is suggested that bauxite and preflysch materials had the same source and were deposited in the Middle Carboniferous on the offshore carbonate shoals.     

Tectonic analysis of progressive secondary deformation in the hinge of a major Caledonian fold nappe,north-western Ireland

Polyphase deformation chronologies established within the mid-crustal portions of orogenic belts have classically been attributed to regional-scale ‘events’ which generate distinct structural sequences that can be directly correlated across large tracts of the orogenic belt. However, concepts of progressive deformation in which minor structures may be continually generated, amplified and redeformed within a unifying kinematic framework suggest that regional correlation of minor structures is both misguided and misleading. Detailed structural analysis of lower amphibolite facies Dalradian metasediments in north-west Ireland does, however, demonstrate that a coherent and meaningful deformation chronology can be established within the framework of individual fold nappes. Protracted deformation has resulted in the generation of a series of overprinting, secondary structures (D₄–D₉), which are kinematically linked to the continued structural evolution and south-east directed translation of the crustal-scale (D₃) Ballybofey (fold) Nappe. Secondary (D₄) crenulation axes initiated at an oblique angle to the direction of nappe transport both rotate and amplify into larger scale folds, which are subparallel to transport and demonstrate successive stages of diachronous folding. Continued nappe-related deformation induces southwards verging contractional (D₅) folds, which are particularly well developed and focused into reactivated ductile (D₃) thrust zones generated during the initial stages of nappe translation. Subsequent to thickening-induced ductile extension and collapse of the nappe, a return to contractional tectonics is marked by major episodes of broad, open buckle folding developed orthogonal to both the overturned limb (D₇) and upper limb (D₈) of the nappe. Detailed structural analysis and investigation of secondary folds and overprinting fabrics provides a valuable insight into the protracted kinematic evolution of major fold nappes.     

云南白秧坪矿化集团中区成矿构造动力学分析

云南白秧坪帮化集中区位于兰坪－思茅印支燕山、喜马拉芽期成矿带的北段，成矿以中生代海相过渡到际相的碳酸岩建筑、碎屑岩建筑夹火山岩建造为基础，喜马拉芽期构造推覆有热液改造为主导作用。通过野外地质调查，利用共轭剪切破裂解析出断块掀斜前后的构造动力学特征。结果表明，本区喜马拉芽期构造运动有3期，早期表现为云龙－江城褶皱系边部发生较强烈褶皱变形，盆地内部以挠曲变形为主；中期为脆性变形期，整个褶皱出现SN向冲断系统，前期褶皱部分得以强化，其余地区广泛出现宽缓褶皱；晚期为松驰－挤压交替的调整期。     

藏北改则新生代早期逆冲推覆构造系统

藏北改则及邻区新生代早期发育大型逆冲推覆构造系统,由不同方向的逆冲断层、不同时代的构造岩片、不同规模的飞来峰和构造窗、不同类型的褶皱构造组成。羌塘中部发育羌中薄皮推覆构造,石炭系板岩和二叠系白云质灰岩自北向南逆冲推覆于上白垩统与古近系红层之上,形成大型逆冲岩席和弧形逆冲断层,原地系统古近纪红层下伏三叠系—侏罗系海相烃源岩。羌塘南部发育南羌塘薄皮推覆构造,导致班公—怒江蛇绿岩、三叠系—侏罗系海相地层及侏罗纪混杂岩自北向南逆冲推覆于古近纪红层与下白垩统海相沉积岩层之上,形成三条蛇绿岩片带、大量飞来峰和厚度较大的构造片岩。中新世早期火山岩层和湖相沉积呈角度不整合覆盖逆冲断层、褶皱构造和逆冲岩席,不整合面上覆火山岩年龄为23.7～19.1Ma,指示中新世早期改则及邻区基本结束了强烈逆冲推覆构造运动。估算羌中逆冲推覆构造的推覆距离约100～115km,南羌塘逆冲推覆构造的推覆距离约82～110km;新生代早期改则逆冲推覆构造系统近南北方向逆冲推覆总距离为182～225km,对应地壳缩短率为(50.3±2.7)%。     

南天山库车秋里塔格中段构造结变形特征和变形机理

本文通过遥感影像解译和地表调查,利用二维地震剖面,结合钻、测井数据,分析秋里塔格中段构造结的构造特征,利用轴面分析和多余面积方法计算构造缩短量。研究表明,秋里塔格中段构造结的构造变形受3个滑脱面,即中新统吉迪克组(N1j)膏盐岩、古?始新统库姆格列木组(E1?2km)膏盐岩和侏罗系(J)煤层控制。构造结东部发育两个滑脱面,分别位于中新统吉迪克组膏盐岩和侏罗系煤层,形成浅层的滑脱褶皱和深层的断层转折褶皱,构成复合型背斜; 西部发育一个滑脱面即库姆格列木组膏盐岩,膏盐岩聚集形成南秋里塔格滑脱褶皱。库车塔吾背斜向西倾伏,背斜下伏沿吉迪克组膏盐岩和侏罗系煤层发育的断层(F1和F 3)的滑移量向西减小,如东部A?A′剖面上断层F3的滑移量为7.7 km,西部C?C′剖面上为3.8 km; 南秋里塔格背斜向东倾伏,背斜下伏沿库姆格列木组膏盐岩发育的断层的滑移量向西增大,东部的A?A′剖面断层不发育,西部的D?D′剖面构造缩短量达5.1 km。构造结地区3个滑脱面交汇： ①东、西段卷入变形的断层和褶皱在此发生了叠加干涉, ②断层的滑移量在此汇合转换, ③库车塔吾背斜、南秋里塔格背斜和托克拉克坦背斜在此交汇,交汇处发育多个高角度的逆冲剪切断层和表皮褶皱,形成了秋里塔格中段复杂构造结,构造结地区未发育大规模走滑断层。根据上述认识,我们提出了秋里塔格中段构造结的三维构造模型。     

柯坪塔格推覆构造几何学、运动学及其构造演化

大量野外构造地质调查和深部构造解释表明柯坪塔格推覆构造由多组倒转复式背斜、复式箱状背斜构成的推覆体及其前缘逆冲断裂组成 ,由寒武系—第四系组成的推覆体由北向南逆—斜冲 ,平面上构成向南凸出的弧形推覆构造 ;普昌断裂由各不相连的逆冲斜冲断裂段组成 ,而不是完整的一条走滑断层 ,各推覆体前缘逆冲断裂与各推覆体的普昌断裂段共同构成统一的前缘逆冲斜冲逆冲断裂和推覆构造系统 ;普昌断裂段以西的推覆体具有向东抬升、向西倾覆的鼻状构造特征 ,普昌断裂段以东的推覆体具有向西抬升、向东倾覆的鼻状构造特征 ,普昌基底隆起带是巴楚隆起隐伏在柯坪塔格推覆构造之下的部分。各推覆体前缘断裂在深部均归并于统一的寒武系底部的滑脱面 ,其南浅北深 ,东浅西深 (普昌隆起带以西 )或西浅东深 (普昌隆起带以东 ) (6 10km ) ,埋深较大区发育多组滑脱面。柯坪塔格推覆构造的形成时期为晚第四纪 ,为现今活动的推覆构造系统。文中认为各推覆体向南西的倾覆端基底滑脱面和中新生界内部的滑脱面没有贯通 ,是未来 6级以上地震的发震构造部位。     

雪峰造山带北段地质构造特征——以慈利—安化走廊剖面为例

以慈利—安化走廊带为例, 对雪峰造山带北段西部地质构造特征进行了调查研究。研究表明, 雪峰造山带在廊带上可分为北部武陵断弯褶皱带和南部雪峰基底拆离带。武陵断弯褶皱带内主要发育北东东—东西向褶皱和同走向逆断裂, 另有少量北东向和北北西向右行平移断裂、北东东—东西向正断裂; 雪峰基底拆离带发育东西—北东向褶皱和同走向逆断裂、正断裂以及少量北东向平移断裂。武陵断弯褶皱带变形主要受控于板溪群底界面向北的滑脱及其导生的逆冲; 雪峰基底拆离带变形主要受控于切穿冷家溪群褶皱基底的断裂拆离与逆冲, 拆离与逆冲的方向总体由南向北, 但南缘总体逆冲方向指向南, 从而组成背冲构造样式。上述褶皱和断裂形成于武陵运动、加里东运动、印支运动、早燕山运动等挤压事件, 白垩纪伸展事件, 古近纪中晚期区域北东—北北东向挤压以及古近纪末—新近纪初北西向挤压等构造事件, 其中以加里东运动和印支运动形成的褶皱和同走向逆断裂最为重要。雪峰造山带北段与中段—南段一样具背冲构造样式, 但受加里东期近南北向挤压的区域大地构造背景影响, 北段逆冲、增厚和抬升作用的强度与幅度更大。      

扬子板块与华南板块对接带萍乡区段构造特征

扬子板块与华南板块对接带由江西中部通过,其中萍乡地区自燕山运动末期以来,在北部以逆冲推覆作用为主,形成了大规模的逆冲推覆构造,南部在重力滑脱作用下,形成变质-岩浆热穹隆和一系列滑褶构造带及滑块构造,中部则形成相互叠覆的南、北构造对接带。在这些推、滑覆体之下多处掩覆了含煤岩系,这为寻找隐伏煤田开辟了一个新天地,同时,南、北构造对接带也是钴多金属矿有利的成矿区段。      

塔里木盆地库车前陆褶皱带中段盐相关构造特征与油气聚集

塔里木盆地北部库车前陆褶皱带古新统一始新统发育盐岩层系，将库车前陆褶皱带构造变形和圈闭样式分为三层，即盐上构造、盐层(盐间)构造和盐下构造。盐上构造包括盐上背斜、盐上逆冲断层及断层相关褶皱、盐上背冲断块构造、强制褶皱、盐上逆冲断层遮挡构造和盐推覆构造等；盐层(盐间)构造主要包括盐枕构造、盐间断褶构造、盐焊接构造和外来盐席等；盐下构造主要有背冲断块构造、断层相关褶皱、叠瓦冲断带和双重构造等。库车前陆褶皱带盐构造的形成可能受挤压作用、重力滑动和重力扩展作用多重控制。笔者等讨论了盐相关构造油气成藏条件和模式，认为库车前陆褶皱带盐岩层变形与丰富的圈闭构造形成密切相关，烃源岩主要位于盐下，盐岩层作为优质盖层构成石油和天然气藏最优越的遮挡条件，断裂对盐下、盐间和盐上油气成藏都起重要控制作用，但盐下是最有利的油气聚集场所。