Tectonic Evolution of the Middle Frontal Area of the Longmen Mountain Thrust Belt,Western Sichuan Basin,China

By analyzing the balanced cross sections and subsidence history of the Longmen Mountain thrust belt, China, we concluded that it had experienced five tectonic stages： （1） the formation stage （T3x） of the miniature of Longmen Mountain, early Indosinian movement, and Anxian tectonic movement created the Longmen Mountain; （2） the stable tectonic stage （J1） where weaker tectonic movement resulted in the Longmen Mountain thrust belt being slightly uplifted and slightly subsiding the foreland basin; （3） the intense tectonic stage （J2-3）, namely the early Yanshan movement; （4） continuous tectonic movement （K-E）, namely the late Yanshan movement and early Himalayan movement; and （5） the formation of Longmen Mountain （N-Q）, namely the late Himalayan movement. During those tectonic deformation stages, the Anxian movement and Himalayan movement played important roles in the Longmen Mountain＇s formation. The Himalayan movement affected Longmen Mountain the most; the strata thrust intensively and were eroded severely. There are some klippes in the middle part of the Longmen Mountain thrust belt because a few nappes were pushed southeastward in later tectonic deformation.

Differential Tectonic Deformation of the Longmen Mountain Thrust Belt,Western Sichuan Basin,China

Field investigation and seismic section explanation showed that the Longmen Mountain Thrust Belt has obvious differential deformation： zonation, segmentation and stratification. Zonation means that, from NW to NE, the Longmen Mountain Thrust Belt can be divided into the Songpan- Garz~ Tectonic Belt, ductile deformation belt, base involved thrust belt, frontal fold-thrust belt, and foreland depression. Segmentation means that it can be divided into five segments from north to south： the northern segment, the Anxian Transfer Zone, the center segment, the Guanxian Transfer Zone and the southern segment. Stratification means that the detachment layers partition the structural styles in profile. The detachment layers in the Longmen Mountain Thrust Belt can be classified into three categories： the deep-level detachment layers, including the crust-mantle system detachment layer, intracrustal detachment layer, and Presinian system basal detachment layer; the middle-level detachment layers, including Cambrian-Ordovician detachment layer, Silurian detachment layer, etc.; and shallow-level detachment layers, including Upper Triassic Xujiahe Formation detachment layer and the Jurassic detachment layers. The multi-level detachment layers have a very important effect on the shaping and evolution of Longmen Mountain Thrust Belt.

The Deep Geophysical Structure of the Middle Section of the Longmen Mountains Tectonic Belt and its Relation to the Wenchuan Earthquake

Investigation of the deep geophysical structure of the Longmen Mountains tectonic belt and its relation to the Wenchuan Earthquake is important for the study of earthquakes.By using magnetotelluric sounding profiles of the Luqu-Zhongjiang and Anxian-Suining; seismic sounding profiles of the Sichuan Maowen-Chongqing Gongtan,the Qinghai Huashi Gorge-Sichuan Jianyang,and the Batang-Zizhong; and magnetogravimetric data of the Longmen Mountains region,the deep geophysical structure of the Songpan-Ganzi block,the western Sichuan foreland basin,and the Longmen Mountains tectonic belt and their relation was discussed.The eastward extrusion of the Qinghai-Tibet Plateau thrusts the Songpan-Ganzi block upon the Yangtze block,which obstructs the eastward movement of the Qinghai-Tibet Plateau.The Maoxian-Wenchuan,Beichuan-Yingxiu,and Anxian-Guanxian faults of the Longmen Mountains fault belt dip to northwest with different dip angles and gradually converge in the deeper parts.Geophysical structure suggests that an intracrustal low-velocity,low-resistivity,and high-conductivity layer is common between the middle and upper crust west of the Longmen Mountains tectonic belt but not in the upper Yangtze block.The Sichuan Basin has a thick low-resistance sedimentary layer on a stable high-resistance basement; moreover,there are secondary paleohighs and depression structures at the lower part of the western Sichuan foreland basin with characteristic of high magnetic anomalies,whereas the Songpan-Ganzi block has a high resisitivity cover of upper crust and continues to a low-resistance layer.Considering the Longmen Mountains tectonic belt as the boundary,there are Bouguer gravity anomalies of ＂one belt between two zones.＂ Thus,we infer that there is a corresponding relation between the inferred crystalline basement of the Songpan block and the underlying basin basement of the Longmen Mountains fault belt.Furthermore,there may be an extensive ancient Yangtze block,which is west of the Ruoergai block.In addition,the crust-mantle ductile shear zone under the Longmen Mountains tectonic belt is the main fault,whereas the Beichuan-Yingxiu and Anxian-Guanxian faults at the surface are earthquake faults.The Wenchuan Ms 8.0 earthquake might be attributed to the collision of the Yangtze block and the Qinghai-Tibet Plateau.The eastward obduction of the eastern edge of the Qinghai-Tibet Plateau and eastward subduction of its deeper part under the influence of the collision of the Indian,Pacific,and Philippine Plates with the Eurasia Plate might have caused the Longmen Mountains tectonic belt to cut the Moho and extend to the middle and upper crust; thus,creating high stress concentration and rapid energy release zone.

Application of the Material Balance Method in Paleoelevation Recovery: A Case Study of the Longmen Mountains Foreland Basin on the Eastern Margin of the Tibetan Plateau

We applied the material balance principle of the denudation volume and sedimentary flux to study the denudation-accumulation system between the Longmen Mountains （Mts.） and the foreland basin. The amount of sediment in each sedimentation stage of the basin was estimated to obtain the denudation volume, erosion thickness and deposit thickness since the Late Triassic Epoch, to enable us to recover the paleoelevation of the provenance and the sedimentary area. The results show the following： （1） Since the Late Triassic Epoch, the elevation of the surface of the Longmen Mts. has uplifted from 0 m to 2751 m, and the crust of the Longmen Mts. has uplifted by 9.8 km. Approximately 72% of the materials introduced have been denuded from the mountains. （2） It is difficult to recover the paleoelevation of each stage of the Longmen Mts. foreland basin quantitatively by the present-day techniques and data. （3） The formation of the Longmen Mts. foreland basin consisted of three stages of thrust belt tectonic load and three stages of thrust belt erosional unload. During tectonic loading stages （Late Triassic Epoch, Late Jurassic-Early Cretaceous, Late Cretaceous-Miocene）, the average elevation of Longmen Mts. was lower （approximately 700-1700 m）. During erosional unloading stages （Early and Middle Jurassic, Middle Cretaceous and Jiaguan, Late Cenozoic）, the average elevation of Longmen Mts. was high at approximately 2000-2800m.

Episodic Orogeny Deduced from Coeval Sedimentary Sequences in the Foreland Basin and Its Implication for Uplift Process of Longmen Mountain,China

Longmen Mountain located at the boundary between the Sichuan Basin and Tibetan Plateau,representing the steepest gradient of any edges of the plateau.Three endmember models of uplift process and mechanism have been proposed,including crustal thickening,crustal flow,and crustal isostatic rebound.Here we use coeval sedimentary sequences in the foreland basin to restraint uplift process and mechanism in the Longmen Mountain.The more than 10,000 m thick Late TriassicQuaternary strata filled in this foreland basin and can be divided into six megasequences that are distinguished as two distinct types.The first type is the wedge-shaped megasequences which are sedimentary response of strong active thrust loading events,characterized by a high rate of subsidence and sediment accumulation,coarsening-upward succession and a dual-sourced sediment supply.This type includes Late Triassic,Late Jurassic to Early Cretaceous and Late Cretaceous to Paleogene megasequences.The second type is the tabular megasequences,characterized by the low rate of subsidence and sediment accumulation,finingupward succession,and a single-sourced sediment supply,which is sedimentary response of isostatic rebound and erosion unloading.This type includes the Early to Middle Jurassic,Middle Cretaceous and Neogene to Quaternary megasequences.Basing on sedimentary,active tectonic,geomorphic evidence,we infer that the direction has been reversed from SSWdirected sinistral strike-slip to NNE-directed dextral strike-slip during 40-3.6 Ma,and since 3.6 Ma,the Longmen Mountain thrust belt belong to times of isostatic rebound and erosional unloading with NNEdirected dextral strike-slip.This suggests that crustal isostatic rebound is a primary driver for uplift and topography of the present Longmen Mountain.The Wenchuan(Ms8.0) earthquake,which ruptured a large thrust fault with NNE-directed dextral strikeslip along the range front,is an active manifestation of this crustal isostatic rebound process with dextral strike-slipping and shortening.This process may be the cause for the Wenchuan Earthquake and the apparent paradox of high relief,little shortening,the relative dearth of historical seismicity in the region.

Characteristics of Nappes and Segmentation of the Longmen Mountains Thrust Belt, Western Sichuan Basin, China

In order to reveal the nature of the segmentation of Longmen Mountains Thrust Belt caused by the three nappes (Jiaoziding, Jiudingshan, and Baoxing Nappe), several methods are applied in this paper, including field investigation, seismic explanation and balanced crossed section, etc. Results of research reveal that nappes in Longmen Mountains vary in geometry, kinematics, and dynamics. Jiaoziding Nappe has generally behaved in a ductile manner, whereas Jiudingshan Nappe has been rigid, and the rheology of Baoxing Nappe has been intermediate between that of the other two nappes. The development of nappes has resulted in tectonic segmentation of Longmen Mountains: the main structural style of the northern segment is thrust faulting, with Jiaoziding Nappe representing a giant syncline. Given its ductility, it absorbed lots of stress, with the least amount of tectonic shortening in the SE part of the nappe. In the middle segment, the deformation is controlled by the rigid Jiudingshan Nappe, whose frontal area records lots of tectonic shortening. Deformation in the southern segment is intermediate in character between that of the other two segments, characterized by horizontal zonation, as demonstrated by fault development, and vertical stratification, which indicates that fault development was controlled by lithology.

Meso-Cenozoic Tectonic Events Recorded by Apatite Fission Track in the Northern Longmen-Micang Mountains Region

There is a cross-cutting relationship between the E-W trending structures and the NE- trending structures in the northern Longmen-Micang Mountains region, which reflects possible regional tectonic transition and migration. Apatite fission track （AFT） analyses of 15 samples collected from this area yield apparent ages varying from 30.3±4.2 Ma to 111.7±9.0 Ma and confined-track-lengths ranging from 10.6±0.3 pm to 12.4±0.1 μm. Four specific groups were identified on the basis of the Track Age Spectrum Calculation （TASC） patterns, i.e., 143-112 Ma, 93.6-88 Ma, 42-40 Ma and -25.6 Ma. These age groups correspond to the spatial distributions of datasets and may represent four tectonic events. Together with the regional deformation patterns, the four age groups are interpreted to indicate tectonic superposition, transition and migration during the Meso-Cenozoic with the following possible order： （1） the Micang Mountains belt was dominated by the E-W trending structure during 143-112 Ma; （2） the contraction of the Longmen Mountains belt from the NW to the SE during 93.6-88 Ma led to the superposition of the NE-trending structures over the E-W trendinding structures; （3） dextral strike-slip shear dominated the Longmen Mountains belt at 42-40 Ma; （4） westward migration of the active tectonic belt occurred from 93.6-25.6 Ma in a break-back sequence in the northern Longmen Mountains belt. The Late Cenozoic tectonics in the northern Longmen Mountains belt are characterized by the dextral strike-slip shear and the occurrence of westward break-back sequence of deformations. As a result, north-south differences in deformations along the Longmen Mountains belt were intensified since the Miocene time and strains were mainly accumulated in the hinterland of the Longmen Mountains instead of being propagated to the foreland basin.

Structural Styles of Longmen Mountain Thrust Belt,SW China

Through field geological investigation and seismic interpretation of the Longmen （龙门） Mountain thrust belt, we summarized the following structural styles： thrust belt, fault-related fold （fault bend fold, fault propagation fold, and fault decollement fold）, pop-up, triangle zone, duplex, superimposed fold, ductile deformation structures, reverse thrust fault, klippe, decollement structure,etc.. These structural styles have evident distribution characteristics; they had zonation and segmentation in plane. The zonation presents as the thrust nappe tectonic zone to the west of Tongjichang （通济场） fault, fault-related folds between Tongjichang fault and Guankou （关口） fault, and low and mild folds to the east of Guankou fault. The segmentation is evidenced as the scale of reverse thrust faults was minor between Tongjichang No. 1 fault and Tongjichang No. 2 fault. The distance between these two faults became long in the Daynanbao （大园包） structure, and there developed typical fault propagation fold and pop-up between these two faults. Furthermore, the structures had stratifi-cation in profile. The salt layer of T21 provided good conditions for the formation and development of large listric thrust faults; the thrust fault slipped in the salt layer and formed decollement structures and fault-related fold. At the same time, there formed duplex and reverse thrust faults between the two decollement layers.

Fault Characteristics in Longmen Mountain Thrust Belt, Western Sichuan Foreland Basin, China

Through field geological survey, the authors found that abundant thrust faults developed in the Longmen （龙门） Mountain thrust belt. These faults can be divided into thrust faults and strike-slip faults according to their formation mechanisms and characteristics. Furthermore, these faults can be graded into primary fault, secondary fault, third-level fault, and fourth-level fault according to their scale and role in the tectonic evolution of Longmen Mountain thrust belt. Each thrustfault is such as composed of several secondary faults, Qingchuan （青川）-Maowen （茂汶） fault zone is composed of Qiaozhuang （乔庄） fault, Qingxi （青溪） fault, Maowen fault, Ganyanggou （赶羊沟） fault, etc.. The Longmen Mountain thrust belt experienced early Indosinian movement, Anxian （安县） movement, Yanshan （燕山） movement, and Himalayan movement, and the faults formed gradually from north to south.

Crustal Uplift in the Longmen Shan Mountains Revealed by Isostatic Gravity Anomalies along the Eastern Margin of the Tibetan Plateau

This study examines the relationship between high positive isostatic gravity anomalies （IGA）, steep topography and lower crustal extrusion at the eastern margin of the Tibetan Plateau. IGA data has revealed uplift and extrusion of lower crustal flow in the Longmen Shan Mountains （the LMS）. Firstly, The high positive IGA zone corresponds to the LMS orogenic belt. It is shown that abrupt changes in IGA correspond to zones of abrupt change of topography, crustal thickness and rock density along the LMS. Secondly, on the basis of the Airy isostasy theory, simulations and inversions of the positive IGA were conducted using three-dimensional bodies. The results indicated that the LMS lacks a mountain root, and that the top surface of the lower crust has been elevated by 11 km, leading to positive IGA, tectonic load and density load. Thirdly, according to Watts＇s flexural isostasy model, elastic deflection occurs, suggesting that the limited （i.e. narrow） tectonic and density load driven by lower crustal flow in the LMS have led to asymmetric flexural subsidence in the foreland basin and lifting of the forebulge. Finally, based on the correspondence between zones of extremely high positive IGA and the presence of the Precambrian Pengguan-Baoxing complexes in the LMS, the first appearance of erosion gravels from the complexes in the Dayi Conglomerate layer of the Chengdu Basin suggest that positive IGA and lower crustal flow in the LMS took place at 3.6 Ma or slightly earlier.

Pn-wave velocity and anisotropy of the western Sichuan and Longmen Mountain region,China

Pn-wave velocity and anisotropy beneath the western Sichuan and adjacent region are inverted by a tomography method using arrival data from earthquakes recorded by the Sichuan Earthquake Network. It is followed by an analysis of the uppermost mantle structure of the Longmen Mountain region and the deep tectonics of the Ms8.0 earthquake in Wenchuan. Our results reveal a prominent velocity contrast across the Longmenshan fault: low velocities in the Songpan-Ganzi mountain range and high velocities in the Sichuan basin. They indicate that the Songpan-Ganzi mountain range is underlain by a weak uppermost mantle, which is easy to create ductile flow, whereas the Sichuan basin has a mechanically strong uppermost mantle. These features reflect a difference of the lithospheric mantle beneath tectonically active mountain range and stable basin. The boundary between the high and low velocities is coincident with the Longmenshan fault. Bordered by Wenchuan, the northern part of the boundary is parallel to the fault belt and aftershock distribution, but the southern part of the boundary deviates from the fault belt and is bent toward the Sichuan basin. The Ms8.0 earthquake in Wenchuan is located at the flexural part between them, where a strong heterogeneity is shown in the velocity variation across and parallel to the boundary. This area is reasonably believed as a potential location for regional stress accumulation and release in the Longmen Mountain region, reflecting a prominent character in the deep structure around Wenchuan. Pn-wave anisotropy reveals the mantle flow that is consistent with the eastward movement of the Tibetan Plateau. This process provided deep dynamic sourse for tectonic deformation in the Longmen Mountain region and for the occurrence of the Wenchuan Ms8.0 earthquake, but it is clearly affected by the strong lithosphere of the Sichuan basin. There is no correlation between the Pn-wave anisotropy and the Longmenshan fault, hence, this fault system is only restricted within the crust. However, the mantle flow beneath the southeastern Tibetan Plateau shows a clockwise rotation along the Xianshuihe fault. We infer that these two fault systems may be controlled by different deep dynamics.

DEFORMATIONAL AND METAMORPHIC HISTORY OF THE CENTRAL LONGMEN MOUNTAINS, SICHUAN CHINA

The Longmen Mountains and adjacent regions on the eastern margin of the Tibetan plateau can be divided into three tectonic units: the eastern Songpan—Garzê fold belt, the Longmen Mountains (Longmen Shan) Thrust—Nappe belt and the Western Sichuan foreland basin that occupies the western part of the Sichuan basin. The Longmen Shan Thrust—Nappe belt is subdivided by six northwest\|dipping major listric thrusts, with accompanying duplexes and imbricate fans, into five large\|scale nappes (Chen & Wilson, 1996). In the inner Longmen Shan, the nappe units have incorporated both Mesoproterozoic basement and Sinian (Neoproterozoic) to Triassic cover sequences as “thick\|skinned" horses. Whereas, in the frontal Longmen Shan, Sinian to Cretaceous cover sediments have been stripped from the basement as “thin\|skinned" fold and thrust sheets, including extensively distributed klippen structures. Pre\|thrusting extension during Devonian to middle Late Triassic times resulted in syndepositional normal faults. Structural inversion of these faults initiated the “Peng Xian—Guan Xian basement complex", Jiuding Shan and Tangwangzhai nappes, during an early episode of the Indosinian Orogeny (Norian to Rhaetian). This was followed by episodic thrusting during latest Triassic to Early Cretaceous times to develop the Guan Xian—An Xian and Southeastern Marginal nappes that have incorporated sediments from the neighbouring foreland basin into the frontal part of the Thrust—Nappe belt. Differential thrusting occurred across the Thrust—Nappe belt during a Late Miocene reactivation of the pre\|existing faults.

龙门山中段北川通口剖面中二叠统栖霞组地层沉积特征

川西龙门山前带栖霞组是近期油气勘探的热点,龙门山中段栖霞组勘探程度低,明确其地层沉积特征,对于天然气勘探具有重要的指导意义。基于对龙门山中段北川通口剖面实测,结合薄片鉴定和主量、微量元素分析,对栖霞组层序地层及沉积特征进行了研究,明确了沉积环境。北川通口剖面梁山组—栖霞组识别出2个三级和1个四级层序界面,划分为1个三级层序和2个四级层序。梁山组—栖霞组发育滨岸—沼泽、开阔台地及台地边缘3种沉积相,开阔海、滩间海、台内滩和台缘滩4种亚相,台内生屑滩、台缘生屑滩和台缘砂屑、生屑滩3种微相。栖一段至栖二段下部整体处于开阔台地,栖二段中上部则处于台地边缘,建立了龙门山中段栖霞组镶边碳酸盐岩台地沉积模式。栖霞组沉积期整体处于温暖、湿润、富氧、古生物大量繁殖的海水—陆表海沉积环境,栖二段中上部高位体系域Sr/Ba比值整体最低,MgO/CaO比值最高,平均古水温高,V/(V+Ni)、V/Cr低值,古盐度m值明显较高,代表远离陆地的浅水、高能台地边缘沉积环境,厚大高能台缘砂屑、生屑滩发育,是规模优质滩相孔隙型白云岩储层发育有利部位。龙门山中段栖二段与北段、南段具有相似的沉积相带和储集条件,是栖霞组油气勘探的有利区带,有望成为实现规模增储的重要战略接替阵地。

龙门山山前带中段分层差异变形特征及成因

龙门山构造带是研究陆内构造变形活动的理想场所之一。龙门山山前带的构造变形整体具有纵向分段、横向分带、垂向分层的特点。为了明确龙门山山前带中段分层变形特征,并进一步探讨分层差异变形成因。本文应用浅表地质露头、数字高程与三维地震数据等资料,开展龙门山山前带中段构造样式的识别及分层差异解析,绘制典型剖面构造演化图,计算关键层位缩短量。研究表明:龙门山山前带发育多种类型的断层相关褶皱,其中,上三叠统叠瓦逆冲构造以及深部逆冲楔在龙门山山前带中段广泛发育。总体来看,龙门山山前带中段大致以嘉陵江组为界,呈叠瓦逆冲构造系统与深部中小尺度冲断构造系统垂向叠置的构造特征,滑脱层对分层变形起着控制作用。此外,岩性也影响着分层变形,嘉陵江组膏盐岩等区域性分布的软弱层在变形中发挥着传递应变的滑脱层作用;多期次变形的叠加也是导致龙门山山前带中段分层差异变形的重要原因。

龙门山地区的鸟类多样性与区系组成

鸟类多样性是生态系统重要的监测指标,为掌握龙门山地区鸟类资源情况,2021年3月至2021年5月采用样线法对龙门山地区进行了调查,并结合相关文献资料检索,统计出龙门山地区分布有鸟类20目77科280属580种,其中雀形目鸟类43科350种,占所有鸟类总数的60.3%,非雀形目34科230种(39.7%)。有国家重点保护野生鸟类129种;IUCN物种红色名录中评估为极危、濒危和易危的分别有2种、5种和19种;被列入CITES附录中的有68种;中国特有鸟类38种。从区系构成上看,龙门山地区鸟类组成兼具东洋界和古北界成分,东洋界种类295种(50.9%)占比高于古北界种类230种(39.7%)、广布种55种(9.5%)。

龙门山前陆褶皱冲断带的平衡剖面分析

对龙门山前陆褶皱冲断带形成的正反转构造过程有过许多分析 ,但明显缺乏直接的证据和定量的分析。本文在地层资料分析的基础上 ,借助平衡剖面分析验证龙门山形成的正反转构造过程 :志留纪至中三叠世受多条倾向北西的同沉积断裂控制 ;晚三叠世以来遭受北西—南东向挤压、抬升和剥蚀 ,形成逆冲推覆构造。在晚三叠世和新生代的两期板块碰撞的影响下 ,龙门山产生了两期褶皱冲断作用 ,但在南、北两段表现出显著不同的变形过程。龙门山北段表现为复杂的逆冲推覆构造 ,能明确划分出两期构造变形 ,晚三叠世的变形强烈 ,缩短率达 31.7% ;而新生代的变形较弱 ,缩短率仅为 10 .5 %。南段则表现出基底卷入的叠瓦状冲断的特点 ,主要体现新生代的构造变形 ,晚三叠世的构造变形基本上被改造 ,南段整体缩短率达 2 6 .2 %。

川西龙门山褶皱冲断带分带性变形特征

通过野外地质考察和地震资料解释,将龙门山褶皱冲断带划分为5个构造带,即青川-茂汶断裂以西为松潘-甘孜构造带,青川-茂汶断裂与北川-映秀断裂之间为韧性变形带,北川-映秀断裂与马角坝-通济场-双石断裂之间为基底卷入冲断带,马角坝-通济场-双石断裂与广元-关口-大邑断裂之间为前缘-褶皱冲断带,广元-关口-大邑断裂以东为前陆坳陷带,在构造变形特征上,各条断裂在演化上具有前展式特征,在松潘-甘孜构造带和韧性变形带构造变形强烈,形成推覆构造带等构造变形样式,在前缘-褶皱冲断带和前陆坳陷带,变形强度较弱,形成背冲断块或断层相关褶皱等构造,西北部区域的变形表现为塑性变形特征,向南东方向渐变为塑-脆性变形和脆性变形,在剖面上各条断裂所形成的深度向盆地方向逐渐递减。龙门山褶皱冲断带的分带性变形特征是由多种因素共同影响的结果,这些因素主要有板块构造背景的决定作用、多套滑脱层的控制作用和岩性因素的制约作用。

汶川地震的地表破裂与逆冲-走滑作用

2008年5月12日在龙门山映秀-北川断裂带发生的8.0级特大地震,属于逆冲-走滑型地震。作者以地表破裂为切入点,在映秀-北川断裂和彭灌断裂的关键部位,对断错山脊、洪积扇、河流阶地、边坡脊、断层陡坎、河道错断、冲沟侧缘壁位错、小路位错、公路位错、公路拱曲、构造裂缝、断层偏转、擦痕、挤压脊、坡中槽等汶川地震所导致的地表破裂和断裂带开展了详细的野外地貌测量,标定了映秀-北川断裂带和彭灌断裂的垂向断距和水平断距,结果表明汶川地震的地表破裂带沿北东东向延伸,走向介于NE30°～50°之间,倾向北西,倾角介于30°～40°之间。其中北川-映秀断裂带的破裂带从映秀向北东延伸达180～190 km,属于单侧多点破裂型,以逆冲-右行走滑为特点,垂直位错为1.60～6.00 m,水平位错为0.20～6.50 m;彭灌断裂的地表破裂出露于彭州磁峰场-绵竹汉旺之间,长度为30～40 km,以逆冲-右行走滑为特点,垂直位错为0.39～2.00 m之间,水平位错为0.20～0.70 m。表明该地震地表破裂带存在逆冲运动分量和右行走滑运动分量,逆冲运动分量略大于或等于右行走滑运动分量。根据历史地震和活动构造地貌的年龄测定资料,作者认为该区单条断裂的强震复发间隔在1～3ka。在此基础上,初步编制了龙门山地区的地质动力模型图,并认为下地壳物质在龙门山的近垂向挤出和垂向运动,导致了龙门山断裂带的向东逆冲运动、龙门山构造带抬升和汶川特大地震。

龙门山构造带北段地震、地质综合解释

在龙门山推覆构造带东缘的盆地区中已发现了多个具有一定规模的气田,但在推覆带上还未有突破。由于龙门山逆掩断裂带地理条件十分恶劣,地震勘探很难得到好的地震反射资料,难以查清推覆构造带下的构造形态,阻碍了该区油气资源的勘探进程。2003年在龙门山构造带北段广元地区开展的“变线元多线(宽线)剖面采集”试验,于龙门山“前、后山带”获得了相当好的地震反射剖面,在推覆带发现了保存完好的大型背斜构造。从山前带的东边界开始,这一隐伏构造带向西延展的宽度在25 km以上,直到后山带的中段。隐伏构造的发现,使近期在该区找到大型油气田的希望成为现实。文章依据断层相关褶皱理论模式对宽线剖面进行综合解释,结合区域地质研究成果解析了龙门山构造北段的变形历史。

龙门山中段构造特征与汶川地震

通过地表地质调查、钻井资料和各种地球物理资料分析,剖析了龙门山中段的构造特征和岩石圈结构特征的特殊性:(1)10 Ma以来龙门山中段隆升幅度和速率最大;(2)龙门山中北段喜马拉雅期构造活动较南段弱;(3)龙门山中段主构造带之下存在有大于10 000Ω.m的高阻异常体;(4)龙门山中段具有+0.015 cm/s2的剩余(60 km×60 km^120 km×120 km)布格重力异常;(5)龙门山中段为正的均衡重力异常区。在此基础上,结合汶川地震主震和余震的分布特征,得出:(1)四川汶川地震主震和绝大多数余震震源多位于映秀-北川主断层下盘(扬子地块上),断层活动的主动盘是下盘,映秀-北川主断层的活动为俯冲兼左旋走滑;(2)此次地震是扬子板块向西俯冲(L型俯冲)、四川盆地顺时针旋转和青藏高原隆升逆冲相互作用的产物;(3)四川汶川地震有可能是构造地震的一种新类型,其孕育巨大能量的最重要条件为地块坚硬(能量不易释放)和位于地块边缘(构造作用强烈,有能量来源)。最后,建议有关部门和地学界重视中国主要地块边缘的重力场,尤其是剩余布格异常和均衡重力异常的研究。这也许对地震的长期预测有所帮助。