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前陆褶皱冲断带构造特征研究

Structural Characteristics of the Thrust Fold of Foreland

【作者】 刘殊

【导师】 马宗晋;

【作者基本信息】 中国地震局地质研究所 , 构造地质学, 2007, 博士

【副题名】以米仓山、龙门山前陆盆地及其褶皱带为例

【摘要】 研究目的和意义:随着油气勘探的深入,前陆褶皱冲断带的研究成为地学研究的热点。90年代后,安第斯山前冲断带发现一系列油气田,再次掀起了造山带前缘冲断构造研究高潮。1999年克拉2大型油气田的发现,促成了中国国内前陆盆地褶皱冲断带研究热潮。在此背景下,龙门山前、米仓前褶皱冲断带成为国内油气勘探的热点地区,引起众多地质学家的关注。2000年以来,中石油、中石化在山前带投入巨资进行油气勘探,得到大量的山前带的构造剖面,盆山耦合关系得到清晰的展现,取得重大的突破。目前条件下对前陆盆地褶皱冲断带进行研究,具有重要的理论意义和实践意义。龙门山前陆褶皱冲断带的研究已有相当长的历史,在构造变形特征、发育演化利形成的动力学机制等方面已经取得了很多较为明确的认识,但是对造山带分段性机理的研究较少,对龙泉山前隆的性质研究更少。龙门山北段发育侏罗山式复杂推覆带、中段发育推覆冲断褶皱带、南段发育叠瓦冲断带;中、上三叠世时期,中段山前为凹陷、两端为古隆起;龙泉山前隆只是存在于乐山—德阳的盆地中部。总体上,前陆盆地系统在龙门山中段较完善。其控制性机理一直缺乏必要的研究。在前陆盆地凹陷附近的斜坡上,已经发现大型的新场气田。显然,对龙门山前陆盆地构造差异性进行研究,具有理论上的创新,对实践也具有指导作用。米仓山构造位置位于上扬子板块向北凸出的边缘,米仓山北邻东西向秦岭造山带,西侧为松潘—甘孜造山带和北东向龙门山推覆构造带,东侧为大巴山北西—南东向弧形推覆构造带,南侧为四川盆地低缓变形区,米仓山实际上就是三个造山带中的一个构造结,对四川盆地的构造、以及沉积响应起着重要作用,是构造研究理想的场所。米仓山构造带是一个复杂叠合的造山带,具有多期次构造形成,多方向应力场复合的特征。米仓山也是一个较独特的造山带,关于米仓山造山带的性质研究一直较少。米仓山造山带模式也不同于龙门山和大巴山,和现有的造山带模式也有不同之处。研究表明,其形成过程由于扬子板块北端有一个米仓山—汉南刚性地体,可能在碰撞过程中被挤压而冲起。也有可能是刚形体两端形成断层,被推覆带铲出地表。对米仓山前构造样式的分析,直接能够指导山前盆地的油气勘探。对米仓山造山模式的研究,将丰富造山带构造理论。研究内容:(1)重点研究龙门山分段性形成机理。探讨推覆带内刚性地体和板块在前陆盆地构造演化中的影响;探讨盆地内构造差异在对山前构造样式的控制作用。分析前隆的控制因素。(2)采用构造结概念,分析米仓山地体在川东北盆地、山前构造演化中的作用。以米仓山地体为中心,分析三个地体:米仓山汉南地体-碧口地体—佛坪地体之间的碰撞和拼接关系;分析三个板块:秦岭—松潘甘孜—杨子板块之间的碰撞关系;(3结合龙门山、大巴山的构造演化,探讨米仓山前中、新生代以来的多边界盆山耦合关系。(4)详细对比米仓山前、大巴山前、龙门山前构造样式差异,分析造山带山前构造样式差异的控制性因素。(5)针对米仓山独特造山带性质,探讨刚性地体在造山带中的作用。研究思路:以龙门山前、米仓山两个互相关联的造山带的褶皱冲断带为研究对象。对比分析两个造山带的构造特征,研究造山带不同的形成机理;对比分析不同类型的盆山耦合构造样式,研究不同类型的构造样式控制性因素。以板块构造理论和大陆动力学为指导,结合模拟试验结果,运用盆山统一体的学术思想对褶皱冲断带进行研究。研究方法和技术:(1)在盆山结合带部位,利用野外地质调查对地表构造进行研究,利用石油地震勘探剖面分析深部构造特征,对比分析深、浅部构造的关系之间的表现形式,以及深、浅部构造之间的相互控制关系。(2)利用已有钻井资料、结合最新地震资料,建立两个造山带山前逆冲构造的几何学模式。(3)利用地震剖面所揭示的前陆褶皱冲断带特征,采用平衡剖面技术来恢复周缘山系的演化,建立造山带运动学模式;在较为简单的构造部位,直接用层拉平描述构造演化过程。(4)借用模拟试验结果,和典型的造山带山前构造进行对比,探索米仓山和龙门山山前构造样式形成的理论依据。(5)重点关注造山带附近的刚性体,研究刚性体在板块中的作用。主要成果和认识:龙门山前陆褶皱冲断带分段性因素研究取得如下成果和认识:(1)、龙门山北段:碧口地体起重要作用。三叠世以来,米仓山-汉南地体和碧口地体拼接;稍晚或者同时,米仓山-汉南地体和佛坪地体拼接;在晚三叠世晚期,三个地体拼接完成,龙门山北端造山形成天井山-摩天岭古陆;碧口地体碰撞在四川盆地内形成一系列NE向构造雏形,这些构造对后期的盆山耦合产生重大的影响,是龙门山山前构造分段性的主要原因之一。(2)、龙门山中段:在中段发育推覆带、山前凹陷和前隆,为比较完整的前陆盆地系统,与两端有明显区别。川西凹陷前陆盆地的形成由两个因素控制:动力来自松潘-甘孜刚性板块向东挤压;盆地因素为,从早三叠世开始,中段仍为海相盆地、两端为古陆。推覆带内的刚性地块和山前凹陷两者结合,成为川西凹陷前陆盆地的主要发育因素。龙泉山前隆只发育在龙门山中段、中南段。(3)、山前带内构造样式受到盆地构造控制:如果山前盆地存在隆起,容易发育薄皮推覆带,即南段新生代叠瓦冲断带、北段印支期侏罗山式推覆带;山前为凹陷则发育双重构造和反冲构造,即中段构造特征。米仓山前山前褶皱冲断带研究(包括大巴山前隐伏推覆带)取得如下认识和成果:(1)、米仓山是扬子板块北端的构造结。在构造结附近,三叠纪以来,以米仓山汉南地体为中心,米仓山汉南地体—碧口地体—佛坪地体先后相互碰撞;碧口地体和米仓山地体的碰撞相对在前;佛坪、碧口地体对米仓山汉南地体的碰撞形成复合应力场,大致方向为NW→SE;前后各个期次动力方向变化不大,山前构造为加强型。构造形成主体时间在晚燕山—喜山期。(2)、将米仓山前构造带划分为基底冲断带、山前冲断带、山前反冲三角带、山前褶皱带;建立盆山耦合样式:盆山之间以双重构造、反冲三角带过渡,推覆带发育程度相对低。(3)、米仓山山前构造样式的控制因素为:山前沉积盖层较厚;其次存在两个较厚滑脱层。两个原因的组合,使得米仓山前发育双重构造和反冲三角带。米仓山前基底异常也有一定作用,在碰撞过程中,在刚性体前端产生较强的剪切应力,形成冲断断层;由于嘉陵江膏岩层的存在,剪切应力不能向上传递,断层在膏岩层内滑脱,由此形成双重构造。(4)、米仓山前的构造分段性受到在早期的背斜、向斜控制。(5)、米仓山造山带是俯冲板块上的一个造山带,和通常造山带处于仰冲盘有较大的不同。米仓山造山带的形成机理为:尽管米仓山位于俯冲下盘,但扬子板块北端存在一个古刚性地体,在扬子板块和秦岭板块的碰撞过程,沿着早期火成岩边缘形成冲断层,基底冲出地表形成造山带—米仓山造山带。(6)、米仓山和大巴山的构造相互关系为:将米仓山看作一个刚性楔形体,楔入秦岭板块,因此米仓山和大巴山属于同一构造背景。在板块碰撞中,板块突出部—即刚性地体首先碰撞;刚性对碰结中间为残余洋。随着秦岭—扬子板块的拼接逐渐完成,残余洋逐渐关闭,时间稍晚。在刚性地体附近,构造变形自然发生旋转,形成各种弧形构造。7、NE向通南巴构造带上发育大量的NW向断层,是大巴山改造的产物。通南巴构造带中间发育NW向涪阳坝断裂带,在卫星影像上发现涪阳坝断裂延伸到普光气田。通过平衡剖面分析认为::早期是拉张断层,其后在大巴山推覆挤压作用下,拉张断层复活、反转,形成逆冲断裂带;这条断裂带上盘发现有生物礁,地表构造和地下构造具有一定联系,推测大巴山山前隐伏推覆带、盆地内存在大型礁滩相储层。

【Abstract】 The purpose of research: Along with the exploration of hydrocarbon reservoirs, the research of thrust fold of foreland become a geological hot point. A series of oil reservoir were discovered in the thrust fold of Andean foreland in the 1990s, which pop up a worldwide high tide of research on the thrust fold again. A domestic upsurge about the research of foreland thrust is also ignited by the discovery of the Kela-2 large field in 1999. Nowadays, the thrust fold of foreland of Longmen Mountains and Micang Mountain have became the focused area of exploration, and also attracts a lot of concern of geologists. In the 21st century, PetroChina and Sinopec have launched tremendous capital into the exploration in thrust fold of foreland for reservoir, and have attained a lot of seismic data which have described the coupling relationship between the basin-range clearly. Breakthrough has also been got in many fields. Under the present condition, the research about the thrust fold of foreland bears very important meanings in theory and practice.A number of geologists have studied Longmen Mountains in a very long time, and have got a lot of results, such as tectonic deformation, evolution of the Mountains, and dynamic character, and so on. However, there is very little concern about the segmentation character, and there’ s even less study about the Longquan forebulge. The difference between the segments of Longmen Mountain foreland is very prominent. The structural style in the north section is nappe of Jura type, and style in middle is a thrust fold. In the south imbricate fault. The basin before the foreland is also very different. In the early and middle Triassic, the basin in the middle section was a depressed basin, in contrast, north and south basin were palaeohigh. Longquan forebulge, the distribution range of which is from Leshan to Deyan, exists in the middle section only. The typical foreland basin system exists in the middle section. Oppositely, north and south foreland basin systems are not so typical. The controlling factors of all these difference lack essential research at all times. Xinchang gas field has been found on the ramp of the foreland basin, which means that the study about the differences of foreland possesses theoretical innovation, and also will steer the exploration in the thrust fold of foreland.The tectonic situation of the Micang Mountain is in the north marginal zone of Yangzi plate which is bulgy northward. Micang Mountain is adjacent to Qinling orogenic belts in the north, and closed to Songpan-Ganzhi orogenic belts and Longmen nappe in the west. There lie ark-like Dabashan Mountains in the east and distorted area of low amplitude in the south. As a fact, Micang Mountain is a structural knot among all these three orogenic bels, and bears key function to the tectonics and deposition of Sichuan basin. So, Micang Mountain is a perfect area to study structures. The tectonic belt of Micang Mountain is a complicated and superimposed one which was built up in many times by multi-directional stress. The orogenic model of Micahn Mountain is very unique as well, which is different from Longmen and Daba Mountains, and also different from general style of ordinary orogenic belts. The characteristics of Micang orogen lacks necessary research for a long time. Study of this thesis has shown that there is a Micang-Hannan rigid block situated in the north marginal zone of Yangzi plate. When the plates collided in Triassic, the basement of Yangzi plate was extruded up, or scraped up by the overthrust on the two sides of the rigid block. The conclusion of structure style will instruct exploration of reservoirs directly and enrich orogen theory as well.Contents of research:(1)The highlight of this thesis is to study the mechanism of Longmen Mountains’ formation, and discuss how and why the rigid block in the nappe dominate the evolution of foreland, and probe into what is the controlling factor which influences the structural style of the foreland region. The key factor of forebulge evolution is also analyzed.(2) The concept of structure knot is adopted to analyze what kind of the function Micang-Hannan rigid block possesses in the evolution of foreland and the deposition in the basin before Micang Mountain. The collision and assembly relationship among three rigid blocks, Micang-Hannan rigid block, Foping rigid block and Bikou rigid block, is analyzed, which focuses on Micang-Hannan rigid block. The collision of three plates—Qingling, Songpan-Ganzi, and Yangzi, is also discussed.(3) Integrating evolution of Longmen Mountains and Micang Mountains, the research discusses the coupling relationship among the multiple barriers between the basin and range in the Mesozoic era and Cenozoic era.(4) Carrying through a detailed interpretation to compare the differences among the Longmen Mountains, Micang Mountains, and Daba Mountains; Analyzing what is the key factor which dominate the style of foreland tectonic.(5) Aiming at the particular characteristic of Micang Mountains to discuss how and why a rigid block control orogen evolution.Thinking of Test and Evaluation: Aiming at the three orogenic betls which are interrelated with each other, the research takes thrust folds as the study target before the Longmen Mountains and Micang Mountain, and explores the characteristics and formation mechanism of thrust fold. This work analyze different structure styles and different couplings between the range and basin, takes the theory of plate tectonics as guide, integrates simulation test and exerts united theory about the Mountain and basin to analyze the orogenic tectonic style.Research methodologies and Technique:(1) In the belt of transition of range and basin, the study analyzes surface structure on the base of field survey, takes the full advantage of seismic data to describe the characteristics of deep seated structures. Then research goes on to compare the structural pattern in the shallow and deep layer and get their interinhibitive relationship.(2) Making use of available drilling data and newest seismic data, introducing newest interpretation method to constitute the geometry pattern and kinematics style of thrust fold of two orogenic belts.(3) By means of analyzing the characteristics of superimposed tectonic deformation in the shallow and deep strata which is revealed by seismic profile, using balanced section to buildup evolution style of Mountains in the periphery. Flattening method is also used to analyze simple structure of foreland and describe evolution of Mountains directly.(4) Drawing support from simulation test, comparing the structure pattern studied with general tectonic patterns of ordinary orogenic belts, analyzing the style of foreland thrust studied and finding out the theory evidence.(5) Paying stress attention to function of rigid block near the orogenic zone, seeing about the effect of rigid block.Main conclusions:The main conclusions about segmentation character of Longmen Mountains are as follows:(1)In the north segment: The Bikou rigid block possesses very important function. Since the early Triassic, Micang rigid block got assembly with Bikou rigid block. Some times later or at the meantime, Micang rigid block was pieced together with Foping rigid block. In the late Triassic, these three rigid blocks fulfilled assembly with each other. Because of the collision and assembly of these three rigid blocks, an ancient land of Muotianling and Tianjingshan became into being in the north of Longmen Mountains, and a series of embryonic tectonics were generated which yielded grand influence on the late evolution of couplings between the range and basin.(2)In the middle segment: The nappe and depression and forebulge in the middle segment constitute a typical foreland basin system. The deformation of Chuangxi depression of the foreland basin is controlled by two factors. The first one is that the dynamic came from Songpan-Ganzi plate which pusheed towards east. The second one is the basin factor, which is there is marine basin in the middle, but old land on the tow terminal side since Triassic. The rigid block integrated with the depression in the basin is the main reason which caused the development of foreland basin. The Longquan forebulge also developed in the middle foreland basin.(3) The structural styles are also controlled by structures in the basin. The result is that if there is anticline before the foreland, a thin skinned nappe structure, that is imbricated thrust zone in the south and nappe of Jura style in the north, will come into being. If there is a syncline before the foreland, a foreland basin developed, that is Chuangxi foreland basin system, and the structure styles in the middle are duplex and recoil structure.The main conclusions got from the study of Micang Mountains (including Daba Mountains):(1) Micang Mountains is a structural knot in the north fringe of Yangzi plate. There are three rigid blocks near this knot which takes Micang rigid block as center. Since the Triassic, Micang rigid block collided with Bikou rigid block and Foping rigid block one after another. The collision of Micang rigid block with Bikou rigid block may occur a little earlier. The collision of these three rigid blocks generated a compound stress, the orientation of which was NW→SE and did not change a lot. Therefore, the structural style of foreland formed in later time succeeded to the earlier structure pattern, which is called reinforced structure. The predominant period of orogenic development was late Yenshanian and Himalayaian era.(2) The structural belt of the Micang Mountain foreland can be divided into sole thrust, thrust of Mountain front, recoiling triangle zone, and folded belt of Mountain front. The coupling structural style between the Mountain and basin is duplex or recoiling triangle, and the nappe structure does not develop very well.(3) The controlling factor in the Micang Mountain front is: Firstly, there is a thick capping formation, and secondly, there are two thick decollement layers. That these two factors integrated with each other built up duplex and recoiling triangle zone in the Micang Mountains front. The blocks in the basement possessed also some function. When collision happened, a shear stress field came into being which engendered thrust faulting. Because of soft gypsum rock of Jialingjiang formation, shear stress cannot transmit into the upper layer and duplex is generated by shear stress in the down strata.(4) The section character in the foreland of Micang Mountains was controlled by anticline and syncline which formed in the earlier time.(5) The orogenic belts of Micang Mountain are in the descending slab of the Yangzi, which is different from those in the obduction belts. The formation mechanism of Micang Mountain orogenic belts is: In despite of position situated in the descending plate, there is a palaeo-rigid block in the border of the Yangzi plate. When the Yangzi plate collided with Qingling plate, thrust fault toke place along the fringe of early igneous rock and the basement breakthrough the earth’ s surface, Micang orogenic belts came into being.(6) The structure relationship between the Micang Mountains and Daba Mountains can be pictured in this way: The Micang rigid block was taken as wedge and wedging into the Qingling plate, so the orogenic evolution of these two orogen was under the same structural setting. During collision, the salient of plates encountered with each other in the first time, and there was a remnant ocean basin between the collision points. The remnant ocean basin closed up gradually in the later time. Tectonic deformation circulated certainly near the rigid block and ark-like structure came into being.(7) There are a lot of faults whose orientation are NW and are generated by Daba Mountains’ movement in Tong-Nan-Ba structural belt. In the middle of Tong-Nan-Ba structure belt there is a Fuyangba fault zone which extends into Puguang field on the satellite picture. The analysis through balanced section has attained the conclusion: This Fuyangba fault zone is a tensional fault which was revived and reversed by push and extrusion of Daba Mountains’ movement. In this way a thrust belt came into being. The reef has been founded in the hanging wall of the thrust belt, and the surface structure has some relationship with underground structure. So a reasonable prospecting can be made that there may be a big reservoir of reef flat facies under the cryptic nappe structure of Daba orogen in the basin.

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