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Structural Characteristics Of The Thrust Fold Of Foreland

Posted on:2008-05-13Degree:DoctorType:Dissertation
Country:ChinaCandidate:S LiuFull Text:PDF
GTID:1100360212495129Subject:Structural geology
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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.
Keywords/Search Tags:foreland basin, thrust belt, Longmen Mountains, Micang Mountains, Daba Mountains, Tong-Nan-Ba structure belt, structure knot, duplex, recoiling triangle zone, structural block, imbricate thrust, nappe of Jura style
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