| Among the Songpan-Garze plateau, there are four adjacent peaks that comprise the Great-snow Mountain Range, which is called "Four-girl Mountains". They are located in Rilong Town on the border of Xiaojin County and Wenchuan County, Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province, it is260km from Chengdu. The four mysterious peaks of the Four-Girl Mountain are surrounded by more than10snowy peaks, among which, the4th peak is the highest, which is aslo called "the Queen of Shu Mountains" with an elevation of6250meters, which is the second highest peak in the West Sichuan Plateau, second only to Gonggar Mountain with an elevation of7556meters. Another, the elevations of the1st peat,2nd peak and3rd peak are5355meters,5454meters and5664meters respectively. Like four girls, they are as white as ice and as pearly colored as jade, and are very pretty and charming. They are collectively called "the Four-Girl Mountain". The precipice of the4th peak is so high that nobody has ever climbed it. Today, it is a scenic area at national Four-A level and National Geopark, and has become a tourist site frequented by domestic and foreign mountain clembers. It is a tourist site with multiple functions for people to enjoy sightseeing, mountain climbing, exploration, scientific investigation, holidays and outing.The Tibetan Plateau uplift are a focus of geologists all over the world.The process of Songpan-Garze terrane uplift in east Tibetan Plateau at late Cenozoic which plays an important role in environmental effects of the Chengdu economic zone. The Four-Girl Mountain uplift process is a wonderful example of the Songpan-Garze terrane uplift, and its study of uplift have never engaged by predecessors. The research results of its uplift have important scientific and economic significance.This thesis is focusing on field investigation Quaternary and granites in the Four-Girl Mountain area, based on a large of information comprehensive collection and analysis in the Songpan-Garze terrane about the surface geology, geophysics, GPS measurements, regional stress field, and so on. This thesis comprehensively summarized the process of surface uplift in the Four-Girl Mountain area, according to study on river terraces of the Min River, the Dadu River, Yalong River, and plateau planation surface, plateau loess, karst, biotic community, and so on.This thesis comprehensively analyzed the process of crustal uplift in the Four-Girl Mountain area and its adjacent, according to study on thermochronology and fission track of the Siguniangshan Granites, the Nianbaoyuze Granites, the Sexilaka Granites, the Mengtonggou Granites, the Yanggonghai Granites, and so on. And this thesis deeply discussed the effect of plate tectonics, thrust faulting, isostatic gravity anomaly to uplifting process in the Four-Girl Mountain area,according to the comprehensive analysis research of seismic wave of crust and upperMantle,Magnetotelluric, regional force of gravity and airborneMagnetic, and variations of GPS horizontal vector and velocity field, and so on.1. The Songpan-Garze Plateau uplift is characterized by episode, whole coherence, non-entire synchronous in the Four-Girl Mountain area.(1) The pisode uplift is that uplift process could be divided into multistage evolution, namely planation surface phase, mountain phase, freezing physiognomy phase in the Four-girl Mountain area. Miocene epoch is forming period of the planation surface. The A episode of the Qingzang tectonic movement is representative of uplift start in the Pliocene,about3.6Ma. The uplift accelerated markedly since the late Pleistocene, but the uplift rate became low at Holecene, suggesting that it is now in a new stage of uplift acceleration inthe Four-Girl Mountain area.Five apatite fission track samples were collected from various altitude in steep slope relief on the Four-Girl Mountain granites of theMain thrust fault hanging wall in the middle segment of the Longmen mountain thrusting fault zone. Its ages and length are analyzed that fission track ages from the steep slope relief area of elevation3071-4460meters are ranging from12.8±1.6Ma to3.1±0.6Ma. The Yanggong Lake granites shows that the apatite fission track ages of five samples are from1.4±0.5Ma to0.3±0.2Ma which is relatively younger. The Xilaka granites in Seda county shows that the apatite fission track ages of four samples are from144.7±13.0Ma to17.8±2.2Ma which is relatively older. The Lianbaoyuze granites shows that the apatite fission track ages of two samples are3±0.8Ma and2.7±0.8Ma individually,but its zircon fission track ages of two samples are122±12Ma and91.1±8.6Ma individually which is relatively older visibly. The Mentong canaliculus granites shows that the apatite fission track ages of three samples are6.6±2.0Ma,7.3±1.4Ma,and3.9±1.2Ma individually,and the Laojun canaliculus granites shows that the apatite fission track ages is4.0±3.2Ma. And besides, the Pengguan Complex of the Longmens mountain fault zone in the eastern Four-girl Mountain shows that the apatite fission track ages of two samples are8.7±5.6Ma and10.5±7.2Ma individually, and the Baoxing Complex shows that the apatite fission track ages of two samples are8.7±5.6Ma and10.5±7.2Ma individually. And as evidence of comprehensive analysis of combine the Yanggong lake granites, the Mentong canaliculus granites, the Laojun canaliculus granites, the Xilaka granites, and the Lianbaoyuze granites, thermal history modeling shows that there were multi-stage uplifting and exhumation since middle Miocene.(2) A large amount of work of field surveys and interior research have indicated that are4to8terraces in the Min River Valley,4to7terraces in the Dadu River Valley, and at least12terraces in the Xuanshui River Valley. The whole coherence uplift is that river terraces shows have13times at lowest on territorial plateau uplift during the period of livelong uplift in the Four-Girl Mountain area, and every times of uplift have all recorded by layered landforms depositions of the late Cenozoic, their ages are1130ka、857ka、772ka、586ka、472ka、315ka、253ka、144ka,97ka、77ka、52ka、23ka、11ka respectively.Four ages about1130ka、772ka, 586ka、144ka in which could be correspond to the A,B,C episode of the Kunhuang movement,and Gonghe movement. And two times uplift about1130ka and680ka which can be compared with apatite fission track of granitic gneiss from the Namche Barwa Group in eastern-Himalayan syntaxis, although surface uplift event at680ka in the Four-Girl Mountain area which be recorded only in the Dadu River Valley.The whole coherence also shows that plateau uplift have same time and rate between the Four-Girl Mountain area and the East Himalayan orogenic belt in middle to late Miocene.The Four-Girl Mountain Granites’s average uplift rate is0.17mm/a from12.8Ma to8.2Ma,we can be compare it in times and rates to uplift event of that predecessors had undertaken granite ages of apatite fission track with different height above sealevel,and its rate in accordance with granites of Lhasa, Gaowu, Dazi in same time.(3) Non-entire synchronous uplift is that the Songpan-Garze Plateau uplift had undergone a process from peripheral influence to central significance,namely from its plateau edge to plateau hinterland. Among its edge, the river terraces of Ludin,Maoxian, Songpan had given some records at157ka firstly, whereafter its hinterland the river terraces of Jinchuan, Luhuo, Lixian have given another records at140ka. And uplift rate is different from everywhere more or less at same episode.2. Apatite fission track informations shows that the plateau uplift started since Neogene also. The plateau uplift development have been through four phases in the Four-Girl Mountain area, namely extension epeirocratic period, planation surface period, mountain buliding period, and cryosphere formation period,and it has a history of about200Ma. This includes:(1) It was extension epeirocratic period from Jurassic to Paleogene, hided the Four-Girl Mountain Granite Pluton deeply under the ground. It be considered as being about balanced elevation to the denudation in the Songpan-Garze terrane. Its uplift rate is slow speed, absolute uplift rate is only0.017-0.081mm/a, and relative uplift rate is only0.0039-0.0275mm/a. The Eucalyptus communities from the Relu Formation shows that the Songpan-Garze terrane had been uplifted to the altitudes of about500-600meters above sea level at the terminal Paleogene. Palaeomagnetic informations shows that in Mesozoic tectonics movement form of the Songpan-Garze terrane are quite a different to present GPS monitoring results. This indicate that the plateau uplift must started after Mesozoic in the Four-Girl Mountain and adjacent area. (2) It was planation surface period from early Miocene to early Pliocene.Its absolute uplift rate is more than a bit denudation rate in the Four-Girl Mountain area, plateau planation surface was formed into1000meters above sea level at early Pliocene. Its absolute uplift rate is only0.5-1.625mm/a, and granites denudation rate is only0.1678-0.2878mm/a. And a lot of the granites uplift rate of apatite fission track shows that the forming process of plateau planation surfaces can be roughly divided into18-7.3Ma and6.6-5.1Ma two periods, and two levels planation surfaces were formed individually. The height of two plateau planation surfaces is only about4500-5500meters above sea-level in the Four-girl Mountain area, theMain planation surface in which elevation is about200-300meters shorter than its summit plan. And these two plateau planation surfaces in the Four-girl Mountain area is taller than its adjacent area. The altitude didn’t exceed1000meters when the two plateau planation surfaces were just formed, the end period of two planation surfaces formation is initial period of plateau uplift.(3) It had been unlocked overture of plateau uplift in the Songpan-Garze terrane by the A episode of the Qingzang tectonic movement at Pliocene (about3.6-1.8Ma), it began to mountain buliding period in the Four-Girl Mountain area. Its absolute uplift rate had doubled and redoubled by1.86-2.03mm/a, and granites denudation rate increased also slightly by0.15-0.4mm/a. It lead to that uplift size had achieved about3000meters above sea level at Late Pliocene by the difference between uplift rate and denudation rate, and Dayi Conglomerates were sedimented in the front of the Longmen Mountians.It was beginning to show that the mountain buliding speed more faster than past in Kunhuang tectonic movement of Pleistocene from1.8Ma B.C. to0.126Ma in the Four-Girl Mountain area. Its absolute uplift rates were increasing in that episode in a geometrical progression, its denudation rate had also doubled and redoubled,and its granites uplifting rate was significantly faster than denudation rate and river incision rate. They had been left a lot of redcords by the intermediate-acid intrusive granites, river terrace, alluvial fan and proluvial fan,plateau loess,and so on.This is that granites absolute uplift rate is3.63-17.06mm/a, and denudation rate is1.06mm/a. Its average incision rate is0.535mm/a in the Dadu River Valley, its average incision rate is0.493mm/a in the Min River Valley, and its average incision rate is0.45mm/a in the Xuanshui River Valley, the threee rates are approximate. It caused by plateau uplift rate was significantly more than denudation rate and incision rate in this period, the plateau has been uplifted to an altitude of more than4000meters above sea level at Late C episode of the Kunhuang tectonic movement in the Four-Girl Mountain area, the Mountain landform of the Four-Girl Mountain had basically taken shape, and Yaan Conglomerates were sedimented in the front of the Longmen Mountians. When the plateau uplift size is high enough so that the power of plateau monsoon was effected, and eolian deposits were formed in Ganze loess, Luhuo loess, Jingchuan loess, Lixian loess,and so on.(4) It is cryosphere formation period during the Gonghuo tectonic movement from late Pleistocene to Holocene in the Four-Girl Mountain area, which start age is about40ka, corresponding approximately to11th stage of surface uplift in the Four-Girl Mountain area. Its surface uplift rate is a modest slowdown. The1st to3rd river terraces were widely produced along the Min River, the Dadu River, and the Xuanshui River by the incision action of the Gonghuo tectonic movement. A large amount of alluvial fans and proluvial fans had been washed down from the Min River, Tuo River, and Qingyi River aquo-system, and had been deposited in the front of the Longmeng Mountain, and there are two layers which chengdu clay wrere deposited in the Chengdu plain. All these causes the high mountain landscapes of an altitude of more than6000meters above sea level in the Four-Girl Mountain and adjacent area now.Since the late Pleistocene, its incision rate is0.63-3.18mm/a in the Min River Valley. All the surface uplift rates become slower at the rest of the section. At the Wenchuan County Section and the Yinxiu Town Section of the mainstream of the Min River, of which passes through the Longmen mountain fault bel, the surface uplift rates continues to growt. At the Ludin Section of the Dadu River Valley, the surface uplift rate have begun to drop off significantly which it dropped from1.14mm/a of the past to0.67mm/a of the present. At the Fubian River Section of the Dadu River Valley in Xiaojing County, which it dropped from1.44mm/a of the past to1.13mm/a of the present. At the same time, its incision rate is0.78-1.87mm/a in the Dadu River Valley.3.The granitoid intrusions occurred in the eastern part of the Songpan-Garze terrane from the Late Indo-Chinese epoch to Early Yenshan Epoch. The Four-Girl Mountain Granitic (FGMG) is one of them which these zircons has characterisitc of magma zircons widely. According to the LA-ICP-MS zircon U-Pb dating of the23 crystal zircons from biotite granodiorite of the FGMG which yielded a weighted206Pb/238U mean age of (191±1)Ma, which is interpreted as FGMG’s magma crystallizaton age(Early Yenshan).It was proposed during the regional geological survey that previous the NW-trending Jingchuan-Lixian tectono-magmatic belt, they can’t represent products of the collision between he Yangzi massif and the Songpan-Garze terrane, although the major axis and mineral direction of most of the single granitic masses exist in the parallel arrangement of the NW-trending in the Songpan-Garze terrane. The NE-trending Douhu-Jingchuan-Xiaojing-Heishui tectono magmatic belt should be established newly. The FGMG is one representative granitic mass of the them. It is represent products of part fusing in the low velocity layer exists in the middle-lower crust of the Songpan-Garze terrane what the Songpan-Garze terrane be inserted along the Longmen mountain structural belt generally by the Yangzi massif.4. There are have complex dynamics mechanism why the plateau uplift in the Four-Girl Mountain area. Many factors have positive effect on plateau uplift for example deep tectonic difference, isostatic gravity adjusting, the hanging wall of main thrust. We look at the problem on plateau uplift dynamics mechanism in the Four-Girl Mountain area, we must first consider the its block boundary condition, and that affects the working procedure or process.(1) Block Boundary ConditionThe Songpan-Garze terrane is a great block of inverse triangular. On the plane, its motion of block are influenced or controled by boundary condition, that is surrounded by the Longmen mountain fault zone, the Xianshui river fault zone, the East Kunlun fault zone. Among of them, the Longmen mountain fault zone is a large lower angle thrust fault zone, and consists of a group of NE trending subordinate faults. It passes through the east of the Four-Girl Mountain, which are dominated by reverse slips with dextral strike-slip component. And the Xianshui river fault zone is a sinistral strike-slip faults.The regional tectonic stress field between the two zones shows that the direction of compressive principal stress σ1is in E-W and the tensil stress σ3is in S-N. Now, this analysis accords was supported by result of the GPS surveying in recent years.Vertically, its depth of tectonic influence can reach middle and lower crust, even the top of upper mantle. In this block of inverse triangular, interpretation of geophysical data shows that there exist evident differences in crustal structure and thickness between the Songpan-Garze terrane and the Yangtze block, and middle and lower crust have a series of velocity layers of seismic wave and low resistivity layer of magnetotelluric in the Songpan-Garze terrane. But such velocity layers and low resistivity layer aren’t found in the Yangtze block. It was not until upper mantle these differences start to reduce gap in deep tectonic structure between the Songpan-Garze terrane and the Yangtze Block. The Four-Girl Mountain granites attributed to partial melting occur that have some relevance for the low velocity layers in the depth of20-37kilometres in Four-Girl Mountain area extensively.(2) Process of Plateau UpliftDeep geophysical exploration shows that there are several great "root of mountain" in the Four-Girl Mountain area, and mainly these roots of mountain is made up of many great granitic plutons. And isostatic gravity anomaly have some obvious differences between the Four-Girl Mountain area and its adjacent regions.As as a result of compression and collision of the Indian plate with the Tibet Plateau from south to north, the various spheres and layers in the crust and mantle interior in Tibet Plateau are taken to level lateral movement for responsive adjustment, and the weak layer material of low velocity layer exists in the middle crust and lower crust of the Songpan-Garze Plateau are flowed as lateral movement from west to east also.As as a result of difference material in the crutst and mantle between the Songpan-Garze terrane and the Yangtze block, a powerful near NE-trending regional stress field was produced by that emanates from obstruction of the Yangtze block, when the low velocity layer exists in the middle crust and lower crust of the Songpan-Garze terrane flows eastward to the Four-Girl Mountain area. Add the isostatic gravity adjustment of a great "root of mountain" are superimposed on each other in the Four-Girl Mountain area. Furthermore, it is superimposed on each other which this area is isolated in hanging wall of the Longmen mountain thrusting fault zone. The maximum rolling and maximum skyscraping earth’s surfaces in the Four-Girl Mountain area, which have made as a "the Queen of Shu Mountains", that is to say, it is the2nd highest peak in the West Sichuan Plateau. It is formed under the together actions of mentioned above three forces, and strong weathering processes, rugged denudation, violent down-cutting and exhumation. |
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