Study On Current Movement Characteristics And Numerical Simulation Of The Main Faults Around The Eastern Himalayan Syntaxis

The area around the Eastern Himalayan Syntaxis(EHS) characterized by most intensive tectonic deformation, complicated tectonic types, and well-preserved geological traces. This area contains the Himalalyan, Lhasa, Qiangtang, and Sichuan-Yunnan block , involves the Indian Plate and the main boundary fault, the main central fault, the Yaluzangbu fault, Jiali fault, the Nu jiang fault, Motuo fault and Apalong fault. It can be thought that the area around the EHS is one of the key areas testing the different theories and doctrines about late Cenozoic deformation mechanisms of the Tibetan Plateau. As this area lies in mountains above 4000m with valley-mountain landscapes of Yaluzangbu jiang, Nu jiang, and Lancang jiang, bad working conditions and various reasons such as complicated geological structure, make it difficult to use a single way to determine the movement field of tectonic deformation and to conduct quantitative research of the faults with different natures and different ways of current movement on different structural parts in a short time. Therefore, this thesis uses a various of methods to make a comprehensive analysis of the tectonic deformation and the movement characteristics of main faults in the area around the EHS. They include collecting data of geometric and active features of tectonic blocks and boundary faults by field investigation, crustal deformation and the movement type of the main faults by high- accuracy of GPS observation technology with more observation sites in key tectonic parts, physical parameters and deep geological structure of crust by the comprehensive analysis of geophysical exploration data, discussing the movement features of main faults and the kinetic mechanism of tectonic deformation by large-scale numerical simulation analysis.Ⅰ,Research methodsThis work, uses three methods to study the crustal deformation velocity field and motion styles of the main faults movement:(ⅰ) Traditional geologyField survey is an indispensable way of geological research. It can directly observe the active features of main faults in the historical period. By analysis of previous field investigations and GPS data, several places are selected for field geological investigation, which are the Yaluzangbu fault, Jiali fault and Nu jiang fault and the area with different views in previous studies. During the investigation, the study was focused on the cross section of the main faults which are active since the late Quaternary, to obtain more geological evidence about the movement features of major faults in this area since the late Quaternary. (ⅱ) GPS observation and analysisIn order to study the present tectonic deformation features of the EHS and the surrounding areas deeply, this work analyzed the long-term observation data of GPS sites for "China Crustal Movement Observation Network Project" in the study area systematically. In addition 12 new GPS observation points in the areas of lack GPS sites were built, to conduct two-period observations. In this thesis, GPS velocity profiles across the fault and the fault dislocation model were used for studying the movement characteristics of the main fault of the different structural sections. The former decomposed the velocity vector of GPS measured points on both sides of fault into the velocity components perpendicular to the profiles, which reflect the dip-slip motion of the fault and the velocity components parallel to profiles, which reflect the strike-slip movement of the fault. The difference between the both sides of the fault representes its motion characteristics. The latter used the OKADA dislocation model to retrieve movement rates and characteristics of the fault.(ⅲ) Finite element numerical simulationThe finite element method can reconstruct the geological process and make activity prediction. Based on the three-dimensional finite element method, regarding main faults as boundaries, using the parameters of lithosphere in different areas of the Tibetan Plateau from geological and geophysical data, the tectonic deformation model for the EHS is established. Using the latest GPS observation data of"China Crustal Movement Observation Network", and data of"China Continental Tectonic Environment Monitoring Network Project"and project"The main faults'current movement observation around Eastern Himalayan Syntaxis area"as the boundary constraints of displacement and the movement rates. This work has made a numerical modeling for the main faults'movement features.Ⅱ,Research contents and results(ⅰ) Movement characteristics of the main faults in Late QuaternaryThrough field investigation, it was found that the Yaluzangbu fault was active significantly from the eastern section of Long County, and that the fault has cut the late Quaternary strata in many places. The Jiali fault can be divided into three sections, and the EHS is its boundary. The northwest section (Naqu-Tongmai) displayed dextral strike-slip, with a rate of 3.2～3.7mm/a. The EHS region (Tongmai Section) has dextral strike-slip, and its rate is slower than the northwest section of the Jiali fault. The active characteristic of the southeast section of the Jiali fault (Tongmai-Chayu) was changed, as shown that the moraine ridge near the Galong Temple was sinistral dislocated by the Jiali fault with the slip rate of about 3.8mm / a. The Nu Jiang fault mainly showed the squeezing property from Guoqing to Tiantuo, and showed no significant activity since Late Quaternary. There is no found that the geological profiles dislocated the Late Quaternary deposits and landscape, and shows no significant activity since Late Quaternary. The south section was active obviously, and the Late Quaternary strata were dislocated in many places. The geomorphy of the northwest section of the Jinsha Jiang fault indicates that the direction of a series of river gullies was changed by this fault, which resulted in sinistral twisting river gullies and obvious sinistral movement. Along the middle- south section of the Jinsha Jiang fault, the development of river gullies was affected since the late Quaternary, which resulted in dextral twisting river gullies and many places of the Late Quaternary strata dislocated.(ⅱ) Results of GPS observation profilesThe research of the GPS profiles across the faults and dislocation model revealed the nature of the faults and the movement rates.The Yaluzangbu fault has a clear dextral strike-slip characteristic. The strike-slip rate of Lhasa section of this fault is 2.4 ~ 3.9 mm/a, and the shortening rate is 1.3 ~ 4.7 mm/a. The Linzhi section has the slip rate of 6.2 ~ 6.8 mm/a and the shortening rate of about 0.6 ~ 6.0 mm/a, exhibiting a dextral-compressive motion.The northwest section of the Jiali fault near Naqu shows dextral compression, the strike-slip rate is 4 ~ 5.8mm/a, and the shortening rate is about 4.6mm /a. The fault near Tongmai has weak dextral compression, the strike-slip rate is 1.3～2.0mm/a, and the shortening rate is about 2.5mm/a. The fault near Chayu shows sinistral compression with a strike-slip rate 3.7～4.0mm/a, and shortening rate is about 6.2mm/a.The northwest section of the Nu Jiang fault zone near Nagqu mainly shows the compressive movement at a rate 1.2 ~ 2.0mm/a. The southeast section is mainly of dextral strike-slip with a rate gradually increasing from the north (the strike-slip rate of 2.1m /a) to south (the strike-slip rate of 3.2mm/a).The Jinsha Jiang fault in the vicinity of the Qinghai-Tibet Highway shows sinistral-compressive movement where the strike-slip rate is 3.0 ~ 4.0mm/a, and the shortening rate is about 3.5mm/a. The faults near Cangdu, Jiang Da and Baiyu mainly show dextral-compressive movement with strike-slip rate 3.4 ~ 4.3mm/a, and the shortening rate 1.8 ~ 2.9mm/a. The faults near Batang and Derong mainly show dextral compressive movement, with a strike slip rate 3.0 ~ 3.1mm/a, and the shortening rate 0.4 ~ 2.0mm/a.(ⅲ) Results of the finite element numerical simulationⅰ)The blocks north and east of the EHS rotated clockwise around the EHS. The southeast boundary of the dextral strike-slip fault is not Jiali fault, instead likely the Apalong fault.ⅱ)The Jiali fault was not an overall dextral strike-slip fault. Its northwest section and the Tongmai section at the top of EHS have the dextral compressive properties, and the kinetic property of the southeast section of the EHS has changed from the dextral strike-slip into a sinistral strike-slip movement. If the southeast branch of the Jiali fault was not connected with the Sagaing fault, the simulation results have a better fitting to the GPS observations when the Apalong fault connects the Sagaing fault, and this result indirectly confirms that the Jiali fault may not be connected to the Sagaing fault current, at least not simply connected. The Apalong fault and Sagaing fault may be linked.ⅲ)The EHS is still playing a role at present, which together with the Assam horn affects the current regional tectonic deformation, and many of the fault activity conversions and important tectonic events occurred in or close to the region between them. The change of the movement rates and activity of the Yaluzangbu fault, the Nu jiang fault and the Jiali fault ends close to the Assam horn, so does the Jiali fault. The Apal Long fault is active highly, where occurred the Ms8.6 Chayu earthquake in 1950, all of which may have a relationship with interaction between the EHS and the Assam horn.Ⅲ,Main conclusionsThrough comprehensive analysis, this work has obtained some new recognitions:ⅰ) There is apparent dextral slip on the Yaluzangbu fault since late Quaternary. The activity increases from west to east. The dextral slip rate of its Lhasa section is 2.4~ 3.9mm/a. The dextral slip rate of its Linzhi section is 6.2~ 6.9mm/a.ⅱ) The Jiali fault isn't an overall dextral strike-slip fault. Its movement features and rates are variable in different tectonic parts. Generally, it can be divided into three sections by EHS. The fault in west area of the EHS is the northwestern segment of the Jiali fault, the fault in Yigong-Tong mai area on the top of the EHS is the middle segment, and the fault in the southeast area of the EHS is the southeastern segment. The northwestern segment is right-lateral compressive movement, with a strike slip rate 3.2~ 5.8mm/a. The middle segment has weak dextral compressive movement, with a strike slip rate 1.3~ 2.0mm/a. And the southeast segment is left-lateral compressive movement, with a strike slip rate 3.7~ 4.0mm/a.ⅲ) The Nu Jiang fault has different activities in different tectonic parts since late Quaternary. The main activity of northwestern segment of the Nu jiang fault is compressive movement. The activity of the middle segment of the Nujiang fault is not obvious in geology since late Quaternary, but with a strike slip rate 2mm/a by GPS. The activity of south segment of the Nu jiang fault is obvious. It can be seen that the fault dislocation of the late Quaternary strata is present in many areas, and the strike slip rate 2.3 ~ 3.2mm/a.ⅳ) The Jinsha Jiang fault's activity was also different in different tectonic parts since late Quaternary. In the northwest segment, the Jinsha Jiang fault affected the flow of a series of gullies, causing gullies to experience left-twist, sinistral movement significantly, and the strike slip rate 3.0 ~ 4.0mm/a. The activity is not obvious in middle segment in geology since late Quaternary, but with a strike slip rate 3.4 ~4.3mm/a by GPS. In south segment, the Jinsha Jiang fault affected the development of water-gully systems, causing gullies to undergo right-twist, dextral movement significantly.ⅴ) The northern and eastern blocks of the EHS rotate clockwise around the EHS overal. The southern boundary dextral strike-slip fault is not the Jiali fault, but the Yaluzangbu fault and Apa long fault.ⅵ) Numerical simulation results show that the Jiali fault and Sagaing fault may not be linked, at least not simply connected. The Apa long fault and the Sagaing fault may be connected.ⅶ) EHS still plays a role at present, and affects the current regional tectonic deformation together with the Assam horn. Some fault conversions and important tectonic events occurred between them or very close to the region.