Lithospheric Structure Of Near North-south Striking Rifts In Tibet Plateau And Its Geodynamical Process

Near north-south striking rifts in Tibetan plateau is one of the most obvious tectonic styles in TibetPlateau. With continuous study and cognition, Rifts in Tibetan plateau have recently become the hot focus onTibetan Plateau. At present, there are two main completely different arguments about mechanisms of those rifts.One viewpoint is that the formation mechanism of rifts is relative to uplifting of Tibetan Plateau. The otherrelates to far-ranging rifting in Asian. According to the first viewpoint to rift mechanism, some behavior ofthose north-south strike rifts must exist in the whole plateau, and its tectonic features is all the same. To thelater one, it is all known that Shanxi Graben and Bakal Rifts form under extending environment. But, thosenorth-south strike rift in the plateau form under south-north extrusion tectonic condition during orogeny. Thetwos have different regional tectonic backgrounds.To post the scientific mystery of formation mechanisms of rifts in Tibet Plateau, many earth scientistsfrom all of the world have intensively focused on the problem, surveyed and researched it by geologicalobservation, focus solution to earthquakes, Remote Sense Images and geochemistry analysis. Although manyachievements and cognitions have acquired, they mainly reflected near surface structures. And, the deepsubsurface structure and tectonics backgrounds of those rifts is less known. However, those features is the keyto understand and acknowledge its cause of formation about those rifts. So, some key scientific problem asfollowing had to answer at first.(1),How do those rifts extensively exist in the surface to distributes distribute in the deep crust? (2),Doesexist the deep tectonics that control those rifts' distributions in the inner lithosphere under Tibet Plateau ? If itdoes, where it is? Either Karakulum-Jiali faults along the middle parts of Bankong Co-Nujiang Suture or not?(3),Those rifts are the mark of plateau collapsing when it uplift to an extent or the single result from intenseextrusion-deformation, or the escaping to east under extrusions from northern and southern sides of Tibetplateau? (4),Rifting in the plateau is local range, or regional one that belong to the extending system in thewhole of East Asian, which include ShanXi Graben and Bakal Rifts? If they were one, Tibetan rifts have thethe same extending mechanism as extensive extension in east Asian? Or what is mechanism of the plateaurifts?Presently, Tibet plateau is still strongly active, uplifting and deformation. And the plateau rifts is one ofthe youngest tectonic action in Tibet and its adjacent at geological time scale. So, Recognition of riftsgeodynamic process is to help us with understanding some mechanism of continent-continent collision,continental underthrusting and deformation. Further, we can comprehend the complex dynamical process ofthe earth's formation and evolution.Now, The existing deep sounding data surveyed in Tibet plateau mainly focused on the east-west strikestructure. So, there are very little data that do reflect transvers characteristics cross south-north strike rifts. Inorder to make all use of low-precision Bouguer gravity anomaly data, the author give a bran-new and effectmethod of B-spline Wavelet Transformation to extract veracious deep structure information of those plateaurifts.B-spline Wavelet Transformation have all the basic feature of the ordinary wavelet transformation, but also it well utilize the basic behavior of B-spline function that include smooth, derivative and explicit functionrelationship, by which feature some normal orthogonal wavelet-base is easy to construct. In the potentialtheory, magnitude and intension of some potential anomaly be relative to the location, shape and deep of itssource. And potential representation expression commonly is Guass function that can get the same result byB-spline function. So, utilizing B-spline to construct Gauss function, multiscale-decomposition and the firstorder derivation of the wavelet to extraction and separation of the potential fields in order to achieve marginalfeature of geological objects. According to Liptiszh law that exist in wavelet transformation, we canqualitatively describe dip and approximate depth of fault zones. To overcome disturb and influence frompreponderant margins of three dimensional cube in potential theory, the author reference an idea of directionfilter extensively used in general data process of potential theory to extrude structure information of thedirection. Based on forward analysis of local and regional models, the author explicitly give general regularityfor extracting linear structure information by B-spline wavelet transform. Those models suggest that dataprecision for analysis must be sure to clearly distinguish the marginal variety of object on the basis of Sampletheorem. Moreover, the difference between local model and regional model in various feature indicate that wemust cautiously explain some regional anomalous in potential fields. On the base of above cognitions, weutilize the fast order derivation of B-spline Wavelet to process regional Bougour gravity anomalous of Tibet(1: 25, 000, 000 scale), and well achieve some deep distributions of the plateau rifts. Those clear distributionsregularities suggest that B-spline wavelet transformation have particular advantage to separate and extractfaults distributions from gravity fields.Based on the existing geophysical and geological data, to get all the most information of Tibetan rifts aspossible, the author use many geophysical technologies, such as gravity, areomagnetic, deep seismic soundingsand eqrthquake et all, combined with remote sense image, GPS observation, subsurface geology, tectonicsgeochemistry and geochronology. Thus, we can recognize and understand structures, deformation andgeodynamic process of the inner of the Earth from different way. During this researching for the rifts, we takeaction as following:At first, the author adopted B-spline wavelet transform to separate gravity anomalous (1: 25, 000, 000 scale)and to extract linear structure related to rifts. For areomagnetic anomalous (1: 20, 000, 000 on scale), the authoruse many ways, such as match filter, upward continuation and horizontal derivative to extract linear structureof areomagnetic anomaly. The two results, combined with tectonics from surface geology, depict distributionsof regional structures in rifts areas of Tibetan platean, and analyses the relationship between those lineardistributions and near north-south striking rifts in Tibetan. Second, to acknowledge the regularity of all of riftdistribution, the author study touching relationship between rift and surface tectonics by remote sense imageand learn the local property of rifts. Thirdly, on the base of collecting achievement from GPS observations, seismic activity, seismic anisotropy in the researching area, the author describe the features of structure activityand regional stress fields in rifting area of Tibetan plateau, from surface crust to deeper lithospheric mantle.Consequently, the mechanism of active rift in Tibetan plateau be acquired. Fourth, the author study deepcrustal structure of Yadong-Gulu rifts located in eastern Lassha Terrene. and regional deep structure in wholeof Lassha Terrene with gravity fits integrated with seismic sounding and with seimic sounding achievements.Fifthly, comparing geological and geophysical features of Tibetan rifts with some typical continental riftswhich form in pure extending environments, the author give a specific concepts of those rift formed in Tibetan.And, those rifts are classified according to their features. The author analyses formation mechanism of thosedifferent kinds of rifts and verifies those concept model of rifts mechanisms by seismic soundings data. At last, the author summarizes above-mentioned results and provide the unity geodynamical model of rifts in Tibetanplateau. Combined with surface geology, geochemistry and chronology, the author simply tell the geodynamical process of those rifts. According to this results of Tibetan rifts, the author acquire basicacknowledges as follows for above-mentioned scientific problems about rifts.(1) Rifts with completely coherent characteristics of tectonics do not exist in Tibetan plateau. BecauseYadong-Gulu rift, as the greatest scale in Tibet plateau, is divided into two parts by YaLung Zangbo Suture, namely Yadong-Kangma rift located in Himalaya Terrene and Nimu-Gulu rift related to mantle in LasshaTerrene. Therefore, all of those rifts in Tibetan plateau are limited by those terrenes boundary. Rifts in Tibetanplateau are related to some characteristics of corresponding terrenes, ant its structure and regional stress fields.North-east trending rift in Qiangtang terrene be related to the basement property of Qiangtang Terrene. Andnear north-south striking rift main located in Lassha Terrens, and those rifts are very complex, and are studiedas main objects(2) East-western striking rifts are mainly related to Geji-south Gaize- North Cuoqen-Shenzha insidiousfaults (abbreviated, GGCSF) that located in the north margin of Gangdse batholith, not the middle part ofBangong Cuo-Nujiang suture which are presently accepted at large. The results from seismic activity suggestthat the fore have strong active at presently. Instead, the later have not clearly active. Combined with existingacknowledge, Geji-south Gaize- North Cuoqen-Shenzha insidious faults perhaps become boundary faultzones.(3) According to the vertically extending depth of those rifts in Lassha Terrene, they are divided into twokinds. One is crustal rift, which means the rifts only are located in upper and middle crust. They are mainlylocated in western Lassha Terrene, such as Shenzha-Xietongmen Rift, Dangrerong Cuo-Gu Cuo Rift and soon. Correspondingly, the other kind is mantle rift that relates to mantle activity, and cut through the whole crust.The south end of all rifts in Lassha Terrene are limited by Yalur Zangbo Suture. And, the north end of crustalrift are truncated by east-west trending GGCSF.(4) The strike of pre-existing tectonics in Lassha Terreen are different. The strike of pre-exiting tectonic inwestern Lassha Terrene is mainly north-west strike, and one of that in eastern Lassh Terrene is north-east strike.The boundary is 88E. Rifts in Lassha terrene succeeded to those pre-existing regional tectonics.(5) the depth of Moho under Lassha Terrene is deeper than its south side Himalaya Terrens and the northside Qiangtang Terrene, and mainly become down concave. Moreover, the depth of Moho under eastern LasshTerrene is deeper than western Lassha Terrene. The low velocity zone is snapped as the result of extension inLassha Terrene. Those characteristics reveal that extension in Lassha Terrene is not equal. And the east part ofLassha Terrene is stronger than the west one.(6) North-east trending rift in Qiangtang terrene be related to the basement of Qiangtang Terrene and tocorresponding regional stress field. Rifts located in Lassha Terrene have complex mechanism of theirformation. The mechanism of Nimu-Gulu Rift in eastern Lassha Terrene results from directional fluxion ofsubtance in lithospheric mantle, shearing-slipping in curst and the location. And the mechanism of crustal riftsis the consequences of inhomogeneous ductile slipping of LVZ under extending environments and the effectsof brittle upper ernst and topography.(7) Compared geological and geophysical features of Tibetan rifts with some typical continental riftswhich form in pure extending environments, rifts in Tibetan plateau are named colliding-raising rift orOrogenic Rift. So, rifting in Tibetan plateau is not a part of extending system in far-large East Asian whichinclude Shanxi Graben and Baikal Rifts.(8) Crustal rifting in Lassha Terrene is the result of inhomogeneous ductile slipping along LVZ in themiddle of crust under extending environments and the effects of brittle upper crust and topography. Andmantle rifting formed by lithospherie ductile deformation, isostatic adjust, creeping of substance in uppermantle regional variation of tectonic stress fields. This two mechanism all contribute to oblique subduction of India plate. It lead not only to progressive extrusion stress to north in Tibetan Inner, but to asymmetriceast-west stretching in Lassha Terrene.(9) As for the whole, rifting that happen in Lassha Terrene and Qiangtang Terrene maybe attribute tooblique subduction of India Plate into Asia plate, and being blocked by Tarim Block at Pamir Plateau syntax, and having free boundary at east margin of Tibet, and Tibetan inner matter creeping from west to east. Incourse of creeping, the matter of Tibet stretch asymmetry, because of different material fabric, structure andtectonic stress field. It is the distinction that lead to different rifts kinds between Lassha Terrene andQiangtang. Terrene. The crust rift in west of Lassha Terrene and N-E rift in Qiangtang Terrene are controlled byGGCSF dextral strike-slip.(10)All in a word, some condition under which the rifts form are complicated. All the models that themechanism of uplift, collapse and later strong deformation under extrusion conditions, or matter escaping toeast by extrusions from two sides (north side and south side) in Tibet have single tectonic circumstanc anddepth structure feature. Rifts in Tibet do not belong to fan-asian spread system with Shanxi graben and Bakalrifts. Tibetan rifts results from oblique subduction-collision of India plate, free eastern boundary of Tibet andsome differences among terrenes in Tibet plateau and among inner terrene.(11) The oldest age of Tibet plateau extrusion is 20±3Ma. During this term, the strike-slip dirction ofJiali Faults had been changed latral into dextral, and Honghe-Ailaoshan latral shear faults into dextral shear.The evidence indicate that regional stress fields had radically happened during the term. The time that thestrongest rifting happened be related with corresponding to terrene where it form. Rifting in Himalaya Terrenehappened about 15～13Ma. The time of Nimu-Gulu Rift, which be studied in detail among those rifts in Tibet, happened about 8 Ma, and become more and more younger form south to north. It is disputable that the time ofN-E striking Rifting began. Some scientists considered 13.5 Ma, others is 4Ma.