Thoughts on Pairing Assistance to Tibet for Promoting Agricultural High-quality Development in Southern Tibet from the Perspective of Agricultural Power

In the new era,there is an urgent need to further promote pairing assistance to Tibet,promote the simultaneous construction of a strong agriculture in Tibet and the China's Mainland,and compose a Chinese-style modernization.Southern Tibet,located in the southeastern part of the Tibet Autonomous Region,includes Shannan City and Nyingchi City,is a region assisted by four provincial partners including Hubei Province.This paper introduces the agricultural environment in southern Tibet,studies its agricultural characteristics,and analyzes the main issues of its pairing assistance.Taking forging the strong consciousness of the Chinese national community as the main line,the paper explores strategies for promoting agricultural high-quality development in southern Tibet through pairing assistance to Tibet from the perspective of agricultural power,and proposes some strategies,such as inheriting agricultural cultural heritage,promoting the upgrading of modern seed industry,enhancing the characteristic advantages of highland barley(naked barley)and animal husbandry industries,and developing edible fungi and cold water fish industries.

Characteristics of Metamorphic Rock Magnetic Fabrics in the Nyalam Area of the Southern Tibet and Its Geological Significance, China

This paper reports Precambrian rock magnetic fabrics in the Nyalam area of southern Tibet. The analytical results of magnetic fabrics show that the values of H are high （〉 10% in general）, so the ductile deformations of the Precambrian rock are strong. The orientation of the maximum principal stress inferred from the minimum magnetic susceptibility is nearly S-N, NE-SW and NW-SE. The Flinn diagram of the magnetic fabrics show that the strain pattern is oblate and constrictional type. Magnetic foliation of great majority of rock samples is well developed and the magnetic lineation is poor and the magnetic susceptibility ellipsoid is flattened. The magnetic lineation of the minority rock samples is well developed and the magnetic foliation is poor and the magnetic susceptibility ellipsoid is prolate. According to the geological field and the magnetic fabrics, there are 3 times tectonic stress field in SN directed extruding, NW-SE directed extruding, NW-SE directed extension. It shows that the Nyalam area has undergone process the orientation of SN, NW-SE nappe structure and NW-SE directed extension structure. The change of tectonic stress is reflected by the field characteristics of the Precambrian rock magnetic fabrics that is the direct responding result of the arc-continental, continent-continental collision between the India and Asian continents in the late part of the Late Cretaceous to Late Eocene and subsequently shifted to intra-continental convergent, the plateau uplifting and extension structure stage since the Late Eocene.

Biogenic characteristics of Mesozoic cherts in southern Tibet and its significance

The Xialu chert, which contains abundant biological information, were investigated by major element analysis, micro-Raman, SEM and EPMA. The results show that SiO2 content of chert is 84.12%-93.08%, averaging 89.84%. The close packed structures of low degree crystallinity of quartz indicate the hydrothermal origin. SiO2 of modern hot springs exhibit loose silica pellets and nodular, beaded structures. Under polarization microscope, the presence of biological skeleton structures indicate that biological activities are involved in the hydrothermal deposition, which correspond to the geochemical characteristics: w(SiO2)/ w(K2O+Na2O), w(SiO2)/w(Al2O3) and w(SiO2)/w(MgO), with average values of 295.29, 68.88 and 284.45, respectively. SiO2 is enriched in the organism(radiolarian) centers, the degree order of SiO2 within the biologic structures is much higher than that of outside. The impurity minerals albites are formed earlier than the original deposition. Kaolinites, feldspars and mixture of organic materials display lower degree of crystallinities and accumulate as vermicular aggregates.

Distribution of the low velocity bulk in the middle-to-lower crust of southern Tibet: implications for formation of the north–south trending rift zones

We conducted the ambient noise tomography to image the shallow crustal structure of southern Tibet. The2D maps of phase velocity anomalies at the periods of10–16 s show that the low velocities are mainly confined along or near some of the rift zones. While the maps at the periods of 18–25 s show that the coherent patterns that the low velocities expand outside of the rift zones. It means that the low velocities are prevailing in the middle crust of southern Tibet. According to the previous study of surface wave tomography with teleseismic data,we find that the low velocities from the lower crust to the lithospheric mantle are also restricted to the same rift zones. Thus,the integrated knowledge of the distribution of the low velocities in southern Tibet provides some new insight on the formation of the north–south trending rift zones. Compiling the multidiscipline evidences,we conclude that the rifting was an integrated process of the entire lithosphere in the early stage（＊26–10 Ma）,but mainly occurred within the upper crust due to the weakening a decoupling in the low velocity middle crust in the late stage（later than ＊8 Ma）.

An Important Spreading Event of the Neo-Tethys Ocean during the Late Jurassic and Early Cretaceous: Evidence from Zircon U-Pb SHRIMP Dating on Diabase in Nagarze, Southern Tibet

Widely distributed in Gyangzê-Chigu area, southern Tibet, NW- and nearly E-W-trending diabase（gabbro）-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean. In order to constrain the emplacement age of these dykes, zircons of two samples from diabases in Nagarzê were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean ^206pb/^23SU ages were obtained in this paper, which are 134.9±1.8 Ma （MSWD=0.65） and 135.5 ± 2.1 Ma （MSWD=1.40）, respectively. They not only represent the crystallization age of the diabase, but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous. This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.

Late Triassic Granites From the Quxu Batholith Shedding a New Light on the Evolution of the Gangdese Belt in Southern Tibet

The Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo-Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India-Eurasia collision. It is widely accepted that northward subduction of the Neo-Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U-Pb-Hf isotope data and whole-rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U-Pb dating from two representative samples yields consistent ages of 225.3~=1.8 Ma and 229.9~1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene （47-52 Ma） as previously suggested. Geochemically, the mylonitic granite pluton has a sub-alkaline composition and low-medium K calc-alkaline affinities and it can be defined as an I-type granite with metaluminous features （A/CNK〈I.1）. The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island-arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu-Hf isotopes indicate that the granite has highly positive till（t） values ranging from ＋13.91 to ＋15.54 （mean value ＋14.79）, reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two-stage model ages ranging from 238 Ma to 342 Ma （mean value 292 Ma）, suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams also suggest that the granite was derived from partial melting of the mafic lower crust. Taking into account both the spatial and temporal distribution of the mylonitic granite, its geochemical fingerprints as well as previous studies, we propose that the northward subduction of the Neo-Tethys oceanic slab beneath the Lhasa terrane had already commenced in Late Triassic （-230 Ma）, and that the Late Triassic magmatic events were formed in an active continental margin that subsequently evolved into the numerous sub- terranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.

Early Eocene Radiolarian Fauna from the Sangdanlin, Southern Tibet: Constraints on the Timing of Initial India-Asia Collision

This is a new report on the early Eocene radiolarian fauna from the Sangdanlin section in the Gyirong region, along the southern margin of the Yarlung Zangbo Suture Zone. The Sangdanlin section measured in this study is divided into three lithostratigraphic units from bottom to top： the Zongzhuo, Sangdanlin, and Zheya formations. Abundant radiolarian fossils were obtained from the Sangdanlin section and 54 species of 30 genera were identified and assigned as follows： Cryptamphorella conara-C. macropora the late Cretaceous Zone and Amphis_phaera coronate, Bur）ella tetradica-Bekoma campechensis, and B.bidartensis-B. divaricata the Paleocene-early Eocene Interval Zones. The Paleocene- early Eocene radiolarian zones are comparable to the radiolarian zones RP4-RP8 in New Zealand. Based on the data of radiolaria and lithofacies, it is suggested that the Zongzhuo Formation should be deposited along the base of the north-facing, continental slope of the Greater Indian continental margin, and the Sangdanlin Formation should be a deep marine, sedimentary sequence located in a foreland basin. The early Eocene radiolarian fauna in the Sangdanlin Formation constrains the initial age of the India-Asia collision to no later than 53.6 Ma.

Burial Records of Reactive Iron in Cretaceous Black Shales and Oceanic Red Beds from Southern Tibet

One of the new directions in the field of Cretaceous research is to elucidate the mechanism of the sedimentary transition from the Cretaceous black shales to oceanic red beds. A chemical sequential extraction method was applied to these two types of rocks from southern Tibet to investigate the burial records of reactive iron. Results indicate that carbonate-associated iron and pyrite are relatively enriched in the black shales, but depleted or absent in red beds. The main feature of the reactive iron in the red beds is relative enrichment of iron oxides （largely hematite）, which occurred during syn-depostion or early diagenesis. The ratio between iron oxides and the total iron indicates an oxygen-enriched environment for red bed deposition. A comparison between the reactive iron burial records and proxies of paleo-productivity suggests that paleo-productivity decreases when the ratio between iron oxides and the total iron increases in the red beds. This phenomenon could imply that the relationship between marine redox and productivity might be one of the reasons for the sedimentary transition from Cretaceous black shale to oceanic red bed deposition.

Response of Reactive Phosphorus Burial to the Sedimentary Transition from Cretaceous Black Shales to Oceanic Red Beds in Southern Tibet

The mechanism of sedimentary transition from the Cretaceous black shales to the oceanic red beds is a new and important direction of Cretaceous research. Chemical sequential extraction is applied to study the burial records of reactive phosphorus in the black shale of the Gyabula Formation and oceanic red beds of the Chuangde Formation, Southern Tibet. Results indicate that the principal reactive phosphorus species is the authigenic and carbonate-associated phosphorus （CAP） in the Gyabula Formation and iron oxides-associated phosphorus （FeP） in the Chuangde Formation which accounts for more than half of their own total phosphorus content. While the authigenic and carbonate-associated phosphorus （CAP） is almost equal in the two Formations; the iron oxidesassociated phosphorus is about 1.6 times higher in the Chuangde Formation than that in the Gyabula Formation resulting in a higher content of the total phosphorus in the Chuangde Formation. According to the observations on the marine phosphorus cyde in Modern Ocean, it is found that preferential burial and regeneration of reactive phosphorus corresponds to highly oxic and reducing conditions, respectively, leading to the different distribution of phosphorus in these two distinct type of marine sediments. It is the redox-sensitive behavior of phosphorus cycle to the different redox conditions in the ocean and the controlling effects of phosphorus to the marine production that stimulate the local sedimentary transition from the Cretaceous black shale to the oceanic red beds.

Triassic Sequence Stratigraphy and Sea-Level Changes in Qomolongma Area, Southern Tibet, China: From Epicontinental Sea to Rift Basin

The Triassic in the Qomolongma area, southern Tibet, was deposited under an extensional tectonic setting from the Pangea supercontinent to continental rifting. From the Induan to Rhaetian, 12 depositional sequences (3rd order) have been recognized, which can be grouped into 5 sequence sets and in turn make up a well defined mesosequence (2nd order). Among the recognized marine transgressions, those at 250 Ma, 239 Ma, 231 Ma and 223 Ma respectively are particularly of significance and can be correlated widely across continents. The study shows that in Triassic the Qomolongma area experienced a sedimentary evolution from epicontinental sea to rift basin with the turning point at ca 228 Ma. During the early and middle epochs, the area was under epeiric sea, with carbonate ramp to mixed shoal environments predominant. In the late Carnian, the strong extension initiated listric faulting, thus resulting in rapid basement subsidence and the onset of a rift basin. From the late Norian to Rhaetian, it manifested as a rapid basin filling process in the area. Coupled with long term sea level fall, the excessive terrigenous influx led to the shift of environment from deep water prodelta to shore and finally to fluvial plain.

CRETACEOUS AND TERTIARY BOUNDARY IN THE TINGRI REGION OF SOUTHERN TIBET

Marine uppermost Cretaceous to lowermost Tertiary sequence occurs in the Gamba and Tingri regions of southern Tibet. In previous work, the sequences in both regions were divided into the Zongshan, Jidula and Zhepure Formations (Wan, 1985; Hao and Wan, 1985; Xu, Wan, et al. 1990). The K/T boundary in Gamba was studied by Wan Xiaoqiao (1988). This paper deals with the boundary in the Tingri region. Willems (1993) discussed the Cretaceous to Tertiary sequence in the Tingri region. In his work, the Upper Cretaceous to Lower Tertiary passage beds involved the Zongshan, Zhepure Shanpo, Jidula and Zhepure Shan Formations. The lithologies in the Tingri and Gamba regions are generally comparable, and we describe the boundary sequence in the Tingri region as follows:1\ Zongshan Formation (Campanian—Middle Maastrichtian) Hao and Wan (1985) first described this formation in Tingri. It consists of limestones equivalent to the strata in Gamba. The nomination of the Zongshan formation has been used in both regions. The Formation in Gamba ranges from Campanian to Late Maastrichtian age. In Tingri, however, the upper beds were referred to the Zhepure Shanpo Formation because they are clastic sediments (Willems, 1993). Planktonic foraminifera are abundant, such as Globotruncana spp. and Omphalocyclus sp.

PALEOCEANOGRAPHY AND EVOLUTION OF THE CENO-TETHYS: MICROPALEONTOLOGICAL EVIDENCE FROM PELAGIC SEDIMENTS IN THE YARLUNG ZANGBO SUTURE ZONE, SOUTHERN TIBET

The Ceno\|Tethys is a southern branch of the Tethyan ocean and existed in the Mesozoic and early Cenozoic times. The evolution of the Ceno\|Tethys has been discussed based mainly on geological and paleontological data obtained from shallow marine sediments on the both sides of the ocean. Pelagic sediments deposited in a deep ocean basin of the Ceno\|Tethys are often incorporated in suture zones. However, geological and paleontological data from the pelagic sediments, which enable us to elucidate the paleoceanography and tectonic evolution of the Ceno\|Tethys, are still limited. The Xialu chert crops out about 30 km south of Xigaze and occupies the southern marginal part of the E—W trending Yarlung Zangbo Suture zone. As a result of radiolarian biostratigraphic research along two continuous sections, we identified seven different aged radiolarian assemblages from pelagic and hemipelagic sediments ranging early Middle Jurassic (Aalenian) to Early Cretaceous. The Aalenian radiolarian fauna is the oldest known record so far for the Xialu chert. It is noteworthy to point out that the fauna came from a chert sample which contains no terrigenous elements other than clay minerals. There is a possibility that radiolarian faunas older than Aalenian age will be discovered from the Xialu chert in the future.

Characteristic of crustal structure beneath the rifts in southern Tibetan plateau

The method of 2-D travel time inversion, which can be applied to determining 2-D velocity structure and interfaces simultaneously, is used in this paper to reprocess the data of Paiku Co-Pumoyingcuo seismic profile across the Nyima-Tingri rift and Shenzha-Dinggye rift. P-wave velocity structure and interfaces beneath the profile are obtained. The interfaces in the crust near Tingri and Dinggye which are located on rifts have a tendency to uplift, and velocities of middle and lower crusts are high. Low velocity layer in upper crust has an offset. Compared with the distribution of the earthquakes in this region, it is speculated that normal faults near Tingri and Dinggye extend to the upper mantle. Apparently it is affected by deep material： the uplift of mantle causes partial melting in the crust, thus the thickness of crust in this area becomes thin, and tension failures occur in this region easily. On the basis of the characteristics of the earthquakes＇ distribution and the structures of the crustal velocity and interfaces, materials from the mantle still uplifts and the failures are still active.

Finite element modelling of the pull-apart formation: implication for tectonics of Bengo Co pull-apart basin, southern Tibet

The tectonic deformation and state of stress are significant parameters to understand the active structure, seismic phenomenon and overall ongoing geodynamic condition of any region. In this paper, we have examined the state of stress and crustal deformation during the formation of the Beng Co pull-apart basins produced by an enéchelon strike-slip fault systems using 2D Finite Element Modelling (FEM) under plane stress condition. The numerical modelling technique used for the experiments is based on FEM which enables us to analyze the static behavior of a real and continues structures. We have used three sets of models to explore how the geometry of model (fault overlap and pre-existing weak shear zone) and applied boundary conditions (pure strike-slip, transpressional and transtensional) influence the development of state of stress and deformation during the formation of pull-apart basins. Modelling results presented here are based on five parameters: 1) distribution, orienttation, and magnitude of maximum (σH max) and minimum (σH max) horizontal compressive stress 2) magnitude and orientation of displacement vectors 3) distribution and concentration of strain 4) distribution of fault type and 5) distribution and concentration of maximum shear stress (σH max) contours. The modelling results demonstrate that the deformation pattern of the en-échelon strike-slip pull-apart formation is mainly dependent on the applied boundary conditions and amount of overlap between two master strike-slip faults. When the amount of overlap of the two master strike-slip faults increases, the surface deformation gets wider and longer but when the overlap between two master strike-slip faults is zero, block rotation observed significantly, and only narrow and small surface deform ation obtained. These results imply that overlap between two master strike-slip faults is a significant factor in controlling the shape, size and morphology of the pull-apart basin formation. Results of numerical modelling further show that the pattern of the distribution of maximum shear stress (τmax) contours are prominently depend on the amount of overlap between two master strike-slip faults and applied boundary conditions. In case of more overlap between two masters strike-slip faults, τ max mainly concentrated at two corners of the master faults and that reduces and finally reaches zero at the centre of the pull-apart basin, whereas in case of no overlap, τmax largely concentrated at two corners and tips of the master strike-slip faults. These results imply that the distribution and concentration of the maximum shear stress is mainly governed by amount of overlap between the master strike-slip faults in the en-échelon pull-apart formation. Numerical results further highlight that the distribution patterns of the displacement vectors are mostly dependent on the amount of overlap and applied boundary conditions in the en-échelon pull-apart formation.

A TAXONOMICAL AND ECOLOGICAL OVERVIEW OF RECENT AND HOLOCENE OSTRACODES OF THE NAM CO REGION, SOUTHERN TIBET

We present a list of living and (sub) fossil Ostracoda (Crustacea) from Holocene sediments from Lake NamCo,Southern Tibet, including descriptions, distribution and ecological data of the important taxa from the Nam Cocatchment. Species associations from lake surface sediments, sediment cores and outcrop samples consist of eight speciesincluding Candona candida ( O.F. Müller, 1776 ) , Candona xizangensis ( Huang, 1982 ) , Eucypris afghanistanensis( Hartmann, 1964) , Fabaeformiscandona danielopoli ( Yin & Martens, 1997 ) , Ilyocypris cf. mongolica ( Martens, 1991 ) ,? Leucocythere dorsotuberosa ( Huang, 1982 ) , Leucocytherella sinensis ( Huang, 1982 ) and Limnocythere inopinata ( Baird,1843). The dominant species are ? L. dorsotuberosa and L. sinensis. They show morphological variations with different degreesof ornamentation. We regard the variable noding and rib formation as intraspecific variability possibly driven byenvironmental factors. In general,the number and degree of ribs increases with water depth in ? L. dorsotuberosa. Thus,theornamentation may present a means to reconstruct(paleo) water depths. Our list is intended to serve as a taxonomical andpaleoecological primer for future paleolimnological work.

The Cenomanian-Turonian Anoxic Event in Southern Tibet: A Study of Organic Geochemistry

The Cenomanian-Turonian oceanic anoxic event (C/T OAE) is developed in southern Tibet. Organic geochemical study of the Cenomanian-Turonian sediments from the Gamba and Tingri areas shows that the mid-Cretaceous black shales in southern Tibet are enriched in organic carbon. The molecular analyses of organic matter indicate marine organic matter was derived from algae and bacteria. In the Gamba area, the organic matter is characterized by abundant tricyclic terpanes and pregane, which are predominant in 191 and 217 mass chromatograms, respectively. Pristane/phytane (Pr/Ph) ratios in the C/T OAE sediments are less than 1, demonstrating the domination of phytane. The presence of carotane can be regarded as a special biomarker indicating oxygen depletion in the C/T OAE sediments in the Tethyan Himalayas. In anoxic sediments, β-carotane and γ-carotane are very abundant. The β- and γ-carotane ratios relative to nC 17 in the Cenomanian-Turonian anoxic sediments vary from 32.28～ 42.87 and 5.10～ 11.01.

LONG TERM HISTORY (DEXTRAL TO SINISTRAL AND DUCTILE TO BRITTLE) OF THE DAMXUNG—JIALI SHEAR ZONE IN SOUTHERN TIBET

The Damxung—Jiali shear zone is a shear zone in Southern Tibet that trends W—E for>180km.It appears to have a varied history of both displacement sense and deformation conditions.We have examined microscopic and field data (including fault plane populations) in the westernmost 40km of the shear zone (on the northern margin of the Damxung graben near to the route of the INDEPTH II deep seismic line). The shear zone outcrops along the southern flank of an W—E trending chain of hills in which all valleys run N—S, providing a series of sections of exposure through the shear zone. The shear zone is 3～5km wide and has varying degrees of deformation. It cuts across the main trends of, and does not seem to be restricted to any particular lithology. The principal fabric in the shear zone is steeply to moderately south dipping. The bulk of the shear zone is phyllite and lower grade schist with a consistent degree of both ductile and brittle strain. A mylonitized carbonate unit that is sometimes present within the phyllites may have locally provided a mechanical instability as indicated by the higher strain. Elsewhere a conglomerate unit contains carbonate clasts that are stretched (prolate) up to 30∶1.The most spectacular unit forms lozenges of quartzifeldspathic rich rock (several 100m wide & 2～5km long) whose microfabric indicates deformation mechanisms (e.g., wholesale grainsize reduction) that operated around 400～500℃; the highest grade of the deformation fabrics. The lozenges are remnant evidence of higher temperature strain (at higher strain rate?) that is preserved only in the quartzifeldspathic rock. They are interpreted to be mega\|scale boudins resulting from their strength contrast with the weaker phyllites and schists during later lower temperature deformation. All the presently preserved sense of shear indicators are observed to be left\|lateral, and bulk shortening directions resolved using slip data from fault plane populations are consistent with this sinistral displacement.

GEOLOGICAL EVENT ACROSS THE BOUNDARY OF THE LATEST PALEOCENE TO EARLIEST EOCENE IN TINGRI, SOUTHERN TIBET

The marine early\|middle Early Paleogene in the northern Tethys Himalayas was found in southern Tibet in mid 1980s (Hao and Wan, 1985), and the bio\|, litho\|, and chrono\| stratigraphy were patterned by then (Hao and Wan, 1985; Willems et al., 1993, 1996). It seems that there is not an abrupt boundary between the Paleocene and Eocene (P/E) because they are composed of carbonate rocks. This similarity is widespread in the world. They are well examined in many ocean DSDP and ODP logs in similar continuous sediments. But in fact, it is sure that there is a geologic event between the Paleocene and Eocene around the world. Foraminifer extinction, stable isotope record, clay mineral changes, etc. verify the geologic event existence at the latest Paleocene. Unlike the K/T boundary, there is at present little direct or indirect evidence of such catastrophes coincident with the transition (Zachos et al., 1993). This work first shows that there could be also a geologic event across the boundary of latest Paleocene to earliest Eocene exposed on land, certainly in China territory.The P/E transitional sediment is exposed and remeasured at Gongzha, northwestern flank of the Zhepure Shan Syncline, west to Tingri. Here the P/E transitional stratum is approximately equivalent to the upper Member IV and the entire Member V (Willems et al., 1993, 1996). The exact boundary between the Paleocene and Eocene, i.e. between the Unit 16 and Unit 17, is determined by benthic foraminiferal zonations. The fossil zone Alveolina beginning at Unit 17 is just above the boundary that is partly different from Willems et al (1993, 1996).

DISCOVERY OF THE LATEST MARINE NON-CARBONATE SEDIMENT WESTERN TINGRI, SOUTHERN TIBET

The marine non\|carbonate sediment is exposed at the southwestern flank of the Zhepure Syncline where it lies about 4 km east to the typical cross\|section, Gongzha cross\|section (Hao and Wan, 1985; Willems et al., 1993, 1996) and ca.75km north to the Qomolangma. There are few people who can approach the place of the cross\|section, even so are endemic Tibetan. The cross\|section is named of the Qumiba cross\|section in this abstract. The new discovery, conformed on the massive Zhepure Formation limestone, here is named of Qumiba Formation. It is composed of two series of terrigenous sediments: Enba Member—lower gray shales intercalating with sandstones, and Zhaguo Member—upper reddish shales interbedding with sandstones. Abundant nannofossils with chronological meanings are first found out.Enba Member Common calcareous nannofossils in the Lutetian such as Chiastozygus barbatus, Chiasmolithus sp., Discoaster barbadiensis, Helicosphaera compacta, Reticulofenstra bisecta, Sphenolithus radians are processed in sample S12W1\|S17W1, in which the Helicosphaera compacta and Sphenolithus radians are part of the indicators of the nannofossil zone NP15. So it is proposed that Units S12\|17, 110 m in thickness, could have been deposited during Middle\|Late Lutetian.

THE ORIGIN OF DOUBLE HIGH CONDUCTIVITY LAYERS IN THE CRUST OF SOUTHERN TIBET AND ITS GEODYNAMIC CONSEQUENCE

Geoelectrical investigations in Tibetan Plateau show that there are two high conductivity layers (HCLs) in the crust of southern Tibetan Plateau. The first HCL is at a depth of 15～20km, and the second at 45～60km. In the central region of the Plateau, such as north Qiangtang and Bayan Har, there is only one HCL in crust at the depth about 15～20km.The origin of HCL in upper crust of southern Tibet is mainly caused by free saline aqueous fluids. The seismic investigation by project INDEPTH in southern Tibet demonstrates that the bright spot reflection at about 15km depth is caused by 10% volume of free aqueous fluids (Makovsky and Klemperer, 1999). Seismic and heat flow research indicates the temperature in 15km depth of Tibet is from 400℃ to 650℃. The high concentration of Cl - in water of geothermal spring in southern Tibet means that the aqueous fluid in crust is saline water. The experiment on conductivity of NaCl solution shows that the conductivity is more than 4.5 S·m -1 under 400MPa and 300～650℃ condition (Quist and Marshall, 1968). This p\|T condition corresponds to the pressure and temperature range at 15～20km depth of Tibet.