The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite,Tibet,in China shows that the rodingitization involved continuous changes in fluid composition during differe...The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite,Tibet,in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction.The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures.Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock(^10 m×30 m),and they record wide variations in bulk composition,mineralogy,and texture.The mineral assemblages,from center to margin,vary from(1)relics of primary clinopyroxene(Cpx_(r))and primary amphibole(Amp_(r))+newly formed late amphibole(Act)+primary plagioclase(Pl_(r))+clinozoisite+prehnite+albite+chlorite+titanite+ilmenite(R1 rodingite),through(2)relics of primary clinopyroxene(Cpx_(r))+newly formed late clinopyroxene(Cpx_(n))+primary and late amphiboles(Amp_(r)+Act)+clinozoisite+prehnite+albite+chlorite+titanite(R2 rodingite),to(3)newly formed late clinopyroxene(Cpx_(n))and amphibole(Act)+clinozoisite+prehnite+albite+chlorite+titanite(R3 rodingite).As a result of the metasomatic process of rodingitization,the content of CaO in the whole rock chemical composition from R1 to R3 increases,SiO_(2) decreases,and Na_(2)O+K_(2)O is almost completely removed.Massbalance diagrams show enrichments in large ion lithophile elements such as Rb,Cs,Ba,and Pb as well as Ni during rodingitization.The central part of the rodingitized intrusion(R1 rodingite)was only slightly affected by metasomatism.On the other hand,the contents of the rare earth elements(REEs),high field strength elements(HFSEs;e.g.Zr,Nb,Ta,Hf,and Y),and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization.Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H_(2)O-saturated,CO_(2)-rich environment.The estimated conditions of metamorphism were-281-323℃and 0.4-3.9 kbar,representing the subgreenschist facies.In this environment,REEs and HFSEs were soluble in the fluids and partly removed.Moreover,these prehnite rodingites formed in a progressively reducing and less alkaline environment,as indicated by decreases in f(O_(2))and bulk-rock Fe^(3+)/Fe^(2+) ratios,and the records of fluidΔpH from the center to the margin of the studied rodingitized intrusion.展开更多
Podiform chromitites are characteristically occurred in ophiolites(e.g.,Thayer,1964;Dickey,1975).However,the metallogenic processes for podiform chromitites are still unclear.Early models involved fractional crystalli...Podiform chromitites are characteristically occurred in ophiolites(e.g.,Thayer,1964;Dickey,1975).However,the metallogenic processes for podiform chromitites are still unclear.Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers(Dickey,1975;Boudier and Coleman,1981),but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites(Dick,1977;Dick and Bullen,1984).Recent studies by the majority of authors have suggested that melt-rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites(Zhou and Robinson,1994;Zhou et al.,1996,2005,2014;Robinson,2008;Page and Barnes,2009;Uysal et al.,2009,2012;González-Jiménez et al.,2011,2015).Based on the occurrence of some ultrahigh pressure minerals(e.g.diamond and coesite)in chromitites,it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling(Yang et al.,2007;Yamamoto et al.,2013).Although geat progresses have been made towards understanding the genesis of podiform chromitites,some fundamental issues in remain unanswered.For examples,what are the major controls on the size of chromitites?And why some ophiolites contain large podiform chromitite bodies,whereas most ophiolitic massifs are essentially chromitite-barren? The Yarlung-Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world.It contains fresh peridotites as well as different-sided podiform chromitites.The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China.In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small-scale chromitite bodies.Such characteristics make the Yarlung-Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites. The Luobusha chromitites are large lens and enclosed in dunite.In contrast,the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes.The closely spatial association of the chromitites and dunite envelopes,together with their textural features,support a petrogenetic model that the chromitites from the Luobusha,Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite.In terms of chemical composition of chromite,there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo.Chromite from the Luobusha chromitites has high Cr#(71-82),whereas Chromite in the Dazhuqu chromitites show relatively low Cr#(16-63),and chromite in the Dongbo chromitites includes low Cr#(11-47)and high Cr#(70-81)types.For the Dongbo and Dazhuqu massifs,linear trends of Cr#with Mg O,Fe Ot,Ni,Ga,V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites,suggesting that the melt-rock reaction may provide major budget of Cr for the chromitites.The similar compositions at a given Cr#in chromite from these rocks also demonstrate that the chromitites may have been formed by in-situ crystallization of chromite under low melt/rock ratio.In contrast,the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites,implying that the formation of the chromitite bodies requires a continual replenishment of Cr-rich melts from deeper mantle.Fractionation and accumulation of chromite from a large volume of Cr-rich melt may play an important role on the formation of the Luobusha chromitites.MORB-normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr-rich boninitic melt at surpra-subduction zone(SSZ)setting.However,the Dongbo and Dazhuqu chromitites have formed originally from a MORB-affinity melt at a mid-ocean ridge(MOR)environment. In summary,the Luobusha chromitites crystallized from a Cr-rich melt in a dynamic conduit,where fractional crystallization and crystal settling play a key role in formation of the large chromitites.In contrast,the small-scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts.Podiform chromitites can be formed in MOR environment,whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.展开更多
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 subdu...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<1.1). 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 εHf(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 subterranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.展开更多
Widely distributed in Gyangze-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 orde...Widely distributed in Gyangze-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 Nagarze were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean 206Pb/238U 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.展开更多
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 ...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 Amphisphaera coronate, Buryella 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.展开更多
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 (3...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.展开更多
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 F...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.展开更多
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 Co...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.展开更多
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...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.展开更多
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 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.展开更多
The method of 2-D travel time inversion,which can be applied to determining 2-D velocity structure and in-terfaces simultaneously,is used in this paper to reprocess the data of Paiku Co-Pumoyingcuo seismic profile acr...The method of 2-D travel time inversion,which can be applied to determining 2-D velocity structure and in-terfaces simultaneously,is used in this paper to reprocess the data of Paiku Co-Pumoyingcuo seismic profile across the Ny-ima-Tingri rift and Shenzha-Dinggye rift. P-wave velocity structure and interfaces beneath the profile are obtained. The in-terfaces 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.展开更多
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...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.展开更多
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 th...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.展开更多
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...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).展开更多
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 ...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.展开更多
基金supported by the National Natural Science Foundation of China(Grant No.41572044)the SGSTSP of the Chinese Academy of Sciences(Grant No.XDB03010201)+1 种基金SDUST Research Fund(Grant No.2015TDJH101)financial support from Chinese Scholarship Council(Grant No.201808370192)。
文摘The analysis of early stage rodingite from the ultramafic rocks of the Xialu Massif in the Xigaze Ophiolite,Tibet,in China shows that the rodingitization involved continuous changes in fluid composition during different stages of subduction.The early stage prehnite-bearing rodingite was produced at low pressures and temperatures along extensional fractures.Samples of rodingite were collected along a profile from the center to the margin of a rodingitized intrusive igneous rock(^10 m×30 m),and they record wide variations in bulk composition,mineralogy,and texture.The mineral assemblages,from center to margin,vary from(1)relics of primary clinopyroxene(Cpx_(r))and primary amphibole(Amp_(r))+newly formed late amphibole(Act)+primary plagioclase(Pl_(r))+clinozoisite+prehnite+albite+chlorite+titanite+ilmenite(R1 rodingite),through(2)relics of primary clinopyroxene(Cpx_(r))+newly formed late clinopyroxene(Cpx_(n))+primary and late amphiboles(Amp_(r)+Act)+clinozoisite+prehnite+albite+chlorite+titanite(R2 rodingite),to(3)newly formed late clinopyroxene(Cpx_(n))and amphibole(Act)+clinozoisite+prehnite+albite+chlorite+titanite(R3 rodingite).As a result of the metasomatic process of rodingitization,the content of CaO in the whole rock chemical composition from R1 to R3 increases,SiO_(2) decreases,and Na_(2)O+K_(2)O is almost completely removed.Massbalance diagrams show enrichments in large ion lithophile elements such as Rb,Cs,Ba,and Pb as well as Ni during rodingitization.The central part of the rodingitized intrusion(R1 rodingite)was only slightly affected by metasomatism.On the other hand,the contents of the rare earth elements(REEs),high field strength elements(HFSEs;e.g.Zr,Nb,Ta,Hf,and Y),and some highly compatible elements such as Cr and Sc decreased slightly during rodingitization.Thermodynamic modeling based on equilibrium mineral assemblages indicates that the rodingite of the Xialu Massif formed in an H_(2)O-saturated,CO_(2)-rich environment.The estimated conditions of metamorphism were-281-323℃and 0.4-3.9 kbar,representing the subgreenschist facies.In this environment,REEs and HFSEs were soluble in the fluids and partly removed.Moreover,these prehnite rodingites formed in a progressively reducing and less alkaline environment,as indicated by decreases in f(O_(2))and bulk-rock Fe^(3+)/Fe^(2+) ratios,and the records of fluidΔpH from the center to the margin of the studied rodingitized intrusion.
文摘Podiform chromitites are characteristically occurred in ophiolites(e.g.,Thayer,1964;Dickey,1975).However,the metallogenic processes for podiform chromitites are still unclear.Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers(Dickey,1975;Boudier and Coleman,1981),but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites(Dick,1977;Dick and Bullen,1984).Recent studies by the majority of authors have suggested that melt-rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites(Zhou and Robinson,1994;Zhou et al.,1996,2005,2014;Robinson,2008;Page and Barnes,2009;Uysal et al.,2009,2012;González-Jiménez et al.,2011,2015).Based on the occurrence of some ultrahigh pressure minerals(e.g.diamond and coesite)in chromitites,it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling(Yang et al.,2007;Yamamoto et al.,2013).Although geat progresses have been made towards understanding the genesis of podiform chromitites,some fundamental issues in remain unanswered.For examples,what are the major controls on the size of chromitites?And why some ophiolites contain large podiform chromitite bodies,whereas most ophiolitic massifs are essentially chromitite-barren? The Yarlung-Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world.It contains fresh peridotites as well as different-sided podiform chromitites.The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China.In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small-scale chromitite bodies.Such characteristics make the Yarlung-Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites. The Luobusha chromitites are large lens and enclosed in dunite.In contrast,the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes.The closely spatial association of the chromitites and dunite envelopes,together with their textural features,support a petrogenetic model that the chromitites from the Luobusha,Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite.In terms of chemical composition of chromite,there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo.Chromite from the Luobusha chromitites has high Cr#(71-82),whereas Chromite in the Dazhuqu chromitites show relatively low Cr#(16-63),and chromite in the Dongbo chromitites includes low Cr#(11-47)and high Cr#(70-81)types.For the Dongbo and Dazhuqu massifs,linear trends of Cr#with Mg O,Fe Ot,Ni,Ga,V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites,suggesting that the melt-rock reaction may provide major budget of Cr for the chromitites.The similar compositions at a given Cr#in chromite from these rocks also demonstrate that the chromitites may have been formed by in-situ crystallization of chromite under low melt/rock ratio.In contrast,the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites,implying that the formation of the chromitite bodies requires a continual replenishment of Cr-rich melts from deeper mantle.Fractionation and accumulation of chromite from a large volume of Cr-rich melt may play an important role on the formation of the Luobusha chromitites.MORB-normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr-rich boninitic melt at surpra-subduction zone(SSZ)setting.However,the Dongbo and Dazhuqu chromitites have formed originally from a MORB-affinity melt at a mid-ocean ridge(MOR)environment. In summary,the Luobusha chromitites crystallized from a Cr-rich melt in a dynamic conduit,where fractional crystallization and crystal settling play a key role in formation of the large chromitites.In contrast,the small-scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts.Podiform chromitites can be formed in MOR environment,whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.
基金supported by the China Postdoctoral Science Foundation(M2017612220)the Shandong Province Natural Science Foundation(Doctoral Funds,ZR2017BD033)
文摘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<1.1). 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 εHf(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 subterranes, paleo-island-arcs and multiple collision phases that form the present southern Tibet.
文摘Widely distributed in Gyangze-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 Nagarze were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean 206Pb/238U 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.
基金supported by the Strategic Project of Science and Technology of Chinese Academy of Sciences (XDB050105003)the State Scholarship Fund of China (41272030)+1 种基金the National Basic Research Program of China (2012CB822001)IGCP608
文摘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 Amphisphaera coronate, Buryella 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.
基金This work is supported by the National Key Basic Research Program （2006CB701406）；Natural Science Foundation of China for Youth （40403003） ；Key Project of the Natural Science Foundation of China （40332020）, and is a contribution to IGCP 463.
基金This paper is supported jointly by the National Natural Science Foundation( No.4982 5 10 2 ) and the Ministry of Science and Te
文摘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.
基金supported by the Natural Science Foundation of China for Youth(40403003)National Key Basic Research Program(2006CB701406)Key Project of Natural Science Foundation of China(40332020),and is a contribution to IGCP 463/555.
文摘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.
基金the Deutsche Forschungsgemeinschaft grant Schw 671/8-1
文摘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.
文摘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.
基金Projects(41273040,41303025)supported by the National Natural Science Foundation of China
文摘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.
基金supported by the National Natural Science Foundation of China (No. 40674052)
文摘The method of 2-D travel time inversion,which can be applied to determining 2-D velocity structure and in-terfaces simultaneously,is used in this paper to reprocess the data of Paiku Co-Pumoyingcuo seismic profile across the Ny-ima-Tingri rift and Shenzha-Dinggye rift. P-wave velocity structure and interfaces beneath the profile are obtained. The in-terfaces 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.
文摘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.
基金Acknowledgements This work was supported by China Geological Survey （Grant No. H45C004002, 1212010784007） and the Project of the National Natural Science Foundation of China （Grant No. 40272012）.
文摘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.
文摘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).
文摘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.
