In southern Tibet, both the South Tibetan detachment system (STDS) and north-south trending rifts (NSTRs) are prominent extensional structures. There is debate about whether they overlap temporally or not, which would correspond, respectively, to the tectonic transition being an active process driven by internal tectonic forces or a passive process where one structure triggers the other in response to external gravitational or stress relaxation mechanisms. Resolving this controversy is essential for reconstructing the dynamic transition from syn-collisional crustal thickening to post-collisional extension. The Yadong rift, located in the central Himalaya and Lhasa terrane, truncates the STDS and thus provides a key structure to unravel this problem. Combining structural analysis with newly determined dates of zircon U-Pb (similar to 22 Ma, similar to 18 Ma and similar to 14 Ma), mica 40Ar/39Ar (similar to 14.2-13.5 Ma), and apatite fission track (similar to 8.4-7.3 Ma), our study indicates that ductile deformation associated with this rift ceased by similar to 14.2-13.5 Ma followed by brittle deformation that overlapped with STDS activity (similar to 14.2-11 Ma). Furthermore, compiled data on the activation timings of both structures along the Himalayas reveal that the timings of brittle motion on the STDS and ductile motion on the NSTRs overlap spatiotemporally along the Himalayas, with an eastward younging pattern, which challenges the popular idea that the transition from N-S to E-W extension represents an abrupt geodynamic shift. Instead, our findings support a refined interpretation of the mechanism involving eastward lithospheric flow driven by lateral detachment of the Indian slab, coupled with asynchronous gravitational potential energy gradients.

40Ar/39Ar同位素定年技术是一种在K/Ar同位素定年法基础上发展出来的另一种同位素定年方法,利用在核反应堆中对含钾样品进行快中子照射,达到使39 K衰变成39 A r,测定40 A r*/39 A r比值来计算样品的年龄.随着现代稀有气体同位素分析新技术研究的迅速发展,含钾样品的40 A r/39 A r同位素定年技术依靠其高准确性和高精度,已经发展成为当今最先进可靠和最具主要价值的岩石矿物同位素定年技术手段之一,从而被人们广泛地应用于各种重要的地质事件的精确定年,并为地质年代表提供"金钉子"般的年龄制约.首先总结了40 A r/39 A r同位素定年技术的发展历史、原理、样品处理分析等,然后重点阐述了应用40 A r/39 A r法这一同位素定年技术在青藏高原生长演化研究中如何解决关键性的科学问题,最后对同位素定年技术的发展作出了展望.

Post‐collisional ultrapotassic magmatism received special attention because it is an important indicator for mantle geodynamics in a collisional orogen. So far, no mantle‐derived magmatism has been identified within the Himalayan belt, hindering our understanding of the architecture and thermal state of the deep‐seated lithosphere in this area during the orogenesis. Here, we report the first discovery of mantle‐derived ultrapotassic rock in the Himalayan region. The middle Miocene (ca. 13 Ma) lamprophyre dike intruded the Tethyan Himalaya sedimentary cover of the Ramba dome on the shoulder of the Yadong‐Gulu rift. Geochemical analyses suggest that the ultrapotassic melt was derived from the subcontinental lithospheric mantle of the Indian plate, within a relatively shallow depth in the spinel stability field. The ultrapotassic magmatism indicates asthenospheric upwelling in the middle Miocene, a response to the mantle unrooting process beneath the Himalayan orogen.

We reconstruct the rise of a segment of the southern flank of the Himalaya-Tibet orogen, to the south of the Lhasa terrane, using a paleoaltimeter based on paleoenthalpy encoded in fossil leaves from two new assemblages in southern Tibet (Liuqu and Qiabulin) and four previously known floras from the Himalaya foreland basin. U-Pb dating of zircons constrains the Liuqu flora to the latest Paleocene (ca. 56 Ma) and the Qiabulin flora to the earliest Miocene (21- 19 Ma). The proto-Himalaya grew slowly against a high (similar to 4 km) proto-Tibetan Plateau from similar to 1 km in the late Paleocene to similar to 2.3 km at the beginning of the Miocene, and achieved at least similar to 5.5 km by ca. 15 Ma. Contrasting precipitation patterns between the Himalaya-Tibet edifice and the Himalaya foreland basin for the past similar to 56 m.y. show progressive drying across southern Tibet, seemingly linked to the uplift of the Himalaya orogen.

As the boundary thrust between India and Asia in southern Tibet, the Zhongba–Gyangze Thrust (ZGT) emplaced the Yarlung Zangbo Suture Zone (YZSZ) units in the hanging wall southward onto Tethyan Himalaya sequences (THS) of the northern Indian continental margin in the footwall. Detailed field investigation, electron backscatter diffraction (EBSD) analysis, detrital zircon U–Pb geochronology and 40Ar–39Ar thermochronology were conducted to understand the evolution of the ZGT in Sangsang area, central southern Tibet. The shear zone of ZGT is located within the sedimentary-matrix mélange of YZSZ that is mainly composed of matrix of blueschist with meta pelagic–hemipelagic siliceous and siliciclastic rocks and blocks of basalt, limestone and sandstone. Penetrative F1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope. Strong lattice preferred orientation (LPO) fabric initiated by the low-temperature (350−450°C) (010)[001] slip system was detected by EBSD in the sodic amphiboles of the blueschist. The 40Ar–39Ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of ZGT occurred between 71 and 60Ma. In the THS, a newly documented younger unit preserving detrital zircons from the southern Asian margin lies above the Triassic–Cretaceous sequences that carries only detrital zircons from the Indian continent. This unit is dated to be ~61Ma by the detrital zircon ages, similar to the Sangdanlin Formation and representing the Yarlung Zangbo Foreland Basin (YZFB) system. The ZGT had probably been active due to the initial India–Asia collision and acted as the frontal thrust controlling the development of YZFB.

A geologic investigation was undertaken in the Hoh-Xil–Songpan-Ganzi (HXSG) complex, northern Tibet in order to better understand magma genesis and evolution during the late stages of Paleo-Tethys ocean closure. The HXSG complex is composed of vast accumulations of Middle–Upper Triassic marine gravity flow deposits that were extensively intruded by igneous rocks. These early Mesozoic rocks exposed in this area record a rich history of accretionary tectonics during the amalgamation of the Tibetan Plateau terranes. Eight plutons sampled from the western HXSG complex yield zircon U–Pb ages that range from 225 to 193 Ma. Muscovite 40Ar/39Ar ages for the Hudongliang and Zhuonai Lake plutons yield ages of 210.7 ± 2.5 Ma and 212.7 ± 2.5 Ma, respectively. These plutonic rocks can be subdivided into two geochemically distinct groups. Group 1 (221–212 Ma: Dapeng Lake, Changhong Lake and Heishibei Lake plutons) is composed of high-K calc-alkaline rocks that have strongly fractionated REE patterns with high (La/Yb)N ratios (91–18) and generally lack Eu anomalies (Eu*/Eu = 1.02–0.68). Rocks in Group 1 display pronounced negative Nb–Ta and Ti anomalies on primitive mantle-normalized spidergrams. Group 1 rocks exhibit high Sr (782–240 ppm) and low Y (6.3–16.0 ppm) contents with high Sr/Y ratios (84–20). Based on Sr–Nd–Hf isotopic data (87Sr/86Sri = 0.7079–0.7090, εNd(t) = − 7.7–− 4.7, εHf(t) = − 5.7–− 0.8) and low MgO contents (MgO = 1.10–2.18%), Group 1 rocks are geochemically similar to adakitic rocks and were probably derived from partial melting of the downgoing Paleo-Tethys oceanic slab and overlying marine sediments. Group 2 plutons (225–193 Ma: Daheishan, Yunwuling, Zhuonai Lake, Malanshan and Hudongliang plutons) display lower P2O5 with increasing SiO2 and are medium-K to high-K I-type calc-alkaline bodies with low Sr (14–549 ppm) and high Y (22.3–10.5 ppm) contents. Group 2 rocks have variable fractionated REE patterns ((La/Yb)N = 3–38) and negative Eu anomalies (Eu*/Eu = 0.02–0.86). Together with Sr–Nd–Hf isotopes (87Sr/86Sri = 0.7072–0.7143, εNd(t) = − 6.6–− 2.0, εHf(t) = − 0.6–+ 3.0), Group 2 rocks are most likely formed by partial melting of the juvenile crustal sources. Collectively, these data suggest that the Hoh-Xil turbidites were underlain by more continental arc crust than previously thought. We propose that rollback of the subducting Paleo-Tethys oceanic slab led to partial melting of overlying continental arc fragments which developed beneath the HXSG gravity flow deposits.

The timing of uplift of the Tibetan Plateau remains controversial, with estimates varying from Eocene to more recent than Pliocene. In particular, the paleoaltimetry of the Qiangtang terrane, on the central Tibetan Plateau, is completely unknown. Here, we present new stable isotope results of fluvial/lacustrine carbonate cement, pedogenic carbonate and marl from the Kangtuo and Suonahu formations deposited between ∼51 and ∼28 Ma in the Heihuling–Bamaoqiongzong area of the northern Qiangtang terrane. The lithofacies associations indicate that the Kangtuo formation was deposited in alluvial fan and fluvial floodplain environments, and the Suonahu formation was deposited in near shore lacustrine, playa-lake and channelized fluvial environments. Carbon and oxygen isotope values, coupled with the sedimentary facies interpretations, point to evaporation and low respiration rates in the Eocene–Oligocene paleosols, suggesting an arid climate in the high Qiangtang area at the time. The δ18Opsw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ18Oc (PDB) values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as above 5000 m by at least the middle Oligocene (28 Ma), similar to the present elevation in this area. The aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high Lhasa terrane established by the middle Oligocene blocked the northward transport of tropical moisture. Maintenance of high elevation (>5000 m) from at least the Oligocene to the present suggests that the Qiangtang crust was already thickened by that time, and that underthrusting of India beneath Asia since then has continued to provide additional material while at the same time driving Asian lower crust eastward by crustal flow.

The provenance of the upper Paleocene to Early Eocene strata in Tethys Himalaya records early history of crustal thickening and sedimentation. This paper reports detailed in-suit detrital zircon U-Pb ages and whole rocks Nd-isotopes data from Upper Paleocene to Lower Eocene Jiachala Formation of the Tethys Himalaya in Gyantze, southern Tibet. The Jiachala Formation consists of sandstone and mudstone. The largest detrital zircon age population is within the range of 350~80 Ma, 900~470 Ma, and 1300~950 Ma. There are also some scatted ages between 2800~1500 Ma. Sandstones of the Jiachala Formation yield 87Sr/86Sr=0.707505-0.713174, 143Nd/144Nd=0.512206-0.512355, εNd(0) =-5.52~-8.43. Detrital sediments were mainly derived from the Xigaze forearc basin and Upper Triassic Langjiexue Group, and subordinately from the Yarlung-Zangpo suture zone. Our study indicates that the Jiachala Foramtion was deposited in peripheral foreland basin, and significant crustal thickening occurred during Early Eocene.

The Biluoxueshan deformation zone is an Early Eocene dextral transpressive zone located around the Eastern Himalayan Syntaxis, and it consists mainly of granite, mylonitic granite, mylonitic gneiss, and schist. Structural evidence indicates that the granitic plutons were emplaced into the zone as subvertical sheeted bodies, then progressively deformed and partly mylonitized. The zone is characterized by a vertical to subvertical foliation and a subhorizontal stretching lineation. Kinematic criteria indicate a dextral sense of shear. A solid-state tectonic fabric is preserved in the mylonitic and magmatic rocks. The submagmatic and weak solid-state fabrics in these granitic plutons all are concordant, and they record horizontal shortening, subvertical extension, and a significant component of dextral simple shear. The plutons are interpreted in terms of syntranspressional magmatism. The SIMS U–Pb ages of zircons in the granite plutons are 57–46Ma, and the ages of the mylonitic granites range from 34 to 22Ma. We interpret the timing of the transpressive deformation to be between 57 and 22Ma, and it may have started in the Late Paleocene and/or Early Eocene, corresponding to a period of crustal thickening due to India–Asia convergence. Movements dominated by dextral ductile shear occurred during the Early Oligocene to Early Miocene, accompanied by the continuous intrusion of granites. The total displacement along the zone is greater than 100km. 40Ar/39Ar analysis of both the mylonites and the undeformed granites provides additional limits on the timing of vertical exhumation deformation, which is most consistent with a Middle Miocene cooling event along the transpressional zone (19–16Ma). The Biluoxueshan shear zone, therefore, displays a complex geological history of deformation since ca. 57Ma, and it has played a significant role in the overall intracontinental deformation, rather than merely being a strike-slip fault for the southeastwards differential extrusion of the Indochina block.

The ~800-km-long dextral Karakoram fault zone (KFZ) bounds SW Tibet and, despite a number of recent studies, its spatial and temporal evolution and consequent role in continental deformation remains under debate. In order to better constrain fault evolution we present U–Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) and 40Ar/39Ar analyses of granitic samples from the southern portion of the Karakoram Fault. Zircons from the ductile shear zone in the Namru region of the Ayilari granite batholith record a crystallization age compatible with Gangdese magmatism belt (also known as the Trans-Himalayan magmatic Belt, THB) followed by a medium temperature cooling event between about 12.8–11.5Ma. And zircons from the ductile shear zone in the Zhaxigang region of the Ayilari granite batholith record a crystallization age of 18.7Ma. The Ayilari granite batholith near Namru appears to have experienced neither the 32Ma nor 25–22Ma magmatic events reported by previous studies. Biotite 40Ar/39Ar ages provide evidence that the 12.8–11.5Ma cooling event coincides with the initiation of the KFZ cutting through the Ayilari granite, both at Namru and Zhaxigang. Consequently, we conclude that the previously reported 32Ma, 25–22Ma and 22–14Ma events bear no relationship to the activity along the Karakoram fault, but resulted from either local metamorphic events caused by regional tectonics (such as activity on the south Kailas thrust fault), or from magmatism around Shiquanhe (also known as Gar). This study supports the conclusion that the KFZ, at least in the Zhaxigang–Namru region, propagated along strike to the Gar–Menshi area as a thin-skinned structure at around 13Ma.

Fossil tooth enamel from herbivores is considered one of the best proxies for paleoclimate and paleoelevation reconstructions, due to its low susceptibility to diagenetic alteration. A synthesis of oxygen and carbon isotope analyses of modern tooth enamel from herbivores such as Tibetan yaks, asses and antelopes is assessed. The average δ13C(PDB) value of herbivore tooth enamel in the Lhasa and south Qiangtang terrains is −11.3% ± 1.1%, whereas in the north Qiangtang and Hoh Xil terrains value is −10.2% ± 1.4% consistent with the current C3-dominant ecotype in the Tibetan Plateau. In addition, the average δ18O(PDB) values from tooth enamel show a northward systematic increase from −11.8% ± 3.4% in the Gyirong Basin, to −11.1% ± 1.1% in the Lhasa and southern Qiangtang terrains, and then to −9.0% ± 1.1% in the northern Qiangtang and the Hoh Xil areas, similar to those of the local river water and precipitation variation trends. It is suggested that δ18O(PDB) values of tooth enamel should not be used directly to evaluate the paleo-elevation of the Tibetan Plateau, because oxygen isotopic composition from tooth enamel is a combination of drinking water (precipitation- and river-sourced) and diet (plant-sourced). Moreover, an empirical relationship between oxygen isotope of tooth enamel and modern river water, and as well as elevation is recommended, which can be used in future studies of Tibetan Plateau paleoelevation.

Various sources of 21Ne and 22Ne exist in surface rocks: cosmogenic, in situ nucleogenic from internal U and Th, trapped crustal nucleogenic and trapped atmospheric. This paper reports the first measurement, in China, of cosmogenic 21Ne and 22Ne in surface bedrocks. We developed a unique sample pre-treatment procedure that effectively removed inclusions inside quartz grains, and thus maximally reduced nucleogenic contributions of 21Ne and 22Ne. Step-heating experiments show that concentrations of cosmogenic 21Ne and 22Ne in summit bedrock samples R9202 and R9203 from Grove Mountains, Antarctica, are (3.83±0.87)×108 and (5.2280.51)×108 atoms/g, respectively. The corresponding minimum exposure ages are 2.2±0.5 and 3.0±0.3 Ma. This indicates that the ice sheet in East Antarctica was uncovered the crest of Mount Harding, a typical nunatak in Grove Mountains, since at least mid-Pliocene.

The Grove Mountains, lying in the interior of East Antarctica, consist of 64 nunataks. Geomorphic characteristics of the nunataks suggest that past ice sheet elevations have overtopped the summits of the Grove Mountains. Cosmogenic 21Ne, 10Be and 26Al dating yields surface exposure ages of five bedrock samples taken from the crest of Mount Harding, a typical nunatak in the Grove Mountains. Using multi-nuclide fitting, we have calculated the time that the ice sheet retreated below the crest of Mount Harding; all data point to the late Miocene, ∼6.3Ma ago. The results provide the first land-based evidence of the elevation of the East Antarctic Ice Sheet in the Grove Mountains in Late Miocene, which reached 2300m, 200m higher than the current ice sheet level. The higher than current ice sheet elevations during the late Miocene together with contemporaneously higher temperatures in the Southern Ocean suggest that moisture transport plays an important role in ice sheet expansion in the interior of East Antarctica.

The ages of the east-west trending mafic-ultramafic dykes of southeast Tibet Plateau range between 145 Ma and 130 Ma. In Zhegu area, the subalkaline ultramafic picritic dykes are interpreted to have been highly partial melted from garnet lherzolite mantle source in the depth of 150 ∼ 180 km, 5∼6 GPa; alkaline ultramafic dyke tends to be the result of low partial melting in the spinel-garnet transitional zone; the mafic dykes' magmas have experienced partial crystallization at relatively low pressures and no longer stand for the primary magmas. Subalkaline ultramafic picritic dykes in southeast Tibet suggest the high degree partial melting event, indicating the existing of a mantle plume carrying huge energy and/or volatile in the northern Indian continental margin during late Jurassic to early Cretaceous.

利用氧同位素作为古高度计重建造山带的古高度是近年发展起来的应用比较广泛的方法.本文通过对青藏高原河水δ18Ow(SMOW)的空间分布特征分析,表明高原南北δ18Ow(SMOW)由于水汽来源和水汽循环方式不同存在显著差异.以中央分水岭山脉为界,南部δ18Ow(SMOW)平均值为-15.6‰左右,北部为-8.6‰左右;南部氧同位素值随高度的平均变化率为-0.24‰/100m,北部为-0.15‰/100m.分别建立了藏北地区和藏南地区河水氧同位素和高度的关系,同时应用可可西里及昆仑山口现代食草动物牙齿釉质、尼玛盆地现代土壤碳酸盐的氧同位素值对所建立的经验模型进行了检验,表明这两个模型分别应用于藏北和藏南地区古高度的恢复是可行的,为今后青藏高原古高度研究工作的开展提供了定量的计算方法.

The Karakorum fault zone (KFZ), composed of strike-slip faults, has played an important role in intra-continental deformation during the Cenozoic convergence between the Indian and Eurasian plates. However, the spatial and temporal evolution of the KFZ remains under debate. This paper reports new zircon U-Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) and biotite Ar-40/Ar-39 ages for samples from the fault zone to clarify the timing of events. The ages of Zircon U-Pb fall mainly around 47-50 Ma, which corresponds to a period of important magmatic activity of the Gangdese granitic belt (also known as the Trans-Himalayan magmatic belt). A metamorphic event followed in the period around 32 Ma, and that was then followed by a medium temperature cooling event between about 12 and 7.7 Ma. Biotite Ar-40/Ar-39 ages provide evidence that the 12-7.7 Ma cooling event coincides with the initiation of the KFZ cutting through the Ayilari granite. The Ayilari granite pluton does not seem to have experienced the separate 25-23 Ma cooling event, as shown in previous studies 60-80 km west of the current study area [Lacassin, R., Valli, F., Arnaud, N., Leloup, PH., Paquette, J.L, Li, H, Tapponnier, R, Chevalier, M.L. Guillot. S.. Maheo, G., Xu, Z., 2004. Large-scale geometry, offset and kinematic evolution of the Karakorum fault Tibet. Earth and Planetary Science Letters 219 (3-4), 255-269.; Valli, F., Arnaud, N., Leloup, H.P., Sobel, E.R., Maheo, G., Lacassin, R., Guillot, S., Li, H., Tapponnier, P., Xu, Z., 2007. Twenty million years of continuous deformation along the Karakorum fault western Tibet: a thermochronological analysis. Tectonics 26, doi: 10.1029/2005TC001913.: Valli, F., Leloup, P., Paquette, J., Arnaud, N., L!, H., Tapponnier, P., Lacassin, R., Guillot, S., Liu, D., Deloule, E., Xu, Z., Mahe'o, G., 2008. New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault. Tectonics 27, doi:10.1029/200TrC002184.]. We conclude that the 25-23 Ma cooling event bears no relationship to the activity of the KFZ, but resulted from a local metamorphic event caused either by a regional tectonic event (such as activity of the south Kailas thrust fault) or by magmatism around Shiquanhe (also known as Gar). The study of the KFZ around Namru supports the opinion that the KFZ propagated along the fault strike to the Gar-Menshi area around 12 Ma. (C) 2009 Elsevier B.V. All rights reserved.

A newly recognized east-west trending province of 43 to 28 Ma volcanic rocks occurs in the southern Qiangtang terrane of central Tibet. The lavas are Na-rich calc-alkaline in composition, relatively primitive, and locally host ultramafic and mafic xenoliths. Foliated mafic granulite xenoliths from similar to 28 Ma lavas equilibrated at temperatures in the range of 980 to 1260 degrees C, indicating that the southern Qiangtang terrane lower crust was deformed and heated to very high temperatures during or before the Oligocene. In the northern Qiangtang terrane is a parallel suite of volcanic rocks of coeval age. However, here, the volcanic rocks are (ultra)potassic in composition and underlain by a hot (T > 800 degrees C) metasedimentary-bearing lower crust. We suggest that both suites of Qiangtang lavas were derived from a primitive mantle source and that the enriched nature of the northern Qiangtang lavas reflects contamination by partial melts of metasedimentary lower crust. This contrasts with the conventional interpretation that Tibetan potassic lavas were solely derived from an ancient, enriched mantle lithospheric source. While removal of lithospheric mantle seems to be required to produce the high temperature melts, Eocene-Oligocene volcanism was coeval with thrust reactivation along bounding suture zones, implying that mantle dynamics were linked to intracontinental subduction. (c) 2006 Elsevier B.V. All rights reserved.

Granites sampled from Garzê-Litang thrust, Longmen Shan thrust, Garzê and Litang strike-slip faults in the eastern Tibetan Plateau have been analyzed with apatite fission track thermochronological method in this study. The measured fission track apparent ages, combined with the simulated annealing modeling of the thermal history, have been used to reconstruct the thermal evolutionary histories of the samples and interpret the active history of the thrusts and faults in these areas. Thermal history modeling shows that earlier tectonic cooling occurred in the Garzê-Litang thrust in Miocene (∼20–16 Ma) whereas the later cooling occurred mainly in the Longmen Shan thrust since ∼5 Ma. Our study suggests that the margin of eastern Tibetan Plateau was extended by stages: through strike-slip faults deformations and related thrusts, the upper crust formed the Garzê-Litang margin in the Miocene epoch and then moved to the Longmen Shan margin since ∼5 Ma. During this process, the deformations of different phases in the eastern Tibetan Plateau were absorbed by the thrusts within them and consequently the tectonic events of long-distance slip and extrusion up to hundreds of kilometers have not been found.