East Tibet Research Articles

Assessment of Simulations of Snow Depth in the Qinghai-Tibetan Plateau Using CMIP5 Multi-Models

The depths of snow in the Qinghai-Tibetan Plateau simulated by 19 phase five of the Coupled Model Inter-Comparison Project (CMIP5) models are assessed in comparison with the observation ones from passive microwave satellite remote-sensing (PMSR) and 72 weather stations. The annual regional mean snow depths (ARMSDs) from 1851 to 2005 indicate decreases for most of the models but slight increases for CMCC-CM, FGOALS-g2, and CanESM2. The simulated ARMSD anomalies vary from −10 cm to 20 cm, and the amplitudes exhibit obvious decreases over the recent 50 years. In the period from 1986 to 2005, the spatial distributions of the annual mean snow depth (AMSD) simulated by MRI-CGCM3, CCSM4, CESM1-BGC, CESM1-CAM5, and CESM1-WACCM are quite similar to those from the PMSR observations. But the maximum and mean AMSD values simulated by all 19 models are much larger than those observed by PMSR. The inter-annual variation of snow, which occurs primarily in the Middle and East Tibetan Plateau, is simulated relatively well by inmcm4. From 1960 to 2005, the ARMSDs simulated by most of the models show decreases but are increasing according to actual observations. Only five models produce positive trends that agree with the observations. According to the selected indices, the CCSM4, MIROC5, MRI-CGCM3, and FGOALS-g2 models are selected as relatively good models for predicting the plateau snow, and the 21st century changes in the snow climate in the Qinghai-Tibetan Plateau are projected using these four models.

COMPARING ICESAT/GLAS BASED ELEVATION HEIGHTS WITH PHOTOGRAMMETRIC TERRAIN HEIGHTS FROM UAV-IMAGERY ON THE EAST TIBETAN PLATEAU

Abstract. Digital elevation models (DEMs) and height measurements are broadly used in environmental studies. Two common elevation sources are the Ice Cloud and land elevation Satellite (ICESat), which acquired laser range measurements with the Geoscience Laser Altimeter System (GLAS) across the globe and elevation data from the Shuttle Radar Topography Mission (SRTM). Current developments of small unmanned aerial vehicles (UAV) provide the opportunity to collect aerial images of remote areas at a high spatial resolution. These can be further processed to digital surface models by stereophotogrammetry and provide a reliable data source to evaluate coarse scale Digital Elevation Models (DEMs). This study compares ICESat/GLAS and SRTM90 elevation data against photogrammetric terrain heights within GLAS footprints on high altitudes on the East Tibetan Plateau. Without vegetation-bias, we were able to examine height differences under different topographic conditions and of different acquisition dates. Several resampling techniques were applied to SRTM90 data and averaged height within each footprint was calculated. ICESat/GLAS heights (n = 148) are most similar to UAV data based elevations with an averaged difference of −0.8m ±3.1m. Results furthermore indicate the validity of ICESat/GLAS heights, which are usually removed from analyses by applying different quality flags. Smallest difference of SRTM90 to UAV based heights could be observed by a natural neighbour resampling technique (averaged 3.6m ±14m), whereat other techniques achieved quite similar results. It can be confirmed that within a range of 3,800–4,200m above mean sea level the ICESat/GLAS heights are a precise source to determine elevation at footprint geolocation.

Topographic architecture and drainage reorganization in Southeast China: Zircon U-Pb chronology and Hf isotope evidence from Taiwan

The uplift of Tibet Plateau and the marginal sea spreading have had important influence on the tectonic, landform and drainage system in East Asia, although the marginal sea spreading in shaping the topography and drainage reorganization in East Asia has been still controversial. Here we present U-Pb age and Hf isotopic composition of detrital zircon grains from Cenozoic sedimentary rocks in Taiwan to understand how the provenance and river systems evolved. Our data show that the U-Pb age spectra of detrital zircon grains in Paleogene sandstones are dominated by Yanshanian (180–67Ma) zircon grains and with subordinate or nil Proterozoic and Archean zircon grains. These results are in contrast to those in Miocene rocks that are dominated by the Indosinian (257–205Ma) zircon grains together with Yanshanian, Proterozoic and Archean population. The initial Hf isotope ratios [εHf(t)] of the zircon grains also display systematic change in Paleogene and Neogene strata. Our data demonstrate that the Hsuehshan Range and Western Foothills in Taiwan have the same sedimentary sources. The source region of Paleogene strata was mainly located at the coast in southeast China and migrated inland over time. The source might have reached the Lower Yangtze region during early Miocene. Although the mechanism of transport of sediments from the Lower Yangtze region to Taiwan is unclear, we speculate that the Minjiang River might have been larger in Early Miocene than the present and might have delivered inland material along the boundary of Yangtze and Cathaysia Blocks to Taiwan. These were then captured by the Yangtze River systems at some time after Late Miocene. This change corresponds to the time of the drainage reorganization in East Tibet, such as Yangtze River, and the regional subsidence resulting from the opening of marginal sea. The combined effects of Tibet uplift and opening of marginal sea might have shaped the topography and river system reorganization in East Tibet. The evolution of topography and drainage systems in southeast China seems to be mainly controlled by the opening of marginal sea.

Geochemical composition of the last deglacial lacustrine sediments in East Tibet and implications for provenance, weathering, and earthquake events

Abstract Identifying and understanding the respective influence of climate and tectonic processes becomes a frontier topic in tectonically active mountain ranges. In the eastern Tibetan Plateau, earthquake occurred frequently and usually led to formation of many dammed lakes. In this study, a last deglacial section of lacustrine sediments at Xinmocun in eastern Tibet is selected for conventional X-ray fluorescence (XRF) and Scanning XRF (SXRF) elemental analysis. The high correlation of major and trace element abundances between the Xinmocun lacustrine samples and the loess-soil samples from the Chinese Loess Plateau (CLP) supports the previous view of the eolian origin of the Xinmocun lacustrine sediments. Analysis of major element abundances, ratios, and trace element ratios indicates that the dust provenance of the Xinmocun lacustrine sediments is different from that of the CLP, and is similar to that of the loess at Ganzi and Hongyuan nearby the study area. The significant variations of most geochemical elements and their close coupling with grain-size variations cannot be reasonably explained by the changes in transport dynamics of eolian dust, and is possibly caused by the intermittent changes in available dust provenance. Frequent earthquakes triggered abundant landslides and provided large amounts of dust for the study area. Accordingly, in the >16 μm fraction of the Xinmocun grain-size record, many element abundances and their ratios, such as SiO2/Al2O3, TiO2/Al2O3, CaO/Al2O3, Sr/Al2O3, Rb/Sr, and Na2O/Al2O3, can be regarded as sensitive indicators of earthquake events in the tectonically active regions. SXRF measurements at the U-channel surface can provide some cost-effective indicators of seismic events, such as Si/Al, Ti/Al, Ca/Al, Sr/Al, Zr/Rb, and Rb/Sr. Significant variations in most element abundances and their ratios of the Xinmocun lacustrine sediments correlate closely with those of its grain-size record. They all show no increasing or decreasing trends with time. These suggest that tectonic activities characterized by seismic events possibly had a major role on the landscape erosion in the eastern Tibetan Plateau, while the climatic influence seems minor.

Middle Jurassic syn-kinematic magmatism, anatexis and metamorphism in the Zheduo-Gonggar massif, implication for the deformation of the Xianshuihe fault zone, East Tibet

The Xianshuihe fault (XSHF) zone is an important lithospheric structure in east Tibetan Plateau. The Zheduo-Gonggar massif, about 120km long and 13–18km wide, developed along its eastern segment. Based on geological mapping, meso- and micro-structure studying and geochronology testing, the massif is divided into 5 units: (1) Intrusions of 216–204Ma, (2) Intrusions of ∼170Ma, (3) mid-Jurassic sillmanite-grade metasediments, (4) Oligo-Miocene migmatite zone, (5) Intrusions less than 20Ma ago. Intrusions of 216–204Ma related to the Songpan–Garze folding and distributes on the southern segment of the massif. The middle Jurassic intrusions which were found for the first time in the region, includes the syn-kinematic Zheduoshan granite and a suit of leucogranite. The former constrained by the youngest U–Pb age of 171Ma which provides a minimum shear age of the XSH fault. The latter, U–Pb age of 170Ma, is a common feature of convergent orogens or intracontinental subduction. The Oligo-Miocene migmatite zone, about 80km long and 1–3km wide, developed on the eastern edge of the massif. Intrusions less than 20Ma ago is located on the northern segment of the massif. It had been widely researched that the latter two units are associated to the Cenozoic deformation of the XSH fault. While the mid-Jurassic syn-kinematic magmatism, anatexis and metamorphism along the XSH fault suggests a more strong Mesozoic tectono-thermal event in the fault zone. The discoveries provide us with a new perspective on understanding the Mesozoic and Cenozoic deformation of the XSH fault zone, southeastern Tibet.

Mesozoic–Cenozoic evolution of the Danba dome (Songpan Garzê, East Tibet) as inferred from LA-ICPMS U–Pb and fission-track data

The eastern margin of the Tibetan Plateau is a key region to understand the formation and tectonic evolution of the plateau. Most studies concentrated on the Longmen Shan range that corresponds to the tectonically very active boundary between the Tibetan Plateau and the Sichuan Basin. This work addresses the deformation and exhumation pattern in the poorly understood metamorphic Danba dome situated at the conjunction between the southern Longmen Shan range and the major strike-slip Xian Shui He fault. The Early Mesozoic and Cenozoic evolution of the dome are then compared to the regional evolution of eastern Tibet. The about 830Ma zircon U–Pb ages obtained on the Gezong and Gongcai gneiss as well as U–Pb ages on inherited zircons in the Early Mesozoic granites confirm the occurrence of Neo-Proterozoic magmatic episodes in the Danba basement at 800–900Ma and 650–750Ma. Following the closure of the Palaeotethys Ocean, granitoids were emplaced in the Danba metamorphic basement and Triassic cover from about 230Ma to about 170Ma. The western part of the Lian He Kau granitic massif, emplaced (between 210 and 200Ma) within the Triassic cover sequence of the dome was only slightly affected by the Early Mesozoic deformation. However, in the eastern, deeper level compartment of the massif, ductile deformation occurred between 200 and 190Ma, contemporaneous with the onset of the Barrovian-type metamorphism dated at 200–180Ma. That deformation is compatible with the Early Mesozoic deformation observed in the Longmen Shan range and with the development of the large-scale décollement level in the Songpan Garzê area. Apatite fission track data then indicate that, following the Oligocene–Middle Eocene onset of eastward motion of the Songpan Garzê terrane, the Danba dome developed as a crustal-scale antiform during the Late Miocene–Early Pliocene (10–4Ma). The exhumation was controlled by the Lian He Kau fault (a major transpressive structure parallel to the Xian Shui He fault) and the associated Manai thrust faults. Regionally, while the Oligocene–Middle Miocene exhumation phase seems to have been of regional extent, the Late Miocene to Present day phase is localised along the Longmen Shan range thrusts as well as along the back-thrusts formed by the Lian He Kau, Manai and associated faults.

An Assessment of Design of Observation Network over the Tibetan Plateau Based on Observing System Simulation Experiments (OSSE)

An Observing System Simulation Experiment (OSSE) was conducted to test the rationality and regional representation of the layout of the upper-air observation system over the Tibetan Plateau and neighboring areas and to consider the actual significance of observation stations. The simulations were validated primarily by comparing results from corresponding experiments wherein simulated observations were assimilated into the initial fields or not. The results showed that there were significant differences in prediction for the different layouts. The layout with both existing and planned observation stations was better than the ones with only existing or only planned stations in terms of forecast accuracy, especially around the areas with new observations added. Specifically, for winds, there were large improvements in Xinjiang, Qinghai, Gansu Corridor, Tibet, Yunnan and mid-west and northeast Sichuan. For temperature and relative humidity, the most significant improvement was in Tibet, southwest and east Qinghai, mid-east Sichuan, and mid-north Yunnan. For heights, the major improvements were in Xinjiang, Qinghai, east Tibet, mid-west Sichuan and Yunnan. The results also validated the layout of the observations and indicated the necessity to add planned observations over the Tibetan Plateau and neighboring areas.

Accelerated foliar litter humification in forest gaps: Dual feedbacks of carbon sequestration during winter and the growing season in an alpine forest

Can forest gaps lead to constrained litter decomposability by redistributing heat and moisture conditions, thereby increasing carbon sequestration from plant to soil via litter humification in alpine forests? We studied mass losses, humic substances, humic acid, fulvic acids, as well humification degrees and humification ratios in six foliar litters with a field litterbag experiment from the gap center, canopy gap and expanded gap to the closed canopy in winter and the growing season in an alpine forest of the east Tibetan Plateau. Humification degrees of 18%–45% for birch, 15%–40% for fir, 8%–30% for willow, 14%–26% for cypress, 9%–25% for larch and 7%–19% for azalea foliar litter were observed in forest gaps and the closed canopy over one year of incubation. Small amounts of humic substances accumulated in winter, whereas considerable humic acid accumulated, but fulvic acid mineralized during the growing season. Compared with the closed canopy, foliar litter humification in forest gaps was lower in winter but greater in the growing season, implying a dual role of forest gaps in carbon sequestration between winter and the growing season. Carbon sequestration could be accelerated in forest gaps. Reduced snow cover under a scenario of winter warming would stimulate soil carbon storage in these alpine forests.

Study on crustal magnetic anomalies and Curie surface in Southeast Tibet

In this paper, the Potsdam model POMME-6.2 is used to investigate the distributions of crustal magnetic anomalies and Curie surface in Southeast Tibet. The Curie surface is compared with the regional heat flow, Bouguer anomaly, Moho depth, and seismicity. The results show that the magnetic anomalies and Curie surface are both consistent with the geological structure. Sichuan Basin exhibits a high positive anomaly, while orogenic belts such as the Longmenshan, northwestern Sichuan, and western Yunnan, exhibit weak positive or negative anomalies. The distribution of magnetic anomaly confirms that escape flow from east Tibet branches into northeastward part and southward part on west Sichuan Basin, due to resistance by the rigid basin. The depth of Curie surface ranges from 20 to 34km. The Curie surface beneath the Longmenshan, Xiaojiang and Lijiang–Xiaojinhe faults is shallow, with the uplift strike consistent with the faults. The Curie surface beneath Sichuan Basin and the central Bayan Har massif is deep, with sheet-like depressions. Strong earthquakes primarily occurred in the areas with the uplift of Curie surface. The heat flow values near Tengchong, Lijiang, Dali and Kunming are high and the Curie surface there is shallow.

Unprecedented recent warming rate and temperature variability over the east Tibetan Plateau inferred from Alpine treeline dendrochronology

Despite instrumental records showing recent large temperature rises on the Tibetan Plateau (TP), only a few tree-ring temperature reconstructions do capture this warming trend. Here, we sampled 260 trees from seven Alpine treeline locations across the southeast TP. Standardized tree-ring width chronologies of Abies squamata and Sabina squamat were produced following Regional Curve Standardization detrending. The leading principal component of these records is well correlated with the regional summer (JJA) minimum temperature (MinT) (R2 = 0.47, P < 0.001, 1953–2009). Hence we produce a regional summer MinT reconstruction spanning the last 212 years. This reconstruction reveals a long-term persistent warming trend, starting in the 1820s, at a rate of 0.45 ± 0.09 °C/century (1820–2009). This trend is also detected since the 1820s in the Asian summer MinT reconstruction produced by the PAGES 2K project, with a very close warming rate (0.43 ± 0.08 °C/century, 1820–1989). Our record also displays an enhanced multi-decadal variability since the mid-twentieth century. The 1990s–2000s are the warmest of our whole record, due to the superposition of the gradual warming trend and decadal variability during this interval. The strongest decadal cooling occurs during the 1950s and the largest warming trend during the 1970s. The magnitude of warming from 1973 to 2003 was larger than the total warming trend from 1820s to 2009. Extreme events are also more frequent since 1950. The pattern of multi-decadal variability has similarities with the Atlantic multi-decadal oscillation, suggesting common causality. CMIP5 historical simulations fail to capture both the magnitude and timing of this multi-decadal variability. The ensemble CMIP5 average produces a steady warming trend starting in the 1970s, which only accounts for about 60 % of the observed warming trend during this period. We conclude that TP summer temperature could reflect a climate response to increased greenhouse gas concentrations, however modulated by multi-decadal variations common with the Atlantic sector.

OSL and pollen concentrate 14C dating of dammed lake sediments at Maoxian, east Tibet, and implications for two historical earthquakes in AD 638 and 952

Formation of dammed lakes provides exceptionally important information of continental geological processes, responding to tectonic and climatic influences. Establishing accurate geochronological frameworks within lake strata is challenging because the stratigraphy is often bereft of biostratigraphy and directly dateable material. Optically Stimulated Luminescence (OSL) and AMS 14C dating of pollen concentrates are well-established tools for dating lake sediments. Whether they are suitable for lake sediments in high-alpine settings remains uncertain. In this study, OSL and AMS 14C dating of pollen concentrate were conducted on the Diaolin section in a high-alpine setting in the eastern Tibetan Plateau. Good match of both dating results suggests that they are fit for dating lake sediments in high-alpine settings. More than 300 g of sediment is required for preparation of pollen concentrates. During the pretreatment, ∼3% NaOH solution should be added to the sample, and then heated until just boiling (≤5 min) because NaOH treatment easily destroys pollen grains. Applying the heavy liquid flotation with specific gravity of 1.74–1.76 is useful to isolate relatively pure pollen grains. Sieving with a 20-μm and 63-μm mesh can concentrate pollen grains substantially. The OSL and AMS 14C dating yielded the basal age of the Diaolin section (650 AD). This indicates that the dammed-lake formed around 650 AD, probably caused by the earthquake occurring in the study area in 638 AD. The seismites characterized by soft-sediment deformation and phyllite layer happened at 780–980 AD, probably corresponding to the earthquake occurring on November 20, 952 AD in the study area.

Tectonics, topography, and river system transition in East Tibet: Insights from the sedimentary record in Taiwan

The Cenozoic in East Asia is marked by major changes in tectonics, landscapes, and river systems, although the timing and nature of such changes remains disputed. We investigate the geochemistry and neodymium isotope character of Cenozoic mudstones spanning the breakup unconformity in the Western Foothills of Taiwan in order to constrain erosion and drainage development in southern China during the opening of the South China Sea. The La/Lu, Eu/Eu*, Th/Sc, Th/La, Cr/Th, and εNd values in these rocks show an abrupt change between ∼31 and 25 Ma. Generally the higher εNd values in sediments deposited prior to 31 Ma indicate erosion from Phanerozoic granitic sources exposed in coastal South China, whereas the lower εNd values suggest that the main sources had evolved to inland southern China by ∼25 Ma. The SHRIMP U-Pb ages of zircons from a tuff, together with biostratigraphy data constrain the breakup unconformity to be between ∼39 and 33 Ma, suggesting that the seafloor spreading in the South China Sea commenced before ∼33 Ma. This is significantly older than most of the oceanic crust preserved in the deeper part of the basin. Diachronous westward younging of the breakup unconformities and provenance changes of basins are consistent with seafloor spreading propagating from east to west. Initial spreading of the South China Sea prior to ∼33 Ma corresponds to tectonic adjustment in East Asia, including extrusion of the Indochina block and the rotation and eastward retreat of the subducting Pacific Plate.

The Deformation Features of Crustal Structure Beneath the East Tibetan Margin and Neighboring Areas

Shear wave splitting analysis is one of the most commonly used techniques in structural seismology. In this study, the splitting parameters of Moho converted P-to-S phase (Pms), the time difference between maximal energy of qS1 and qS2 and their amplitude ratio were obtained. The data were from 29 broad-band stations across the Longmenshan fault belt. Subsequently, the deformation features of the crustal structure beneath the east Tibetan margin and Sichuan basin were analyzed. Our results show that the time delay and the time difference between maximal energy of qS1 and qS2 in east Tibetan margin is obviously larger than Sichuan basin. However, the amplitude ratio between qS1 and qS2 beneath Longmenshan fault is much smaller than other tectonic sub-regions. These results may suggest the existence of fluid beneath the Longmenshan fault.

Crustal structure across the Kunlun fault from passive source seismic profiling in East Tibet

The crust beneath the northeastern (NE) Tibetan Plateau records the imprints on Paleozoic Kunlun orogen and far field effects from continental collision and convergence between the Indian and Eurasian plates. A passive source seismic profile was conducted across eastern Kunlun mountains (also called Animaqing suture belt). Receiver function imaging and H-k stacking results with this dataset demonstrate that (1) crust slightly thins from about 64 km under the Songpan–Ganzi terrane to about 56–62 km under the Qaidam–Kunlun block and the Qilian block; (2) Moho topography is relatively smooth with gradual undulation beneath east Kunlun orogeny belt, in contrast to 2–5 km Moho step at the AKMS/Kunlun fault from previous deep seismic soundings or even no Moho topography variation from recent deep seismic reflection profiling; (3) the average crustal Vp/Vs ratios display an increasing trend to the Kunlun fault belt from 1.76 to 1.85 in Songpan–Ganzi terrane, while decreases from 1.83 to 1.65 away from the fault belt in Qaidam–Kunlun block; (4) The high Vp/Vs ratio in Songpan–Ganzi terrane, is similar to previous results from the profile across Longmenshan fault belt. The normal to low Vp/Vs ratio distribution in Qaidam–Kunlun block supports the viewpoint that the deformation occurring in NE Tibet is predominated by upper-crustal thickening; and (5) the thickness of non-seismogenic layer varies slightly about 40 km between 90° and 100°E, then decreases to less than 20 km around 106°E along the Kunlun fault. The change of the non-seismogenic layer thickness is consistent with the change of the lower crust thickness derived from deep seismic sounding. The increased crustal thickness may be due to the differences in the thickness of the crust of the two plates before their collision, and/or largely achieved by thickening of the lower crust, perhaps indicating a crust flow mechanism operating more strongly in the western region.

Provenance and earthquake signature of the last deglacial Xinmocun lacustrine sediments at Diexi, East Tibet

Well-preserved lacustrine sediments are found in some areas in East Tibet. This region is characterized by a windy and semi-arid climate, alpine valleys, and frequent earthquakes. Measurements of rare earth elements, observations from a scanning electron microscope and a high-resolution record of grain-size measurements allowed us to compare fine sediments from the Xinmocun section in the Diexi Lake, with loess from the Chinese Loess Plateau and South China. Results indicate that fine grains of the Xinmocun lacustrine sediments were transported by wind and trapped in the lake, whereas the >16μm fraction was likely from local sources. The grain-size changes within the section repeatedly show abrupt coarsening and upward fining, probably due to palaeoearthquake events. Large earthquakes in the study area often caused rockfalls and landslides, exposing fine sediments that had accumulated on mountains' slopes. The fine grains were then retransported by wind to the Diexi Lake. Optically stimulated luminescence dating of the Xinmocun section indicates continuous deposition from 18.65 to 10.63ka. These results indicate that palaeoearthquakes in the study area had a mean recurrence interval of ~0.32ka. Therefore, we propose that lacustrine sediments in a tectonically active region have the potential to record a continuous history of palaeoearthquakes. Palaeoearthquakes probably produced numerous rockfalls and landslides in alpine valleys and provided significant sources of regional eolian dust.

Density structure and isostatic state of the crust in the Longmenshan and adjacent areas

Abstract In this study, we've analyzed some new gravity data obtained from two profiles across the Longmenshan (LMS) Fault system in order to determine the density structure and isostatic state of the crust beneath the Songpan–Ganzi block (SG) and the Sichuan basin (SC). According to our research, Bouguer gravity anomalies along the two profiles range from − 450 mGal in the SG to − 80 mGal in the SC. And density structural models show obvious differences between the SG and the SC. Compared with the SC, the SG has lower average density in the crust, and it has a low-density layer with thickness of 5 km in the middle crust. In the SC area, however, the crustal density increases gradually with depth and there exists no low-density layer in the crust. The Moho interface decreases from 60 km beneath the SG in east Tibet to 40 km beneath the SC. In the LMS region, the topography, Bouguer gravity, and Moho depth change abruptly in a narrow tectonic belt. The crustal isostatic state, which is derived from the Airy compensation model based on the topography, gravity and deep seismic sounding data, indicates that the SG on the northwest side of the LMS is not in an isostatic state, compared with the isostatic crust beneath the SC. The isostatic equilibrium of the crust under the northeast section of the LMS is not achieved as that in the southwest section of the LMS. Based on the research, we infer that the non-homogenous collision between the Tibetan Plateau and the Yangtze Craton has caused the strong variation of the Moho depth and the special isostatic state of the crust in this area.

Zircon U–Pb geochronology of the Konggar granitoid and migmatite: Constraints on the Oligo-Miocene tectono-thermal evolution of the Xianshuihe fault zone, East Tibet

Abstract The Konggar massif, about 120 km long and 13 – 18 km wide, developed along the eastern segment of the Xianshuihe fault zone in East Tibet. It consists of two rock units: the Konggar granitic pluton and an elongate migmatite zone about 70 km long and 1 – 3 km wide. A metamorphic event was well recorded by the growth rims of zircons in the migmatite, their SHRIMP U–Pb dating of two rock samples taken from leucosome and melanosome yields respectively the ages of ca. 31.75 Ma and ca. 26.9 Ma. The SHRIMP U–Pb dating of zircons from two rock samples of the granitic pluton yields respectively the crystalline ages of 17.35 Ma and 14.4 Ma. These new data, together with the previously zircon U–Pb ages, the mica Ar–Ar dating ages and whole rock Rb–Sr age of the granitoid, allows to constraining the tectono-thermal evolution history of the Xianshuihe fault zone. It is inferred that this fault zone suffered along its eastern segment from high temperature metamorphism and migmatization during the Oligocene (in 32 – 27 Ma), which was followed by magma intrusion during the Miocene (in 18 – 12 Ma); sinistral shearing began to occur at about 10 Ma and continues to present-day.

Study of methods for extraction of remote sensing information of rocks and altered minerals from Luobusha ophiolite in east Tibet

With Luobusha ophiolite as the study case and on the basis of remote sensing data of ETM and ASTER,the authors used the ratio and principal component analysis methods to extract the altered mineral information of iron-stain alteration(magnetite and olivine) and that of hydroxyl alteration(serpentine and chlorite) respectively so as to study the utilization of remote sensing technology to recognize and extract the rock and altered mineral information of ophiolite.In addition,pure pixel index(PPI) method was employed to extract endmember spectrum.In combination with the available field lithologic information,dunite was distinguished from peridotite by methods of spectral angle mapper(SAM) and spectral feature fitting(SFF).On such a basis,the spatial distribution of Luobusha ophiolite rock and its main altered minerals was determined.The results of comparative analysis show that rock and mineral information extracted by different methods and different data types can be verified reciprocally.The extracted information of dunite and peridotite is well consistent with the extracted information of iron-stain alteration and hydroxyl alteration and is also coincident with the result of the ground survey.The results of this study show that the methods proposed in this paper are feasible for extraction of ophiolite rock and mineral remote sensing information,and have good geological effect.

Calculation and Analysis of Annual Total Solar Radiation in Complicated Topography

Abstract. Solar radiation is the most important energy source in the Earth. And its spatial distribution has critical effects on the life and environmental process on the Earth. The characteristics of spatial distribution of total solar radiation in the complicated topography in the mountain area of east Tibetan Plateau was simulated in the present study. The three kinds of radiation, i.e. direct solar radiation, diffusive solar radiation and total solar radiation were calculated based on the existing astronomical parameters and experiential formula. The calculation of topographical shading was modified by using the 30m-resolution DEM data. The results revealed in general that the total solar radiation tended to decrease northwest-southeastwardly. At regional scale, the latitudinal zonality of the spatial distribution of annual total solar radiation differed insignificantly. Local topography affected profoundly the spatial distribution of annual total solar radiation. The annual total solar radiation in complicated topography was relatively less, whereas there was significant inner difference.

Crustal anisotropy from Moho converted Ps wave splitting analysis and geodynamic implications beneath the eastern margin of Tibet and surrounding regions

In addition to crustal thickening, distinctly different mechanisms have been suggested to accommodate the huge convergences caused by the continental collision between India and Eurasia. As the transition zone between the two grand tectonic domains of Asia, the Tethys and the Pacific, east Tibet and its surrounding regions are the ideal places to study continental deformation. Pervasive rock deformation may produce anisotropy on the scale of seismic wavelengths; thus, seismic anisotropy provides insight into the deformation of the crust and mantle beneath tectonically active domains. In this study, we calculated receiver function pairs of radial- and transverse-components at 98 stations located in Sichuan and Yunnan provinces, China. We selected 7423 pairs with high signal-to-noise ratio (SNR) and unambiguous Moho converted Ps phases (Pms) to measure the Pms splitting owing to the crustal anisotropy. Both the crustal thickness and the average crustal Vp/Vs ratio were calculated simultaneously by the H–k stacking method. The geodynamic implications were also investigated in relation to surface geological features, GPS velocities, absolute plate motion (APM), SKS/SKKS splitting, and other seismological observations. In addition to the fast polarization directions (FPDs) of the crustal anisotropy, we observed a conspicuous sharper clockwise rotation around the eastern Himalayan syntaxis than was revealed by GPS velocities. The distributed FPDs within and near the main active fault zones also favored the directions parallel to the faults. This implied that the deformation of a continuous medium revealed by GPS motions is a proxy for the deformation of the brittle shallow crust only, while the main active faults and the deep crustal interiors both play important roles in the deep deformation. Our results suggest that the deformation between the crust and upper mantle within the northernmost section of the Indochina block is decoupled due to the large difference in the directions between the observations related to the crust (GPS and crustal anisotropy) and mantle (APM and mantle anisotropy). Focusing on the transition zone between the plateau and the South China and Indochina blocks, we suggest that the motion of the Central Yunnan sub-block is a southeastward extrusion by way of tectonic escape. There is less deformation in the deep crust and the motion is controlled by the active boundary faults of the Ailaoshan–Red River shear zone to the west and the Xianshuihe–Xiaojiang fault to the east; the lower crustal flow within the plateau southeastward reached the Lijiang–Xiaojinhe fault, but further south it was obstructed by the Central Yunnan sub-block.