Southeastern Tibet Plateau Research Articles

Syn-subduction crustal shortening produced a magmatic flare-up in middle Sanjiang orogenic belt, southeastern Tibet Plateau: Evidence from geochronology, geochemistry, and structural geology

The Permo-Triassic Jomda-Weixi-Yunxian continental margin arc belt in the southeastern Tibetan Plateau was formed by the subduction of the main Paleotethyan branch underneath the Cathaysian-affinity blocks along the Longmu Co-Shuanghu-Changning-Menglian suture. This belt consists of three segments with distinctive magmatism history. Available geochronological data revealed that the northern and southern segments of this belt comprise several volcanic successions intercalated with terrestrial clastics suggesting long-lived and pulsed magmatism. In contrast, the middle segment is made of a single volcanic pile of dacite/rhyolite and minor basalt with thickness of >1.6km. Numerous granodiorite batholiths and a few diabase dykes intruded the volcanic pile. Detailed zircon U-Pb dating indicates that the volcanic rocks and intrusive rocks were formed during a short time period of ~7millionyears (~244–251Ma), defining a magmatic flare-up. Petrographic and geochemical data including Sr-Nd isotopics demonstrate that the felsic magmas were derived from ancient crust, while the basalt and diabase are sourced from the mantle wedge induced by subduction of the Paleo-Tethys. Structural studies reveal syn-subduction crustal shortening and thickening of the middle segment. The thickened crust likely has prevented the subduction-induced basaltic magma from eruption; they alternatively were accumulated in the bottom of the crust yielding a magma chamber. The chamber heated, softened, and melted the crust, forming felsic magmas. When the crust was weakened to a critical threshold, the felsic magmas rose up and erupted to the surface, resulting in the magmatic “flare-up”. This study suggests that syn-subduction shortening and thickening of the overriding crust seems to have played a significant role in the development of magmatic flare-ups along a continental margin arc-system. Other well-studied examples include the Mesozoic Serra Nevada and the late Miocene to Holocene Altiplano-Puna Plateau, central Andes.

Rainfall characteristics and thresholds for periglacial debris flows in the Parlung Zangbo Basin, southeast Tibetan Plateau

The Parlung Zangbo Basin in the southeastern Tibet Plateau is affected by the summer monsoon from the Indian Ocean, which produces large rainfall gradients in the basin. Rainfall data during 2012–2015 from five new meteorological stations are used to analyse the rainfall characteristics. The daily rainfall, rainfall duration, mean rainfall intensity, and peak rainfall intensity are consistent, but sometimes contrasting. For example, these values decrease with increasing altitude, and the gradient is large downstream and small upstream, respectively. Moreover, the rainfall intensity peaks between 01:00 and 06:00 and increases during the afternoon. Based on the analysis of 14 debris flow cases in the basin, differences in the rainfall threshold differ depending on the location as sediment varieties. The sediment in the middle portions of the basin is wet and well structured; thus, long-duration, high-intensity rainfall is required to generate debris flows. Ravels in the upstream area are arid and not well structured, and short-duration rainfall is required to trigger debris flows. Between the above two locations, either long-duration, low-intensity rainfall or short-duration, high-intensity rainfall could provoke debris flows. Clearly, differences in rainfall characteristics and rainfall thresholds that are associated with the location must be considered in debris flow monitoring and warnings.

Late Quaternary paleoseismology of the Milin fault: Implications for active tectonics along the Yarlung Zangbo Suture, Southeastern Tibet Plateau

How the eastward motion of crust in the central Tibetan Plateau is accommodated in the remote regions of the eastern Himalayan syntaxis remains uncertain. Although the Yarlung Zangbo suture (YZS) forms a striking lineament in the topography of the region, evidence for recent faulting along this zone has been equivocal. To understand whether faults along the YZS are active, we performed a geological investigation along the eastern segments of the YZS. Geomorphic observations suggest the presence of active faulting along several segments of the YZS, which we collectively refer to as the “Milin fault”. Paleoseismologic data from trenches reveal evidence for one faulting event, which is constrained to occur between 5620 and 1945 a BP. The latest faulting event displaced alluvial surface T2 by ~7m. The offset on this earthquake place the minimum value on the vertical slip rate of ~0.3mm/yr. Empirical relationships between surface rupture length, displacement and magnitude, suggest that magnitude of the latest event could have been Mw ~7.3–7.7. On the basis of this slip rate and the elapsed time since the last event, it is estimated that a seismic moment equivalent to Mw ~7.0 has been accumulated on the Milin fault. It is pose a threat to the surrounding region. Our results suggest that shortening occurs in the vicinity of the eastern Himalayan syntaxis, and part of eastward motion of crust from the central Tibetan Plateau is absorbed by uplift of the eastern Himalayan syntaxis.

Nature ESR signals of quartz E' center shed new light on river sediments provenance: A case study in southeast margin of the Tibet Plateau

Host rock samples and deposits collected from the upper Yangtze River and the Dadu River, which both drained the southeastern Tibet Plateau and nature electron spin resonance (ESR) signals of quartz E' center were determined with ESR spectrometry. Statistical analyses of the results show that nature signal intensity of quartz E' center as a new indicator can be used to fingerprint the source of detrital quartz in modern river sediments. Firstly, nature signal intensities of quartz E' center both in bedrocks and its deposits show lower values (less than 11 s. u.) in granitic drainages than those in metamorphic drainage and sedimentary drainage (more than 19 s. u.). Secondly, nature signal intensities of quartz E' center in deposits will be enhanced by sunlight bleaching from source to sink. Thirdly, with the increasing of transportation distance accumulation nature signal intensities of quartz E' center in deposits are changed according to the different mixture ratios of samples which attributed the different types of host rocks. The researches indicate that nature signal intensities of quartz E' center have a significantly dependence on lithology both in bedrocks and in deposits, and nature signal intensities of quartz E' center can reflect the mixture ratio of deposits with different types of sourced bedrocks.

Late Early-Cretaceous quartz diorite–granodiorite–monzogranite association from the Gaoligong belt, southeastern Tibet Plateau: Chemical variations and geodynamic implications

Geochemical variations in granitic rocks may be controlled by their source rocks, melting reactions and subsequent magmatic processes, which resulted from various geodynamic processes related to subduction, collision, or slab break-off. Here we report new LA–ICP-MS zircon U–Pb ages and Hf isotopes, whole-rock chemistry and Sr–Nd isotopes for the late Early Cretaceous quartz diorite, granodiorite and monzogranite in the Gaoligong belt, southeastern Tibet Plateau. The zircon U–Pb dating yield ages of 113.9±1.6, 111.7±0.8, and 112.8±1.7Ma for the quartz diorite, granodiorite, and monzogranite, respectively, which are coeval with bimodal magmatism in the central and northern Lhasa sub-terrane. There are the distinct sources regions for the quartz diorite and granodiorite–monzogranite association. The quartz diorites are sodic, calc-alkaline and have high Mg# (52–54) values. They also have elevated initial 87Sr/86Sr (0.707019 to 0.709176) and low εNd(t) (−5.16 to −7.63), with variable zircon εHf(t) values (+5.65 to −9.02). Zircon chemical data indicate a typical crustal-derived character with high Th (142–1260ppm) and U (106–1082ppm) and moderate U/Yb ratios (0.30 to 2.32) and Y content (705–1888ppm). Those data suggest that the quartz diorites were derived from partial melting of ancient basaltic lower crust by a mantle-derived magma in source region. The granodiorite–monzogranite association has high-K calc-alkaline, weakly peraluminous characters. They show lower Nb/Ta (5.57 to 13.8), CaO/Na2O (0.62 to 1.21), higher Al2O3/TiO2 (24.4 to 44.4) ratios, more evolved whole-rock Sr–Nd and zircon Hf isotopic signatures, all of which suggest derivation from mixed basaltic and metasedimentary source rocks in a deep crustal zone. We propose that the granitic magmatisms at ca. 113–110Ma in the Gaologong belt was triggered by the slab break-off of Bangong–Nujiang Tethyan oceanic lithosphere.

Black carbon aerosol and its radiative impact at a high‐altitude remote site on the southeastern Tibet Plateau

AbstractAerosol black carbon (BC) was measured with an Aethalometer™ at Lulang, a high‐altitude station in southeastern Tibetan Plateau (TP), from July 2008 to August 2009. Daily mean BC loadings varied from 57.7 to 5368.9 ng m−3 (grand average ± standard deviation = 496.5 ± 521.2 ng m−3), indicating a significant BC burden even at free tropospheric altitudes. BC loadings were highest during the premonsoon and lowest during the monsoon, and peaks in BC were coincident with high atmospheric boundary layers. Daily peaks in BC occurred from 08:00 to 10:00 local time with minor fluctuations at other times. The BC mass absorption efficiency (MAE) was calculated from elemental carbon concentrations obtained from a thermal/optical reflectance method and absorption coefficients from the Aethalometer™, and values ranged from 6.1 to 31.7 m2 g−1 (average = 16.6 ± 5.7 m2 g−1). Strong variations in the MAEs during the monsoon can be ascribed to large uncertainties due to low BC and babs and possibly coatings on the BC. High MAEs during premonsoon pollution events were likely due to internal mixing during transport. The mean direct surface radiative forcing (DRF) estimated from a radiation model was −19.9 (±7.4) W m−2 for the full aerosol population and −3.9 (±1.8) W m−2 for a BC only scenario. The BC DRF during a case study (−36.0 W m−2) was much stronger than the typical, and the BC contribution to the forcing was higher (~50%) than usual (~20%). These results show that BC can at times account for a relatively large fraction of the aerosol surface heating over the southeast TP, which may affect both climate and hydrological cycles.

Variations of chemical features in a monsoon glacier watershed, southeastern Tibet Plateau

The pH, EC (electric conductivity), δ18O and major ions in melt water, precipitation, runoff and groundwater samples which were collected in Baishui basin were analyzed. Based on chemical analysis, the characteristic variations are analyzed in this paper. The melt water, precipitation, runoff and groundwater were Ca2+/Mg2+/HCO3− type with similar ion compositions but showed different ion concentrations. Normally the content is high in groundwater and runoff water but low in melt water and precipitation. From the upper basin glacier downward, there was a general increasing trend for the total ion concentrations in basin. HCO3 − and Ca2+ accounted for more than 80 % of total ions in samples, indicating that ions mainly came from limestone and water typical of carbonate solution water. Water–rock interaction is the significant contributor to water geochemistry. The concentration of inorganic ion in melt water had an obvious diurnal variation with temperature change. The δ18O values increases with the altitude decrease; the similarity of δ18O in runoff and groundwater samples was due to their frequent transformation in karst regions.

Early-summer temperature variations over the past 563 yr inferred from tree rings in the Shaluli Mountains, southeastern Tibet Plateau

We developed a tree-ring chronology (AD 1446–2008) based on 75 cores from 37 Abies squamata Mast. trees from the Shaluli Mountains, southeastern Tibet Plateau, China, using signal-free methods, which are ideally suited to remove or reduce the distortion introduced during traditional standardization. This chronology correlates best with regional temperatures in June–July, which allowed us to develop a June–July temperature reconstruction that explained 51.2% of the variance in the instrumental record. The reconstruction showed seven cold periods and five warm periods. Cold periods were identified from AD 1472 to 1524, 1599 to 1653, 1661 to 1715, 1732 to 1828, 1837 to 1847, 1865 to 1876 and 1907 to 1926. Warm intervals occurred from AD 1446 to 1471, 1525 to 1598, 1716 to 1731, 1848 to 1864, 1877 to 1906 and 1927 to present. The reconstruction agrees well with nearby tree-ring-based temperature reconstructions. Spatial correlation analyses suggest that our reconstructions provide information on June–July temperature variability for the southeastern Tibetan Plateau and its vicinity. Spectral analyses revealed significant peaks at 2–6, 10.7, 51.2, 102.2 and 204.8yr. The temperature variability in this area may be affected by ENSO, the Pacific Decadal Oscillation and solar activity.

Rock slope stability evaluation in static and seismic conditions for left bank of Jinsha River Bridge along Lijiang-Xamgyi’nyilha railway, China

Jinsha River Bridge is located along the Lijiang-Xamgyi’nyilha railway on the southeastern Tibet plateau; it is an area with a high prevalence of earthquakes. The bridge abutments were designed to be constructed in river bank slopes, where rocks are controlled by two sets of joint planes that significantly influence the stability of the left bank slope. According to the engineering-geological conditions and the characteristics of discontinuities, strength properties of the rock mass were obtained based on Barton model and direct shear test. Numerical analyses were performed using FLAC3D software to examine the slope’s response to seismic loading. Then in order to evaluate the damage trends of the rock mass under the different loading conditions, a calculation model based on the geological parameters and slope stability was simulated and analyzed using the discrete element numerical simulation program UDEC (Universal Distinct Element Code), and the effect of degradation of discontinuities on the slope stability was investigated. The results show that the destruction of rock mass under the gravity, bridge foundation, and seismic load are mainly concentrated within 30 m depth of slope, and the slope under loading may slide along joint planes. In addition, the dynamic analysis by amplification of the input loading indicates that instability occurs to the bank slope at a height of about 200 m, and rock blocks will fail under seismic load. Therefore, to prevent the slope from deformation under the engineering loading and strong earthquakes, the bridge foundation should be strengthened.

Topographic Constraints on Deviatoric Stress Field in the Indo-Eurasian Collision Region: Seismo-Tectonic Implications

The deviatoric stress field are computed from the inversion of Gravitational Potential Energy (GPE) for the Indo-Eurasian plate collision region including the Himalaya and the Tibet Plateau. The resulting stress pattern in combination with stress and strain rates obtained by inverting, respectively, the focal mechanism solution of large earthquakes and GPS derived plate motions are used to study the nature of the present-day deformations. A narrow belt bordering the Himalayan collision zone from the south is characterized by strong compressive stresses. The variations in stress pattern along this belt coincide with arc-normal ridges extending into the Himalaya and are able to explain arc-parallel segmentation of seismicity. Gravitational collapse seems to play an important role in the southeastern Tibet Plateau. Depth sensitivity of the seismic derived stresses and GPS derived surface strain rates coupled with evidence of arcuate shaped high electrical conductivity favour strong ductile flow around the Eastern Himalaya Syntaxis (EHS) at mid-crustal depth. The deflection of crustal flow indicted by the viscous resistance offered by the rigid Sichuan basin adds to the traction stresses to cause clockwise rotation of the block around EHS.

Study on Pn Velocity and Anisotropy in the Uppermost Mantle of the Eastern Himalayan Syntaxis and Surrounding Regions

AbstractThe Eastern Himalayan Syntaxis (EHS) is a singular point of continental collision between the Indian and Eurasian plates, and has been a focus of geoscientific researches. We added the ISC data of 1964~2006 to a previous Chinese data set used by Pei et al., and 349475 Pn arrivals were selected in all. We have obtained seismic velocities and anisotropy in the uppermost mantle around EHS and surrounding regions by performing tomographic inversion of Pn travel times. Comparing Pn velocity variation with geological results, we find that they have distinct correlation: particularly high velocities always exist in old stable regions such as Sichuan Basin and Indian Plate. Low Pn velocities lie in active tectonic regions, volcanic areas and magmatic rock regions, for example, eastern Tibetan Plateau and North‐South Seismic Zone, southern Yunnan province and northern Indochina. The Pn anisotropy shows that the fast directions of Pn turn around the EHS from NE in southeastern Tibet Plateau, then SE to NS in North‐South Seismic Zone. But the direction changes sharply to EW in the northern Indochina. The rotation of Pn fast direction may be a result of mass extrusion from Tibet under simple shear, which is caused by mass flow in southeastern Tibetan Plateau and North‐South Seismic Zone relative to stable EHS and Sichuan Basin. The EW Pn direction probably results from pure shear, which is caused by compression in NS and extension in EW in Burma back‐arc extensional area. The polarization directions of SKS have similar rotation to Pn, but in northern Indochina the area of EW‐oriented fast direction of Pn has a southward offset of about 3° than SKS, which may result from the faster movement of upper lithosphere in this area.

An outline of avalanches in the southeastern Tibet Plateau, China

Avalanches are a widespread hazard that cause serious damage in the southeastern Tibetan Plateau. Abundant solid precipitation and high relief are favourable for avalanche occurrence. The monsoon climate brings a lot of precipitation which, however, is not well distributed. Avalanches are most common around the margins of the plateau. Wet snow avalanches occur most frequently. Based on data on precipitation and topographical relief, a map is drawn showing the areal distribution of avalanche hazards.

An outline of avalanches in the southeastern Tibet Plateau, China

Avalanches are a widespread hazard that cause serious damage in the southeastern Tibetan Plateau. Abundant solid precipitation and high relief are favourable for avalanche occurrence. The monsoon climate brings a lot of precipitation which, however, is not well distributed. Avalanches are most common around the margins of the plateau. Wet snow avalanches occur most frequently. Based on data on precipitation and topographical relief, a map is drawn showing the areal distribution of avalanche hazards.