Himalayan Orogeny Research Articles

Silicate weathering in the Ganges alluvial plain

The Ganges is one of the world's largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny, weathering and global climate change. Three regions can be recognised in the Ganges basin, with the Himalayan orogeny to the north and the plateaus of peninsular India to the south together delimiting the Ganges alluvial plain. Despite constituting approximately 80% of the basin, weathering processes in the peninsula and alluvial plain have received little attention. Here we present an analysis of 51 water samples along a transect of the alluvial plain, including all major tributaries. We focus on the geochemistry of silicon and its isotopes. Area normalised dissolved Si yields are approximately twice as high in rivers of Himalaya origin than the plain and peninsular tributaries (82, 51 and 32 kmol SiO2 km−2 yr−1, respectively). Such dissolved Si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the DSi mobilised during the initial weathering process is retained in secondary clay minerals. However, the silicon isotopic composition of dissolved Si (expressed as δ30Si) varies from +0.8‰ in the Ganges mainstem at the Himalaya front to +3.0‰ in alluvial plain streams and appears to be controlled by weathering congruency, i.e. by the degree of incorporation of Si into secondary phases. The higher δ30Si values therefore reflect decreasing weathering congruency in the lowland river catchments. This is exploited to quantify the degree of removal using a Rayleigh isotope mass balance model, and consequently derive initial silica mobilisation rates of 200, 150 and 107 kmol SiO2 km−2 yr−1, for the Himalaya, peninsular India and the alluvial plain, respectively. Because the non-Himalayan regions dominate the catchment area, the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment (41% and 34%, respectively).

A Laser Based Fluorination (BrF5) System for the Extraction of Oxygen (O2) from Silicate Rocks of Himalaya and δ18O Measurements: Method Establishment and Implications

Oxygen forms the majority of minerals constituting the terrestrial and planetary objects. Its chemical and isotopic compositions often bear signatures of the processes occurring within these objects on different time scales. Silicates are the common rocks in the Himalaya, the major feature on the Earth, which has witnessed different stages of metamorphism and got reworked during its evolution. Therefore, oxygen isotopes in silicate rocks can be used to reveal the processes pertaining to the Himalayan orogeny. These processes include petrogenesis, metamorphism, subduction and associated magmatic events, etc. Oxygen isotopic composition (δ18O) is needed to be measured in silicate rocks which require special arrangements. Towards this, a vacuum line of stainless steel coupled with a computer controlled CO2 laser system was fabricated, for the first time in India, to extract oxygen (O2) from rock forming mineral in the BrF5 environment. The line is equipped with suitable valves and traps for the efficient recovery of oxygen from the silicate minerals and reliable handling of hazardous oxidants (i.e., BrF5, etc.) that are used as fluorination agents to break Si=O bonds. This extraction system is interfaced with a continuous flow stable isotope ratio mass spectrometer and is being used successfully for the oxygen isotopic (δ18O) measurements in silicate and oxide minerals. Here we report the δ18O results on a set of silicate rocks and standard reference materials including Norway olivine, Gee Whiz Garnet standard from University of New Mexico, olivine from Norway Fan (Lab Standard), silicate rocks from North West Himalaya (India) and NBS-28 quartz a standard reference material from International Atomic Energy Agency, Vienna. The average δ18O values (relative to Vienna standard mean ocean water) and 2SD are 6.4 ± 0.3 ‰ (N = 17) for Norway olivine, 12.7 ± 0.2 ‰ (N = 10) for Gee Whiz Garnet and 9.5 ± 0.3 ‰ (N = 12) for NBS-28 quartz. In addition, some of the quartz vein and quartzites from the Garhwal Himalaya were also measured to establish the measurement protocols and procedures. Silicate rocks from the Himalaya near the main central thrust zone were tried to fix the end-member composition for oxygen isotopes (δ18O). Such information is helpful to infer about the origin of quartzite and their relationship with possible protoliths in the region. Further, this technique finds applications in the analysis of biogenic silica (diatoms and phytoliths, etc.) for paleoclimatic reconstruction.

The rise and fall of continental arcs: Interplays between magmatism, uplift, weathering, and climate

Continental arcs, such as the modern Andes or the Cretaceous Sierra Nevada batholith, are some of the highest topographic features on Earth. Continental arc volcanoes may produce more CO2 than most other types of volcanoes due to the interaction of magmas with sedimentary carbonates stored in the continental upper plate. As such, global flare-ups in continental arc magmatism may amplify atmospheric CO2 concentrations, leading to climatic warming. However, the high elevations of continental arcs may also enhance orographic precipitation and change global atmospheric circulation patterns, possibly increasing the efficiency of chemical weathering and drawdown of atmospheric CO2, which may subdue the climatic warming response to volcanic activity. To better evaluate the climatic response, we develop models that integrate magmatic crustal thickening, topographic uplift, isostasy and erosion. The topographic response is used to predict how soil formation rates, soil residence times, and chemical weathering rates vary during and after a magmatic episode. Although magmatism leads to crustal thickening, which requires topographic uplift, highest elevations peak ∼10 My after magmatism ends. Relatively high elevations, which enhance erosion and chemical weathering of the continental arc, persist for tens of million years after magmatism ends, depending on erosion kinetics. It has recently been suggested that the Cretaceous–Paleogene greenhouse (high atmospheric CO2 and warm climate) coincided with a global chain of continental arcs, whereas mid- to late Cenozoic icehouse conditions (low atmospheric CO2 and cold climate) coincided with a lull in continental arc activity after 50 Ma. Application of our models to the Sierra Nevada (California, USA) continental arc, which represents a segment of this global Cretaceous–Paleogene continental arc, reproduces the observed topographic and erosional response. Our models require that the newly formed continental arc crust remained high and continued to erode and weather well after (>50 My) the end of magmatism. Thus, in the aftermath of a global continental arc flare-up, both the total volcanic inputs of CO2 decline and the average weatherability of continents increases, the latter due to the increased proportion of widespread remnant topography available for weathering and erosion. This combination leads to a decrease in the long-term baseline of carbon in the ocean/atmosphere system, leading to cooling. Mid-Cenozoic cooling is often attributed solely to increased weathering rates associated with India–Eurasian collision and the Himalayan orogeny. However, the total area of now-extinct Cretaceous–Paleogene continental arcs is 1.3–2 times larger than that of the Himalayan range front and the Tibetan plateau combined, suggesting that weathering of these remnant volcanic arcs may also play a role in drawing down CO2 through silicate weathering and subsequent carbonate burial. In summary, if global continental arc flare-ups lead to greenhouse conditions, long-lived icehouse conditions should follow in the aftermath due to decreased CO2 inputs and an increase in regional weathering efficiency of remnant arc topography.

Structural model of the Balkassar area, Potwar Plateau, Pakistan

Balkassar is an important hydrocarbon producing area of the Potwar Plateau, Pakistan. Two-dimensional seismic reflection data of the area revealed tectonically controlled, distinct episodes of (1) normal faulting in the basement followed by (2) reverse faulting in the cover sequence. Himalayan orogeny and associated diapirism of the Precambrian Salt Range Formation have produced many salt-cored anticlines in the Potwar Plateau, and one such salt-cored anticline is present in the Balkassar. This anticline has NE–SW-oriented axis, and both the SE and NW limbs are bounded by reverse faults. The basement normal faults indicate Jurassic rifting and splitting of Pangaea. We interpret reverse faults with dip angles of about 60°–75° in the cover sequence, having both hinterland and foreland vergence. Both NW- and SE-dipping faults are present in contrast to the only southward-directed thrusts of previous models. Duplexes and triangle zones, which are common in the northern part of the Potwar Plateau, are not developed in the Balkassar area due to comparatively less crustal shortening in the area. The present interpretation can help in understanding the complex structures in other parts of the Potwar Plateau for hydrocarbons exploration and also in deformed foreland basins worldwide that display similar characteristics but are considered to be dominated by low-angle thrust tectonics.

Neogene marine isotopic evolution and the erosion of Lesser Himalayan strata: Implications for Cenozoic tectonic history

An extensive, northward deepening blanket of Neoproterozoic and Cambrian sedimentary rocks once extended from the Himalayan margin far onto the Indian craton. Cambrian deposits of this “upper Lesser Himalayan” succession, which include deposits of the “outer” Lesser Himalaya tectonic unit, are enriched in radiogenic 187Os. They make up part of a proximal marine facies belt that extends onto the craton and along strike from India to Pakistan. By contrast, age-equivalent facies in the Tethyan Himalaya are more distal in nature. Neoproterozoic to Cambrian strata of the upper Lesser Himalayan succession are now missing in much of the Lesser Himalaya, with their erosion exposing older Precambrian Lesser Himalayan strata. We suggest that exhumation and weathering of the upper Lesser Himalaya and related strata caused dramatic changes in the 187Os/188Os and 87Sr/86Sr Neogene record of seawater starting at ∼16 Ma. First-order estimates for the volume of upper Himalayan strata, as well as the volume of all LH rock eroded since this time, and geochemical box modeling, support this idea. Exhumation at 16 Ma is a fundamental event in the evolution of the Himalayan orogeny and the geochemical evolution of the oceans, and will be a critical part of the construction of future models of Himalayan thrust belt evolution.

Stratigraphic characterization and structural analysis of the northwestern Hazara Ranges across Panjal Thrust, northern Pakistan

The study area is a part of the Western Himalayan fold-and-thrust belt in northern Pakistan and lies in the hanging wall of Panjal Thrust between Hazara-Kashmir and Indus syntaxes (Fig. 1). The present study uses stratigraphic appraisal and structural analysis to understand the tectonic evolution of the western Hazara Ranges. Based on detailed field mapping and structural analysis, the Western Hazara Ranges are broadly divisible into three structural domains: Sherwan, Aluli, and Pind Khan Khel. Each of these three structural domains comprises a major synclinal fold structure that exposes the Early Cambrian Abbottabad Formation in the core and Precambrian Tanawal Formation in the limbs. The syncline folds are doubly plunging and upright in nature. The basement rocks in the hanging wall of Panjal Thrust belong to Tanawal Formation which dominantly comprises of quartzites in the study area but changes to quartz schists and garnetiferous schists in the northern parts of the Hazara arc, while in the footwall, the Hazara Formation which is dominantly slates with some minor limestone is present as a basement sequence. Both blocks with different basement rocks are overlain by similar Early Cambrian Abbottabad Formation. This study attempts to explain the difference of basement rocks across Panjal Thrust through a tectonic evolution model, which suggests that the Pan-African orogeny is responsible for the basement stratigraphic changes while the Himalayan orogeny folded the Early Cambrian sequence in broad synclines and anticlines. The present exposures of Early Cambrian sequence in the core of synclines resulted due to subsequent erosion of anticlines and preservation of synclines.

Tectono-geomorphic and environmental set-up deduced during deposition of Mio-Pleistocene sediments in NW Himalaya, India

Detailed pedofacies characterization along-with lithofacies investigations of the Mio-Pleistocene Siwalik sediments exposed in the Ramnagar sub-basin have been studied so as to elucidate variability in time and space of fluvial processes and the role of intra- and extra-basinal controls on fluvial sedimentation during the evolution of the Himalayan foreland basin (HFB).Dominance of multiple, moderately to strongly developed palaeosol assemblages during deposition of Lower Siwalik (~12–10.8Ma) sediments suggest that the HFB was marked by Upland set-up of Thomas et al. (2002). Activity of intra-basinal faults on the uplands and deposition of terminal fans at different times caused the development of multiple soils. Further, detailed pedofacies along-with lithofacies studies indicate prevalence of stable tectonic conditions and development of meandering streams with broad floodplains. However, the Middle Siwalik (~10.8–4.92Ma) sub-group is marked by multistoried sandstones and minor mudstone and mainly weakly developed palaeosols, indicating deposition by large braided rivers in the form of megafans in a Lowland set-up of Thomas et al. (2002). Significant change in nature and size of rivers from the Lower to Middle Siwalik at ~10Ma is found almost throughout of the basin from Kohat Plateau (Pakistan) to Nepal because the Himalayan orogeny witnessed its greatest tectonic upheaval at this time leading to attainment of great heights by the Himalaya, intensification of the monsoon, development of large rivers systems and a high rate of sedimentation, hereby a major change from the Upland set-up to the Lowland set-up over major parts of the HFB.An interesting geomorphic environmental set-up prevailed in the Ramnagar sub-basin during deposition of the studied Upper Siwalik (~4.92 to <1.68Ma) sediments as observed from the degree of pedogenesis and the type of palaeosols. In general, the Upper Siwalik sub-group in the Ramnagar sub-basin is subdivided from bottom to top into the Purmandal sandstone (4.92–4.49Ma), Nagrota (4.49–1.68Ma) and Boulder Conglomerate (<1.68Ma) formations on the basis of sedimentological characters and change in dominant lithology. Presence of mudstone, a few thin gravel beds and dominant sandstone lithology with weakly to moderately developed palaeosols in the Purmandal sandstone Fm. indicates deposition by shallow braided fluvial streams. The deposition of mudstone dominant Nagrota Fm. with moderately to some well developed palaeosols and a zone of gleyed palaeosols with laminated mudstones and thin sandstones took place in an environment marked by numerous small lakes, water-logged regions and small streams in an environment just south of the Piedmont zone, perhaps similar to what is happening presently in the Upland region/the Upper Gangetic plain. This area is locally called the ‘Trai region’ (Pascoe, 1964). Deposition of Boulder Conglomerate Fm. took place by gravelly braided river system close to the Himalayan Ranges. Activity along the Main Boundary Fault led to progradation of these environments distal-ward and led to development of on the whole a coarsening upward sequence.

Detrital zircon U-Pb-He double dating: A method of quantifying long- and short-term exhumation rates in collisional orogens

A U-Pb-He double-dating method is applied to detrital zircons with core-rim structure from the Ganges River in order to determine average short- and long-term exhumation rates for the Himalayas. Long-term rates are calculated from the U/Pb ages of metamorphic rims of the grains that formed during the Himalayan orogeny and their crystallization temperatures, which are calculated from the Ti-in-zircon thermometer. Short-term rates are calculated from (U-Th)/He ages of the grains with appropriate closure temperatures. The results show that short-term rates for the Himalayas, which range from 0.70 ± 0.09 to 2.67 ± 0.40 km/Myr and average 1.75 ± 0.59 (1 σ) km/Myr, are higher and more varied than the long-term rates, which range from 0.84 ± 0.16 to 1.85 ± 0.35 km/Myr and average 1.26 ± 0.25 (1 σ) km/Myr. The differences between the long-term and short-term rates can be attributed to continuous exhumation of the host rocks in different mechanisms in continental collision orogen. The U/Pb ages of 44.0 ± 3.7 to 18.3 ± 0.5 Ma for the zircon rims indicate a protracted episode of ∼25 Myr for regional metamorphism of the host rocks at deeper crust, whereas the (U-Th)/He ages of 42.2 ± 1.8 to 1.3 ± 0.2 Ma for the zircon grains represent a protracted period of ∼40 Myr for exposure of the host rocks to shallower crustal level. In particular, the oldest (U-Th)/He ages of the zircon grains are close to the oldest U/Pb ages for the rims, indicating that some parcels of the rocks that contain zircons were rapidly exhumed from deep to shallow levels in the stage of collisional orogeny. On the other hand, some parcels of the rocks may have been carried upwards by thrust faults in the post-collisional stage. The parcels could be carried upwards by the thrust faults that steepen as they near the surface, or by transient movement faults so that areas of rapid exhumation became areas of slow exhumation and visa versa on a time scale of a few Myr in order to maintain the continuous exhumation. In this regard, the Ganges River must be preferentially sampling areas that are currently undergoing above average rates of uplift.

The depositional setting of the Late Quaternary sedimentary fill in southern Bannu basin, Northwest Himalayan fold and thrust belt, Pakistan.

Geostatistical variogram and inversion techniques combined with modern visualization tools have made it possible to re-model one-dimensional electrical resistivity data into two-dimensional (2D) models of the near subsurface. The resultant models are capable of extending the original interpretation of the data to depict alluvium layers as individual lithological units within the 2D space. By tuning the variogram parameters used in this approach, it is then possible to visualize individual lithofacies and geomorphological features for these lithologic units. The study re-examines an electrical resistivity dataset collected as part of a groundwater study in an area of the Bannu basin in Pakistan. Additional lithological logs from boreholes throughout the area have been combined with the existing resistivity data for calibration. Tectonic activity during the Himalayan orogeny uplifted and generated significant faulting in the rocks resulting in the formation of a depression which subsequently has been filled with clay-silt and dirty sand facies typical of lacustrine and flood plain environments. Streams arising from adjacent mountains have reworked these facies which have been eroded and replaced by gravel-sand facies along channels. It is concluded that the sediments have been deposited as prograding fan shaped bodies, flood plain, and lacustrine deposits. Clay-silt facies mark the locations of paleo depressions or lake environments, which have changed position over time due to local tectonic activity and sedimentation. The Lakki plain alluvial system has thus formed as a result of local tectonic activity with fluvial erosion and deposition characterized by coarse sediments with high electrical resistivities near the mountain ranges and fine sediments with medium to low electrical resistivities towards the basin center.

Tibetan garnet records early Eocene initiation of thickening in the Himalaya

Tectonic reconstructions of the Himalayan orogeny depend on the age at which crustal thickening commenced. To investigate this age, we analyzed garnet from middle crustal rocks exposed in the north Himalayan Mabja and Kangmar gneiss domes of Tibet using Lu-Hf geochronology. Garnet yielded Lu-Hf ages of 54–52 Ma in Mabja and 51–49 Ma in Kangmar samples. On the basis of microstructural and major element and rare earth element zoning observations, the Lu-Hf ages are interpreted as recording garnet growth during contractional deformation in the middle crust at 54.3 ± 0.6 Ma, followed by variable recrystallization during subsequent high-temperature ductile extension. The new Lu-Hf ages are the first to confirm that crustal thickening and contraction in the Tibetan Himalaya was broadly synchronous with the early Eocene collision between Greater India and the Eurasian plate.

Gas potential of Proterozoic and Phanerozoic shales from the NW Himalaya, India: Inferences from pyrolysis

Organic richness and kerogen properties of sixty-seven shales, obtained from the outcrops and underground mines of Jammu, Kashmir and Ladakh regions of Northwest Himalaya, India, have been studied to evaluate their gas generation potential using Rock Eval pyrolysis. Ranging in age from the Proterozoic to Tertiary, organic matter content and characteristics of the carbonaceous and coaly shales vary widely, indicating that sedimentary and burial history significantly affected the preservation and maturation of organic components in rocks.The total organic carbon (TOC) content ranges from 0.01 to 1.2% in the Permian–Jurassic and Paleozoic–Tertiary shales of Ladakh to as high as 32.5% in the Eocene shales/coaly shales from Jammu. The thermo-labile hydrocarbons (S1) and those from cracking of kerogen (S2), released from the pyrolysis of Eocene Subathu shales, are observed to be high (from 0.1 to 2.6 and from 0.5 to 15.5mgHC/g rock, respectively). Rock Eval thermal maturity parameters, indicated by Tmax (temperature at highest yield of S2; >490°C) and calculated vitrinite reflectance (1.5 to 3.7 Ro %), suggest a post-mature, dry gas stage for the hydrocarbon generation. Based on hydrogen index (HI) and Tmax correlations, organic matter in the Subathu shales is characterized by a late metagenetic gas prone Type III kerogen and a fair to excellent gas potential is exhibited by these shales.The interbedded shale units in the Proterozoic Sirban Limestone Formation, occurring as an isolated inlier in Jammu, show TOC values from 0.1 to 1.4% with quite low S1, S2 and HI values. Thermal maturity of shales within the stromatolitic Sirban succession indicates a post-mature and/or already spent hydrocarbon stage. The Plio-Pleistocene carbonaceous clays, lignites and mudstones from the Lower Karewa Group of Kashmir basin are organically rich with TOC content ranging from 5.86 to 29.4%. A thermally immature, Type II/Type III kerogen is indicated by the HI (109 to 278mgHC/g TOC) and Tmax (399 to 427°C) plots of the Karewa samples. The exposed Lower Triassic Black shales from the Permian–Triassic boundary sections in Kashmir are comparatively lean in organic matter, with TOC values ranging from 0.18 to 0.93%, whereas the Permian–Jurassic and Paleozoic–Tertiary shales from Ladakh have a TOC content ranging from 0.01 to 1.22%. The pyrolyzable organic carbon is <0.1% and the residual organic carbon contributes significantly to the TOC content of these shales. The other Rock Eval parameters (S1, S2, HI) are quite low and indicate a poor source potential. The majority of shales from the Tethys and Trans Himalayan regions appear to have undergone metamorphism and exhumation of organic carbon associated with the Himalayan orogeny.

Ensemble forecast of climate suitability for the Trans-Himalayan Nyctaginaceae species

Oxybaphus himalaicus Edgew. (Nyctaginaceae) is the only native species of Oxybaphus occurring outside of the Western Hemisphere. Unfortunately, very little is known about the distribution of this species in Nepal, despite it being endemic to the Trans-Himalayas. In the earlier collections it was mis-identified as another very common Nyctaginaceae species Boerhavia diffusa L. We used species distribution modelling technique to determine the climate space of these two species. We used an SDM ensemble method in the BiodiversityR R-package to identified the bioclimatic variable that defines the climate space for B. diffusa and O. himalaicus. Species distributions were modelled using a subset of least correlated variables from bioclimatic data and elevation dataset. The most favourable conditions for B. diffusa were centrally located at well-drained and hot lowlands of Nepal whereas that of O. himalaicus was located in highlands valleys. Discontinuities in suitable climatic conditions coincided with biogeographical barriers associated with topographic transitions. The rising slope of mountains limit the climatic suitability of B. diffusa particularly due to low temperature and high aridity, whereas this condition was suitable for O. himalaicus. But presence of high Himalayas fragmented the suitable climatic condition for this species. We speculate that historic climatic changes associated with changes in the geology of the Himalayas, formed by tectonic forces and sculpted by weathering and erosion, are responsible for the fragmentation of habitat of this species. The extensive study of such a species could provide insight into the history of species colonization in the Himalayas, as well as the potential distribution of Himalayan species.

FORMATION CONDITIONS OF JIMUSAER OIL SHALE AT THE NORTHERN FOOT OF BOGDA MOUNTAIN, CHINA

The Jimusaer oil shale, located in the northern foot of Bogda Mountain, was deposited in the Lucaogou Formation in the Upper Permian. In this paper the effect of paleostructure, paleoclimate and sedimentary environment on the mineralization of oil shale in the study area was inves- tigated. The results show that the Jimusaer oil shale was developed in the stable paralic lacustrine basin formed in the Late Middle Permian, and cropped out with the uplift of Bogda Mountain during the Himalayan orogeny. The Lucaogou Formation, which is composed of six lithologic segments, was deposited in a warm humid to dry hot climate and fresh to brackish water environment indicated by values of Sr/Cu, Sr/Ba and Mn/Fe ratios, which vary segment by segment. There are also similar variety regularities shown, which implicate that the changes of paleoclimate influenced directly water salinity in the study area. The Lucaogou Formation includes two oil shale segments which were developed respectively in the shallow to semi-deep lake and semi-deep to deep lake, and the environment of large-area, moderate saline deep water provided favourable conditions for generation and preservation of oil shale.

Thin lithosphere–asthenosphere boundary beneath Eastern Indian craton

The lithosphere–asthenosphere boundary (LAB) separates the hard and rigid outer layer of the earth (lithosphere) and the weaker, hotter, and deeper part of the upper mantle (asthenosphere) and plays a pivotal role in plate tectonics. However, its definitive detection, especially beneath the cratons, is proving elusive. One of the geophysical tools used to map the LAB beneath the cratons is through magnetotelluric (MT) observations. The resistivity at boundary falls in the range of 5–25 Ω-m and can be explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Here, we report thickness of the LAB in one of the oldest dated ancient cratons of India—Eastern Indian Craton (EIC) of ~ 3.3 Gyr, from MT studies. The two prominent phase-sensitive strike directions, one each for crust and mantle, and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath EIC. Our results show that the LAB beneath the EIC is at 95 km. The region is interesting as the electrical properties of the crust and mantle and the Moho depth are similar to those of the Slave Craton, Canada (~ 4.0 Gyr) but the depth of the LAB beneath the EIC is half that of the Slave craton. As cratonic signatures, depicted by ultrapotassic rocks from Gondwana coal fields close to EIC, are preserved at least till early Cretaceous (117 Ma) it is likely that Himalayan orogeny could have played a major role in delamination of the lithospheric roots of the EIC in addition to attendant seismicity.

The Ban Houayxai epithermal Au–Ag deposit in the Northern Lao PDR: Mineralization related to the Early Permian arc magmatism of the Truong Son Fold Belt

The Ban Houayxai Au–Ag deposit is located in the Truong Son Fold Belt in the northern Lao PDR, mainland SE Asia. The deposit is hosted within an Early Permian volcano-sedimentary unit which is a part of a Late Carboniferous–Early Permian (310–270Ma) volcanic–plutonic sequence of the Truong Son Fold Belt. The main Au–Ag mineralization stages (i.e., Stages 2 and 3) at Ban Houayxai consist of hydrothermal veins/breccias associated with a carbonate–quartz–sulfides (pyrite–sphalerite–galena–chalcopyrite)–sericite–chlorite–electrum–native silver–stephanite mineral assemblage. Geochronological studies from LA-ICP-MS zircon U–Pb and adularia Ar–Ar and K–Ar dating identified Early Permian, Cretaceous and Eocene ages. The Early Permian age is related to the emplacement of the mineralized volcanic arc sequence, whereas the Cretaceous and Eocene ages represent resetting probably related to Yanshanian and/or Himalayan Orogenies. A regional tectonic reconstruction indicates that the Au–Ag epithermal deposit at Ban Houayxai was formed as a result of southward subduction of an oceanic crust attached to the South China Terrane beneath the Indochina Terrane during the Early Permian. From the metallogenic point of view, the recognition of the old epithermal system at Ban Houayxai implies that the Truong Son Fold Belt possesses high potential for the presence of deeper magmatic-related deposits such as porphyry and porphyry-related skarn deposits.

Microfacies analysis and paleoenvironmental significance of palustrine carbonates in the Thakkhola-Mustang Graben (Nepal Himalaya)

The Thakkhola-Mustang Graben represents the extensional tectonic phase of the Tibetan Plateau uplift and whole Himalayan orogeny. It is situated at the northern side of the Dhaulagiri and Annapurna Ranges and south of the Yarlang Tsangpo Suture Zone. Stratigraphically, the oldest sedimentary units are the Tetang and Thakkhola Formations (Miocene), while the Sammargaon, Marpha and Kaligandaki Formations lying disconformably above these formations represent Plio-Pleistocene units. In this study, different lacustrine carbonates and calcretes were investigated within different lithological units and depositional environments to interpret the palaeoenvironmental and palaeoclimatological evolution of the area.Geological mapping, construction of columnar sections and carbonate sampling were carried out in the field, and stable oxygen and carbon isotope analyses and thin section analyses were done in the laboratory. Lacustrine facies contained abundant pelletal, charophytic algae, oncolitic algal micritic palustrine limestones with ostracods, and micritic mudstones with root traces. Stable carbon and oxygen isotope analysis from the carbonates show a range of δ13C values from −0.6‰ to 11.1‰ (V-PDB) and δ18O values from −13.5‰ to −25‰ (V-PDB).Discontinuous growth of oncolites and spherical pellets (25–40μm in diameter) in micritic limestone, algal mats and charophyte algae indicate the presence of both shallow and deep water carbonates. Ostracods in dark micritic carbonates indicate quiet and calm water conditions. Microfabrics of the carbonates suggest that they were deposited in a flat and shallow lacustrine environment. The δ18O values of the investigated limestones of the Thakkhola-Mustang Graben suggest that it attained the current elevation level prior to the east-west extension of the Himalaya.

Carboniferous inherited grain and age zoning of monazite and xenotime from leucogranites in far-eastern Nepal: Constraints from electron probe microanalysis

Chemical Th-U-total Pb isochron method (CHIME) of monazite and xenotime from three leucogranites in far-eastern Nepal revealed the presence of Carboniferous inherited grains of monazite and intense intrusion of leucogranites around 18-16 Ma in the High Himalaya. In garnet-bearing sillimanitemuscovite- biotite leucogranite, most monazite grains have inherited cores with the chemical dates of ~504-418 and ~342-272 Ma, which were overgrown by Early Miocene mantles and rims of 17.9 ± 1.5 Ma. Early Ordoviciain and Carboniferous ages are rarely found in the same euhedral-subhedral cores. In addition to the previously recognized Early Paleozoic magmatism, monazite cores with Carboniferous ages in Early Miocene leucogranites provide evidence for two periods of magmatism at the base of the High Himalaya prior to the Cenozoic Himalayan orogeny. In muscovite-biotite leucogranite, no inherited domains were observed in monazite and xenotime grains. They yielded the CHIME monazite and xenotime dates of 16.1 ± 2.0 and 19.8 ± 6.5 Ma, respectively. Monazite grains adjacent to xenotime have significantly lower concentrations of UO2 and Y2O3 compared to those isolated from xenotime. These results imply that xenotime influences Y and U contents in monazite, reflecting local equilibrium system. In aplitic leucogranite, monazite grains yielded the mean of apparent chemical date of 18.0 ± 2.2 Ma. The CHIME monazite ages of ~18-16 Ma in three leucogranites reflect the timing of melt crystallization.

Large rivers and orogens: The evolution of the Yarlung Tsangpo–Irrawaddy system and the eastern Himalayan syntaxis

The eastern Himalayan syntaxis has experienced some of the highest rates of deformation and erosion in the orogen during the Late Cenozoic, and the Yarlung Tsangpo, Brahmaputra, Irrawaddy, Salween, and Mekong rivers are the key erosional systems in that region. The Yarlung Tsangpo drains southern Tibet and the deep Siang River gorge through the eastern Himalayan syntaxis before joining the Brahmaputra in northeastern India. It has been proposed that the Yarlung Tsangpo drained into other large rivers of southern Asia, such as the Irrawaddy, Salween and Red River. We have used uranium/lead dating and hafnium measurements of detrital zircons from Cenozoic sedimentary deposits in Central Myanmar to demonstrate that the Yarlung Tsangpo formerly drained into the Irrawaddy River in Myanmar through the eastern syntaxis, and that this ancient river system was established by (at least) the Middle–Late Eocene. The Yarlung Tsangpo–Irrawaddy river disconnected in the Early Miocene driven by increased deformation in the eastern syntaxis and headward erosion by tributaries of the Brahmaputra. Our results highlight the significance of the sedimentary record of large orogen-parallel rivers and provide key chronological constraints on landscape evolution during the Early Miocene phase of the Himalayan orogeny.

Upper Oligocene–Lower Miocene Gangrinboche Conglomerate in the Xigaze Area, Southern Tibet: Implications for Himalayan Uplift and Paleo-Yarlung-Zangbo Initiation

The Gangrinboche Conglomerate, exposed along the Yarlung-Zangbo suture zone, records a crucial stage of the Himalayan orogeny. The type section of these strata in the Xigaze area, southern Tibet, including the Qiuwu and Dazhuka Formations, was studied by integrated stratigraphic, sedimentologic, petrographic, and geochemical techniques. Palynological data and detrital zircon U-Pb ages indicate that the Qiuwu Formation was deposited during the latest Oligocene to the earliest Miocene (most probably ∼23 Ma), while the overlying Dazhuka Formation was deposited during 23–18 Ma. The Qiuwu Formation was deposited in a deltaic setting, and detritus was entirely derived from the Gangdese magmatic arc in the north. The Dazhuka Formation, in contrast, was deposited in mainly braided river environments and contains clasts derived from both the Gangdese arc in the north and the Himalayan orogen in the south. Clasts derived from the south first occur at the base of the Dazhuka Formation and increase in abundance upsection to become predominant at the top of the formation. This indicates active Early Miocene uplift and accelerated erosion of the Himalayan belt. Paleocurrent data from the Dazhuka Formation show westward axial sediment transport, which together with mixed provenance from both sides of the basin indicates that a paleo-Yarlung-Zangbo River running parallel to the suture zone initiated at the very start of the Miocene, although with flow direction opposite to that of the present. Flow reversal and establishment of the modern eastward-flowing course must have occurred later on in the Neogene, possibly initiating rapid uplift and focused erosion of the Namche-Barwa syntaxis. Basin subsidence at the close of the Paleogene and subsequent development of a major longitudinal paleo-Yarlung-Zangbo took place contemporaneously with initiation of the South Tibetan Detachment System and Main Central Thrust farther to the south, probably reflecting onset of the “hard collision” phase of the Himalayan orogeny.

Formation mechanisms of secondary hydrocarbon pools in the Triassic reservoirs in the northern Tarim Basin

The northern Tarim Basin (Tabei) is a rich composite hydrocarbon accumulation region with multi-layers of oil and gas reservoirs in the Ordovician, Carboniferous and Triassic strata. In the Triassic, the Tabei region was dominated by non-marine sedimentation that developed deltaic sandstone reservoirs with favourable petrophysical properties for hydrocarbon accumulations. Hydrocarbons in the Triassic hydrocarbon pools are dominated by oil and condensate gas. Analysis of the hydrocarbons, in combination with well testing and drilling data, reveals that oils in the Triassic hydrocarbon pools are commonly biodegraded and originated mainly from the middle-upper Ordovician source rocks. The predominant dry gases in the reservoirs are cracked from oils derived from the Cambrian source rocks. The origin of hydrocarbons in the Triassic pools is similar to those in the underlying reservoirs. Fluid inclusions and K–Ar dating confirm that the hydrocarbons were accumulated during the Himalayan orogenic movement. Because the middle–upper Ordovician source rocks already experienced peak hydrocarbon generation in the late Hercynian orogeny (late Permian), the Triassic hydrocarbon pools are from secondary adjustment of earlier oil and gas pools from the underlying Ordovician and Carboniferous reservoirs. Due to the rapid subsidence of the Kuqa foreland basin to the north of the Tabei region since the beginning of the Himalayan orogenic movement, the Triassic reservoir strata was changed from south-dipping to north-dipping, resulting in the formation of plunging anticlines and hydrocarbon migration toward the south. At the same time, fault reactivation occurred in the area, causing the hydrocarbons from the deeper reservoirs to migrate into the Triassic structural traps through faults and unconformities, and thus formed the widespread secondary hydrocarbon pools. The structure inversion during the Triassic and the formation of nose structures since the Himalayan orogeny (250 ∼ 290 Ma, Permian) controlled the secondary accumulation in the Triassic reservoirs. The reformation process of the hydrocarbon pools continued until the late Himalayan orogeny when the gas was charged and developed condensate pools in the eastern area.