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Abstract: The Ramba gneiss dome, one of the north Himalayan gneiss domes, is composed of three tectono-lithologic units separated by an upper and a lower detachment fault. Low-grade metamorphic Tethyan Himalayan sedimentary sequence formed the upper unit above the brittle upper detachment fault. Mylonitic gneiss and a leucogranite pluton made up the lower unit beneath the ductile lower detachment fault. Mylonitic middle-grade garnet-, staurolite- and andalusite-schist constituted the middle unit between the two faults, which may be that the basal part of the upper unit experienced detachment shear. The Ramba dome underwent three episodes of deformation in its tectonic evolution. The first episode was a top-down-to-north-northwest sliding possibly related to the activity of the south Tibetan detachment system (STDS). The second episode was the dominant deformation related to a east–west extension, which resulted in a unique top-down-to-east kinematics and the major tectonic features of the dome. The third episode was a collapse sliding toward the outsides of the dome. The Ramba gneiss dome is possibly a result of the east–west extension and magmatic diapir. The lower detachment fault is probably the main detachment fault separating the sedimentary sequence from the crystalline basement during the east–west extension in the dominant deformation episode. The diapir of the leucogranite pluton formed the doming shape of the Ramba gneiss dome. This pluton intruded in the core of the dome in a late stage of the dominant deformation, and its Ar–Ar cooling ages are about 6Myr. This indicates that the dominant deformation of the dome happened at the same time of the east–west extension represented by the north–south trending rifts throughout the northern Himalaya and southern Tibet. Therefore, the formation of the Ramba gneiss dome should be related to this east–west extension. [Copyright &y& Elsevier]

Abstract: The granitic gneisses in the cores of north Himalayan gneiss domes (NHGD) and at the crest of the high Himalayas form part of the greater Himalayan crystalline complex (GHC). These rocks are characterized by the assemblage of quartz, plagioclase, K-feldspar, biotite, muscovite and minor garnet. Here we present results from LA-ICPMS zircon U–Pb dating of six granitic gneiss samples, which reveal formation ages of ca. 500–473Ma and suggest a long-lived magmatism during early Paleozoic in the GHC. Geochemical data show that these rocks are characterized by high SiO2 (70.93–74.59wt.%), K2O (4.22–5.91wt.%), A/CNK values (>1.1) and low Na2O/K2O (0.42–0.83), and enrichment in Rb, Th, U, depletion in Ba, Nb, Ta, Sr, Zr, and strong negative Eu anomalies. These features suggest that the protoliths of the gneisses are high potassium calc-alkaline and peraluminous S-type granites derived from partial melting of crustal materials. The high initial 87Sr/86Sr (>0.706) and negative ε Nd(t) (−6.2 to −10.6) of the granitic gneisses compare with those of the metasedimentary rocks in the GHC, indicating that the granites were generated from partial melting of the sedimentary protoliths. Based on the geochronological, whole-rock geochemical and Sr–Nd isotopic data presented in this study, we suggest that the granites formed in a back-arc setting with a continental arc-affinity related to the subduction of the Proto-Tethyan Oceanic lithosphere. The mafic magmatic underplating, triggered by the subduction, rollback and the break off of Proto-Tethyan Ocean slab, resulted in partial melting of the crust in the GHC. Integrating results from previous studies, we propose an Andean-type orogeny along the margin of the Gondwana. [Copyright &y& Elsevier]

The continuous melting of valley glaciers can impact the water levels of glacial lakes and create glacial lake outburst floods (GLOFs). The Xixabangma massif is one of the most populated areas in the Himalayas and has suffered from multiple GLOFs. To estimate the glacier melting rate in the past four decades and analyze the outburst risk of glacial lakes in the Xixabangma massif, we determined changes in glacier mass balance, glacier area and glacial lake area based on KH-9 images, TanDEM-X images, Landsat images, SRTM DEM and ICESat-2 elevations. Our results show that, from 1974 to 2018, the total glacier area shrank from 954.01 km2 to 752.46 km2, whereas the total glacial lake area grew from 20.90 km2 to 38.71 km2. From 1974 to 2000, 2000 to 2013 and 2013 to 2018, the region-wide glacier mass balance values were −0.16 m w.e./a, −0.31 m w.e./a and −0.29 m w.e./a, respectively. Three glacial lakes, named Gangxico, Galongco and Jialongco, respectively, expanded by 127.14%, 373.45% and 436.36% from 1974 to 2018, and the mass loss rates of their parent glaciers from 2000 to 2013 increased by 81.72%, 122.22% and 160.00% relative to those during 1974 to 2000. The dams of these three lakes are unstable, and their drainage valleys directly connect to a major town and its infrastructure. Due to current high-water levels, possible external events such as ice collapse, landslide, heavy rainfall and earthquakes can easily trigger GLOFs. Hence, we deemed that the Gangxico, Galongco and Jialongco glacial lakes are dangerous and require special attention. [ABSTRACT FROM AUTHOR]