Abstract Sulfides play a crucial role in the distribution of chalcophile elements in the Earth's mantle. In this work, combined petrography and mineral chemistry of sulfide and diopside in a pyroxenite from an ultramafic–alkaline–carbonatite complex of NE India, related to the Kerguelen plume, was carried out and a considerably high sulfur concentration in the parental melt of the pyroxenite was obtained. Two types of sulfide, with similar compositions, were detected in pyroxenite: Type A are multifaceted polygons, elliptical and spherical in shape, occurring as poikilitic inclusions in diopside; and Type B are intergranular sulfides of irregular shapes in silicate grains. These sulfides are often partially replaced by magnetite. Mineral chemistry suggests that both types of sulfide are products of re-equilibration of high-temperature monosulfide solid solution and represent a low-temperature ( c. 400°C) mineral phase of the Cu–Fe–S system. Petrographical features suggest that the sulfides were separated as immiscible melt droplets at the time of sulfur saturation and fractionation of diopside in the coexisting silicate magma. Our study implicates that both high- and low-temperature sulfides can form in the plume-associated ultramafic rocks.

Abstract The permanent GNSS station located at the Everest Pyramid Laboratory of EvK2CNR recorded its position coordinates during the earthquakes at the Gorkha (25 April 2015) and Ghorthali zones (12 May 2015) at an interval of every 30 s. The data recorded over three days prior to and after the earthquakes were analysed and the movement indicated a shifting of the GNSS station point from its original position every 30 s. From an accurate analysis of the coordinates of the station determined using GNSS Bernese software, it is possible to detect the movements of the station during the seismic events. The shifts in the GNSS point were summed to provide an integral function (PIF, Pyramid Integral Function) that can be computed for each of the three components. Comparing them with the displacement record of the GURALP broadband seismic station (IO-EVN) of the OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, OGS, Trieste), located at the Pyramid, it is possible to establish a correlation, particularly with the vertical and north components; the maxima of the PIF coincide with the time of occurrence of the earthquakes.

Abstract Group velocities for a period range of 6–60 s for the fundamental mode of the Rayleigh wave passing across the Himalaya–Karakoram–Tibet orogen are used to delineate the structure of the upper lithosphere using the data from 35 broadband seismic stations. 2D tomography velocity maps of group velocities were obtained at grids of 1° separation. Redefined local dispersion curves are inverted non-linearly to obtain 1D velocity models and to construct a 3D image of the S-wave structure down to a depth of 90 km. The Moho discontinuity is correlated with c. 4.0 km s −1 S-wave velocity. The results depict a NE-dipping trend of the Moho depth from c. 40 km beneath the frontal part of the Himalaya to up to c. 70–80 km beneath the collision zone before shallowing substantially to c. 40 km beneath the Tarim Basin. The study also reveals thick deposits of sediments in the Indo-Gangetic plains and the Tarim Basin. A broad low-velocity zone at mid-crustal depth in the western Tibetan Plateau, the Karakoram region and the surface-collision part of the India–Eurasia tectonic plates is interpreted as the effect of partial melting and/or the presence of aqueous fluid. The high velocities in the southern deeper part indicate that the lower crust and uppermost mantle of the Indian Plate are dense and cold.

Abstract The tectonic framework of NW Himalaya is different from that of the central Himalaya with respect to the position of the Main Central Thrust and Higher Himalayan Crystalline and the Lesser and Sub Himalayan structures. The former is characterized by thick-skinned tectonics, whereas the thin-skinned model explains the tectonic evolution of the central Himalaya. The boundary between the two segments of Himalaya is recognized along the Ropar–Manali lineament fault zone. The normal convergence rate within the Himalaya decreases from c. 18 mm a −1 in the central to c. 15 mm a −1 in the NW segments. In the last 800 years of historical accounts of large earthquakes of magnitude M w ≥ 7, there are seven earthquakes clustered in the central Himalaya, whereas three reported earthquakes are widely separated in the NW Himalaya. The earthquakes in central Himalaya are inferred as occurring over the plate boundary fault, the Main Himalayan Thrust. The wedge thrust earthquakes in NW Himalaya originate over the faults on the hanging wall of the Main Himalayan Thrust. Palaeoseismic evidence recorded on the Himalayan front suggests the occurrence of giant earthquakes in the central Himalaya. The lack of such an event reported in the NW Himalaya may be due to oblique convergence.

Abstract New apatite fission track (AFT) ages have been obtained from a synformal nappe of the Higher Himalayan Crystallines emplaced over the Lesser Himalayan metasedimentary zone of the Arunachal Himalaya, India. The AFT cooling ages within the nappe range between 5.0 ± 0.8 and 14.4 ± 1.3 Ma. Modelled exhumation rates calculated from these cooling ages vary from 0.25 ± 0.12 to 0.69 ± 0.25 mm a −1 , which indicates slow exhumation since the Middle to Late Miocene. The AFT cooling ages are younging on both the northern and southern flanks of the synform and the oldest ages are confined to the core. The close mimicking of a shallow crustal exhumation pattern with the synformal structure suggests a strong control of the development of the synform on the exhumation path of the rocks and hence a tectonics–exhumation linkage in the central Arunachal Himalaya. Comparison of these AFT ages with the regional thermochronological record of the Eastern Himalaya reflects a variation in exhumation rates with strike. The AFT age pattern in the central Arunachal Himalaya does not match the pattern of precipitation, which suggests an absence of climate-driven tectonic deformation via focused erosion.

Abstract Crystalline klippen over the Lesser Himalayan Metasedimentary Sequence (LHMS) zone in the NW Himalaya have specific syn- and post-emplacement histories. These tectonics also provide a means to understand the driving factors responsible for the exhumation of the rocks of crystalline klippen during the Himalayan Orogeny. New meso- and microscale structural analyses, and thermochronological studies across the LHMS zone, Ramgarh Thrust (RT) sheet and Almora klippe in the eastern Kumaun region, NW Himalaya, indicate that the RT sheet and Almora klippe were a part of the Higher Himalayan Crystalline (HHC) of the Indian Plate which underwent at least one episode of pre-Himalayan deformation and polyepisodic Himalayan deformation in ductile and brittle–ductile regimes. The deformation temperature pattern within the Almora klippe records a normal thermal profile from its base to top but an inverted thermal profile from the base of Almora klippe down towards the LHMS zone. New fission-track data collected across the RT sheet and Almora klippe along Chalthi–Champawat–Pithoragarh traverse in the east Kumaun region document the exhumation of both units since Eocene times. Zircon fission-track (ZFT) ages from the Almora klippe range between 28.7 ± 2.4 and 17.6 ± 1.1 Ma, and from the RT sheet between 29.8 ± 1.6 and 22.6 ± 1.9 Ma; and the apatite fission-track (AFT) ages from the Almora klippe range between 15.1 ± 1.7 and 3.4 ± 0.5 Ma, and from the RT sheet between 8.7 ± 1.2 and 4.6 ± 0.6 Ma. The age pattern and diverse patterns of the exhumation rates reflect a clear tectonic signal in the RT sheet and the Almora klippe which acknowledge that the Cenozoic tectonics influenced the exhumation pattern in the Himalaya.