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Abstract: The Himalayan orogeny, a result of the collision of India and Eurasia, provides direct evidence of strain accommodation and large-scale rheological behavior of the continental lithosphere. Knowledge of the timing of the India–Eurasia collision is essential to understand the physical processes involved in collisional systems. Here we present a geochronological and multi-isotopic study on rocks from the upper crust of the Kohistan Paleo-Island Arc that formed in the equatorial part of the Neo-Tethys Ocean. In situ U–Pb geochronology and Hf isotopes in zircon, and whole-rock Nd and Sr isotopic data of plutonic rocks from the Kohistan-Ladakh Batholith, are used to construct a continuous record of the isotopic evolution of the source region of these granitoids that are related to both the subduction of the oceanic lithosphere and subsequent arc–continent collisions. We demonstrate that profound changes in the source region of these rocks correspond to collisional events. Our dataset constrains that the Kohistan–Ladakh Island Arc initially collided along the Indus suture zone with India at 50.2±1.5Ma, an age generally attributed to the final India–Eurasia collision for the entire Himalayan belt. In the western Himalaya, the final collision between the assembled India/Arc and Eurasia however, occurred ∼10Ma later at 40.4±1.3Ma along the so-called Shyok suture zone. We present evidence indicating that a similar dual collision scenario can be extended to the east and conclude that a final India/Arc–Eurasia collision at ∼40Ma integrates crucial aspects of the magmatic, tectonic, and sedimentary record of the whole Himalayan mountain belt. [Copyright &y& Elsevier]

Abstract: The Tibetan Plateau is the largest orogenic system on Earth, and has been influential in our understanding of how the continental lithosphere deforms. Beneath the plateau are some of the deepest () earthquakes observed within the continental lithosphere, which have been pivotal in ongoing debates about the rheology and behaviour of the continents. We present new observations of earthquake depths from the region, and use thermal models to suggest that all of them occur in material at temperatures of . Thermal modelling, combined with experimentally derived flow laws, suggests that if the Indian lower crust is anhydrous it will remain strong beneath the entire southern half of the Tibetan plateau, as is also suggested by dynamic models. In northwest Tibet, the strong underthrust Indian lower crust abuts the rigid Tarim Basin, and may be responsible for both the clockwise rotation of Tarim relative to stable Eurasia and the gradient of shortening along the Tien Shan. [Copyright &y& Elsevier]

Abstract: We surveyed the Owen Fracture Zone at the boundary between the Arabia and India plates in the NW Indian Ocean using a high-resolution multibeam echo-sounder (Owen cruise, 2009) for search of active faults. Bathymetric data reveal a previously unrecognized submarine fault scarp system running for over 800km between the Sheba Ridge in the Gulf of Aden and the Makran subduction zone. The primary plate boundary structure is not the bathymetrically high Owen Ridge, but is instead a series of clearly delineated strike-slip fault segments separated by several releasing and restraining bends. Despite an abundant sedimentary supply by the Indus River flowing from the Himalaya, fault scarps are not obscured by recent deposits and can be followed over hundreds of kilometres, pointing to very active tectonics. The total strike-slip displacement of the fault system is 10-12km, indicating that it has been active for the past ~3 to 6Ma if its current rate of motion of 3±1mmyr−1 has remained stable. We describe the geometry of this recent fault system, including a major pull-apart basin at the latitude 20°N, and we show that it closely follows an arc of small circle centred on the Arabia–India pole of rotation, as expected for a transform plate boundary. [ABSTRACT FROM AUTHOR]

Abstract: Provenance tools are applicable to many problems in sedimentary geology as they help unravel the tectonic and metamorphic history of the hinterland and provide insights into the erosional pathways and origins of sediments. In many cases it is more appropriate to use single grain approaches as opposed to bulk sediment methods in order to discover the precise input of the contributing geological terranes since the input of subordinate sources may not be detectable in bulk rock studies. Using a selection of modern river sediments we characterise the Sm–Nd isotopic composition of individual detrital apatites from the main Himalayan geological terranes. Our analyses allow us to effectively distinguish between apatites derived from the Eurasian Plate (relatively high εNd values and low 147Sm/144Nd ratios), from those derived from the Indian Plate (low to high εNd values and moderate to high 147Sm/144Nd ratios). We then apply this approach to Tertiary Indus Basin sedimentary rocks to attempt to better determine the timing of India–Eurasia collision. We find that detrital apatites in the Tertiary Indus Basin have been sourced solely from Eurasia, lacking a mixed India–Eurasia provenance input which would document the India–Eurasia collision. This study illustrates the use of this relatively novel provenance tool and provides a sound framework for similar studies in the future concerning the tectono-metamorphic-erosional evolution of the Himalaya. [ABSTRACT FROM AUTHOR]

Abstract: Estimating depth to the bottom of the magnetic crust (synonymously called the Curie isotherm) on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations. There is still no consensus on the minimum survey area required to arrive at a reliable estimate of the Curie isotherm depth. From the available aeromagnetic data over India, depth to the Curie isotherm is estimated using spectral techniques. We found that the significant spectral maxima required to calculate the depth to the Curie isotherm existed only for 4°×4° blocks in southern peninsular India up to 18 °N latitude and for 5°×5° blocks for the rest of the region in Central India. As aeromagnetic data coverage over India is incomplete, we also calculate the magnetic crustal thickness from the lithospheric model of the CHAMP satellite data for the whole country, using an iterative forward modeling approach. The calculated Curie isotherm is shallow in the mobile belts and deeper in the cratons in both the derivations. As the Curie isotherms calculated from the two data sets collected at very different altitudes using totally different techniques, match reasonably well, it lends credence to the methodology adopted. The derived Curie isotherm depth map is in accordance with the basic structural trend of the major tectonic units within the Indian subcontinent. A comparison is made of the calculated Curie isotherm depth with Moho depths along available DSS profiles over India and it is found that the Curie depth is generally shallower than the Moho depth implying that it possibly represents a thermal boundary rather than a compositional change. Further, we find that high magnitude earthquakes are associated with high gradients in Curie depth. [Copyright &y& Elsevier]

Abstract: Heat flow in the actively deforming Central Indian Basin is on average 30 mW/m2 higher than the theoretical 55 mW/m2 heat flow expected from plate cooling of a Cretaceous oceanic lithosphere. Strong spatial correlation between the anomaly and the active thrust fault network at local (faults) and regional scales suggests two potential tectonically driven mechanisms activated at the time of initiation of deformation: friction-to-heat conversion or exothermic serpentinization. We quantitatively examine both processes using an updated geometry of the thrust fault network and simple thermal models. Friction generated heat is limited in all cases: at shallow levels, shear stresses remain small, while heat generated at deeper levels does not contribute significantly to the surface heat flow since permanent regime is not reached. In the exothermic serpentinization model, a maximum anomaly of 20 to 30 mW/m2 is reached 2 to 6 Myr after the onset of widespread serpentinization, depending on the efficiency of the water circulation. The amount and timing of heat release can fully explain the present-day surface heat flow of the Central Indian Basin, provided vigorous hydrothermal circulation closely followed the onset of deformation. Based on a reprocessed multichannel seismic line, we suggest that faults cutting through the entire crust and across the Moho discontinuity drive water at mantle levels and trigger the exothermic serpentinization reaction. We interpret sub-Moho reflectors imaged at depths of 8 to 15 km below the top of the crust – and coinciding with the location of the maximum reaction rate coefficient of serpentinization – as serpentinization fronts. We discuss the significance of this pulse of serpentinization in terms of timing of deformation, weakening and transient rheology of the oceanic lithosphere. [Copyright &y& Elsevier]

Multibeam swath bathymetry data acquired over an area of about 30 000 km2, together with magnetic and single channel seismic data, have been analyzed to understand the tectonic evolution of the Andaman basin, northeast Indian Ocean. Swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc-parallel seamount chains and N–S-trending fault systems and a relatively smooth eastern part. A SW–NE-trending spreading ridge bisects the basin. An offset of about 11.8 km at 94°21′E longitude divides the spreading ridge into SW and NE segments. High-resolution bathymetry and magnetic data enabled us to identify two finer-scale segments over the SW part of the ridge that display distinct topographic fabric compared to the NE segment. Thin sediment cover and presence of axial seamounts characterize SW segments of the ridge whereas the NE segment has thick sediment cover and well-maintained rift valley. Kinematics of basin evolution, derived from the identification of the seafloor spreading magnetic anomalies, suggest very slow initial opening rates of about 1.6 cm/yr and an increase in the spreading rate to about 3.8 cm/yr from anomaly 2 to present, although this interpretation relies only on magnetic anomalies in the two westernmost segments. It is suggested that the true seafloor spreading started about 4 Ma in the Andaman backarc basin, rather than the 11 Ma postulated previously. We infer based on the integrated analysis of swath bathymetry, magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the Sagaing and Semengko fault systems. Extension by seafloor spreading during the past 4 Myr resulted in the formation of the deep Andaman backarc basin. This phase has also experienced westward propagation of the spreading center. [Copyright &y& Elsevier]

How the collision between India and Asia is related to processes deeper in the mantle is unclear. Here we compare geological reconstructions of block motions within Asia since ≈50 Ma with the tomographically imaged three-dimensional (3-D) morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure. Past positions of the convergent margin show remarkable similarities with slab geometry at specific depths. The striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision. The slab contours match the progressive deformation of Asia’s margin, including India’s indentation and Sundaland’s extrusion. Ever since the onset of collision, the Indian plate appears to have overridden its own sinking mantle and it does not seem, at present, to underthrust Tibet significantly north of the Zangbo suture. If correct, this observation would provide further evidence against models of plateau build-up involving Indian lithosphere. The tomographic images beneath India confirm that Asian deformation has absorbed at least ≈1500 km of convergence since collision began. From the match between the southeastward motion of Sundaland between 40 and 20 Ma and the principal change in slab structure between 700 and 1100 km depths, we infer that lateral advection in the mantle is small and that the sinking rate beneath Sunda was ∼2 cm/yr in the lower mantle and ∼5 cm/yr above the transition zone. [Copyright &y& Elsevier]

The evolution of the Himalayan foreland is the result of continent–continent collision and related large-scale tectonics in the region. The initial foredeep basin sequences are exposed in limited areas of the western Himalaya, which makes these areas very significant in unraveling the earliest evolution of the foreland system. The basal interval of the Himalayan foredeep is exposed in the Jammu area (India), which preserves silicified breccia formed by the erosion of hanging walls of shallow faults. Two Paleocene sections are analyzed that suggest the existence of growth faults which developed in response to the India–Asia collision in the Late Paleocene (∼57.9–54.7 Ma). The pebble-size clasts and their derivation entirely from the basement demonstrate rapid sedimentation in response to rapid subsidence at the onset of basin evolution. The angular unconformity showing effects of erosion associated with a thin soil horizon may be due to a forebulge at the site of the unconformity. The reworked bauxite above this soil horizon demonstrates erosion of another forebulge from the cratonward side. [Copyright &y& Elsevier]