Continental Margin Of India Research Articles

Petrogenesis of alkaline rocks from Murud-Janjira, in the Deccan Traps, Western India

A late-stage rift-related tholeiite-alkalic suite of igneous intrusions cut the Deccan Traps lavas at the western Indian continental margin. The suite comprises intrusives that can be grouped into ten lithotypes on the basis of their mutual relationships. Tholeiitic types predate the alkaline rocks and greatly predominate, however, the alkaline members exhibit more diversity in mineralogy and chemistry, and are amongst the rare magmatic rocks from the Deccan that host both mantle and lower crustal xenoliths. The mineralogy of most rock types is dominated by clinopyroxene. The diversity of the alkaline rocks could be mainly accounted for by fractional crystallization and mixing between evolved and primitive melts under varying P-T conditions. Sodic and potassic lamprophyres are amongst the most primitive samples with high Mg #, FeO/MgO < 1, high Cr and also with relatively high Ba, Sr, Zr and Nb. They are the most deeply derived magmas within the Deccan Traps as is evident from the mantle and lower crustal xenoliths entrained by them. They possibly represent low degree melts of incompatible element-enriched mantle source rocks. The nephelinites are strongly porphyritic and despite their high Mg #s can be regarded as evolved magmas that have been responsible for the formation of the tephriphonolite daughter. The nephelinites have undergone contamination by lower crustal granulites. The composite intrusions of microdiorites with their complexly zoned mineralogy dominated by plagioclase and amphiboles/micas represent hybrid rocks that have resulted from mixing between tholeiitic and trachytic melts partly at depth and partly at shallow crustal levels.

Signal enhancement and multiple suppression using Radon transform: an application to marine multichannel seismic data

Radon filters are often used for removal of multiple reflections from normal move-out-corrected seismic reflection data. In the conventional Radon transform, integration surfaces are hyperbolic rather than linear. This specific hyperbolic surface is equivalent to a parabola in terms of computational expense, but more accurately distinguishes multiples from primary reflections. The forward transform separates seismic arrivals by their differences in travel time move-out. Multiples can be suppressed by an inverse transform of the seismic data. Examples show that multiples are effectively attenuated in pre-stack and stacked seismograms. Based on the parabolic Radon transform, a new method is utilized for missing offset restoration, resampling and regularization of pre-stack individual common depth point (CDP) gathers. The method is also valid for resampling spatially aliased seismic data. Restoration of missing offsets and trace interpolation is an interesting and important problem in seismic data processing. Here we present an application of Radon transform on a multichannel seismic data set from the western continental margin of India (WCMI), which shows remarkable signal enhancement.

Did the Kohistan-Ladakh island arc collide first with India?

Research Article| March 01, 2009 Did the Kohistan-Ladakh island arc collide first with India? Shuhab D. Khan; Shuhab D. Khan † 1Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA †E-mail: sdkhan@uh.edu Search for other works by this author on: GSW Google Scholar Douglas J. Walker; Douglas J. Walker 2Department of Geology, University of Kansas, Lawrence, Kansas 66045-7613, USA Search for other works by this author on: GSW Google Scholar Stuart A. Hall; Stuart A. Hall 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA Search for other works by this author on: GSW Google Scholar Kevin C. Burke; Kevin C. Burke 3Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA Search for other works by this author on: GSW Google Scholar Mohammad T. Shah; Mohammad T. Shah 4National Center of Excellence in Geology, University of Peshawar, Peshawar, 25120, Pakistan Search for other works by this author on: GSW Google Scholar Lisa Stockli Lisa Stockli 5Department of Geology, University of Kansas, Lawrence, Kansas 66045-7613, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2009) 121 (3-4): 366–384. https://doi.org/10.1130/B26348.1 Article history received: 19 Oct 2007 rev-recd: 17 Jul 2008 accepted: 18 Jul 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shuhab D. Khan, Douglas J. Walker, Stuart A. Hall, Kevin C. Burke, Mohammad T. Shah, Lisa Stockli; Did the Kohistan-Ladakh island arc collide first with India?. GSA Bulletin 2009;; 121 (3-4): 366–384. doi: https://doi.org/10.1130/B26348.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Kohistan-Ladakh block occupies the northwestern corner of the Himalayan mountains and has long been recognized to represent an island arc constructed on ocean floor during Jurassic and Cretaceous times. Because the Kohistan-Ladakh block now lies within the Asian continent, it is important to know how and when it became sandwiched between India and the rest of Asia. We have found from analysis of paleomagnetic data that in Late Cretaceous–early Paleocene times, the Kohistan-Ladakh island arc could not have been far from the equator. India was close to the equator, but the southern margin of Asia was more than 3000 km to the north. Our new U-Pb zircon age results from rocks of the Kohistan-Ladakh block show that calc-alkaline volcanic arc igneous activity ended in the Kohistan-Ladakh arc by 61 Ma. We interpret that cessation to date the collision of Kohistan with India. This new timing is confirmed by evidence that a Southern Hemisphere enriched DUPAL mantle source was involved in the generation of the latest Cretaceous Teru Volcanic Formation rocks of the Kohistan-Ladakh arc. Further confirmation of the collision of the Kohistan arc with India in early Paleocene times comes from evidence of the timing of obduction of ophiolites and from the unconformity of postcollisional sedimentary rocks onto the Indian continental margin in northwestern and western Pakistan. Final incorporation of India, now carrying the Kohistan-Ladakh block in its NW corner, into Asia took place at the Shyok suture. The best evidence for the timing of that suturing ca. 50 Ma comes from two postcollisional granites (ages 47 Ma and 41 Ma) in northern Kohistan, which show in their zircon isotopic compositions evidence of the involvement of ancient Asian continental crust that did not exist under Kohistan before the suture formed. The 50 Ma age for Shyok suturing against the then-active Karakoram Andean arc fits well with the extension of the suture beyond the eastern end of the Kohistan-Ladakh block to join the precisely dated ca. 51 Ma Yarlung–Tsang Po suture between India and the southern (Lhasa block) margin of Tibet, which at that time was also occupied by an Andean arc, the Gangdese arc. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cenozoic kinematic history of the Kohistan fault in the Pakistan Himalaya

Detailed mapping suggests a new scenario for the kinematic history of the Late Cretaceous Kohistan arc complex and for the cause of Cenozoic metamorphism and exhumation along the Indian continental margin in Pakistan. The earliest deformational phases resulted in early Cenozoic amphibolite-facies metamorphism, superposed tight to isoclinal F1/F2 folds, and development of strong penetrative rock fabric within the Indian plate in response to southwestward, and then southeastward, overthrusting of ophi-olitic melange in the Indus suture zone and West Pakistan fold belt, respectively. This deformation included the development of the Banna and Malakand thrust faults. Prograde metamorphism ended following this deformational phase, and the early structures were deformed along N-trending, open, upright, F3 folds and domes. This was followed by ESE- to E-directed thrusting of the Kohistan arc complex onto the Indian continent along the dominantly brittle Kohistan fault. Thrusting ended in the late Oligocene–early Miocene when the Kohistan arc assumed its present position along the northern margin of India. South-verging structures began to form on the Indian plate for the first time in the late Oligocene–early Miocene. Deformation had expanded to the foreland by the middle to late Miocene with development of the Panjal-Khairabad thrust. Neogene structures in the metamorphic zone include N-trending, east-side-up, high-angle fault zones that deform or reactivate the Kohistan fault, and NW-trending, east-side-up reverse faults, both of which are seismically active. In the tectonic scenario outlined here, shortening along the Kohistan fault occurred subsequent to metamorphism, and was directed toward the ESE-E throughout its history. The southern termination of the fault is considered to be a right-slip sidewall ramp such that the Kohistan arc never advanced much father south on the Indian plate than its present position. Broad flexure, folding, and erosion in front of and south of the actively advancing Kohistan arc complex contributed to exhumation of the Indian plate. This differs from interpretations that state that Kohistan was thrust in a southward direction; that underthrusting beneath Kohistan was the primary cause of early Cenozoic metamorphism on the Indian plate; and that top-side-north extensional reactivation of the Kohistan fault contributed to exhumation.

Occurrence of volcanic ash in the Quaternary alluvial deposits, lower Narmada basin, western India

This communication reports the occurrence of an ash layer intercalated within the late Quaternary alluvial succession of the Madhumati River, a tributary of the lower Narmada River. Petrographic, morphological and chemical details of glass shards and pumice fragments have formed the basis of this study. The ash has been correlated with the Youngest Toba Tuff. The finding of ash layer interbedded in Quaternary alluvial sequences of western Indian continental margin is significant, as ash being datable material, a near precise time-controlled stratigraphy can be interpreted for the Quaternary sediments of western India. The distant volcanic source of this ash requires a fresh re-assessment of ash volume and palaeoclimatic interpretations.

Occurrence of gas hydrates along the continental margins of India, particularly the Krishna‐Godavari offshore basin

The presence of gas hydrates along the Indian continental margins has been inferred mainly from the bottom simulating reflection/reflector (BSR) and the gas hydrate stability zone thickness map of India. Multidisciplinary investigations have been carried out in the Krishna‐Godavari offshore area along the eastern continental margin of India which is known for its hydrocarbon potential. Processed multibeam data provided a high resolution seafloor mosaic with a fine scale geomorphology. Deep tow digital side scan sonar, multifrequency chirp sonar and 3.5 KHz sub‐bottom profiler records depict various kinds of gas escape features over the regions where BSRs are prominent. Geochemical analyses of the 5 m‐long cores show a general decrease trend in the porewater sulphate concentration, while the gas chemistry reveals an increase trend of methane concentration with core depth. Total Organic Carbon varies from 0.6 to >2.0% and CaCO3 from 5.0 to >29%. Observed geophysical, geochemical and microbial proxies suggest the likely presence of gas hydrates in the Krishna‐Godavari offshore area. Recent drilling work carried out onboard JOIDES Resolution confirmed the presence of massive (>80 m thick) accumulation of gas hydrates, and fully developed gas hydrate system in the Mahanadi offshore area and the Andaman Sea.

Formation and tectonic evolution of the Cretaceous–Jurassic Muslim Bagh ophiolitic complex, Pakistan: Implications for the composite tectonic setting of ophiolites

The Muslim Bagh ophiolitic complex Balochistan, Pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the Indian and Eurasian plates. These ophiolites were obducted onto the Indian continental margin around the Late Cretaceous, prior to the main collision between the Indian and Eurasian plates. The upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes. Each pluton has a transitional dunite-rich zone at its base, and new geochemical data suggest a similar mantle source region for both the plutons and dykes. In contrast, the lower nappe consists of pillow basalts, deep-marine sediments and a mélange of ophiolitic rocks. The rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid-ocean ridge. The basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the Indian plate. The two nappes of the Muslim Bagh ophiolitic complex are thus distinctively different in terms of their age, lithology and tectonic setting. The recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved.

Mapping of Muslim Bagh ophiolite complex (Pakistan) using new remote sensing, and field data

TETHYS is a relational GIS database that combines geophysical, remote sensing, geochemical, and geochronological data, developed as a flexible resource for studying magmatic and geodynamic responses to continental collisions. In this paper, we demonstrate utility of our database by integrating field, remote sensing, and structural data, for detailed mapping and tectonic emplacement of Muslim Bagh ophiolite of western Pakistan. This ophiolite forms the uppermost part of a nappe pile which accreted onto the Indian continental margin during the closure of the Neo-Tethys during a pre-terminal collision that predated the final closure of Tethys during the major collision between India and Eurasia. Utilizing the TETHYS, Landsat, ASTER imagery, and a digital elevation model developed from the ASTER data are used to characterize the lithology and structure of the area. Use of image processing techniques improved the geologic map of the area, for a better understanding of the tectonic emplacement of the Muslim Bagh ophiolite. For the first time we report that the dikes in the Muslim Bagh ophiolite are cutting the metamorphic sole. Our preliminary geochemical data for sheeted dike complex suggest chemical affinities with arc-related rocks. This observation suggests that dikes were intruded in an island arc environment soon after the ophiolite was formed.

Post-collisional adakites in south Tibet: Products of partial melting of subduction-modified lower crust

Post-collisional (26.2 to 10.1 Ma) adakites occur within the Lhasa terrane of the southern Tibetan Plateau in an E–W trending, 1500 km long, magmatic belt. Outcrops are small and restricted within N–S-trending rift zones (grabens); they include both extrusive and intrusive facies. The adakites have high SiO 2 (59–70 wt.%), Al 2O 3 (15–18 wt.%) and Sr (317–1133 ppm) contents and Sr/Y ratios (44–162), and low Y (4.2–12.9 ppm) and HREE (e.g. Yb < 0.9 ppm) concentrations. Their MORB-normalised incompatible element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The combined trace element and Sr–Nd–Pb isotope characteristics of the adakites suggest that their source was mafic-intermediate lower crust formed during a preceding stage (153–40 Ma) of active continental margin magmatism. Lower crustal melting was primarily induced by the conduction of heat from contemporaneous potassic–ultrapotassic magmas produced by partial melting in the asthenosphere or lower lithosphere. Trace element modelling calculations suggest that the adakites are the products of 5–10% partial melting of garnet-bearing amphibolite facies meta-igneous rocks. Differences in the LILE contents and Sr–Nd isotope compositions of adakites sampled to the east and west of 86° E can be attributed to variable degrees of partial melting of the lower crust and mixing between potassic–ultrapotassic magmas and lower crustal melts. Extensional collapse of the Tibetan Plateau may have contributed to partial melting of the lower crust and the formation of the potassic–ultrapotassic magmatism by decompression melting of a thin asthenospheric mantle wedge above a subducted slab of Indian continental margin lithosphere. The oldest age of the post-collisional adakites and contemporaneous potassic–ultrapotassic magmatism in the Lhasa terrane may, therefore, provide constraints on the timing of initiation of tectonic collapse in the southern part of the plateau.

Fluid flow related features as an indicator of potential gas hydrate zone: western continental margin of India

Multichannel seismic reflection data from the continental margin of western India suggest the potential presence of fluid expulsion features, which may or may not be associated with gas hydrates. No typical bottom simulating reflector was observed on the reflection seismic section. As a result we look for other evidence in seismic sections in a small corridor of the western continental margin of India in order to establish the presence of gas hydrates. We study features including venting through the seafloor, pockmarks, sea floor collapse, faults acting as migration paths for fluid flow, transparent gas-charged sediment, reduction in amplitude strength, diapirism and mud-volcano. Presence of all these gas-escape features on a seismic section implies the probable presence of methane within the zone of hydrate stability field.

Occurrence of arsenic (V) in forearc mantle serpentinites based on X-ray absorption spectroscopy study

We examined As concentration, mineralogical site, and oxidation state in the serpentinites associated with the Tso Morari eclogitic rocks in the Indus suture zone, northwest Himalaya, to examine how highly fluid-soluble elements like As are transferred from slabs to arc magmas in subduction zones. The serpentinite samples represent hydrated peridotites at the base of the mantle wedge beneath Eurasia, and were exhumed from a depth of ∼100 km during subduction of the Indian continental margin. The bulk serpentinite samples contain total As concentrations ranging from 6 to 275 ppm. Arsenic in the samples is most likely present with magnetite and antigorite, since electron probe analysis yielded up to 90 ppm As in these minerals. X-ray absorption near-edge structure spectra indicate that As in serpentinites is mostly As(V) and that the neighboring atoms of As are O, although there are minute grains of sulfides and arsenides. The ratio of As(V) to total As is greater in samples with higher As contents, suggesting that the major source of As was oxidized As(V), introduced to the mantle wedge. Arsenic(V), originally adsorbed on Fe-oxides in slabs and overlying sediments, was most likely liberated during their subduction and incorporated subsequently in the overlying mantle wedge. Our data constrain the introduction of As into the mantle wedge at relatively shallow levels, much shallower than 25 km. Arsenic incorporated in the serpentinites was transported to deeper levels by mantle flow downward along the subduction zone, to be subsequently exhumed together with eclogitic rocks.

Migration of methane at the diapiric structure of the western continental margin of India — insights from seismic data

Investigations are being carried out to locate the indirect evidences of gas occurrences in the Indian Continental Margins by identifying features like pockmarks, shale diapirs, venting as well as bottom simulating reflectors (BSR). We investigate one such case of a shale diapirs. Occurrence of free gas and gas hydrates is reported using a complex trace analysis to study the variation of reflection strength and instantaneous frequency along individual reflections. A frequency analysis over the flank of the shale diapir indicates attenuation of high frequencies, possibly representing the migration of fluids (probably methane) along the fault pathways. Similarly a plot of reflection strength along an individual reflector shows the migration pathways and significant blanking.

Subsidence history of the north Indian continental margin, Zanskar–Ladakh Himalaya, NW India

Detailed geological field mapping has allowed the restoration of two full stratigraphic sections through the highly deformed Mesozoic and Early Tertiary fold and thrust belt of the north Indian continental margin. The two sections, which are representative of a proximal and a distal facies on the margin, have been backstripped using standard techniques. Profiles of the tectonic subsidence and uplift through the pre-collisional history of the margin have been constructed and compared with the predictions of simple thermal and mechanical models. The pre-collisional history can be explained by a thermal model with an initial age of rifting of 270 Ma and a stretching factor, β , of c . 1.2. This model accounts for the general exponential decrease in the backstripped tectonic subsidence. The model fails, however, to completely explain the subsidence and uplift history of the margin since the late Cretaceous. The history during this time is characterized by uplift at the most proximal location and an increase in subsidence at the most distal location. We attribute these differential vertical movements to flexural loading of the north Indian margin by obduction of the Spontang ophiolite. The best fit model is one in which a 70 km wide wedge-shaped load, tapering from 10 to 0 km thick, is emplaced on rifted lithosphere with an elastic thickness, T e , of 5–10 km. These results, which are in accord with the late Cretaceous timing of obduction and the structure of the Spontang ophiolite, provide new constraints on the T e structure of extended continental lithosphere 120–150 Ma after a rifting event.

Dating of Sediment Layers and Sediment Accumulation Studies along the Western Continental Margin of India: A Review

Modern surface sediments and organic matter/peat in sediment cores along the western continental margin of India have been dated by excess 210Pb and conventional or accelerator mass spectrometry 14C techniques. We have established an extensive database of these radiometric dates and related parameters, and have calculated sedimentation rates. Modern sedimentation rates are higher (1.8-19.0 mm/yr) on the continental margin of northwestern India and decline (0.01-2.6 mm/yr) toward the southwest. This trend is attributed to differences in load-carrying capacity of the rivers and lithology of the adjacent hinterland. Sedimentation rates decrease away from the coast, except for higher sedimentation rates at few places on the mid-slope compared to inner slope regions. The data reveal lateral as well as temporal variations in accumulation rates, which are explained in terms of environment of deposition and paleomonsoon conditions. The onset of intensive monsoonal conditions during the Holocene is reflected in very high sedimentation rates on the inner shelf. Peat and carbonized wood layers in the inner shelf cores are of Early Holocene age and indicate marine transgression. Age inversions in the cores of continental shelf and slope regions are attributed to either sediment slumping or reworking and redeposition of foraminifera shells. This study provides additional information to the worldwide database for continental margin sedimentation, and is useful for understanding Late Quaternary sedimentation records, sea level changes, the Holocene marine transgression event, the role of Last Glacial Maxima (LGM), and climatic changes in the input of terrigenous sediments to continental margins.

Preferential distribution along transcontinental corridors of kimberlites and related rocks of Southern Africa

Regional and local structural controls on the emplacement of 1326 Southern African kimberlites and related rocks (kimberlites sensu lato, 11% of which are dated) are analysed using a framework of lineaments defined by combining geology, aeromagnetics, gravity and geomorphological data. Spatial analysis of occurrences within clusters of kimberlites less than 100km across resolves variable trends, depending on the age and position of the cluster; but on a regional scale the distribution of these clusters is statistically controlled by four lineament trends: 040°, 096°, 134° and 165°. Similar regional trends are observed as aspect lineaments that can be followed over large distances from modelling the variation in dip direction of the Southern African topography. These observations suggest that different geological parameters exert a control on the distribution of kimberlites. Local structures may include en-echelon fault arrays, Riedel, R’-, P- or T-structures within trans-continental lithosphere structures (cryptic continental corridors). Many cryptic continental corridors are collinear with fracture zones along the Atlantic and Indian continental margins of Southern Africa, and may have found their origin in events resulting from plate reorganization during the break-up of the supercontinent Gondwana. Fault resistance may have rapidly changed the stress state of the African continent causing the deep lithospheric faults to be the loci of episodic extension, allowing kimberlite fluids to ascend through the faults and cluster within near-surface structures. A progressive age variation of kimberlite magmatism in Southern Africa may be attributed to stress propagation along deep lithospheric fractures.

Reconstructing the total shortening history of the NW Himalaya

The onset of India‐Asia contact can be dated with both biostratigraphic analysis of syn‐collisional sedimentary successions deposited on each side of the Indus Suture zone, and by radiometric dating of Indian crustal rocks which have undergone subduction to great depths in the earliest subduction‐collision stages. These data, together with paleomagnetic data, show that the initial contact of the Indian and Asian continental margins occurred at the Paleocene/Eocene boundary, corresponding to 55 ± 2 Ma. Such dating, which is consistent with all available geological evidence, including the record of magnetic anomalies in the Indian ocean and decrease of magmatic activity related to oceanic subduction can thus be considered as accurate and robust. The sedimentary record of the Tethys Himalaya rules out obduction of oceanic allochtons directly onto the Indian continental margin during the Late Cretaceous. The commonly inferred Late Cretaceous ophiolite obduction events may have thus occurred in intraoceanic setting close to the Asian margin before its final emplacement onto the India margin during the Eocene. Granitoid and sedimentary rocks of the Indian crust, deformed during Permo‐Carboniferous rifting, reached a depth of some 100 km about 1 Myr after the final closure of the Neo‐Tethys, and began to be exhumed between 50 and 45 Ma. At this stage, the foreland basin sediments from Pakistan to India show significant supply from volcanic arcs and ophiolites of the Indus Suture Zone, indicating the absence of significant relief along the proto‐Himalayan belt. Inversion of motion may have occurred within only 5 to 10 Myr after the collision onset, as soon as thicker and buoyant Indian crust chocked the subduction zone. The arrival of thick Indian crust within the convergent zone 50–45 Myr ago led to progressive stabilization of the India/Asia convergent rate and rapid stabilization of the Himalayan shortening rate of about 2 cm yr−1. This first period also corresponds to the onset of terrestrial detrital sedimentation within the Indus Suture zone and to the Barrovian metamorphism on the Indian side of the collision zone. Equilibrium of the Himalayan thrust belt in terms of amount of shortening versus amount of erosion and thermal stabilization less than 10 Myr after the initial India/Asia contact is defined as the collisional regime. In contrast, the first 5 to 10 Myr corresponds to the transition from oceanic subduction to continental collision, characterized by a marked decrease of the shortening rate, onset of aerial topography, and progressive heating of the convergent zone. This period is defined as the continental subduction phase, accommodating more than 30% of the total Himalayan shortening.

Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet

Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.

Slope failures along the western continental margin of India: a consequence of gas-hydrate dissociation, rapid sedimentation rate, and seismic activity?

Along the western continental margin of India (WCMI), several bottom simulating reflectors were identified on analogue single-channel seismic records, some of these located in areas where slumping and mass wasting were observed. The causes, consequences and degree of geographic variation of these geomorphic processes are assessed in terms of possible gas-hydrate dissociation during Pleistocene sea-level changes, high sedimentation resulting in underconsolidation, and seismotectonic activity prevailing along the WCMI margin. One consequence of possible gas-hydrate dissociation along the continental slope could be sediment failure and mass transport down the slope. By contrast, in the flat deep-sea areas, gas-hydrate dissociation may have led to gas seepage and the development of pockmarks at the seafloor.

The Toba Volcanic Event and Interstadial/Stadial Climates at the Marine Isotopic Stage 5 to 4 Transition in the Northern Indian Ocean

The Toba volcanic event, one of the largest eruptions during the Quaternary, is documented in marine sediment cores from the northeastern Arabian Sea. On the crest of the Murray Ridge and along the western Indian continental margin, we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5–4 boundary with the chemical composition of the “Youngest Toba Tuff.” Time series of the Uk′37-alkenone index, planktic foraminiferal species, magnetic susceptibility, and sediment accumulation rates from this interval show that the Toba event occurred between two warm periods lasting a few millennia. Using Toba as an instantaneous stratigraphic marker for correlation between the marine- and ice-core chronostratigraphies, these two Arabian Sea climatic events correspond to Greenland interstadials 20 and 19, respectively. Our data sets thus depict substantial interstadial/stadial fluctuations in sea-surface temperature and surface-water productivity. We show that variable terrigenous (eolian) sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record. Within the provided stratigraphic resolution of several decades to centennials, none of these proxies shows a particular impact of the Toba eruption. However, our results are additional support that Toba, despite its exceptional magnitude, had only a minor impact on the evolution of low-latitude monsoonal climate on centennial to millennial time scales.

Constraints on India-Eurasia collision in the Arabian Sea region taken from the Indus Group, Ladakh Himalaya, India

Abstract The Indus Group is a Paleogene, syntectonic sequence from the Indus Suture Zone of the Ladakh Himalaya, India. Overlying several pre-collisional tectonic units, it constrains the timing and nature of India’s collision with Eurasia in the western Himalaya. Field and petrographic data now allow Mesozoic-Paleocene deep-water sediments underlying the Indus Group to be assigned to three pre-collisional units: the Jurutze Formation (the forearc basin to the Cretaceous-Paleocene Eurasian active margin), the Khalsi Flysch (a Eurasian forearc sequence recording collapse of the Indian continental margin and ophiolite obduction), and the Lamayuru Group (the Mesozoic passive margin of India). Cobbles of neritic limestone, deep-water radiolarian chert and mafic igneous rocks, derived from the south (i.e. from India), are recognized within the upper Khalsi Flysch and the unconformably overlying fluvial sandstones of the Chogdo Formation, the base of the Indus Group. The Chogdo Formation is the first unit to overlie all three pre-collisional units and constrains the age of India-Eurasia collision to being no younger than latest Ypresian time (&gt;49 Ma), consistent with marine magnetic data suggesting initial collision in the Arabian Sea region at c. 55 Ma. The cutting of equatorial Tethyan circulation north of India at that time may have been a trigger to the major changes in global palaeoceanography seen at the Paleocene-Eocene boundary. New 40 Ar/ 39 Ar, apatite fission-track and illite crystallinity data from the Ladakh Batholith and Indus Group show that the batholith, representing the old active margin of Eurasia, experienced rapid Eocene cooling after collision, but was not significantly reheated when the Indus Group basin was inverted during north-directed Miocene thrusting (23–20 Ma). Subsequent erosion has preferentially removed 5–6 km ( c. 200°C) over much of the exposed Indus Group, but only c. 2 km from the Ladakh Batholith. Reworking of this material into the Indus fan may complicate efforts to interpret palaeo-erosion patterns from the deep-sea sedimentary record.