Indus-Yarlung Zangbo Research Articles

P n tomographic velocity and anisotropy beneath the Tibetan Plateau and the adjacent regions

We present a tomographic velocity and anisotropy model of the uppermost mantle beneath the Tibetan Plateau and the adjacent regions. The investigation analyzed 105,385 Pn phase readings from the International Seismological Centre (ISC) and the China Earthquake Data Center. The average Pn velocity under the study area is approximately 8.15 km/s, with velocity perturbations up to 3–4%. We find high Pn velocities under the Indian Plate and in the Tarim and Sichuan basins, low Pn velocities under the Hindu Kush and in Myanmar and the adjacent region, and especially low Pn velocities under the area north of the Indus-Yarlung Zangbo suture. The high Pn velocity anomalies of the Indian Plate are discontinuous at the collision region in the east-west direction, indicating that the Indian Plate probably subducts in a piecewise manner. Distributions of Pn velocities are used to validate mechanisms for the subduction of the Indian Plate presented in previous studies. In addition, Pn anisotropy is obtained simultaneously with Pn velocity. At plate collision zones, the fast Pn anisotropy direction is parallel to the direction of the collision edge. We validate the existence of Pn anisotropy under these regions and discuss the relationship of anisotropy with tectonic structure and plate movement.

Eclogite from the Kumon range, Myanmar: Petrology and tectonic implications

This paper presents the first report on the occurrence of eclogite from the Kumon range of the Western province in Myanmar, which is in the southeastern extension of the Himalayan orogenic belt. The eclogite is mainly composed of omphacite, garnet, hornblende/edenite/katophorite/taramite, biotite, quartz, and rutile. The garnet grains in the eclogite usually show textures of barrier reef, atoll, and table reef types, and have a wide compositional range of Alm58–70Sps1–2Prp9–16Grs14–31. Omphacite grains that occur as garnet inclusions and as isolated crystals in the matrix have similar compositions of Jd34–45 and Jd37–44, respectively. Lesser amounts of jadeitic clinopyroxene (Jd21–38), phengite, biotite, albite, and quartz occur in the lagoon of barrier reef and atoll garnet grains. The matrix omphacite is partly replaced by symplectite of sodic clinopyroxene of Jd20–29 and albite. The lower limits of the pressure/temperature during the eclogite stage, which are defined by the assemblage of garnet, omphacite (Jd40–45), and quartz, are 1.2–1.3GPa/530–615°C. The finding of eclogite from Myanmar suggests the possibility of a wide occurrence of high-pressure metamorphic rocks in the ophiolite zone along the southeastern extension of the Indus-Yarlung Zangbo suture in Myanmar and Indochina.

Anorthitic plagioclase and pargasitic amphibole in mantle peridotites from the Yungbwa ophiolite (southwestern Tibetan Plateau) formed by hydrous melt metasomatism

Plagioclase crystals with an anorthite content up to An 99 have been discovered in mantle peridotites from the Yungbwa ophiolite, which crops out along the Indus–Yarlung Zangbo Suture, southwestern Tibetan Plateau. Amphiboles of pargasitic composition also occur in these plagioclase peridotites. Microtextures support the view that both plagioclase and amphibole were formed metasomatically by hydrous melts entering the Yungbwa peridotites, during which pyroxene was dissolved and olivine was precipitated. The extreme Ca-rich nature of the plagioclase indicates that the metasomatic melts had high Ca/Na ratios, whereas low Ti contents in accompanying spinel and amphibole indicate severe depletion of Ti in the hydrous melts. This suggests that the metasomatic melts were derived from a refractory mantle source. Clinopyroxene trace element data indicate that the metasomatic melts were also enriched in light rare earth elements relative to both middle and heavy rare earth elements. Therefore, the melts show geochemical characteristics similar to arc magmas. This implies that the mantle peridotites in the Yungbwa ophiolite experienced a two-stage evolution; they were formed at a mid-ocean ridge and subsequently entered a subduction zone setting. Their history thus records the opening and closing of the Neo-Tethys Ocean.

REE and PGE Geochemical Constraints on the Formation of Dunites in the Luobusa Ophiolite, Southern Tibet

The Luobusa ophiolite, Southern Tibet, lies in the Indus–Yarlung Zangbo suture zone that separates Eurasia to the north from the Indian continent to the south. The ophiolite contains a well-preserved mantle sequence consisting of harzburgite, clinopyroxene (cpx)-bearing harzburgite and dunite. The harzburgite contains abundant pods of chromitite, most of which have dunite envelopes, and the cpx-bearing harzburgites host numerous dunite dykes. Dunite also exists as a massive unit similar to those of the mantle–crust transition zones in other ophiolites. All of the dunites in the ophiolite have a similar mineralogy, comprising mainly olivine with minor orthopyroxene and chromite and traces of clinopyroxene. They also display similar chemical compositions, including U-shaped chondrite-normalized REE patterns. Mantle-normalized PGE patterns show variable negative Pt anomalies. Detailed analysis of a chromite-bearing dunite dyke, which grades into the host cpx-bearing harzburgite, indicates that LREE and Ir decrease, whereas HREE, Pd and Pt increase away from the dunite. These features are consistent with formation of the dunite dykes by interaction of MORB peridotites with boninitic melts from which the chromitites were formed. Because the transition-zone dunites are mineralogically and chemically identical to those formed by such melt–rock reaction, we infer that they are of similar origin. The Luobusa ultramafic rocks originally formed as MORB-source upper mantle, which was subsequently trapped as part of a mantle wedge above a subduction zone. Hydrous melts generated under the influence of the subducted slab at depth migrated upward and reacted with the cpx-bearing harzburgites to form the dunite dykes. The modified melts ponded in small pockets higher in the section, where they produced podiform chromitites with dunite envelopes. At the top of the mantle section, pervasive reaction between melts and harzburgite produced the transition-zone dunites.

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.

Tomographical evidence for Indian Plate underthrusting only beneath Tethyan Himalaya

Tomographical inversion was performed using the data recorded by 51 seismographys deployed on a profile that followed southern Xizang (Tibet) high-way through a Sino-French Joint seismic experiment. The results indicate that the underthrusting Indian Plate is limited to the south of Indus-Yarlung Zangbo Suture (IYS) beneath Tethyan Himalaya, extends vertically to 150 km deep with relatively high angle near Gala, and becomes horizontally northward. The seismic velocities beneath Kangmar, Gangdise and Yangbajain-Golug rift in the middle of the lithosphere show low velocity features, which may indicate the existence of high temperature and partial melting. The results from tomography strongly suggest that the continent-continent subduction occurred only beneath Himalaya and was confined to the south of IYS since the collision between India and Eurasia.

Shear Wave Anisotropy of the Upper Mantle Beneath the Region from Tingri of Tibet to Golmud of Qinghai

AbstractMeasurements of shear wave splitting at 43 three–component seismic stations show very big difference in anisotropy on both sides of the Indus–Yarlung Zangbo suture (ITS), but little difference on both sides of the older Bangong–Nujiang suture (BNS) and the Jinsha River suture (JS) to its north. Obvious discrepancy exists between the anisotropy direction and the superficial tectonic trends, which is not explicable directly by the coherent upper mantle deformation usually supposed to occur in consistency with the trend of a collisional belt. On the other hand, strong spatial relationships are observed from the anisotropy results, such as the orthogonal directions of anisotropy on both sides of ITS and the good correlation between the region of larger magnitude of anisotropy and the zone of inefficient Sn propagation of Qiangtang as well as the systematic rotation of the directions of anisotropy, which should testify some much more complicated aspects of the continental convergence mechanism To the best of our data, we tend to suppose that the Qinghai—Tibet plateau might result from a mechanism complicated by the coexistence of Argand's underthrusting and Dewey's diffuse deformation.

Age of initiation of collision between India and Asia: A review of stratigraphic data

The collision of India with Asia is perhaps the most profound tectonic event to have occurred in past 100 Ma. It is responsible for the uplift of the Himalayas and Tibetan Plateau and has been argued to have been responsible for geological, geochemical, and climatological consequences of global extent. Yet the age of initiation of this collision remains poorly constrained. The literature is replete with estimates that range from the Late Cretaceous (> 65 Ma) to latest Eocene (< 40 Ma) with little consensus in between. This paper reviews the available stratigraphic evidence from the Himalayan region, and concludes that only in the western Zanskar-Hazara region is the age well constrained as starting in the Late Ypresian (∼ < 52 Ma). To the east only in the Malla Johar region of the Tethyan Himalayas have potentially syn-collisional sediments been recognized south of the Indus Yarlung Zangbo suture. However, here the correlation of the upper part of the Sangchamalla Flysch is contentious, with correlations ranging from Late Cretaceous to Middle Eocene (Lutetian). In the most eastern sections of Tertiary rocks thus far recognized within the Tethyan Himalayas north and east of Everest (Mount Qomolangma) normal, shallow shelf-type carbonates extend into the Lutetian, without evidence of a change in sedimentation to the top of the section, so the start of collision must be still younger. Along-strike of the Indus Yarlung Zangbo suture thick submarine delta-fan complexes derived from erosion of the Himalayan-Tibet system provide independent estimates that agree with a diachronous collision initiating in the late Ypresian in the west and progressing into and perhaps through the Lutetian in the east. The stratigraphic and magmatic history along the north side of the suture are compatible with such a diachronous history. This diachroneity has important implications for estimates of the accommodation of strain within this orogenic system.

Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle

The Luobusa ophiolite in the Indus—Yarlung Zangbo suture of southern Tibet hosts the largest known chromite deposit in China. The podiform chromitites occur in a well-preserved mantle sequence consisting of harzburgite with abundant lenses of dunite. The harzburgites have relatively uniform bulk-rock compositions with mg-numbers [100 Mg/(Mg + Fe)] ranging from 89 to 91 and show flat, unfractionated, chondrite-normalized platinum group element (PGE) patterns. Their accessory chromite varies widely in cr-number [100Cr/(Cr + Al)] (18–66). These rocks are essentially residua left after extraction of mid-ocean ridge basalt (MORB)-type magmas. The podiform chromitites display nodular, massive, disseminated and banded textures and typically have dunite envelopes that grade into the surrounding harzburgite and diopsidic harzburgite with increasing pyroxene contents. They consist of relatively uniform chromite with high cr-numbers (74–82), have strongly fractionated, chondrite-normalized PGE patterns with enrichment in Os, Ir and Ru relative to Rh, Pt and Pt, and are believed to have formed from a boninitic magma produced by a second stage of melting. Dunites contain accessory chromite intermediate in composition between those of harzburgite and chromitite and are believed to be the products of reaction between new boninitic magmas and old MORB-type peridotites. The melt-rock reaction removed pyroxene from the peridotites and precipitated oli-vine, forming dunite envelopes around the chromitite pods. The melts thus became more boninitic in composition and chromite saturated, leading to precipitation of chromite alone. The interplay of melt-rock interaction, chromite fractionation and magma mixing should lead to many fluctuations in melt composition, producing both massive and disseminated chromitites and phase layering within individual podiform bodies observed in the Luobusa ophiolite.

Tectonic framework of the Himalaya, Karakoram and Tibet, and problems of their evolution

The Himalaya, the Karakoram and Tibet were assembled by the successive accretion to Asia of continental and arc terranes during the Mesozoic and early Tertiary. The Jinsha and Banggong Sutures in Tibet join continental terranes separated from Gondwana. Ophiolites were obducted onto the shelf of southern Tibet in the Jurassic before the formation of the Banggong Suture. The Kohistan—Ladakh Terrane contains an island arc that was accreted in the late Cretaceous on the Shyok Suture and consequently evolved into an Andean-type batholith. Further east this TransHimalayan batholith developed on the southern active margin of Tibet without the prior development of an island arc. Ophiolites were obducted onto the shelf of India in the late Cretaceous to Lower Palaeocene before the closing of Tethys and the formation of the Indus—Yarlung Zangbo Suture at about 50 Ma. Post-collisional northward indentation of India at ca.5 cm a-1 since the Eocene has redeformed this accreted terrane collage; palaeomagnetic evidence suggests this indentation has given rise to some 2000 km of intracontinental shortening. Expressions of this shortening are the uplift of mid-crustal gneisses in the Karakoram on a late-Tertiary breakback thrust, folding of Palaeogene redbeds in Tibet, south-directed thrust imbrication of the foreland and shelf of the Indian Plate, north-directed back-thrusts along the Indus Suture Zone, post-Miocene spreading and uplift of thickened Tibet, giving rise to N—S extensional faults, and strike-slip faults, which allowed eastward escape of Tibetan fault blocks.

Spasmodic rift reactivation and its role in the pre-orogenic evolution of the Himalayan region

The evidence of several phases of pre-orogenic mafic magmatism of rift tectonic setting, associated with epeirogenic uplift, erosion and transgression, in the exposed Middle Proterozoic to Paleocene lithostratigraphic column of the Himalayas suggests that the region was the site of episodic rift reactivation for at least 1.5 Ga. The magmatism shows an overall progressive trend in the intensity, volume and spatial extent from the oldest (Middle Proterozoic) to the youngest (Early Cretaceous) phase. Also, the locus of magmatic activity shows a gradual south to north shift: the earliest eruptions were along the peripheral part of the basin, the middle ones moved towards the inner parts and the last phase occurred in the axial part of the basin. It is suggested that this peripheral to axial shift of magmatism indicates a gradual shift of the stress regime from an initial diffused pattern to progressive accentuation and final concentration along a narrow belt now represented by the Indus-Yarlung Zangbo Suture Zone. The long history of spasmodic tensional tectonics has certain critical regional and global implications. The regional implications include 1. (1)the rift-associated upper mantle volatile flux and high heat flow in the region inducing lithospheric buoyancy and epeirogenic uplift, erosion, transgression over the flank regions, and subcrustal anatexis; 2. (2)the control on lithological-geochemical associations (e.g., the deposition of carbonate rocks following each phase of rifting); and 3. (3)the evolution of the basin from an early wide depositional basin to finally a narrow linear trough by repeated rift reactivation. The global implication is examined in the light of possible mechanisms of rift initiation and its episodic reactivation. The convection current and mantle “plume” mechanisms within the plate tectonics framework are at variance with both the geological constraints of the region and the general considerations of these processes. Most importantly, the nature and age of the Indus-Yarlung Zangbo ophiolite and other geological data cannot be reconciled with the plate tectonics proposition that the ophiolite represents “ocean floor” rocks of the wide oceanic Tethys. Instead, the data indicate that the ophiolite represents mafic-ultramafic diapirism in an extremely attenuated lithosphere. The extraordinarily close synchroneity of the rifting spasms in the Himalayan region and the times of change in the Earth's rotation rate strongly point to some close relationship and the involvement of some basic aspect of Earth dynamics in the initiation and repeated reactivation of rifting in the Himalayan region.