Nujiang Suture Research Articles

Ages and geochemistry of the Renacuo granitoids in the Gaize area, central Tibet: implications for the northward subduction of the Bangong Suture Ocean

Subduction of Bangong Suture Ocean and collision between the Lhasa and Qiangtang blocks were keys to better understand the development of central Tibet. However, the initiation and closure timing, and nature and structure of the Bangong suture are still poorly constrained. Mesozoic intrusions are widely distributed in central Tibet. Petrology, geochronology and geochemistry of the Renacuo pluton and their microgranular enclaves in the Gaize area to the north of the Bangong–Nujiang suture zone, south of the Longmu Co–Shuanghu suture zone, are used to constrain the tectonic history of the Bangong suture. Results of our SHRIMP U–Pb dating of the zircons are as follows: quartz diorite is 139 ± 4 Ma, MSWD = 1.6; the microgranular enclave is 147.3 ± 3.3 Ma, MSWD = 0.45. The evolutionary trend of the major elements, trace elements and rare earth elements of the microgranular enclaves and the host rocks are almost similar, suggesting that the enclaves and the host rocks may have had the same source. They most likely originated from magma mixing between mantle-derived and crust-derived melts. On the basis of our mapping and geochronologic studies, we suggest that the formation of the Renacuo pluton is consistent with previously suggested northward subduction of the Bangong Suture Ocean, which occurred in the Late Jurassic–Early Cretaceous. Copyright © 2015 John Wiley & Sons, Ltd.

Evolution of the Bangong–Nujiang Tethyan ocean: Insights from the geochronology and geochemistry of mafic rocks within ophiolites

The Bangong–Nujiang suture zone (BNSZ) is located in the central Tibetan Plateau, and represents the relict of the Bangong–Nujiang Tethyan Ocean (BNTO). This paper presents zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotope data for the ophiolitic rocks from the Rutog, Dongco, Dongqiao, Amdo, and Dengqen areas (from west to east) in the BNSZ. Zircon U–Pb ages obtained from five gabbros and one leucogabbro from the five areas are 169±2, 167±2, 187±2, 184±2, 177±3, and 164±2Ma, respectively. Mafic rocks (gabbros and basalts) in the BNSZ can be divided into MORB-like and OIB-type, based on geochemical data. The MORB-like rocks are tholeiitic and can be further divided into N- and E-MORB subtypes, marked by depleted and flat REE patterns, respectively. All MORB-like rocks show supra-subduction zone (SSZ) geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements, coupled with high and positive whole-rock εNd(t) values (+8.1 to +5.4). The OIB-type rocks, which formed in the Early Cretaceous (~132–108Ma), are exposed in the Dongco, Dongqiao, and Tarenben areas, and they consist mostly of alkali basalts with within-plate geochemical characteristics and positive εNd(t) values (+5.9 to +4.5). On the basis of these new data, combined with information from the literature, we further support that the BNTO was an intra-oceanic arc–backarc basin that existed between the North Lhasa subterrane (NLT) and the South Qiangtang subterrane (SQT) during the Early–Middle Jurassic. This basin may have been active until the Early Cretaceous, when the OIB-type basalts erupted.

Zircon U-Pb Age and Geochemical Characteristics of Ore-bearing Granodiorite Porphyry in the Duobuza Porphyry Copper Deposit, Tibet

Abstract The Duobuza deposit is the first porphyry-type copper deposit discovered with giant prospect in the Bangongco metallogenic belt. Geochemical data indicates that the ore-bearing Duobuza granodiorite porphyry is high-K calc-alkaline to shoshonitic and peraluminous composition. The ore-bearing granodiorite porphyry is enriched in large-ion lithophile elements(LILE) such as Rb, K, Th, La, Ce and Sr, and depleted in high-field-strength elements(HFSE) such as Nb, Ta, P, and Ti. The rare-earth element (REE) patterns show enrichment in light REEs relative to heavy REEs. The major, rare-earth, and trace elements of the ore-bearing granodiorite porphyry show characteristics of adakites, formed in an island arc setting. The laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb age of the ore-bearing granodiorite porphyry is 123.4±1.2 Ma (MSWD = 1.7), which also represents the age of the copper-mineralization. Together with the age data of the early Cretaceous magmatic rocks in the Bangongco–Nujiang suture zone and the middle-northern Gangdese, it indicates that there was bidirectional (northward and southward) subduction of the Bangongco–Nujiang ocean during 120 Ma, and the Duobuza deposit was related to this event.

Petrology, geochemistry, and geochronology of boninitic dikes from the Kangqiong ophiolite: implications for the Early Cretaceous evolution of Bangong–Nujiang Neo-Tethys Ocean in Tibet

The Kangqiong ophiolite is exposed in the central–western part of the Bangong–Nujiang suture zone (BNSZ) of central Tibet. This study reports new data for boninitic dikes with the aim of reconstructing the geodynamic and petrogenetic evolution of the Kangqiong ophiolite. Ten samples of boninitic dikes that cross-cut the mafic cumulates have very low TiO2 (0.34–0.42%) contents and high MgO (6.65–8.25%) contents. LA-ICP-MS U–Pb analyses of zircon from the boninitic dikes yield an age of 115 Ma. They are characterized by positive εHf(t) values varying from +13.1 to +15.0. Taking into account the geochemical characteristics of the mantle section, the Kangqiong ophiolite should be generated in a fore-arc spreading setting resulting from intra-oceanic subduction. Based on our data and previous studies, we propose that the BNSZ represents the major suture and records the Early Cretaceous intra-oceanic subduction of the Bangong–Nujiang Neo-Tethys Ocean, and the Shiquan River–Yongzhu–Jiali ophiolitic mélange belt represents a back-arc basin. These two belts, together with the northern Lhasa subterrane should, represent an Early Cretaceous intra-oceanic subduction system and back-arc basin in central Tibet that is similar to present-day active intra-oceanic subduction systems in the western Pacific Ocean. The final closure of the Bangong–Nujiang Neo-Tethys Ocean might have taken place later than the Early Cretaceous.

Petrogenesis of diabase from accretionary prism in the southern Qiangtang terrane, central Tibet: Evidence from U–Pb geochronology, petrochemistry and Sr–Nd–Hf–O isotope characteristics

AbstractThe Bangong–Nujiang suture (BNS) between the Lhasa and Qiangtang terranes is an important boundary and its petrogenesis is controversial. Diabase from the accretionary prism in the southern Qiangtang terrane yields a zircon U–Pb age of 181.3 ± 1.4 Ma. All the diabases show tholeiitic basalt compositions, gentle enrichment patters of light rare earth elements (REE), variable enrichment in incompatible element concentrations (e.g. Th and Rb), and no anomaly in high field strength elements (e.g. Nb and Ta), similar to that of enriched mid‐ocean ridge basalt (E‐MORB). They have relatively homogeneous whole rock Nd (εNd(t) = 7.3–9.1) and zircon Hf–O isotopic compositions (εHf(t) = 14.8–16.1, and δ18O = 4.57–6.12‰), possibly indicating melting of the depleted mantle and no significant crustal contamination during the petrogenesis. The element variations suggest that the diabases were formed by plume–ridge interaction at a mid‐ocean ridge within the Bangong–Nujiang ocean.

Late Triassic island-arc–back-arc basin development along the Bangong–Nujiang suture zone (central Tibet): Geological, geochemical and chronological evidence from volcanic rocks

A major debate related to the evolution of the Tibetan Plateau is centered on whether or not an island arc–back-arc basin system occurred along the Bangong–Nujiang suture zone, central Tibet. Here we present new zircon U–Pb geochronology, rare earth elements (REEs) and bulk-rock geochemistry of these magmatic rocks in the Amdo area, the middle Bangong–Nujiang suture zone, central Tibet, to identify significant and new records of Mesozoic tectonomagmatic processes. Zircon U–Pb dating using LA-ICP-MS techniques yields a concordant age with a weighted mean 206Pb/238U age of 228.6±1.6Ma (n=7, MSWD=1.19) for the Quehala basalts, and a mean age of 220.0±2.1 (n=8, MSWD=1.5) for the Amdo pillow lavas. On the normalized REE patterns of zircon, significant Ce enrichment indicates that the magma sources of these magmatic rocks have been subjected to modification of slab-derived fluid. Geochemical features suggest that the Quehala basalts (ca. 228Ma), displaying an island arc tholeiites (IATs) affinity, resulted from partial melting of an relatively enriched mantle wedge in the subduction zone, whereas the Amdo pillow lavas (ca. 220Ma) characterized by both arc-like and N-MORB-like geochemical characteristics occurred as associated back-arc basin basalts (BABBs) at the spreading center of back-arc basin after the formation of island arc tholeiites. In conclusion, the volcanic rocks in the Amdo area have documented the magmatic processes from early-stage subduction to development of associated back-arc basin, confirming the occurrence of intra-oceanic subduction within the Bangong–Nujiang Tethys during the late Triassic. Furthermore, the spatial relationships among the Quehala formation, Tumengela formation and Amdo pillow lavas indicate likely northward subduction of the Bangong–Nujiang Tethyan Ocean during the Late Triassic to middle Jurassic.

The evolution of the Bangong–Nujiang Neo-Tethys ocean: Evidence from zircon U–Pb and Lu–Hf isotopic analyses of Early Cretaceous oceanic islands and ophiolites

We conducted in situ U–Pb analyses of zircons from three basalts and one gabbro from the Zhonggang oceanic island, one basalt from the Zhaga oceanic island, and one gabbro from the Kangqiong ophiolite (all located in the middle segments of the Bangong–Nujiang suture zone of Tibetan Plateau), as well as in situ Hf isotope analyses of zircons from one gabbro from the Zhonggang oceanic island to constrain the tectonic evolution of the Bangong–Nujiang Neo-Tethys ocean. All samples contain numerous inherited zircons, and all the zircons contain magmatic oscillatory zoning and have Th/U ratios exceeding 0.4. Moreover, the average ΣREE content of these zircons is less than 2000ppm, and they display clear negative Eu and variable positive Ce anomalies, indicating a magmatic origin. LA–ICP–MS U–Pb dating of the zircons revealed three clear peaks in the age distribution, at 248–255, 162–168, and117–120Ma; Lu–Hf isotopic analyses of zircons from the gabbro of the Zhonggang oceanic island yielded a 269Ma crust–mantle separation age. Taking into account the regional geology, previous data, and our new analyses, we infer that the middle and western segments of the Bangong–Nujiang Neo-Tethys ocean had initially opened in the late Permian (254–269Ma) and that the ocean opened substantially between the late Permian and the Early Triassic (248–255Ma). In addition, we infer that the initiation of subduction of the Bangong–Nujiang Neo-Tethys ocean took place at ~162–168Ma, which is Middle Jurassic. The117–120 Ma age is the time when the oceanic islands and ophiolites were formed, indicating that the Bangong–Nujiang Neo-Tethys ocean was, to some extent, still open at that time.

Petrology and U–Pb zircon geochronology of bimodal volcanic rocks from the Maierze Group, northern Tibet: Constraints on the timing of closure of the Banggong–Nujiang Ocean

We present new zircon U–Pb dates, major and trace element chemistry, and Hf isotopic compositions for bimodal volcanic rocks of the Maierze Group (MG) in the Maierze area of the Southern Qiangtang–Baoshan block, northern Tibet. We discuss the implications of these data for the evolution of this region. The MG bimodal volcanic rocks consist of basalts and dacites that yield LA-ICP-MS zircon U–Pb ages of 122 and 120Ma, respectively. The MG basalts have light rare earth element (LREE)-enriched chondrite-normalized REE patterns (LaN/YbN=13–14), high Ti/V ratios (45–64), high Zr (190–270ppm) and Nb (22–41ppm) concentrations, and Zr/Y ratios (7–9) that are similar to those of within-plate basalts. The MG basalts also have low MgO and total Fe2O3 (TFe2O3) concentrations, significant enrichments in the LREE and the light ion lithophile elements (LILEs; Rb, Ba, Th, U and Pb), and weak depletions in the high field strength elements (HFSE; Nb, Ta, and Ti), all of which are clearly evident in primitive-mantle-normalized multi-element variation diagrams. The MG dacites are more enriched in the LILE (e.g., Rb, Ba, Th, U, K, and Pb) and more depleted in the HFSE (e.g., Nb, Ta, and Ti) than the MG basalts. Moreover, the dacites have variable zircon εHf(t) values (−6.3 to +6.3). These features indicate that the parental magma for the MG basalts was likely derived from an enriched lithospheric mantle source that was contaminated by subduction-related fluids or melts. In contrast, the MG dacites were derived from mixing of the MG basaltic magma with a second magma derived from partial melting of the continental crust. The geochemical and Hf isotopic characteristics of the MG bimodal volcanic rocks suggest that they formed during the initial stages of development of a back-arc basin. From south to north, the Bangong–Nujiang suture zone, the Duolong gold-rich porphyry copper deposit, and the Maierze bimodal rocks are interpreted to represent a remnant of a complete volcanic arc system that comprises the Bangong–Nujiang Ocean, an island arc, and a back-arc basin, respectively. The fact that this volcanic arc system was still active during the Early Cretaceous indicates that closure of the Bangong–Nujiang Ocean occurred later than the previously suggested period of between the Late Jurassic and Early Cretaceous.

Geochemistry, Geochronology, Sr‐Nd Isotopic Compositions of Jiang Tso Ophiolite in the Middle Segment of the Bangong‐ Nujiang Suture Zone and Their Geological Significance

AbstractThe Jiang Tso ophiolite, situated in the middle segment of the Bangong‐ Nujiang Suture Zone, is a part of the easternmost Qieli Lake ophiolite subzone and is close to the south of Pung Lake ophiolite. The rock association of Jiang Tso ophiolite is relatively complete and is mainly composed of metamorphic peridotite, gabbro and diabase. Comparing with N‐MORB, the ophiolite is high in Mg and low in Ti, K, Na, P, and is depleted in Nb, Ta, Hf, Th and enriched in Rb, Sr and Ba. Geochemical characteristics of the Jiang Tso ophiolite indicate it is of a supra‐subduction zone type formed in the spreading ridge of back arc basin. The SHRIMP U‐Pb dating of zircons from the gabbro yielded a weighted average age of 188.1±4.1 Ma (MSWD=1.4), indicating the Jiang Tso ophiolite was formed in the late stage of early Jurassic. The Sr, Nd isotopic compositions show that the Tethyan mantle domain is the depleted mantle (DM), with enriched mantle domain II (EM II). They have the same Sr, Nd isotopic composition with the India Ocean MORB type.

Age and nature of the late Early Cretaceous Zhaga Formation, northern Tibet: constraints on when the Bangong–Nujiang Neo-Tethys Ocean closed

This article reports the depositional environment, provenance, and U–Pb zircon age constraints for the newly identified Zhaga Formation in northern Tibet and uses these to better understand the tectonic evolution of the Bangong–Nujiang suture. One transect across the Zhaga Formation was investigated. The Zhaga Formation is ~2 km thick, dominated by greywacke and conglomerate at the base, basalt and limestone in the middle, and greywacke and shale at the top. Greywacke in the Zhaga Formation typically contains 70–75% quartz, 5% feldspar, 3–5% rock debris, and >15% matrix, with normal grading and convolute bedding, basal flow structures, and distinct Bouma sequences interpreted as bathyal to abyssal turbidites. One rhyolite sample and one greywacke sample from the studied transect were collected for zircon U–Pb dating. The rhyolite yields a concordia age of 118 Ma, and the greywacke yields nine age peaks of 247, 330, 459, 541, 611, 941, 1590, 1871, and 2482 Ma, indicating that the Zhaga Formation formed during the late Early Cretaceous and the provenance of its detritus was the Qiangtang area. These data, combined with the Early Cretaceous ocean islands, indicates that the Bangong–Nujiang Neo-Tethys Ocean must have been open during the late Early Cretaceous. We conclude that the Bangong–Nujiang Neo-Tethys Ocean closed after the late Early Cretaceous and not during the Late Jurassic or the early Early Cretaceous as proposed by previous workers.

Structural model of the lithosphere–asthenosphere system beneath the Qinghai–Tibet Plateau and its adjacent areas

The deep structure of the lithosphere–asthenosphere system, as imaged from geophysical data, of the Qinghai–Tibet Plateau, the highest on the Earth, provides important clues in understanding its orogenic processes. Here we reconstruct the main features of the structure of the crust and upper mantle from surface wave tomography in the Qinghai–Tibet Plateau and its adjacent areas, in order to understand the modality of the convergence and collision process between the Indian and Eurasian plates. Based on Rayleigh waves dispersion theory, we collected long period and broad-band seismic data from the global and regional seismic networks surrounding the study area (20°N–50°N, 70°E–110°E). After applying instrument response calibration and filtering, group velocities of the fundamental mode of Rayleigh waves are measured using the frequency–time analysis (FTAN). Combining the published dispersion data, a 2-D surface-wave tomography method is applied to calculate the lateral variations of group velocity distribution at different periods, in the range from 8s to 150s. The Hedgehog non-linear inversion method is performed to obtain shear wave velocity (Vs) versus depth models of the crust and upper mantle for 181 cells, with size 2°×2°. In order to identify the cellular representative models, we applied the local smoothness optimization method (LSO). Fairly detailed structural models of the lithosphere–asthenosphere system have been defined. The Vs models demonstrate the lateral variation of the thickness of the metasomatic lid between the south and north of the Bangong–Nujiang Suture (BNS) and the west and east of Tibet. The variation in thickness of the metasomatic lid may suggest that the leading edge of the subducting Indian slab reaches up to BNS.

Petrology, geochemistry, and geochronology of the Zhonggang ocean island, northern Tibet: implications for the evolution of the Banggongco–Nujiang oceanic arm of the Neo-Tethys

This study presents new data relating to the tectonic evolution of the Zhonggang ocean island, within the Mesozoic Banggongco–Nujiang suture zone of northern Tibet, and discusses the implications of these data for the evolution of this region. Thirteen basalt and ten gabbro samples were collected from a sampling transect through this area; these samples have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and are enriched in highly incompatible elements, yielding primitive-mantle-normalized trace-element variation patterns that are similar to ocean island basalts (OIB). A gabbro dike intruded into basalt of the Zhonggang ocean island and was overlain by basaltic conglomerate, suggesting that this dike was formed after the basalt, but before the basaltic conglomerate. The gabbro dike yields an LA–ICP–MS zircon U–Pb age of 116.2 ± 4.1 Ma, indicating the timing of formation of the Zhonggang ocean island, and suggesting in turn that the Banggongco–Nujiang Neo-Tethys Ocean remained open at this time. These data, combined with the geological history of the region, indicates that the Banggongco–Nujiang Neo-Tethys Ocean opened between the late Permian and the Early Triassic, expanded rapidly between Late Triassic and Middle Jurassic time, and finally closed between the late Early and early Late Cretaceous.

Petrology, geochemistry, and geological significance of the Nadong ocean island, Banggongco–Nujiang suture, Tibetan plateau

We studied oceanic mafic igneous rocks of the Mesozoic Banggongco–Nujang suture zone in western Tibet to constrain the tectonic evolution of these rocks and the region as a whole. Two transects were accomplished. Seven basalt samples from the base of the Nadongshan transect (N1 basalts) have flat chondrite-normalized rare earth element (REE) and primitive-mantle-normalized trace element variation diagrams that are similar to MORB. Two basalt samples from the base of the Nadongshan transect (N1 basalts), ten gabbro samples from the middle of Nadongshan transect (N2 gabbros), four basalt samples from the bottom of Tanjiuxiama transect (T1 basalts), and four basalt samples from the top of the Tanjiuxiama transect (T2 basalts) are alkali basalts and have light rare earth element (LREE)-enriched chondrite-normalized REE patterns, and have primitive-mantle-normalized trace element variation diagrams that are enriched in highly incompatible elements, similar to OIB. LREE concentrations increase from N1 basalts to the T1 and T2 basalts, which have (La/Yb)N up to 16 and have even higher (Ce/Sm)N. These data indicate that the Nadong ocean island is an Azores-type ocean island that formed during the mature stage of development of the Banggongco–Nujiang Ocean. The conformable nature of the Nadong ocean island with the Mugagangri Group flysch indicates that the Banggongco–Nujiang Ocean was never a large ocean.

Mapping crustal structure beneath southern Tibet: Seismic evidence for continental crustal underthrusting

Receiver function imaging along a temporary seismic array (ANTILOPE-2) reveals detailed information of the underthrusting of the Indian crust in southern Tibet. The Moho dips northward from ~50km to 80km beneath the Himalaya terrane, and locally reaches ~85km beneath the Indus–Yalung suture. It remains at ~80km depth across the Lhasa terrane, and shallows to ~70km depth under the Qiangtang terrane. An intra-crustal interface at ~60km beneath the Lhasa terrane can be clearly followed southward through the Main Himalaya Thrust and connects the Main Boundary Thrust at the surface, which represents the border of the Indian crust that is underthrusting until south of the Bangong–Nujiang Suture. A mid-crustal low velocity zone is observed at depths of 14–30km beneath the Lhasa and Himalaya terranes probably formed by partial melt and/or aqueous fluids.

Geochronology of the Dong Tso Ophiolite and the Tectonic Environment

AbstractThe wedge shaped Dong Tso ophiolitic block is distributed near the transition point from the western to the middle sub–belt of the Bangong–Nujiang suture zone. The ophiolite is characterized by well–developed cumulate rocks that are mainly composed of cumulate and massive gabbros. In the cumulate gabbros, the adcumulate amphiboles are distributed extensively around the plagioclase and residual pyroxene grains; hence, the rocks are named adcumulate amphibole–gabbro. In this study, the formation age of the ophiolite has been estimated to be 166 ± 4 million years (Ma) by the sensitive high–resolution ion microprobe (SHRIMP) II U–Pb isotopic analysis of the zircons from the adcumulate amphibole–gabbro; the 40Ar/39Ar plateau age was estimated to be 148.19 ± 1.53 Ma, which should represent the emplacement time of the ophiolite, by isotopic dating of the pure amphibole mineral from the amphibole–schist. Two different suits of volcanic lavas have been recognized in this work. The purple colored pillow basalts have high TiO2 and P2O5 contents, and are rich in light rare earth elements (LREEs), large–ion lithospheric elements (LILEs) and high–field–strength elements (HFSEs), the characteristics that are the typical of the oceanic island basalt (OIB). On the other hand, other massive basaltic andesites of celadon color are poor in MgO; rich in Fe2O3, LREEs, LILEs, and HFSEs; and especially characterized by negative Nb and Ta anomalies, the properties that establish the andesites as continental arc volcanic rocks. It is concluded that hotspots had developed in the old Dong Tso basin, the oceanic basin that had been developing from middle Jurassic (166 Ma) or even before and emplaced northward in late Jurassic (about 148 Ma).

Early Jurassic high-pressure metamorphism of the Amdo terrane, Tibet: Constraints from zircon U–Pb geochronology of mafic granulites

The Amdo terrane holds one of the important keys to understand the tectonic evolution of Central Tibet. Here we report the results from a combined study of Cathodoluminescence imaging, LA–ICPMS U–Pb dating, and trace element analysis of zircons from high-pressure (HP) mafic granulites from the Amdo terrane. Zircons from the mafic granulite are rounded or anhedral and show sector or planar domains with low Th/U ratios, low REE contents, and flat HREE patterns, and can be divided into two types. The first type of zircons shows negative Eu anomalies, indicating that they coexisted with garnet and symplectitic plagioclase during the retrograde stage of metamorphism, whereas the second type possesses positive Eu anomalies, implying that they formed during the peak-metamorphic stage of the HP granulite. U–Pb data yielded a weighted mean 206Pb/238U age of 190.7±3.0Ma (MSWD=2.8, n=8) for zircons with positive Eu anomalies, interpreted as the time of the peak metamorphism, and a mean age of 181.4±1.8Ma (MSWD=2.2, n=16) for zircons with negative Eu anomalies corresponding to the amphibolite-facies retrogression. Combining with previous data, we suggest that the Amdo terrane became a microcontinent in the Tethys Ocean during the Permian–Triassic rifting between the Lhasa and Qiangtang terranes, and then the Amdo terrane subducted to about 50-km-depth beneath the Qiangtang terrane experiencing HP granulite-facies metamorphism in the Early Jurassic at 190Ma. The region was then uplifted to the mid-crustal levels (~20km) with an exhumation rate of ca. 3mm/year. The presence of Early Jurassic HP granulite shows that the Amdo terrane is related to the Bangong–Nujiang Suture zone instead of with the Mid-Qiangtang Suture zone.

New constraints on crustal structure and Moho topography in Central Tibet revealed by SinoProbe deep seismic reflection profiling

Abstract From October 2009 to May 2010, through the support of the SinoProbe project a series of deep seismic reflection profiles were conducted. In order to image deep crustal structure of central Tibet, these profiles extend from the northern Lhasa terrane to the Qiangtang terrane by crossing the Bangong–Nujiang suture (BNS). The Moho depth varies from 75.1 km beneath the northmost Lhasa terrane to 68.9 km beneath southmost Qiangtang terrane. There is a 6.2 km sharp Moho offset across the BNS. Within 25 km to the north of the BNS, the Moho rises smoothly to 62.6 km. Distinct Moho reflector lies at 62.6–67.3 km beneath the Qiangtang terrane. The boundary of the middle and upper crusts and that between the middle and lower crusts interpreted from INDEPTH velocity model correspond to the strong reflection horizons at about18.8 km and 31.3 km depth beneath south Qiangtang terrane, respectively. A north-dipping series of reflection packages in the mid- to lower crust may mark subduction of the Lhasa terrane beneath the Qiantang terrane. The central portion of the reflection profile exhibits an antiformal structure at 14.1 km in the upper crust, which corresponds with the blueschist-bearing metamorphic belt.

Subduction of the Bangong-Nujiang Ocean: constraints from granites in the Bangong Co area, Tibet

We report geochronologic, whole-rock geochemical and Sr–Nd isotopic analyses of the granites that are exposed to both the north and the inside of the Bangong–Nujiang Suture (BNS) zone as well as the implications for the Mesozoic history of Tibet. To the north of the BNS, the Larelaxin pluton consists of I-type quartz diorite and highly fractionated I-type biotite granite. The Larelaxin pluton is enriched in large-ion lithophile elements (LILE) but depleted in high-field-strength elements (HFSE); therefore, it exhibits the features of volcanic arc rocks. The initial Sr (0.7102 to 0.7215) and negative εNd (t) (−2.91 to −5.20) values imply a mixture of depleted mantle and continental crust. The mean 206Pb/238U zircon age is 168 Ma; we therefore propose that the Bangong–Nujiang Ocean (BNO) had already been subducted beneath the Qiangtang terrain by the middle Jurassic. Inside the BNS, the Rutog granites intruded into the Lagongtang and Duoren formations, which show a continental margin and a forearc basin sedimentary facies, respectively. The mean 206Pb/238U zircon age is 101 Ma. The Rutog granites are monzogranites with a high Na/K ratio (Na2O/K2O > 1) and a high LILE/HFSE ratio, and A/CNK < 1.1. The high Sr/Y ratio (22 to 56) implies that these granites are adakitic. The low initial Sr (0.7044 to 0.7055) and positive εNd (t) (+1.46 to +2.70) values indicate that the protolith of the Rutog granites originated mainly from a depleted source. We attribute the Rutog plutonism to the development of an oceanic arc during the continuing northward subduction of the BNO and propose that the Rutog adakitic granites were formed by melting of the subducted BNO crust with limited crustal contamination. Copyright © 2013 John Wiley & Sons, Ltd.

A model of the crust and mantle structure down to 700 km depth beneath the Lhasa to Golmud transect across the Tibetan plateau as derived from seismological data

A 700 km deep seismic velocity cross-section beneath the Lhasa to Golmud transect across the Tibetan plateau is presented. In contrast to the first version of this cross-section, which comprised an 800 km wide swath centred on the Lhasa to Golmud transect, due to the recent proliferation of publications concerning the mantle structure beneath Tibet, this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect. The results from the recent INDEPTH IV project indicate that the crustal thickness change from 70 km beneath the Songpan–Ganzi terrane and Kunlun mountains to 54 km beneath the Qaidam basin is located about 100 km north of the Kunlun Fault and almost 45 km north of the North Kunlun Thrust. The Qaidam basin Moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent Songpan–Ganzi lower crust underthrusting or flowing northward beneath the Qaidam basin Moho. Thus the high Tibetan plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath the stronger Qaidam crust. Beneath the crust, high-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong–Nujiang suture. Northwards, Asian lithospheric mantle is overlain by a low-velocity, less dense, warm Tibetan plate consisting of an upper lithospheric and a lower asthenospheric part. The apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north Tibet is slower, less dense and warmer than under south Tibet, in agreement with the observed uppermost mantle velocities. This, in turn, could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau.

Seismic Attenuation Characteristics along the Hi-CLIMB Profile in Tibet from Lg Q Inversion

We investigate the Q of crustal Lg waves along the Himalayan-Tibetan Continental Lithosphere during Mountain Building (Hi-CLIMB) profile in Tibet using a reliable two-station method. Four earthquakes of M ≥5:5 recorded along a 800-km- long densely-spaced seismic profile are used to extract Lg-wave spectra. One hundred seven high quality interstation paths that allow us to use the standard two-station method for Lg Q measurement are selected from 2369 possible pairs. By using those 107 interstation Q0 (1 Hz Lg Q) values as input, lateral variations in attenuation char- acteristics are derived through inversion. The region has Q0 estimates ranging from 88 5 to 165 15, with the lowest value in the Qiangtang terrane and the highest across some part of the lower Himalaya. The lowest value is attributed to the high Poisson's ratio, higher temperatures, as well as partial melts in the crust. The results do not contain any detectable variation in Q0 either across the Himalaya-Tibet collision zone (i.e., the Indus-Tsangpo suture zone) or along the major Bangong- Nujiang suture zone (BNSZ). Consistent low Q0 values along the Hi-CLIMB profile manifest the presence of omnipresent partial melting of the crust throughout the plateau.