Variation of Moho Depth across Bangong-Nujiang Suture in Central Tibet—Results from Deep Seismic Reflection Data

There is a long-term dispute at Moho depth across the Bangong-Nujiang suture (BNS). Due to the complicated and changeable seismic geological condition, it is not easy to acquire images of the reflective Moho in central Tibet. In the support of the SinoProbe project, a series of deep seismic reflection profiles were conducted to image Moho structure across the BNS and the Qiangtang terrane. These profiles extend from the northern Lhasa terrane to the Qiangtang terrane crossing the BNS. Both shot gathers and migration data show clear Moho images beneath the BNS. The Moho depth varies from 75.1 km (~24 s TWT) beneath the northmost Lhasa terrane to 68.9 km (~22 s TWT) beneath southmost Qiangtang terrane, and rises smoothly to 62.6 km (~20 s TWT ) at ~28 km north of the BNS beneath the Qiangtang terrane. We speculate that the Moho appears a 6.2 km sharp offset across the BNS and becomes ~12.5 km shallower from the northmost Lhasa terrane to the south Qiangtang terrane at ~28 km north of the BNS. The viewpoint of Moho depth across the BNS based on deep seismic reflection data is inconsistent with the previous 20 km offset.

The Beila Ophiolite from the Bangong-Nujiang Suture Zone, Northern Tibetan Plateau

The Beila ophiolite is located in the middle part of the Bangong-Nujiang suture zone,northern Tibetan plateau.It is a complete ophiolite suite,and plays a key role in understanding the evolution of the Bangong-Nujiang suture zone,as well as the Meso-Tethys Ocean.The Beila ophiolite was composed of peridotite,serpentinite,gabbro,pillow basalt,and minor rodingite.Peridotites comprisemainlymedium–tocoarse–grained serpentinized harzburgites and minor plagioclase-bearing lherzolites and dunites.There are some felsic-ultramafic dykes within the peridotite and they are mainlypegmatoidal pyroxenites,coarse to fine-grained gabbros,and diabases.Gabbros included isotropic and cumulate gabbros,and they commonly contain minor pegmatoidal gabbros veins.Pillow basalts and basaltic andesites overlaid on the margin of the serpentinized peridotites.Rodingite occurs as lenses and/or dykes within the host serpentinized peridotites.Zircon SHRIMP U–Pb dating for two rodingite samples yielded the ages ranging from172 to 164 Ma.Whole-rock geochemical and zircon Hf isotopic data show that the Beila ophiolite shows SSZ-type ophiolite affinity.Finally,we suggest that the Beila ophiolite was generated in an initial subduction process at the middle Jurassic(164–172 Ma).

The Characteristics of Yongzhu–Guomang Lake Ophiolitic Melange in Bangong-Nujiang Suture, Xizang(Tibet),China

Yongzhu–Guomang Lake ophiolitic melange exposed about 100 km with large scale and complete ophiolitic uint in Xainza County,Xizang(Tibet).It is connected with Nam Lake,Kaimeng ophiolitic mélange to the east,and

The Cretaceous of the Eastern Bangong-Nujiang Suture Zone (Tibet):Tectono-Sedimentation

The ophiolite-bearing Bangong-Nujiang zone (BNZ) traversing central Tibet from east to west separates the Qiangtang block in the north from the Lhasa block in the south. The Cretaceous of the area includes Chuanba Formation (K 1c ), Duoba Formation (K 1d ), Langshan Formation (K 1l ) and Jiangba Formation (K 2j ). The K 1c is composed of black shale, sandy pelite, siltstone, sandstone, coal beds and volcanic rocks, of shallow marine facies. The K 1 d consists of terrestrial siliciclastics intercalated with some calcareous sandstone beds bearing Orbitolina sp. indicating marine influence. The K 1j is carbonate platform deposits of shallow marine and lagoon. The K 2j is characterized by terrestrial thick massive red conglomerate. An active margin related to B-subduction zone is considered to be the geological setting of the Cretaceous sedimentation.

Peridotites and Chromitites from the Dingqing Ophiolite in the Eastern Segment of Bangong–Nujiang Suture Zone, Tibet: Occurrence Characteristics and Classifications

The Dingqing ophiolite is located in the eastern segment of the Bangong-Nujiang suture zone. This suture zone is W–E trending parallel with the Yarlung–Zangbo suture zone and is an strategic area for exploring chromite deposits in China. The Dingqign ophiolite is distributed in near SE-NW direction. According to the spatial distribution, the Dingqing ophiolite is sudivided into two massifs, including the East and the West massifs. The Dingqing ophiolite covers an area of nearly 600 km2. This ophiolite is composed of peridotite, pyroxenite, gabbro, diabase, basalt, plagiogranite and chert(Fig. 1). The peridotite is the main lithology of the Dingqing ophiolite. The peridotite covers about 90% of the total area of the Dingqing ophiolite. The Dingqing ophiolite is dominated by harzburgite with a small amounts of dunite. The Dingqing harzburgite displays different textures, such as massive, Taxitic, oriented and spherulitic textures(Fig. 2d–i). These four types of harzburgite occur in both the East and West massifs, especially in the Laraka area of the eastern part of the East massif. Dunites have different occurrences in the field outcrops, such as lenticular or stripshaped, thin-shell and agglomerate varieties(Fig. 2a–c). On the basis of detailed field work, we have discovered 83 chromitite bodies, including 27 in the East massif and 56 in the West massif. According to the occurrence scale and quantity of the chromitite bodies, we have identified four prospecting areas, namely Laraka, Latanguo, Langda and Nazona. Chromitites in the Dingqing ophiolite show different textures, including massive, disseminated, veined and disseminated-banded textures(Fig. 3). On the basis of the Cr#(=Cr/(Cr+Al)×100) of chromite, we have classified the Dingqing chromitite into high-Cr, medium high chromium type, medium chromium type and low chromium type chromitite(Figs. 4, 5). Among them, low chromium type chromitite Cr# is extremely low, ranging from 9.23 to 14.01, with an average of 11.89;TiO2 content is 0.00% to 0.04%, and the average value is 0.01%, which may be a new output type of chromitite. These different types of chromitites have different associations/assemblages of mineral inclusions. The inclusions in high chromium type chromitite are mainly clinopyroxene and a small amount of olivine;medium high chromium chromitite are mainly amphibole, a small amount of clinopyroxene and phlogopite;while low-chromium chromite rarely develops mineral inclusions, and micron-sized clinopyroxene inclusions are common in olivines which are gangue mineral in it. These different types of chromite ore bodies have a certain correspondence with the field output, and may also restrict their genesis. This part will be further developed in the follow-up work.

Petrology, geochemistry and geochronology of the Zhongcang ophiolite, northern Tibet: implications for the evolution of the Bangong-Nujiang Ocean

Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ^(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).

青藏高原西部班公-怒江缝合带下方地壳结构与地块拼合模式

利用远震接收函数偏移成像方法获得青藏高原西部Hi-Climb项目剖面北段地壳结构转换波成像。结果显示班公-怒江缝合带下方拉萨地体上地壳向N仰冲,下地壳向N俯冲,而羌塘地块上地壳向S仰冲,下地壳向S俯冲,可能意味着青藏高原西部拉萨地块和羌塘地块具有复杂的拼合过程。结合前人的岩石学研究成果,建立了新特提斯北洋盆洋壳S向俯冲、距今60～50Ma印度板块与欧亚板块碰撞后,拉萨地块的下地壳向羌塘地块下俯冲,而后印度板块俯冲到羌塘地块下方的地块拼合模式。

Origin of Qushenla Formation Volcanic Rocks in the Nawucuo Area,Northern Tibet,and Constraints on the Subduction Polarity of the Bangong-Nujiang Tethys Ocean

The Qushenla Formation volcanic rocks are widely exposed in the northern margin of the Bangong-Nujiang suture zone(BNSZ).Research on these rocks is of great significance for understanding the tectonic evolution of the Bangong-Nujiang Tethys Ocean(BNTO).In this study,a systematic geological survey was conducted on the Qushenla Formation volcanic rocks that are widely exposed in the Nawucuo area,in the northern margin of the western segment along the BNSZ.The whole-rock geochemistry,zircon U-Pb dating,and in situ zircon Lu-Hf isotopes were carried out in this study,aiming to constrain the formation age,rock genesis,magma source and tectonic setting of the volcanic rocks.The zircon U-Pb dating shows that the Qushenla Formation volcanic rocks in the western BNSZ erupted during the period of 120–108 Ma,i.e.,Early Cretaceous.The Qushenla Formation volcanic rocks are a suite of intermediate-basic volcanic and pyroclastic rocks belonging to the medium-K calc-alkaline series.They are relatively enriched in light rare earth elements(LREEs)and incompatible elements such as Rb,K,La,Th,Sm,and Hf,whereas depleted in heavy REEs(HREEs)and high field strength elements(HFSEs)such as Nb,P,Zr,and Ti.The in situ zirconεHf(t)values of the volcanic rocks range from 8.95 to 12.57,with an average of 10.40.The Mg#,Th/La and Th/Ce values are between those of the mantle-derived magma and the continental crust.The formation of the Qushenla Formation volcanic rocks can be explained through the following process:(1)As the nospheric materials that upwelled during the rollback of the subducting Tethys Ocean slab induced the large-scale partial melting of the mantle wedge and the formation of the initial basaltic magma.(2)These mantle-derived magmas ascended and induced the partial melting of the lower crust to generate peraluminous melts.(3)The mixing of the peraluminous melts and mantle-derived melts generated the initial magma with homogeneous Sr-Nd isotope compositions.(4)Last,the eruption of the magma produced the widespread Qushenla Formation volcanic rocks at the surface.When combining this information with the regional geological background,it is believed that the Qushenla Formation volcanic rocks,the Meiriqicuo Formation volcanic rocks and the Late Jurassic–Early Cretaceous intrusive rocks together constitute the tectonic magmatic arc of the active continental margin on the southern margin of Qiangtang,which was formed in the tectonic setting of the northward subduction of the Bangong-Nujiang oceanic crust beneath the Qiangtang Block.

Petrological, geochemical and chronological constraints for the tectonic setting of the Dongco ophiolite in Tibet

The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O+ K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from +2.7―+5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ^(39)Ar/^(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.

Deep dynamical processes in the central-southern Qinghai-Tibet Plateau—Receiver functions and travel-time residuals analysis of north Hi-Climb

Teleseismic receiver functions and travel-time residuals along the north Hi-Climb broadband seismic array in the central-southern Qinghai-Tibet Plateau show that the lithosphere structures in the central and western Qinghai-Tibet Plateau are different. In the central Qinghai-Tibet Plateau, the Indian Plate is northward subducted beneath the Qiangtang block and arrives at the greatest depth beneath the central-southern Qiangtang block. The delaminated Indian lithospheric slab remains beneath the central Lhasa block to a depth possibly greater than that of the upper interface of the mantle transform zone. In the western Qinghai-Tibet Plateau, the Indian lithospheric plate is gently northward subducted and may have arrived to the south of Tarim plate. Due to the resistance from the gently northward subduction of the Indian mantle lithosphere in the western Qinghai-Tibet Plateau, the upwelling mantle material be-neath the Qiangtang block moves mostly toward the east to bring about the lateral eastward flow of the deep mantle hot material in the central Qinghai-Tibet Plateau.