Qiangtang Block Research Articles

Uplift and exhumation of the Chakabeishan ore district in the northern margin of the Qaidam Basin, China: Constraints from fission track thermochronology

The Chakabeishan area is located in the northern margin of the Qaidam Basin, Northwest China, and is an important lithium mining area. However, there is a relatively lower research degree and lack of data constraints of tectonic uplift and exhumation. Using fission track technology, new apatite and zircon fission track results have been reported for the first time in order to reveal the tectonic processes and uplift histories of this area. A total of 11 sample results were obtained, these ages could be divided into four groups, namely, 159–144 Ma, 114–100 Ma, 72 Ma and 52–32 Ma, revealing four periods of tectonic activities. Thermal history modelling provided three episodes of uplift at 120–60 Ma, 60–18 Ma and 18–0 Ma. In the first stage, the Southern Qilian Mountains was reactivated and subducted southward due to collision of the Lhasa and the Qiangtang blocks, resulting in rapid uplift in this area. The second stage is tectonic quiescence period corresponding to a relatively steady uplift. The third stage showing rapid uplift caused by the collision between the Indian and Eurasian Plates. The average uplift rate and total uplift amount of the three stages calculated according to the geological thermal histories are 0.039 km/Myr and 3.27 km, respectively. Using the age–closure temperature method, the exhumation rate has been calculated as 0.034–0.148 km/Myr from zircon closure–temperature of 250 °C to apatite closure–temperature of 100 °C and 0.049–0.076 km/Myr form the apatite closure–temperature of 100 °C to 15 °C of surface temperature. The exhumation rate is fastest from the Late Cretaceous to Eocene and has then changed a little since the Eocene, with the average exhumation rate of about 0.059 km/Myr. Synthesizing the zircon ages and geological characteristics, this area is characterized by multi–stages mineralization, and the metallogenic age is mainly in the Late Jurassic–Cretaceous.

Metallogeny in the Bangong–Nujiang belt, central Tibet, China: A review

The Bangong–Nujiang metallogenic belt consists of scattered Tethyan oceanic blocks, mainly distributed underneath the margins of the Qiangtang and Lhasa terranes in central Tibet. A new world-class metallogenic belt has been reported in this region recently, based on the geological mapping and ore deposit prospecting over the last two decades. It currently comprises inferred resources of 30 Mt Cu and 500 t Au, together with several Cr–Ni, Fe, and W (Mo) resources, forming a significant potential area for future mineral exploration. These metals are mainly hosted in porphyry copper, skarn copper, skarn iron, orogenic gold, quartz-vein tungsten, and ophitic chromite deposits. The mineral deposits in the Bangong–Nujiang metallogenic belt have been widely recognized in different localities, including the southern edge of the southern Qiangtang block, part of the north Lhasa block, and even part of the central Lhasa block, indicating they were formed in variable geological settings, from the initial opening, subduction, and collision to the extension of the Bangong–Nujiang Ocean. Specifically, five major tectonic events contributed to mineralization, including the stage 1 (240–165 Ma) initial opening of the Bangong–Nujiang Ocean, stage 2 (165–145 Ma) oceanic subduction, stage 3 (145–100 Ma) close of the ocean, stage 4 (100–65 Ma) continent–continent collisional orogenesis, and stage 5 (65–0 Ma) post-orogenesis. At stage 1, Cr–Ni deposits were formed during the initial opening of the ocean; porphyry–epithermal Cu (Au), skarn Fe, and minor orogenic Au deposits were formed at stage 2 and stage 3; a younger pulse of a few porphyry–skarn Cu ± Mo and orogenic Au deposits were formed during stage 4; finally, W(Mo) deposits were generated in stage 5. In general, porphyry Cu systems, orogenic Au, and skarn Cu polymetallic deposits that occurred in the subduction and post-collision settings related W(Mo) deposits have the most potential for future exploration. An in-depth investigation of several scientific problems, such as addressing the tectonic setting, magmatism, and metallogeny of this region and genetic linkage of these deposit preservations to plateau uplift, is essential for the future success of exploration in the Bangong–Nujiang metallogenic belt.

Tectono-magmatic response to the geometric evolution of slab breakoff in the Paleo-Tethys Ocean: Constraints from Late Triassic granites in the Qiangtang block, northern Tibet

Slab breakoff can explain the postcollisional igneous and metamorphic events in many collisional orogens and triggers the transition from initial continental collision to deep subduction. However, the relationship between the deep-seated geometry of slab breakoff and shallow tectono-magmatic events remains unclear. In this study, we report new geochronological, geochemical, and Hf isotope data for granitic rocks from the Qiangtang block. Three granitic samples yielded zircon U-Pb ages of 226−214 Ma and mostly gave negative zircon εHf(t) values of −20.1 to +0.57. These granitoids are K-rich, calc-alkaline, magnesian, and metaluminous to weakly peraluminous. We propose that the granites were generated by partial melting of compositionally heterogeneous lower crust in the Southern Qiangtang block. Our new data, along with previously published data for igneous and metamorphic rocks in the Qiangtang block, suggest that the Longmu Co−Shuanghu suture zone represents a remnant of the closure of the Paleo-Tethys Ocean, and that the Late Triassic tectono-magmatic evolution was controlled by slab breakoff after continental collision. The contrasting magmatic and metamorphic ages show that the two stages of tectono-magmatic activity in the Late Triassic were due to a change in the subduction angle of the continental lithosphere from steep (225−215 Ma) to shallow (215−205 Ma) during slab breakoff. Furthermore, we infer that slab breakoff of the oceanic lithosphere caused extensive crustal melting and reworking and initiated far-field subduction. Our results provide new insights into the slab breakoff processes that control the geometry of subducting continental lithosphere and related postcollisional magmatic and metamorphic events in collision zones.

Crustal structure beneath the northern part of the southeastern Tibetan Plateau revealed by a seismic dense nodal array

The continental convergence between the Indian and Eurasian plates in the Cenozoic not only caused the formation of the Tibetan Plateau (TP) but also pushed a large volume of continental lithosphere out of the plateau southeastwards. It has been strongly debated for many years how to transport mass from the central plateau to the southeastern TP. In the area between the central plateau and its southeastern margin, we deployed a southwest-northeast-trending dense seismic nodal array to record teleseismic waveforms. Receiver function methods were used to estimate the crustal bulk properties and to image the crustal structure. Our results show that the crustal thickness is approximately 60 km and the crustal average Vp/Vs ratio is approximately 1.78, which is less than the predicted values of the crustal channel flow model. In the crustal structure image, the Moho beneath the southwestern part of the profile dips northeast and extends below the Moho beneath the northeastern part of the profile. Beneath the southwestern part of the profile, a lower crustal northeast-dipping interface is also imaged, which is likely linked with the Jinsha River suture. Our results suggest that the most significant crustal thickening in the southeastern TP was produced by the northwards or northeastwards subduction of the Qiangtang block, which occurred in the late Oligocene to early Miocene, as indicated by palaeo-altimetric studies. This finding implies that the modern landform in the southeastern TP resulted from southeastwards block extrusion along strike-slip faults and more monsoonal precipitation and seasonal climate variation since 10 Ma.

Middle Devonian (Givetian) coral-stromatoporoid patch reefs from the Lazhuglung Formation, Xizang (Tibet) and their palaeoecological and palaeogeographical implications

We carried out, for the first time, detailed studies on the coral-stromatoporoid patch reefs from the Lazhuglung Formation near Shouxinghu in the Lazhuglung-Badangco area, Ngari Prefecture of Xizang (Tibet) Autonomous Region, China. On the basis of 1085 thin sections prepared from three patch reefs, the fossil components and reef structures are depicted. The patch reefs are found to encompass diverse fossil organisms, including tabulate and rugose corals, stromatoporoids, brachiopods, gastropods, ostracods, nautiloids, calcified cyanobacteria, and tubeworms. Tabulate corals and stromatoporoids are the predominant reef-builder, particularly Alveolites obtortus and Gerronostromaria gromotuchense. The patch reefs are Middle Devonian Givetian in age based on the tabulate coral and stromatoporoid assemblage. Extensive biotic interactions between the reef organisms are found in all three patch reefs, which are beneficial for the reef formation and development. The ecological features of the corals and stromatoporoids indicate a mesophotic ecosystem that is widespread in the Givetian. In addition, the common species and similar characteristics between the faunas in the Lazhuglung Formation, North Qiangtang Block and those in South China suggest a close palaeobiogeographic relationship. This study enhances our understanding on fossil components, reef structure and tectonic affiliation of the Palaeozoic strata in the Lazhuglung-Badangcuo area.

Paleomagnetic constraint on the formation of the Eastern Himalayan Syntaxis: A new late Eocene result from the Mangkang area of the eastern Tibetan Plateau

The Eastern Himalayan Syntaxis (EHS) is a major structural and geomorphological unit in Eastern Tibet; however, its mechanism of formation and the extent of its northward effect of compression remain unclear. To address these questions, we conducted a paleomagnetic and geochronologic study of the late Eocene trachyte intrusive mass in the Mangkang area of Tibet. The U-Pb SHRIMP II age estimate of the trachyte intrusive mass is ∼40 Ma, and the estimated mean paleomagnetic direction we obtained is D=27.0°, I=36.8°, k=42.9, α95=1.3°, and n=282 specimens. These results indicate that the Mangkang area was located at a paleolatitude of 20.5 ± 0.9° N during the late Eocene. A comparison of paleolatitudes of the Qiangtang block with those inferred from the apparent polar wander paths (APWP) of India and Eurasia suggests that the Eastern Himalayan Syntaxis was formed after 35 Ma when Eurasia was penetrated by ∼600 km by northward-moving India. This is consistent with geologic evidence of intra-continental shortening in the foreland of the Eastern Himalayan syntaxis. The northward counterclockwise penetration of the corner of India also resulted in the large right-lateral strike-slip system of the Sagaing fault (SGF), with ∼400-600 km of displacement between the EHS and the Shan-Thai block. Thus, the northward wedging of the EHS indicates minimal southward displacement of the Shan-Thai block. Our results link the tectonic activity of the EHS to the lateral extrusion of the southeastern margin of the Tibetan Plateau, providing a new perspective on the evolution of this region since the late Eocene.

Detrital zircon geochronology of Jurassic strata in the Jianggalesayi–Washixia Basin, northwestern Tibet Plateau: Implications for the uplift history of the Altyn Tagh Orogen

Jurassic terrestrial sediments of the Jianggalesayi–Washixia Basin in the northwestern Altyn Tagh Orogen record abundant information regarding the evolution and uplift of the orogen. Investigation of this orogen is critical to establishing whether the Tarim and Qaidam basins were connected during the Jurassic. In this study, we present 474 single‐grain laser‐ablation–inductively coupled plasma–mass spectrometry detrital zircon U–Pb ages obtained from five Jurassic sandstone samples collected from four sections in the basin. Detrital zircons from the Lower Jurassic strata yield age populations of 997–726 and 551–509 Ma, indicating that the zircons were derived mainly from the southern–central Central Altyn Tagh Massif. Zircon age populations from the lower Middle Jurassic strata are 1024–838, 778–643, and 543–357 Ma, which suggest that the sediments were derived predominantly from the northern Central Altyn Tagh Massif and the southern South Altyn Tagh subduction–collision complex belt (SATSB). Zircon age populations from the upper Middle Jurassic strata are 980–830, 556–424, and 277–245, suggesting that these sediments were derived from the central SATSB. Detrital zircons from the Upper Jurassic strata display age groups of 515–387 and 325–226 Ma, indicating that these sediments originated from the northern SATSB. Uplift of the Altyn Tagh Orogen was initiated during the Middle to Late Triassic as a synchronous far‐field response to the collision of the Qiangtang Block with the Eurasian Plate. By the Early Jurassic, the Qaidam Basin was no longer connected with the Tarim Basin. During the Late Jurassic to Early Cretaceous, the collision of the Lhasa Block with the southern margin of the Eurasia Plate led to further uplift of the Altyn Tagh Orogen.

Moho depth inversion in the Tibetan Plateau from high-precision gravity data

The Tibetan Plateau (TP) is the youngest orogenic belt formed by continental collision on the Earth. It is a natural laboratory for studying continental dynamics such as continental convergence, plate subduction, and plateau uplift. Studying its deep structure has always been a hot issue in geological research. The Moho is the interface between the crust and the mantle and is an important interface in the structure of the Earth's circle. Its depth, lateral variation, and fine structure of the crust-mantle interface are important parameters to reveal the process of lithospheric dynamics in the TP. The study of the Moho surface generally has two methods: seismic and gravity. Seismic detection has the characteristics of high precision, but they are limited along a few cross-sectional lines and have a high cost. It is not suitable and cannot be carried out in a large area of the TP; Gravity inversion can obtain large-area Moho depth, but affected by gravity data and inversion methods, the accuracy is lower than that of seismic. In this paper, a high-precision gravity field model is selected. With the constraint of seismic observation, the Parker-Oldenburg interface inversion method is used, and the Bott iteration method is introduced to improve the inversion efficiency. The high-precision Moho depth in the TP is obtained, consistent with the seismic detection results. The research results show that the shape of the Moho in the TP is complex, and the variation range is large, reaching 60-80 km. In sharp contrast with the adjacent area, there is a clear sharpness zone at the plateau's edge. In the TP's interior, the Moho's buried depth is characterized by "two depressions and two uplifts". The Moho in the south of the Yarlung Zangbo River (YZR) inclines to the north, and the Moho in the north depresses downward, interpreted as the Indian plate subducts to the north below Tibet. The Moho depression on the north side of the Qiangtang block (QTB), up to 72 km deep, may be related to the southward subduction of the lithosphere. The Moho uplift of the QTB has the same strike as the Bangong-Nujiang suture zone (BNS), which may reflect that the area is compensated by a low-density and low-velocity mantle.

Long-Term Spatial–Temporal Evolution of Seismicity of the 2010 Ms 7.1 Yushu, Qinghai, China Earthquake

On 14 April 2010, a destructive earthquake (Ms 7.1) occurred along the Ganzi–Yushu fault (GYF), which is the boundary between the Songpan–Ganzi and Qiangtang Blocks of the eastern Tibetan Plateau. In this study, we present an effective workflow for microearthquake detection and location that assembles three main methods including a graphics processing unit-based match and locate technology (GPU M&L), a deep-neural-network-based noise-reduction technique (DeepDenoiser), and a U-net-based phase picking method (PhaseNet). Following this workflow, a high-resolution catalog has been constructed based on continuous seismic data from 2008 to 2018 recorded by 13 seismic stations deployed by Qinghai seismic network in Yushu, Qinghai. The GPU M&L is first applied to detect earthquakes which are usually hard to pick by routine methods due to their low signal-to-noise ratios. Then the DeepDenoiser is applied to remove noises from events we detected by GPU M&L, providing a better picking of seismic phases. The PhaseNet and hypoDD are conducted to pick seismic phases and relocate detected events, respectively. The spatial–temporal evolution of the foreshock sequence of the Yushu earthquake suggests that a triggered cascade model is responsible for the nucleation of the mainshock. The mainshock and aftershocks southeast of the mainshock mainly rupture on the primary northwest (NW)-trending GYF while the foreshocks and aftershocks on the NW segment rupture along the northeast (NE)-trending fault normal to the strike of the GYF. The average cumulative slip rate of the GYF estimated from six repeating earthquakes is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.4~\pm ~1.7$ </tex-math></inline-formula> mm/year, which is consistent with the geodetic and geological observations.

3D velocity and anisotropy of the southeastern Tibetan plateau extracted by joint inversion of wave gradiometry, ambient noise, and receiver function

With the deformation pattern of the southeastern Tibetan Plateau remaining controversial, its anisotropic structure and the three-dimensional (3D) velocity model are powerful tools for understanding this issue. The surface wave dispersion curve (DC) and receiver function (RF) are sensitive to absolute velocity and elasticity contrasts, respectively. In this work, we propose a new method for the joint inversion of 3D shear velocity (Vs) and anisotropy using the datasets of RFs and DCs, which are obtained by combining the wave gradiometry method and ambient noise tomography. Because DCs and RFs are divided into back-azimuth bins, and both datasets in each bin are jointly inverted to obtain velocity in the corresponding direction, the method is called the azimuth-dependent joint inversion method. By applying this new method to the Temporary West Sichuan Array, we obtained a high-resolution 3D Vs model and anisotropic structure of the southeastern Tibetan Plateau. In our model, the vertically consistent fast propagation direction in the Songpan-Ganzi block (SGB) and the north Chuandian block (NCDB) suggest that the deformation in this region is largely coupled vertically. Our model provides anisotropic evidence of the Yangtze crystalline basement extending to the SGB. The low-velocity zone (LVZ) was much thicker and more prominent in the NCDB than in the SGD. Anisotropy in the middle lower crust of the NCDB does not show any sign of an LVZ flowing from the SGB or the Qiangtang block to the NCDB. These observations are contrary to those of the crust flow model, suggesting that the LVZ was generated locally. The mushroom-shaped high-velocity structure beneath the southern Chuandian block and boundary fault zone may be a relic of the Emeishan paleo-mantle plume.

The Late Triassic Longmenshan lateral foreland thrusting: New insights from geological evidence and 3-D particle discrete-element simulation

This work proposes a new lateral foreland thrusting model based on geological evidence and 3-D particle discrete-element simulation to explain the Longmenshan southeastward thrusting during the closure of the Songpan-Ganze basin. The Late Triassic NE−SW compression caused by the northward movement of the Qiangtang Block and the resulting differential shortening within the wedge-shaped Songpan-Ganze terrane produced southeastward topographic gradient. The thick sedimentary pile, driven by the horizontal tectonic force and the deviatoric stress generated from gravitational effect, and decoupled from the subducting basement by the low-strength décollement, was laterally extruded and resulted in the southeastward Longmenshan thrusting. Therefore, the Longmenshan thrust belt is a lateral foreland thrust belt of the Songpan-Ganze terrane. For the first time, 3-D particle discrete-element simulation was used for the geological study of the Longmenshan area, and it clearly reproduces the dynamical process of the Longmenshan southeastward thrusting and well predicts the Xiaojin Arcuate Zone. The particle discrete-element simulation results verify the new model and reveal that two key factors facilitate the lateral foreland thrusting: the wedge-shaped geometry that produces differential shortening and lateral topographic gradient, and the low-strength décollement that decouples the extruded sedimentary pile from the basement. The lateral foreland thrust belt, which is unique in its tectonic location and dynamic behavior, is a new kind of foreland thrust belt that is different from the pro-foreland and retro-foreland thrust belts, and it provides new insight into the tectonic evolution of collisional orogens.

Lithospheric extension of the accretionary wedge: An example from the Lanling high-pressure metamorphic terrane in Central Qiangtang, Tibet

Deciphering the exhumation mechanism of high-pressure−low-temperature (HP-LT) metamorphic rocks can provide important insights into the tectonic evolution of oceanic subduction zones at active continental margins. Here, we present a multidisciplinary study examining the exhumation tectonics of the Permo−Triassic eclogite-bearing HP-LT terranes of the Central Qiangtang metamorphic belt (CQMB) in the central Tibetan Plateau. Field geological relations and microscopic observations show that the HP-LT rocks in the Lanling area are separated from the epimetamorphic late Paleozoic ophiolite mélange of the hanging wall by low-angle detachment faults and primarily exhibit five stages of deformation. The HP-LT terranes were exhumed as a metamorphic core complex and show pervasive synexhumation top-to-the-SW and -S shear structures. The results of the petrological and mineralogical analysis and pseudosection modeling of retrograde eclogites indicate that these rocks are characterized by synexhumation mineral growth pulses with hairpin-type decompressional pressure-temperature trajectories. A compilation of previous geochronological data and our 40Ar/39Ar dating results of synexhumation minerals and shear bands in HP-LT rocks indicate continuous exhumation at ca. 244−215 Ma prior to the continental collision between the North Qiangtang Block (NQB) and South Qiangtang Block. Moreover, the CQMB is likely an autochthonous accretionary wedge resulting from northward subduction of the Paleo-Tethys Ocean beneath the NQB. Combined with ca. 243−230 Ma mantle upwelling to the north of Lanling, we infer that the CQMB, namely, the accretionary wedge of the Paleo-Tethys Ocean, experienced Middle−Late Triassic lithospheric extension in the NW-SE direction.

Permian foraminifers from the exotic limestone blocks within the central Qiangtang Metamorphic Belt, Tibet and their geological implications

• The age of the exotic limestone blocks within the Central Qiangtang Metamorphic Belt is late Kungurian to Roadian. • The exotic limestone blocks came from the Shuanghu type of Permian sequences in the South Qiangtang Block. • A strong subduction and accretion process happened during the closure of the Longmu Co-Shuanghu Paleotethys Ocean. The Central Qiangtang Metamorphic Belt (CQMB) is a subject of long-standing controversy for decades with respect to its origin and tectonic evolution. In this study, two foraminiferal faunas are reported from the exotic limestone blocks in the Jiaomuchaka and Jiangaizangbu sections within the CQMB. In total, 9 species of fusulines and 32 species of smaller foraminifers have been discovered in the Jiaomuchaka section, and 10 species of fusulines and 15 species of smaller foraminifers have been documented in the Jiangaizangbu section. In terms of the presence of the fusulines Pseudodoliolina ozawai , Presumatrina neoschwagerinoides , Neoschwagerina simplex and Afghanella sp., an age of late Kungurian to Roadian can be assigned to the faunas. A comparative study on the stratigraphic sequence and foraminiferal faunas between the exotic limestone blocks and contemporaneous deposits in the adjacent areas reveals that the exotic limestone blocks resemble strata of the Lugu Formation in the Shuanghu and Minzhuochaka areas in the South Qiangtang Block. The presence of these exotic limestone blocks within the CQMB suggests that the in-situ deposits of the Lugu Formation have been partly subducted beneath the North Qiangtang Block during the closing of the Paleotethys Ocean along the Longmu Co-Shuanghu Suture Zone.

How and How Much Did Western Central Tibet Raise by India–Asia Collision?

AbstractThis study uses geochemical mohometers to investigate the paleo‐crustal thickness and elevation of western Tibet through the Cretaceous to Eocene. The northern Lhasa Block and southern Qiangtang Block (QB) had a similar paleo‐crustal thickness of ∼55 km during the Late Cretaceous, but the latter had reached its present‐day thickness (∼70 km) and elevation (∼5.5 km) by the mid‐Eocene, which was not caused by underthrusting of Indian crust according to isotopic evidence. Our study demonstrates the western and central QB shared a similar early Paleogene tectonomagmatic evolution with the formation of a continues proto‐plateau in central Tibet, resulting from crustal shortening and subsequent lithospheric foundering. This in turn implies underthrusting of the Indian lithosphere beneath the western QB after Eocene, given the present‐day lithospheric difference between the western and central QB. Our reconstruction of paleo‐crustal thickness and elevation, however, suggests the Indian underthrusting caused minimal further crustal thickening and uplift.

Paleomagnetism and geochronology of upper Eocene volcanic rocks from the western Qiangtang block: Constraints on the post-collisional shortening in western Tibet

Understanding the dynamics of the uplift of the Tibetan blocks requires constraints on the timing and magnitude of the crustal shortening accommodated by the different blocks. However, the estimates of the magnitude of post-collisional intra-Asian convergence range from >3000 km to a few hundred kilometers. Here we present new paleomagnetic and geochronological results from the Meisu Formation lavas on the southwestern margin of the western Qiangtang block, western Tibet. Zircon U/Pb data reveal that these volcanic rocks erupted during the late Eocene (∼40 Ma). Following progressive thermal demagnetization, stable characteristic remanent magnetizations (ChRMs) were successively isolated from 28 sites. These ChRMs passed the fold and reversal tests, consistent with a primary remanence. The paleopole at 50.2°N, 163.2°E with A 95 = 5.9° yields a paleolatitude of ∼29.5 ± 5.9°N at ca. 40 Ma for the southern margin of the western Qiangtang block (33.2°N, 80.9°E). A comparison of the Eocene latitudes between the western Qiangtang and Tarim block indicates a ∼ 700 km of post-40 Ma latitudinal crustal shortening between them at the longitude of ∼80°E. Our review of published Cretaceous-Paleogene paleopoles from western Tibet suggests that the western Qiangtang block was positioned at a stable latitude during ∼116–30 Ma. Our paleomagnetic compilation also indicate a discrepancy in the latitude (∼9.7 ± 4.3°) of western Lhasa and Qiangtang during the interval of 132 and 67 Ma, which vanished at ca. 30 Ma. We suggest a crustal shortening model to interpret this 1076 ± 477 km discrepancy, which was accommodated by the interior of the Lhasa block, the Bangong-Nujiang suture zone, and/or an eastward extrusion of Indochina from an original position between the Lhasa and Qiangtang blocks during ∼67–30 Ma. We conclude that the crustal shortening caused by the ongoing India-Asia collision spread progressively northwards into Asia. We emphasize that reliable paleomagnetic data from Paleocene and lower Eocene units in the western Qiangtang and Lhasa blocks have the potential to further refine these crustal shortening estimates. • We provide late Eocene paleomagnetic results from western Qiangtang. • The western Qiangtang block had a paleolatitude of 29.5 ± 5.9°N at ca. 40 Ma. • ∼700 km of post-40 Ma N-S crustal shortening between western Qiangtang and Tarim. • ∼1100 km of N-S crustal shortening between western Lhasa and Qiangtang during ∼67–30 Ma. • Crustal shortening spread progressively northwards into Asia.

Placing Another Piece of the Tethyan Puzzle: The First Paleozoic Paleomagnetic Data From the South Qiangtang Block and Its Paleogeographic Implications

AbstractThe origin and drift history of the South Qiangtang block (SQB) is critical to understanding the internal dimensions and kinematics of the late Paleozoic to early Mesozoic Tethyan realm and the subsequent evolution of the Qinghai‐Tibet Plateau, but so far no paleomagnetic data constrain the SQB in Paleozoic time. Here we present a paleomagnetic study of the middle Permian volcanic and sedimentary rocks from the SQB. Well‐defined high‐temperature components of magnetization were isolated from 6 sites of tuffaceous sandstone and 9 sites of basalt, but their directional populations are distinct. On the basis of a conglomerate test and comparisons with younger paleomagnetic data, we argue the basalts preserve a primary magnetization, whereas the sandstones may have been remagnetized during the Triassic. The magnetic directions from the basalt indicate that the SQB was located at a latitude of 22.3 ± 5.8°S during the middle Permian. Together with existing paleomagnetic data from other Tethyan continental blocks, this new constraint reveals that during the middle Permian the Neo‐Tethys Ocean spanned ∼10–20° of latitude and the Paleo‐Tethys Ocean spanned ∼50°, but also that the latter was a composite ocean system which included at least two important branches. In further considering the geological records of the SQB, we propose the block rifted from the northern margin of Gondwana during opening of the Neo‐Tethys in early Permian time, and then drifted rapidly northward until ultimately colliding with the North Qiangtang block and the southern margin of Laurasia during final closure of the Paleo‐Tethys in the Triassic.

Thrust Structure and Bidirectional Paleocurrent in the Jingzhushan Formation during the Late Cretaceous in the Nyima Basin, Tibet Plateau, China: Approach of Magnetic Fabric and Zircon Chronology

(1) Background: The widely developed thrust structures in the Jingzhushan Formation of late Cretaceous in Nyima Basin are related to the collision which leads to orogeny between the Lhasa and Qiangtang blocks. Clarifying the tectonic properties of the Nyima Basin is of great significance to characterize the evolution of the collision between the Lhasa block and the Qiangtang block. (2) Methods: Combined with mineralogy and petrology and U-Pb zircon dating of extrusive rocks, a detailed magnetic fabric test and rock magnetic experiment were carried out on the red sandstones of the Late Cretaceous Jingzhushan Formation in Nyima Basin. (3) Results: The sedimentary magnetic fabrics developed at the three sampling locations in the southern part of the basin are related to paleocurrents. The strained magnetic fabrics developed at one sampling site in the north, including pencil-like fabrics and tensile linear fabrics, are related to SE-NW tectonic stress. The average magnetic susceptibility value of the strained fabric with sandstone is lower than that of the sedimentary fabric, and its size is controlled by mica paramagnetic minerals. Ferromagnetic minerals are magnetite or hematite. In addition, the age of sandstone in the Jingzhushan Formation from the Nyima Basin matches the age of the eruptive rock in the Jingzhushan Formation from Coqen Basin, which is 92 Ma. (4) Conclusions: The strained fabric is developed near the thrust fault, which can explain the widely developed thrust structures in the basin. The Nyima Basin of the Jingzhushan Formation in the Late Cretaceous was an ocean-continent transitional foreland basin developed under the background of arc-arc “soft collision”.

Formation of late Miocene silicic volcanic rocks in the central Tibetan Plateau by crustal anatexis of granulites

Partial melting of granulites generates melts, which causes the modification of continental crust. However, the detailed mechanisms of this partial melting and related magmatic processes are debated. In this study, we investigated how crustal melting of granulites generated late Miocene (ca. 6.0 Ma) silicic volcanic rocks in the Chibuzhangcuo area of the Qiangtang Block, Tibet. Various peritectic textures (e.g., quartz inclusions in clinopyroxene and fluorhydroxyl phlogopite) suggest that the pyroxenes were derived from a residual granulite phase and peritectic reactions, and were then entrained into the partial melts. The F-rich mica, granulite mineral assemblages, and high titanomagnetite–ilmenite (811–975 °C) and zircon saturation (836–867 °C) temperatures indicate the silicic magmas were high-temperature and water-poor. The geochemical and isotopic compositions of the silicic volcanic rocks are similar to those of middle–lower crustal intermediate–mafic granulite xenoliths in Cenozoic volcanic rocks in the Qiangtang Block. We suggest that these volcanic rocks were generated by partial melting of hydrous mineral-bearing granulites, triggered by an underlying thermal anomaly associated with the high crustal heat-flow of the central Tibetan Plateau. This work indicates that the occurrence of restitic hydrous mineral and perturbation of thermal anomaly are the primary conditions to cause the partial melting of granulite. As the melting went on, the residual granulite phase and peritectic minerals were likely to be preferentially entrained into the melt due to the erosion effect caused by the continued melt flow at the sites of melting.

Paleoearthquakes of the Yangda-Yaxu fault across the Nujiang suture and Lancang river suture zone, southeastern Tibetan Plateau

The WNW-trending Yangda-Yaxu fault (YYF) is located in the interior of the Qiangtang block (QTB). The YYF cuts through the Nujiang suture and Lancang river suture zone and divides Nujiang fault (NF) and Lancangjiang fault (LCJF) into two sections with significantly different activity levels, suggesting that the YYF may function as a specific structure in this region. In addition, a recent work argues that the YYF plays an important role in strain partitioning in southeastern Tibet and poses a high surface-faulting risk to the Sichuan-Tibet railway. However, no M ≥ 5.0 earthquakes have been recorded, and no palaeoseismic research has been conducted along the fault, leading to limited knowledge regarding its rupture behavior, which is essential for understanding regional tectonic deformation and assessing the regional seismic potential. In this study, we constrained the timings and recurrence intervals of late Quaternary paleoseismic events along the YYF for the first time. Through trench excavations and exposure cleaning combined with radiocarbon dating, five faulting events were identified, namely, E1 through E5 from youngest to oldest (831–1,220, 3,307–6,703, 9,361–10,286, 12,729–14,651, and before 14,651 yr BP). The recurrence interval of major earthquakes along the YYF follows a quasi-periodic pattern with an interval of ∼4,000 yr. Combining the clear linear geomorphic features along the fault and the paleoearthquake results in this paper, we believe that YYF is a newly-generated active fault, and has a significant control effect on the late Quaternary evolution of the NF and the LCJF. Further analysis revealed that the YYF also plays an important role in accommodating crustal deformation.

Eocene adakitic quartz monzonites and granite porphyries from the northern Qiangtang Block, central Tibet: Partial melting of sediment-rich mélange?

The timing and mechanism of crustal thickening and initial surface uplift of the Tibetan Plateau remain disputed. Here, we report zircon U–Pb geochronological and O isotopic and whole-rock geochemical and Sr–Nd isotopic data for Eocene (41–37 Ma) granite porphyries and quartz monzonites from the Qoima Co area of the northern Qiangtang Block, central Tibet. The granite porphyries are characterized by high silica content (72 wt%), adakitic trace-element compositions, enriched Sr–Nd isotope signatures [(87Sr/86Sr)i = 0.7074, εNd (t) = −4.5], and a mean zircon δ18O value of +6.28 ± 0.85‰. The quartz monzonites exhibit high K2O content (5.1–6.8 wt%) and K2O/Na2O ratios (1.3–2.3). They also display adakitic geochemical characteristics, such as low Y (12–25 ppm) and Yb (1.1–1.8 ppm) content, high Sr/Y (98–187) and La/Yb (59–134) ratios, negligible Eu and positive Sr anomalies, and depletion in Nb and Ta. The quartz monzonites have (87Sr/86Sr)i ratios ranging from 0.7069 to 0.7078, εNd (t) values of −5.1 to −2.9, and mean δ18O values ranging from +6.27 ± 0.64‰ to +7.91 ± 0.32‰. We suggest that the granite porphyries were most probably derived by the partial melting of thickened, sediment-bearing lower crust and that the quartz monzonites were most likely generated by the partial melting of sediment-rich mélange. Combining these results with the existing tectonic and geophysical data, we conclude that continental subduction and mélange underplating may have been responsible for crustal shortening and tectonic thickening.