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The Xiazha Formation recently identified in the Yarlung-Zangbo suture zone of southern Tibet has been considered as crucial evidence for early (pre-Cretaceous) subduction of Neo-Tethyan oceanic lithosphere. In this study, we carried out an integrated stratigraphic, petrographic, and geochemical study of the Xiazha Formation, which consists of abyssal chert, siliceous mudrocks, and calcareous turbidites deposited onto basaltic basement. Basalt geochemistry indicates alkali to tholeiitic magmas generated in a within-plate oceanic setting. The siliciclastic fraction contained in calcareous turbidites consists of plagioclase and mafic-rock fragments derived from the underlying basalts, and includes Cr-spinel grains with relatively high TiO 2 and moderate Al 2 O 3 concentrations pointing to an OIB source. Based on these data, and virtual lack of detritus of continental origin, we infer that the Xiazha Formation was deposited in oceanic island/plateau within the Neo-Tethyan domain. Its stratigraphy records an evolution from basaltic eruptions to the development of a carbonate platform surrounding the basaltic island, and finally to drowning of the atoll. The Xiazha Formation was not deposited in a subduction-related basin and, therefore, cannot be used as evidence for pre-Cretaceous initiation of the Yarlung subduction zone. • Earliest Cretaceous oceanic island developed in the Neo-Tethys Ocean. • The Xiazha Formation in the Yarlung suture shows intact stratigraphic continuity. • It was deposited on an oceanic island rather than in a subduction setting. • The Xiazha Formation records the evolution of the oceanic island through time. • The Xiazha Formation cannot be used as evidence for pre-130 Ma subduction. [ABSTRACT FROM AUTHOR]

• Explored the variation law of fracture gas-steam pressure with heat flow. • The stress intensity factors under the action of fracture gas-steam pressure and heat flow stress is established. • The influence of heat flow incidence angle and heat flow density on crack initiation is analyzed. • Provided certain theoretical guidance for engineering fractures with geothermal flow. The construction of tunnel projects in the Sichuan-Tibet region has led to the occurrence of rock fractures caused by high temperature and high humidity, which are inevitable challenges. When there is fracture water in rock fractures and it encounters a heat flow, the water vaporizes into fracture steam, which undergoes thermal expansion to generate fracture gas-steam pressure. The heat flow direction is at an angle to the crack, causing heat flow stress in the rock. To understand the mechanism of crack initiation and propagation in rocks under the joint action of uniform heat flow-induced fracture gas-steam pressure and heat flow stress, the control equation for fracture gas-steam pressure is derived and expressions for stress intensity factor, crack temperature field, stress field, and crack initiation strength criterion are established based on the theory of complex variable functions. The key parameters are analyzed and discussed, finding that under low heat flux density, small incidence angles, and large crack inclination angles, K I values are higher, and the crack initiation is mainly in tension. Under high heat flow density, large incidence angles, and crack inclination angles near 45°, K II values are higher, and the crack initiation is mainly in shear. The coupling effect of K I and K II is more sensitive to heat flow density and inclination angle. The critical initiation angle and initiation strength increase with the increase of heat flow density and incidence angle, with heat flow density being the determining factor. [ABSTRACT FROM AUTHOR]

• Based on the modified MTS criterion, the rock crack initiation criterion under the combined action of thermal stress and fracture air-vapor pressure is established by the superposition method. • The expansion pressure of the fracture gas in the crack is controlled by the temperature and the gas saturation, and the temperature is the dominant effect. • K I , K II are controlled by thermal stress and fracture gas-vapor pressure, in which thermal stress is the dominant factor. • With the increase of temperature, the crack initiation form tends to be tensioned from shearing, and the gas expansion pressure promotes the tensioned part. Near the surface of the Sichuan-Tibet line tunnel surrounding rock under construction is in a sub-hot and humid environment, and long-term accumulation will produce damp-heat diseases. The significant temperature difference leads to the thermal stress of the surrounding rock, and the surrounding rock fracture water vaporizes to create fracture air-vapor pressure, which induces crack initiation. The state equation of gases, thermoelastic mechanics, and the theory of linear elastic fracture mechanics are used to study the crack initiation law of rock under the joint action of sub-elevated temperature stress and fracture air-vapor pressure. The stress intensity factor's expression under the two joint actions is derived from the superposition principle. Then the initiation criterion considering both internal influencing factors (rock properties, crack geometry parameters (inclination angle)) and external influencing factors (far-field stress, temperature, humidity factors) is derived. The theory is used to quantitatively analyze the distribution law of tangential stress at the crack tip and the crack initiation form. The effects of each parameter on critical crack initiation angle and crack initiation strength are discussed. The theoretical results are further verified by numerical simulation. The results show that in the sub-elevated temperature range, the thermal stress and the fracture air-vapor pressure jointly affect the crack initiation law, and thermal stress is the dominant factor. With the temperature increase, K I > K II , the critical crack initiation angle increases gradually, the crack initiation form changes from shear to compression-shear, and finally tends to tension. The fracture air-vapor pressure promotes the tension part. The maximum tangential stress increases linearly with temperature. The maximum tangential stress and critical crack initiation angle increase first and then decrease with the rise of crack inclination angle and reach the maximum at 45°. The research results supply a theoretical basis for preventing and treating the rock crack propagation surrounding the tunnel under sub-elevated temperatures and high humidity. [ABSTRACT FROM AUTHOR]

Podocarpium is an extinct genus in Fabaceae with rich fossil records in Eurasia dating back to the Eocene. However, the diversification and biogeographic histories of Podocarpium are poorly known due to a lack of fossils in some key regions, such as the Tibetan Plateau, an area recently shown to be crucial for floristic exchanges worldwide in the geological past. Here, we describe well-preserved fossil pods of Podocarpium from the late Eocene (~ 35 Ma) of the Lunpola Basin, central Tibetan Plateau, China. Together with detailed morphological observation and geometric morphology analysis, these fossil pods are named Podocarpium tibeticum W.-C. Li, J. Huang et T. Su sp. nov., characterized by asymmetrical elliptical valves, olecranon fruit beak, and a clearly oblique base. This material comprises the oldest fossil record of Podocarpium from the Tibetan Plateau and adjacent regions. The discovery of P. tibeticum shows Podocarpium was present in central Tibet by the late Eocene. Together with previous fossil records, it indicates that Podocarpium may have originated in East Asia, migrated into the central valley of Tibet in the late Eocene, and then spread westward to Europe through a low-latitude path of island chains along the Neotethys. Results of paleoecological niche modeling demonstrate that Podocarpium had an almost worldwide potential distribution range in the Eocene, but this sharply contracted thereafter until by the Pliocene it was restricted to a small area of East Asia. Generally, this finding further supports the idea that what is now the central Tibetan Plateau was a globally significant hub for Paleogene floristic exchange. • First report of Podocarpium in central Tibetan Plateau. • Shows a new fossil pod species of Podocarpium. • Enhances understanding of its historic distribution globally. • Podocarpium is sensitive for dry and cool climates. [ABSTRACT FROM AUTHOR]

• The back-arc and arc-type magmatisms (ca. 838–748 Ma) were newly identified in the Rutong and Shiquanhe region, west of Tibet. • The North Lhasa terrane may separat from the Indian continent, finally constituted the northern segment of the East African Orogen during the Neoproterozoic. • The BNSZ retains geological records of the evolution of the Mozambique Ocean in the Neoproterozoic. Due to limited exposure of the Precambrian rocks in the Tibetan Plateau, their affinities and evolutions are inadequately studied. Here we report the newly identified Neoproterozoic metabasites (including metagabbros and metadolerites) at the western end of the Bangong-Nujiang suture zone (BNSZ) and the west part of the North Lhasa terrane. The Rutong Dong (RTD) metagabbros and Shiquanhe Nan (SQN) metadolerites yield zircon U-Pb ages of 838 Ma and 748 Ma, respectively. The RTD metagabbros are tholeiitic and exhibit both MORB- and arc-like geochemical characteristics, and we consider these to have formed in a back-arc basin setting. Combined with positive zircon ε Hf (t) (+6.3 to +13.8) and whole-rock ε Nd (t) (+4.4 to +5.3) values, the geochemical features of the RTD metagabbros indicate that they originated from a depleted mantle source enriched by subduction-related components (e.g., Th and U). The SQN metadolerites are mainly of transitional character (between tholeiitic and calc-alkaline), as well as representing typical calc-alkaline basalts. Their arc-like geochemical characteristics, as well as low positive whole-rock ε Nd (t) (+1.7 to +3.4), and positive zircon ε Hf (t) (+4.3 to +5.9) values, indicate generation by partial melting of an enriched subduction components modified mantle in an arc-type setting. Based on previous research, we propose that the North Lhasa terrane likely separated from India at ca. 925–860 Ma by the opening of the Mozambique Ocean. As the subduction zones were active at ca. 838–730 Ma in this ocean, a series of subduction-related magmatic events occurred, that is recorded in this study of the Rutong-Shiquanhe region, and other parts of the BNSZ and North Lhasa terrane. Consequently, we suggest that the BNSZ not only preserves information of the Meso-Tethys but also retains geological records of the Neoproterozoic Mozambique Ocean. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Shiquanhe-Yarlun Zangbo geothermal field enriched in rare alkali metal resources. • Himalayan leucogranite are typical of S-type granites. • Partial melting of metasediments and intense water–rock interaction promote RAMs enrichment. • Strong adsorption controls the hyper enrichment of rare metals during silica precipitation. Geothermal waters in the Shiquanhe-Yarlun Zangbo geothermal field of southern Tibet are highly enriched in rare alkali metals (RAMs), which form a globally unique geyserite Cs-deposit. Boron isotopes, together with the geochemistry of a local leucogranite, geothermal springs and geyserite were investigated to constrain the migration and accumulation of RAMs in the geyserite cesium deposit. The δ11B values of −9.7‰ to −8.4‰ in tourmalines from the Himalayan leucogranite have shown typical S-type granites. A mass balance calculation indicates that the magma of the S-type granite contained < 10 % dehydrated fluid, which resulted in an enrichment of RAMs by 4–5 times during the partial melting of crustal materials, compared to that in the UCC. The linear relationships of ΣRAMs vs. B and Na + K vs. soluble Si, indicate that further enrichment of RAMs in the geothermal fluids was promoted by intense water–rock interaction at temperatures of 210 to 250 °C in the geothermal system reservoir. A strong adsorption process controlled the hyper enrichment of RAMs (especially Cs) during silica precipitation from the upwelling geothermal water, with the sharp decreases in temperature and pressure leading to the enrichment of Cs in fresh geyserites. Despite loss of RAMs during dehydration of the geyserite during phase transformation from opal-A, opal-CT, and chalcedony to quartz, freshly deposited geyserites have great potential for exploration of RAMs resources in this area. [ABSTRACT FROM AUTHOR]