Spatial distribution and inventory of natural gas hydrate in the Qiongdongnan Basin,northern South China Sea

Natural gas hydrate is a potential clean energy source and is related to submarine geohazard,climate change,and global carbon cycle.Multidisciplinary investigations have revealed the occurrence of hydrate in the Qiongdongnan Basin,northern South China Sea.However,the spatial distribution,controlling factors,and favorable areas are not well defined.Here we use the available high-resolution seismic lines,well logging,and heat flow data to explore the issues by calculating the thickness of gas hydrate stability zone(GHSZ)and estimating the inventory.Results show that the GHSZ thickness ranges between mostly~200 and 400 m at water depths>500 m.The gas hydrate inventory is~6.5×109-t carbon over an area of~6×104 km2.Three areas including the lower uplift to the south of the Lingshui sub-basin,the Songnan and Baodao sub-basins,and the Changchang sub-basin have a thick GHSZ of~250-310 m,250-330 m,and 350-400 m,respectively,where water depths are~1000-1600 m,1000-2000 m,and2400-3000 m,respectively.In these deep waters,bottom water temperatures vary slightly from~4 to 2℃.However,heat flow increases significantly with water depth and reaches the highest value of~80-100 mW/m2 in the deepest water area of Changchang sub-basin.High heat flow tends to reduce GHSZ thickness,but the thickest GHSZ still occurs in the Changchang sub-basin,highlighting the role of water depth in controlling GHSZ.The lower uplift to the south of the Lingshui sub-basin has high deposition rate(~270-830 m/Ma in 1.8-0 Ma);the thick Cenozoic sediment,rich biogenic and thermogenic gas supplies,and excellent transport systems(faults,diapirs,and gas chimneys)enables it a promising area of hydrate accumulation,from which hydrate-related bottom simulating reflectors,gas chimneys,and active cold seeps were widely revealed.

Formation of the Zengmu and Beikang Basins,and West Baram Line in the southwestern South China Sea margin

The Zengmu and Beikang basins,separated by the West Baram Line(WBL)in the southwestern South China Sea margin,display distinct geological and geophysical features.However,the nature of the basins and the WBL are debated.Here we explore this issue by conducting the stratigraphic and structural interpretation,faults and subsidence analysis,and lithospheric finite extension modelling using seismic data.Results show that the WBL is a trans-extensional fault zone comprising normal faults and flower structures mainly active in the Late Eocene to Early Miocene.The Zengmu Basin,to the southwest of the WBL,shows an overall synformal geometry,thick folded strata in the Late Eocene to Late Miocene(40.4-5.2 Ma),and pretty small normal faults at the basin edge,which imply that the Zengmu Basin is a foreland basin under the Luconia and Borneo collision in the Sarawak since the Eocene.Furthermore,the basin exhibits two stages of subsidence(fast in 40.4-30 Ma and slow in 30-0 Ma);but the amount of observed subsidence and heat flow are both greater than that predicted by crustal thinning.The Beikang Basin,to the NE of the WBL,consists of the syn-rift faulted sub-basins(45-16.4 Ma)and the post-rift less deformed sequences(16.4-0 Ma).The heat flow(~60 mW/m2)is also consistent with that predicted based on crustal thinning,inferring that it is a rifted basin.However,the basin shows three stages of subsidence(fast in 45-30 Ma,uplift in 30-16.4 Ma,and fast in 16.4-0 Ma).In the uplift stage,the strata were partly folded in the Late Oligocene and partly eroded in the Early Miocene,which is probably caused by the flexural bulging in response to the paleo-South China Sea subduction and the subsequent Dangerous Grounds and Borneo collision in the Sabah to the east of the WBL.

Ultrafast Quasiparticle Dynamics and Electron-Phonon Coupling in(Li0.84Fe0.16)OHFe0.98Se

Distinctive superconducting behaviors between bulk and monolayer FeSe make it challenging to obtain a unified picture of all FeSe-based superconductors.We investigate the ultrafast quasiparticle(QP)dynamics of an intercalated superconductor(Li1-xFex)OHFe1-ySe,which is a bulk crystal but shares a similar electronic structure with single-layer FeSe on SrTiO3.We obtain the electron-phonon coupling(EPC)constant(0.22±0.04),which well bridges that of bulk FeSe crystal and single-layer FeSe on SrTiO3.Significantly,we find that such a positive correlation betweenλA1 g and superconducting Tc holds among all known FeSe-based superconductors,even in line with reported FeAs-based superconductors.Our observation indicates possible universal role of EPC in the superconductivity of all known categories of iron-based superconductors,which is a critical step towards achieving a unified superconducting mechanism for all iron-based superconductors.

Observation of a bi-critical point between antiferromagnetic and superconducting phases in pressurized single crystal Ca0.73La0.27FeAs2

One of the most strikingly universal features of the high-temperature superconductors is that the superconducting phase emerges in the close proximity of the antiferromagnetic phase, and the interplay between these two phases poses a long-standing challenge. It is commonly believed that, as the antiferromagnetic transition temperature is continuously suppressed to zero, there appears a quantum critical point, around which the existence of antiferromagnetic fluctuation is responsible for the development of the superconductivity. In contrast to this scenario, we report the observation of a bi-critical point identified at 2.88 GPa and 26.02 K in the pressurized high-quality single crystal Ca_(0.73)La_(0.27)FeAs_2 by complementary in-situ high pressure measurements. At the critical pressure, we find that the antiferromagnetism suddenly disappears and superconductivity simultaneously emerges at almost the same temperature, and that the external magnetic field suppresses the superconducting transition temperature but hardly affects the antiferromagnetic transition temperature.

Cenozoic tectonic subsidence in the Southern Continental Margin, South China Sea

We analyzed two recently acquired multi- channel seismic profiles across the Dangerous Grounds and the Reed Bank area in the South China Sea. Reconstruction of the tectonic subsidence shows that the southern continental margin can be divided into three stages with variable subsidence rate. A delay of tectonic subsidence existed in both areas after a break-up, which was likely related to the major mantle convection during seafloor spreading, that was triggered by the secondary mantle convection below the continental margin, in addition to the variation in lithospheric thickness. Mean-while, the stage with delayed subsidence rate differed along strikes. In the Reed Bank area, this stage is between 32-23.8 Ma, while in the Dangerous Grounds, it was much later （between 19-15.5 Ma）. We believe the propagated rifting in the South China Sea dominated the changes of this delayed subsidence rate stage.

The deep thermal structure of the lithosphere in the northwestern South China Sea and its control on the shallow tectonics

The northwestern sub-basin of South China Sea(SCS)is a unique tectonic unit formed in the early spreading of the SCS.The northwestern Sub-basin has a series of complex geological structures such as seamounts and fault zones surrounded by the Xisha Trough,the Zhongsha Massif,and the Pearl River Valley.These extensional structures and magmatic activity in the northwestern sub-basin are closely related to the lithospheric structure and its deformation.However,details of the deep lithosphere structure are still poorly known.Here,we obtained detailed data of water and Moho depth using sonar buoys,Extended Spread Profiles(ESP),Ocean Bottom Seismometer(OBS),both Multi-beam and land-sea joint seismic surveys in the northwestern sub-basin and its surrounding areas.Then we adopted a thermal isostasy method to calculate the depth of the Lithosphere-Asthenosphere Boundary(LAB)in the northwestern sub-basin of the SCS and its surrounding regions.Results show that the range of LAB depth is~25–110 km.The shallowest burial depth is 25–60 km occurring in the ocean basin.The depth increases to 60–110 km toward the continental margin.The lithospheric structure on the north and south sides of the Xisha Trough is symmetrical and shows the deep structure and thermal features of aborted rifts.The LAB depth in the Zhongsha Trough and the Zhongsha Massif increased from 60 to 70 km southwestwards,consistent with the trend of surface morphology.The LAB depth to the west side of the Pearl River Valley is 60–80 km,and the thinning of the lithosphere is related to the distribution of faults,depressions and the magmatic activity.The LAB depth in the northwestern sub-basin and the eastern subbasin is less than 60 km with the thinnest part being less than 46 km.Combining ocean drilling,seismic investigation,and seafloor topography,we show that the ocean basin of the northwestern sub-basin of the SCS locates within the 46 km isobath of the LAB.The formation of the rifted valleys and discrete blocks surrounding the ocean basins is both controlled by the regional tectonic movement and the deep thermal state,where their lithospheric structures show strong heterogeneity.

Quantum phase transition and destruction of Kondo effect in pressurized SmB6

SmB_6 has been a well-known Kondo insulator for decades, but recently attracts extensive new attention as a candidate topological system. Studying SmB_6 under pressure provides an opportunity to acquire the much-needed understanding about the effect of electron correlations on both the metallic surface state and bulk insulating state. Here we do so by studying the evolution of two transport gaps(low temperature gap E_l and high temperature gap E_h) associated with the Kondo effect by measuring the electrical resistivity under high pressure and low temperature(0.3 K) conditions. We associate the gaps with the bulk Kondo hybridization, and from their evolution with pressure we demonstrate an insulator-tometal transition at ~4 GPa. At the transition pressure, a large change in the Hall number and a divergence tendency of the electron-electron scattering coefficient provide evidence for a destruction of the Kondo entanglement in the ground state. Our results raise the new prospect for studying topological electronic states in quantum critical materials settings.

Low-temperature magnetic anomalies of oxygen-doped T′-type 214 compounds

A new low_temperature anomalous magnetism at about 29 K for the oxygen doped (Eu 1- x Y x ) 2CuO 4 and Sm 2CuO 4, prepared under high pressure, has been investigated. This anomaly originated from different mechanisms to weak ferromagnetism (WF) behavior of T′ phase, and should be assigned to the ferromagnetic clusters formed by doped holes.