Characteristics of crustal variation and extensional break-up in the Western Pacific back-arc region based on a wide-angle seismic profile

The marginal sea and back-arc basins in the Western Pacific Ocean have become the focus of tectonics due to their unique tectonic location.To understand the deep crustal structure in the back-arc region,we present a 545-kmlong active-source ocean bottom seismometer(OBS)wide-angle reflection/refraction profile in the East China Sea.The P wave velocity model shows that the Moho depth rises significantly,from approximately 30 km in the East China Sea shelf to approximately 16 km in the axis of the Okinawa Trough.The lower crustal high-velocity zone(HVZ)in the southern Okinawa Trough,with V_(p) of 6.8-7.3 km/s,is a remarkable manifestation of the mantle material upwelling and accretion to the lower crust.This confirms that the lower crustal high-velocity mantle accretion is developed in the southern Okinawa Trough.During the process of back-arc extension,the crustal structure of the southern Okinawa Trough is completely invaded and penetrated by the upper mantle material in the axis region.In some areas of the southern central graben,the crust may has broken up and entered the initial stage of seafloor spreading.The discontinuous HVZs in the lower crust in the back-arc region also indicate the migration of spreading centers in the back-arc region since the Cenozoic.The asthenosphere material upwelling in the continent-ocean transition zone is constantly driving the lithosphere eastward for episodic extension,and is causing evident tectonic migration in the Western Pacific back-arc region.

Geochemistry of the upper Permian sandstones in the Dalong Formation in the South Yellow Sea Basin,East China:implications to provenance,weathering,and tectonic setting

To reconstruct the paleoenvironment,the petrography and geochemical compositions of the upper Permian Dalong Formation(Fm)(P_(3)d)sandstones in the Central Uplift,South Yellow Sea Basin,was investigated,after which the provenance,tectonic setting,weathering conditions,and diagenetic history were determined.The Dalong Fm is mainly composed of sandstones with a few mudstones and limestones.The petrographic analysis showed that the P_(3)d sandstones are in low maturity and could be classified into feldspar sandstone and lithic sandstone on average framework composition of Q_(63)F_(16)L_(21).The major and trace element results show that Dalong Fm sandstones are enriched in Rb and depleted in Cr and Ni compared with the upper continental crust(UCC).The chemical weathering intensity is weak to low moderate in the source area as indicated by the chemical index of alteration(CIA)and other results from a variety of methods.The diagenetic process included three main stages:eodiagenesis,mesodiagenesis,and telodiagenesis.Tectonic discrimination plots show that the source areas were an active continental margin(ACM)and a passive continental margin(PCM),which may have been related to the northward movement of the Lower Yangtze Plate(LYP)and the collision between the LYP and the North China Plate(NCP).

Characteristics and classification of paleozoic tight reservoirs in the central uplift of the South Yellow Sea Basin

Several sets of Paleozoic tight reservoirs are developed in the Central Uplift of the South Yellow Sea Basin.A qualitative analysis of the microscopic pore structure of the tight reservoir rocks was carried out through cast thin slice and scanning electron microscopic image observation.Based on reservoir pet-rophysical properties,thirty core samples in the Central Uplift of the South Yellow Sea Basin were selected for high-pressure mercury intrusion(HPMI)analysis,which was then combined with fractal calculation to classify and evaluate the tight reservoirs.The analysis of the HPMI curves and related parameters shows that the Paleozoic tight reservoirs can be divided into three types:Type-A,Type-B and Type-C.Type-A sandstone reservoirs contain pores with size mostly ranging between 0.01 and 0.1 mm,followed by pores with size range of 0.001-0.01 m m,and relatively fewer pores larger than 0.1 m m.The Type-B reservoirs are carbonate rocks with extremely heterogeneous pore size distribution,which is closely related to the development of dissolution pores and microfractures.Type-C sandstone reservoirs are dominated by nanopores and submicron pores that distribute more heterogeneously than pores in Type-A reservoirs.The pore distribution in sandstone reservoirs shows significant fractal characteristics and is closely related to the pore size.The heterogeneity of nanopore distribution has a negative cor-relation with porosity and median pressure and a relatively weak correlation with permeability.Our study has important implications for petroleum exploration in the South Yellow Sea Basin.

Characterization and Monitoring of Transient Enamine Radical Intermediates in Photoredox/Chiral Primary Amine Synergistic Catalytic Cycle

Enamine-derived radicals are crucial intermediates in singly occupied molecular orbital(SOMO)catalysis.However,observing them directly is elusive and remains a long-standing challenge.Here,an advanced time-resolved electron paramagnetic resonance technique was employed to characterize and monitor the key intermediates in photoredox transformations by primary aminocatalysis on a microsecond timescale.The transient enamine radical cation,generated by single electron transfer(SET),and the deprotonated form ofα-imino radical intermediates were directly observed for the first time,both spectroscopically and kinetically.In reactions with styrene,enamine radical cation was found to be faster thanα-imino radical by one order of magnitude.This revealed the subtle role of deprotonation associated with secondary enamine radical cation in the photoredox transformations by primary aminocatalysis.