Assessment of acoustic backscatter intensity surveying on deep-sea ferromanganese crust: Constraints from Weijia Guyot, western Pacific Ocean

Near-bottom observation data from the manned deep submersible Jiaolong with high-precision underwater positioning data from Weijia Guyot,Magellan Seamounts in the Western Pacific Ocean are reported.Three substrate types were identified:Sediment,ferromanganese crust,and ferromanganese crust with a thin cover of sediment.The ferromanganese crusts show clear zoning and their continuity is usually disturbed by sediments on areas of the mountainside with relatively gentle slope gradients.The identified substrate spatial distributions correspond to acoustic backscatter intensity data,with regions of high intensity always including crust development and regions of low intensity always having sediment.Therefore,acoustic backscatter intensity surveying appears useful in the delineation and evaluation of crust resources,although further more work is needed to develop a practicable methodology.

Tectonic-thermal history and hydrocarbon potential of the Pearl River Mouth Basin,northern South China Sea:Insights from borehole apatite fission-track thermochronology

The Pearl River Mouth Basin(PRMB)is one of the most petroliferous basins on the northern margin of the South China Sea.Knowledge of the thermal history of the PRMB is significant for understanding its tectonic evolution and for unraveling its poorly studied source-rock maturation history.Our investigations in this study are based on apatite fission-track(AFT)thermochronology analysis of 12 cutting samples from 4 boreholes.Both AFT ages and length data suggested that the PRMB has experienced quite complicated thermal evolution.Thermal history modeling results unraveled four successive events of heating separated by three stages of cooling since the early Middle Eocene.The cooling events occurred approximately in the Late Eocene,early Oligocene,and the Late Miocene,possibly attributed to the Zhuqiong II Event,Nanhai Event,and Dongsha Event,respectively.The erosion amount during the first cooling stage is roughly estimated to be about 455-712 m,with an erosion rate of 0.08-0.12 mm/a.The second erosion-driven cooling is stronger than the first one,with an erosion amount of about 747-814 m and an erosion rate between about 0.13-0.21 mm/a.The erosion amount calculated related to the third cooling event varies from 800 m to 3419 m,which is speculative due to the possible influence of the magmatic activity.

Quantitative relationship between surface sedimentary diatoms and water depth in North-Central Bohai Bay,China

To study the quantitative relationship between surface sedimentary diatoms and water depth,67 surface samples were collected for diatom analysis on eight profiles with water depth variation from the muddy intertidal zone to the shallow sea area in North-Central Bohai Bay,China.The results showed that the distribution of diatoms changed significantly in response to the change in water depth.Furthermore,the quantitative relationship between the distribution of dominant diatom species,their assemblages,and the water depth was established.The water depth optima for seven dominant species such as Cyclotella striata/stylorum,Paralia sulcata,and Coscinodiscus perforatus and the water depth indication range of seven diatom assemblages were obtained in the study area above the water depth(elevation)of-10 m.The quantitative relationship between surface sedimentary diatoms and water depth provides a proxy index for diatom-paleo-water depth reconstruction in the strata in Bohai Bay,China.

Formation and evolution of the South China Sea since the Late Mesozoic:A review

The existing genetic models of the South China Sea(SCS)include an extrusion model of the Indochina Peninsula,a back-arc extension model,and a subduction and dragging model of the Proto-South China Sea(PSCS).However,none of these models has been universally accepted because they do not fully match a large number of geological phenomena and facts.By examining the regional tectonics and integrating them with measured data for the SCS,in this study,a back-arc spreading-sinistral shear model is proposed.It is suggested that the SCS is a back-arc basin formed by northward subduction of the PSCS and its formation was triggered by left-lateral strike-slip motion due to the northward drift of the Philippine Sea Plate.The left-lateral strike-slip fault on the western margin caused by the Indo-Eurasian collision changed the direction of the Southwest Sub-basin's spreading axis from nearly E–W to NE–SW,and subduction retreat caused the spreading ridge to jump southward.This study summarizes the evolution of the SCS and adjacent regions since the Late Mesozoic.

New high-resolution 2D seismic imaging of fluid escape structures in the Makran subduction zone,Arabian Sea

Seabed fluid escape is active in the Makran subduction zone,Arabian Sea.Based on the new highresolution 2D seismic data,acoustic blanking zones and seafloor mounds are identified.Acoustic blanking zones include three kinds of geometries:Bell-shaped,vertically columnar and tilted zones.The bellshaped blanking zone is characterized by weak and discontinuous reflections in the interior and upbending reflections on the top,interpreted as gas chimneys.Vertically columnar blanking zone is interpreted as side-imaged gas chimneys associated with focused fluid flow and topped by a seafloor anomaly expressed as a localized reflection discontinuity,which may together serve as a vent structure.Tilted acoustic blanking zone could be induced by accretionary thrust activity and rapid sedimentation surrounding slope.Seafloor mounds occur at the sites of bell-shaped acoustic blanking zone and may be associated with the material intrusion.Bottom simulating refectors(BSRs)are widely distributed and exhibit a series of characteristics including diminished amplitude,low continuity as well as local shoaling overlapping with these acoustic blanking zones.The large amount of gases dissociated from the gas hydrates migrated upwards and then arrived at the near-seafloor sediments,followed by the formation of the gas hydrates and hence the seafloor mound.

Diapir structure and its constraint on gas hydrate accumulation in the Makran accretionary prism, offshore Pakistan

The Makran accretionary prism is located at the junction of the Eurasian Plate,Arabian Plate and Indian Plate and is rich in natural gas hydrate(NGH)resources.It consists of a narrow continental shelf,a broad continental slope,and a deformation front.The continental slope can be further divided into the upper slope,middle slope,and lower slope.There are three types of diapir structure in the accretionary prism,namely mud diapir,mud volcano,and gas chimney.(1)The mud diapirs can be grouped into two types,namely the ones with low arching amplitude and weak-medium activity energy and the ones with high arching amplitude and medium-strong activity energy.The mud diapirs increase from offshore areas towards onshore areas in general,while the ones favorable for the formation of NGH are mainly distributed on the middle slope in the central and western parts of the accretionary prism.(2)The mud volcanoes are mainly concentrated along the anticline ridges in the southern part of the lower slope and the deformation front.(3)The gas chimneys can be grouped into three types,which are located in piggyback basins,active anticline ridges,and inactive anticline ridges,respectively.They are mainly distributed on the middle slope in the central and western parts of the accretionary prism and most of them are accompanied with thrust faults.The gas chimneys located at different tectonic locations started to be active at different time and pierced different horizons.The mud diapirs,mud volcanoes,and gas chimneys and thrust faults serve as the main pathways of gas migration,and thus are the important factors that control the formation,accumulation,and distribution of NGH in the Makran accretionary prism.Mud diapir/gas chimney type hydrate develop in the middle slope,mud volcano type hydrate develop in the southern lower slope and the deformation front,and stepped accretionary prism type hydrate develop on the central and northern lower slope.The middle slope,lower slope and deformation front in the central and western parts of the Makran accretionary prism jointly constitute the NGH prospect area.