Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

The study of morphology, distribution, and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes, geodynamics, and tectonic evolution of continental margins. Previous studies concentrated partly on the magma-rich rifted basins, where the lateral magma transport mainly affects the igneous complexes' connection and distribution. However, due to seismic wave shielding effects of the large shallow magmatic bodies, the underlying igneous complexes and their corresponding magma plumbing systems in the magma-poor rifted margins are still in debate. In this study, 2D/3D seismic data and well data are utilized to describe the morphology, architecture, and spatial-temporal distribution of igneous complexes in the Lingshui sag of the Qiongdongnan basin, northwestern South China Sea margin. The identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills. Twenty-six cone-shaped mounds that overlie intruded sills through internal disturbed conduits were also described. Drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents. A uniform Bottom Mounds Horizon of these vents suggests that they probably formed at the same time. Constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers, the magma emplacement was dated to the middle Miocene (ca. 14.6 Ma). Most of the hydrothermal vents are distributed along the F2 fault zone and have direct linkage with the underlying sills, while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust, where the crust thins to 6–10 km. The sills intruded into different layers, from the lower Oligocene to the lower Miocene and the emplaced depth of sills is 1.2–6.3 km, whether or not they feed any vents above. Unlike most of the large volume and laterally linked sills found in the magma-rich rifted margins, the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport, which might coexist with numerous polygonal or small faults and interference reflections. This work highlights the critical role of basin structures in controlling the distribution of post-rift igneous complexes in magma-poor margins, including thinned continental crust, sedimentary thickness, and faults

Seismic characteristics and evolution of post-rift igneous complexes and hydrothermal vents in the Lingshui sag (Qiongdongnan basin), northwestern South China Sea

The study of morphology, distribution, and characteristics of igneous complexes has great significance to the understanding of magma plumbing processes, geodynamics, and tectonic evolution of continental margins. Previous studies concentrated partly on the magma-rich rifted basins, where the lateral magma transport mainly affects the igneous complexes' connection and distribution. However, due to seismic wave shielding effects of the large shallow magmatic bodies, the underlying igneous complexes and their corresponding magma plumbing systems in the magma-poor rifted margins are still in debate. In this study, 2D/3D seismic data and well data are utilized to describe the morphology, architecture, and spatial-temporal distribution of igneous complexes in the Lingshui sag of the Qiongdongnan basin, northwestern South China Sea margin. The identified igneous complexes include 98 intrusive sills and feeder dykes beneath some of the isolated sills. Twenty-six cone-shaped mounds that overlie intruded sills through internal disturbed conduits were also described. Drilled well samples and seismic expressions suggest that these mounds are hydrothermal vents. A uniform Bottom Mounds Horizon of these vents suggests that they probably formed at the same time. Constrained by biostratigraphic data and sedimentation rate of underlying and overlying sedimentary layers, the magma emplacement was dated to the middle Miocene (ca. 14.6 Ma). Most of the hydrothermal vents are distributed along the F2 fault zone and have direct linkage with the underlying sills, while the large sill complexes that are connected with limited vents are mainly present above the hyperextended continental crust, where the crust thins to 6–10 km. The sills intruded into different layers, from the lower Oligocene to the lower Miocene and the emplaced depth of sills is 1.2–6.3 km, whether or not they feed any vents above. Unlike most of the large volume and laterally linked sills found in the magma-rich rifted margins, the scattered distribution of sills at different levels indicates that dykes probably play an important role in magma transport, which might coexist with numerous polygonal or small faults and interference reflections. This work highlights the critical role of basin structures in controlling the distribution of post-rift igneous complexes in magma-poor margins, including thinned continental crust, sedimentary thickness, and faults

Genesis and evolution of the mass transport deposits in the middle segment of the Pearl River canyon, South China Sea: Insights from 3D seismic data

Utilizing newly acquired 3D seismic data, piston core, bathymetry and wells, this study investigated the geomorphological characters, genesis and evolution of two main phases of Quaternary mass transport deposits (MTDs) in the Pearl River canyon's middle segment. The older MTDs_1 almost covers the whole trough zone of Pearl River canyon's middle segment with an area of 1570 km(2), which is sourced from north, west and south. Whereas the younger MTDs_2 has a much smaller coverage area of 840 km(2) and is mainly from the northern slope. These MTDs could be divided into western and northern slope-attached MTDs and southern slope-detached MTDs based on their source areas. Within the MTDs-dominated study area, coarse sediments are presented at the area connecting to the slope channels and canyons. These two phases of MTDs probably occurred at similar to 0.79 Ma and similar to 0.54 Ma according to a published dating result. The homogeneous, organic-rich fine-grained sediments have high compressibility, preconditioning the generation of weak layers for submarine failures on the low gradient slope in the Pearl River canyon's middle segment. Our results showed that slope canyon types, variations of sediment supply and the regional tectonic evolution exerted important controls on the generation and evolution of these MTDs. The eastern slope canyons with little slope fans have a steeper slope at the canyon mouths, resulting the northeastern MTDs to be prone to retrograde landwards. The change of shelf-channel system from un-incised type to incised type increased the sediment delivery efficiency, resulting in larger scale MTDs_1 in the Pearl River canyon. While the subsequent decrease in the number of incised shelf channels cut down the sediment delivery into the deep-water and reduced the younger MTDs' scale as well. The variation of sediment supply had a greater impact on the development of slope-attached MTDs. The rapid subsidence of Baiyun Sag and tectonic activities of Dongsha Rise during the Quaternary favored the occurrence of the submarine landslides on low gradient slopes in the study area. However, the weakening of the tectonic activities resulted the Quaternary MTDs' scale to be decreased upwards, especially for the southern slope-detached MTDs. The complex topography formed by the slope-attached MTDs has a greater potential to capture the subsequent turbidity currents from the shelf-edge deltas, bringing the generation of potential good turbidite reservoirs in the deep-water region. (C) 2017 Elsevier Ltd. All rights reserved

Oceanographic consequences of the Bransfield Strait (Antarctica) opening

The Bransfield Strait (Antarctica) is an important region for evaluating changes in Weddell Sea shelf waters on geological time scales because of its restricted connections to the surrounding ocean. However, the detailed oceanographic consequences of the opening of the strait remain unclear. We present bottom-current-related sedimentary features in the Bransfield Strait and examine the impact of the strait’s opening on deep-water circulation. Our findings show that the ocean circulation started to resemble that of the present day after a period of volcanic activity, possibly around the Middle Pleistocene. Coeval changes in Bransfield Strait morphology and an increase in seafloor irregularities due to the formation of volcanic chains finally determined new pathways for the Bransfield deep and bottom waters, enhanced due to the new climatic scenario of 100 k.y. cycles. The fact that “modernlike” oceanic circulation occurred only during previous interglacial periods demonstrates the significant impact of 100 k.y. climate cycles on the thermohaline changes of Antarctic deep waters. Hence, establishing a modern-day circulation model would enable researchers to assess paleoproductivity and local upwelling that have profoundly influenced the marine ecosystem of the Antarctic Peninsula after the Middle Pleistocene3,61