Displays

TS6.2

The West Pacific regime is dominated by a convergent plate setting, but develops two thirds of the world’s marginal basins which have different histories and causes. Some are built on continental crust and some formed by seafloor spreading. Some began to form in Mesozoic time and others began in Cenozoic time. Many are filled with sediments and volcanics and some of these contain hydrocarbon deposits. Some are no longer actively extending but others are still tectonically active and pose hazards to nearby coastal communities. The purpose of this session is to present our modern understanding of these marginal basins, how they formed, how they subsided, how they were filled, how they died, and the economic benefits and potential hazards they present.
In this session, we welcome all contributions that deal with marginal basins in the West Pacific and/or try to answer to the questions related to the evolution of marginal basins in convergent plate settings. We particularly encourage multi-disciplinary studies that address the issues of inheritance on the rifting process, the discuss modes of breakup, the role of magmatism in lithospheric breakup and the contribution of sedimentation and source to sink processes in marginal margins.

Public information:
(1) For attenders, you are encrouraged to download and read through the related present materials before the session, and prepare your comments and questions (in text) in advance to avoid delays;
(2) For presenters, please give a short summary of your research, and be prepared to answer questions. It will be better to have some of the answers on your conclusions and methods typed in advance.
(3) Provide your contacts to audience, thus the discussion could continue after the session.

Share:
Co-organized by GD7
Convener: Weiwei Ding | Co-conveners: Dieter Franke, Jiabiao Li, Gianreto Manatschal, Zhen SUN
Displays
| Tue, 05 May, 08:30–10:15 (CEST)

Files for download

Session materials Download all presentations (79MB)

Chat time: Tuesday, 5 May 2020, 08:30–10:15

D1347 |
EGU2020-6456
| solicited
| Highlight
Western Pacific marginal basin plate tectonics: overview, questions, and insights from mantle structure
(withdrawn)
Jonny Wu and John Suppe
D1348 |
EGU2020-1600
| Highlight
Zhi Wang and Jian Wang

Arc magmatism and megathrust earthquake occurrence in a subduction zone are deemed to attribute to many factors, including structural heterogeneities, fluid contents, temperature, depth of subducting oceanic crust, and etc. However, how these factors affect them is unclear. The extensive arc magmatism observed on the island arcs is considered to be an indicator on chemical exchange between the wedge mantle and the surface in a subduction zone. Megathrust earthquake frequently occurrence is also considered to be attributed to the slab melting and associated interplate coupling of the subducting plate. The Western Pacific subduction zone is regarded as one of the best Laboratory for seismologists to examine these processes due to the densest seismic networks recording numerous earthquakes. Some of the previous studies suggest that the island-arc magmatism is mainly contributed to the melting of peridotite in the mantle wedge due to fluids intrusion from the dehydration process associated with the subducting oceanic crust. They further argued that the oceanic plate could only provide water to the overlying mantle wedge for arc magmatism, but not melt itself due to that it is too cold to melt at a depth between 100 and 200km. However, some of other studies revealed that the hydrated basalt derived from the mid-ocean ridge will be melted with high T and water saturated on the upper interface of the sbuducting plate in the mantle wedge. We consider that the three-dimensional (3-D) P- and S- wave velocity (Vp, Vs) and Poisson’s ratio (σ) structures of the subduction zone, synthesized from a large-quantity of high-quality direct P-, and S-wave arrival times of source-recive pairs from the well located suboceanic events with sP depth phase data could provide a compelling evidence for structural heterogeneity, highly hydrated and serpentinized forearc mantle and dehydrated fluids in the subduction zones. In this study, we combined seismic velocities and Poisson’s ratio images under the entire-arc region of the Western Pacific subduction zone to reveal their effects on megathrust earthquake generation and arc magmatism. We find that a ~10 km-thick low-velocity layer with high-V and high-Poisson’s ratio anomalies is clearly imaged along the upper interface of the subducting Pacific slab. This distinct layer implies partial melting of the oceanic crust due to the deep-seated metamorophic reactions depending on the source of fluids and temperature regime. Such a process could refertilize the overlying mantle wedge and enrich the peridotite sources of basalts under the island arc. Significant low-V and high-Poisson’s ratio anomalies were observed in the mantle wedge along the volcanic front, indicating melting or partial melting of peridotite-rich mantle and then yield tholeiitic magma there. The present study demonstrates that the combined factors of fluid content, mineral composition and thermal regime play a crucial role in both slab melting and arc-magmatism under the Western Pacific subduction zone.

D1349 |
EGU2020-1404
| Highlight
Florian Schmid, Heidrun Kopp, Michael Schnabel, Anke Dannowski, Ingo Heyde, Michael Riedel, Martin Engels, Anouk Beniest, Ingo Klaucke, Nico Augustin, Philipp Brandl, Colin Devey, and Mark Hannington

The northeastern Lau Basin is one of the fastest opening and magmatically most active back-arc regions on Earth. Although the current pattern of plate boundaries and motions in this complex mosaic of microplates is fairly well understood, the structure and evolution of the back-arc crust are not. We present refraction seismic, multichannel seismic and gravity data from a 300 km long east-west oriented transect crossing the Niuafo’ou Microplate (back-arc), the Fonualei Rift and Spreading Centre (FRSC) and the Tofua Volcanic Arc at 17°20’S. Our P wave tomography model shows strong lateral variations in the thickness and velocity-depth distribution of the crust. The thinnest crust is present in the Fonualei Rift and Spreading Center, suggesting active seafloor spreading there. In the much thicker crust of the volcanic arc we identify a region of anomalously low velocities, indicative of partial melts. Surprisingly, the melt reservoir is located at ~17 km distance to the volcanic front, supporting the hypothesis that melts are deviated from the volcanic arc towards the FRSC in sub-crustal domains. We identify two distinct regions in the back-arc crust, representing different opening phases of the northeastern Lau Basin. During initial extension, likely dominated by rifting, crust of generally lower upper-crustal velocities formed. During an advanced opening phase, likely dominated by seafloor spreading, crust of higher upper-crustal velocities formed and is now up to 11 km thick. This thickening is the result of magmatic underplating, which is supported by elevated upper mantle temperatures in this region.

D1350 |
EGU2020-20752
| solicited
| Highlight
Guoliang Zhang, Ji Zhang, and Shuai Wang

The Caroline Rise has played an important role in the tectonic frame of the western Pacific, however, the nature and origin of the Caroline Rise has long been unclear. The boundary between the Pacific plate and the Caroline plate has long been unclear, thus, it unclear which plate is underneath the Caroline Rise. In this study, we confirmed that the Caroline Rise represents an oceanic plateau formed as a large igneous province based on seafloor sampling. In this study, we have age-dated and analyzed the whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the basalt samples from the Caroline Plateau. The basalt samples are classified into two groups, the alkali group and the tholeiite group. The results of age-dating indicate older ages for the tholeiite group than the alkali group. The tholeiite group basalts are apparently older than the Caroline Islands and are close to the basalts of Ontong Java Plateau in trace element compositions. We suggest that the tholeiite group basalts represent the main stage volcanism and the alkali group basalts represent the late stage volcanism of the Caroline Plateau. The alkali group basalts show trace element and isotope compositions similar to those of the Caroline Islands to the east. The tholeiitie group basalts have involved significant amount of depleted asthenosphere components, which suggests interactions of the Caroline plume with the Caroline basin spreading center. The MORB-like depleted geochemical nature of the Caroline tholeiite group basalts indicates formation of the Caroline Plateau under the young and thin Caroline plate lithosphere. Our results of age and geochemistry of the Caroline Plateau/Seamount system could be explained by the activities of the Caroline hotspot. This work was financially supported by the National Natural Science Foundation of China (91858206, 41876040).

D1351 |
EGU2020-6525
| Highlight
Liyan Tian, Si-Yu Hu, and Xuan-Ce Wang

The circulation of seawater within the oceanic crust promotes the extensive chemical variations of the lithosphere prior to its entering subduction zones as well as the development of the biosphere. A good understanding of the chemical variations during hydrothermal circulation is essential to further decipher the biological activities in such extreme environments. Epidote is a common byproduct, but a good indicator for hydrothermal activities during the hydrothermal alteration of oceanic crust.

This study presents the petrographic and geochemical features of epidote from depth of 850-910 m (below the surface) in the northern South China Sea margin to provide insights into the possible chemical variations in hydrothermal systems in subsurface. Eight samples with obvious epidote veins were chosen from the altered basalts in Hole 1502B of IODP Expedition 368. They cover a range with different depth and occurrences, including epidote veins, composite epidote-calcite veins, and composite epidote-silica veins. Sulfide mineralization is widespread and dominated with pyrite, chalcopyrite and sphalerite. Scanning Electron Microscopy images show that the epidote-calcite vein samples display obvious zonation structure in epidote, and the others not. The major element concentrations of Fe also show variations with epidote zonation. We further carried out in situ trace element concentration measurement on epidote minerals by Laser Ablation-Induced Coupled Plasma-Mass Spectrometry. In Chondrite-normalized diagrams, all epidote mineral samples show flat patterns with significant positive Eu anomalies, which may relate to highly oxidized conditions maximising Eu3+ incorporation. We therefore propose that the zonation of epidote may reflect the pulse of hydrothermal activities, one of which is likely to be associated with the precipitation of chalcopyrite and sphalerite.

D1352 |
EGU2020-3557
Philipp Brandl, Anna Kraetschell, Justin Emberley, Mark Hannington, Margaret Stewart, Sven Petersen, and Alan Baxter

The offshore regions of Eastern Papua New Guinea and the Solomon Islands include several active and remnant arc and backarc systems that formed in response to complex plate tectonic adjustments following subduction initiation in the Eocene. Although there has been extensive exploration for offshore petroleum resources, and more than 54 research cruises have investigated or transited the region since 1993, a comprehensive regional geological map, including the deep marine areas, has not been available at a scale that permits quantitative analysis of the basin history. We present the first map that depicts interpreted assemblage- and formation-level lithostratigraphic units correlated across the marine basins and adjacent land masses. The mapped assemblages and large-scale formations are based on a compilation of land-based geological maps, marine geophysical data (hydroacoustics, magnetics, and gravity) integrated with the results of geological sampling, ocean drilling, seismic surveys, and seabed observations.

More than 400,000 km2 of the map area covered by ship-based multibeam and other geophysical data were inspected to derive the offshore geological units. In areas with limited data, the units were extrapolated from well-documented formations in adjacent regions with more complete information, including on land. This approach follows closely the techniques used for remote predictive mapping in other regions of the Earth where geological information is sparse. Geological boundaries were constrained by ship-based multibeam data reprocessed at 35-m to 50-m resolution and integrated with the Global Multi-Resolution Topography (GMRT) gridded at 100 m. Lithotectonic assemblages were assigned on the basis of plate structure, crustal type and thickness, age, composition, and sedimentary cover and further refined by bathymetric and geophysical data from the literature and cruise reports. The final compilation is generalized and presented here at 1:1 М. Our new approach integrates conventional mapping on land with remote predictive mapping of the ocean floor.

The newly compiled geological map illustrates the diversity of assemblages in the region and its complex geodynamic evolution. The resolution of our map allows to perform quantitative analyses of area-age relationships and thus crustal growth. Further geoscientific analyses may allow to estimate the regional mineral potential and to delineate permissive areas as future exploration targets.

D1353 |
EGU2020-6710
Cuimei Zhang, Xiong Pang, Ming Su, Jinyun Zheng, Hongbo Li, Yale Gu, Jiangyang Zhang, and Yanghui Zhao

The interaction between magmatic and extensional processes related to the formation of rifted margins has been and still is highly debated. The interpretation of magmatic additions, timing of onset and budget of magma during rifting and lithospheric breakup remain controversial and poorly understood. In contrast, the emplacement of magmatic additions in rift systems with high sedimentation rates provides an exceptional perspective towards resolving some of these problems.

In this paper, we present two new high-resolution seismic profiles imaging the complete transition from the hyperextended crust to oceanic crust in the northern South China Sea (SCS). Based on the observation of magma-related structures and the interrelationship with the sedimentary sequence, we define forms and timing of magmatic additions. We show that magmatic activity initiated during necking and then propagated together with the seaward formation of “new” basement , as indicated by the occurrence of sills and laccoliths during hyperextension, and ENE striking cone-shaped volcanos during the final breakup stage before the establishment of an embryonic and then steady-state oceanic crust.

First order estimations of the magmatic budget in order to decipher the magmatic evolution show that it strikingly increased during final hyperextension and the breakup stage and lasted until 23.8 Ma. Thus, magmatic activity continued even after cessation of rifting. This study enables for the first time to provide a semi-quantitative estimate of when, where and how much magma formed during final rifting and breakup at a magma-intermediate margin.

D1354 |
EGU2020-1553
Jianye Ren, Chao Lei, and Junxia Zhang

Bohai Bay, East China Sea and South China Sea are three of the largest-scale Cenozoic petroleum-rich sedimentary basins along the Chinese continental margin. For the past decades, the wealth of geological and geophysical data was acquired by the petroleum industries, which provide an opportunity to have a synthetic study on these basins.

(1) Structure and stratigraphic framework for the Cenozoic basins in the Bohai Bay, the East China Sea and the South China Sea are revealed to be different. The Bohai Bay basin was imaged to be a pull-apart basin, through which a regional-scale strike-slip fault went. The South China Sea was controlled by extension, which generated a serial of deepwater basins on the hyper-extended crust adjacent to the oceanic crust, most of which was controlled by the detachment faults. Between the Bohai Bay basin and East China Sea is the East China Sea, at the deep level of which a serial of thrust faults occurred. It indicated the regional compression from the pacific plate toward the East China.

(2) Based on the different structure and stratigraphic sequence in the basins along the Chinese continental margin, the basins evolutions were reconstructed. In Late Paleocene to Middle Eocene, distributed faulting occurred along the Chinese continental margin. Subsequently, in Late Eocene the evolution of these three basins were observed to be different. The Bohai Bay Basin was strongly influenced by the oblique strike-slip faulting, and lasted to the latest Late Oligocene, followed by the thermal subsidence in Miocene and a pulse of acceleration subsidence since Pliocene. In contrast to Bohai Bay basin, the continental shelf basin of the East China Sea experienced a long-time compression in the context of back-arc setting, and subsequently has a regional subsidence since Early Pliocene. The continental crust of the South China Sea was thinned since Late Eocene and eventually broke apart in Oligocene to form oceanic crust, where detachment faults bounded a serial of deepwater basins.

The different in basin structures and evolutions since Late Eocene was consistent with the event of plate organization in the western Pacific at that time. Before the event, Chinese continental margin was influenced by the interaction of Eurasian and Pacific plates, e.g. double-plate system. The subduction and related retreat of Pacific plate led to the back-arc extension of the Chinese continental margin, generating widely distributed grabens and half grabens filled with sediments. After this event, the Chinese continental margin was deformed by the interaction between India, Eurasian, Pacific and Philippine Sea plates, e.g. multi-plate system. In this context, several dynamic forces affected the evolution of the Chinese continental margins was observed, e.g. the collision between India and Eurasia, the change of the subduction direction of the Pacific plate, the subduction collision of the proto-South China Sea, the northward movement of the Philippine Sea plate. These complex plate reorganizations lead to the different genetic type of basins in Chinese continental margin.

D1355 |
EGU2020-5610
Zhen Sun, Fucheng Li, Ning Qiu, and Longtao Sun

The South China Sea (SCS) is one of the largest marginal seas in the western Pacific. Widespread onshore outcrops of the late Mesozoic granitic and volcanic rocks suggest that the SCS was once an active margin associated with Paleo-Pacific or proto South China Sea subducted toward South China in the late Mesozoic. It transitioned into rifting after late Cretaceous and then spreading in Oligocene. IODP drilling indicates that SCS transitioned from subduction into sea spreading in a short time (no more than 30 Myrs) and show diachronous breakup both temporally and spatially. What controlled this tectonic process? In order to answer these questions, we used a combination of data sources, including reprocessed magnetic data, drilling/dredging samples, depositional environment and deformation style on multi-channel seismic profiles to constrain the possible spatial distribution of the Mesozoic volcanic arc first and found that the southwest part of the Mesozoic volcanic arc distributes on both sides of the southwest SCS sub-basin, while the northeast part remains nearly in its original location. The results suggest that the initial breakup sites for the SCS margin might be arc area in the southwest and fore-arc area in the northeast during the opening of SCS basin. Mathematical modeling experiments suggest that several circumstances may cause fore-arc breakup, including: steepening of the subducting plate, a pre-existing rheologically/tectonically weak zone in the arc-front/fore-arc in the subduction plate, seamount or plateau subduction and damaging of the fore-arc area. Also if the subducting slab is young or the subduction time is short, fore-arc breakup will occur. Further analysis suggested that along with the rifting, two sources of magma may have contributed to the rifting. One is supposed to be decompressive melting, the other one is deep sourced and constitute the high velocity lower crustal magmatic underplating, which is supposed to be related with the subduction slab break off.

D1356 |
EGU2020-6436
Fei Wang and Weiwei Ding

South China Sea (SCS) is not only the crucial pathway for transporting terrigenous materials from Eurasia to the Western Pacific Ocean since the early Oligocene, but also the dominant accumulation and preservation place as a result of limited material exchange between the semi-closed oceanic basin and the open ocean since the middle Miocene. Diverse factors, including global climate changes, eustatic sea level change, regional and local tectonic events, et al., controlled the sedimentary dispersal and accumulational patterns in the oceanic basin of the SCS, which can be revealed by the calculation of sediment budget at different geological times, as the sediment budget can illustrate directly the sediment influx, storage, loss in a basin system (Hapke et.al, 2010).

By interpreting the multichannel seismic profiles covering the whole oceanic basin with constraints from International Ocean Discovery Program (IODP) Expeditions 349, 367 and 368, we reconstructed the sequence stratigraphy framework of the study area, and then calculated the sedimentary budget at different geological time. This work aims to quantitate the sedimentary dispersal and accumulation in the oceanic basin for the first time.

Until now we have completed the sequence boundary identification and dating, as well as the division of sedimentary units of all multichannel seismic profiles. The grid data of different sequence boundaries have been obtained and posted on the bathymetric map, and by the time-depth conversion with appropriate function in different region referred from the drilling results of IODP expeditions, we have figured out the thickness of each sedimentary unit. In the following we will do the decompaction correction before calculating the sedimentary budget of the whole oceanic basin at different times. This work could increase our understandings on the major controlling factors and possible material sources of the deposition process.

 

D1357 |
EGU2020-2273
Yanghui Zhao, Weiwei Ding, Jianye Ren, Jiabiao Li, Dianjun Tong, and Jiangyang Zhang

It has been widely reported that extension of the upper crust measured from faulting is far less than the lower crustal thinning at rifted continental margins. This phenomenon is referred to as “extension discrepancy”. However, recent studies found out not all rifted margins had experienced a crustal thinning increasing with depth. Here, we use observations from 3D seismic reflection data that cover the Baiyun Rift, to explore the extension discrepancy between the brittle extension and crustal thinning when the crust of the Northern South China Sea margin thinned from 30km to <12km. To achieve this, first, we restored the rift system of the Baiyun Rift in the absence of post-rift sediments and water loading. Subsequently, we applied alternative methods based on the fault geometries and the crustal thickness ratios to compare the deformation of the brittle crust and the whole crust. Results show (1) the upper crustal faulting was sufficient to explain the whole crust thinning in the basin center, indicating no extension discrepancy; (2) near the rift flanks, the upper crustal faulting is greater than the whole crustal thinning, indicating inverse discrepancy. In the northeast of the Baiyun Rift where detachment faulting occurred, magmas passively upwelled and thickened the crust due to isostasy. Consequently, the lower crust was exhumed locally during the detachment faulting. These results indicate the hyper-thinning process of the continental crust in the Northern South China Sea was substantially dominated by tectonic extension rather than thermal thinning.

D1358 |
EGU2020-9137
Pan Luo, Jianye Ren, Xi He, Chao Lei, Junjie Xu, Gianreto Manatschal, Nick Kusznir, and Peng Chao

Our study focuses on the Zhongjianna (ZJN) (Phu Kham) Basin, located at the western termination of the South China Sea (SCS) and separated from the Indochina continent by the N-S striking East Vietnam Boundary Fault Zone, which is a large scale strike-slip fault system. The sedimentary infill history of the ZJN basin records the complete evolution and interaction of the Indochina-SCS system and allows the tectonic and kinematic evolution of the basin to be understood.. The discovery of hyper-extended continental crust and mantle exhumation in this basin leads to the question of what is the relative role of large-scale strike-slip and orthogonal faulting in controlling crustal thinning in the ZJN basin.  

  Our preliminary results confirm the existence of hyperextended continental crust flooring the ZJN basin. Two different types of structures can be identified in this area: extension related deformation in the eastern part and strike-slip related deformation in the western part. The analysis of fault geometries and kinematics linked to timing and subsidence rates suggest that the N-S-orientated strike-slip structures dominated the continental shelf and slope area on the west side of the basin. In the basin, however, most faults strike NE-SW and are parallel to the mid-ocean ridge. Thus, it appears that the ZJN basin resulted from the partitioning between strike-slip and orthogonal extension.

In our presentation we show the results of our seismic interpretation, strain and subsidence analysis and discuss the interaction between strike-slip and orthogonal extension in setting up the hyper-extended ZJN basin and its implications for the large scale tectonic and geodynamic framework.

D1359 |
EGU2020-8920
Peng Chao, Gianreto Manatschal, Nick Kusznir, Pauline Chenin, Jianye Ren, and Xiong Pang

In the last two decades, knowledge of the South China Sea (SCS) rifted margins has significantly evolved. However, there are still many open questions related to when, how, and under what conditions major crustal thinning (necking) and lithospheric breakup occurred and how these processes are recorded in the stratigraphic and magmatic tape recorder. In this study, we aim to explore the tectono-sedimentary-magmatic evolution of rift systems during crustal thinning and lithospheric breakup. Our study is based on observation of conjugate margins architecture along high resolution long offset reflection seismic sections through the Northwest SCS. We focus on crustal thinning and lithospheric breakup, and the transition to first oceanic crust and the birth of an oceanic spreading centre. We describe the Northwest SCS crustal architecture, define extensional domains (proximal, necking, hyper-extended, OCT, oceanic domain) and margin architecture (upper and lower plate). We determine the tectono-sedimentary evolution and discuss the evolution of deformation modes through time and space by linking the tectono-stratigraphic-magma evolution with the observed crustal thinning. These results have important implications for understanding the deformation history and processes in time and space, and enable the analysis and linkage of the tectono-stratigraphic evolution of rift systems with the observed crustal thinning and breakup processes.

D1360 |
EGU2020-1727
Limei Tang and Ling Chen

We present new major and trace element chemistry and Sr, Nd, and Pb isotope data from basalts, recovered from the forearc setting of the Yap Trench in the western Pacific, and discuss their melt evolution and petrogenesis within the framework of the geodynamic interactions among the Caroline Plate, the Caroline ridge, and the Philippine Sea plate. These rocks have mid-ocean ridge basalt (MORB)-like geochemical features, including medium Fe contents, tholeiitic affinity, high TiO2 values at a given Fe2O3/MgO ratio, Ti/V, Nb/Y, Ba/Yb, and Ba/Th ratios similar to those of back-arc basin basalts (BABB), and trace element patterns commonly displayed by MORB and BABB lavas. However, these basalts are characterized by highly radiogenic Sr and Pb contents, reminiscent of western Pacific sediments. We suggest that forearc magmatism was responsible for the origin and petrogenesis of these rocks. Forearc magmatism was induced by the shrinking of the Philippine Sea plate, which squeezed out the underlying back-arc basin asthenosphere with Indian–type ambient mantle characteristics to invade the forearc mantle of the Yap Trench and causes lithospheric extension. Upwelling and decompression melting of this mantle produced MORB-like lavas in the narrow forearc setting. An apparent slab tear or gap in the subducting plate facilitate the penetration of the mantle outflow. The collision of the Caroline Ridge subducted more sediments into the mantle wedge. Melting of the subducted sediments and the invasion of the Indian-type asthenosphere into the forearc account for the highly radioactive Sr and Pb isotopes of the MORB-like lavas.

D1361 |
EGU2020-4396
| Highlight
Xin Shan, Xuefa Shi, and Shuqing Qiao

Hybrid event beds and a debrite are identified in a core on the mid-shelf of East China Sea. Four units are divided according to abrupt boundary identification, with assistance of grain size analysis. The hybrid event beds typically comprise four internal divisions from the base to the top: (1) structureless muddy sand (H1a, high density turbidite); (2) massive muddy sand with mud clasts (H1b, higher density turbidite); (3) linked debrite (H3); (4) homogeneous mud (H5, fluid mud). The radiocarbon ages of the core were in the range of 3890–8526 yr BP. Based on correlation with other surrounding cores, the depositional age of hybrid event beds and the debrite may be less than 500 yr BP. The TOC and δ13C values in event beds suggest a local erosional regime. The average δ13C value for turbidite (H1a and H1b) is similar to the H3 division in the hybrid event beds, implying that the organic matter in the H1a, H1b and H3 may come from the same source area. The REE data reveals the sediment source is initially from Korean rivers. Bi-plots of (La/Lu)UCC vs. (La/Y)UCC, (La/Y)UCC vs. (Gd/Lu)UCC, (La/Yb)UCC vs. (Gd/Yb)UCC and (La/Yb)UCC vs. (Sm/Nd)UCC of four units in the core are concentrated in the similar range, indicating these event beds have the same source area. Both regimes that partial transformation from a debris flow and erosional bulking are suggested. It is unlikely that the debris flow is triggered by a hyperpycnal flow or a tsunami, because both can carry continental and/or coastal signals which have not been recognized in the core. Typhoon can be a probable triggering mechanism.

 

Acknowledgements

This paper was supported by National Program on Global Change and Air-Sea Interaction (Grant No. GASI-GEOGE-03), National Natural Science Foundation of China (Grants No U1606401 and No. 41706063) and the Taishan Scholar Program of Shandong.

D1362 |
EGU2020-4608
Minghui Zhao, Jean-Claude Sibuet, Jonny Wu, Longtao Sun, and Jiazheng Zhang

The Huatung basin (HB), located between the Philippine Sea plate (PSP) and the South China Sea (SCS), has likely existed near tectonically-active plate boundaries since the early Cenozoic. It may record SCS evolution from the SCS rifting phase to today, and is a key region to understand the broad geodynamic interactions between the SCS and PSP. A left-lateral shear plate boundary between the SCS and PSP followed the Gagua ridge and was active before 56 Ma. A slight compressive component along the Gagua ridge might have occurred from 40 to 30 Ma, giving rise to the topographic uplift of Gagua ridge and adjacent ridges with possibly some underthrusting of the PSP below the HB. A significant compressive episode also occurred along a second fracture zone around 23 Ma ago. The Manila trench inception occurred along the PSP-SCS plate boundary before the end of SCS spreading, involving the subduction of the younger SCS beneath the older HB. Later the intra-oceanic Luzon arc formed and collided in a sub-parallel fashion with the Eurasian continent around 5-6 Ma ago to form Taiwan. The PSP/EU motion was oblique with respect to this plate boundary during SCS opening. However, we have no direct evidence of the HB age (early Cenozoic or early Cretaceous) and if the PSP underthrusted below the HB. We propose to carry a deep seismic refraction survey and dredge sampling of basement units to clarify this problem. This work is supported by the Chinese National Natural Science Foundation (contracts 91958212, 41730532, 41576070 and 41676043).

D1363 |
EGU2020-6264
Bin Wu, Xiaodan Wu, Xuefa Shi, Shuqing Qiao, Shengfa Liu, Limin Hu, Jihua Liu, Yazhi Bai, Aimei Zhu, Narumol Kornkanitnan, and Somkiat Khokiattiwong

The seasonal reversal of monsoon climatology modulates precipitation, currents, river influx and a variety of biogeochemical processes. In the present study, we evaluated the role of tropical monsoon pertaining to fluvial discharge, sediment load, coastal current and water stratification on seasonal organic carbon dynamics during four sampling campaigns in the Upper Gulf of Thailand (UGoT), SE Asia. This study demonstrates that particulate organic carbon (POC) is closely correlated with the river influx of suspended sediment, which is generally regulated by the local rainfall. Higher POC is found near the large estuarine section (Chao Phraya River, CHAO) during southwest monsoon period and the small estuarine section (Mae Klong River, MK) during the tropical cyclones impacted November 2013. POC in the estuarine sections is influenced more by the seasonal shift than the coastal sections. Land-derived organic matter prevails in the small estuarine and coastal sections, while marine-derived organic matter dominates in the CHAO and MK impacted estuarine sections. Total organic carbon (TOC) however displays less significant seasonal monsoon variations than POC. Further, TOC tends to accumulate in the sub-silt fraction of sediments, which mainly occurs in the small estuarine and eastern coastal sections and is obviously influenced more by marine-derived factors. TOC in surface sediment of the CHAO and MK influenced sections however displays more seasonal variations with prevailing river input as evidenced by coarser sediment and higher C/N ratios. Moreover, the almost year round water stratification across the region acts as the barrier in retaining organic carbon in the estuaries and their vicinities from dispersal into the lower portion of Gulf of Thailand. High sedimentation rate (~1.1 cm·yr-1) further facilitates the organic carbon burial in the study area. The delivery, dispersal and burial of organic carbon are closely associated with the climate controlled precipitation, and thus the tropical monsoon climatology under the global warming in particular is an important factor influencing the organic carbon in the UGoT.

Acknowledgements

This study was supported by National Programme on Global Change and Air-Sea Interaction (GASI-02-IND-CJ05, GASI-GEOGE-03), the Natural Science Foundation of China (U1606401), the Qingdao National Laboratory for Marine Science and Technology (2016ASKJ13), the China-Thailand cooperation project “Research on Vulnerability of Coastal Zones”, and the Taishan Scholar Program of Shandong.

D1364 |
EGU2020-8782
Shengfa Liu, Xuefa Shi, Xisheng Fang, Yanguang Dou, Yanguang Liu, and Xuchen Wang

We present a paleoclimatic reconstruction for the Holocene by clay mineral analyses of sediments from core MZ02 retrieved from the mud area of the inner continental shelf of the East China Sea (ECS). The clay minerals mainly consist of illite (66%-79%) and chlorite (12%-19%), with minor kaolinite (7%-13%) and smectite (0-6%). Provenance analysis suggests that the illite-dominated clay minerals were derived mainly from the detrital outputs of the Changjiang, Minjiang, and small rivers from Taiwan Island. Our study indicates that the sea level rise since the last glacial, the strength of the Taiwan Warm Current (TWC) and Chinese Coastal Current (CCC) have controlled the dispersal and deposition of clay minerals on the ECS, that in turn determined the clay mineral compositions in the core sediments. During 13,000-9500 BP, due to the lower sea level and shorter distance between these three estuaries and core MZ02, fine sediments on the inner shelf of the ECS were primarily supplied by mixed provenances from the Changjiang, Taiwanese, and Minjiang rivers. During the early Holocene (9500-6200 BP), stronger sediment reworking and erosion at the shelf edge was responsible for the increased lateral transport of fine sediments in the ECS, which lead to a dominance of the sediment source from the Changjiang, while the Taiwanese and Minjiang rivers only provided minor components of detrital sediment to the shelf. Increased strength of TWC might have played an important role in the sediment dispersal and deposition on the inner shelf of the ECS during 6200-2400 BP, with a dominance of more than 60% sediments transported from Taiwanese rivers. Furthermore, our study implies that the Asian monsoon and the weakening of TWC were linked to the abrupt increase of Changjiang and Minjiang derived terrigenous detritus materials since 2400 BP.

 

Acknowledgments

This work was supported by National Nature Science Foundation of China (No.41106063), Science and Technology Basic Special Program of China (No.2008FY220300), Marine Public Welfare Research Project of China (No.200805063), China Postdoctoral Science Foundation (No.20100481304) and Coastal Investigation and Research Project of China (No. 908-01-CJ12).

D1365 |
EGU2020-8919
Qiao Shuqing, Shi Xuefa, Yonggui Yu, Limin Hu, Lin Zhou, Guangbo Ren, Gang Yang, Zhengquan Yao, and Naishuang Bi

The fluvial sediment to the sea is the base of coastal geomorphology and biogeochemical processes, and its transport is an important pathway to the global biogeochemical cycle. The Yellow River is one of globally well-known large rivers because of high sediment load and Chinese Mother River. Its channel shifts frequently because of high sediment load and steep river-channel gradient in the lower reaches . The terminal channel has shifted more than 50 times since 1855 and the last two changes in 1976 and 1996. Furthermore, Yellow River Conservancy Commission has began to implement Water-Sediment Regulation Scheme (WSRS) since 2002, to increase the main channel discharge capacity and to reduce deposition in the reservoirs and river channel. Surface sediment, multi-core and gravity sediment cores, remote sensing images and bathymetric data near the Yellow River delta were collected to study the impact of WSRS and river terminal change together with the water and sediment discharge at the gauging station. Especially, 7Be, 210Pb and 137Cs, grain size, sediment color and TOC/TN was measured to show sedimentary record of WSRS and channel shift on inter-and intra-annual time scale. The results show that the fresh sediment from Yellow River  during 2014 WSRS period can be transported eastward more than 80 km off the rivermouth, while cannot pass 38° easily. Meanwhile the sediment can penetrate as deep as 12 cm. The subaerial delta area is mostly stable after 2002, and its balance is mainly controlled by the surrounding artificial coastline. The subaqueous delta changed from trapping about 4.6×108 t to being eroded ~ 3.1×108 t and 1.1×108 t each year during the three stages of 1976-1996, 1996-2002 and 2002-2014. It is proposed that the subaerial delta area will change little except for the Q8 outlet area, while the subaqueous delta evolution mostly depend on the Huanghe material besides the hydrodynamic conditions. In addition, the aim of WSRS to scour the lower riverbed will recede in future. This study deepens our understanding of the fluvial sediment disperse pattern and sedimentation under the influence of human activities and hydrodynamic conditions.

Acknowledgements
This study was supported by National Programme on Global Change and Air-Sea Interaction (GASI-GEOGE-03) and the Natural Science Foundation of China (U1606401).
D1366 |
EGU2020-9119
wu zhaocai

In order to grasp the Cenozoic extension and tectonic deformation characteristics of the crust in the southern of the South China Sea, a newly acquired multi-channel seismic profile (DZ02) acrossing the conjugate continental margin of the Southwest Subbasin, Nansha block, and Nansha Trough is explained. Four stratigraphic units (syn-rift unit, drift unit1, drift unit2 and post-rift unit) were determined with six sequence boundaries (Tg, T70, T60, T40, T20, T10). Based on the differences in tectonic units and the features of stratigraphic and structural in the southern of the South China Sea, it is divided into five structural belts from northwest to southeast, which are the northern continental margin extension zone, the Southwest Subbasin, the Nansha intracontinental extension zone, the Nansha forebulge zone, and the Nansha trough. The fault derived and whole crustal extension factors of the Nansha block are also calculated. The results show that in time, the Nansha block has undergone two phases of extension, namely the syn-rift period and the seafloor spreading period. The syn-rifting stage accounted for about 69% of the total extension, and the seafloor spreading stage of the South China Sea accounted for about 26%. In space, the whole crust extension factor is greater than the fault derived extension factor in most areas. By comparing with the multi-channel seismic profile of the eastern part of the Nansha block imply that the crustal extension process is synchronous, but the extent of the extension in the western of Nansha is always greater.

D1367 |
EGU2020-13579
Ming Su, Zhixuan Lin, Ce Wang, Hui Chen, Shan Liu, and Kunwen Luo

Late Miocene-to-present sedimentary succession, consisting of deep-water channels, submarine canyons, shelf-edge delta and clinoform, mass transport complex (MTC), turbidites, pelagic sediments and so on, has accumulated in the Qiongdongnan Basin (QDNB) and supplied from several sediment potential provenances, including surrounding tectonic uplifts and drainage system (e.g. Hainan island, Red River system and Shenhu Uplift). This multiple sediment source system with a variety of space-time distribution is more likely to result in different transported pathways to connect with accumulated zone of the QDNB. To investigate spatial and temporal variation of sediment transported patterns since the late Miocene, the primary dataset in this study is high-resolution 2D seismic profiles that are used to interpret several types of sedimentary features and to determine the distribution within the basin. Integrated analysis of core samples and well log data summarized from previous studies is allow for acquiring high-resolution vertical information about physical and chemical properties of different types of sedimentary features. Depended on characterization and spatial distribution of depositional models, the sediment delivered pattern could be classified into three major types. (1) the downslope transports suggest that sediments were transported by gravity flows and slope failures from high topographic areas to deposit at the basinfloor, and basinward prograding deposition at the shelf or tectonic uplifts, channels/canyons developed along the slope and submarine fans formed at the lower slope are the products of downslope transports shown in the SE-trending seismic profiles; (2) the canyon-axial transports are associated with geomorphology of the Central Canyon System (CCS) across the QDNB from SWW to NEE. Abundant sediments originated at the Red River system were supplied from the west, resulting in dominantly onlap-filling turbidites with a series of erosional discordance within the head area and western segment of the CCS; (3) the combined transport is a mixture of downslope and canyon-axial sediment transports. A large volume of MTDs source from the Hainan Island in the north was transported southward and impeded by the Southern Uplift, so they tended to widen the canyon and continuously deliver eastward along CCS. These three types of sediment transported patterns since the late Miocene in the QDNB might be helpful for predicting distribution of different sedimentary characteristics, which has economic significance in the industrial field.

D1368 |
EGU2020-11862
Ziying Xu, Jun Wang, Hongfang Gao, and Yongjian Yao

We give a review of the up-to-date research situation about The Zhongnan-Liyue Fault Zone (ZLFZ), than analyze the spatial distribution and tectonic deformation feature of the ZLFZ based on the geophysical data including topographic, seismic, gravity and magnetic data. The results show that the ZLFZ has obvious north-south segmentation characteristics in in the South China Sea Basin. The north section, which is between northwest sub-basin and east sub-basin, is a narrow zone with the width of ~16 km, and is NNW trend from 18°N,115.5°E to 17.5°N,116°E. Meanwhile ,the south section, which is between southwest sub-basin and east sub-basin, is a wide zone with the width of 60-80 km, and is NNW trend from the east of ZhongshaBank to the west of LiyueBank. The main fault of the ZLFZ is NNW trend along the seamounts ridge of Zhongnan. the ZLFZ of transition region is NNE trend from the north section to the south section. According the sub-basin’s sedimentary thickness and oceanic crust thickness exist obvious difference, on both sides of the ZLFZ, we speculate that the ZLFZ play an important role on geological structure of sub-basin. According to the chang of crustal structure, We speculate that the ZLFZ is at least a crustal fracture zone.

Key words: South China Sea Basin; Zhongnan-Liyue Fault Zone; Spatial distribution; Tectonic deformation 

Foundation item: National Natural Science Foundation of China (41606080, 41576068); The China Geological Survey Program (GZH201400202, 1212011220117, DD20160138, 1212011220116).

D1369 |
EGU2020-12566
Penggao Fang, Geoffroy Mohn, Julie Tugend, and Nick Kusznir

    The Valencia Trough is commonly included as part of the set of western Mediterranean Cenozoic extensional basins that formed in relation with the Tethyan oceanic slab rollback during the latest Oligocene to early Miocene. It lies in a complex tectonic setting between the Gulf of Lions to the North-West, the Catalan Coastal Range and the Iberian chain to the West, the Balearic promontory to the East and the Betic orogenic system to the South. This rifting period is coeval with or directly followed by the development of the external Betics fold and thrust belts at the southern tip of the Valencia Trough. Recent investigations suggest that the Valencia Trough is segmented into two main domains exhibiting different geological and geophysical characteristics between its northeastern and southwestern parts. The presence of numerous Cenozoic normal faults and the well-studied subsidence pattern evolution of the NE part of the Valencia Trough suggest that it mainly formed coevally with the rifting of Gulf of Lion. However, if a significant post-Oligocene subsidence is also evidenced in its SW part; fewer Cenozoic rift structures are observed suggesting that the subsidence pattern likely results from the interference of different processes.

    In this presentation, we quantify the post-Oligocene subsidence history of the SW part of the Valencia Trough with the aim of evaluating the potential mechanisms explaining this apparent subsidence discrepancy. We analyzed the spatial and temporal distribution of the post-Oligocene subsidence using the interpretation of a dense grid of high-quality multi-channel seismic profiles, also integrating drill-hole results and velocity information from expanding spread profiles (ESP). We used the mapping of the main unconformities, especially the so-called Oligocene unconformity, to perform a 3D flexural backstripping, which permits the prediction of the post-Oligocene water-loaded subsidence. Our results confirm that the post-Oligocene subsidence of the SW part of the Valencia Trough cannot be explained by the rifting of the Gulf of Lions. Previous works already showed that the extreme crustal thinning observed to the SW is related to a previous Mesozoic rift event. Here, we further highlight that if few Cenozoic extensional structures are observed, they can be interpreted as gravitational features rooting at the regionally identified Upper Triassic evaporite level. Backstripping results combined with the mapping of the first sediments deposited on top of the Oligocene unconformity show that they are largely controlled by the shape of Betic front with a possible additional effect of preserved Mesozoic structures. At larger scale, we compare the mechanisms accounting for the origin and subsidence at the SW part of the Valencia Trough with those responsible for the subsidence of its NE part and the Gulf of Lions.

D1370 |
EGU2020-21429
Lijie Wang, Fuyuan Li, Baojin Zhang, Ziying Xu, and Zhe Wang

Volcanism occurs close to the rifting active areas, especially in the passive continental margins. Their occurrence can have considerable impacts on the continental lithosphere breakup process, hydrocarbon accumulation system in a basin, and regional heat flows. The Xisha massif surrounded by two hyper-extended continental crust and three oceanic basins that the area is underlain by stretched continental crust in the northwestern South China Sea margin. Sporadic Cenozoic volcanic samples and structures from wells, seismic data, and multi-beam data in the Xisha massif have been previously recognized. This study focuses on describing the igneous structures and mapping the volcanic distributions. With the use of drilled wells with lithologic and stratigraphic information, 2D multiple channel seismic data, and multi-beam data, the occurrence of three phases Cenozoic volcanism were mapped. The first episodic volcanism during the rifting to spreading stage in the South China Sea occurred together with Mesozoic granitic pluton. The drilling samples in the well CK-2 show that Late Eocene to Early Miocene basaltic pyroclastic rocks beneath the thick Miocene reefal limestone. Only five mound shaped structures from seismic profiles located on the basement highs and entirely overlapped by the following carbonates. The second episodic volcanism occurred during Middle Miocene that features volcanic group, isolated volcanic mounds, lava flows, and hydrothermal vents associated with sills in the northwestern Xisha massif. The volcanic groups are mainly present above a NE-SW trended sag and the long axis trends NW, the same as the Middle Miocene active fault orientation in the Qiongdongnan basin. The volume of the largest volcanic group is ca. 504 km3. From the intruded strata and deformational structures of volcanic mounds, sills, and laccolith, we found the third episodic volcanism occurred during Pliocene on a small scale. The igneous bodies mainly distributed in the southern Xisha massif. Distribution and volume of igneous bodies show that Middle Miocene stage magmatic activity is more intense than the others, where volcanism is dominant. Comparing with tectonic stress filed, continental crust structures, and sediment thickness, we found the distribution of volcanos is probably related to NE-SW stretching stress filed during Middle Miocene. A high vertical pressure caused by 20-25 km’s crust thickness and only ca. 1-3 km thickness sediment layer may build a good vertical gradient for magma transport. We indicate the intense Middle Miocene volcanism in Xisha massif is also related to the high velocity layers in the lower crust and cased the high heat flows. These phenomena probably coincided with more magma intruded in the lower crust when plenty of post spreading magmatism emplacement in the SCS margin.