Li Sanzhong

TECTONIC TRANSITION AND PLATE RECONSTRUCTIONS OF THE EAST ASIAN CONTINENTAL MAGIN

It is a hot debate on the Meso-Cenozoic dynamics of the East Asian continental margins,since the nature,type and evolution of its continental margins are not well-known..This paper systematically summarizes the results of plate reconstructions on the East Asian continental margin since Permian over the past 40 years with emphasis on the new research results in the past 10 years.The paper mainly focuses on the variation in types of the East Asian continental margin since Late Triassic,the transitions among different tectonic domains.Our data suggests that the East Asian continental margins as a whole experienced a pre-Triassic passive continental margin,a Late Triassic-Early Cretaceous continental magmatic arc of the Andean-type active continental margin,a Late Cretaceous-Eocene strike-slipping Andean-type active continental margin with of pull-apart basins,and a post-Oligocene Japanese-type active continental margin successively.Tectonic trasition is important for understanding of the formation of the Bohaisx Bay,the Yellow Sea,the East China Sea and the South China Sea basins in East China.The changes of the continent-ocean distribution,plate configuration and dynamics at the various stages are also discussed.

BASIN DYNAMICS AND BASIN GROUPS OF THE SOUTH CHINA SEA: BASIN DYNAMICS AND BASIN GROUPS OF THE SOUTH CHINA SEA

The South China Sea was formed in Cenozoic.Basins in the South China Sea could be divided into different types,such as continental shelf basins,continental slope basins and deep sea basins,which are closely related with the rifting and transition of the continental margins,and thus could be grouped into strike-slip-related pull-apart and extensional basins,extrusion-escape-related rifting basins,extension-and subduction-related buckling basins.They developed further into some ridge-spreading-related oceanic sub-basins in different stages.These basins were formed not under a single dynamics but under a very complex and changeable dynamics.These basins have suffered from a series of Cenozoic tectonic movements,such as the Shenhu,the South China Sea,the Dongsha,resulting in the formation and spatial-temperal migration of complex angular unconformities in these basins.Accompanying with the tectonic migrations and evolutions in these basins,faulting,magmatism,shifting of sedimentary subsidence centers and depocenters,oil accumulation and geohazards show a significant feature of jumping.The complex dynamic settings of the South China Sea have caused many controversies on their origin.The plate dynamic factors include the Pacific Plate subduction and the indentation of the Philippine Sea Plate along the Taiwan Orogen to the east side of the South China Sea,and the Indian Plate oblique subduction and mid-ocean ridge subduction to the west side.They may also be responsible for the uplifting of the Tibetan Plateau and the related extrusions of continental blocks to the north side.At the same time,the mantle dynamics of deep-seated magma underplating,delamination,mantle plume and mantle-hydrated process should not be ignored.At last,this paper proposes one strike-slipping pull-apart model to explain the onset of the South China Sea Basin which is closely related to the tectonic evolution of the Pacific Tectonic Domain.

BASIC STRCUTURAL PATTERN AND TECTONIC MODELS OF THE SOUTH CHINA SEA: PROBLEMS, ADVANCES AND CONTROVERSIES: BASIC STRCUTURAL PATTERN AND TECTONIC MODELS OF THE SOUTH CHINA SEA: PROBLEMS, ADVANCES AND CONTROVERSIES

The South China Sea is located at a juncture among the Eurasian,the Indian-Australian and the Pacific plates,being the largest continental marginal sea along the East Asian continental margin.It has experienced a complex transition of continental marginal types.The northern margin was an Andean-type continental margin before 80 Ma,then gradually transferred into a rifted continental margin in the Eocene and a typical passive continental margin since the middle Miocene.The eastern margin was an open water before 17 Ma and gradually became a subduction zone of the trench-arc-basin system by one-way subduction to double-way subduction,when the South China Sea became semi-closed basin since 6 Ma.The western boundary gradually transformed into a continental margin with strike-slipping faulting or transform faulting since 34 Ma.The southern margin was an asymmetric rift-type continental margin corresponding to the northern margin before 34 Ma,and became a passive continental margin during 34~16 Ma and gradually a thrusting-type continental margin after 16 Ma.Complex dynamic settings of the South China Sea have caused many controversies on their origin.The plate dynamic factors include either the Pacific Plate subduction and the indentation of the Philippine Sea Plate along the Taiwan Orogen to the east side the South China Sea,or the Indian Plate oblique subduction and mid-ocean ridge subduction to the west side.They may also be responsible for the uplifting of the Tibetan Plateau and the related extrusions of continental blocks to the north side.At the same time,the mantle dynamics of deep-seated magma underplating,delamination,mantle plume and mantle-hydrated process should not be ignored.

CENOZOIC TECTONICS AND DYNAMICS OF BASIN GROUPS OF THE NORTHERN SOUTH CHINA SEA: CENOZOIC TECTONICS AND DYNAMICS OF BASIN GROUPS OF THE NORTHERN SOUTH CHINA SEA

There are a lot of Cenozoic basins from west to east in the northern margin of the South China Sea,including the Beibu Gulf,the Qiongdongnan,the Pearl River Mouth and the Taixinan basins.Previous studies suggested that these basins are derived gradually during the rifting of the southeast continental margin of the South China Block and the formation of the northern passive margin of the South China Sea.However,a large number of seismic profiles revealed that some major basin-controlling faults in the northern margin of the South China Sea are landward-dipping,which are clearly inconsistent with the major seaward dipping faults of a typical passive continental margin basin.Thus,the Mckenzies extension model of passive continental margin remains controversial for the formation mechanism of the northern marginal basins of the South China Sea.This article,based on the comparison between a large number of field observation and marine seismic data,reveals that the northern continental margin of the South China Sea is not a passive continental margin before 34Ma.The early faults were well developed in NE—NNE-striking,corresponding to the strike-slipping faults in the southeast South China.These dextral,right-step strike-slipping faults are NE—NNE striking,controlling the secondary faults with near-EW or NEE strikes and the distribution of tectonic units in the pull-apart basins.Therefore,a series of Cenozoic basins in the northern margin of the South China Sea is dextral,right-step pull-apart basin group,having close relation in dynamics.This pull-apart model is consistent with the migration of sedimentary-subsidence centers and depocenters,tectonic jumping and magmatic migration within Cenozoic basins.The northern continental margin of the South China Sea actually became a typical passive continental margin since 15 Ma,when the South China Sea oceanic crust terminated spreading.At last,all structures in the northern continental margin of the South China Sea are offset by the late,diffusive NWW-striking faults resulting from the gradual indentation of the Philippines Plate to the Eurasian Plate along the Luzon Arc and the Taiwan Orogenic Belt between 10 Ma and about 5 Ma.Basin dynamics in northern continental margin of the South China Sea is mainly related to the dynamics of the Pacific Plate.

Paleoproterozoic amalgamation of the NorthChina Craton and the assembly of the Columbia supercontinent

Throughout the 1990’s, the plate reconstruction of the Supercontinent Rodinia was the major field of supercontinental reconstruction, and became a hot research topic in the international community. However, in the course of the Supercontinent Rodinia reconstruction, we found three aspects are inconsistent with the idea that the Supercontinent Rodinia is the first supercontinent in the Earth geological history: (1) Between many continental blocks in the Supercontinent Rodinia such as between North America and Siberia, North America and Antarctica, North America and Australia, South America and the Baltic, West Africa and South America and so on, is not assembled by the Grenville-period orogenic belts of about 1 Ga, indicating that these continental blocks had merged together before the formation of the Rodinia; (2) All the large blocks within the Rodinia contained older continental collision orogenic belts of 2.1–1.8 Ga than the Grenville orogenic belt; (3) During the 1.8–1.1 Ga (700 Ma), the Earth had basically no large-scale continental collisional events. On the contrary, the global collisional orogenic events of 2.1–1.8 Ga on the Earth resulted in 2.1–1.8 Ga of orogenic belts’ distribution between the old continents and in their internals are global, such as 2.1–2.0 Ga Transamazonian-Eburnean collision zone between the South America and West Africa Continental blocks, 1.9–1.8 Ga Trans-Hudson collision zone in the North America continental block, the Nagssugtoquidian collisional zone of 1.9–1.8 Ga within the Greenland continental block, the Kola-Karelia collision zone of 1.9–1.8 Ga within the Baltica continental block, the Akitkan collision zone within the Siberia Block, 2.0–1.9 Ga Limpopo collisional zone between South African Kaapvaal and Zimbabwe blocks, 2.0–1.9 Ga Capricorn collision zone between the West Australian Yilgarn and Pilbara cratons, 1.850 Ga Trans-North China Orogen in the Central North China Craton and others. Therefore, based on the mentioned-above, Zhao et al. (1998) first proposed in the 15th Australian Geological Congress that these widely-distributed 2.1–1.8 Ga collision zones in the Earth may record a global continent-continent collision event, resulting in the formation of one pre-Rodinia supercontinent. The supercontinent named the “Hudson” because the Trans-Hudson orogenic belt in the North American Continent is the result of the most typical continent- continent collision zone.

CENOZOIC BASIN-CONTROLLING FAULTS AND THEIR BEARING ON BASIN GROUPS FORMAION IN THE SOUTHERN SOUTH CHINA SEA: CENOZOIC BASIN-CONTROLLING FAULTS AND THEIR BEARING ON BASIN GROUPS FORMAION IN THE SOUTHERN SOUTH CHINA SEA

There are many small and medium-sized Cenozoic sedimentary basins in the southern South China Sea,such as the Nanweixi Basin,the Beikang Basin,the Liyue Basin,the Zengmu Basin,the Nansha Trough,the Brunei-Sabah Basin,the Northwest Palawan Basin and other basins from north to south.In general,NE-trending faults control the formation of the half grabens with faulting in the north and overlapping in the south.However,single grabens are mainly developed in the northern continental slope,while two-layered basins developed in the south of the region,of which the lower layer is the graben and the upper layer the imbricated nappes.According to the nature of main controlling faults and the transition of the basins at different evolutionary stages,these basins can be divided into three groups: the rift basin group(the Nanweixi,Beikang and Liyue basins),the rift-pull-apart-foreland superimposed basin group(the Zengmu basin) and the rift-foreland basin group(the Nansha Trough,Brunei-Sabah,the Northwest Palawan basins).The formation of these basin groups is closely related to major marginal faults of the basins and secondary faults in the sub-basins.Based on the nature of the faults,the basin-controlling fault belts can be subdivided into three types: tensile,shear and compressional,including the extensional fault belt in the northern edge of the Nansha Trough,the strike-slip fault belts at the west edge of the Wanan Basin and the Lupar and the Tingjia-Lee Jun Fault belts,and the thrust fault belt in the southern edge of the Nansha Trough.Tectonic events which happened in southern South China Sea,such as the South China Sea Movement in Oligocene and the Nansha Movement in the Middle Miocene,affected the basins to various degrees.As the result,there was the inversion from a rift basin/pull-apart basin to a marine foreland basin,as well as the inversion of fault belts from normal faults to reverse faults or strike-slip faults.The kinetic mechanism of the basin groups varies in different stages.In the early stage,it may be related to the continental rifting of the southeast edge of the Eurasia continent,and the subduction and dragging of the Paleo-South China Sea slab resulted in the splitting of southern South China Sea from the South China continent,and then,half garbens formed with faulting in the north and overlapping in the south.In the later period(about 16 Ma),the north-directed imbricated thrust nappes propagated forward in the southern South China Sea due to the collision of the southern Australian Plate to the northern Eurasian Plate,which resulted in the transition of basin types and the counterclockwise rotation of the Borneo Block.

UNCONFORMITIES IN THE BEIBUWAN BASIN AND THEIR IMPLICATIONS FOR TECTONIC EVOLUTION: UNCONFORMITIES IN THE BEIBUWAN BASIN AND THEIR IMPLICATIONS FOR TECTONIC EVOLUTION

The Cenozoic Beibuwan Basin is located in the southwest margin of the South China Block.Many depositional breaks and unconformities have been found in the Cenozoic sequence.The characteristics and distribution pattern of these conformities are described in details in this paper.According to their scale and behaviors,these unconformities can be divided into three orders.And based on ages and their relationship with the rifting stages of the basin,there are three main types of unconformities,i.e.rifting phase unconformity,rifting-depression phase unconformity and inversion phase angular unconformity.The basal unconformity of the basin recorded the initial rifting event of the basin.Being the first-order unconformity,it formed in the rifting phase,and was in conjunction with the sudden decrease in the subduction rate of the Pacific Plate.The unconformity beneath the Weizhou Formation was interpreted in this paper as a rifting phase third-order unconformity formed by the Zhuqiong Movement.It is regionally associated with the collision of the Indian Plate to the Eurasian Plate and the turn of the subduction direction of the Pacific Plate.The unconformity at the bottom of the Xiayang Formation is the transitional interface of the basin structural systes.As a second-order unconformity,it corresponds to the continuous opening of the South China Sea Basin.The unconformity at the bottom of the Dengloujiao Formation is considered as the result of the tectonic inversion of the basin in the later stage.It is a third-order angular unconformity formed in the inversion phase.From a regional point of view,it is related to the Indian-Australian Plate,rather than the subduction of the Philippine Sea Basin.It is concluded that the formation of unconformities is closely related to the rifting of the South China continental margin and the multiple-order and multiple-phase transtension or transpression.It is believed that the intensively strike-slip faulting along the South China continental margin is the result from the reorganization of the regional plate tectonic pattern.

Meso-Cenozoic Evolution of Earth Surface System under the East Asian Tectonic Superconvergence

The East Asian geological setting has a long duration related to the superconvergence of the Paleo-Asian, Tethyan and Paleo-Pacific tectonic domains. The Triassic Indosinian Movement contributed to an unified passive continental margin in East Asia. The later ophiolites and I-type granites associated with subduction of the Paleo-Pacific Plate in the Late Triassic, suggest a transition from passive to active continental margins. With the presence of the ongoing westward migration of the Paleo-Pacific Subduction Zone, the sinistral transpressional stress field could play an important role in the intraplate deformation in East Asia during the Late Triassic to Middle Jurassic, being characterized by the transition from the E-W-trending structural system controlled by the Tethys and Paleo-Asian oceans to the NE-trending structural system caused by the Paleo-Pacific Ocean subduction. The continuously westward migration of the subduction zones resulted in the transpressional stress field in East Asia marked by the emergence of the Eastern North China Plateau and the formation of the Andean-type active continental margin from late Late Jurassic to Early Cretaceous (160-135 Ma), accompanied by the development of a small amount of adakites. In the Late Cretaceous (135-90 Ma), due to the eastward retreat of the Paleo-Pacific Subduction Zone, the regional stress field was replaced from sinistral transpression to transtension. Since a large amount of late-stage adakites and metamorphic core complexes developed, the Andean-type active continental margin was destroyed and the Eastern North China Plateau started to collapse. In the Late Cretaceous, the extension in East Asia gradually decreased the eastward retreat of the Paleo-Pacific subduction zones. Futhermore, a significant topographic inversion had taken place during the Cenozoic that resulted from a rapid uplift of the Tibet Plateau resulting from the India-Eurasian collision and the formation of the Bohai Bay Basin and other basins in the East Asian continental margin. The inversion caused a remarkable eastward migration of deformation, basin formation and magmatism. Meanwhile, the basins that mainly developed in the Paleogene resulted in a three-step topography which typically appears to drop eastward in altitude. In the Neogene, the basins underwent a rapid subsidence in some depressions after basin-controlled faulting, as well as the intracontinental extensional events in East Asia, and are likely to be a contribution to the uplift of the Tibetan Plateau.

VARIATION OF ANCIENT WATER TEMPERATURE AND ITS MECHANISM IN THE NORTHERN SOUTH CHINA SEA SINCE 16 MA

The research of surface water temperature of paleo-oceans is usually focused on Quaternary,rarely earlier than 2.5Ma.Based on the data obtained from sites 1147 and 1148 of ODP 184 Cruise,using long-chain alkenones and oxygen isotopes of benthic foraminifera as proxies,we calculated the marine oxygen isotope composition and the seawater temperature changes since 16Ma in this paper.The planktonic foraminifera oxygen isotope method was adopted to explore the mechanism of ancient seawater temperature change.The results show that the trends of water temperature change in the northern South China Sea kept step with the global climate changes.During the formation of the northern hemispheric ice sheets,the sea surface temperatures are changed synchronously with the benthic foraminiferal δ18O,which indicate the size of the high-latitude ice sheet.It shows the special nature of ancient climate changes in such a tropical sea as the South China Sea and provides new evidence for the further study of the ancient global climate change in the low-latitude tropical sea.

RELATION OF YANSHANIAN EXTRUSION TECTONICS TO THE FORMATION OF THE BOHAI BAY BASIN:NUMERICAL SIMULATION BASED ON FLAC~(2D)

The formation and evolution of the Meso-Cenozoic Bohai Bay Basin is closely related with the Paleo-Asian Ocean Tectonic Domain,the Pacific Tectonic Domain and the Tethys Tectonics Domain.Previous structural analysis revealed that extrusion tectonics is one of the significant regimes to the formation of the Bohai Bay Basin.However,its geodynamics is difficult to be separated from the dynamics of plate boundaries.Therefore,this paper focused on their relation between its geodynamics and the regional stress fields by simulation based on FLAC2D.The calculated result indicates that the northeast part of the Jiaoliao Block escaped northeastward and the southwest part eastward during Mesozoic while the Bolu Block escaped southwestward,and all these processes contribute to the last configuration of the Bohai Bay Basin.The results also reveal that the Pacific Tectonic Domain plays the most important role for the basin.Simultaneously,the North China Craton rotated anticlockwise with the variation in regional stress fields during different structural evolutionary stages.This result is in accordance with the Yanshanian tectonic outline of the study area.