South China Sea Oceanic Crust Research Articles

Formation and Tectonic Evolution of Ophiolites in the Sabah Area (Borneo, SE Asia)

Zircon U-Pb dating, rock geochemistry, Sr-Nd-Pb, and zircon Hf isotope analyses were conducted on the ultrabasic and basic rocks of ophiolites in the Sabah area (Borneo, SE Asia). The zircon U-Pb ages of ultrabasic and basic rocks range from 248 to 244 Ma, indicating that the ophiolites already existed in the early Triassic. The rare earth elements of basic rocks in Central Sabah show N-MORB-type characteristics and E-MORB-type characteristics in the northwest and southeast. The εNd(t) values of basic rocks range from 3.66 to 8.73, and the εHf(t) values of zircon in ultrabasic rocks are between −10.2 and −6.1. Trace element analysis shows that the magmatic source was influenced by melts and fluids from the subducting plate of the Paleo-Tethys Ocean. The tectonic evolution of the Sabah area can be traced back to the Early Triassic. At that time, the fast subduction of the Paleo-Tethys Ocean plate and the retreating of the Paleo-Pacific plate resulted in the upwelling of mantle material in relatively small extensional settings, leading to the formation of the ophiolites. From the Jurassic to the Early Cretaceous, the Paleo-Pacific plate was intensely subducted, and the ophiolite intrusion in the Sabah area moved to the continental crust of South China or the Sundaland margin as fore-arc ophiolites. From the Late Cretaceous to the Miocene, with the expansion of the Proto-South China Sea and South China Sea oceanic crust, the ophiolites in the Sabah area drifted southward with microplate fragments and sutured with East Borneo.

A revolution in understanding SE Asia geodynamics since 20.5–18 Ma

We present an updated evaluation of SE Asian geodynamics that includes the interactions of the South China Sea (SCS) marginal basin with surrounding plates since the end of SCS spreading 20.5–18 Ma. Newly available Ar39/Ar40 ages of SCS oceanic crust drilled at IODP U1431 near the SCS East basin extinct spreading center are older than 18 Ma. Conversely, the oldest ages of the Luzon arc and forearc at Taiwan's Lanyu island, Coastal range and Lichi mélange are 17–18 Ma, suggesting that onset of the Manila subduction zone may have begun a few m.y. earlier. Before ∼20.5 Ma, the northern part of the Manila transcurrent fault (MTF), considered as the western boundary of the Ryukyu subduction zone, was a left-lateral lithospheric-scale shear zone. From ∼20.5–18 Ma to ∼7 Ma, this portion of MTF was connected to the Manila trench. Since ∼7 Ma, the MTF extended into the Taiwan Longitudinal valley and continued southwards to north Luzon island as near-vertical, left-lateral shear zone. Today, south of ∼24°N, the MTF protrudes down to 30 km depths and terminates above the deeper Manila slab. Since ∼7 Ma, the whole MTF shifted 400 km westward with respect to Eurasia and rotated ∼23° clockwise to become oriented ∼NS north of 16°N latitude. We identify a tear fault in the Eurasian (EU) plate north of the Ryukyu trench that is located south of the Myako and Yonaguni islands. Since ∼10 Ma, the tear continuously progressed westward within EU crust, with the Philippine Sea plate progressively subducting northwestward between the two lips of the tear fault. A RFF (ridge-fault-fault) triple junction was active in the EU crust before 20.5 Ma, from 10 to 7 Ma, and since 2 Ma. This triple junction was always located on the MTF with one branch of the MTF on each side of the triple junction, and the third branch being the spreading center.

Mid-Miocene uplift and tectonic-sedimentary evolution of the Taiwan accretionary prism: Constraints from sedimentary records in the Hengchun Peninsula, southern Taiwan

The young Taiwan orogenic belt is a classic example of a modern arc–continent convergence zone, which displays a longitudinal transition from active collision in the north to oceanic subduction in the south. However, the timing of its initial exposure above the sea level and the early-stage uplift mechanism has been a point of much debate. Here we applied major and trace element, Nd isotope and heavy minerals analysis on the Miocene sedimentary rocks from the Hengchun Peninsula to trace sediment provenance and to further constrain the tectonic-sedimentary evolution of the Taiwan accretionary prism. Results show that the Hengchun mudstones (∼11.6–6.5 Ma) are characterized by higher CaO (1.42 wt %) and MgO (2.57 wt %) contents, Cr/Zr (0.52) and Co/Th (1.58) ratios, and less negative ϵNd (t) values (−12 to −7) than those from the Chinese passive continental margin sediments. It indicates that the Chinese mainland was not exclusive sediment contributor for the Hengchun sediments. Combined with the high content of Cr-spinel heavy mineral (mean value ∼ 28.2%) in the Hengchun Miocene sandstones, and the presence of mafic lithic grains and N-MORB-type mafic rocks of 26-22 Ma in the Hengchun Peninsula, it is clear that the subducting South China Sea oceanic lithosphere should be also considered for the origin of the Hengchun sediments. The most likely source of these oceanic clasts being transported to the Hengchun Peninsula is the equivalent prism to the north, which had involved oceanic fragments during the subduction process. Consequently, Taiwan accretionary prism should have been already partly exposed prior to ∼11.6 Ma, considerably earlier than most previous estimates and the onset of arc-continent collision. Underplating of seamounts and/or the thinned Chinese continental margin beneath the overlying Taiwan accretionary prism, together with wedge extrusion tectonics, may have played an important role for this early-stage uplift, which now is also observed in the sandbox experiments, marine observation and analogue modeling. Our study not only better integrates the subduction and then erosion history of the South China Sea oceanic crust with sedimentary records in Taiwan, but also signifies a modern example from Taiwan that highlights the importance of soft exhumation mechanism in worldwide plate convergence zones relative to hard collision.

Plagioclase-regulated hydrothermal alteration of basaltic rocks with implications for the South China Sea rifting

IODP Hole U1502B penetrates >180 m into the crystalline basement generated at a rifting margin of the South China Sea (SCS), which is the first confirmed intermediate-type margin between the magma-rich and -poor endmembers. The recovered lavas show petrographic characteristics of mid-ocean ridge basalt (MORB), but suffered pervasively from hydrothermal alteration. This sequence represents the oldest SCS oceanic crust ever drilled in-situ, and offers a globally unique window to explore the hydrothermal processes during continental breakup. Here, 50 whole-rock samples representative of Hole U1502B were analyzed for major and trace elements and Sr-Nd isotopes, presenting the first report of hydrothermally altered basalts for the SCS. The protolith appears to be tholeiite, enriched MORB, and little affected by crustal contamination due to the constant mantle-values of εNd. However, the altered rocks are characterized by significant Ca depletion and 87Sr/86Sr modification. Major processes are identified to be tightly involved with plagioclase: chloritization and albitization. Both reactions are responsible for the Ca-loss and Sr-mobility, and for the resultant Mg- and Na-uptakes, respectively. Environments varying from the peripheral to the deep parts within a discharge zone are evidenced by the co-existence of hydro-fracturing brecciation, enrichments of base metals (Zn, Cu), high water/rock ratios (~1–25), and lower greenschist facies alteration (albite + chlorite + epidote ± quartz ± pyrite). This variability can be attributed to detachment-related faulting, allowing deeply channeled pathways of fluids. Besides, such tectonic effects permit a penetration of Hole U1502B into the porosity boundary of seismic layer 2A/2B, corresponding to either 1) the lithologic lava–dike transition and/or 2) an alteration front of hydrothermal circulation. In any case(s), our results imply a more complex and longer SCS rifting than previously thought.

Formation of Hengchun Accretionary Prism Turbidites and Implications for Deep‐water Transport Processes in the Northern South China Sea

AbstractLocated at the end of the northern Manila Trench, the Hengchun Peninsula is the latest exposed part of Taiwan Island, and preserves a complete sequence of accretionary deep‐sea turbidite sandstones. Combined with extensive field observations, a ‘source‐to‐sink’ approach was employed to systematically analyze the formation and evolutionary process of the accretionary prism turbidites on the Hengchun Peninsula. Lying at the base of the Hengchun turbidites are abundant mafic normal oceanic crust gravels with a certain degree of roundness. The gravels with U‐Pb ages ranging from 25.4 to 23.6 Ma are underlain by hundreds‐of‐meters thickness of younger deep‐sea sandstone turbidites with interbedded gravels. This indicates that large amounts of terrigenous materials from both the ‘Kontum‐Ying‐Qiong’ River of Indochina and the Pearl River of South China were transported into the deep‐water areas of the northern South China Sea during the late Miocene and further eastward in the form of turbidity currents. The turbidity flow drastically eroded and snatched mafic materials from the normal South China Sea oceanic crust along the way, and subsequently unloaded large bodies of basic gravel‐bearing sandstones to form turbidites near the northern Manila Trench. With the Philippine Sea Plate drifting clockwise to the northwest, these turbidite successions eventually migrated and, since the Middle Pleistocene, were exposed as an accretionary prism on the Hengchun Peninsula.

Sedimentary records from Hengchun accretionary prism turbidites on Taiwan Island: Implication on late Neogene migration rate of the luzon subduction system

Although plate motion models for the Philippine Sea Plate (PSP) are relatively well established, they remain to be confirmed by reliable geological records. Oblique convergence between Eurasia and the PSP has resulted in the uplift of Taiwan Island along the plate boundary. As the latest exposed portion of Taiwan, the Hengchun Peninsula features well-preserved deep-sea turbidites with rounded mafic conglomerate clasts. We apply an integrated source-to-sink analysis and broad field investigations to generate a reliable kinematic reconstruction linking the Hengchun turbidite outcrops with South China Sea (SCS) borehole sediments. Deposited in the lowermost turbidite layer, the Hengchun rounded gabbro and basalt clasts resemble the SCS oceanic crust based on our U–Pb geochronological (~23.6–25.4 Ma) and elemental geochemical results (normal mid-ocean ridge basalts), along with previous magnetic anomaly constraints. The multimodal U–Pb spectra of the matrix-supported sandstones suggest a mixed provenance including central Vietnam, the Pearl River tributaries and potential intrabasinal uplifts, which have not received prior consideration. During the late Miocene, abundant terrigenous sediments from the eastward-flowing “Kontum-Ying-Qiong River” and Pearl River were transported into the submarine environment. The extensive turbidity currents scoured and eroded the SCS mid-ocean ridges, which feature numerous low-angle detachment faults. These materials of various sources were transported eastwards for thousands of kilometers prior to their incorporation into the accretionary prism adjacent to the Manila Trench. From the late Miocene to Pleistocene, the PSP moved northwestward at estimated velocities of ~61–65 mm/yr, facilitating the final uplift and exposure of the accretionary prism.

A reinterpretation of the metamorphic Yuli belt: Evidence for a middle-late Miocene accretionary prism in eastern Taiwan

The Yuli metamorphic belt has been the topic of petrological and geochronological studies for over 40 years and has been interpreted as a Cretaceous melange. Our study utilizes zircon U-Pb dating of schist and exotic blueschist blocks in the Yuli belt. These new ages indicate that these metamorphic rocks are actually middle Miocene in age and may represent the deeper structural levels of an accretionary prism. Several distinctive detrital zircon U-Pb age populations are recognized from 14 siliceous schists of melange-hosted rocks that are similar in age population to the Cretaceous, Eocene-Oligocene, and Miocene strata of Taiwan. The wide range of ages is interpreted as a product mixing of various sedimentary strata prior to metamorphism. Three blueschists of a volcanic-arc protolith enclosed within the host rocks yield crystallization ages of 15.4 ± 0.4, 15.5 ± 0.3, and 16.0 ± 0.2 Ma based on zircon U-Pb dating. In consideration of the new data regarding the Cretaceous-Miocene host rocks and the middle Miocene exotic blueschist blocks, it strongly suggests that the Yuli belt formed at deeper levels of an accretionary wedge during subduction of South China Sea oceanic crust at the middle-late Miocene. Subsequently, the rapid uplift of the metamorphic belt was probably related to doubly vergent wedge extrusion due to the Pliocene arc-continent collision.

Arc-continent collision of the Coastal Range in Taiwan: Geochronological constraints from U-Pb ages of zircons

Abstract The oblique arc-continent collision between the Luzon arc and the southeastern margin of the Eurasian continent caused the uplift of Taiwan. The Coastal Range in eastern Taiwan is the northern section of the Luzon arc in the collision zone and thus records important information about the arc-continent collision. In this paper, we determine and analyze the U-Pb ages of magmatic zircons from the volcanic arc and clastic zircons from the fore-arc basin in the Coastal Range. For the volcanic arc in the Coastal Range, the eruption ages range from 16.8–5 Ma. Given that the initial subduction of the South China Sea oceanic crust (17 Ma) occurred before the Luzon arc formed, we conclude that the volcanic activity of the Coastal Range began at 16.8 ± 1.3 Ma; it was most active from 14 to 8 Ma and continued until approximately 5 Ma. The U-Pb chronology also indicates that the initial stage of arc-continent collision of the Coastal Range started at approximately 5 Ma, when the northern section of the Luzon arc moved away from the magmatic chamber because of the kinematics of the Philippine Sea Plate.

Cenozoic stratigraphy of Taiwan: Window into rifting, stratigraphy and paleoceanography of South China Sea

Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at ∼14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminiferal abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs (−2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.

Magnetic recording of the Cenozoic oceanic crustal accretion and evolution of the South China Sea basin

We review and discuss some of the recent scientific findings made on magnetic data in the South China Sea (SCS). Magnetic anomalies bear extremely rich information on Mesozoic and Cenozoic tectonic evolution. 3D analytical signal amplitudes computed from magnetic anomalies reveal very precisely relict distributions of Mesozoic sedimentary sequences on the two conjugate continental margins, and they are also found very effective in depicting later-stage magmatism and tectonic transitions and zonation within the SCS oceanic crust. Through integrated analyses of magnetic, gravity and reflection seismic data, we define the continent-ocean boundary (COB) around the South China Sea continental margin, and find that the COB coincides very well with a transition zone from mostly positive to negative free-air gravity anomalies. This accurate outlining of the COB is critical for better tracing magnetic anomalies induced by the oceanic crust. The geometrically complex COB and inner magnetic zonation require the introduction of an episodic opening model, as well as a transform fault (here coined as Zhongnan Fault) between the East and Southwest Sub-basins, while within the East and Southwest Sub-basins, magnetic anomalies are rather continuous laterally, indicating nonexistence of large transform faults within these sub-basins. We enhance magnetic anomalies caused by the shallow basaltic layer via a band-pass filter, and recognize that the likely oldest magnetic anomaly near the northern continental margin is C12 according to the magnetic time scale CK95. Near the southern continental margin, magnetic anomalies are less recognizable and the anomaly C12 appears to be missing. These differences show an asymmetrical opening style with respect to the relict spreading center, and the northern part appears to have slightly faster spreading rates than to the south. The magnetic anomalies C8 (M1 and M2, ∼26 Ma) represent important magnetic boundaries within the oceanic basin, and are possibly related to changes in spreading rates and magmatic intensities. The magnetic evidence for a previously proposed ridge jump after the anomaly C7 is not clear. The age of the Southwest Sub-basin has yet to be further examined, most favorably with deep-tow magnetic surveys and ocean drilling. Our magnetic spectral study shows that the shallowest Curie points are located around the eastern part of the Southwestern Sub-basin, whereas within the East Sub-basin Curie depths are smaller to the north of the relict spreading center than to the south. This pattern of Curie depths is consistent to regional heat flow measurements and later-stage volcanic seamount distributions, and we therefore reason that Curie-depth variations are closely associated with later-stage magmatism, rather than with crustal ages. Although magnetic anomalies located around the northern continent-ocean transition zone (COT) are relatively quiet, this area is not a typical magnetic quiet zone since conceptually it differs markedly from an oceanic magnetic quiet zone. The relatively quiet magnetic anomalies are seemingly associated with a shallowing in Curie isotherm and thinning in magnetic layer, but our comprehensive observations suggest that the well-preserved thick Mesozoic sedimentary rocks are major causes for the magnetically quiet zone. The high similarities between various low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies clearly confirm that deeper magnetic sources (in the lower crust and the uppermost mantle) have contributions to long-wavelength surface magnetic anomalies in the area, as already inferred from magnetically inversed Curie depths. The offshore south China magnetic anomaly (SCMA) becomes more prominent on low-pass filtered marine and air-borne magnetic anomalies and satellite magnetic anomalies, indicating very deeply-buried magnetic sources beneath it.

Temporal and spatial records of active arc-continent collision in Taiwan: A synthesis

Well-documented stratigraphy and clearly defined geodynamics in Taiwan, where some of the best records on arc-continent collision have been preserved, offer a unique example for the study of collision belts worldwide. The oblique arc-continent collision in Taiwan caused a simultaneous and sequential migration of four tectonic processes. Beginning from 16 to 15 Ma, subduction of the South China Sea oceanic crust beneath the Philippine Sea plate resulted in volcanism in the Coastal Range and formation of an accretionary prism in the Central Range. Beginning in the latest Miocene–earliest Pliocene, the subduction was followed by initial arc-continent collision, as supported by the following: unroofing and erosion of the deformed accretionary prism, and deposition of sediments thus derived in the adjacent accretionary forearc (5 Ma) and slope basins (4 Ma); waning of volcanism (north, 6–5 Ma; south, 3.3 Ma); buildup of fringing reefs on the gradually quiescent volcanoes (north, 5.2 Ma; south, 2.9 Ma); arc subsidence by strike-slip faulting and the development of pull-apart intra-arc basins (north, 5.2–3.5 Ma; south, 2.9–1.8 Ma); thrusting of forearc sequences to generate a collision complex starting from 3 Ma; and clockwise rotation of the arc-forearc sequences (north, 2.1–1.7 Ma; south, 1.4 Ma). The collision propagated southward and reached southern Taiwan by 5 Ma, as evidenced by the successive deformation of the associated accretionary wedge en route. Afterward, the advanced arc-continent collision stage appeared in the earliest Pleistocene, as marked by the westward thrusting and accretion of the Luzon arc-forearc against the accretionary wedge (north, 1.5 Ma; south, 1.1 Ma) and exhumation of the underthrust Eurasian continent rocks (north, 2.0–1.0 Ma; south, 1.0–0.5 Ma). The final stage of the tectonic process, arc collapse-subduction, began by 1 Ma off the northern Coastal Range. The geologic records compiled and presented in this study strongly support the scenario of a continuous southward migration of tectonic processes and a change in sediment source and structural style. Most importantly, the model has a broad potential for reconstructing and predicting the evolution of arc-continent collision through space and time.

Continent–Ocean Transition of the Northern South China Sea and off Southwestern Taiwan

The northeasternmost South China Sea (SCS) involves four geological provinces: the SCS oceanic crust, Southeast Asian continental margin, Taiwan orogenic belt and Manila subduction zone (Figure 1). The SCS is one of the largest marginal seas of Southeast Asia. The continental margin of the northern SCS has obliquely collided with the northern tip of the Luzon Arc (Philippine Sea plate), which moves northwestwards with respect to Eurasia, whereas the SCS oceanic plate is subducting eastwards beneath the Philippine Sea plate. From Luzon to Taiwan, the convergent setting along the Manila Trench shows a transition from subduction to collision in Taiwan Island. Obviously, studying the continent–ocean transition zone of Eurasia will help to understand the geological processes involved in the collision between the Eurasian and Philippine Sea plates. It will also help to provide reliable geodynamic reconstructions of the SCS. However, in the past, the crustal nature of the northern SCS, the deformation of the northernmost SCS continental margin and the geodynamic evolution of the northern portion of the Manila subduction zone were scarcely known. This Special Issue contains a collection of 10 papers that present recent data and new insights in the northeastern SCS and off southwestern Taiwan.

New Bathymetry and Magnetic Lineations Identifications in the Northernmost South China Sea and their Tectonic Implications

The seafloor spreading of the South China Sea (SCS) was previously believed to take place between ca. 32 and 15 Ma (magnetic anomaly C11 to C5c). New magnetic data acquired in the northernmost SCS however suggests the existence of E–W trending magnetic polarity reversal patterns. Magnetic modeling demonstrates that the oldest SCS oceanic crust could be Late Eocene (as old as 37 Ma, magnetic anomaly C17), with a half-spreading rate of 44 mm/yr. The new identified continent–ocean boundary (COB) in the northern SCS generally follows the base of the continental slope. The COB is also marked by the presence of a relatively low magnetization zone, corresponding to the thinned portion of the continental crust. We suggest that the northern extension of the SCS oceanic crust is terminated by an inactive NW–SE trending trench-trench transform fault, called the Luzon–Ryukyu Transform Plate Boundary (LRTPB). The LRTPB is suggested to be a left-lateral transform fault connecting the former southeastdipping Manila Trench in the south and the northwest-dipping Ryukyu Trench in the north. The existence of the LRTPB is demonstrated by the different patterns of the magnetic anomalies as well as the different seafloor morphology and basement relief on both sides of the LRTPB. Particularly, the northwestern portion of the LRTPB is marked by a steep northeast-dipping escarpment, along which the Formosa Canyon has developed. The LRTPB probably became inactive at ca. 20 Ma while the former Manila Trench prolonged northeastwards and connected to the former Ryukyu Trench by another transform fault. This reorganization of the plate boundaries might cause the southwestern portion of the former Ryukyu Trench to become extinct and a piece of the Philippine Sea Plate was therefore trapped amongst the LRTPB, the Manila Trench and the continental margin.

Subsidence nature of a strike-slip related basin: an example learned from the Sarawak Basin

The subsidence nature of a strike-slip related basin is the least Wlderstood as compared to the rift basin and the basin formed by the lithospheric flexure which is al~o ~owr:t ~ a foreland basin. In brief, the rift basin of McKenzie model has a subsidence profile w~h is characterised by fast initial subsidence and followed by a slower thermal subsidence. In contrast, th,~ forelapd basin is charactensed by a slower initial subsidence and followed by rapid subsidence to the en4 of basin formation. The subsidence profile ofSarawak Basin was selected for this discWJs~on ,as the seismic interpretations concluded that the basin was formed by strike-slip tectonism, contra4ictmg ~ a foreland basin in terms of its tectonic origin; i.e. it was created by lithospheric flexure by th~ ~w>ducijon of South China Sea oceanic crust beneath the NW Sarawak continental crust. The study has been conducted using commercial software, Basin Modelling System Version 4 by Platte River Associates. The result of the study shows that the burial hjstory curves for the wells representative of the Sarawak Basin show many of the profiles with e,ai-Jy r~pid subsidence fonowed by a later phase where basement subsidence is slower, indicative of rifted style qftectonic origin. These are followed either by a series of later compressional basin inversion or cpntinued with thermal subsidence similar to a rift basin profile. The evaluation of stretching factors and heat-flow shows a direct reJati.onship through out the basin which are consistent with the origin of a basin dominated by strike-slip ~ctonics. The finding of this study helps in understanding the nature of subsidence in the strike-slip r:el~~e,~ ,b~in which concurrently challenges earlier models for a subduction-related origin for the S~awak Basin.

Tectonic evolution of accretionary prism in the arc-continent collision terrane of Taiwan

The thick sedimentary and meta-sedimentary rocks west of the Eocene-Paleozoic metamorphic basement of Taiwan represent an accretionary prism developed between the Eurasian continent and the Philippine Sea plate. The accretionary prism consists of a subduction wedge in the east and a collision prism in the west. The deep-marine subduction wedge developed during the eastward subduction of the South China Sea oceanic crust since the Early Miocene. In the Central Range, a regional unconformity with mylonite structure occurred between the Miocene deep-marine slates-turbidites and the Paleozoic-Eocene metamorphic basement. The unconformity marks the tectonostratigraphic break between the overlying subduction wedge and the underlying underthrust Eurasian continent. The subduction wedge extends from the western Central Range southwards through the Hengchun Peninsula to the offshore Hengchun Ridge. Subduction of the South China Sea oceanic crust further led to the oblique arc-continent collision starting about 6.5 Ma in northern Taiwan. During the collision, the shallow-marine passive margin and foreland sequences were progressively incorporated to the collision prism by a series of west-vergent thrusts in the Hsüehshan Range and the Western Foothills. The Kaoping Slope west of the subduction wedge of the Hengchun Ridge represents the modern collision prism in the active arc-continent collision zone. The collision prism is juxtaposed against the subduction wedge to the east along the Lishan-Laonung-Hengchun fault, which extends offshore to the 30-km-wide fault zone between the Kaoping Slope and the Hengchun Ridge. Before the onset of the arc-continent collision, the Lishan-Laonung-Hengchun fault developed along the northern part of the proto-Manila trench and acted as the thrust front located to the west of the subduction wedge. At present, the thrust front has migrated southwestward to the west of the collision prism. The arc-continent collision propagated southwards and yielded the time-transgressive deformations from north to south to result a south tapering configuration of Taiwan. This paper is the first to recognize the consistent occurrence of the subduction wedge and collision prism onshore and offshore Taiwan. This allows reconstruction of the tectonic evolution of the accretionary prism during the subduction and collision tectonics of Taiwan.

New Marine Data from Vietnam Margin Limit the Amount of Extrusion of Indochina During the Opening of the South China Sea: ABSTRACT

A total of 9300 km of high resolution, wide coverage multibeam (Simrad EM12) bathymetric data have been acquired offshore Vietnam during the Ponaga cruise of the R/V L'Atalante in May 1993. Gravity and magnetic measurements, 6-channel seismic data, as well as 6 dredges also have been obtained. East of central Vietnam, the margin displays northeast-southwest tectonic structures typical of a passive margin. The depth of the basement of the Nha Trang basin suggests that it could be of oceanic nature, with a 20 to 30 Ma age compatible with the age of the South China Sea oceanic crust located further east. Southeast of South Vietnam, the authors identified the western tip of the fossil axis of the South China Sea. It constitutes a propagating ridge into a highly stretched continental crust, partly intruded by volcanics. East of 110[degrees]30[prime]E, tilted blocks are symmetric with respect to the oceanic axis, whereas west of 110[degrees]30[prime]E they are mostly tilted toward the south, which suggests the occurrence a large listric normal fault associated with a large amount of extension. The normal faults bend progressively to a more northerly direction when approaching the north-south scarp that bounds the Conson basin. This geometry is compatible withmore » a right-lateral motion, and the normal faults associated with the oceanic propagator suggest that the dextral motion is synchronous with at least the last phase of spreading in the South China Sea (23-16 Ma). Since recent offshore oil data have established that the prolongation of the Red River fault within the Gulf of Tonkin was affected by left-lateral motion from the Oligocene to the upper Miocene, the results suggest that the change from left-lateral motion in the Gulf of Tonkin to right-lateral motion along the Vietnam margin occurs because the South China Sea basin opens more rapidly than the extrusion of Indochina. Thus, the total amount of extrusion of Indochina probably does not exceed 100 or 200 km.« less

Tectonic evolution of the NW Sabah continental margin since the Late Eocene

The NW Sabah continental margin in the northern part ofNW Borneo, consists of 2 main elements: a Tertiary trench-associated sedimentary basin with up to 12 km of siliciclastic sediments and the Southern South China Sea Platform, separated by the deep and relatively narrow, NE-trending NW Sabah Trough. The Tertiary sedimentary sequences were deposited during two main phases of basin development: 1. a pre-early Middle Miocene phase of generally deep-marine clastic sedimentation (Stages I, II and III). The sediments were subjected to strong compression to form an imbricate wedge related to the subduction of the South China plate. This was concomitant with the counter-clockwise rotation of the Borneo plate, and the relative motion of the two plates may have produced N-S wrench faults. 2. a post-early Middle Miocene phase of clastic shelf/slope deposition (Stage IV) which prograded northwestward over the underlying imbricate wedge and separated from the latter by a major regional unconformity. On the basis of differences in structural styles and sedimentation histories, the NW Sabah continental margin can be subdivided into 7 tectonostratigraphic provinces: (1) the CrockerAccretionary Prism, (2) the Inboard Belt, (3) the Outboard Belt, (4)the East Baram Delta, (5) the NW Sabah Margin, (6) the NW Sabah Trough and (7) the Southern South China Sea Platform. Some 90 exPloration and exploratory appraisal wells have been drilled in the NW Sabah offshore. Apart from minor oil and gas shows in the pre-early Middle Miocene deep­ marine sediments, all commercial accumulation~ discovered to date have been found in the Middle Miocene or younger clastic reservoirs in the Inboard Belt, Outboard Belt and East Baram Delta. The other tectonostratigraphic province have not been drilled to date. The tectonic evolution of the NW Sabah continental margin occurred in four stages: 1. The Late Eocene to early Middle Miocene subduction of the South China Sea oceanic crust beneath Borneo with deposition and subsequent of deep-marine sediments into an accretionary prism. 2. The collision and subduction of the South China Sea attenuated continental crust with Borneo in early Middle Miocene which led to the regional uplift and erosion of the accretionary prism resulting in the Deep Regional Unconformity. This was followed by NW progradation over the Inboard Belt from Middle Miocene to early Late Miocene. 3. Cessation of active subduction in middle late Miocene was accompanied by major tectonic activities. The Inboard Belt was subjected to strong compressional deforma­ tion, probably associated with deep-seated major N-S shear zones. The area was strongly folded, uplifted and eroded resulting in the Shallow Regional Unconformity.

Geodynamic evolution of the Taiwan-Luzon-Mindoro belt since the late eocene

The structural framework of the Taiwan-Luzon-Mindoro belt (or festoon) is described, following three major transects: the Luzon transect with active subduction and active island arc; the Taiwan transect with active collision; the Mindoro transect with active subduction and inactive collision. Based on this geological study and on available geophysical data, a model for the geodynamic evolution of this portion of the Philippine Sea and Eurasia Plates boundary is proposed in a succession of reconstructions between the Late Eocene and the Present. The major geodynamic events are: 1. (1) beginning of the opening of the South China Sea (S.C.S.) in Lower Oligocene times, contemporaneous with obduction of the Zambales and Angat ophiolites on Luzon. 2. (2) subduction of a Mesozoic (?) oceanic basin along the proto-Manila trench from the Upper Oligocene to the Lower Miocene. 3. (3) obduction of the South China Sea oceanic crust onto the Chinese and Reed Bank—Calamian passive margins in Middle Miocene time (14–15 Ma) related to a major kinematic reorganization (end of opening of the S.C.S.). 4. (4) beginning of collision between the Luzon microblock and the two margins of the S.C.S. in the Upper Miocene (~ 7 Ma); collision is still active in Taiwan whereas it stopped in Mindoro during the Pliocene.

Middle Oligocene oceanic crust of South China Sea jammed into Mindoro collision zone (Philippines)

Mindoro Island, south of Luzon (Philippines), is a complex junction between the Manila Trench and the collision zone of the North Palawan block with the western Philippines mobile belt. Middle Oligocene ophiolites recognized in this suture are coeval with the oldest magnetic anomalies of the South China Sea basin. These ophiolites are part of a complex pile of terranes thrust above the North Palawan block at the lower–middle Miocene boundary. These ophiolites are interpreted as fragments of South China Sea oceanic crust jammed between two distinct continental blocks during the counterclockwise rotation of Luzon.