South China Sea Slab Research Articles

Seismic signature of subduction termination from teleseismic P- and S-wave arrival-time tomography: The case of northern Borneo

Studies attempting to gain new insights into the last stage of the subduction cycle are typically challenged by limited direct observations owing to a lack of recent post-subduction settings around the world. Central to unravelling how the subduction cycle ends is an understanding of crust and mantle processes that take place after subduction termination. Northern Borneo (Malaysia) represents a unique natural laboratory because it has been the site of two sequential subduction episodes of opposite polarity since the mid-Paleogene. The region exhibits several enigmatic post-subduction (after ∼ 10 Ma) features, including: subsidence followed by rapid uplift, localised intraplate volcanism, possible orogen collapse, and a pluton that emerged to become the third highest peak in southeast Asia, Mt Kinabalu (4095 m). Arrival-time residuals from distant earthquake data recorded by the nBOSS seismic network have been used to investigate P- and S-wavespeed variations in the crust and underlying upper mantle beneath northern Borneo. Our 3-D tomographic images consistently show a high-velocity perturbation in western Sabah that we associate with an upper-mantle remnant of the Proto South-China Sea slab, thus providing important constraints for tectonic reconstructions of SE Asia. The tomographic models, combined with other seismological and geological information, reveal evidence for lithospheric removal in eastern Sabah via a drip instability. Our results suggest that lithospheric drips can be smaller than previously thought, yet their effects on the post-subduction evolution of continental lithosphere can be significant.

Late Cretaceous fore–arc spreading in the northern Kuching Zone of West Borneo, SE Asia: Constraints from the Pakong Mafic Complex

• The Pakong Complex formed at ∼ 100–78 Ma, more widely than expected. • The Pakong Complex comprises N-MORB- and E-MORB-type mafic rocks with arc-like imprints. • Fore-arc spreading resulted in the generation of the Pakong Complex. • The Cretaceous tectonic evolution of West Borneo was related to the paleo-Pacific subduction . The tectonic evolution of West Borneo and its paleogeographic correlation with other blocks/terranes in Southeast Asia during the Mesozoic remain unclear. Here we present newly obtained zircon U–Pb ages and whole-rock elemental and Sr–Nd isotopic data for the Pakong mafic rocks along the Lupar Line in the northern Kuching Zone of West Borneo. The zircon U–Pb dating results show that the Pakong gabbros were formed at 82.0 ± 1.9 Ma, while the diabases were emplaced at 78.5 ± 0.9 Ma. Geochemically, all the Pakong mafic rocks can be divided into two groups. The Group 1 samples show an affinity to N–MORB with negative anomalies in Nb, Ta and Ti, while Group 2 is characterized by E–MORB–like REE pattern with slight enrichment in LREE but similar depletion in Nb, Ta and Ti to Group 1. Isotopically, the former (ε Nd (t) = 6.23–9.87) has more depleted Nd isotopic compositions than the latter (ε Nd (t) = 4.25–5.71). The combined signatures indicate that the N–MORB–like mafic rocks (Group 1) were derived from 10 to 30% partial melting of a depleted lherzolite mantle modified by subduction–related fluids and that the E–MORB–like mafic rocks (Group 2) were the results of mixing between the Group 1 magmas and the OIB-type melts. Integrating with regional data, we propose the Late Cretaceous fore–arc spreading development along the Lupar Line in the northern Kuching Zone of West Borneo, which was induced by the rollback of the subducting Paleo–Pacific plate rather than the Proto–South China Sea slab.

Mantle transition zone discontinuities beneath Taiwan and its adjacent areas: Implications for slab subductions

How deep the two opposite dipping subduction zones in the vicinity of Taiwan, northwestward-dipping Philippine Sea Plate (PSP) and eastward-dipping South China Sea Slab (SCS), penetrated down into mantle, are hotly on debate and insufficiently imaged by seismic tomographic inversions, either due to inadequate quantities of seismic data or limited vertical resolution of the techniques used by previous investigations. Here we employ an unprecedented number of 55,847 high-quality P-to-S radial receiver functions recorded by 304 broadband seismic stations located in SE China and adjacent areas to image the topography of the mantle transition zone (MTZ) discontinuities (d410 and d660) to delineate the configurations of the subducted slab segments and analyze the impacts of thermal and water content anomalies on the MTZ. The close-to-normal wavespeed-corrected d410 and a ~ 10 km depression of the corrected d660 to the east of northern Taiwan are revealed, indicating that the PSP breaks off above the d410 and the delaminated PSP drops into the lower MTZ along with slab dehydration. To the east of southern Taiwan, where the SCS begins to subduct eastward, the uplift of the apparent d410 is nearly twice as much as that of the d660. The positive wavespeed anomaly above the d410 and low temperature in the vicinity of the d410 are enough to contribute to the uplift of MTZ discontinuities, suggesting that the SCS just reaches the d410, rather than penetrating the entire MTZ.

P-wave tomography of subduction zones around the central Philippines and its geodynamic implications

High-resolution tomographic images are obtained by inverting a large number of arrival-time data of local earthquakes and teleseismic events to depict the 3-D crustal and upper mantle structure beneath the central Philippines. Our tomographic results show that the subducted South China Sea slab beneath the southern segment of the Manila Trench steepens and tears, resulting in migration of the locus of active volcanism in the Macolod Corridor, due to the collision between the Palawan microcontinental block and the Philippine Mobile Belt. The subduction of the Philippine Sea Plate along the Philippine Trench started at 10–12°N or south of 12°N, the central part of the trench, from at least ∼10Ma estimated from our tomographic images. Our results reveal clearly a high-velocity anomaly in and around the mantle transition zone, which is interpreted as the subducted Proto South China Sea slab that sinks deeper southeastward, being well consistent with geological results that the age of collision between the Palawan microcontinental block and the Philippine Mobile Belt becomes younger from the south to the north. This collision zone can be divided into northern and southern segments, demarcated by the salient point of the collision zone, which is probably the boundary between the South China Sea slab and the Proto South China Sea slab, and may be ascribed to the complete consumption of the two slabs.

Present‐Day Crustal Movement and Focal Mechanism Solutions, and Plate Interaction Since Late‐Miocene in the Eastern South China Sea

AbstractThe plate convergent belt in the eastern South China Sea (SCS) is an important window for studying the tectonic evolution of the SCS and the Philippine Sea (PhS) since late‐Miocene. We studied the plate interactions of this area since the late‐Miocene based on block kinematics, morphology of the subducted SCS slab and focal mechanism solutions (FMS). At the late stage of the late‐Miocene, the westward motion of the PhS plate was first impeded by its collision with the Palawan micro continental plate in the south, and then by the collision of the Luzon arc with the South China continental margin in the north. Consequently, the middle part between the two collision zones became a passage where westward movement of the PhS plate continued relatively smoothly. Because of these two impediments, the rate of westward motion increased northwards to its maximum in the northern Luzon and then decreased northwards, leading to the westward migration of the Manila trench. The subducted SCS slab, however, did not roll back accordingly due to the SE‐directed mantle flow. As a result, the subducted slab suffered strong push and slightly reverse bending of the subducted slab occurred in the contact face between PhS and SCS lithosphere. According to this model, morphology of the Manila trench was shaped by the differential westward motion rates from south to north of the northern Luzon, and the width of the subducted slab played little role. Furthermore, the double arc in the north Luzon arc was also induced by a differential westward moving velocity along the arc. A shear zone whose location coincides with the east arc might have facilitated its formation.

Tomographic evidence for a slab tear induced by fossil ridge subduction at Manila Trench, South China Sea

A tomographic travel-time inversion has been applied to trace the subducted slab of the South China Sea (SCS) beneath the Manila Trench. The dataset, taken from the International Seismological Centre (1960–2008), is composed of 13,087 P-wave arrival times from 1401 regional earthquakes and 8834 from 1350 teleseismic events. The results image the different morphology of the subducted SCS slab as a high-velocity zone. The subducting angle of the slab varies along the trench: at 16° N and 16.5° N, the slab dips at a low angle (24° ~ 32°) for 20–250 km depth and at a moderate angle (50°) for ~250–400 km depth. At 17° N, the slab dips at a low angle (32°) to near 400 km depth, and at 17.5° N and 18° N the slabs are near vertical from 70 ~ 700 km depth, while at 20° N the high-velocity anomalies exhibit features from horizontal abruptly to near vertical, extending to 500 km depth. The dramatic steepening of the slab between 17° N and 17.5° N may indicate a slab tear, which is coincident with the axis of a fossil ridge within the SCS slab at around 17° N. In addition, low-velocity zones in the three profiles above 300 km depth may represent the formation of the slab window, induced by ridge subduction and slab tear, initiating upward mantle flow and resulting in the partial melting of the edge of the slab. The slab tear could explain the volcanic gap and geochemical difference between the extinct Miocene and Quaternary volcanoes in the Luzon Arc, the much higher heat flow around the fossil ridge, and the distribution of most of the adakites and the related porphyry Cu-Au deposits in the Luzon area. Based on the geometry and morphology of the subducted slab and certain assumptions, we calculate the initial time of ridge subduction, which implies that ridge subduction and slab tear possibly started at ~8 Ma.

Post-rift uplift and focused fluid flow in the passive margin of northern South China Sea

An important post-rifting tectonic event with extensive uplift and fluid flow took place at the northern margin of the South China Sea (SCS) during the late Miocene. Based on an analysis of two- and three-dimensional seismic data plus drilling well data, the tectonic event mainly affected the Dongsha Rise and the adjacent depressions. Tectonic deformation likely occurred in the Dongsha Rise, the eastern part of the Panyu Swell and several depressions of the Zhu I Sag. The post-rifting deformation in the study area was characterized by faulting, erosion, igneous activity and hydrothermal fluid flow. The tectonic movement caused abundant secondary NW and WNW tensional and transtensional faults, with lengths of 1 to 10 km. It also created a clear angular unconformity which marks the time of the tectonic movement Late Neogene igneous rocks are found in the strata in the Dongsha Rise only close to the continent-ocean transition zone. Geothermal activity occurred in the Liuhua carbonate platform and adjacent depressions. Deformation is more intense in the eastern area than in the western area. According to sequence stratigraphy, we infer that the tectonic event mainly occurred in the late period of Late Miocene and ended at the Mio/Pliocene boundary (5.5 Ma). The dynamic mechanism for tectonic movement could be associated with subduction of the South China Sea slab beneath the Philippine Sea Plate at the Manila trench. When subduction of lower density crust within the continental-ocean transition zone was initiated, large resistance stresses may have led to lithosphere bending and crustal uplift in the Dongsha area. (C) 2014 Elsevier B.V. All rights reserved.

Tomography of the southern Taiwan subduction zone and possible emplacement of crustal rocks into the forearc mantle

This paper investigates velocity structure of the active plate boundary in southern Taiwan by joint analysis of gravity anomaly and seismic arrival time data. P and S–P arrival time data from 3238 earthquakes were used. In addition to CWBSN permanent networks, seismic data include the Central Weather Bureau permanent networks and a temporary network consisting of 11 ocean bottom seismometers (OBSs) that was deployed to detect the aftershocks of the 2006 (ML7.1) Henchun earthquake occurred beneath southern Taiwan. The total available OBS data set consists of ~700 detected earthquakes, from which around 500 could be well located where about 450 events have been used in simultaneous inversion for hypocenters, three-dimensional Vp and Vp/Vs models for the study area. The main objective of incorporating gravity analysis is used to improve the velocity model for the offshore area, where it is poorly sampled by local earthquakes. This study found a low-velocity zone existing above the subducting South China Sea slab in the mantle wedge. Based on gravity modeling and our resulting velocity and Poisson's ratio models suggest that the subduction complex, which is characterized with a low P-wave velocity and low Poisson's ratios beneath the southern Taiwan. This duplex structure is characterized by a zone of low P-wave velocities in the range of 6.2–6.8km/s between 25 and 40km depth. It also shows that earthquake hypocenters do not fall within this low velocity zone. We have also used seismic tomography velocities to estimate the volume percentage of serpentinite and silica concentrations in southern Taiwan. The calculated serpentinite is about 30% and the volume percentage of quartz estimated is about 20% at the base of the forearc lower crust.

Reappraisal of the Arc-Arc Collision in Taiwan

Al though it is evident that Tai wan has been formed by the collision of the west-facing Luzon arc with the Eurasian continental mar gin, there re main a lot of enigmas in this collision. The major ones are: (1) a trans form fault presently connecting the Manila and Ryukyu Trenches in the Philip pine Sea-Eurasia relative motion direction is missing, and in stead, the Ryukyu Trench ex tends near off shore E. Tai wan, (2) the western edge of the intermediate-depth seismicity associated with the Philip pine Sea plate subduction beneath NE Tai wan has a NNW trend, not NW, and (3) a large negative Bouguer gravity anomaly, with undulation of its amplitude in a wave length of 60-80 km, exists along the southernmost Ryukyu forearc. We propose a new model of the collision in Tai wan to re solve these enigmas, assuming that the southern Ryukyu forearc was migrating to the south west with respect to Eurasia for the past several m.y. and the Luzon arc has been colliding with this actively migrating Ryukyu forearc. The northern most Luzon arc is divided into two parts by the NNW line directing along the Philip pine Sea-Ryukyu forearc motion from its initial intersection point with the Ryukyu Trench; the part west of this line has been obducted on the Ryukyu forearc-Eurasian mar gin, producing the collision orogen in Taiwan, and the part east of it has been subducted beneath the Ryukyu forearc. This evolutionary scenario re solves enigmas (1) and (2) kinematically. This model also predicts that the South China Sea slab has to be torn by the west ward component of the motion of the subducting Philip pine Sea slab to Eurasia. This would have brought large lateral compression in the shallow portion of the Philip pine Sea slab at its western border, which might lead to buck ling of the slab causing the ob served undulated gravity anomaly.

The P wavespeed structure in the mantle to 800 km depth below the Philippines region: geodynamic implications

P waves from earthquakes south of Taiwan, recorded by the BATS seismic array and CWB seismic network, were used define the P wavespeed structure between depths of 100 and 800 km below the Philippines region. The presence of a low wavespeed zone in the upper mantle is inferred, although the details are unclear. Wavespeeds in the uppermost mantle are low, as expected for seismic energy propagating within an oceanic plate. The estimated depths of the 410- and 660-km discontinuities are 325 and 676 km respectively. The unusually shallow depth of the upper discontinuity below and to the east of Luzon is inferred by clearly resolving the travel-time branch produced by refraction through the transition zone. A possible explanation for the northern part of the region covered is that seismic energy reaches its maximum depth within or close to the cool, subducted oceanic South China Sea slab where subduction has been slow and relatively recent. Further south, however, the presence of a broken remnant of the South China Sea slab, formed during a period of shallower subduction, is suggested at depths below 300 km to explain the broad extent of the elevated 410-km discontinuity. The 660-km discontinuity is slightly deeper than usual, implying that low temperatures persist to lower mantle depths. The wavespeed gradients within the transition zone between depths of 450 and 610 km are higher than those predicted by both the pyrolite and piclogite models of the mantle, possibly due to the presence of water in the transition zone.

The crust and upper mantle structure beneath southeastern China

We analyzed teleseismic waveforms recorded by 44 stations in the Fujian and Taiwan provinces of China and obtained 5344 high quality receiver functions. The crustal thickness ( H) and average crustal V P / V S ratio ( k) beneath every station were estimated using the H– k stacking method. Crustal thicknesses near the Fujian Province range from 28.3 to 32.8 km with an average of 31.1 km, and the corresponding V P / V S ratios vary from 1.70 to 1.84 with a mean of 1.76. From inland to offshore of the Fujian Province, the crustal thicknesses decrease and Poisson's ratios increase. These may indicate decreasing SiO 2 and increasing calc-alkaline contents in the crust. The discontinuity structures such as the Moho, subducting slab, the 410- and 660-km discontinuities (hereafter we call them the 410 and the 660) are also studied using common converted point (CCP) stacking of receiver functions. Along two NW–SE lines of central and northern Taiwan, the CCP stacking results show a western dipping structure at depths above 50 km, suggesting that the Philippine Sea plate is probably subducting beneath the Eurasian continent plate near the central and northern Taiwan. The CCP stacking results show sharp and flat 410- and 660-km discontinuities, and the transition zone thickness (TZT) is the same as that of ambient mantle beneath Fujian and Taiwan Strait, but thickens in the east of Taiwan. These results suggest that (1) the subducting Eurasian continent plate is confined to the depths above 410 km beneath Fujian and Taiwan Strait; and (2) the South China Sea slab may reach the transition zone beneath the east of Taiwan.

Development of three‐dimensional basement structure in Taiwan deduced from past plate motion: Consistency with the present seismicity

Using colored clay, we examined geometrical evolution of the three‐dimensional basement structure in Taiwan due to relative plate motion under the following assumptions: the motion of the Philippine Sea plate relative to the Eurasian plate has been constant during the last 15 Myr, the Ryukyu and Manila trenches had been connected by a transform fault before the collision between the Luzon arc and Eurasian continental margin, and the collision started at 5 Ma. The basement structure obtained from the clay model was fully consistent with observed deep seismicity, which led us to the following conclusions. There are no oceanic slabs beneath central Taiwan. Instead, the Eurasian continental margin is underthrusting beneath it. The Philippine Sea slab extends northwestward from the Ryukyu trench, while the South China Sea slab extends southeastward from the Manila trench. The downdip length of the South China Sea slab becomes shorter from south to north.

New insights on 3-D plates interaction near Taiwan from tomography and tectonic implications

Recent tomographic results are used to trace the South China Sea and Philippine Sea subducting slabs, south and northeast of Taiwan along the Manila and Ryukyu trenches, respectively. In particular, the 3-D plates interaction beneath Taiwan is discussed based on a close-up view of the tomographic sections together with earthquake hypocenters distribution. Our study indicates that: (1) the east-dipping South China Sea slab can be followed to the north, up to the latitude of Hualien, (2) the Eurasian plate subducts beneath most part of the Taiwan island down to the 670 km-depth discontinuity, (3) the north-dipping Philippine Sea slab can be followed slightly west of the longitude of Hualien. Both plates thus interact beneath northern Taiwan where the arc–continent collision is paroxysmal. (4) Slab detachment is envisaged at the northern edge of the subducted Eurasian plate beneath the Coastal Range of Taiwan, which may facilitate the northwestward motion of the Philippine Sea plate with respect to Eurasia. Slabs geometries obtained from tomographic sections allow us to reconstruct the Late Neogene plate kinematics and dynamics in this region. Our main conclusions are: (1) The size of the original South China Sea was about twice its present size. (2) The subducted part of the West Philippine Basin i.e. the largest oceanic basin of the Philippine Sea Plate, extends only 400 km north of the Ryukyu Trench. (3) Slab detachment might have occurred 3–5 my ago beneath the central and northern Ryukyu Arc along a weak zone that is aligned with the Gagua Ridge: an ancient plate boundary. (4) The Ryukyu Trench has propagated westward from 126°E of longitude (southeast of Miyako Island) to the locus of the present arc–continent collision, along a major lithospheric tear that cut through the continent–ocean boundary first, and then through the continental lithosphere. As a consequence, the southern Ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my. It has been elaborated onto a passive margin which corresponded to the edge of the Eurasian continental shelf bordering the South China Sea prior to Pliocene. (5) The Southern Okinawa Trough rifted during the last 2±1 Ma following the propagation of the subduction zone. (6) Subduction of the Eurasian margin probably stopped recently beneath northern Taiwan and a flip of subduction from west to east is being initiated near Hualien.