The South China Sea (SCS) located at the intersection of the three intercontinental plates of Eurasia, IndiaAustralia, and the Pacific Oceanis, is a typical marginal sea basin formed by the seafloor spreading under the tectonic background of plate convergence. Many crustal-scale studies indicate that the SCS basin has undergone asymmetric spreading, multi-phase ridge jumps, and intense post-spreading volcanic activity. Due to the lack of seismic data in the oceanic basin of the SCS, it remainsunclear about the scale and basin control of the Zhongnan fault, the magma source depth of the SCS basin, and the transport channel after the cessation of seafloor spreading. Phase velocity derived from ambient noise surface wave tomography may provide useful information to shed light on the mechanisms of the aforementioned problems. From October 2019 to July 2020, a 3D Ocean Bottom Seismometers (OBS) passive seismic observation experiment was carried out by the Second Institute of Oceanography, Ministry of Natural Resources (SIOMNR) in a broad area of the SCS. Based on the seismic ambient noise data recorded by 16 OBSs in the SCS basin, we inverted the phase velocity images over a period range of 10–20 s using ambient noise surface wave tomography. Our results indicate that the Zhongnan fault zone is a lithospheric-scalefault, which played a regulating role in the last oceanic ridge transition of the SCS basin from the East Subbasin to the Southwest Subbasin. In addition, the low-velocity body in the north flank of the Southwest Subbasin extends from the post-spreading seamounts on the ocean crust to the uppermost mantle (i.e., about 10–30 km), which indicates an oblique magma migration during the postspreading volcanism.
SUMMARY The orientation of an ocean-bottom-seismometer (OBS) is a critical parameter for analysing three-component seismograms, but it is difficult to estimate because of the uncontrollable OBS posture after its deployment. In this study, we develop a new and effective method to estimate the OBS orientation by fitting the amplitude of direct P wave of teleseismic receiver functions. The reliability of this method is verified using synthetic data and observed waveforms recorded at land seismic stations in Shandong Province, China. Our extensive synthetic tests show that our new method is little affected by a thin sedimentary layer that has a low S-wave velocity. The orientations of OBS stations that we deployed in the Yap subduction zone in the Western Pacific Ocean are estimated and corrected using our new method. After the correction, the direct P waves of teleseismic receiver functions show very good consistency. The effects of white and coloured noise in different levels, epicentral distance and backazimuth are also investigated, and the results show that these factors have small effects on the new method. We also examine the effect of sensor tilting on estimation of the OBS orientation, and find that a tilting correction should be made before the misorientation correction. We compare the OBS orientations determined with the new method and other methods and find that they are generally consistent with each other. We also discuss advantages and shortcomings of various methods, and think that our new method is more robust than the existing methods.
We construct a complete density transection based on the velocity structures across the Zhongsha Bank in the South China Sea. Gravity modelling of the lateral density contrasts between tectonic units helps us to determine the structural attributes and boundaries between continental blocks and deep basins. The configuration of the continent–ocean boundary (COB) around the Zhongsha Bank is mapped based on the gravity/magnetic anomaly and crustal structures. A low-density mantle is found beneath the Zhongsha Bank and the oceanic basins, and this mantle is associated with the high heat-flow background. The COB orientation is northeast-east in the north of the bank, with faulted linear structures. In further southeast, where there is a more intact crust, the COB orientation changed to north-northeast. The reconstructed density model and gravity/magnetic map indicate that the Zhongsha Bank is conjugated with the Liyue Bank by a rifted basin, where the crust had experienced localized deformation before the seafloor spreading. Because of the insufficient magmatism in the oceanic basin, the spreading ridge propagates into the weakened continental lithosphere between the two continental blocks, thus completely separating the Zhongsha Bank from the Liyue Bank. Seafloor spreading ridge jumps within the South China Sea may also be affected by the heterogeneous lithosphere beneath the continental blocks and oceanic basins.
Abstract We determined P‐wave tomographic images by inverting a large number of arrival‐time data from 2749 local earthquakes and 1462 teleseismic events, which are used to depict the three‐dimensional morphology of the subducted Eurasian Plate along the northern segment of the Manila Trench. Dramatic changes in the dip angle of the subducted Eurasian Plate are revealed from the north to the south, being consistent with the partial subduction of a buoyant plateau beneath the Luzon Arc. Slab tears may exist along the edges of the buoyant plateau within the subducted plate induced by the plateau subduction, and the subducted lithosphere may be absent at depths greater than 250 km at ∼19°N and ∼21°N. The subducted buoyant plateau is possibly oriented toward NW‐SE, and the subducted plate at ∼21°N is slightly steeper than that at ∼19°N. These results may explain why the western and eastern volcanic chains in the Luzon Arc are separated by ∼50 km at ∼18°N, whereas they converge into a single volcanic chain northward, which may be related to the oblique subduction along the Manila Trench caused by the northwestern movement of the Philippine Sea Plate. A low‐velocity zone is revealed at depths of 20–200 km beneath the Manila Accretionary Prism at ∼22°N, suggesting that the subduction along the Manila Trench may stop there and the collision develops northward. The Taiwan Orogeny may originate directly from the subduction of the buoyant plateau, because the initial time of the Taiwan Orogeny is coincident with that of the buoyant plateau subduction.
Abstract Collision of oceanic plateaus with trenches has played a key role in the continental growth and plate tectonic reorganizations throughout the Earth's history. However, the understanding of collision between an initially rifted plateau with a trench is still deficient. Here we conduct the first seismic tomography and receiver function analyses in the Yap subduction zone, western Pacific Ocean, where the initially rifted Caroline Plateau is colliding with the Yap Trench. Our results reveal horizontal and overturned slabs south and north of the Sorol Trough, respectively, which may be caused by slab breakoff followed by underplating and eastward mantle flow with ultra‐slow convergence. The distinct slab morphologies could be responsible for the short‐lived arc volcanism (11‐7 Ma), horst‐graben structures, different Bouguer gravity anomalies and stress regimes in the Yap subduction zone. The overturned slab may cause the incoming plateau to be stretched to facilitate the initial plateau subduction.
Abstract Interaction of oceanic plateaus with trenches plays a vital role in subduction activities and tectonic evolutions. The Yap trench is a rare case of an oceanic plateau subduction system. However, the knowledge of the impacts of plateau-trench interaction on subduction activity is still insufficient, due to a lack of seismological observations. Using ocean-bottom seismometer data near the Yap trench from April 2016 to May 2017, we conduct seismicity analyses in the Yap subduction zone by utilizing a machine-learning algorithm and matched-filter detections. The pattern of seismicity in the Yap trench exhibits characteristics similar to typical active subduction zones. The seismicity delineates a steep subducting slab, which may have resulted from the blocking of the buoyant Caroline Plateau. The majority of earthquakes are shallower than 80 km in the event-detectable area in the Yap trench, much shallower than the potential slab depth of 350 km from the previous seismic tomography images.
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