Xuelin Qiu

Crustal structure across the Xisha Trough, northwestern South China Sea

Located at the northwestern part of the South China Sea (SCS) between the Hainan and Xisha (Paracel) Islands, the Xisha Trough represents a failed rift in conjunction with the opening of the SCS between 32 and 17 Ma. From west towards east within a scale of several hundred kilometers, it presents all major stages of the rifting process, and thus, provides an ideal place to study the rifting process in great details. In the autumn of 1996, a joint team of Sino-German scientists carried out a wide-angle seismic experiment across the Xisha Trough with 10 ocean bottom hydrophones (OBH) along a 237-km NNW-SSE-oriented profile, which was surveyed in 1987 with multi-channel seismic (MCS) method by BGR of Germany and SOA of China. Favorable weather conditions and the powerful 4 x 12-1 air gun array rendered very good quality data with seismic signals observed at the offset of up to 110 km, A detailed velocity-depth model was obtained by using an interactive trial-and-error 2D ray-tracing method. Interpretation of the NICS data published by BGR provides very good geometrical constraints of the complex upper crustal structure, which is characterized by fault blocks, half-horsts and half-grabens filled with syn- and post-rift Cenozoic sediments. The velocity model in turn confirms the major structure outlined by the interpretation of the MCS data, showing a varying sedimentary layer between I and 4 km of thickness and velocities between 1.7 and 4.5 km/s. The P-wave velocity of 5.5 km/s on the top of the crystalline basement is relatively low, suggesting strong weathering. Within the crystalline crust, the velocity increases downward continuously to 6.8 km/s at the bottom of the crust without a clear differentiation in the middle crust, showing clearly its continental nature even beneath the Xisha Trough. The Moho is marked by a sharp first-order interface with a velocity of 8.0-8.1 km/s at the uppermost mantle. The Moho depth is 15 km beneath the center of the trough and increases gradually to more than 25 kin towards north and south, corresponding to an extreme thinning of the pre-rift continental crust from more than 25 kin under the coast line to only 8 kin beneath the Xisha Trough. The similar velocity structure of the continental nature on both sides of the Xisha Trough suggests a homogeneous pre-rift continental setting. The sharp Moho and the lack of high velocity body (HVB) in the lower crust imply no magmatic underplating, which is very different to interpretations across the eastern part of the continental margin in northern South China Sea. The intense faulting in the upper crust, the strong but rather symmetrical crustal thinning centered at the Xisha Trough and the close neighborhood to the open SCS NW subbasin suggest a pure shear rifting, which failed most likely at the phase shortly before the continental breakup

Characteristics of surface heat flow in the South China Sea

A total of 592 heat flow measurements, ranging from 8 to 192 mW/m(2), 78 percent of which are between 50 and 100 mW/m(2), have been collected in the South China Sea (SCS). To overcome shortcomings such as the uneven distribution of the measurements and the occurrence of abnormal heat flow values, the tectonic evolution of different areas and their crustal thickness have been combined in analyzing geothermal characters. The results show that the oceanic basins, where they are floored by oceanic crust, the western part of the southern margin and the western fault system have high heat flow values. Heat flow along the northern margin of the SCS increases from 61 mW/m(2) on the shelf to 73-80 mW/m(2) in the slope area, and in the Xisha-Zhongsha area increases from about 70 mW/m(2) to about 85 mW/m(2) from the NW to the SE. Heat flow in the Nansha (Spratley) Islands is about 60 mW/m(2) or even higher, and decreases from the NW to SE; The average heat flow of the Mekong Basin is about 60 mW/m(2), which is similar to that of the Beibuwan Basin on the northern margin. Heat flow on the eastern margin and on the eastern part of the southern margin is lower, especially in the Luzon Trough, where the average heat flow is lower than 40 mW/m(2). Observed heat flow values in the SW subbasin are generally lower than predicted theoretically from the age of the ocean floor, unlike values in the eastern subbasin. A high heat flow zone in the lower slope area of the northern margin is recognized for the first time

Mapping the crustal structure under active volcanoes in central Tohoku, Japan using P and PmP data

[1] We present high-resolution tomographic images of the crust under the active arc volcanoes in the central part of Northeast Japan (Tohoku) determined by using arrival times of first P waves and post-critically reflected waves from the Moho discontinuity (PmP). Results of detailed resolution analyses show that the addition of PmP data can improve significantly the resolution of the lower crustal structure. After the PmP data are included in the tomographic inversion, the low-velocity (low-V) anomalies in the lower crust under the active volcanoes can be better imaged. The low-V zones are clearly visible in the entire crust beneath the volcanoes extending from the surface down to the Moho discontinuity. Arc-magma related, deep, low-frequency microearthquakes are located around the low-V zones in the lower crust under the volcanoes, which occurrence is probably associated with the movement of fluid magma under the volcanoes.

Seismic observation of an extremely magmatic accretion at the ultraslow spreading Southwest Indian Ridge

The oceanic crust is formed by a combination of magmatic and tectonic processes at mid-ocean spreading centers. Under ultraslow spreading environment, however, observations of thin crust and mantle-derived peridotites on the seafloor suggest that a large portion of crust is formed mainly by tectonic processes, with little or absence of magmatism. Using three-dimensional seismic tomography at an ultraslow spreading Southwest Indian Ridge segment containing a central volcano at 50°28′E, here we report the presence of an extremely magmatic accretion of the oceanic crust. Our results reveal a low-velocity anomaly (−0.6 km/s) in the lower crust beneath the central volcano, suggesting the presence of partial melt, which is accompanied by an unusually thick crust (~9.5 km). We also observe a strong along-axis variation in crustal thickness from 9.5 to 4 km within 30–50 km distance, requiring a highly focused melt delivery from the mantle. We conclude that the extremely magmatic accretion is due to localized melt flow toward the central volcano, which was enhanced by the significant along-axis variation in lithosphere thickness at the ultraslow spreading Southwest Indian Ridge.

Along-axis variation in crustal thickness at the ultraslow spreading Southwest Indian Ridge (50°E) from a wide-angle seismic experiment

The Southwest Indian Ridge (SWIR) is characterized by an ultraslow spreading rate, thin crust, and extensive outcrops of serpentinized peridotite. Previous studies have used geochemical and geophysical data to suggest the presence of a thicker crust at the central and shallowest portions of the SWIR, from the Prince Edward (35°30′E) to the Gallieni (52°20′E) fracture zones. Here we present a new analysis of wide-angle seismic data along the ridge 49°17′E–50°49′E. Our main conclusions are as follows: (1) we find an oceanic layer 2 of roughly constant thickness and steep velocity gradient, underlain by a layer 3 with variable thickness and low velocity gradient; (2) the crustal thickness varies from ∼5 km beneath nontransform discontinuities (NTDs) up to ∼10 km beneath a segment center; (3) the melt supply is focused in segment centers despite a small NTD between adjacent segments; (4) the presence of a normal upper mantle velocity indicates that no serpentinization occurs beneath this thick crust. Our observation of thick crust at an ultraslow spreading ridge adds further complexity to relationships between crustal thickness and spreading rate, and supports previous suggestions that the extent of mantle melting is not a simple function of spreading rate, and that mantle temperature or chemistry (or both) must vary significantly along axis.

Numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence: Discussions on the evolution of the Tainan Basin

Existing evidence shows that an Oligocene erosion event occurred on the northern continental margin of the South China Sea, and the Tainan Basin area might be at the center of this event, followed by a rapid tectonic subsidence in the late Oligocene and early Miocene period. The rapid tectonic subsidence is mainly thermal-controlled, and the effect of the Yichu Fault on the Tainan Basin is limited to the basins eastern part. We develop a 2-D thermal-mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the Tainan Basin. Our modeling indicates that the Oligocene uplift, erosion, and subsequent rapid subsidence could be caused by a thermal event, and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults. However, it also suggests that other mechanisms might be needed to jointly account for the observed erosions. Citation: Shi, X., H. Xu, X. Qiu, K. Xia, X. Yang, and Y. Li (2008), Numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence: Discussions on the evolution of the Tainan Basin, Tectonics, 27, TC6003, doi: 10.1029/2007TC002163.

Three-dimensional tomographic model of the crust beneath the Hong Kong region

We present the first three-dimensional seismic velocity model of the Hong Kong region, including the Dangan Island fault zone (DIFZ). The crust beneath Hong Kong is predominantly igneous, and is characterized by relatively high seismic velocity. Further south, we observe an elongated velocity anomaly beneath part of the DIFZ where significant seismicity has been recorded. This anomaly demarcates the contrasting seismic velocity structures on the opposite sides of the fault zone from the surface to a depth of at least 20 km, suggesting that the DIFZ extends to the lower crust and dips subvertically. Our model also indicates that there is an abrupt change in the along-strike crustal structure of the fault zone, with a significantly higher seismic velocity in the region west of the seismically active area. We anticipate that this first three-dimensional subsurface tectonic model of Hong Kong and the DIFZ will help assess the influence of crustal heterogeneities on the spatial pattern of intraplate earthquakes in this densely populated region of south China.

Seismic phases from the Moho and its implication on the ultraslow spreading ridge

The Moho interface provides critical evidence for crustal thickness and the mode of oceanic crust accretion. The seismic Moho interface has not been identified yet at the magma-rich segments (46°–52°E) of the ultraslow spreading Southwestern Indian Ridge (SWIR). This paper firstly deduces the characteristics and domains of seismic phases based on a theoretical oceanic crust model. Then, topographic correction is carried out for the OBS record sections along Profile Y3Y4 using the latest OBS data acquired from the detailed 3D seismic survey at the SWIR in 2010. Seismic phases are identified and analyzed, especially for the reflected and refracted seismic phases from the Moho. A 2D crustal model is finally established using the ray tracing and travel-time simulation method. The presence of reflected seismic phases at Segment 28 shows that the crustal rocks have been separated from the mantle by cooling and the Moho interface has already formed at zero age. The 2D seismic velocity structure across the axis of Segment 28 indicates that detachment faults play a key role during the processes of asymmetric oceanic crust accretion.

Distribution and identification of the low-velocity layer in the northern South China Sea

The low-velocity layer (LVL), closely related with tectonic activities and dynamic settings, has always been a hot topic in the deep crustal structure studies. The deep seismic (OBS/OBH) and onshore-offshore experiments have been extensively implemented in the northern South China Sea (SCS) since the 1990s. Six seismic profiles were finished on the northern margin of SCS by domestic and international cooperations. The features of crustal structures were revealed and five velocity-inversion layers were discovered. Among them three LVLs with 3.0-3.5 km.s(-1) velocity are located in the sedimentary structure (2.0-6.0 km in depth and 2.0-4.6 km in thickness) of the Yinggehai Basin and Pearl River Mouth Basin. They were identified by the reflective and refractive phases for their shallow depth. The other two LVLs with 5.5-6.0 km.s(-1) velocity generally existed in the middle crust (7.0-18.0 km in depth) with an about 2.5-6.0 km thickness in the transitional crustal structure of the northeastern and northwestern SCS. They were detected by the refractive phase from their overlain and underlying layers. We explored the possible tectonic formation mechanisms combining with previously reported results, which provided evidence for the formation and evolution of SCS.

Postseafloor Spreading Volcanism in the Central East South China Sea and Its Formation Through an Extremely Thin Oceanic Crust

P-wave velocity models were obtained by forward and inverse modeling from 38 ocean bottom seismometers deployed in the central East sub-basin of the South China Sea (SCS). Four types of crust have been defined; a) thin oceanic crust ( 5 km), b) typical oceanic crust (5-6 km), c) thick oceanic crust hosting post-spreading volcanoes ( 6 km) with significant intrusive roots, and d) thick oceanic crust with enhanced spreading features ( 6 km) but without significant roots. Within the central East sub-basin, the thin oceanic crust, only identified inside a 80-km wide zone, is located within an overall 150-km wide domain characterized by N055° seafloor spreading trends. The post-spreading volcanoes were formed during a N-S tensional episode around 6-10 Ma, several millions of years after seafloor spreading ceased in the SCS. Seafloor spreading (N055° and N145°) and post-spreading (N000° and N090°) features are observed in the morphology of some of these volcanoes. The rupture of the brittle thin oceanic crust was focused where the crust was the weakest, i.e. at the intersection of the extinct spreading ridge with former fracture zones. From geological and geophysical arguments, we suggest that the post-spreading volcanism might have been influenced by the Hainan plume activity through a buoyancy-driven partial melting mechanism.