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Crustal thickness and composition are closely related to geology and tectonic evolution of the region. Studying the differences in the crustal thickness and composition of the South China Block (SCB) is important to gain a comprehensive understanding of multi-phase amalgamation, breakup, reworking, and regional geodynamic processes. In this study, teleseismic data from 135 high-density portable broadband stations from SinoProbe were processed using Common Conversion Point (CCP) stacking and H-κ stacking methods. The Moho depth and P-wave and S-wave velocity ratio (Vp/Vs) were studied. Our results revealed several insights about the tectonic processes in the SCB. First, the crustal structure and Vp/Vs ratios of the Cathaysia Block, Jiangnan Orogenic Belt, and Yangtze Block were significantly different. The average depth of the Moho in the Cathaysia Block was approximately 31 km, and the Vp/Vs ratios increased from the inland area (1.66) to the coastal area (1.78), indicating the oceanward increase of mafic proportion in the lower crust, which is related to the influence of the Paleo-Pacific Plate westward subduction. Second, the crustal thickness of Jiangnan Orogenic Belt deepens from east ca. 31 to the west ca. 42 km and the Vp/Vs ratios varied from 1.75 to 1.64, illustrating a relatively felsic crust, which could have been related to the Mesozoic upper crustal thickening under compression followed by the lower crust removal under the extensional background. Third, the average crustal thickness of the Yangtze Block was 42 km, and the Vp/Vs ratios ranged from 1.64 to 1.84, presenting a positive correlation between the Moho depth and the Vp/Vs ratio, which is explained by the relatively thick mafic lower crust. Based on the variations in the crustal structure and Vp/Vs ratios of the profile, we inferred that the central part of the Jiangnan Orogenic Belt was bounded by the Jiangshan-Shaoxing-Hengyang in the east and the Jiujiang-Shitai-Jishou in the west. The small-scale Moho depth undulations at the intersection of the Cathaysia Block and the Jiangnan Orogenic Belt could be related to the Mesozoic compression-extension geodynamic transformation. [ABSTRACT FROM AUTHOR]

Abstract: To understand the formation and the tectonic process of the Mesozoic middle–lower Yangtze metallogenic belt (YMB), the SinoProbe program deployed a quasi-linear passive source seismic array across the belt. We performed receiver function profiling and measurement of shear-wave splitting parameters with the collected data. Our results show that the Moho depth varies significantly along the profile and that a “mantle uplift” exists right beneath the YMB. We also found that the lower crust of the YMB is different from that of its adjacent areas in structure on the receiver function profile. It possesses seismic anisotropy with direction roughly parallel to the belt. Our SKS/SKKS shear-wave splitting results also show similar belt-parallel azimuthal anisotropy right beneath the YMB. We interpret the seismic anisotropy in the lower crust of the YMB as the result of mineral crystal alignment caused by melting and belt-parallel flow in the Mesozoic ore-forming process. Besides, we observed a nearly south-dipping converter extending from shallow to lower crust beneath the Hefei Basin, which most possibly resulted from the Mesozoic crustal extension. We interpret the “mantle uplift” and the crustal extensional structures to be consequences of asthenospheric upwelling during the Mesozoic ore-forming process. Our results suggest that the lower crust of the YMB was most likely one part of the multi-level metallogenic magma system in the Mesozoic magmatism and mineralization processes, and the formation of the metallogenic belt to be a result much similar to the MASH (Melting, Assimilation, Storage and Homogenization; cf. Hildreth and Moorbath, 1988; Richards, 2003) process. First, the asthenospheric upwelling resulted in a crustal extensional environment; then the melts from the upwelling asthenosphere intruded into the lower crust of the YMB, and assimilation occurred when they mixed with in situ lower crustal materials, which led to the formation of adakitic-like magma; the adakitic-like magma rose up along extensional structures, and reacted with country rocks to form mineral deposits. [Copyright &y& Elsevier]

The Cathaysia Block is located in South China, along the western margin of the Pacific plate, and well known for its widespread granitoids and rich poly-metallic mineralization. Despite the numerous studies conducted in this region, some key questions are still in debate, which revolve around the detailed crustal architecture, the boundary between the western and eastern part of Cathaysia Block, and the geodynamic evolution and metallogenic settings in the Cathaysia Block. In this study, we have conducted a dense broad-band seismic observation in the Cathaysia Block, and obtained the fine crustal structure of it by using P-wave receiver functions and analyzing other geophysical and geological studies comprehensively. Our study has following findings: (1) the crust of the Cathaysia Block is ~31.5 km thick, thinner than that of the global continent, and is interpreted to have been thinned since the Late Mesozoic; (2) the Vp/Vs ratio of the Cathaysia Block is lower than that of the global continent, which implies there is no extensive underplating beneath the Cathaysia Block, or that the magma may be confined at bottom of the crust and formed a velocity gradient belt around Moho; (3) Zhenghe-Dapu fault is marked as the structural boundary between the western and eastern Cathaysia Block; (4) the western and eastern Cathaysia Block have different intra-crustal structures, petrochronology features and mineralization settings; (5) the subduction of the paleo-Pacific plate and the following roll-back could be responsible for the geodynamic evolution of the crustal deformation, as well as the widespread magmatism and mineralization. • Fine crustal structure of Cathaysia is obtained by new seismic survey. • Zhenghe-Dapu fault is the boundary between the east and west Cathaysia. • No extensive magmatic underplating beneath the Cathaysia Block. • Two-stage paleo-Pacific subduction led to the crustal structure of Cathaysia. [ABSTRACT FROM AUTHOR]

The South China block (SCB) is located at the southeastern margin of the Eurasian plate, where intense Mesozoic magmatism and metallogenic belts occurred. The mechanism of mineralization is still unclear and debated. To clarify this issue, we determine a robust 3-D model of P-wave tomography of the upper mantle beneath the SCB by using 77,969 high-quality travel-time data of 934 teleseismic events recorded at 315 permanent and 72 portable stations in the SCB. Our results show that high-velocity (high-V) zones exist in the upper mantle under the Yangtze block and in the mantle transition zone under the eastern segment of the Jiangnan orogen. The former reflects thicker cratonic lithosphere, whereas the latter may reflect the deeply subducted paleo-Pacific plate. Low-velocity (low-V) anomalies are revealed in the upper mantle beneath the Cathaysia block, which are located above the high-V anomaly in the mantle transition zone. These low-V anomalies may reflect hot upwelling flows in the upper mantle, which might be related to the large-scale magmatic activities in the late Mesozoic. Our results suggest that the low-V hot mantle materials beneath the Cathaysia block have risen to the shallow lithosphere along magmatic channels such as fault zones, which eroded the nearby cratonic root of the Yangtze block and the overlying lithosphere and destroyed the lithosphere beneath the eastern SCB. The hot upwelling materials mineralized in the shallow part and so formed many metallogenic belts in the SCB. • The 3-D upper-mantle structure under the South China block (SCB) is studied using teleseismic tomography. • The Yangtze and Cathaysia blocks exhibit higher and lower P-wave velocities, respectively. • Hot mantle upwelling under the Cathaysia block caused lithospheric thinning and rich metallogenic belts in the eastern SCB. [ABSTRACT FROM AUTHOR]

We studied the crustal structure and tectonics in the north Tibetan Plateau from the Songpan-Ganzi terrane to the Qaidam Basin using teleseismic receiver-function imaging, across a major lithospheric boundary, the Kunlun-Qaidam boundary, where previous studies suggest a ~15–20-km change in crustal thickness from thicker crust in the Kunlun Mountains to thinner crust in the Qaidam Basin. We report P receiver functions for 70 stations, largely the International Deep Profiling of Tibet and the Himalaya (INDEPTH), phase IV, experiment. Our most dense station coverage is located along the roughly north-south INDEPTH-IV active-source seismic profile at approximately 95° E longitude. Azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and V p /V s from across the study area. Receiver functions show strong converters that we interpret as the Moho at ~70 km depth beneath the Qiangtang, Songpan-Ganzi terranes and Kunlun Mountains and at ~50 km depth beneath the central Qaidam Basin. This large change in crustal thickness occurs >50 km north of the North Kunlun strike-slip fault, on which the 2001 M8.1 Kunlun earthquake occurred. Receiver functions for some of the stations north of the thickness change at the Kunlun-Qaidam boundary also show a deeper ~70-km bright converter in addition to the 50-km converter. The two converters appear to overlap by up to ~30 km in some locations along the south Qaidam Basin. We combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north Tibetan Plateau including crustal flow and crustal injection. At depths imaged here, shallower than ~100 km, we see no evidence of southward subduction of Eurasian lithosphere. • Seismic data reveals lithospheric structure and composition in North Tibet. • The crustal thickness changes from 70 to 50 km beneath the Kunlun-Qaidam boundary. • North Tibet may have thickened as a result of crustal flow or injection. [ABSTRACT FROM AUTHOR]

In this study, we conduct ambient noise tomography and teleseismic two-plane-wave tomography to probe the crustal and uppermost mantle structures in the Middle–Lower Yangtze River region. The data used include 14 months (from July 2012 to August 2013) of continuous vertical component ambient noise data and 219 teleseismic earthquakes recorded at 138 broadband seismic stations from Chinese provincial networks and 19 temporary seismic stations deployed by China University of Geosciences (Beijing). First, we apply ambient noise tomography to the collected ambient noise data to generate Rayleigh wave group and phase velocity maps at 5–42 s periods and two-plane-wave tomography method to earthquake data to generate intermediate-to-long period phase velocity maps at 20–143 s periods. We then combine the short-to-intermediate period velocity maps from ambient noise and the intermediate-to-long period phase velocity maps from earthquake data to generate broadband phase velocity maps at periods from 5 to 143 s. By inverting these 5–143 s Rayleigh wave velocity maps, we construct a 3-D shear wave velocity model from the surface to ∼250 km depth in the Middle–Lower Yangtze River region. The 3-D model shows that in the upper crust, the basin regions, including the JiangHan, HeHuai, SuBei, HeFei and NanYang basins, are all featured with low velocities, and the mountain regions with high velocities. In the uppermost mantle, a low-velocity zone at ∼100–200 km depth is observed beneath the Middle–Lower Yangtze River Metallogenic Belt. Moreover, our tomographic results show that the NingWu and NingZhen ore districts are clearly characterized by the strongest low velocity anomaly in the uppermost mantle at ∼70–200 km depth. The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest JiuRui ore district to northeast NingWu ore districts. The change pattern of the low-velocity zone in the depth extent and the amplitude is consistent with the fact that peak ages of magmatic events along the Middle–Lower Yangtze River Metallogenic Belt progressively become younger and younger from 148 Ma in the southwest to 125 Ma in the northeast. The observed low-velocity zone may represent the cooling hot upper mantle which was partially molten in the past resulting from partial melting of the paleo-Pacific plate or of an enriched mantle source induced by the westward subduction of the paleo-Pacific plate. The upwelling of the mantle-derived magmas may result in the formation of these granitic rocks and coeval ores deposits along the Middle–Lower Yangtze River Metallogenic Belt. [ABSTRACT FROM AUTHOR]