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A 300 km long seismic reflection profile was acquired across the Middle and Lower Reaches of the Yangtze River (MLY) metallogenic belt and its adjacent areas. The objective of the survey was to establish the deep architecture and geodynamic framework of the region. Results based on the interpretation of the deep seismic data include (1) Tan-Lu fault appears as a subvertical thrust fault or transpression fault with its deep portion dipping toward the southeast; (2) the Zhangbaling uplift is squeezed out along this fault; (3) complex upper crustal deformation structures beneath the Chuquan depression include both kink bands, thrusts, imbrication and fold structures reflecting contraction deformation, and detachment fault and normal-fault structures reflecting extensional deformation; (4) the “crocodile” reflection structure emerging beneath the Tan-Lu fault and Ningwu–Lishui volcanic basin, which represents the decoupled deformation process of the upper and lower crust associated with intra-continental subduction; (5) further to the southeast, the upper crust deformation shows a large-scale “wave-form” pattern, making crustal scale syncline and anticline; (6) the entire section of the reflection Moho is clearly discernible at depth of 30.0–34.5 km, and the Moho beneath the Middle and Lower Reaches of Yangtze River metallogenic belt is shallowest, while the Moho beneath the North China block is deeper than that beneath the Yangtze block. The Moho offsets could be seen beneath the Ningwu volcanic basin. The seismic reflection data suggest that lithosphere delamination and asthenosphere upwelling that may result from the Mesozoic intra-continental orogenesis is responsible for the formation of large scale magmatism and mineralization in the MLY metallogenic belt. [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]

Abstract: Five intersecting reflection seismic profiles were shot over the Lu–Zong volcanic basin, one of the seven ore districts in the middle and lower Yangtze River Metallogenic Belt in east China. The seismic data reveal diversified reflective patterns in the upper crust, which are interpreted as horsts and grabens in general. The middle crust presents both thrust related structures and boudinage and pinch-and-well structure on different profiles, suggesting a strong structural heterogeneity, which developed due to a change from compressional to extensional movement. Below the central part of a volcanic basin, the lower crust shows an obvious reflective anisotropy both at the crust–mantle and the brittle–ductile transition in the crust. This feature may be explained by magma system emplaced along a NE-trending fault or weak zone at different depths. The Tan-Lu fault zone is present as a northwest-dipping fault extending to the middle crust. The lower crust beneath the Tan-Lu fault and peripheries is possibly of Dabie affinities. The Lu–Zong volcanic basin is clearly defined as an asymmetric graben with master faults on both the east and north flanks. A contrasting reflection character suggests a division of the basin into northern and southern parts separated by a concealed fault. The relatively thin volcanic sequences, inferred to be 800–1000m in thickness, cover Triassic and Paleozoic sequences at depth, that provide an exploration target in the northern part for deep porphyry and skarn deposits. The dominant deformation reflects NW–SE-oriented compression and extension formed during the Yanshanian orogenic episode. The contraction deformation is characterized by thrust faults with imbrication of crustal slices, whereas the extension deformation is featured by normal faults, regional detachments and basins. Underplating of mantle-derived magma and a multi-layer magma intrusion model are proposed to explain both seismic features and the NE-striking Lu–Zong volcanism and igneous intrusions in the region. [Copyright &y& Elsevier]

Highlights: [•] The crustal thickness and Poisson’s ratio are estimated at 252 stations in the eastern Tibet. [•] Our results show the strong heterogeneous and complex crust structure across the eastern Tibet. [•] Lateral variations of crustal thickness and Poisson’s ratio correlate well with different tectonic features. [Copyright &y& Elsevier]

A 3-D P-wave velocity structure down to 750 km beneath the eastern South China Block is obtained by using a large number of high-quality teleseismic data. Based on our results and those of previous studies, we suggest that late Mesozoic magmatism and lithospheric thinning in the eastern South China Block could be attributed to the subduction of the Paleo-Pacific Plate and its eastward rollback. [Display omitted] • A 3-D P-wave velocity model beneath the eastern South China Block is obtained. • The velocity structures beneath the eastern South China Block are very complicated. • The Paleo-Pacific Plate has had a profound impact on the eastern South China Block. The eastern South China Block (ESCB) is characterized by intense magmatism in the late Mesozoic. To further investigate the mechanism of late Mesozoic magmatism, we apply the tomographic method to construct a P-wave velocity model down to 750 km by using 40,989 P-wave arrival times collected from 687 teleseismic events recorded by 191 stations. Our model shows that (1) a high-Vp anomaly is observed beneath the lower Yangtze Block (YZB) and northern Cathaysia Block (CAB) in the crust and uppermost mantle, which could represent the present lithosphere; (2) two high-Vp anomalies are observed in the upper mantle beneath the YZB, which could be the detached lithosphere and the relict of the subducted SCB; (3) the break-off of the subducted Eurasian Plate has penetrated the 410 km discontinuity beneath central and northern Taiwan Island; (4) the stagnant Paleo-Pacific Plate in the MTZ is mainly beneath the lower YZB; (5) the low-Vp anomaly in the upper mantle that is immediately above the stagnant Paleo-Pacific Plate may represent the thermal asthenosphere caused by plate dehydration; and (6) the low-Vp anomaly beneath the southern CAB extends from the crust downward to the MTZ, which may represent an 'upward channel' for thermal materials. Based on our results and those of previous studies, we suggest that late Mesozoic magmatism in the ESCB could be highly associated with the subduction of the Paleo-Pacific Plate and its eastward rollback. We also suggest that thermal/chemical erosion may be responsible for lithospheric thinning in the ESCB. [ABSTRACT FROM AUTHOR]

• Tomography with a dense seismic array reveals the upper-mantle structure in SE China. • A low-velocity zone with high Poisson's ratio exists under the western Cathaysia block. • The low-velocity zone reflects thermal flow causing the late Mesozoic mineralization. Our study region is located in the Cathaysia block (CB), Southeast China. The most significant feature of the CB is the widespread Mesozoic magmatism and poly-metallic mineralization. However, there is still no consensus on their genesis and related tectonic processes. In this study, we determine high-resolution (0.3° in the horizontal direction) tomographic images of Vp and Vs and Vp/Vs ratio in the crust and upper mantle using 6773 P-wave and 4526 S-wave travel-time data of teleseismic events recorded by a broadband linear seismic array. The array was deployed by our team in the middle part of the CB, consisting of 81 portable stations with an interval of 5–8 km. Our tomographic results reveal two obvious low-velocity (low-V) zones with high Vp/Vs ratio (high-σ) beneath the Nanling ore belt and the Wuyishan terrane, which may reflect magmatic channels formed by partial melting that may provide heat for metal mineralization. Two high-V anomalies with low-σ are located beneath the western Wuyishan terrane and the eastern CB, which may represent refractory lithosphere or remnants of delaminated lithospheric material caused by underplating of the magma due to the subduction of the paleo-Pacific plate and its following rollback. There exists an anomaly with low-Vp, high-Vs and low-σ at depths of 150–300 km beneath the western Wuyishan terrane, which may be related to the reserved trace of magmatic conduits. On the basis of the tomographic results, we propose a geodynamic model to explain the deep mechanism of mineralization beneath the CB. [ABSTRACT FROM AUTHOR]

A mineralization zone exists in the Middle and Lower Yangtze River (MLYR) region. Previous studies have shown that this zone might be caused by the rich ejection of magma in the Mesozoic. We have applied the teleseismic tomography method to determine a 3-D P-wave velocity structure of the mantle down to 500km depth beneath this region by using 14,740 P-wave arrival times collected from 519 teleseismic events recorded by 46 portable and 47 static seismic stations located in this region. The relative residual times used for the tomography are calculated by the modified multi-channel cross-correlation method which increases not only the efficiency but also the data precisions up to 0.01s. The grid space is set as 1°×1° horizontally and 50–100km vertically. Our tomographic results show that the lithosphere with high velocity anomalies is separated into two parts: one locates above the depth of 100km and the other at the depths between 250km and 400km, and the asthenosphere with low velocity anomalies locates between these two parts of lithosphere, which indicates that the lithosphere delaminated together with the asthenosphere upwelling. The detachment of lithosphere might relate to the subduction of the western Pacific slab. Therefore, our study provides clear evidence to geoscientists for understanding the deep dynamic process beneath this region and a three-stage geodynamic process is developed based on the new evidence. [ABSTRACT FROM AUTHOR]