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Xu Y.,Beijing University of Technology | Xu Y.,Chinese Academy of Geological Sciences | Lu Q.-T.,Chinese Academy of Geological Sciences | Zhang G.-B.,Beijing University of Technology | And 4 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2015

The Middle-Lower Yangtze River Metallogenic Belt (MLYRMB) is one of the most important metallogenic belts in China and plenty of researches have done in this belt. In this paper, we determined a 3-D S-wave velocity structure of the mantle down to 600 km depth beneath the MLYRMB by using 5524 S-wave relative travel times collected from 163 teleseismic events recorded by 73 static seismic stations located within this belt and its adjacent areas. The tomography method used in our study is the TOMOG3D, which has been widely used in a lot of tomography studies and gained a lot of achievements. In the data processing, we chose the data with high signal to noise ratio according to 3 criteria and used an automatic method called MMCC to pick up the relative travel times precisely. Besides that, we have also applied the crust correction method to reduce the effect caused by the crustal inhomogeneity, in order to make the final result more accurate. Our result shows: (1) there exist a low velocity anomaly in the upper mantle and the trend of the anomaly is consist with MLYRMB, which maybe the upwelling of asthenosphere; (2) there exist a high velocity anomaly in the mantle transition zone and the bottom of upper mantle which maybe the subduction of paleo-Pacific plate and the delaminated lithosphere; (3) the low velocity anomaly beneath the belt has a trend that is gradually shallower from the south to the north, which means the up welling of asthenosphere start from south to north. Combining a lot of studies about geophysics, geochemistry, geology and so on, we assumed that the formation of the MLYRMB in Mesozoic was mainly caused by the delamination of lithosphere. © 2015, Science Press. All right reserved.

Zheng D.,China University of Geosciences | Zheng D.,Beijing University of Technology | Li H.,China University of Geosciences | Li H.,Beijing University of Technology | And 4 more authors.
Geophysical Journal International | Year: 2016

The crustal and upper mantle velocity structure in the northeastern Tibetan Plateau is obtained from joint analysis of receiver functions and Rayleigh wave dispersions. The resulting velocity model reveals a close correlation between the thick (>60 km) crust and the presence of an intracrustal low-velocity zone beneath the Qiangtang and Songpan-Ganzi terranes as well as the northwestern Qilian orogen. However, the high Vp/Vs ratio of the crust is found only beneath the Qiangtang and Songpan-Ganzi terranes. The crustal low velocity zone does not appear in the west Qinling and southeastern Qilian orogens, which have a relatively thin (~50 km) crust, indicating that crustal channel flow is not the primary mechanism by which the northeastern Tibetan Plateau grows. A continuous low velocity zone from the mid-to-lower crust down to 160 km beneath the eastern Kunlun fault suggests an induced local mantle upwelling after partial detachment of the lithosphere. © The Authors 2015.

Wang Q.,Geological Exploration Technologies Institute of Anhui Province
Scientia Geologica Sinica | Year: 2014

On the shallow covered area of Bengbu-Fengyang, we explained the geological construction using the gravity and magnetic anomalies, and ascertained the tectonic system. We deemed that the Bengbu anticlinorium is outstanding southward. The trend identical with the Bengbu upwarping tectonic line is NEE direction on the west area. The trend is near E-W direction in the Fengyang County, which is in the middle of the area. On the east area, the trend is NE-trending. The left-lateral tractive effects of Tanlu fault zone is the main reason of the axis changing. We determined the hidden boundary of the Bengbu rock mass and the range of the Fengyang-Zongpu tectonic magmatic hydrothermal zone, and drew the map of the Bengbu-Fengyang area geophysics deducing bedrock tectonic outline. It is suggested that the intrusive contact zone of the Bengbu rock mass, the tectonic magmatic hydrothermal activity and the intersections of the important fault zones are the favorable prospecting zone. The prospecting direction is put forward.

Zhang Z.,Zhejiang University | Wu L.,Zhejiang University | Wang R.,Kunming Institute of Exploration | Zhang J.,Geological Exploration Technologies Institute of Anhui Province
Zhongnan Daxue Xuebao (Ziran Kexue Ban)/Journal of Central South University (Science and Technology) | Year: 2013

The first kind of Fredholm integral equation in the downward continuation was solved by adopting conjugate gradient method with the assumption that the coefficient matrix is positive definite. The theoretical model test shows that downward continuation has a very fast convergence rate but suggests poor ability to compress the noise.compared with integral iteration method, conjugate gradient method indicates a faster convergence rate, as well as divergence rate, and is not so applicable. In order to acquire stable solutions the regularization of the operator of the ill-posed problem was carried out and the problem was transformed to solve the minimum value of the least squares estimations. After using the conjugate gradient method to solve the least squares problem, a method was implemented for downward continuation based on CGNR method that has a better ability to suppress noise. Model test demonstrates that this method has strong ability in restraining noise. This method convergence fast and has obvious computation advantage by a comparison with least square steepest descent method.

Pan F.-B.,Wuhan University | Pan F.-B.,Zhejiang University | Zhang H.-F.,Wuhan University | Xu W.-C.,Wuhan University | And 3 more authors.
Lithos | Year: 2016

Mafic intrusive rocks are widely exposed in the Motuo tectono-magmatic belt, southeast Lhasa terrane. LA-ICP-MS U-Pb zircon dating shows that they have magma crystallization ages of 69 and ca. 50 Ma. These mafic intrusive rocks are characterized by variable SiO2 (44.60-57.60 wt.%), high Al2O3 (17.19-20.86 wt.%), and low MgO (1.85-5.38 wt.%) with Mg# of 31-55. Their chemical composition is comparable with low-MgO high-Al basalts to basaltic andesites. They are enriched in LILEs (Rb, Ba, K) and LREE and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with weakly evolved Sr-Nd-Hf compositions (whole-rock (87Sr/86Sr)0 = 0.7064 to 0.7086, εNd(t) = -3.41 to +1.22, and zircon εHf(t) = -3.8 to +6.4). The mafic rocks were derived from partial melting of metasomatized lithospheric mantle. Geochemical and Sr-Nd-Hf isotopic data show that they were insignificantly modified during magma emplacement. We provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern Lhasa terrane. Sr-Nd isotopic composition of the Motuo Late Cretaceous-Eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane. The foundered ancient materials may subsequently re-fertilize the lithospheric mantle not only in the eastern Lhasa terrane but also in the surrounding areas. © 2015 Elsevier B.V.

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