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Le B.M.,Tongji University | Yang T.,Tongji University | Gu S.,Seismological Bureau of Hainan
Journal of Asian Earth Sciences | Year: 2015

The origin of the widespread volcanism at the Leizhou-Hainan (Leiqiong) region in the Southern China remains obscure. We take advantage of the highly active seismicity and dense seismic networks surrounding this region to investigate its upper mantle and Mantle Transition Zone (MTZ) structure. Over 5000 P-wave waveforms whose raypaths bottom at depths around the MTZ are collected, and traveltimes of their first arrivals are hand-picked. By matching the traveltime curve variation over the epicentral distance range from 10° to 35°, we first construct a 1-D upper mantle and MTZ velocity structure for the region. This initial model is then refined by forward modeling, in which the observed triplicated waveforms from selected earthquakes are compared with the synthetic seismograms with varying velocity structure. In our preferred model for Leiqiong, the P-wave velocities deeper than 200. km at the upper mantle are 0.8-1.2% lower than the IASP91, and 0.6% slower in the MTZ, while the top and bottom boundaries of the MTZ depresses 12. km and slightly uplifted, respectively, compared to the global averages. This model provides independent constraints on the structure beneath Leiqiong, suggesting a thermal anomaly within the MTZ and a lower mantle origin for the volcanism seen in this region. © 2015 Elsevier Ltd.

Ma R.-Y.,CAS Qinghai Institute of Salt Lakes | Ma R.-Y.,University of Chinese Academy of Sciences | Han F.-Q.,CAS Qinghai Institute of Salt Lakes | Ma H.-Z.,CAS Qinghai Institute of Salt Lakes | And 11 more authors.
Acta Geoscientica Sinica | Year: 2015

A study of the hydrochemical characteristics and boron isotope distribution of salt lakes in Hoh Xil region was conducted on the basis of the survey in 2009 and 2010. Besides the Mingjing Lake, Xijinwulan Lake, Lexiewudan Lake, Salt Lake and Cangcuo Lake discovered earlier, the Duoxiu Lake, Chacuo Lake, Bucha Lake and Guomucuomaderi Lake are all salt lakes discovered recently, whose salinities are equal to or higher than 50 g/L. These discoveries have brought the total number of salt lakes in this region to 9. Except for the Lexiewudan salt lake, which is of the chloride type, the remaining 8 salt lakes are of the sulfate type; the boron and lithium are both abundant in the lake water and surrounding waters, and B-Li elements indicate a collaborative growth and decline relationship, suggesting that the material sources, migration conditions and enrichment environment of B is similar to characteristics of Li in this area. The results of Positive Thermal Ionization Mass Spectrometry measurements show that δ11B values are in the range of +4.77‰~+12.52‰, by far lower than the values of seawater, implying that these salt lakes belong to terrigenous nature, which is consistent with the results of previous research on these areas. There are a large number of springs exposed on the northern bank of the Lexiewudan salt lake and Bucha salt lake, and the springs are the primary material sources of these two lakes according to hydrochemical and boron isotopic analysis. The regional tectonic features and boron isotope geochemistry characteristics reveal that the boron element in the Xinqingfeng hot spring is mainly from lixiviation of volcanic rocks in the depth. ©, 2015, Science Press. All right reserved.

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