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Cui J.,State Key Laboratory for Continental Tectonics and Dynamics | Lin W.,Japan Agency for Marine - Earth Science and Technology | Wang L.,Chinese Academy of Geological Sciences | Gao L.,China University of Geosciences | And 9 more authors.
Tectonophysics | Year: 2014

The Wenchuan Earthquake Fault Scientific Drilling Project was implemented rapidly after the great 12 May 2008 earthquake (Mw 7.9) to better understand rupture mechanisms of the seismic faults. The first borehole of the project, WFSD-1 was located in Hongkou Township, Dujiangyan City, Sichuan Province, China on the hanging wall of the Yingxiu-Beichuan fault, which underwent large dextral and vertical displacement during the earthquake. The near-vertical borehole was 1201m long. In-situ stresses were measured in rock samples from depths between 424 and 1173m drilling depth by the anelastic strain recovery (ASR) method. The average trend of the maximum principal stress σ1 was N309° (ranging from N291° to N325°), rotated with increasing borehole depth from NW-SE to WNW-ESE. The magnitude of the in-situ maximum principal stress was estimated to be 35.3MPa at the depth of 1173m. The relations between horizontal and vertical stresses are vertical stress σv>maximum horizontal stress σH>minimum horizontal stress σh at depths above 424m, σH>σh>σv at depths from 424m to 800m, and σH>σv>σh below the depth of 800m. These indicate that the stress states are in a normal faulting stress regime above 424m; a reverse faulting regime from 424m to 800m and a dextral strike-slip regime below 800m. The stress measurements from WFSD-1 are consistent with the focal mechanism solution in which the mainshock was dominated by thrusting accompanied by dextral strike-slip motion. The orientations of σ1 are roughly consistent with the tectonic displacement direction of the Longmenshan area, which provides further evidence of NW-SE movements representing compression of the Songpan-Ganzi block toward the Sichuan basin. © 2013 Elsevier B.V. Source


Gong Z.,Chinese Academy of Geological Sciences | Gong Z.,State Key Laboratory for Continental Tectonics and Dynamics | Li H.,Chinese Academy of Geological Sciences | Li H.,State Key Laboratory for Continental Tectonics and Dynamics | And 4 more authors.
Tectonophysics | Year: 2015

We investigate the drilling mud gas from 70m to 1200m borehole depth of the Wenchuan earthquake Fault Scientific Drilling Hole-1 (WFSD-1) in the context of rapid response fault zone drilling to large earthquakes. Complete depth profiles are accomplished for CH4, He, H2, CO2, N2, Ar, O2 and N2/Ar. 222Rn is also available for a limited depth. The volcanic hanging wall generates high Rn and He, whereas the CH4 is low. The sedimentary footwall yields higher CH4 and CO2, with CH4 peaks up to 2.5vol.% in the intensely fractured shale and siltstone from 640m to 715m.The drilling mud gases distribute asymmetrically across the co-seismic slip plane at 589.2. m, with the bellow 117. m true depth manifests as a prominent gas anomaly zone. Gas concentrations in this zone are much higher and vary more violently than the upper 200. m. The low P-wave velocity and resistivity imply that the strata are highly fractured, and gases intruded the borehole mainly by dissolving in water. We suggest that the gas distributions are partially lithology affected, but mostly fracture constrained according to the lithology variations, subsidiary fault zone and fracture distribution retrieved from the core. Considering the low deformation rate and seismicity in this region before the earthquake, together with the ductile behavior of the clay-rich fault gouge under slow tectonic loading, and the decreasing gas anomalies away from the slipping plane, we propose that the fractures were mostly generated during the Wenchuan earthquake. It indicates that the Wenchuan earthquake damaged the footwall more than the hanging wall, and the 117. m depth marks the width of the fracturing zone. The differential mechanic property between the hanging wall and the footwall, and the asymmetric stress during the rupturing may contribute to the asymmetric fracture distribution. © 2014 Elsevier B.V. Source


Yu S.Y.,Chinese Academy of Geological Sciences | Yu S.Y.,State Key Laboratory for Continental Tectonics and Dynamics | Zhang J.X.,Chinese Academy of Geological Sciences | Zhang J.X.,State Key Laboratory for Continental Tectonics and Dynamics | Hou K.J.,Chinese Academy of Geological Sciences
Acta Petrologica Sinica | Year: 2011

In this article, we report the geochemical, zircon U-Pb dating and Hf isotopic data for tonalite and granite from DuIan area, the North Qaidam Mountains. Zircon U-Pb dating result indicates that the tonalite was emplaced at 432 ∼434Ma, which shows a geochemical resemblance to adakite: ( 1 ) high SiO 2, enriched in Na 2O and depleted in K 2O, and peraluminous: (2) high Sr and low Y, and thus corresponding to high Sr/Y: (3) high La/Yb amd low Yb: (4) depleted in HREE, and thus strongly partition between HREE and LREE; (5) enriched in LILE including Ba and Sr, depleted in HFSE such as Nb and Ta. In combination with the high Σ m(t) values (3. 3 ∼ 10. 7) and young crustal model age of 732 ∼ 1207Ma, the tonalite was suggested to derived from partial melting of thickened basaltic lower crust. Zircon U-Pb dating result indicates that the granite was emplaced at 382 ∼391Ma, and the granite shows geochemical character similar to S-type granite as followings: ( 1 ) high SiO 2and K 2O, and low MgO, with K 2OZNa 2O values greater than 1.0: (2) peraluminous with A/CNK greater than 1. 0; (3) slightly partition between LREE and HREE ( La/Yb) = 2. 22 ∼9. 62) , with negative Eu anomalies ranging from 0. 27 to 0. 81: (4) strongly depleted in Nb, Ta, Sr and Ti, and slightly depleted in Ba. On the basis of the age interval between granite and HP/UHP metamorphic event, in combination with the geochemical characteristic, the granite was suggested to derive from the partial melting of its country rock Al-rich gneiss with input of minor mantle material in post -collisional extension stage. Source


Pei J.L.,Chinese Academy of Geological Sciences | Pei J.L.,State Key Laboratory for Continental Tectonics and Dynamics | Li H.B.,State Key Laboratory for Continental Tectonics and Dynamics | Si J.L.,State Key Laboratory for Continental Tectonics and Dynamics | And 4 more authors.
Acta Petrologica Sinica | Year: 2011

The pulse uplift of Tibetan Plateau is a compressing uplift event resulted from the inner continent subduction of Indian Plate since ca 55Ma. The uplift of northern Tibetan plateau occurred in the Late Cenozoic definitively influenced the tectonic formation and the large scale aridity. The Mazartagh fault zone lies in the central Tarim basin (latitude; N38°40.911', longitude; E80° 18.484'), about 300km long in the west-east direction. Lower Pleistocene section is a complete sedimentary sequence and wellexposed section in the south of Mazartagh Mountain. This study extends this section, and 9 sites 90 samples were collected from grey yellow mudstones and siltstone. The high resolution magnetostratigraphic results and magnetic susceptibility suggest the age of these strata is from 2.2 ~ 0.1 Ma. Magnetic susceptibility was measured on 209m depth surface outcrop of this section, at 10 ~ 40cm intervals by using the Bartington MS2 surface sensor. The coincident correlation of magnetic susceptibility to deep-sea oxygen isotope records suggested that this section recorded the global climatic variety. The increase elevation of Tibet might have enhanced the Tarim basin aridification. The magnetic susceptibility peak value area may imply that the uplift in the northern margin of Tibet at ca. 1. 8Ma, 1.2Ma 1 0.9Ma, 0. 6SMa. The uplift of the Tibetan plateau at ca 0.9Ma may have resulted in the increase in dust sources and dust transport capability, as associated with the upper sand layer L9 coarsest loess deposition. Source


Li Y.,State Key Laboratory for Continental Tectonics and Dynamics | Yang J.S.,State Key Laboratory for Continental Tectonics and Dynamics | Zhao O.,State Key Laboratory for Continental Tectonics and Dynamics | Zhao O.,China University of Geosciences | And 2 more authors.
Acta Petrologica Sinica | Year: 2011

The Baer ophiolite crops out in the western segment of the Yarlung-Zangbo suture zone (YZSZ) , about 1200km west of Lhasa. It extends more than 40km, in an E-W and is 3∼4km wide. The Baer peridotites consist of cpx-harzburgites and minor lherzolites. According to the texture and structure characteristics in peridotites, the mineral assemblage can be divided into three generations; ( 1 ) The first generation, the mineral assemblage of the residual mantle, includes olivine, orthopyroxene and clinopyroxene porphyroclast; (2) The second generation includes olivine, orthopyroxene, clinopyroxene and spinel. They are always along the first generations mineral porphyroelasts. They are the products of partial melting and melt-peridotites reaction in the peridotites. ( 3 ) The third generation is amphibole, which are the products of mantle metasomatism. Three generation mineral assemblages are interpreted to suggest that the Baer peridotites experienced a two-stage evolution. At first, they were formed at a MOR (mid-ocean ridge) setting and subsequently entered a SSZ (super-subduction zone) setting. Compared to the characteristics of the YZSZ peridotites, we argue that there are two type ophiolites in the YZSZ. The peridotites in the middle section in the YZSZ may be formed in a complex SSZ setting, where the peridotites in the west and east section maybe formed in MOR setting but reconstructed by SSZ. Such tectonic evolutions features maybe attribute to the heterogeneity and complexity of tectonic evolution along the Neo-Tethys Ocean. Source

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