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Li H.,State Key Laboratory of Continental Tectonic and Dynamics | Li H.,Chinese Academy of Geological Sciences | Van der Woerd J.,Institute Of Physique Du Globe Of Strasbourg | Sun Z.,Chinese Academy of Geological Sciences | And 10 more authors.
Gondwana Research | Year: 2012

The 400km-long Karakax left-lateral strike-slip fault is the westernmost segment of the Altyn Tagh fault. It separates northwestern Tibet to the south from the Tarim basin to the north. The western section of the Karakax fault exhibits clear co-seismic surface ruptures of past large earthquakes. Geomorphic offset measurements from the field and high-resolution Ikonos images along 1.5km across the Sanshiliyingfang fan and along 55km of the fault, range from 3 to 28m, with distinct clusters at 6±2(3), 14±2, 19±2 and 24±3m. The cluster of the smallest offsets around 6m (full range from 3 to 10m) distributed over a minimum length of 55km, is attributed to the last largest surface rupturing event that testifies of the occurrence of a magnitude Mw 7.4-7.6 earthquake along the Karakax fault. We interpret the other offset clusters as the possible repetition of similarly sized events thus favoring a characteristic slip model for the Karakax fault. In a 3m-deep trench dug across the active trace of the fault we can identify the main rupture strands of the last and penultimate events. The penultimate event horizon, a silty-sand layer, has been radiocarbon dated at 975-1020AD (AMS 14C age). It is proposed that large Mw 7.4-7.6 events with co-seismic slip of about 6m rupture the Karakax fault with a return time of about 900years implying an average slip-rate of about 6-7mm/years during the late Holocene. These results suggest that the Karakax fault is the largest left-lateral strike-slip fault at the rim of northwestern Tibet accommodating eastward movement of Tibet due to the India-Eurasia collision. © 2011 International Association for Gondwana Research. Source


Li H.,State Key Laboratory of Continental Tectonic and Dynamics | Li H.,Chinese Academy of Geological Sciences | Wang H.,State Key Laboratory of Continental Tectonic and Dynamics | Wang H.,Chinese Academy of Geological Sciences | And 17 more authors.
Tectonophysics | Year: 2013

Scientific drilling in active faults after a large earthquake is ideal to study earthquake mechanisms. The Wenchuan earthquake Fault Scientific Drilling project (WFSD) is an extremely rapid response to the 2008 Ms 8.0 Wenchuan earthquake, which happened along the Longmenshan fault, eastern margin of the Tibetan Plateau. In order to better understand the fault mechanism and the physical and chemical characteristics of the rocks, the WFSD project will eventually drill 5 boreholes along the two main faults. This paper focuses on the first hole (WFSD-1), which started just 178. days after the earthquake, down to a final depth of 1201.15. m. Petrological and structural analyses of the cores allowed the identification of fault-related rocks in the Yingxiu-Beichuan fault (fault gouge, cataclasite, and fault breccia), and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined.We found 12 fault zones in the entire core profile, with at least 10, including the Yingxiu-Beichuan fault zone, with a multiple cores structure and minimum width of ~. 100. m. The co-seismic slip plane of the Wenchuan earthquake at depth (corresponding to the Yingxiu-Beichuan fault zone at the outcrop), as well as its PSZ, was expected to be located at the bottom of the fault zone (at 759. m-depth). Instead, it was found at ~590. m-depth with 1. cm-wide fresh fault gouge, as determined by logging data such as temperature, natural gamma ray, p-wave velocity and resistivity, combined with the fresh appearance, magnetic susceptibility, and microstructure of the gouge. The Wenchuan earthquake slip plane has a dip angle of ~. 65°, showing the high-angle thrust feature. The distribution of fault gouge with several meters thick, the location of the Wenchuan earthquake's PSZ and the thickness of fresh gouge all imply a correlation between the width of the fault zone and the number of seismic events. © 2012 Elsevier B.V. Source


Li H.-B.,State Key Laboratory of Continental Tectonic and Dynamics | Li H.-B.,Chinese Academy of Geological Sciences | Xu Z.-Q.,State Key Laboratory of Continental Tectonic and Dynamics | Xu Z.-Q.,Chinese Academy of Geological Sciences | And 15 more authors.
Geology in China | Year: 2013

The May 2008 Ms 8.0 disastrous Wenchuan earthquake occurred in the eastern margin of Tibetan Plateau, producing 270 and 80 km-long different kinematics features coseismic surface ruptures along the Yingxiu-Beichuan and Anxian-Guanxian faults, respectively. Drilling in active fault zones, especially after large earthquakes, is an effective way to study earthquake mechanisms, in order to better understand the fault mechanisms during the earthquake process as well as physical and chemical characteristics of rocks. The Wenchuan earthquake Fault Scientific Drilling project was carried out on November 6, 2008 (178 days after the Wenchuan earthquake). Five boreholes were ultimately drilled along the Yingxiu-Beichuan and Anxian-Guanxian faults, ranging from 600 m to 3000 m in depth. This paper focuses on the cores of the first hole (WFSD-1). Petrological and structural analysis of the cores allowed the identification of the distribution and combination of fault-related rocks in the Yingxiu-Beichuan fault, and the Principle Slip Zone (PSZ) location of the Wenchuan earthquake was determined, which provided a basis for further research on earthquake mechanism. The detail analysis of the cores reveals the existence of a fault zone, which is about 100 m wide from 575m to 759 m in depth and consists of gouge, cataclasite, and fault breccia. From the top to the bottom there exist mainly cataclasite zone, fault gouge and fault breccias mixed zone, breccias zone and fault gouge zone. This fault zone lies between the Neoproterozoic Pengguan complex and Triassic Xujiahe sedimentary rocks, which is the Yingxiu-Beichuan fault zone, mainly distributed in Xujiahe Formation. The Principle Slip Zone (PSZ) of the Wenchuan earthquake was found at ∼589 m with ∼1 mm fresh fault gouge, which lies in the upper part of Yingxiu-Beichuan fault zone, as evidenced by logging data, and clay mineral analysis, in combination with the microstructure of the gouges. There exist similar structures between Pengguan complex and Triassic Xujiahe Formation in Hongkou outcrop with the thickness of about 190m, lying at the bottom of the Yingxiu-Beichuan fault zone. Hence, The Wenchuan earthquake fault slip cut the Yingxiu-Beichuan fault zone obliquely, indicating that Wenchuan earthquake fault might be a new high-angle thrust fault with a dip angle of 62°-65°, which did not totally follow the slip zone of ancient earthquake and might be related to the earthquake magnitude. It is the result of the long term earthquake activity and evolution from about 10-15 million years ago to the present, which formed the 100-190 m-wide Yingxiu-Beichuan fault zone. Source

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