Characteristics Of Borehole Cores Tectonic Deformation In North Longmenshan

On 2pm 12 th May 2008, the Wenchuan earthquake(Mw7.9) struck the Longmenshan area, eastern of Tibetan Plateau. The Wenchuan earthquake takes the price of blood to provide geologists with the opportunity for learning earthquake mechanism. After the happening of the Wenchuan earthquake, the Ministry of Science and Technology、Ministry of Land and Resources、China Earthquake Administration rapidly organize and implement the Wenchuan earthquake Fault Scientific Drilling Project, for better understanding the Wenchuan earthquake fault.According to the different geological background and rupture characteristics between south and north segments of Yingxiu-Beichuan fault, boreholes are distributed in south and north, and the forth borehole of the Wenchuan earthquake Fault Scientific Drilling project(WFSD-4) is located in the north segment, which location is in Nanba town, Pingwu county, Sichuan province. WFSD-4 is just 1.5 km from the Yingxiu-Beichuan fault surface rupture zone, the total depth of the borehole is 2338.77 m and the depth of cores taking is 511.8m. There are a lot of significant tectonic phenomenon. The thesis is based on the Wenchuan earthquake Fault Scientific Drilling project, takes the cores of the WFSD-4 as the principle study object. Taking advantage of cores recording、comprehensive logging、structural deformation research, combining with the research of surface tectonic deformation, discussing the cores characteristics of WFSD-4 and tectonic deformation history of North Longmenshan, the main conclusions are below:(1)Taking advantage of the cores recording and comprehensive logging, the strata information of the WFSD-4 cores is recognized, the strata belongs to Cambrian system, concludes Youfang formation and Qiujiahe formation. The depth of Youfang formation in borehole is 0~2065m, the lithology is mainly metamorphic standstone; And the depth of Qiujiahe formation in borehole is 2065~2334.62 m, the lithology is mainly slate.(2)There are seven secondary fault zones occurred in WFSD-4, the fault zones are mainly distributed between 2070 and 2225 m, they are FZ2054(2054.72~2055.26)、FZ2080(2078.60~2086.04)、FZ2115(2115.76~2119.87)、FZ2137(2137.02~2138.61)、FZ2158(2157.61~2158.83)、FZ2189(2188.74~2189.20)、FZ2207(2207.52~2208.03), the fault related rocks are mainly fault breccias, these fault zones are all located in the hanging wall of Beichuan fault.(3)There are a lot of significant tectonic phenomenon in WFSD-4 cores. Take advantage of the tectonic and structural deformation study, combining with the research of the surface tectonic deformation, the deformation history of north Longmenshan is obtained. The first phase is with the characteristic of cleavage S1 develops, S1 is kind of ductile deformation, there are flaky minerals like sericite occurs on the S1 face, a lot of quartz and calcite veins inject in S1, pressure shadows occur in several core segment. The second phase is with the characteristic of cleavage S1 folds, which make the cleavage S2 come into being, and the S2 incise S1, the boudin occur in several core segment, this phase is also ductile deformation. The third phase is with the characteristic of cleavage S2 folds, the parallel folds on surface show the obvious NW-SE compressional deformation, which belongs to a kind of brittle deformation. The fourth phase is with the characteristic of kink develops, which reason is the gravity during regional surface uplift, it belongs to brittle deformation.