摘要
龙门山构造带地势陡峻,沟壑纵横,位于陡峭的青藏高原东缘和低缓平坦的四川盆地之间,处于地形地貌、地壳厚度、重磁等地球物理特性变化的陡变带,在约50km的范围内地形高差可达约4000-5000m。陡峭的地势以及复杂的岩性结构使得人们至今对龙门山构造带深部结构缺乏全面而深入和准确的认识,也正如此,人们无法深入认识2008年5月12日在龙门山地区发生的震惊世界的汶川地震(Ms8.0)发震机理和深部结构。龙门山构造带主要由三条断裂组成:汶川-茂县断裂(龙门山后山断裂)、映秀-北川断裂(龙门山中央断裂)和灌县-安县断裂(龙门山前山断裂)。汶川地震(Ms8.0)造成NE走向的映秀-北川断裂和灌县-安县断裂同时破裂,并主要产生240~270km和~80km长的地表破裂带,其中前者具有逆冲 右旋走滑的运动学特征,后者具有纯逆冲的运动学特征,一次逆冲型地震产生如此复杂的地表破裂带在世界上还是第一次报道,这种复杂的地表破裂带如何形成?其深部结构的认识是理解地表破裂带形成以及汶川地震机制的关键。本研究以国家科技专项《汶川地震断裂科学钻探项目》(WFSD)为依托,利用科钻岩心、地球物理综合测井、震后跨断裂的人工地震反射剖面以及震前地震反射剖面的综合研究揭示汶川地震断裂深部(地下~10km深度)结构特征以及探讨与地表破裂带的成因关系。(1)通过汶川科钻一号孔(WFSD-1)、二号孔(WFSD-2)和三号孔(WFSD-3)岩心和综合地球物理测井研究,发现,位于映秀-北川断裂上盘的一号孔和二号孔中的断裂岩都集中分布在第一套彭灌杂岩之下的三叠系须家河组中,产状较陡;而在灌县-安县断裂上盘的三号孔中,三叠系须家河组中的断裂岩分布相对较零散,产状也较缓。这在一定程度上反映了映秀-北川断裂的活动性强于灌县-安县断裂。(2)汶川科钻二号孔岩心中彭灌杂岩与三叠系须家河组叠置、重复出现,反映着逆冲岩片非常发育,地壳缩短作用非常强烈,并且可能被高角度的映秀-北川断裂错断。(3)通过地球物理剖面研究,揭示出映秀-北川断裂北段在~10km深度以上产状NW倾、倾角~50-70。,断裂切割了唐王寨推覆体,造成~3km的累积垂直位错,显示出映秀-北川断裂北段足一条陡倾的高角度逆冲断裂,并且断裂的走滑活动可能占居主要分量,与汶川地震地表破裂运动学、余震分布在断裂两侧及其震源机制解所反映的特征一致;(4)从地球物理剖而研究上揭示出映秀-北川断裂南段在~7-8km深度以上产状NW倾、倾角~45-56°,显示出映秀-北川断裂南段是一条高角度逆冲断裂,断裂的活动可能以逆冲运动为主,伴随小的水平滑动,为一斜向逆冲的断裂,与汶川地震地表破裂运动学、余震主要分布在断裂上盘及其震源机制解所反映的特征一致;(5)通过地球物理剖面研究,灌县-安县断裂在~10km以上呈‘断坡-断坪-断坡’式、低角度向深部延伸,与映秀-北川断裂在~15km或以上相连,一起向下延伸到~20km的滑脱面上,从而使它们在一次地震中同时被激活;(6)映秀-北川断裂深部(~10km以上)高角度的几何学特征,与汶川科钻1号孔和2号孔揭示的高角度几何学特征(~1500m深度范围内,倾角变化由地表到地下从~75°—65°—60°)一致,不仅制约着汶川地震地表破裂带的线性展布,而且还制约着破裂带具有水平运动以及断层两盘破碎强烈的对称变形特征;而灌县-安县断裂深部低角度(从地表到地下~506m深度和~1250m深度位置,倾角由60°—45°—35°)的几何学特征,不仅制约着地表破裂带主要表现为纯逆冲的运动学性质,而且还制约着破裂带随地形变化而变化的非线性展布,以及只有断裂上盘出现强烈破碎的不对称变形结构。
The Longmenshan tectonic belt is located at the transition zone between the Eastern Tibet and the Sichuan basin, where the landform, crustal thickness, magnetic and other geophysical characteristics change abruptly. The altitude difference in about50km distance across this belt can reach as much as4000-5000m. The steep topography and complex lithology of this belt makes it still lacks an accurate and comprehensive understanding about its deep structure. As a result, people can not well understand the seismogenic mechanism and the deep structure of the Wenchuan earthquake(Ms8.0), which happened on May12,2008, along this belt. The longmenshan tectonic belt is mainly composed of three faults, which are Wenchuan-Maoxian fault (Back Longmenshan fault), Yingxiu-Beichuan fault (Central Longmenshan fault), and Guanxian-Anxian fault (Frontal Longmenshan fault), respectively.The Wenchuan earthquake (Ms8.0) ruptured along both the Yingxiu-Beichuan fault and the Guanxian-Anxian fault, and produced two surface rupture zones, which are240-270km and~80km long respectively. The former rupture zone is characterized by thrusting accompanying right-lateral strike-slip motion. The latter is a pure thrust. One thrust type earthquake event ruptured two different faults and produced such a complex surface rupture zone, it is the first time that this phenomenon has been reported in the world. How does the complex rupture zone formed? Deep structure of the seismogenic faults is the key to understand the earthquake mechanism and the formation of the complex surface rupture zones.This dissertation is based on the "Wenchuan Earthquake Fault Scientific Drilling" of the National Science and Technology Planning Project. By analyzing the structures in the cores, comprehensive logging, interpretation of seismic reflection profiles that are deployed after the earthquake and old profiles before the earthquak, deep structures (~10km underground) of the Longmenshan tectonic belt are studied here. The relationship between the deep structure and the surface rupture is also discussed. The main conclusions of this study are listed as follow.(1) By cores study and comprehensive geophysical logging of the WFSD-1, WFSD-2, and WFSD-3boreholes, we found that the fault rocks in WFSD-1and WFSD-2(located in the hanging wall of the Yingxiu-Beichuan fault) are intensively distributed in Triassic Xujiahe formation, with a steep fault angle, whereas fault rocks in WFSD-3(located in the hanging wall of the Guanxian-Anxian fault) are loosely distributed in Triassic Xujiahe formation, with a more gentle fault angle. What we found indicate the activity of Yingxiu-Beichuan fault is much stronger than the Anxian-Guanxian fault.(2) The overlapping and repeated occurrences of Pengguan complex and Xujiahe formation in WFSD-2demonstrate thrust sheets are developed. Crustal shortening is intensive. and may be cut by the high angle Yingxiu-Beichuan fault.(3) Interpretation of seismic profiles reveal that the north segment of the Yingxiu-Beichuan fault dip~50°-70°to NW in~10km underground. The fault offset the Tangwangzhai nap for about3km. This indicates the north segment of the Yingxiu-Beichuan fault is a high angle thrust fault, which is coincided with the kinermatics of the surface ruptures, distribution of aftershocks, and focal mechanism of aftershocks.(4) Geophysical profiles revealed that the south segment of the Yingxiu-Beichuan fault is a high angle oblique thrust fault, with NW dipping angle of~45-56°at~7-8km depth. This south segment of the Yingxiu-Beichuan fault has main thrust movement feature, with slight horizontal movement. This fault character is coincided with the kinermatics of the surface ruptures, distribution of aftershocks, and focal mechanism of aftershocks.(5) Geophysical profiles revealed that the Guanxian-Anxian fault has characters of "fault ramp-fault flat-fault ramp" and low angle extends at10km depth of the Guanxian-Anxian fault constrain the pure thrust movement properties. the no-linear distribution of the Wenchuan Earthquake surface rupture zone by the topographic change, and asymmetric deformation characteristics of the two sides of the fault.