摘要
龙门山断裂带位于青藏高原东缘,主要由三条NE-SW走向逆冲断裂组成,由西向东分别为:汶川-茂县断裂(后龙门山断裂)、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)。汶川地震发生在龙门山断裂带上,造成了映秀-北川断裂带和灌县-安县断裂带同时产生地表破裂,引起了国际地学界广泛关注。为了查明汶川地震的发震机理及其地震过程中的物理、化学变化和扑捉余震信息,快速实施了国家专项“汶川地震断裂带科学钻探计划”(WFSD)。其中汶川地震断裂带科学钻探1号孔(WFSD-1)位于映秀-北川断裂带上盘、以约80o的倾斜角垂直于地表破裂带走向方向打下1200米深的钻孔,并全程取芯用以研究2008年汶川地震产生破裂的龙门山映秀-北川断裂带(汶川地震断裂带)的结构及破裂机制。本文在对WFSD-1岩芯和汶川地震地表破裂带研究以及WFSD-1钻孔温度测量的基础上,对龙门山断裂带的组成、结构特征以及断裂带中断层泥与地震活动性进行分析讨论。研究发现,汶川地震断裂带科学钻探1号孔岩芯中FZ590为汶川地震主滑移带,并识别出汶川地震过程中形成的约2cm厚的新鲜断层泥;地震摩擦过程中形成的残余热,是判断地震主滑移面的重要因素,并且残余热会随时间增长而逐渐扩散至消失。从WFSD-1岩芯的断裂密度剖面和钻孔测井数据来看,映秀-北川断裂带厚度约450m,总体走向N40o-43oE、倾向NW、倾角60o-65o,岩芯破裂密度剖面显示出映秀-北川破碎带具有以主滑移带为中心的对称性破碎带结构,主滑移带两侧有很多次级小断裂所构成,这些小的断裂由断层角砾岩、碎裂岩和/或断层泥组成,个别还可见假玄武玻璃,其中断层泥的厚度由1mm到25cm不等,与映秀-北川断裂带地表出露岩石特征表现基本一致。通常一次地震只能形成几mm至2cm厚的断层泥,推断映秀-北川断裂带中每个断层泥带至少发生过1次到13次地震,总厚度约150cm的断层泥至少发生过183次地震,说明沿着映秀-北川断裂带重复发生过多次强地震活动;从断层泥的分布及其内部平行且颗粒度不同的层状构造的显微特征来看,每次地震活动并不完全沿袭老的地震断裂主滑移面滑动,而是沿着断层泥边部区域滑动。整个断裂带中断层泥分布特征来看,地震断裂活动具有向断裂下盘迁移的趋势,且断层泥的厚度与断层活动性成正比关系,暗示着断裂带的宽度与地震活动次数和演化历史有着成因上的直接联系,这种断裂生长过程反映了地震主滑移面迁移与山脉生长的关系;石英碎粒溶测年研究得出映秀-北川断裂带自上新世至晚更新世发生多次地震断裂活动,形成了该区规模巨大、类型复杂多样的断裂岩,表明龙门山断裂带是一条经常重复发生强地震活动的地震断裂带,多次地震活动所形成的水平缩短及地表抬升是龙门山形成的主要因素。
Longmen Shan fold-and-thrust belt locate at the eastern margin of the Tibetan Plateau, which comprising the Yingxiu-Beichuan fault, Guanxian-Anxian fault and Maoxian-Wenchuan fault. The Wenchuan earthquake (Ms 8.0) struck the Longmen Shan area, which produced a large co-seismic surface rupture zone along the Yingxiu-Beichuan and Guanxian-Anxian fault zones, and aroused the attention to tectonic structures and fault activity. In order to investigate the faulting mechanism and physical chemistry change during the earthquake, and to get the information of aftershoke, the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project was rapidly carried out. The WFSD-1 was drilled 1200-m-deep on the Yingxiu-Beichuan fault, with about 80 degree-tilt angle perpendicular to the direction of the surface rupture zone of 2008 Mw 8.0 Wenchuan earthquake. The WFSD-1 penetrated the Longmenshan fault, and core samples were recovered from between 6- and 1201.15-m depth. Based on the research of WFSD-1 core and the co-seismic surface rupture zone, as well as the temperature measurements in WFSD-1 borehole, the composition features and structures of the Longmenshan fault belt, together with the infault gouge and its relationship with seismic activity are discussed.According to our reaserch, we identify that FZ590 most probably corresponds to the slip surface of the Wenchuan earthquake, which is the principal slip zone (PSZ) during Wenchuan arthquake and produced approximately 2cm fresh fault gouge; frictional heat anomaly is one of the most important factor to indentify the principal slip zone. From the fracture density profile of WFSD-1 core and the logging data, about 450-m Yingxiu-Beichuan fault zone is recognized, with a general trend of N40o-43oE, NW tendency; 60o-65odip angle. The fracture density profile shows a symmetrical structure of fracture characteristic with the principal slip zone (PSZ) as the central, which is composed of many small sub-faults (damage zone) that consist of fault breccia, cataclasite and/or fault gouge, and small amounts pseudotachylite in some faults. The thickness of the gouge in the fault zone ranges from several millimeters to 25 centimeters, consistenting with the fault characteristics recorded in the surface rupture zone. Only several millimeters to approximately 2 centimeters gouge can be formed in one large earthquake, from the results of the Taiwan Chelungpu-fault Drilling Project (TCDP) and WFSD Project, so we can infer that each layer of gouge in Yingxiu-Beichuan fault zone might be produced by at least 1 to 13 large earthquakes. The total thickness of the gouge in this area is about 150 cm, indicating at least 183 earthquake events, and suggesting that strong earthquakes repeatedly occurred along the Yingxiu-Beichuan fault zone.Based on the distribution and micro-layers with different grain size in gouge, each earthquake does not completely slip along the PSZ of the older earthquake, but rather along the edge of the gouge. According to the gouge statistics of the whole fault zone, seismic events have the obvious tendency towards the foot wall, and the thickness of gouge is proportional to the activity of the fault, indicating that the width of fault zone is directly related to the number and evolution history of earthquakes, This fault-grow process reflects the the relationship between fracture stress migration and the growth of mountain. Consequently, we suggest that crustal shortening related to repeated great seismic events, may be a key component of the geodynamics that drive ongoing mountain building on the eastern Tibetan Plateau.