龙门山造山带早期断裂活动的古地震制约——来自汶川科钻(WFSD)岩心的证据

强地震是断裂活动的表现形式,可以诱发地表沉积层序顶部未固结的软沉积物发生变形,形成新的变形层(即震积岩***)。因此,在连续沉积剖面中赋存的多层震积岩应是断裂活动的直接证据。川西前陆盆地中的软沉积物变形记载了龙门山断裂带的活动信息,对认识龙门山造山带演化历史具有重要意义。本文通过"汶川地震断裂带科学钻探"一号孔(WFSD-1)和三号孔(WFSD-3)连续岩心剖面的岩性分析和构造研究,识别出11段不同深度的液化角砾岩层,它们是地震触发成因的软沉积物变形岩层。11个液化角砾岩段厚度从~20m至102m不等,分布在晚三叠世须家河组二-五段。这些液化角砾岩层记录了龙门山前陆盆地形成过程中晚三叠世断裂活动特征及趋势。这些厚度不等的震积岩粗略指示约2~20万年的地震活动长周期(地震幕),以及约4至70万年的间震期(地震幕的间隔时间),反映了龙门山断裂早期脉动式(幕式)活动特征。从不同段液化角砾岩层分布间隔规律来看,地震活跃期间隔(即间震期)越来越短,显示龙门山造山带断裂活动越来越强的趋势。结合前人地表软沉积物变形研究,我们认为龙门山造山带在晚三叠世经历了多期次的正断-逆冲活动的造山作用(至少经历14个地震活跃期),形成龙门山雏形及前陆盆地。

Early fault activities of the Longmen Shan orogen constrained by paleoearthquakes: Evidence from the WFSD drilling cores

Strong earthquakes, produced by large faulting, can induce the unconsolidated soft sediments deformed, and lithified into a new sedimentary rock named as seismites. The seismites are the records of paleoearthquakes, which are the direct evidence of early fault activities. The soft-sediment deformation in West Sichuan Basin records the activities of the Longmen Shan orogenic belt, which is of great importance for understanding the Longmen Shan evolution history. Here in this paper, according to the lithologic analysis and structure research from the continuous core profiles of the Wenchuan earthquake Fault Scientific Drilling hole 1 (WFSD-1) and hole 3 (WFSD-3), 11 sections of liquefied breccia layers are recognized at different depth intervals. These liquefied breccias are soft-sediment deformation rocks induced by large earthquakes, with thickness ranging from ~20m to 102m distributed in the Second to Fifth sections of the Xujiahe Formation. These liquefied breccias documented the faulting characteristics of the formation process of the Longmen Shan foreland basin. These layers of seismites may imply a 20, 000 to 200, 000 years'' seismic active period and 40, 000 to 700, 000 years'' interseismic periods, implying that the early fault activities in the Longmen Shan orogen possessed of a periodical episodic feature. Viewed from the intervals between the different liquefied breccia sections, it seems that the intervals among seismic periods in Longmen Shan orogen have become shorter, in other words, the fault activity has become stranger. Based on our study and the previous surface soft-sediment deformation research, the Longmen Shan orogen has experienced multiple episodes of normal-thrust fault activity orogeny (at least 14 seismic active periods), which formed the primary Longmen Shan and the foreland basin.