【摘要】 龙门山地区上三叠统包括卡尼阶马鞍塘组、诺利阶小塘子组与瑞替阶须家河组。通过野外研究在小塘子组与须家河组的多个层位中首次识别出丰富的地震触发成因的软沉积物变形,包括液化变形(液化角砾岩液化滴状体、液化底劈、液化均一层等);塑性变形(卷曲变形,软布丁)以及与重力作用相关的负载、球-枕、枕状层上述软沉积物变形是龙门山地区晚三叠世构造运动的响应,它们是印支期松潘-甘孜地体与扬子板块裂开、碰撞逆冲走滑伴生的地震事件的记录。通过古地震事件与沉积事件结合分析,提出龙门山地区印支期的构造运动过程为:晚三叠世中期(印支构造期中期)松潘-甘孜地体与扬子板块开始裂开,古断裂走向近NS,断裂活动产生的地震于小塘子组浅海沉积中触发一系列软沉积物变形;晚三叠世晚期(印支构造期晚期)松潘-甘孜地体与上扬子板块发生陆内俯冲,地震诱发须家河组湖相沉积物变形;晚三叠世末期(印支构造期末期)松潘-甘孜地体左旋走滑逆冲于上扬子板块之上,形成松潘-甘孜山与川西前陆盆地,二者边界即现今的汶川-茂县断裂,印支期的造陆与造山伴随古地震发生。依据已识别的软沉积物液化变形位置与汶茂断裂的距离,估算出诺利阶小塘子组古地震震级约为Ms7.2;目前小塘子组液化变形记录远非距汶茂断裂最远的液化点,因而实际的古地震震级远远大于Ms7.2,应存在更强的地震事件。龙门山发生的毁灭性大地震(如2008,5,12汶川大地震Ms8)实际上于晚三叠世期间早已频繁发生,现今龙门山的活动地震带是中生代古地震带的延续。更多还原

【Abstract】 The Upper Triassic Series of the Longmenshan region includes the Carnian Ma’antang Formation,the Norian Xiaotangzi Formation and the Rhaetian Xujiahe Formation.Field study firstly revealed existence of abundant soft-sediment deformations triggeried by earthquakes in several horizons of the Xiaotangzi and Xujiahe formations.Deformation includes liquefaction-induced deformation(liquefied breccia,liquefied droplet,liquefied diapir,and liquefied homogeneous layer,etc.),plastic deformation(convolute deformation,soft pudding),and gravity related load,ball-and-pillow and pillow beds.The above soft-sediment deformations were responses to the Late Triassic tectonic movement in the Longmenshan region and are the records of seismic events associated with the breakup,collision and obduction strike-slip between the Songpan-Ganzi terrane and the Yangtze plate in the Indosinian period.Integrated analysis of paleoseismic events and sedimentary events displays a complete tectonic movement process in the Longmenshan region during the Indosinian period:at the middle stage of the Late Triassic(the middle stage of the Indosinian tectonic period),breakup of the Songpan-Ganzi terrane and the Yangtze plate resulted in a near NS-trending paleo-fault,and earthquake generated by the fault movement triggered a series of soft-sediment deformations in the neritic sediment of the Xiaotangzi Formation;at the late stage of the Late Triassic(the late stage of the Indosinian tectonic period),the intracontinental subduction of the Songpan-Ganzi terrane and the Upper Yangtze plate occurred,and the earthquake induced lake sediment deformations in the Xujiahe Formation;and at the late stage of the Late Triassic(the last stage of the Indosinian tectonic period),the Songpan-Ganzi terrane thrusted onto the Upper Yangtze plate through sinistral strike-slip obduction to form the Songpan-Ganzi Mountain and the western Sichuan Foreland Basin,their boundary is the Wenchuan-Maoxian fault now,and the epeirogeny and orogeny associated with paleoseism occurring in the Indosinian period.Based on the locations of the identified liquefaction-induced soft-sediment deformations and their distances to the Wenchuan-Maoxian fault,it was estimated that the magnitude of the paleoseism occurred in the Norian Xiaotangzi Formation was about Ms7.2.The current records of liquefaction-induced deformations in the Xiaotangzi Formation are far from the farthest liquefaction point away from the Wenchuan-Maoxian fault,so the actual magnitude of the paleoseism was far larger than Ms7.2 and there should have been stronger seismic events.Actually large catastrophic earthquakes(e.g.,Wenchuan large earthquake of Ms8 on May 12,2008) in Longmenshan had already occurred frequently during the Late Triassic,and the currently active seismic belt in Longmenshan is the continuation of the Mesozoic paleoseismic belt.更多还原