断层泥石英微形貌特征在断层活动性研究中的意义

断层泥是断层活动的信息载体,其中的石英颗粒微形貌特征能够反映断层活动方式、期次和活动年代。利用环境扫描电镜,对西藏拉萨那林拉卡断裂白定段断层泥石英样品进行了溶蚀微形貌统计分析及应力痕迹微形貌观察。结果表明:石英颗粒呈现出以钟乳状为主和桔皮状为主的两种截然不同的溶蚀微形貌特征。对照Kanaori Y等关于石英微形貌与年代关系的图谱可知,该断层形成以后至少有过两次再活动,其活动年代分别为上新世至早更新世和中更新世。这与热释光定年法确定该断层最后一次活动时间为287.08 Ka±24.40 Ka的结果相吻合;另外,代表晚期活动特征的应力痕迹微形貌观察结果显示,同一石英颗粒上发育了代表蠕滑和粘滑两种滑动的阶步状刻痕(蠕滑)-撞击碎裂痕(粘滑)组合,以及阶步状刻痕(蠕滑)-贝壳状断口(粘滑)-平直擦线(粘滑)的组合,说明该断层在晚期(中更新世)曾经历了蠕滑和粘滑两种滑动方式。代表蠕滑的阶步状刻痕清晰突显,而代表粘滑的线状擦痕明显受到溶蚀,这说明粘滑运动在先,蠕滑运动在后。此外,擦痕线的叠加、切穿现象还表明,断层在中更新世的滑动至少有3个亚期次。综合应力痕迹微形貌特征认为,3个亚期次滑动的前两次表现为粘滑,第三次为蠕滑或粘滑向蠕滑过渡类型。

THE CHARACTERISTICS AND SIGNIFICANCES OF QUARTZ MICO-APPEARANCE IN THE FAULT GOUGES ON EVALUATION OF THE FAULT ACTIVITY

Gouge is an information carrier for fault activity.The motion mode and date of the fault activity can be identified by observation of micro-shape characteristics of quartz in the gouges.In this paper,dissolved micro-shape and stress micro-shape of the quartz,which were collected from the gouge of the Nalinlaka fault,Lhasa,Tibet,are observed and statistically analysed by ESEM.The results show that there are two kinds of different dissolved micro-shape.Most of them are stalactite-like and orange skin-like.Contrasting the dissolved micro-shape of quartz with the diagram by Kanaori Y et al,it is indicated that there were two stages of fault activities of Pliocene to early Pleistocene and middle Pleistocene respectively.This analysis is consistent with dating value of the fault by thermoluminescence tecnhnique.Also,observations of stress micro-shape show that steps-like nick(by stable creep) and smash mark(by stick slip),or stepslike nick(by stable creep) and shell-like fracture and scrape lines(by stick slip) occurred on same quartz.The stress micro-shape metioned above indicates that the fault has experienced stable creep and stick slip movement before.Furthermore,clear steps-like nick and dissolved scrape lines indicate that stick slip was earlier than stable creep.Again,superposition of three different direction scrape lines on same quartz showed that the fault had experienced three sub-periods of movement in middle pleistocene at least.The colligation of stress micro-shape characteristics reveals that the former two sub-period of movements is characterized by stick slip and the third one is by stable creep or transition of stick slip to stable creep.