黏性沉积物中的古地震触变流动变形*

粒径小于0.005mm的饱和淤泥和黏土等对地震和外力扰动产生的敏感变化特性被称为触变性。地震触发的软沉积物流动变形构造包括液化流动变形与触变流动变形两大类,前者多指沙层和碳酸盐沉积物的液化流动变形,后者指饱和的泥质沉积物触变流动变形。在地层剖面中,饱和淤泥、淤泥质土、黏土、硅泥(胶体)、碳酸盐灰泥等黏性沉积物的触变流动变形构造广布,它们多与沙层等的液化变形构成复合变形构造,但中外地质学家对触变流动变形构造注意较少,往往把它们笼统解释为液化流动构造。近年来地震触发饱和淤泥的触变流动变形现象逐渐引起地质学家的关注。作者对国内多个地层剖面中地震触发的饱和淤泥流动变形记录进行了描述和成因解释,并按照触变流动变形的方向性归纳出4类模式,即①向上流动、②向下流动、③同时向上及向下流动和④近水平方向流动,希望引起从事软沉积物变形和古地震研究的地质学家的关注。

Thixotropic deformation features of cohesive sediments triggered by palaeoearthquakes

Thixotropy in this article refers to the property exhibited by saturated sludges and clays when with particle sizes less than 0.005mm become liquid upon stir or shake during strong earthquakes. The soft-sediment flow deformation structures triggered by earthquakes include liquefied flow deformation and thixotropic flow deformation. The former refers to the liquefaction and flow deformation of sand layers(including fine carbonate sediments);the latter refers to the flow of saturated argillaceous sediments. Thixotropic structures such as mud,clay,siliceous mud,lime mud, etc. are widely distributed in the stratigraphic record,and they mostly form a composite deformation structure with the liquefied deformation structure of the sand layer. Usually the geologists pay more attention to the features of sand layers or misinterpret the thixotropic structures of argillaceous sediment as the liquefied flow structures of sand. Based on the distribution pattern of saturated argillaceous sediments in the stratum,this paper divides the thixotropic flow deformation into four categories: (1)upward,(2)downward,(3)simultaneous upward and downward, and (4) nearly horizontal flows. In this paper, characteristics of these thixotropic features and their forming processes were described and interpreted in combination with field phenomena. Finally,thixotropic flow deformation structures have been systematically summarized. We hope that the thixotropic features would draw attention from geologists engaged in researches on soft sedimentary deformation and palaeoearthquakes.