逆断层正牵引构造对冲积扇沉积过程与沉积构型的控制作用:水槽沉积模拟实验研究

摘要

The normal drag structure controlled by the thrust fault is widely developed in the margin of the compressional basin edge.It formed from the episodic activity of the reverse fault and plays an important role in the depositional process and sedimentary architecture of alluvial fan.In order to understand the controlling effect on the depositional process and sedimentary architecture of alluvial fan by the normal drag structure caused by thrust fault,a set of flume tank experiments have been performed.The study,based on the observation and analysis of the depositional process,shows that large amounts of sediment carried by the high energy stream,rushed out from the synthetic valley and were blocked by the normal drag structure soon afterwards.A fraction of the coarser sediment unloaded at the upstream face of the normal drag structure,the debris flow then separated into two branch channels or deflected to one side of the normal drag structure.Due to the rapid unloading of the coarse sediment,a gravel-sandy bar formed at the upstream face and grew against the stream as debris continued to accumulate (i.e.reverse growth sandy-gravel bar).After rounding the upstream face of the normal drag structure,the branch-channels acted as new sediment source.As a result,two secondary alluvial fan,composed of several debris lobes,formed at the back of the normal drag structure.Controlled by the blocking of normal drag structure,the unloading of sediment on the alluvial fan was visibly different at different locations.Compared with the normal alluvial fan,the sediments of the reverse growth sandy-gravel bar were coarser with poorer sorting,and the sediments of the secondary fans were finer with better sorting.The sedimentary architecture of the alluvial fan also changed with the height of the normal drag structure:the higher the normal drag structure,the stronger and longer control it has on the deposition process.Consequently,the size of the reverse growth sandy gravel bar and secondary fans as well as complexity of the spatial architecture increased with the height of the normal drag structure.When the normal drag structure was completely covered by sediments,the alluvial fan deposition process returned to normal.There is an marked difference in the sedimentary architectures between the alluvial fan controlled by the normal drag structure and a normal one.In the longitudinal section,the former is made of the normal fan lobe,reverse growth sandy-gravel bar and secondary fans,and the latter the gradually evolved debris lobes.In the transverse sections,the former consists mainly of the debris lobes,reverse growth sandy-gravel bar and secondary fans in the proximal,middle and distal part of the alluvial fan,respectively,and the latter uniformly the debris lobes.%逆断层正牵引构造广泛发育于挤压盆地边缘,伴随逆断层的幕式活动而生成,并影响山前冲积扇沉积过程与沉积构型.为进一步认识这种特殊的凸起构造对冲积扇沉积过程及其内部构型的控制作用,利用水槽实验对正牵引构造发育背景下的冲积扇沉积过程进行模拟与观测.研究表明,携带大量沉积物的碎屑流冲出供水槽后很快受到正牵引构造的阻挡,大量的粗粒沉积物快速卸载在正牵引构造的迎水面,形成一个砂砾坝,同时水流被分成两股分支水流.由于较粗粒的沉积物快速在迎水面卸载,砂砾坝迎着水流逐步向物源方向生长,形成逆向(生长)砂砾坝.分支水流绕过正牵引构造后形成两个新的次级物源,在次级物源持续供给下,形成两个由多期碎屑流朵体复合而成的次级扇.受控于正牵引构造的阻挡,冲积扇表面不同位置的沉积物卸载过程差异较大,相较于正常冲积扇沉积体,砂砾坝沉积物偏粗、分选更差,而次级扇沉积物粒度偏细、分选更好;正牵引构造凸起幅度高低也会影响冲积扇沉积构型,凸起幅度越高,正牵引构造对水流的阻挡作用越强、越持久,逆向砂砾坝和次级扇的规模越大、空间结构也越复杂.正牵引构造完全被沉积物覆盖后,扇面沉积特征与一般冲积扇无异.受控于正牵引构造的冲积扇与一般冲积扇的内部构型存在较大差异,在顺物源剖面上前者依次发育碎屑流朵体、逆向砂砾坝及次级扇,而后者则整体以碎屑流朵体为主;在由近端至远端的切物源剖面上,前者依次以碎屑流朵体主控、逆向砂砾坝主控及次级扇主控,而后者则均以碎屑流朵体主控为主.