The controlling effects on depositional process and sedimentary architecture of alluvial fan by normal drag structure caused by thrust fault: insights from flume tank experiments
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.
flume tank experimentsnormal drag structurealluvial fandepositional processsedimentary architecture
NETL
NSTL
万方数据
维普
