The Geomorphology And Sedimentary Records Of The Tectonic Activity In The Lianfeng Fault Zone And Adjacent Areas Since The Late Cenozoic

Geomorphology and sediment are the important basis for studying tectonic activities.The tectonic characteristics and evolution history are different at different time and space scales.In this paper,the Lianfeng fault zone and its adjacent areas in the eastern part of the Sichuan-Yunnan border are taken as the target areas to conduct multi-scale studies on the geomorphological and sedimentary records of the area since the Late Cenozoic.In particular,it provides new geomorphology and tectonic geology evidence for the characteristics and ages of Lianfeng fault since Holocene.These works are helpful to the study of regional active tectonics and the deployment of earthquake disaster prevention work.This study obtained the following points of understanding:1.Based on DEM data,four-level geomorphology were identified in the area from high to low levels(S1-S4),through the statistical elevation histogram and previous research data.In terms of elevation,northwest the area is higher than southeast,and inclines from the northwest to the southeast.S1 is sporadically exposed on both sides of the Zemuhe fault zone and near Jinyang County and Daduo,with an elevation of about 3000-3700m.S2 is the main part of the geomorphic surface in the region,which is widely distributed,with an elevation of about 2300-2700m.S3 is distributed in the eastern and southeastern parts of the region,with an elevation of about 1700-1900 m.S4 distributes along the Anning Valley in the western part of the study area,with an elevation of below of 1500m.2.Differential uplift and subsidence movements on both sides of some faults in the region are analyzed by using elevation histograms at the scales of 10 km*10 km and 5 km*5 km.In the Zemuhe fault zone,the dislocations of the three geomorphic surfaces,S1,S2 and S3 are identified.The vertical dislocations of the geomorphic surfaces have the characteristics of S1>S2>S3.Spatially,the maximum vertical faults of S1 at the northwest end of the fault zone are about 750m and S2 about 500m,followed by those in the middle par,and the vertical faults of S1,S2 and S3 near Puge County are about 550m,400m and 370m.In the Southeast end,the height difference of S1 is the smallest,and that of S1 S2 and S3 are about 300m,210m and 100m,respectively.Combined with predecessors related geomorphological development time,the vertical slip rates of the Zemuhe fault zone since the formation of S1 and S2 are estimated from the middle Pliocene to the end of Pliocene,respectively.From this,It is estimated that the vertical slip rate of the northeast end of the Zemuhe fault zone is about 0.21-0.29 mm/a,that of the middle part is about 0.15-0.21 mm/,that of the southeast is 0.08-0.12 mm/a,and that of the northwest end of the fault zone is about 0.28 mm/a and that of the middle part is about 0.22 mm/a since the formation of S2.The vertical slip rate at the southeastern end of the fault zone is about 0.12 mm/a.Compared with the previous research results(vertical slip rate 0.1-0.85mm/a),it belongs to the same order of magnitude.It is shown that the vertical differential velocity of the two sides of the fault zone is less than 0.3 mm/a on a long time scale.The analysis results of Lianfeng fault zone show that the elevation change trend of the fourth grade geomorphology from Dazhai Township to Yanjiao Township in the southwestern section is the same.The height difference of S1 near Duiping town is about 490 m,and gradually decreases to both ends.The northeast and southwest ends are about 310 m and 300 m respectively.The changes of S2 and S1 are similar,about 380 m in Duiping town,210 m in the northeast and 170 m in the southwest,respectively.3.Based on previous data,geomorphological,sedimentological and chronological studies of river terraces in the southwestern part of Lianfeng fault zone show that the terraces near Laojie in Qiaojia section of Jinsha River are accumulative terraces from the first to the fourth.The heights above the river level are 60-70m,145-190m,260-310m and 350-400m,respectively.The age of OSL in the upper part of T1 terrace is 7.1 ± 0.8ka,and the luminescence dating of the tributary fan sediments is 13.4 ± 0.8ka,which can be compared with that of T2 terrace.Geomorphological and geological evidence of activity in the southwestern segment of Lianfeng fault zone since late Quaternary has been found in terrace deposits.