Study On The Formation Age Of The Dry Hot Valley Based On River Terrace Records In The Qiaojia Section Of Jinsha River

For the formation history of the hot dry valley landscape in Southwest China,there is a huge debate in the academic community.There are two main points of view.One thinks that the dry hot valley is a product of the historical period,caused by the ecological destruction caused by human unreasonable development activities.Another view is that the dry hot valley is a product of geological history,and human activities have only played a reinforcing role.With the deepening of research,more and more scholars tend to favor the latter,but there is no conclusion on which phase of the dry-hot valley was formed during the geological history.Numerous data show that the incineration effect produced by deep river valleys is the basic meteorological condition for the formation of dry and hot valley landscapes.In other words,if deep river valley topography exists,dry and hot valley landscapes should be created.The history of the formation of the dry-hot valley landscape is part of the deep history of the valley landscape.The study of river geomorphology suggests that the lower cut history of the valley can be recorded completely on the terraces on both sides of the valley.Therefore,research on river terraces can obtain important information on the landscape evolution of dry-hot valleys.Years of research have found that 1300 m above sea level is the geographical upper limit of the hot and dry valley landscape of the Jinsha River.Below this limit is the “dredging bushes”,which are dominated by drought-tolerant hot vegetation,and above it is “subtropical forest”.If we can determine the formation history of the valley below 1300 m,we can calculate the age of dry heat valley landscape formation.In the Qiao Jia section of the Jinsha River,the paper found that there are at least 9 terraces.From the aspect of material composition and structure,these terraces all show the shape of base terraces.The bases are mostly Paleozoic carbonate rocks.The fluvial structures and heavy minerals of fluvial facies in terraces at all levels are similar to those of the mainstream or tributaries of the modern Jinsha River,indicating that these terraces are indeed terraces formed by the Jinsha River and its tributaries,and can be used as an information carrier to study the evolution history of the Jinsha River valley.The systematic ESR dating shows that the ages of the terraces formed from old to new are 1.59 Ma,1.29 Ma,1.20 Ma,0.44 Ma,0.29 Ma,0.22 Ma,0.17 Ma,0.087 Ma,0.03 Ma.Based on the top elevation data of fluvial facies sediments from the differential GPS terraces,the mean down-cut rate of the Qiaojia section of the Jinsha River since the Early Pleistocene was 0.205 mm/a.Extrapolating the mean undercut rate to the valley below 1300 m above sea level was 2.77 Ma.That is,the dry and hot valley landscape of the Jinsha River formed at the end of the Pliocene,not the historical period.