The Study Of Denudation-accumulation System Between The Longmen Shan And Foreland Basin

The uplift and denudation of mountain is always the hot spot and the difficulty of the circles of geoscience research. The basin beside the orogenic belt records the history of the uplift and the denudation of the orogenic belt. Denudation-accumulation System between the Longmen Shan and foreland basin is a good matter for research measure. The earthquake happened in 2008 triggered a lot of rockslide, landslide and mud-rock flow. These rockslide, landslide and mud-rock flow give us a perfect data and matter to study the uplift and denudation of mountain, the deposition of the basin beside the orogenic belt.In the study of the denudation-deposition system between Longmen Shan and foreland basin this time, we seriously concerned about the tectonic activities and palaeoclimatic change. Then combine with the conclusion from other research method, make the greater detail depict and analysis to the evolvement process of the denudation-accumulation system between the Longmen Shan and foreland basin. And, try to constitute the geology model of the coupled relation between the Longmen Shan and foreland basin since the late Triassic Period.By analysising and studying the the evolvement process of the denudation-accumulation system between the Longmen Shan and foreland basin since the late Triassic Period, we get some developments and understanding mainly in these ways:1. The Wenchuan Earthquake triggered a great deal of rockslide, landslide and mud-rock flow, and these loose materials were transfered to the river system, then these materials were accumulated, denudated, transferred and accumulated again. So we could get a conclusion that the denudation in the Longmen Shan and the accumulation in the foreland basin were caused by several great earthquake. In the process of materials'conveying and transferring, climate is also a important affect factor.2. The Wenchuan Earthquake did not build the volume of the Longmen Shan, in contrast, because of the great denudation caused by the rockslide, landslide and mud-rock flow, the volume of the Longmen Shan was reduced, and the volume of loose materials is twice larger than the increased volume of the Longmen Shan caused by Earthquake. So the crust uplift caused by the stronge tectonic activities maybe not always make the uplift of paleotopography. The competition between the rate of structure uplift (crust uplift) and the rate of denudation is the determining factor of paleotopography change.3. The surface vertical displacement of the Longmen Shan foreland basin triggered by Wenchuan Earthquake show us a conclusion. In the evolvement process of the Longmen Shan foreland basin, the staged thrusting was the main reason for the uneven flexural subsidence of geosphere, and it make the deep geography in the western of the basin and the flate geography in the eastern of the basin, so the strata filled in this stage look like wedge.4. Based on seriously concerned about the tectonic activities and palaeoclimatic change, we estimate the crust uplift(>9-11km) and the denudation of the Longmen Shan thrust belt(average 5.79km), the research conclusion show that there were three times more quickly crust uplift in the Longmen Shan thrust belt since the late Triassic Period. There are three stages content Ds2(199.6 Ma~155.6Ma),Ds4(112.0 Ma~88.6 Ma)and Ds6(3.6Ma~),when the elevation of the Longmen Shan was higher than other stages.5. The evolvement process of the Longmen Shan foreland basin consist of several stages of thrust belt tectonic load and several stages of thrust belt denudation unload. It can be divided into three tectonic load stages and three denudation unload stages. There are a large gap between these two kinds of stages in the subsidence mechanism of foreland basin and the depositional pattern. In the stages of Ds1(the late Triassic Period), Ds3(the Late Jurassic- Early Cretaceous Period) and Ds5(the late Cretaceous- Miocene Period), the Longmen Shan thrust belt thrust on the geosphere of the foreland basin, and the average elevation is low, about 700-1700m, and the strata filled in the foreland basin looks like wedge. In the stages of Ds2(the early and middle Jurassic Period ), Ds4(the middle Cretaceous Period, Jiaguan Period) and Ds6(the later-cenozoic Period), the Longmen Shan thrust belt thrust was denudated and unloaded, and the average elevation is high, about 2000-2800m, the strata filled in the foreland basin looks like plate object.