| The Qinghai-Tibet Plateau has been subjected to continuous compression and collision from the Indian-Eurasian plate,resulting in the current flat terrain and exceptionally steep edges within the plateau.As the Indian plate continues to subduct northward,it causes the Qinghai-Tibet Plateau to continuously uplift and expand towards the north and east,leading to strong tectonic deformation.The most typical area of the eastern edge of the QinghaiTibet Plateau is located upstream of the Minjiang River,adjacent to the Sichuan Basin.In the south of this area,the topography drops sharply from an elevation of 5000 m to 600 m over a horizontal distance of about 40-50 km,which is due to the effects of tectonic uplifting and erosion.Rivers are one of the carriers that record information about tectonic activities.When tectonic activities occur,the river channel profiles will produce fracture points,and the erosion and evolution of landforms will be driven by upstream migration.By exploring the response mechanism of tectonic uplift to fracture points,we can further understand the specific interaction relationship among tectonics,climate,and landforms.This has important theoretical significance for revealing the uplift mechanism of the eastern edge of the Qinghai-Tibet Plateau,surface erosion processes,and the spatial distribution pattern of modern mountains.This article analyzes the longitudinal profile characteristics of the upstream Minjiang River,studies the spatial distribution pattern of fracture points,and reconstructs the ancient river channel to infer the tectonic uplift history of the upstream Minjiang River.These findings have important theoretical significance for revealing the uplift mechanism of the eastern edge of the Qinghai-Tibet Plateau,surface erosion processes,and the spatial distribution pattern of modern mountains.The main achievements are as follows:(1)Distribution characteristics of fracture points in the upstream Minjiang River: the fracture points mainly generated by tectonic activities are distributed in the two tributaries of Zagu and Heishui River,and two fracture zones at 2400 m and 3500 m have been identified in these two watersheds.Among them,from the distribution of the 2400 m fracture zone,there are fracture points in the main stream of the Minjiang River and the two tributaries during this period,while the distribution of the 3500 m fracture zone is only limited to the two watersheds of Zagu and Heishui River,and there are no fracture points in the main stream of the Minjiang River during this period,which may be due to different tectonic activities.(2)By collecting the 10 Be erosion rate data of the upstream Minjiang River basin,32 erosion rates of watersheds have been obtained.The value of the important parameter K is estimated to be(1.65±0.15)×10-6m0.1/a based on the linear relationship between watershed normalized steepness index and erosion rate.The best reference concavity for the upstream Minjiang River is determined to be 0.45,and the best chi-value interval is 1.2.The uplift rate of the upstream Minjiang River has increased from 0.1 mm/a since 20 Ma ago to the current0.6 mm/a according to the uplift history inversion.The uplift rate of the Zagu River in the watershed has increased from 0.16 mm/a since 8 Ma ago to 0.8 mm/a,and the uplift rate of the Heishui River in the watershed has increased from 0.1 mm/a since 9 Ma ago to 0.7 mm/a.The uplift rate of the Zagu and Heishui rivers is slightly higher than the overall rate.There was a period of overall accelerated uplift in the upstream Minjiang River during the early Pleistocene(around 2 Ma ago).(3)By reconstructing the ancient river channel and estimating migration rates,it is found that the elevation differences between the upstream erosion base level and present-day elevation of the 2400 m fracture zone from the Dujiangyan hydrological station to the upstream of the main streams of the Minjiang River,Heishui River,and Zagu River are(945±89)m,(546±78)m,and(587±84)m,respectively.The vertical downcutting rates of the3500 m fracture zone in the Zagu River and Heishui River watersheds are 0.5 mm/a-1.7mm/a and 0.4 mm/a-1.4 mm/a,respectively.The Zagu River watershed has experienced an average net uplift of 1359 m,and the Heishui River watershed has experienced an average net uplift of 1138 m,indicating that the river channels may have been equally incised.(4)The migration process of the two fractures from the simulation shows that the migration rate of the fracture zone before 0.5 Ma was very fast in the Zagu River and Heishui River watersheds,but once it entered a tributary,the speed significantly decreased,especially after 0.9 Ma.The fracture migration process begins with rapid migration in the main stem,but once it reaches a tributary,the speed significantly decreases.As the distance of transmission increases,the drainage area decreases and the fracture migration rate gradually slows down.(5)Based on the simulation of the fracture migration process and the average speed of fracture migration,the average speed of the 2400 m fracture zone in the upstream Minjiang River is 68.9 km/Ma-232.6 km/Ma,the average speed of the 3500 m fracture zone in the Zagu River watershed is 35.3 km/Ma-119.3 km/Ma,and the average speed of the fracture zone in the Heishui River watershed is 38 km/Ma-127.9 km/Ma.The simulated fracture positions are highly consistent with the actual observed fracture positions.The fitting degree for the 2400 m fracture zone is 0.97,for the 3500 m fracture zone in the Zagu River watershed is 0.98,and for the 3500 m fracture zone in the Heishui River watershed is 0.99.(6)Through the analysis of the spatial relationship between the number of disasters and the fracture position along the upstream of the Minjiang River,it can be found that the number of disaster points downstream of the fracture point is much greater than that upstream.This is because the migration of the fracture point will cause the slope material to loosen,making it easier for disasters to occur along the river.In addition,frequent earthquakes in the area make it more prone to disasters. |
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