关键词: 河流阶地, 金沙江下切速率, 气候变化, 地壳隆升

Abstract:

River terraces are primarily formed by the erosional action of river incision under the influence of vertical movements of the crust or changes in regional erosion base levels, resulting in layered landforms. As products of the long-term evolution of river systems, the formation, development, and evolution of terraces have always been a focal point in Quaternary research. Climate change and tectonic movements play crucial roles in the evolution of river terraces, providing important evidence for studying a region’s paleoclimate and tectonic history, while also indicating the geomorphic evolution of rivers. The ages and elevations of river terraces serve as a crucial window for understanding climate fluctuations and the intensity of tectonic uplift in a specific area. This role cannot be replaced by any other method. Therefore, accurately defining the incision and deposition ages of river terraces is essential for quantitatively reconstructing the development and evolution of rivers, making it a key data point in current research on surface processes and geomorphic evolution.

The study area is located at the southeastern margin of the Qinghai-Xizang Plateau, positioned in the main area of the Jinsha River suture zone at the southwestern edge of the Songpan-Ganzi orogenic belt and the eastern part of the Sanjiang orogenic belt. The regional tectonic setting is complex. Since the late Quaternary, the tectonic uplift at the southeastern margin of the Qinghai-Xizang Plateau has intensified, with accelerated plateau uplift in the post-Pleistocene era accompanied by significant tectonic activity. This has led to substantial incision of rivers in the region, forming multiple layers of overlapping terrace landforms on both sides of the river valleys. These landforms are crucial for quantitatively understanding the plateau uplift process and climate change.

The Jinsha River is one of the main large rivers in the western parts of Sichuan. The river terraces developed in the Jinsha River valley serve as an important evidence for studying the deformation of the plateau crust and climate change. However, there are few Holocene terraces developed in the valley, and their resolution is low. Therefore, current research on the Jinsha River terraces mainly focuses on the orbital time scale(from tens of thousands to millions of years)of climate change and the impact of tectonic uplift, with limited studies on the role of short-term time scales(thousands or hundreds of years)in climate change and tectonic uplift, and a lack of constraints on river incision rates since the late Quaternary. The formation and evolution of river landforms since the Holocene are currently the most important means of studying recent tectonic activities and predicting future climate fluctuations. Therefore, the Baqu River, as a major tributary of the Jinsha River, with the terraces preserved in its valley, has become crucial research material reflecting the climate change and tectonic uplift in the Jinsha River Basin since the Holocene.

The Batang segment of the Baqu River is situated in the midstream valley of the Jinsha River, characterized by a wide valley floor and gentle riverbed slope. Through drilling and shallow seismic exploration to investigate the valley stratigraphy, it was found that the valley sediments can be divided into four layers from top to bottom. The bottom layer consists of Permian strata mainly composed of weathered crystalline limestone, with a core exposure of 22m without reaching the bottom. The third layer is composed of Middle Pleistocene sediments, 68m thick, mainly consisting of large boulders, small gravel, and calcareous clay. The second layer comprises Late Pleistocene sediments, 30m thick, primarily consisting of large gravel and clay. The first layer is mainly composed of fine-grained clay with a small amount of sand and gravel blocks, 10m thick. This indicates that the valley has experienced at least two significant aggradation stages. Using Electron Spin Resonance dating methods, it was determined that these two aggradation events began at approximately 318ka and 143ka, corresponding to Marine Isotope Stages(MIS)10-9 and MIS 6-5, respectively, during glacial melting phases.

Four levels of river terraces are developed within the valley, with T1-T3 being aggradational terraces and T4 being a bedrock terrace. T1 has a terrace height of 5~10m, T2 ranges from 15~25m, T3 ranges from 30~40m, and T4 has a terrace height of 120m. The terrace topography is generally parallel to the longitudinal profile of the modern riverbed, with only minor fluctuations, indicating a predominant overall uplift in the area after terrace formation, with consistent tectonic uplift rates and insignificant differential uplift. Combining Optically Stimulated Luminescence dating, Carbon-14 dating, and cosmogenic nuclide dating methods, it was determined that T1-T3 formed between 1~5ka, specifically 1~2ka, (3.1±0.2)ka, and(4.5±0.4)ka, respectively, while T4 formed around 62ka. A comparison of terrace ages with paleoclimate data revealed that the incision times of T1-T3 corresponded to transitions from cold to warm climates. Calculating the incision rates of terraces based on their ages and terrace heights and comparing them with incision rates in different sections of the Jinsha River, it was found that from the Late Pleistocene to the mid-Holocene, the Baqu River incision rate was(1.5±0.3)mm/a, consistent with other sections of the Jinsha River in western Sichuan. From the mid-Holocene to the present, the incision rate increased to(5.5±0.8)mm/a, approximately four times the incision rate during the Late Pleistocene. While there is a lack of quantified results on river incision rates since the Holocene in surrounding rivers, the enhanced incision rate aligns with the current vertical crustal deformation rates, indicating that intensified crustal uplift since the Holocene may be the primary driver of rapid river incision.

Key words: Fluvial terraces, Jinsha River, Incision rate, Climate change, Crustal uplift