Development of fluvial terraces in Chengdu Plain: Implications for the paleoclimate and Neotectonic Movement

In order to study the paleoclimate and neotectonic movement characteristics of the Chengdu Plain since the Quaternary, a detailed study of the river terrace sequence of the Minjiang River system examined the chronological framework, pollen assemblage characteristics in different geological periods, and geochemical characteristics of vermicular red clay from T4 in Chengdu Plain. Based on a large amount of data, along with field verification, and combined with geological, geomorphological, and chronological data, we identified 5 fluvial terraces in the transverse drainage of the Minjiang River system in the Chengdu Plain. The terrace thicknesses from T5 to T1 are 98~127 m, 59~79 m, 36~52 m, 4~10 m, and 2~5 m, respectively. The results show that the terraces T5~T2 formed at 925±92 ka, 722±77 ka, 462±46 ka, 30.13±2.86 ka and 9.0 ka respectively, which are base terrace, whose development is driven by tectonic movement and climate change, and can be used as the geomorphic symbol of the uplift of the south section of Longquan anticline on the eastern edge of Chengdu Plain since Quaternary. T1 is the accumulation terrace, whose development is mainly driven by climate change. Pollen assemblages show that the Chengdu Plain has been dominated by forest and grassland vegetation since the Quaternary, and the climate has a trend of changing from warm and wet to warm and dry. Terrace data reveal that the south section of Longquan anticline has experienced four intermittent uplifts since the Quaternary, with an uplift height of 127 meters. The uplift of the south section of Longquan anticline reached its peak in the middle of the early Pleistocene, with an uplift rate of 0.089~0.335 mm/a. By the late Early Pleistocene, the uplift rate decreased sharply to 0.027~0.165 mm/a, and then showed a gradually increasing trend, to 0.133~0.322 mm/a in the late Pleistocene Holocene. There is differential uplift between the south and north sections of Longquan anticline. The uplift rate and amplitude of the north section are significantly greater than those of the south section. In modern geomorphology, the north section of Longquan Mountain is dominated by low mountains, and the south section transits to low mountains and hills.