The formation and evolution of the Qiaojia pull-apart basin, North Xiaojiang Fault Zone, Southwest China

Abstract Sedimentary sequences with drastic thickening over short distances have been observed in Qiaojia County, Yunnan Province, Southwest China. These are related to a pull-apart basin controlled by the Xiaojiang strike-slip fault. Our field investigations include determining the surface characteristics of the Qiaojia basin which consists of three terrace sequences and a series of alluvial fans. Several drill holes were used to reveal the internal structure of the basin. The results suggest that the basinal sediments are over 300 m thick. From bottom to top, they can be classified into five different units. We inferred that the units of lacustrine sediments are deposited in a paleo-lake which was formed by a paleo-landslide. Accelerator mass spectrometry radiocarbon dating (AMS 14C dating) was used to estimate the ages of the terrace and lacustrine sediments. We use the results to infer that the paleo-lake has existed about 15,000 years and that the Qiaojia basin was uplifted at an average rate of 3.3 mm/a. Furthermore, we then model the evolution process of the basin and interpreted 6 phases of development.

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