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

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 paleolake 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.