The discovery of thrust nappe structure in Qiashikansayi area on the northern margin of Altun Mountains and its tectonic significance

The Qiashikansayi area of Altun Mountains is located in the western segment of Hongliugou-Lapeiquan structural belt, which is bounded by the NE-trending Altun strike-slip faults to the north and EW-trending northern Altun fault. There volcanic-sedimentary rocks are well developed, which are assigned to Lapeiquan Group and divided into three formations:the lower formation (volcanic rock formation), medium formation (fragmental rock formation) and upper formation (carbonate rock formation). Based on the field study and mapping in Qiashikansayi gully, Goukouquan iron ore area and Beiketan area, the author discovered a series of thick bedded carbonate rocks as an irregular top cover outcropped at high hills or ridge of hills and contacting with fragmental rock or volcanic rock under them by lower angle fault, which constitute the typical thrust outlier structure. The volcanic rock and fragmental rock are dated as Ordovician, while the thick bedded carbonate rocks are not dated by any accurate biostratigraphic or chronologic evidence. It is very probable that thick bedded carbonate rocks may be of Meso-Proterozoic period and the Lapeiquan Group was broken up into two parts:one was a new Lapeiquan Group (only consisting of volcanic rock formation and fragmental rock formation), and the other was carbonate rocks formation of Meso-Proterozoic period. According to the age of Binggou granite body which cut off the thrust nappe structural zone, the author considers that the thrust nappe formed in Middle to Late Ordovician epoch (about 460Ma). All these features are identical to specious tectonic setting of the closure of the Hongliugou-Lapeiquan rift (or back-arc basin or limited ocean basin) and of the collision belt of convergent plate in Middle to Late Ordovician epoch and their evolution history.