| Yanqi basin is a Meso-Cenozoic composite sedimentary basin developed on the basement of Hercynian fold. It looks like a NW trend rhombus on morphology was surrounded by western Huola mountain and eastern Kezile mountain which are in the direction of NW, southern Kuluketage mountain and northern Saaer mountain which are EW trend. The boundaries of the basin include western Tiemenguan fracture, eastern Yushugou-Liuhuanshan fractures, southern Xingeer fracture in the front of Kuluketage mountain and northern Shangshuyuanzi fracture at the southern brink of middle Tianshan. The formation and evolution of Yanqi basin had been controlled or influenced by these fractures in a long period of time. From the characters of structure and geometry, nowadays basin morphology is obviously controlled by structural frame and primary fractures of south Tianshan.Nowadays Yanqi basin is a residual basin due to multi-periods reconstruct, there are great distinctness between Jurassic and nowadays on morphology, In Jurassic the basin's area was bigger than the present basin. Yanqi-Kuche basin reformed through late Yanshan and Xishan movement, there was angular unconformity between tertiary stratum and others that under tertiary. It is difficult to confirm the period of strong extrusion because tertiary stratum straight overlay on the middle Jurassic stratum, So there are many different opinions. The article sufficiency adopt the method of fission track analysis which successfully applied to research the time limit and the process of uplifting, denudation and chilling for planalto, orogen and basin and don't limit to the strata periods. Through the rock thermal evolution chilling event and correlation chronology information from fission track analysis result, then discuss the time limit and process of uplift anddenudation. The testing and analysis results are also to compare with evolution history that has strata distribution and the records of tectonic derformity to attain the more credible results.Firstly, we analysed the outcrop samples around basin, then we used the fission track analysis method to analyse the drilling samples from inner Bohu sag, we subdivided Yanqi basin tectonic movement into two episodes in Cretaceous, late of early Cretaceous and late of late Cretaceous, and the latter is the primary activity period.Through integration analyse the periphery and inner Bohu sag uplift-chilling age of the basin, we confirmed the time limit of tectonic uplift-chilling event is 37~62Ma(Eocene to Paleocene).In the tectonic event of late Yanshan squeezing uplift, the basin periphery was influenced due to squeezing process and uplift before the basin interior;the inner Bohu sag, southern sag and Zhongmachang sag were earlier than northern sag;the squeezing stress gradually transference from periphery to interior of the basin.The fission track analysis of thermal history indicate that Bohu sag of Yanqi basin reached the maximum buried depth and palaeogeothermal is the highest period about 130X1) in the early of early Cretaceous.Then the palaeogeothermal continuance decline, it didn't recover the obviously anneal temperature to apatite fission track again even though Cenozoic burial. Northern palaeogeothermal gradient was about 3.6\'C/lWkn and southern sag was about 3.2~3.3"C/100m before Cretaceous uplift.Kuluketage mountain was strong uplift because of the late of Yanshan period movement. We divided the uplift movement of Kuluketage mountain into three episodes in late Cretaceous through fission track analysis: 35±7Ma, 59~61Ma, 73 ±20 Ma;the primary period was 59~ 61Ma(early palaeocene) when Kuluketage mountain was large scale uplift at the end of Cretaceous, the uplift speed was 39~47 m/Ma. Huola mountain was large scale uplift later than Kuluketage mountain about 75~87Ma(late Cretaceous), the uplift speed was 32.8—38.1 m/Ma.The basin's tectonic evolution divided into four stages since Meso-Cenozoic: Yanqi-Kuche foreland basin;epiorogenic basin developed;squeezing uplift and denudation;Xishan period ramp. |
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