西天山隆升-剥露过程初步研究

Preliminary study on the uplifting-exhumation process of the western Tianshan range,northwestern China.

天山造山带是在古生代造山基础上,在新生代由于印亚碰撞的远程效应而陆内再造成山。本文主要用磷灰石裂变径迹测年分析来限定西天山的隆升-剥露过程。野外系统采集西天山山脉及其伊犁盆地钻孔中的样品,挑选磷灰石进行了裂变径迹测试分析工作,着重开展了磷灰石的温度-时间反演模拟研究。根据温度-时间模拟结果推断,西天山在中新生代期间共发生了3期次的快速冷却作用,分别为早侏罗纪(200～180Ma)、白垩纪中期(115～95Ma)和新生代期间(24Ma以来);根据样品位置分析表明,西天山的隆升-剥露作用并不均一,开始于侏罗纪早期的山脉抬升范围有限,仅局限于中天山;白垩纪中期(115～95Ma),整个西天山山脉和伊犁盆地一起发生整体的抬升和剥露;新生代24Ma以来,西天山的山体块体抬升与山间盆地的断陷同时发育。

The Tianshan range in the middle Eurasia is regarded as a Cenozoic rejuvenated mountain chain based on the Paleozoic Tianshan orogeny in response to the far-field effect of the Indo-Asian collision. This paper mainly present analyses results of apatite fission track both from western Tianshan range and the Ill basin to reveal the unroofing history of the western Tianshan. 19 samples, including granite and volcanic rocks from the Chahanwushu mountain, middle-western Tianshan, sandstones from drilling wells in southern part of the Ili basin, and two volcanic rocks in the northern Tianshan, were collected and apatite was selected for fission track dating in the lab. Temperature-Time modeling of apatite was also completed based on data measurements of the fission track length and age. Analyses results revealed that the western Tianshan has probably experienced three phases of rapid cooling events during the Mesozoic to Cenozoic, i.e. early Jurassic （200 - 180Ma） , middle Cretaceous （115 -95Ma） , and Cenozoic （since ca. 24Ma）. The sampling localities and their cooling processes also suggested that the early Jurassic exhumation of mountains was only limited in the middle Tianshan, and the middle Cretaceous （ 115 -95Ma） unroofing event widely spread the whole Tianshan, including the Ili basin, and the Cenozoic regional differential uplifting of mountains and subsidence of intra-orogeny basins occurred simultaneously since 24Ma.