帕米尔隆升过程中地壳的一种重要缩短机制--以齐姆根构造转换域为例

区域构造显示,在帕米尔－西昆仑前陆地带大致以齐姆根为中心的范围内,山前构造变形呈现分阶段发育的特征,前陆冲断带和前陆沉降带横向不能连接,形成喀什和叶城－和田两个沉降坳陷中心。这是由于造山带前陆的逆冲断裂带向盆地大规模逆掩和推覆过程中大幅度构造叠覆而形成,长距离构造拆离使许多地质记录被掩盖和破坏,给地质构造发展史重建造成很大困难,齐姆根由于地处连接帕米尔、西昆仑2个前缘逆冲断裂系及其前陆沉降带的应力转换和调整部位,构造以斜向走滑为主、逆冲推覆程度相对较弱,因而得以保留许多塔里木西南构造演化过程中形成的地质信息,该区域是正确认识东帕米尔地区构造演化的关键点之一,因此笔者引入“域”的概念,将其命名为“齐姆根构造转换域”,其原型背景是目前帕米尔－西昆仑前缘唯一保存较为完整的沉积盆地,规模不大却显示了较为典型的前陆盆地沉积构造样式,结合区域事件同位素年龄证据,基本可以确定塔西南前陆盆地发育下限不晚于侏罗纪早期,178.4-113.3Ma为初期活动的主要时期。侏罗纪发育有限规模前陆盆地,是在塔里木西南前陆盆地发育的初期阶段,同时盆地的展布受到古近纪以来帕米尔构造结“突刺”状楔入的强烈影响,使侏罗纪原始盆地走向发生偏转。研究表明,该构造转换域与康西瓦断裂一起构成一系列构造应力转换系统,自三叠纪古特提斯闭合以来一直承接着印度板块向北挤压和帕米尔隆升过程产生的巨大地壳缩短量。

AN IMPORTANT MECHANISM OF CRUSTAL SHORTENING DURING UPLIFT OF PAMIR--TAKING QIMUGEN TECTONIC TRANSFORMATION DOMAIN AS AN EXAMPLE

Regional structure clearly indicates that the structural deformation of the Pamir-West Kunlun foreland area, from Kushan River in the west to the region near Xinjiang-Tibet highway in the east with Qimugen in the middle, was completed at different stages. The foreland thrust zone and foreland depression zone are not connected laterally, resulting in formation of Kashi depressin and Yecheng-Hotan depression. The Cenozoic India-Eurasia collision led to formation of large-scale thrust and nappe structures in the foreland area. The structural overlapping and large-distance detachment overprinted and destroyed numerous geological record, resulting in difficulty in reconstruction of regional geology. Qimugen is located at a junction between Pamir and west Kunlun frontal thrust zones, and between two foreland depressions. The structural style here is dominated by oblique strike-slip which functions as stress transformation and adjustment. The relative weaker thrust and nappe structures allow Qimugen area to preserve more geological information for tectonic evolution of the western Tarim basin. It is a key area to reconstruct the regional geological evolution and to understand the mechanism of the uplift of Pamir. "Qimugen tectonic domain" is introduced here to represent a clear linear structure which consists of steeply-deeping strike-slip fault, tight fractures, transcurrent deformation zone, steep-hinge and echelon folds.. The tectonic deformation and geophysical anomalies on both sides show clear discontinuity in Qimugen area. The Qimugen tectonic domain originally refers to the only well-preserved sedimentary basin in the frontal area of Pamir-Kunlun, which shows structural style of typical foreland basins. The isotopic ages indicate that the development of southwestern Tarim foreland basin postdates early Jurassic, and mainly occurs during 178.4-113.3 M at its early stage. The intense "wedging" of Pamir since the Paleocene changes the strike of the Jurassic basin. This tectonic domain works as Kangxiwar fault as a transform system of stress.