晚新生代天山隆升与库车坳陷构造转换的沉积约束

Depositional Record Constraints on Late Cenozoic Uplift of Tianshan and Tectonic Transformation in Kuqa Depression, West China

对库车坳陷库车河剖面不同尺度沉积碎屑组分的系统分析发现 ,晚第三纪以来类磨拉石及其中不稳定碎屑矿物组合开始发育 ,并在中新世后发生显著增加。沉积体系研究表明 ,库车坳陷自白垩纪以来粗粒冲积体系前锋、沉积中心逐步向南迁移 ;但该规律自上新世开始发生了明显转变 ,表现为上新世库车组、上 -更新世西域组冲积体系整体大尺度南移 ,而沉积中心则相对北移 ,盆地北部沉积边界与现存盆山边界大致可以对比 ,并受到南倾同沉积正断裂的控制。对比天山区域构造、古气候演变资料 ,认为上述沉积记录是晚新生代强烈挤压应力作用下天山构造隆升加剧 ,并在上新世以后由于重力作用诱发库车坳陷 -天山浅部构造层伸展、坍塌的结果。对这一构造转换的认识修订了库车坳陷新生代构造调整的序列。

Based on system research on detrital sediments in different scales, it is found that molasse-like deposits, instable (easy abrasive) detrital grains and mineral-assemblages were developed in the Kuqa depression, the south foot of the Tianshan, in Neogene, especially after Miocene. It is also presented that gradual southward migration of alluvial depositional fronts and depositional depocenters with near east-west strike occurred since Cretaceous. However, evident changes of the above migration trend happen since Pliocene, in which large-scale southward migration of alluvial depositional systems but relative northward migration of depositional depocenters occurred simultaneously, and current north depositional boundary is near present basin-mountain boundary between Kuqa depression and Tianshan, which attributes to syndepositional normal faults of southward dip. Combined with data from tectonic and paleoclimate reserches, it is explained that the above depositional records resulted from the tectonic transformation processes in Late Cenozoic, in which gravity-extension and gravity-collapse occurred in the shallow tectosphere of the Kuqa depression-Tianshan since Pliocene due to intensive uplift of the Tianshan under continuous extrusion stress in Late Cenozoic. The conclusion on tectonic transformation processes, presented in the paper, revise the past views on Cenozoic dynamic mechanism of structural accommodation, which was explained with sole southward thrusting pattern of the Tianshan under extrusion stress regimes.