前陆盆地的构造演化及其沉积、地层响应——以库车坳陷下白垩统为例

Research on the tectonic evolution of foreland basins and their responses to deposition and stratigraphy—an example from the Lower Cretaceous in Kuqa Depression

逆冲推覆构造近年来的研究表明,由于逆冲带内超压的反复积聚和释放,冲断活动表现为间歇式前展逆冲方式,且具有早期长周期低前展速率、晚期短周期高前展速率的特点,由此导致作用于前陆盆地的构造负载早期以垂向加载为主、晚期则以递进加载为主。库车坳陷下白垩统记录了一幕完整的前陆盆地构造演化过程,卡普沙良群和巴什基奇克组分别对应于逆冲期和构造宁静期,两者之间发生了地层几何形态的重大转变以及内部削截反射终止点的迁移。与早、晚期冲断活动特征对应,在前隆向前渊的斜坡带,卡普沙良群的下部和上部分别发育大套向逆冲带的前积反射和向克拉通方向的退积超覆反射。在紧邻造山带的前渊带,亚格列木组砾质粗碎屑沉积体标志着逆冲推覆的重新开始;巴西盖组三角洲体系的发育说明快速前展逆冲导致宽缓的楔顶带的形成;巴什基奇克组底部粗碎屑发育及其与巴西盖组之间沉积突变,反映为宁静期岩石圈强烈回弹的结果。最终,前陆盆地构造演化的逆冲期可细分为初始逆冲、?冲、前展逆冲、逆冲减弱—停滞等4个阶段。

According to the recent research on overthrusts at the front of orogenic belts, thrusts and faults propagate intermittently forward to the craton, owing to interplay of accumulating and releasing overpressure. In the early stages, they are characterized by long-period and low-velocity and in the later stage by short-period and high-velocity, resulting in typical overthrust nappe. Therefore, it could be presumed that tectonic loads operate mainly vertically during the early stages and propagate during the later stages. The Lower Cretaceous in the Kuqa Depression recorded an episode of tectonic evolution of a foreland basin. From significant changes in stratigraphic geometry and the migration of erosional truncation sites, we may infer that the Kapushaliang Group and Bashenjiqike Formation in the Kuqa Depression developed during thrust tectonics and quiescence period, respectively. Furthermore, corresponding to thrust-faulting activities in the early and in the later stages, the lower and the upper parts of Kapushaliang Group developed propagation reflection forward to thrust-belt and retrogression reflection forward to craton, respectively. In the foredeep adjacent to the orogenic belt, the coarse gravel sediment in Yageliemu Formation marked the beginning of the thrusting. The delta system in Baxigai Formation implied that a wide and even wedge-top belt was produced by the rapid propagation of thrusting. An abrupt change of deposition between the Bashenjiqike Formation and Baxigai Formation and the coarse gravel sediment at the bottom of Bashenjiqike Formation resulted from violent rebounding of lithosphere. The thrusting period of a foreland basin can be subdivided into four stages： primary thrusting, wedgethrusting, propagation-thrusting, and weakening to termination of thrusting.