塔西南齐姆根楔形构造与走滑构造叠加两阶段演化模式

塔里木盆地西南缘逆冲带由西部近东西方向的喀什逆冲构造带和东部近东西方向柯克亚—和田逆冲构造带以及中间喀什—叶城走滑断层系组成,而北西—南东走向的喀什—叶城走滑断层带及其东侧齐姆根构造则为呈北东方向凸出的弧形构造。齐姆根弧形构造在地表地质和地震剖面均表现为向盆地方向倾斜的单斜形态。而且在塔西南地区,该弧形构造上从白垩系到新近系地层厚度明显大于东西两侧逆冲带同时代地层厚度,表现为异常增厚的特征。为了探讨齐姆根弧形构造特征及地层厚度异常增厚等原因,依据前人的地表地质填图成果,以及塔里木盆地西南缘齐姆根地区及邻区完成的二维地震资料及钻井资料成果,对该区地震剖面资料进行详细的构造解释,提出齐姆根弧形构造单斜之下存在3个隐伏逆冲构造楔形体,即棋盘楔形构造、齐姆根楔形构造和英吉沙楔形构造。地震剖面解释的生长地层指出棋盘楔形构造形成最早,为上新世阿图什组沉积时期;其次发育齐姆根楔形构造,为更新世西域组底部沉积时期;最晚发育英吉沙楔形构造,时间大约在更新世西域组中?上段沉积时期,据此认为齐姆根中深层逆冲构造位移扩展方式为前展式。而且地震剖面解释上也揭示了白垩系到新近系地层厚度异常增厚发育的构造部位及发育规律,说明地层异常厚度变化受区域构造控制,其时间大约为更新世西域组沉积时期。该时期对应于喀什—叶城走滑断层系活动时期,据此我们推测由于走滑断层系区域侧向挤压作用以及东西两侧发育的逆冲构造带起到了限制阻挡作用,三者联合之下造成了齐姆根单斜上含软弱层的各时代地层被挤压屈曲形成弧形形态构造,在弧形构造发育期间岩层之间层间滑脱加厚可导致地层厚度异常增厚。综合前面认识,最后提出齐姆根弧形构造为叠加构造发育地区,经历了早、晚两期构造叠加变形：早期为逆冲楔形构造发育阶段,晚期为走滑作用改造及弧形构造形成阶段。而且沿着齐姆根弧形构造走向,弧形构造北西侧翼部单斜发生逆时针旋转,而弧形构造南东侧翼部单斜发生顺时针旋转,并得到已有古地磁数据支持,属于旋转弧构造类型。该认识支持了帕米尔东侧局部弧形构造是在早期发育的逆冲推覆构造基础上,逐渐叠加有晚期发育的左旋走滑断层而形成弧形构造。

A two-stage evolution model of the wedged-shaped structures superposed with strike-slip faulting for the Qimugen oroclines of the Southwest Tarim Basin,Northwest China

Despite the 2D seismic investigation of a continuous seismic and well data have been applied to study the Qimugen region of the Kashi-Yecheng strike-slip fault in Southwest Tarim Basin foreland fold-thrust belt nearby Pamir range, its structural deformation's styles remain poorly constrained in time and space, hindering its potential for understanding deep to surface processes and oil exploration in Tarim Basin. Based on the iso-dip domain division and axial plane analysis on the calibrated seismic profiles as well as geometrical analysis of the North Qimugen monocline, our interpretation has found that the anomalous thickening layers are found in the monocline from the Cretaceous to the Pliocene Atux formation in shallower part, and the sophisticated wedged-shaped structure developed in the deep part. The sophisticated deep wedged-shaped structures have been interpreted from south to north: the blind Qipan structural wedge, Qimugen structural wedge, and the Yingjisar structural wedge with the Yingjisar anticline in the surface. Both the Qipan and Qimugen structural wedges dies out northward to west of the Yingjisar structural wedge. The onset of growth strata and a sediment accumulation hiatus from the magnetostratigraphic analyses of the Aertashi section can be referred that the Qipan structural wedge was formed. The Qimugen structural wedge had started by 20 Ma, i.e. the deposition of the Xiyu formation. The Yingjisar structural wedge can be associated with the activation of the Kashi-Yecheng strike-slip faulting. The growth strata and relation between three structural wedges can be preliminarily inferred that the deformation sequence of the thrust belt along the eastern margin of Pamir is progressively younger northward. The geometry of the deformation of the Qimugen monocline resembles the behavior of a system of parallel monocline(paper sheets)that distributed simple shear causes strike-slip faulting on parallel sheets. Then, the presence of a rigid backstop is formed by south-north compression at both the top ends of the paper sheets similar to the Kashi-Yecheng strike slip faults in the West Kunlun, and results in their rotation. The paper sheets rotate anticlockwise in the north-segment of the bulged area, whereas they rotate clockwise at the south-segment. Different rates of rotation along strike produces overlap or gaps(shaded in black). In spite of its extreme simplicity, a simple model of paper sheets by strike-slip folding is able to produce overlaps (anomalous thickening layers)in the Qimugen region. Combined with a regional review of existing paleomagnetic studies in Cretaceous to Neogene strata, these results indicate that clockwise rotations along the south-segment at the Aertashi section more than along the north-segment at the Kezi section, in which counterclockwise may possibly exist along the north-segment. These results also show that most of the clockwise rotations along the eastern Pamir occurred after 15 Ma, and did not extend systematically and regionally into the Tarim Basin. Results from growth strata and anomalous thickened by the strike-slip shearing from cretaceous to upper Pliocene combined with existing magnetostratigrphic studies, support a two-stage kinematic model: 1)initial development of the Qimugen deep wedged-shaped structures involved by the Paleozoic and the upper monoclines of from Oligocene to Late Miocene during the Mid-Late Pliocene from 25~20 Ma followed by; 2) development of the strike-slip shearing in the upper monoclines due to dextral Kashi-Yecheng strike-slip faulting from Late Pliocene to present. This suggests that the Southwest Tarim Basin foreland fold-thrust belt can be in episodic manner and has produced two episodic tectonic movements in propagating deformation from the western Tibetan Plateau into the Tarim Basin.