Compression Salt Tectonics And Synkinematic Strata Study In The Middle To Western Kuqa Foreland Basin, Southern Tian Shan

This thesis focuses on the salt tectonics and synkinematic strata in the middle to western Kuqa foreland basin. Integrating surface geology, well data and a grid of seismic reflection profiles, thirteen balanced cross sections were constructed in this paper. Shortening displacements of several profiles were calculated by the axial surface analysis, the Depth-relief method or the Thickness-relief method respectively, and the Cenozoic synkinematic strata (6-8 km thick) in the western Kuqa basin were bed-by-bed restored. According to the shortening displacements and the shape of the synkinematic strata, in combination with the previous magnetostratigraphic data, the salt tectonics evolution and the Cenozoic histories of the Tian Shan were discussed in this paper.The Kuqa basin deposited two sets of salt. They are the Paleocene-Eocene Kumugeliemu salt and the Miocene Jidike salt. The Kumugeliemu salt distributes in the western Kuqa basin, while the Jidike salt distributes in the eastern Kuqa basin.At the middle segment of the Qiulitage fold belt, there are three decollements: the Jurassic coal bed, the Paleocene-Eocene Kumugeliemu salt and the Miocene Jidike Salt involved in deformation. The Kuqadawu anticline is a detachment fold cored by the Jidike salt, which overlies a deep fault-bend fold with a lower decollement flat locates in Jurassic coal bed. The southern Qiulitage anticline is a thin-skinned salt anticline cored by the Kumugeliemu salt.Shortening displacement estimatation shows that the displacements of faults developed by the Jidike salt and the Jurassic coal bed decrease to west, while the displacement of the fault developed by the Kumugeliemu salt is converse. Furthermore, the Kuqadawu anticline plunges to west, the southern Qiulitage anticline plunges to east, so both of the Kuqadawu anticline and the southern Qiulitage anticline plunge with their underlying faults respectively. As a result, (1) the strara involved in deformation were superimposed, (2) the shortening displacements of the three decollements were converged and transformed, and (3) the Kuqadawu anticline, the southern Qiulitage anticline and the Tuokelaketan anticline converged in the middle segment of the Qiulitage fold belt, induced complex deformation over here.At the western segment of the Qiulitage fold belt, thin-skinned compression salt tectonics developed, comprising of numerous allochthonous Kumugeliemu salt. These thin-skinned salt teconics have been influenced by two factors: (1) the distal pinch-out of the Kumugeliemu salt located at south edge of the Qiulitage fold belt, which resulted in an increasing of frictional resistance, was prone to allochthonous salt accumulating in here. (2) Several pre-existing diapirs developed in the western Kuqa basin. These precursor diapirs initiated soon after the Kumugeliemu salt deposition, which had different growth histories, localized the contraction strain of later shortening. If a diapir had a short growth history and was overlain by a thick overburden, it was likely to evolve into a salt dome by later shortening, comprising a 3000-7000 m thickness of allochthonous salt. Conversely, if a diapir was overlain by a thin overburden, it was more likely to form a salt nappe, with northern flank of the diapir thrust over its southern flank.The shape of the synkinematic strata in the westerm Kuqa basin indicates that since the regeneration of the Tian Shan at 20-25 Ma, the shortening deformation of the western Kuqa basin mainly accumulated in the hinterland until the early Miocene, while the intense shortening deformation of the western Kuqa basin iniated since the early Pliocence/ late Miocene (5-6 Ma). Before compression deformation, the middle to western Kuqa bain developed early salt tectonics.The western Kuqa basin has -23 km of total shortening displacements. Accordingly, if the Tian Shan regerated at 20-25 Ma, we get a -1.15/0.92 mm/yr average shortening rate. This is very small than the shortening rate of the modern Tian Shan (-7-8 mm/yr in 80-85°E) estimated by GPS (Abdrakhmatov et al, 1996; Wang et al., 2001). However, two forward kinematic profiles in this paper illustrate that -9 km of the total shortening consumed during the end of Miocene (5.2/5.8 Ma) to the early Pleistocene (2.58 Ma), and -14 km have been absorbed since then, thus obtaining a -3.4/2.8 mm/yr average shortening rate from 5.2/5.8 Ma to 2.58 Ma, and a -5.4 mm/yr average shortening rate since the 2.58 Ma, which is concordant with the shortening rate of the modern Tian Shan estimated by GPS. Therefore, the modern Tian Shan had at least experienced two accelerated events, which began in the late Miocene/early Pliocene and early Pleistocene, respectively.