Cenozoic crustal shortening between the Pamir and Tien Shan and a reconstruction of the Pamir-Tien Shan transition zone for the Cretaceous and Palaeogene

The magnitude of the Late Cenozoic crustal shortening during convergence of the Pamir and Tien Shan was determined using a contemporary pattern consisting of facies zones, palaeomagnetic data (regarding the rotation of tectonic units) and data on the structure of the Tadjik Depression. By Late Cenozoic, Cretaceous and Palaeogene facies zones were cut by the Vakhsh-Trans-Alay overthrust and Darvaz strike-slip faults and a significant part of the Cretaceous-Palaeogene Tadjik Basin was overthrust by the Pamir massif. The sediments of easternmost part of the basin are preserved in the Tarim Depression. The facies zones of the southern slope of the Afghan-Tadjik Basin were deformed and moved northward. A pattern of facies zones indicates a convergence between the Pamir and Tien Shan over a distance of 300-400 km. A number of cross-sections through the Tadjik Depression were used to establish the structure before folding. A rotation of tectonic units, indicated by structural data, conforms to the angles of rotation as determined in palaeomagnetic studies. The data suggest 300 km of convergence between the Pamir and Tien Shan. Stratigraphic, lithological, structural and palaeomagnetic data formed the basis for the construction of the palinspastically-restored palaeogeographic and sedimentologic environments for the Tadjik shallow sea basin which was situated between the Pamir and Tien Shan before their convergence in the Late Cenozoic. The maps were constructed for the eight stratigraphic levels of the Cretaceous and Palaeogene. The Tadjik Sea was a bay in the enormous Turan Sea. In the Early Aptian this bay was located in what is now the Afghan-Tadjik Basin. In Late Cretaceous, the eastern shore of the bay lay 600-700 km further eastward and in the Eocene, marine environments extended even further eastward but after the Rupelian continental environments occupied all of this region. (C) 2000 Elsevier Science B.V. All rights reserved.