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Field investigation shows that the boundary between the Kuqa Basin and the Tianshan Mountains can be divided into two sections with the Yanbulak area as the dividing point. In the western section, the Mesozoic strata overlie unconformably on the Paleozoic rocks. The basin-dipping faults developed in both Mesozoic and Paleozoic rocks. The eastern section is characterized by basin-dipping normal faults separating the Paleozoic strata and Tertiary. The brittle structural analysis was carried out along the basin-range boundary. 360 measurements of striations have been obtained at a total of 25 sites. Paleostress reconstruction indicates that the basin-range boundary was in an extensional condition, with some superimposed strike-slip, during the Late Tertiary. The extension could be explained by the vertical block uplift of the Tianshan Mountains. [ABSTRACT FROM AUTHOR]

Abstract: The Tarim and Junggar basins in central Asia are capped by a thick pile of conglomerates, called the Xiyu Formation, that are commonly linked to a change in climate and/or accelerated uplift near the Plio-Pleistocene boundary. In order to better understand their origin and significance, we carried out a combined structural and magnetostratigraphic study in the Quilitage syncline (southern Tianshan), where the base of the Xiyu conglomerates is observed at both sides of the syncline. A balanced cross-section shows that, even at a local-scale, the base of the Xiyu conglomerates cannot be regarded as a single continuous stratigraphic layer. On the southern flank of the Quilitage syncline, we collected 172 samples collected for magnetostratigraphic dating identify 17 polarity chrons that date the new section from 5.2 to ~1.7Ma and constrain the base of the Xiyu conglomerate here at ~1.7Ma. This is 4.2Ma younger than the age of the Xiyu previously found on the northern limb of the same syncline. Together with other magnetostratigraphic studies carried out around the Tianshan, our study unambiguously demonstrates that the onset of deposition of the Xiyu conglomerates is diachronous, and that the conglomerates are systematically younger toward the basin. Consequently, the Xiyu Formation should not be considered as a chronostratigraphic marker related to any particular tectonic or climatic event, but is instead a prograding gravel wedge that has prograded over the underthrusting forelands. A synthesis of chronologic and structural results yields progradation rates over the last 10Ma on the order of ~2.0mm/yr and ~3.9mm/yr south and north of the Tianshan Mountains respectively. These rates are comparable to the shortening rate across the Tianshan range, suggesting that underthrusting is the main factor governing the progradation rate of the Xiyu Formation. [Copyright &y& Elsevier]