Indosinian intra-continental deformation along the Southern Tianshan: Constraints from growth strata and detrital zircon U-Pb dating in the Kuqa depression

Abstract:

In the Indosinian duration, series of Gondwana related plates pieced together and formed the geologic sketch of China for the first time, i.e. the Yangtze, Sino Korean, Tarim plates, Qiangtang and other sub-plates which had been isolated for a long time. The collision events between the southern margin of Eurasia and the Qiangtang arc reactivated late Paleozoic suture zones in the Tianshan orogen region. However, there are few reports about the initiation timing and tectonic deformation of Indosinian movement in the Tianshan area, which is a key decipher to further understand the early intracontinental deformation history here, especially for the beginning of construction of Eurasia continent in the early Mesozoic. Growth strata record a great deal of information about syn-sedimentary structures, which have been widely used in the study of orogenic belt. Based on detailed field work, the Middle-Late Triassic Kelamayi (T_2-3k) formation, along the Southern Tianshan thrust fault in the northern part of the Kuqa depression, has been identified as typical growth strata. The Kelamayi formation growth strata develop above the overturned strata of the early Triassic Ehuobulake (T_1-2e) formation, and the dip angles change abruptly from nearly vertical to about 15 °. Three sandstone samples were collected in the northern part of the Kuqa depression, KCQ71-1 from the top of the T_1-2e formation, KCQ71-2 from the bottom of the T_2-3k growth strata, and KCQ71-3 from the top of the T_2-3k growth strata respectively. The detrital U-Pb zircon dating of those three samples were conducted. The youngest zircon age of sample KCQ71-2 and KCQ71-3 are 220 Ma, 213 Ma respectively, which indicates the active time of the growth structure is between ~220 Ma to ~213 Ma. The abruptly changes of the detrital zircon ages of T_1-2e formation and T_2-3k formation suggest that there are tectonic events along the Southern Tianshan mountains. Therefore, the coupling relationship between the growth strata of the Kelamayi formation and the Southern Tianshan fold–thrust system indicates that the Indosinian movement in the Southern Tianshan area started at ca. 220 Ma (Late Triassic), stopped at ca.213 Ma. These discoveries provide convincing evidence to understand the intracontinental deformation in the Tianshan area in Indosinian duration.