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Xiang Qin, Xuan-hua Chen, Zhao-gang Shao, Yi-ping Zhang, Yong-chao Wang
Chinese Academy of Geological Sciences, China Geological Survey, Ministry of Natural Resources, Beijing 100037, China
In the Indosinian duration, series of Gondwana-related plates pieced together and formed the geological sketch of China for the first time, i.e. the Yangtze, Sino Korean, Tarim plates, and other sub-plates that had been isolated for a long time. Subduction and collision among the Tarim, Qaidam, and Qiangtang blocks in Triassic, as well as the closure of the Paleo-Tethys Ocean, reactivated Late Paleozoic suture zones in the Tianshan orogen region (Hendrix MS et al., 1992).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 the Eurasia continent in the Early Mesozoic.
The Kuqa Depression, a long and narrow Meso-Cenozoic sedimentary basin, is sandwiched by the Southern Tianshan orogenic belt and the northern margin of Tarim Basin (Fig. 1a).The Triassic strata in the study area, covering the Permian molasse strata in an unconformity relationship, includes the Lower Triassic Ehuobulake Formation, Middle-Upper Triassic Kelamayi Formation, the Upper Triassic Huangshanjie and Taliqike formations (Fig. 1c). There are several main thrust faults in the study area, including the Southern Tianshan thrust fault (F1), the Southern Tianshan thrust front fault—Kuruli fault (F2and F3) and Tiketage fault(F4), and the two unnamed faults (F5and F6) in the Kelasu-Yiqikelike structural belt (Figs. 1a, b). Influenced by the multi-stage plate convergence events along the south margin of the Tarim Plate, these faults have experienced multi-phase tectonic activities, especially the far-field effect of the India-Eurasia collision since the Cenozoic, which blurred traces of the Mesozoic deformation and make it difficult to be identified. Fortunately, the Cenozoic displacement of the Southern Tianshan thrust nappe occurred mainly in the F4zone, which thrust the Carboniferous limestones above the Miocene conglomerates. The Cenozoic deformation of the Kuqa Depression mainly concentrates in the Kelasu-Yiqikelike and Qiulitage structural belt (Figs. 1a, b). As for the Kuruli Syncline area, the well-preserved Triassic strata are weakly deformed, only showing open folds and providing an ideal place for the study of Indosinian syn-sedimentary structures.
Growth strata record a great deal of information about syn-sedimentary structures, which have been widely used in the study of the orogenic belt. Based on detailed fieldwork,the Middle-Late Triassic Kelamayi Formation, constituting the north limb of the Kuruli Syncline, has been identified as typical growth strata (Figs. 1b, f). As shown in section A-B,the growth strata of the Kelamayi Formation develop above the overturned strata of Ehuobulake Formation, and the dip angles change abruptly from nearly vertical to about 15°(Fig. 1f). In the field, the syn-tectonic sedimentary wedge of the Kelamayi Formation was found. The dip angle decreases upward in strata sequence, and the single-layer thickness gradually decreased or pinched out towards the structural high(Fig. 1f). The syn-sedimentary folding geometry has been obscured by the faults F2and F3, thrusting the Xiaocanlike volcanic rocks above the Ehuobulake Formation and the Silurian magmatic rocks above Xiaocanlike volcanic rocks(Fig. 1d). According to the reversed fold geometry (Fig. 1d),it can be assumed that the syn-sedimentary anticline is a kind of fault-propagation fold. The growth unconformity and the fan-shaped geometry indicate that the folding and deformation mode of the growth strata is the limb rotation mode.
The Kelamayi Formation is mainly composed of sandstone, siltstone, and mudstone, which constitute a finingupward sequence (Fig. 1c). A tuffaceous sandstone sample(KCQ71-2) is collected from the bottom of the Kelamayi Formation (see Figs. 1c, f for the location). LA-MC-ICP MS detrital U-Pb zircon dating was conducted in the laboratory of Tianjin Geological Survey Center, China Geological Survey.See Qin X et al., (2020) for specific experimental methods.
From the investigation, the Middle-Late Triassic Kelamayi growth strata was interpreted in the northern Kuqa Depression in the southern margin of South Tian Shan. Along the deformation front of the southern Tianshan fold-thrust system, the typical syntectonic depositional wedge with progressive unconformable bedding of the Kelamayi formation was identified (Figs. 1d, f), and a detrital U-Pb sample KCQ71-2 was collected in the bottom of the Kelamayi growth strata. The weighted average age of the four youngest ages of sample KCQ71-2 is 223.4± 3.1 Ma (Fig.1e; Appendix Table S1), and the distinct oscillatory zones in the CL images indicate that these zircons are magmatic origin (Fig. 1e).Combining with the tuff and volcanic components of the sample KCQ71-2 in Monopolariscope photograph (Fig. 1g),the authors speculate that these zircons may result from synsedimentary volcanic materials, which may either derived from volcanic activity in the areas adjacent to the Tianshan,such as West Kunlun and Eastern Tianshan (Zhu YF, 2007),or from yet undiscovered volcanic sources within the range itself. Therefore, the bottom age of the syntectonic Kelamayi
Fig. 1. a-Geological sketch map of the Kuqa depression, Southern Tianshan, China, FTB-fold and thrust belt; b-simplified geological map of the northern part of the Kuqa depression, Fm.-Formation; c-simplified sedimentary sequences of the northern part of the Kuqa depression;d-cross-section A-B in the northern part of the Kuqa depression. The strata color and strata filling lithology in the section correspond to Fig. 1b and Fig. 1c, respectively; e-the concordia and weighted average age plots of the four youngest ages of sample KCQ71-2; f-typical outcrops of the Kelamayi growth strata, GS-growth strata; g-monopolariscope photograph of sample KCQ71-2, and the white dotted lines represent tuff and volcanic components.
Formation is approximately about 223 Ma, that is to say, the initial activity of the Kuruli fold-thrust system is also close to this timing. These discoveries constrain the initiation of the Indosinian intracontinental deformation in the Tianshan area started at ca. 223 Ma, which is of great significance for the reconstruction of the tectonic evlution of the Tianshan orogen.Similar to the Cenozoic intracontinental deformation in Tianshan, which originated as a result of far-field on-going India-Eurasia collision (Molnar P and Tapponnier P, 1975),the dynamic source of the Triassic intracontinental deformation in South Tianshan primarily originated from the subduction and collision among the Tarim, Qaidam, and Qiangtang blocks in the Triassic (Hendrix MS et al., 1992).
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. 223 Ma (Middle-Late Triassic), which is a little later than the continental collision in the Qinling-Dabie orogenic belt (221-228 Ma) (Li SG et al.,1993). These discoveries provide convincing evidence to understand the intracontinental deformation in the Tianshan area in Indosinian duration.
CRediT authorship contribution statement
Xuan-hua Chen conceived of the presented idea. Xiang Qin developed the theory, performed the investigation and drafted the original mauscipt. Zhao-gang Shao, Yi-ping Zhang and Yong-chao Wang participated in the field investigation and verified the analytical methods. All authors discussed the results and contributed to the final manuscript.
Declaration of competing interest
The authors declare no conflicts of interest.
Acknowledgment
This study was supported by the National Key Research and Development Project (2018YFC0603700) and projects of the China Geological Survey (DD20160083, DD20190011).
Supplementary data
Supplementary data (Appendix Table S1) to this article can be found online at doi:10.31035/cg2021012.
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