GROWING HISTORY AND GEOMORPHIC RESPONSE OF THE EASTERN TERMINATION OF KASHI ANTICLINE, SOUTHWESTERN TIAN SHAN: AN INTEGRATED ANALYSIS OF GEOLOGY, GEOMORPHOLOGY, SEISMIC REFLECTION PROFILE AND MAGNETOSTRATIGRAPHY

doi:10.3969/j.issn.0253-4967.2025.02.20240157

Abstract

Abstract:

The Kashi anticline, located along the leading edge of the Kashi foreland thrust system in the southwestern Tian Shan of China, confronts the Atushi Anticline to the north and connects with the Mingyaole Anticline to the west. The Kashi anticline manifests at the surface as an elongated hillock with a nearly EW strike. The morphology of the anticline is roughly box-shaped, with the southern limb being gentler compared to the northern limb. As a significant component of the Tianshan orogenic belt’s frontal zone, the study of the tectonic evolution of the Kashi Anticline is crucial for understanding the Cenozoic tectonic deformation and crustal shortening processes of the Tian Shan. Previous studies on this fold are primarily focused on its surface-expression part, with little or no focus on its lateral termination that is not expressed on the land surface, which has limited the comprehensive understanding of the anticline’s overall evolutionary process. The study employs the depth-relief area method, combined with high-resolution seismic reflection profile data, to conduct a detailed structural analysis of the eastern termination of the Kashi anticline.
Through meticulous interpretation and quantitative analysis of the seismic profiles, the following key insights have been obtained: Firstly, along the seismic profile, the Cenozoic strata are approximately 6.8km thick, while the Mesozoic-Paleozoic strata are about 2.2km thick. The eastern termination of the anticline detaches along the Paleogene unit, with a depth of ~6.8km. This detachment layer governs the deformation patterns and magnitudes of the overlying strata. Secondly, along the seismic profile, the total shortening of the anticline is estimated to be(882±79)m, of which approximately 94% is attributed to shear shortening, and about 6% is due to curvietric shortening. During the folding process, materials with an excess area of ~3.4km² enter the cross section. Thirdly, according to the analysis of growth strata and published magnetostratigraphic data, the folding of the eastern termination initiates at the age of ~2.1Ma, which implies that the initiation age of the fold should be much older than 2.1Ma. The shortening rate remains at an approximate constant of ~0.4mm/a since the folding initiation. Fourthly, for the pre-growth strata, the uplift of the anticline gradually increases upward with depth, reaching a maximum of approximately 770m at the top boundary of the pre-growth strata. Analysis of the growth strata indicates that the uplift rate either keeps a constant of ~0.4mm/a, or increases significantly from an earlier rate of ~0.1mm/a to a later rate of 0.4 mm/a at the age point of ~1.6Ma. Notably, because the uplift rate is smaller than the sediment rate, the fold exhibits no expression on the surface of the anticline.
Our study exemplifies that an analysis of the buried lateral termination of a fold can well determine the detachment level, shortening and uplift histories, initiation age of the fold, as well as whether or not the excess area enters the cross section during the folding process. These constraints provide a completer and more reliable basis for understanding the entire growing history of the fold. Furthermore, the results demonstrate that the analysis of buried structures in the frontal zones of orogenic belts is indispensable for a comprehensive understanding of regional tectonic deformation characteristics and evolutionary history.

Key words: Tian Shan, Kashi anticline, fold’s lateral termination, growth strata, detachment fold, depth-relief area method

摘要：

喀什背斜位于西南天山喀什前陆冲断带的前缘, 以北与阿图什背斜相望, 向西和明尧勒背斜相接。前人对该背斜的研究主要集中在地表出露部分, 而对隐伏于地下的倾伏端研究较少。文中利用深度-隆升面积法对横切背斜东倾伏端的地震剖面进行了分析, 获得以下几方面认识: 1)背斜东倾伏端的生长受距地表约6.8km的古近系滑脱层控制。2)沿地震剖面, 背斜总缩短量为(882±79)m, 生长过程中有面积约3.4km²的物质流入剖面。3)通过对生长地层的分析和已有的磁性地层学结果, 估算剖面处背斜生长起始时间约为距今2.1Ma, 缩短速率恒定约为0.4mm/a; 由于背斜东倾伏端在距今约2.1Ma开始生长, 因此整个背斜开始生长的时间早于距今2.1Ma。4)剖面处背斜隆升速率可能大致恒定在约0.4mm/a, 也可能在距今约1.6Ma时由之前的约0.1mm/a增大至之后的约0.4mm/a; 由于隆升速率一直小于沉积速率, 背斜并未在地表形成地形起伏。文中研究表明, 对隐伏地下的背斜倾伏端的解析可对滑脱层位置、缩短和隆升历史、生长起始时间及生长过程中是否有盈余面积加入等进行准确限定, 以此可对整个背斜的生长演化进行更为完整可靠的约束。

关键词: 天山, 喀什背斜, 倾伏端, 生长地层, 滑脱褶皱, 深度-隆升面积法

HE Peng-yu, LI Tao, CHEN Zhu-xin, CHEN Jie. GROWING HISTORY AND GEOMORPHIC RESPONSE OF THE EASTERN TERMINATION OF KASHI ANTICLINE, SOUTHWESTERN TIAN SHAN: AN INTEGRATED ANALYSIS OF GEOLOGY, GEOMORPHOLOGY, SEISMIC REFLECTION PROFILE AND MAGNETOSTRATIGRAPHY[J]. SEISMOLOGY AND GEOLOGY, 2025, 47(2): 369-383.

何鹏宇, 李涛, 陈竹新, 陈杰. 西南天山喀什背斜东倾伏端的生长演化和地貌响应: 基于地质地貌、地震反射剖面和磁性地层数据的综合分析[J]. 地震地质, 2025, 47(2): 369-383.

Figures/Tables 8

Fig. 1 The control of uplift-deposition interaction on the folding surface expression (adapted from Daëron et al., 2007).

Fig. 2 Topography, geology and geomorphology of the Kashi anticline.

Fig. 3 Magnetostratigraphic results of the Kashi west section and Kashi Town section at the south limb of the Kashi anticline(see Fig. 2b for location).

Fig. 4 Topographic profile, seismic reflection data, and interpretation results of the section BB' crossing the Kashi anticline.

Fig. 5 Relations between total shortening(S), shear shortening(Sε)and curvietric shortening(Sc).

Fig. 6 A sketch showing the depth-relief area method to calculate the detachment fold shortening (adapted from Epard et al., 1993; LI Shi-qin, 2009).

Fig. 7 Shortening and Uplift histories of the Kashi anticline.

Fig. 8 Sediment-uplift and shortening-uplift graphs of the Kashi anticline.

References 26

[1]	陈杰, Heermance R V, Burbank D W, 等. 2007. 中国西南天山西域砾岩的磁性地层年代与地质意义[J]. 第四纪研究, 27(4): 576-587.
	CHEN Jie, Heermance R V, Burbank D W, et al. 2007. Magnetochronology and its implications of the Xiyu conglomerate in the southwestern Chinese Tian Shan foreland[J]. Quaternary Science, 27(4): 576-587. (in Chinese)
[2]	陈杰, Scharer K M, Burbank D W, 等. 2005. 西南天山明尧勒背斜的第四纪滑脱褶皱作用[J]. 地震地质, 27(4): 530-547.
	CHEN Jie, Scharer K M, Burbank D W, et al. 2005. Quaternary detachment folding of the Mingyaole anticline, southwestern Tian Shan[J]. Seismology and Geology, 27(4): 530-547. (in Chinese)
[3]	管树巍. 2004. 南天山库车、喀什晚新生代冲断褶皱带构造几何学和运动学分析[D]. 杭州: 浙江大学.
	GUAN Shu-wei. 2004. Geometric and kinematic analysis of late Cenozoic fold-and-thrust belt in Kuche and Kashi area, southern Tianshan[D]. Zhejiang University, Hangzhou. (in Chinese)
[4]	李世琴. 2009. 南天山库车前陆盆地中-西段挤压盐构造及同构造沉积地层研究[D]. 杭州: 浙江大学.
	LI Shi-qin. 2009. Compression salt tectonics and synkinematic strata study in the middle to western Kuqa foreland basin, Southern Tian Shan[D]. Zhejiang University, Hangzhou. (in Chinese)
[5]	曲国胜, 陈杰, 陈新发, 等. 2001. 塔里木盆地阿图什-八盘水磨反冲构造系统研究[J]. 地震地质, 23(1): 1-14.
	QU Guo-sheng, CHEN Jie, CHEN Xin-fa, et al. 2001. A study on the back-thrusting system at Atushi-Bapanshuimo in Tarim Basin[J]. Seismology and Geology, 23(1): 1-14. (in Chinese)
[6]	汪新, 贾承造, 杨树锋, 等. 2002. 南天山库车冲断褶皱带构造变形时间: 以库车河地区为例[J]. 地质学报, 76(1): 55-63.
	WANG Xin, JIA Cheng-zao, YANG Shu-feng, et al. 2002. The time of deformation on the Kuqa fold-and-thrust belt in the southern Tianshan: based on the Kuqa River area[J]. Acta Geologica Sinica, 76(1): 55-63. (in Chinese)
[7]	Burbank D, Meigs A, Brozovic N. 1996. Interactions of growing folds and coeval depositional systems[J]. Basin Research, 8(3): 199-223.
[8]	Chen J, Burbank D W, Scharer K M, et al. 2002. Magnetochronology of the Upper Cenozoic strata in the Southern Chinese Tian Shan: Rates of Pleistocene folding and thrusting[J]. Earth and Planetary Science Letters, 195(1-2): 113-130.
[9]	Chen J, Heermance R, Burbank D W, et al. 2007. Quantification of growth and lateral propagation of the Kashi anticline, southwest Chinese Tianshan[J]. Journal of Geophysical Research, 112: B03S16. doi: 10.1029/2006JB004345.
[10]	Daëron M, Avouac J P, Charreau J. 2007. Modeling the shortening history of a fault tip fold using structural and geomorphic records of deformation[J]. Journal of Geophysical Research, 112: B03S13. doi: 10.1029/2006JB004460.
[11]	Eichelberger N W, Nunns A G, Groshong R H, et al. 2017. Direct estimation of fault trajectory from structural relief[J]. AAPG Bulletin, 101(5): 635-653. doi: 10.1306/08231616065.
[12]	Epard J L, Groshong R H. 1993. Excess area and depth to detachment[J]. AAPG Bulletin, 77(8): 1291-1302.
[13]	Gao L, Rao G, Tang P C, et al. 2020. Structural development at the leading edge of the salt-bearing Kuqa fold-and-thrust belt, southern Tian Shan, NW China[J]. Journal of Structural Geology, 140(9): 104184. doi: 10.1016/j.jsg.2020.104184.
[14]	Gonzalez-Mieres R, Suppe J. 2006. Relief and shortening in detachment folds[J]. Journal of Structural Geology, 28:1785-1807. doi: 10.1016/j.jg.2006.07.001.
[15]	Gonzalez-Mieres R, Suppe J. 2011. Shortening histories in active detachment folds based on area-of-relief methods[J]. AAPG Memoir, 94:39-67. doi: 10.1306/13251332M943428.
[16]	Groshong R H. 2015. Quality control and risk assessment of seismic profiles using area-depth-strain analysis[J]. Interpretation, 3(4): SAA1-SAA15. doi: 10.1190/INT-2015-0010.1.
[17]	Heermance R V, Chen J, Burbank D W, et al. 2007. Chronology and tectonic controls of Late Tertiary deposition in the southwestern Tian Shan foreland, NW China[J]. Basin Research, 19:599-632. doi: 10.1111/j.1365-2117. 2007.00339.x.
[18]	Heermance R V, Chen J, Burbank D W, et al. 2008. Temporal constraints and pulsed Late Cenozoic deformation during the structural disruption of the active Kashi foreland, northwest China[J]. Tectonics, 27: TC6012. doi: 10.1029/ 2007TC002226.
[19]	Hubert-Ferrari A, Suppe J, Gonzalez-Mieres R, et al. 2007. Mechanisms of active folding of the landscape(southern Tian Shan, China)[J]. Journal of Geophysical Research: Solid Earth, 112: B03S09. doi: 10.1029/2006JB004362.
[20]	Li T, Chen J, Thompson J A, et al. 2013. Quantification of three-dimensional folding using fluvial terraces: A case study from the Mushi anticline, northern margin of the Chinese Pamir[J]. Journal of Geophysical Research: Solid Earth, 112: B03S09. doi: 10.1029/2006JB004362.
[21]	Li Z G, Li T, Almeida R, et al. 2019. Lateral fault growth in the Kashi anticline(Chinese Tian Shan): Insights from seismic interpretation, shortening distribution, and Trishear Methods[J]. Journal of Geophysical Research: Solid Earth, 124(7): 7303-7319. doi: 10.1029/2018JB017186.
[22]	Poblet J. 2012. D kinematic models of growth fault-related folds in contractional settings[G]// Busby C, Azor A, ed. Tectonics of Sedimentary Basin: Recent Advances, First Edition. Edited by Cathy Busby and Antonio Azor Blackwell Publishing Ltd, Oxford.
[23]	Scharer K M, Burbank D W, Chen J, et al. 2004. Detachment folding in the Southwestern Tian Shan-Tarim foreland, China: shortening estimates and rates[J]. Journal of Structural Geology, 26: 2119-2137.
[24]	Shaw J H, Connors C, Suppe J. 2005. Seismic interpretation of contractional fault-related folds[M]. The American Association of Petroleum Geologists, Tulsa, Oklahoma, USA.
[25]	Suppe J, Chou G T, Hook S C. 1992. Rates of folding and faulting determined from growth strata[M]//McClay K, ed. Thrust Tectonics: London, Chapman & Hall.
[26]	Suppe J, Sabat F, Munoz J A, et al. 1997. Bed-by-bed fold growth by kink-band migration: Sant Llorenc de Morunys, eastern Pyrenees[J]. Journal of Structural Geology, 19(3-4): 443-461.