GeoScienceWorld Lithosphere Volume 2020, Article ID 8294751, 12 pages https://doi.org/10.2113/2020/8294751 Research Article Rapid Eocene Exhumation of the West Qinling Belt: Implications for the Growth of the Northeastern Tibetan Plateau 1,2,3 1,2 1,2 1,2 Yi-Peng Zhang , Wen-Jun Zheng , Wei-Tao Wang , Yun-Tao Tian , 3 1,2 4 1,2 1,2 Renjie Zhou , Bin-Bin Xu, Min-Juan Li, Yong-Gang Yan, Qing-Ying Tian, 1,2 and Pei-Zhen Zhang 1Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat- Sen University, Guangzhou, 510275, China 2Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China 3School of Earth and Environmental Sciences, The University of Queensland, St. Lucia, QLD 4072, Australia 4Gansu Earthquake Agency, Lanzhou 730000, China Correspondence should be addressed to Wen-Jun Zheng; [email protected] Received 5 March 2020; Accepted 7 September 2020; Published 6 November 2020 Academic Editor: Andrea Billi Copyright © 2020 Yi-Peng Zhang et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). Cenozoic exhumation in the northeastern Tibetan Plateau provides insights into spatial-temporal patterns of crustal shortening, erosion, landscape evolution, and geodynamic drivers in the broad India-Eurasia collision system. The NW-SE trending West Qinling Belt has been a central debate as to when crustal shortening took place. Within the West Qinling Belt, a thick succession of Cretaceous sedimentary rocks has been deformed and exhumed along major basin-bounding thrust faults. We present new apatite (U-Th)/He ages from the hanging wall and footwall of this major thrust. Contrasting thermal histories show that rapid cooling commenced as early as ca. 45 Ma and continued for 15–20 Myr for the hanging wall, whereas the footwall experiences continuous cooling and slow exhumation since the late Mesozoic. We infer that accelerated exhumation was driven by thrusting in response to the northward growth of the Tibetan Plateau during the Eocene (ca. 45–35 Ma) based on regional sedimentological, structural, and thermochronological data. 1. Introduction tion of the Asian lithospheric mantle may have resulted in the growth and progressive development of the Tibetan Plateau Ongoing collision between India and Eurasia has exerted a toward the northeast from active plate boundaries [2]. The profound influence on landscape development, fault activa- onset of flexural basins (e.g., Linxia Basin) and provenance tion, and seismicity in East Asia [1–3]. Continuing continental change in the Lanzhou Basin imply pulsed crust shortening convergence may have reactivated pre-Cenozoic orogenic and the growth of the WQB during the late Oligocene [11, belts (e.g., Tian Shan-Altaids, Qilian Shan, and Qinling Belt) 12]. Second, simultaneous contractional deformation in the where preexisting faults of crustal scales exist [2, 4–8]. Studies northeastern plateau margin may have taken place in the have suggested that basin formation, mountain uplift, fault early Cenozoic, accompanying the initial development of activity, and plateau growth may have protracted throughout the Tibetan Plateau, implying a relatively constant bulk strain the Cenozoic Indo-Asian collision, but details regarding crust rate of the Asian lithosphere around the same time [8, 13]. shortening histories are still enigmatic [2, 4–10]. Low-temperature thermochronologic data from hanging wall There are three schools of thoughts regarding the timing blocks of major thrust systems [14, 15], depositional history of deformation along the West Qinling Belt (WQB) in the of major sedimentary basins [16–18], and vertical axis rota- northeastern Tibetan Plateau. First, oblique stepwise subduc- tion of subblock/basins [19] indicate that the onset of moun- Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8294751/5293396/8294751.pdf by guest on 30 September 2021 2 Lithosphere tain building took place along the northeastern plateau mar- middle Triassic clastic rocks are found along the LTF and gin associated with the initial growth of the Tibetan Plateau. Guanggai Shan-Die Shan Fault (GDF). Rocks along the Third, the presence of unconformities between lower Creta- Diebu-Wudu Fault (DWF) include Silurian sandstones and ceous and Tertiary units along the WQB could be related to siltstones, as well as minor Devonian and Carboniferous mountain building and significant uplift during the late Cre- sandstones. The Kangxian-Wenxian-Maqu Fault (KMF) is taceous, which may have no connection with the Cenozoic a secondary fault of the SDSZ and reactivated by Mesozoic growth of the Tibetan Plateau [20, 21]. and Cenozoic intracontinental orogenesis. Whether significant contractional deformation took In the WQB, a regional angular unconformity separates place in the WQB between late Cretaceous and Paleogene lower Cretaceous sediments of the Donghe Group from and whether such events were widespread of WQB remain underlying pre-Jurassic strata (Figures 1(a) and 2(a)). Upper critically unknown. When the Tibetan Plateau grew north- Cretaceous strata are absent. Lower Cretaceous rocks consist ward to the northeastern region remains enigmatic. For clear primarily of purple-red alluvial-fluvial-lacustrine deposits, answers to this question, we used low-temperature thermo- dominated by conglomerate, sandstone, and mudstone, with chronology to examine the exhumation history and possible intercalations of coal measures in the lower part. Paleogene topographic evolution of the WQB by constraining the cool- strata (Guyuan Group), which are separated from the pre- ing of the hanging wall and footwall of a major thrust fault. Cenozoic rocks by an angular unconformity, crop out Our new data, in combination with previously published geo- sparsely within intermontane basins in the WQB [21, 34]. logical data, are analyzed to advance the understanding of The Guyuan Group is dominated by terrestrial red beds orogenic processes in the northeastern Tibetan Plateau. and massive matrix-supported conglomerate. Oligocene ver- tebrate fossils (the Longjiagou fauna) have been reported 2. Geological Setting from the Guyuan Group [21, 34–36]. The Huicheng Basin (HCB in Figure 1) is a northeast- The Qinling Orogen, which trends E-W in central China, was trending intermountain basin in the southeastern WQB, formed during the middle-late Triassic collision between the bounded by the KMF to the south and the Chenxian Fault North China Block and the South China Block [20, 22, 23]. (CXF) to the north. The Huicheng Basin consists of lower The Qinling Belt is divided into the West Qinling Belt Cretaceous rocks (Donghe Group), whose ages are deter- (WQB) and the East Qinling Belt, separated by the Foping mined by spore-pollen assemblages [37, 38]. Rocks are dom- Dome (Figure 1(a)) [23]. The WQB may have experienced inated by purplish-red conglomerate, gray pebbly sandstone, at least two major episodes of shortening during Mesozoic grayish-green mudstone layers with shale interbeds, and coal – time [4, 20, 24 27]. The early episode has been attributed seams. The succession is divided into the Tianjiaba (K1t), to the collision between the North China Block and the South Zhoujiawan (K1z), and Huaya (K1h) Formations, whose ages China Block in the Mesozoic [22, 23, 28, 29], and the later were determined by fossil assemblages, lithofacies associa- stage is thought to be related to the northward growth of tions, and depositional contacts. The lowermost K1t consists the Tibetan Plateau during the Cenozoic [2, 4, 12, 14, 16, of purplish-red massive conglomerate facies and well-sorted, 30]. Deep seismic reflection shows that the active deforma- coarse- to fine-grained glutenites, which have been inter- tion is dominated by upper crustal shortening, middle crustal preted to represent a transition from alluvial fan to sandy shearing, and mantle-derived magmatism since the Oligo- braided river lithofacies. The overlying K1z includes delta cene (Figure 1(b)) [24, 31, 32]. fan lithofacies, consisting of coarse sandstone interbedded The WQB is bounded by the West Qinling Fault (WQLF) with thin siltstone and mudstone. The uppermost K1h and the Shangdan Suture Zone (SDSZ) to the north, the is dominated by a sequence of brown-red mudstone- Mianlue Suture Zone (MLSZ) to the south, the Foping Dome sandstone and thickly bedded massive red conglomerate. to the east, and the Qilian Shan to the west-northwest This formation has been interpreted to represent a shallow (Figure 1(a)). The WQLF and SDSZ, which extend NW-SE lacustrine, sandy braided river and alluvial fan environment. for ~800 km, are characterized by early Paleozoic ophiolitic The minimum total thickness of the Donghe Group is ca. rocks that mark the Shangdan Ocean [28]. Since the Mio- 7000 m [39]. cene, the WQLF has been active as a left-lateral strike-slip Early Cretaceous rocks in the Huicheng Basin were fault. With an estimated slip rate during the Quaternary of folded and thrust faulted (Figures 2(b) and 2(c)). The entire 2–3 mm/yr [30], the WQLF is a major fault that connects basin is an open synclinorium in the hanging wall of the the seismically active Qilian Shan in the west to the tectoni- Mianlue Suture Zone (Figure 3). Limbs of the synclinorium ° ° cally active Weihe Basin, which is located next to the dip 40 –60 (Figures 2(c) and 3). The restored thickness of relatively stable Longzhong Basin (Figure 1(b)). The north- Cretaceous strata and the uncertainties concerning fault dips dipping MLSZ lies north of an arcuate thrust that was active in depth conspire to give the minimum and maximum during the middle-late Triassic collision between the North shortening estimates of ca. 10–20 km (Craddock et al. [40] China Block and the South China Block [23]. Along the and this study). WQB, the elevation gradually decreases toward the east. Four arcuate south-vergent thrust faults are found in the 3. (U-Th)/He Thermochronology WQB (Figure 1(a)) [33].