Geoscience Frontiers 4 (2013) 729e741
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China University of Geosciences (Beijing) Geoscience Frontiers
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Research paper Tectonic evolution of the Dabashan orocline, central China: Insights from the superposed folds in the eastern Dabashan foreland
Wei Shi a,b,*, Jianhua Li b, Mi Tian b,c, Guoli Wu b,d a Key Laboratory of Neotectonic Movement and Geohazard, Ministry of Land and Resources, Beijing 100081, China b Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China c Wuxi Research Institute of Petroleum Geology, Wuxi 214151, China d Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China article info abstract
Article history: The Dabashan orocline is situated in the northwestern margin of the central Yangtze block, central China. Received 29 February 2012 Previous studies have defined the orthogonal superposed folds growing in its central-western segment Received in revised form thereby confirming its two-stage tectonic evolution history. Geological mapping has revealed that more 12 December 2012 types of superposed folds have developed in the eastern segment of the orocline, which probably Accepted 5 January 2013 provides more clues for probing the structure and tectonic history of the Dabashan orocline. In this paper, Available online 26 January 2013 based on geological mapping, structural measurements and analyses of deformation, we have identified three groups of folds with different trends (e.g. NW-, NE- and nearly E-trending folds) and three types of Keywords: Dabashan foreland belt structural patterns of superposed folds in the eastern Dabashan foreland (e.g. syn-axial, oblique, and Superposed folds conjunctional superposed folds). In combination with geochronological data, we propose that the syn- Orocline axial superposed folds are due to two stages of wNeS shortening in the west and north of the Paleo-stress field Shennongjia massif, and that oblique superposed folds have been resulted from the superposition of the Intra-continental orogeny NW- and NE-trending folds onto the early wEeW folds in the east of the Shennongjia massif in the late Late Jurassic Jurassic to early Cretaceous. The conjunctional folds are composed of the NW- and NE-trending folds, corresponding to the regional-scale dual-orocline in the eastern Sichuan as a result of the southwestward expansion of the Dabashan foreland during late Jurassic to early Cretaceous, coeval with the north- westward propagation of the Xuefengshan foreland. Integration of the structure and geochronology of the belt shows that the Dabashan orocline is a combined deformation belt primarily experiencing a two- stage tectonic evolution history in Mesozoic, initiation of the Dabashan orocline as a foreland basin along the front of the Qinling orogen in late Triassic to early Jurassic due to collisional orogeny, and the final formation of the Dabashan orocline owing to the southwestward propagation of the Qinling orogen during late Jurassic to early Cretaceous intra-continental orogeny. Our studies provide some evidences for understanding the structure and deformation of the Dabashan orocline. Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction
The Dabashan orocline is situated at the northern margin of the central Yangtze block; its foreland is a large-scale, southwestward- * Corresponding author. Institute of Geomechanics, CAGS, 11 Minzudaxuenan concaving arc-shaped thrust-fold belt (Fig. 1; Zhang et al., 2001; þ þ Road, Haidian District, Beijing 100081, China. Tel.: 86 10 68425048; fax: 86 10 Dong et al., 2006, 2013; Shi et al., 2012). Currently, there is revived 68422326. E-mail address: [email protected] (W. Shi). interest in understanding the tectonic framework of the Dabashan orocline, due to its special oroclinal structure and gas/oil potential Peer-review under responsibility of China University of Geosciences (Beijing) (inset of Fig. 1; Dong et al., 2005, 2006, 2013; Liu et al., 2006; E.C. Wang et al., 2006; Z.C. Wang et al., 2006; Shi et al., 2007, 2012a; Hu et al., 2008, 2009, 2012; Pei et al., 2009; Wu et al., 2009; Zhang and Dong, 2009; Zhang et al., 2010; Q.S. Li et al., 2011). Previous Production and hosting by Elsevier research established a first-order framework of the structural
1674-9871/$ e see front matter Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2013.01.002 730 .Sie l esineFotes4(03 729 (2013) 4 Frontiers Geoscience / al. et Shi W. e 741
Figure 1. Simplified structural map of the eastern Dabashan orocline (modified from Geological Survey of Hubei Province, 1965a, b, c, 1972, 1975, 1987; Geological Survey of Shaanxi Province, 1966; Geological Survey of Sichuan Province, 1981, 1973) and geological cross-section (after Li et al., 2009). F1 ChengkoueFangxian Fault; F2 XujiabaeYangri Fault; F3 TiexieWuxi Fault; F4 XinhuaeXingshan Fault; F5 Tongchenghe Fault; F6 Yuan’an Fault; F7 Nanzhang Fault. The inset exhibits the study area which is located in the north margin of the central Yangtze block. The black rectangle shows the locations of Fig. 4A, and some black quadrangles present the sites for superposed folds. W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741 731 styles, geometric formation models and tectonic evolution in the massifs (Fig. 2a), coeval with strike-slip activation along the Dabashan orocline (He et al., 1997, 1999; Zhang et al., 2001, 2010; ChengkoueFangxian Fault. However, even after much fieldwork Dong et al., 2005, 2006, 2008, 2011; Li et al., 2006; Liu et al., 2006; evidences for strike-slip activation could not be obtained (Shi et al., E.C. Wang et al., 2006; Shi et al., 2007, 2012a; Tan et al., 2007; Hu 2012a). Based on analysis of the structure and deformation in the et al., 2009, 2012). However, much disagreement is highlighted in central Dabashan orocline, He et al. (1997) insisted the Dabashan the tectonic evolution models of the Dabashan orocline, especially, orocline resulted from dextral shearing because of the eastward regarding its period of formation and its structural geometry, due to extruding of the Dabashan thrust belt during Triassic collisional the profound lack of detailed structural analysis and age dating. At orogeny and the synchronous obstruction of the Micangshan massif least four evolution models related to the orocline have been (Fig. 2b; inset of Fig. 1). Actually, this model excludes the restricting proposed by previous workers. A durative compression model of the ShennongjiaeHuangling massif, and disagrees with the suggests the Dabashan orocline ultimately formed in Indosinian distribution of early Paleozoic veins in the Dabashan thrust belt (Cai et al., 1988) or in Cenozoic (Zhang et al., 2001), due to the long- indicative of sinistral movement (Fig. l; Shi et al., 2012a). Wang et al. term southward thrusting of the Qinling orogenic belt and the (2003) and Meng et al. (2005) outlined a rotation-and-convergence blocking of thrusting propagation by the Hannan and Huangling model (Fig. 2c) using paleomagnetism and deposits recorded in this
Figure 2. The formation models of the Dabashan orocline. (a) Durative compression model (modified from Cai et al., 1988); (b) Rotation-and-convergence model (Wang et al., 2003); (c) Dextral shearing model (He et al., 1997); (d) Superposed deformation model (after Dong et al., 2006). 732 W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741 area (Lin et al., 1985; Yang et al., 1992; Gilder and Courtillot, 1997; Mesozoic basins (e.g. Dangyang and Zigui basins in the east and Yokoyama et al., 2001; Tan et al., 2007), suggesting that the Yangtze west flanks of the Huangling massif, respectively) and two massifs block rotated clockwise and collided with the North China block in (e.g. Shennongjia and Huangling massifs) (Fig. 1). The wNeS cross the Mesozoic, and the Hannan and Huangling massifs contempo- section shows that the Zigui basin is featured by open synclines rarily blocked the thrust-fold propagation of the Dabashan orocline, affecting upper JurassiceSinian strata, and imbricate stacks or tight leading to the formation of the Dabashan orocline. However, it is folds outcrop between the ChengkoueFangxian Fault (F1) and difficult to explain the formation of another arc-shaped belt in XujiabaeYangri Fault (F2) folding SilurianeSinian strata (Fig. 1; Li northeast Sichuan (that is, the Xuefengshan orocline in inset of et al., 2009). The XujiabaeYangri Fault (F2) cleaved the Shennong- Fig. 1). Then E.C. Wang et al. (2006) proposed a compression- jia massif into two parts, indicative of a main detachment layer collapse model on the basis of the similarity of the orocline growing in the Proterozoic basement (Fig. 1). The two NNW- geometry between the Dabashan arc-shaped belt and the Hima- trending grabens, filled with late CretaceousePaleogene red layan orogen, which suggests the Qinling orogen collapsed in the clastic sediment, developed in the east of the Huangling massif Cretaceous due to the rapid reduction of the convergence rate (Fig. 1; Tian et al., 2010), the Yuan’an graben limited by the Tong- between the Yangtze and North China blocks, and the coeval chenghe Fault (F5) and Yuan’an Fault (F6), and the Hanshui half- blocking from the Hannan and Shennongjia massifs. However, this graben growing along the Nanzhang Fault (F7), both these model lacks supporting structural and geochronological evidence. grabens cut the eastern Dabashan foreland belt, suggesting that the Dong et al. (2006) argued a superposed deformation model based foreland belt formed prior to late Cretaceous (Shi et al., 2012b). on the detailed fieldwork, exhibiting a two-stage shortening, wN- orientated shortening in late Triassic to early Jurassic due to the 3. Superposed folds Indosinian movement and wE-striking shortening during late Jurassic to early Cretaceous owing to Yanshanian intra-continental Based on the geologic mapping (1:200,000) and detailed field- orogeny. work, we recognized three sets of folds with different orientation in The large-scale geological mapping and detailed structural the eastern Dabashan foreland, wE-, NW- and NE-striking folds measurements reveal that superposed folds developed in the (Figs. 1 and 3), which are mutually superposed, thereby leading to central-western Dabashan orocline (Dong et al., 2006; Shi et al., the formation of superposed folds with different structural styles, 2007, 2012a; Hu et al., 2009, 2012), which provided many clues e.g. syn-axial, oblique and conjunctional superposed folds. Previous for understanding the issue. Detailed geological mapping and studies showed that the syn-axial superposed fold is composed of fieldwork further reveal more complex superposed folds occurring two sets of syn-trending nonchron folds, and oblique superposed in the eastern segment. What are the structural patterns of the folds resulted from two sets of different-trending nonchron folds superposed folds? And how did they form in the area? In this paper, whose included angle is not equal to 90 (Ramsay and Huber, 1987; we extended our work from the western segment to the eastern Huang, 1988; Ghosh et al., 1992; Yue et al., 1996). Conjunctional segment, and propose an evolution model of the Dabashan arc- superposed folding is due to two sets of synchron folds with shaped belt based on synthesis of structural data, superposed different-trending fold axis (Lee, 1973; Yue et al., 1996; Yue, 1998). deformation. The syn-axial superposed folds mainly occurs this area (Lin between the Shennongjia massif and the ChengkoueFangxian Fault 2. Geological setting or near the XujiabaeYangri Fault, resulting from a two-stage syn- directional shortening (Wu et al., 2009). Outcrop-scale syn-axial The Dabashan orocline is divided into the Dabashan thrust belt superposed folds are mainly characterized by re-bending of the (north Dabashan) and Dabashan foreland belt (south Dabashan) axial plane which strikes east, and developed in Silurian mudstone bounded by the ChengkoueFangxian Fault (Zhang et al., 2010; Shi near the hinge zone of the wEeW CambrianeOrdovician anticline et al., 2012a). The Dabashan foreland belt is located in the north- at station D4121 (Fig. 3, locality B, see the site in Fig. 3A). Another eastern margin of Sichuan basin, marking large-scale orocline syn-axial superposed fold can be observed in Cambrian mudstone geometry and two rigid basements developing in its east and west and shale at site D4051 near the XujiabaeYangri Fault (Fig. 4A, see sides, e.g. HannaneMicangshan and HuanglingeShennongjia the site in Fig. 1), typified by wE-trending hinges of folds, probably massifs, respectively (inset of Fig. 1; Zhang et al., 2001; Dong indicative of the two-stage wN-striking shortening (Fig. 4A), which et al., 2006; Shi et al., 2012a), and a typical double arc-shaped is interpreted in our cartoon (Fig. 4F①). Thus we speculate that structure composed of the Dabashan and the Xuefengshan oro- a two-phase syn-trending shortening occurred in the eastern cline in the west flank of the ShennongjiaeHuangling massif (inset Dabashan foreland owing to the southward thrusting of the of Fig. 1 ; Yue et al., 1996; Huang, 2000; Wu et al., 2009; Shi et al., Dabashan thrust belt and coeval obstructing of the Shennongjia 2012a). The belt is limited by the ChengkoueFangxian Fault (F1) massif during the Dabashan foreland folding. The two stages of to its northeast and TiexieWuxi Fault (F3) to its southwest. Boun- shortening are also documented in the Silurian mudstone chevron ded by the Zhenba Fault (see Shi et al., 2012a) and XujiabaeYangri folds near the XujabaeYangri Fault (site D4050, the location in Fault (F2), the Dabashan foreland belt is divided into the two sub- Fig. 1); here wEeW tight chevron folds were truncated by reverse belts: a basement-involved thrust belt (namely, basement- faults, with hinges of folds and striae showing two stages of nearly detaching belt) and a thin-skinned belt (that is, cover-fold belt) N-trending shortening (Fig. 5EeG), the early shortening leading to (inset of Fig. 1; Hu et al., 2009; Dong et al., 2010; Zhang et al., 2010; folding (Fig. 5F), the late to faulting (Fig. 5G). Shi et al., 2012a). The recent geophysical data (Liu et al., 2006; Q.S. The 1:200,000 geological mapping shows that the regional- Li, 2010; Dong et al., 2013) show the five regional detachment scale oblique superposed fold developed in the east of the layers developed in the Dabashan foreland, Triassic (T1j and T2l) ShennongjiaeHuangling massif, as a result of the superposition of gypsum-salt, Silurian mudstone, Cambrian thin marl and silica- the NW- or NE-trending folds onto the wEeW folds with an stone, Sinian shale and pre-Sinian basement, which intensively included angle of w30 (Figs. 1 and 3A). A typical dome system can affected its structural styles (Liu et al., 2006; Hu et al., 2012; Shi be observed in Ordovician thick-bedded limestone at Site DY73 et al., 2012a). (Fig. 4BeE, the location in Fig. 1). The Silurian black siltstone lying The eastern Dabashan foreland, as a part of the oroclinal over the Ordovician limestone dome was intensively folded Dabashan foreland, largely trends wEeW. It is characterized by two (Fig. 4BeD); the two maximums exhibited on stereographic plots W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741 733
Figure 3. Superposed structures in the eastern Dabashan foreland. (A) Simplified geological map of Nanzhang area (based on Geological Survey of Hubei Province, 1965b) shows three sets of regional-scale folds, wE-, NW- and NE-striking. (B) A syn-axial fold likely reveals two episodes of syn-directional shortening at site D4121. (C) An outcrop-scale conjunctional fold in Triassic limestone at site D4126, composed of two groups of folds, NE- and NW-trending. (D) Presenting two stages of compression that resulted in striae on fault planes, NEeSW and NWeSE. (E) Hinges of folds at site D4126 indicating two-phase shortening. Two black quadrangles in (A) exhibit the sites for the superposed folds.
based on the hinges of folds in the Silurian siltstone, indicative of trending folds mutually interfered with the NE-directed folds the two-group folds with included angle of w40 (Fig. 4E). We here (Fig. 3A), leading to the development of the conjunction speculate that the dome results from oblique superposition of the structure, implying that the two-directional shortening occurred at NE-directed folds onto the wE-trending folds denoting two the same stage, and the dual-orocline also developed in the east of episodes of shortening, wNeS and NWeSE (Fig. 4D, E, F②). the ShennongjiaeHuangling massif except in eastern Sichuan. An The conjunctional superposed folding also occurred to the east outcrop-scale conjunction structure grew in the Triassic thin- of Shennongjia massif and to the north of the Dangyang basin bedded limestone at site D4126 near Nanzhang County (see the (Figs. 1, 3A and 3C). The geological map reveals that the NW- site in Fig. 3A), typical of the two groups of folds with different 734 W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741
Figure 4. Superposed folds and its deformation model in the eastern Dabashan foreland. (A) Syn-axial superposed folds in Cambrian mudstone at station D4051 near the Shennongjia massif (the location in the Fig. 1), probably suggesting two-stage syn-directional shortening. (B) and (C) illustrating a typical dome growing in Ordovician limestone at site DY73 in the east of the Shennongjia massif (the site marked in the Fig. 1), representing two episodes of shortening. (D) and (E) displaying the Silurian black siltstone covering the Ordovician limestone dome was intensively folded, and two sets of folds of different-orientation developed in the strata at site DY73, NE-directed and wE-trending, indicative of two-stage intensive shortening. The hinges of folds were analyzed by using Stereo32. (F) Showing the formation of the two-style superposed folds, ① expresses syn-axial folds owing their origin to syn-directional shortening, and ② represents dome resulting from the different-orientation shortening.
trends, one with an orientation of 005 :59 in axial plane and folds with different trends viz., wEeW, NWeSE and NEeSW. 031 :50 in hinge of fold, the other with a orientation of Based on the analysis of superposed deformation (Shi et al., 035 :70 in axial plane and 343 :46 in hinge of fold (Fig. 3C). At 2012b), the three groups of folds occurred in two episodes, the this site, the hinges of folds largely show two-directional short- wEeWfoldsinthefirst stage, and the others synchronously in the ening, NW- and NE-strikes (Fig. 3E). second stage. In the north of Dangyang basin, the open wE-trending folds affecting Proterozoic strata extend eastwards, and are intensely 4. Paleo-stress fields reshaped by NW- and NE-striking folds, indicating that wE- directed folds probably occurred prior to the others (Figs. 1 and 3A; As discussed above, the wE-trending folds predated the NW- Shi et al., 2012b). Furthermore NW-trending folds and NE-striking trending and NE-oriented folds, and the NW-trending and NE- folds appear to form coevally as analyzed above. oriented folds formed coevally (Figs. 1 and 3A; Shi et al., 2012b), Thus, it is concluded that three types of superposed folds indicating that wNeS shortening occurred prior to the others, and formed in the eastern Dabashan foreland, e.g. syn-axial, oblique, NW-trending shortening is coeval with NE-oriented shortening. and conjunctional, caused by the mutual superposition of the Based on this work, we collected fault-slip vectors in the eastern W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741 735
Figure 5. Superposed deformation in Cambrian limestone at station D4111 (Locality A, B, C and D, see the station in Fig. 1), and in Silurian mudstone at site D4051 (E, F and G, the site in Fig. 1), showing two stages of compression in the eastern Dabashan foreland. (A) wE-trending chevron folds. (B) Two-generation superposed striae on a fault plane reveal that wNeS compression predated NWeSE compression. (C) Two stages of compression based on superposed striae, wNeS and NWeSE compression. (D) The reshaping of the wE- trending folds also exhibits two stages of shortening, first wNeS and second NWeSE. (E) Two-stage wNeS compression, first folding (F) and second faulting (G). Hinges of folds were analyzed using Stereo32.
Dabashan foreland, and reconstructed paleo-stress fields. The limestone show two-stage compression, NEeSW and NWeSE paleo-stress fields were calculated by the fault software from (Fig. 3E), also according to two episodes of folding (Fig. 3D). A Angelier (1984). comparison of these structural data with folding data shows that The chevron folds with the trend of wEeW can be identified in they seem to occur contemporaneously. Cambrian thin-bedded limestone at site D4111 in the north of the Therefore, based on the analysis of superposed deformation and Shennongjia massif (Fig. 5A and D, the location in Fig. 1). Here two fault-slip data collected here, we reconstructed two periods of sets of striae developed on the same bedding plane, and their paleo-stress field in the eastern Dabashan foreland, wNeS superposed relationship suggests NNE-directed compression pre- compression in the first stage, and coeval NEeSW and NWeSE dated NWeSE compression (Fig. 5B and C), coinciding with folding compression in the late stage constituting a conjunctional (Fig. 5D). At site D4126, the striae on fault planes in Triassic compressive stress field (Figs. 6 and 7, Table 1). 736 .Sie l esineFotes4(03 729 (2013) 4 Frontiers Geoscience / al. et Shi W. e 741
Figure 6. Simplified geologic map of the eastern Dabashan foreland, and lower-hemisphere, equal-angle (Wulff) stereographic plots of fault slip vectors exhibiting the conjunctional compressive stress field in late stage composed of the NEeSW and NWeSE compression. The plots are based on the fault software by Angelier (1984). .Sie l esineFotes4(03 729 (2013) 4 Frontiers Geoscience / al. et Shi W. e 741
Figure 7. Simplified geologic map of the eastern Dabashan foreland and lower-hemisphere, equal-angle (Wulff) stereographic plots of fault slip vectors, showing wNeS compression in early stages. The plots resulted from the fault software by Angelier (1984). 737 738 W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741
Table 1 Slip vectors on fault planes measured in eastern Dabashan foreland and their compressive stress field.
Site Longitude Latitude Stratigraphy lithology Vector number s1 (az/pl) s2 (az/pl) s3 (az/pl) R Compressive direction D4000b 110 4403000 31 1803300 ] limestone 4 037/33 133/10 237/55 0.1 NEeSW trending compression D4001b 110 4104100 31 1604800 ] limestone 20 018/06 284/34 117/56 0.9 D4125 111 4804200 31 4604300 O limestone 14 NEeSW 0 00 0 00 D4126 111 48 42 31 46 43 T1 limestone 6 NEeSW DY85 111 4303700 31 4704900 S shale 22 NEeSW DY87 111 4701000 31 4501000 P limestone 3 NEeSW SN209 110 4803900 31 4403600 S shale 11 022/05 292/04 161/83 0.3 SN215b 111 0302100 31 4003300 S mudstone 12 046/01 316/03 138/87 0.9 SN221 110 4405900 31 1702300 O limestone 10 211/16 305/13 073/69 0.9 SN222 110 4401800 31 1901800 S mudstone 7 NEeSW SN224 110 4500100 31 1804900 S mudstone 10 NEeSW SN250 110 4502900 31 2200200 O limestone 6 NEeSW SN252b 110 4405000 31 2203700 ] limestone 7 232/18 088/68 326/12 0.9 SN258 110 4803200 31 2605800 Z dolomite 6 NEeSW 0 00 0 00 SN259b 110 47 54 31 27 16 Pt2 dolomite 6 NEeSW SN281 110 3603300 31 4401800 O limestone 5 NEeSW SN282 110 3601200 31 4403500 O limestone 16 NEeSW SN293 111 1400200 32 0203200 ] limestone 5 NEeSW D4000c 110 4403000 31 1803300 ] limestone 4 121/23 219/19 346/60 0.1 NWeSE oriented compression D4123 111 4804200 31 4604300 O mudstone 3 NWeSE D4125 111 2602600 31 4604600 O mudstone 10 NWeSE 0 00 0 00 D4126 111 48 42 31 46 43 T1 limestone 5 NWeSE 0 00 0 00 DY76 111 48 28 31 45 28 T1 limestone 7 111/03 021/04 237/85 0.8 DY78 111 4802800 31 4502800 S mudstone 5 NWeSE DY91 111 4504500 31 4204500 P mudstone 5 NWeSE SN213 111 0102000 31 4206000 S mudstone 4 NEeSW SN223 110 4402200 31 1900500 S mudstone 7 NWeSE SN229 110 3801800 31 2004500 ] shale 5 129/03 039/11 236/79 0.5 SN251c 110 4501100 31 2200800 O mudstone 4 312/23 072/50 208/31 0.3 SN252c 110 4405000 31 2203700 ] mudstone 3 NWeSE SN253 110 4505900 31 2303200 ] mudstone 5 136/06 227/09 011/79 0.5 0 00 0 00 SN263 110 51 08 31 33 05 Pt2 dolomite 7 118/18 351/61 215/22 0.7 0 00 0 00 SN264 110 51 37 31 34 16 Pt2 dolomite 5 323/01 054/65 232/25 0.7 SN266 110 4501100 31 2200800 O mudstone 6 303/17 035/05 141/72 0.6 SN267 110 4501100 31 2200800 O dolomite 6 NWeSE SN285 110 4501100 31 2200800 ] shale 3 NWeSE SN293 111 1400200 32 0203200 O mudstone 3 NWeSE D4000a 110 4403000 31 1803300 ] limestone 6 355/14 261/14 127/70 0.6 wNeS directed compression D4001a 111 4104100 31 1604000 S siltstone 13 wNeS D4045 109 3800900 31 2600800 S sandstone 16 187/04 277/02 031/86 0.6 D4049 109 3905600 31 3502100 P limestone 19 wNeS D4052 109 3600800 31 4105700 ] mudstone 9 176/13 085/06 332/76 0.6 D4053 109 3503800 31 4205000 ] limestone 7 012/10 234/77 104/09 0.8 D4084 109 3105000 31 4601300 ] limestone 7 162/12 072/14 293/72 0.4 D4088 109 3105400 31 4803100 ] limestone 10 182/27 273/01 006/63 0.5 D4105 110 4004400 32 0002900 ] limestone 6 186/13 277/02 016/77 0.9 D4109 110 4304300 31 5405500 ] limestone 13 182/11 092/02 352/79 0.2 D4110 110 5305400 31 5500600 ] phyllite 6 018/20 287/04 186/69 0.9 D4113 110 4005200 31 5905200 ] limestone 6 177/01 269/64 087/26 0.7 D4115 110 5305500 32 0203400 S slate 9 wNeS D4116 111 0500000 32 0103100 ] limestone 10 174/04 082/20 275/70 0.6 D4119 111 2301200 31 4801600 S mudstone 2 wNeS D4124 111 3003300 31 4503400 S mudstone 20 wNeS DY70 111 1701100 31 3400800 O limestone 7 348/07 257/02 151/83 0.7 DY71 111 1504700 31 3502100 O limestone 11 177/03 267/04 055/85 0.3 DY79 111 1402500 31 4302500 P limestone 4 001/05 237/80 092/08 0.9 DY89 111 4604200 31 4302500 T limestone 6 wNeS DY90 111 4304200 31 4302500 P limestone 4 wNeS SN206 110 5902200 31 4400900 ] limestone 5 wNeS SN208 110 5205600 31 4403700 T limestone 16 wNeS SN210 110 4600400 31 4504700 S mudstone 8 wNeS SN211 110 4601200 31 4602200 S mudstone 6 359/04 090/01 190/86 0.4 SN212 110 5104900 31 4400700 S mudstone 16 184/11 093/09 325/76 0.5 SN215a 111 0302100 31 4003300 S mudstone 4 wNeS SN242 110 3002300 31 2004600 ] shale 9 191/25 084/16 335/59 0.7 SN251a 110 4501100 31 2200800 O limestone 4 wNeS SN252a 110 4405000 31 2203700 ] limestone 9 161/01 067/73 252/17 0.9 0 00 0 00 SN259a 110 47 54 31 27 16 Pt2 dolomite 4 wNeS SN265 110 5501900 31 3904000 ] dolomite 3 wNeS SN283 110 3403800 31 4500500 O limestone 3 wNeS SN284 110 3004600 31 5005400 Z dolomite 3 wNeS s1, s2, s3 represent maximum, intermediate and minimum compressive stress, respectively; az e azimuth, pleplunge, R ¼ (s2 s3/s1 s3), stress ratio; T e Triassic (T1 e early Triassic), P e Permian, S e Silurian, O e Ordovician, ] e Cambrian, Z e Sinian, Pt2 e Meso-Proterozoic. W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741 739
5. Discussion and conclusion 2012b). The Dabashan arc-shaped belt was initiated as a foreland basin along the front of the QinlingeDabie orogen (Fig. 8a; Liu et al., Recently, more and more chronological evidences for timing of 2005; Shi et al., 2012a) due to the collision orogen between the Mesozoic deformation suggest that the Dabashan orocline was Yangtze block and North China block in Indosinian (e.g. Lin et al., intensively reshaped during late Jurassic to early Cretaceous after 1985; Yang et al., 1992; Gilder and Courtillot, 1997; Tan et al., 2007). inter-continental collision orogeny in late Triassic to early Jurassic. Then, in late Jurassic to early Cretaceous, the Dabashan orocline This has been proved by the strata unconformity and sediment was dominated by a radial stress in the Dabashan orocline in which record (Dong et al., 2005, 2006, 2008; Meng et al., 2005; Shi et al., maximum principle stress directions were approximately perpen- 2007, 2012a; Li et al., 2007; Qu et al., 2009; Zhang et al., 2010; Hu dicular to the trend of the belt, e.g. wEeW in the western, NEeSW et al., 2012), and also by isotope dating (e.g. 40Ar/39Ar ages from in the central, and NeS in the eastern segment (Wu et al., Cheng and Yang (2009), Hu et al. (2009), L.P. Liu et al. (2011), X. Liu 2009; Dong et al., 2010; Zhang et al., 2010; Shi et al., 2012), and et al. (2011), S.Z. Li et al. (2010, 2011), Li et al., 2012 and J.H. Li coeval NW-trending compression came from the southwestward (2013); apatite fission-track (AFT) from Shen et al. (2009), Xu propagation of the Xuefengshan foreland (Wu et al., 2009; Liu et al., et al. (2010), J.H. Li et al. (2010), Shi et al. (2012a) and Hu et al. 2010; Shi et al., 2012b), the two bases rapidly exhumed again, and (2012)). On the basis of the detailed structural data and geochro- the Huangling massif was uplifted to the surface (Qu et al., nology, Dong et al. (2006) and Shi et al. (2007) suggested that the 2009; Shen et al., 2009 ; Xu et al., 2010; Hu et al., 2012). The early orthogonal superposed folds occurring in the western Dabashan wE-trending structures were intensively reshaped due to foreland owe their origin to the superposition of wNeS folds onto further southwestward propagation of the Dabashan thrust belt the wEeW folds in late Jurassic to early Cretaceous, and the wN- and the restricting presence of the HannaneMicangshan and orientated shortening in late Triassic to early Jurassic to the Indo- ShennongjiaeHuangling massifs, causing the multi-types of the sinian collisional orogeny, and wE-striking shortening during late superposed folds in the belt, e.g. orthogonal superposed folds in the Jurassic to early Cretaceous to the Yanshanian intra-continental western-central segment, oblique and syn-axial superposed folds orogeny (Dong et al., 2006; Shi et al., 2007, 2012a; Zhang and in the eastern segment. Coevally, the Xuefengshan structural belt Dong, 2009). expanded northwestwardly (Liu et al., 2010), leading to the Here we discriminated three sets of folds (e.g. wE-, NW- and occurrence of the conjunctional superposed folds in the central- NE-striking) in eastern Dabashan foreland resulting from two eastern Dabashan foreland, which denote that the large-scale episodes of shortening, wNeS, and coeval NEeSW and NWeSE. In dual-orocline developed not in the northeast Sichuan, but in the combination with regional structure and geochronology (Hu et al., east of the ShennongjiaeHuangling massif (Fig. 8b). During this 2009, 2012; Qu et al., 2009; Wu et al., 2009; Xu et al., 2009; Zhang time the Dangyang basin was probably separated from the et al., 2010; Shi et al., 2012a, b), we also propose that wE-trending Zigui basin mostly because of the rapid exhumation of the folds formed during late Triassic to early Jurassic, and the others ShennongjiaeHuangling massif (Shi et al., 2012b). Consequently occurred in late Jurassic to early Cretaceous, leading to final the Dabashan belt evolved into an orocline from the orogenic front formation of the eastern Dabashan foreland. of the QinlingeDabie orogen (Fig. 8b). Kinematically and dynami- Thus, by synthesizing the previous structural data and structural cally, the belt is a result of the superposition of the orocline onto the chronology, we point out a two-stage Mesozoic tectonic evolution wE-trending foreland basin, appearing to be due to the multi-plate history in the Dabashan foreland, initiation of the Dabashan oro- convergence in Eastern Asia in Yanshanian (Dong et al., 2007). cline during late Triassic to early Jurassic (Fig. 8a), and final Integrating the previous chronology and deformational records, formation of the Dabashan orocline in late Jurassic to early Creta- we defined three structural types of superposed folds in the eastern ceous (Fig. 8b). Dabashan foreland, establishing its structural framework, e.g. oblique, During the late Triassic to early Jurassic, dominated by wNeS syn-axial and conjunctional superposed folds. The syn-axial super- compression, the HannaneMicangshan and Shengnongjia massifs posed folds were caused by the superposition of two sets of wEeW respectively at the west and east sides of the Dabashan foreland folds, and oblique superposed folds resulted from the reshaping of were rapidly exhumed (Liu and Zhang, 2008; Qu et al., 2009; Xu the wearly EeW folds by the NW- and NE-trending folds; the et al., 2010). It is very likely that the Zigui basin was connected conjunctional folds are composed of the NW- and NE-trending folds. with the Dangyang basin (Liu and Zhang, 2008; Shi et al., 2012b). Thus the Dabashan foreland belt is a compounded deformational belt The ChengkoueFangxian Fault thrusted SSW-directed along the due to the superposition of the intra-continental orogeny onto the front of the Dabashan thrust belt (North Dabashan) (Fig. 1; Shi et al., collisional orogeny in late Jurassic to early Cretaceous. Coevally the
Figure 8. Mesozoic tectonic evolution of the Dabashan foreland. (a) Initiation of the Dabashan orocline due to collisional orogeny in Indosinian (late Triassic to early Jurassic, T3eJ1). (b) Final formation of the Dabashan orocline owing to intra-continental orogeny in Yanshanian (late Jurassic to early Cretaceous, J3eK1). 740 W. Shi et al. / Geoscience Frontiers 4 (2013) 729e741
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