Longmen Shan Fold-Thrust Belt and Its Relation to the Western Sichuan

Home , Longmen Mountains

Longmen Shan fold-thrust AUTHORS Dong Jia Department of Earth Sciences, belt and its relation to the Nanjing University, Nanjing 210093, People’s Republic of China; [email protected] western Sichuan Basin in Dong Jia is a professor in the Department of Earth Sciences of Nanjing University in China. central China: New insights He received his Ph.D. in structural geology from Nanjing University in 1988. His current from hydrocarbon exploration research projects are focused on structural geometries and timing of deformation in the sedimentary basins of China. Dong Jia, Guoqi Wei, Zhuxin Chen, Benliang Li, Qing Zeng, and Guang Yang Guoqi Wei Research Institute of Petro- leum Exploration and Development, Langfang Branch, PetroChina Company Limited, Lang- fang 065007, People’s Republic of China; ABSTRACT [email protected] The Longmen Shan fold-thrust belt is one of the key regions of de- Guoqi Wei is a chief geologist in the Langfang monstrable Mesozoic–Cenozoic tectonic evolution in China, and the Branch of the Research Institute of Petroleum Sichuan Basin was the first natural-gas-producing area in China. In Exploration and Development, PetroChina this article, the structural features of the Longmen Shan belt are Company Limited. He received his Ph.D. from Nanjing University in 2000. His current represented, using both seismic profiles and field data. The complex searches are focused on sedimentary basins in structures of the northeast-trending Longmen Shan fold-thrust belt western China. and its foreland in the western Sichuan Basin are formed by southeast-directed thrusting. Several eastward-verging, rootless Zhuxin Chen Department of Earth Sci- thrust sheets and imbricates of Cambrian–Triassic rocks have been ences, Nanjing University, Nanjing 210093, recognized in the northern Longmen Shan belt. Evidence suggests People’s Republic of China; that the northern Longmen Shan belt experienced at least two major [email protected] periods of deformation in the Late Triassic and Cenozoic. However, Zhuxin Chen received his B.A. degree in the southern Longmen Shan belt is represented by the basement- petroleum geology from the Chinese University involved thrust structures and klippen, and its major periods of of Petroleum in 2001. He is currently finish- deformation were in the latest Cretaceous–early Cenozoic. Sedi- ing his Ph.D. (geology) at Nanjing University, mentary features in the western Sichuan Basin reflect a two-phase where he specializes in seismic interpretation and balancing cross sections. flexural-loading history and illustrate that the Late Triassic foreland basin extends along the foredeep of the entire length of the Benliang Li Research Institute of Petro- Longmen Shan belt, but the uppermost Cretaceous–Paleogene re- leum Exploration and Development, Langfang juvenated foreland basin is restricted in the southern part of the Branch, PetroChina Company Limited, Lang- western Sichuan Basin. fang 065007, People’s Republic of China; Structural geometries suggest that prospective traps are mainly [email protected] developed in the frontal zone of the Longmen Shan fold-thrust belt Benliang Li is a senior geologist in PetroChina and in the southern part of the western Sichuan Basin. One of the Company Limited. He earned a B.A. degree major contributions of this article is finding preexisting Paleozoic in geology from the Chinese Southwestern rift basins under the Cenozoic thin-skinned thrust belts, which Petroleum Institute in 1995 and obtained a represent a new potential hydrocarbon play. Ph.D. in geology from Nanjing University in 2000. His current research focuses on the Mesozoic–Cenozoic basin in western China.

Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. Manuscript received May 10, 2005; provisional acceptance December 1, 2005; revised manuscript received January 18, 2006; final acceptance March 23, 2006. DOI:10.1306/03230605076

AAPG Bulletin, v. 90, no. 9 (September 2006), pp. 1425–1447 1425 Qing Zeng Southwest Oil & Gasfield INTRODUCTION Company, PetroChina Company Limited, Chengdu 610051, People’s Republic of China; The northeast-trending Longmen Shan (Shan = mountains) lies along [email protected] the eastern boundary of the Tibetan Plateau and is one of the key Qing Zeng is a senior research scientist in regions of demonstrable Mesozoic–Cenozoic tectonic evolution of PetroChina Company Limited. He obtained his the China continent (Sengo¨r and Hsu¨ , 1984; Mattauer et al., 1985; B.A. degree in structural geology from Nanjing Huang and Chen, 1987; Xu et al., 1992). This area is trapped among University in 1986. Presently, he is working on the North China, South China, and Qiangtang blocks in the tec- several projects on structural reconstruction tonic collage of China (Figure 1). The recent phase of exploration and trap analysis in the Sichuan Basin. has provided new evidence for structural geometries, deformation Guang Yang Southwest Oil & Gasfield timing, and potential hydrocarbon plays in the western Sichuan Company, PetroChina Company Limited, Basin. Chengdu 610051, People’s Republic of China Guang Yang is the project manager in Petro- China Company Limited. He received his B.A. Tectonic Setting degree in geology from Nanjing University in 1984, followed by his M.S. degree in 1989. The foreland fold and thrust belt of the Longmen Shan is about His interests include petroleum geology, basin 500 km (310 mi) long and 30–50 km (18–31 mi) wide and can be analysis, and entrapment assessment. traced from the Micang Shan of the Qinglin Mountains through the northern and southern Longmen Shan to the Kangdian paleohigh (Figure 1). To the west of the Longmen Shan is a vast triangu- ACKNOWLEDGEMENTS lar region underlain by strongly deformed Middle and Upper Triassic This research was supported by grants from deep-marine strata. These Triassic rocks comprise the Songpan- the National Science Foundation of China Garzeˆ basin, recognized as an ancient remnant ocean basin by Yin (Grant No. 40372091). Seismic profiles and and Nie (1993). To the southeast lies the western Sichuan Basin, drill data for this study, as well as consent to cored by the Yangtze craton (eastern part of the South China block). publish selected data, were generously pro- The Qiangtang block (North Tibet) and one to three accreted vol- vided by PetroChina Company Limited. We are especially grateful to Jia Chengzao, Dai Jinxing, canic arcs of late Paleozoic to early Mesozoic age tectonically bound Deng Qidong, Lu Huafu, Chen Hanlin, and the Songpan-Garzeˆ and Longmen Shan to the southwest (Sengo¨r Xiao Ancheng for their time, support, and geo- and Hsu¨ , 1984; Xu et al., 1992). logic insight. This article benefited from con- The Longmen Shan forms a northeast-trending chain whose structive and very helpful reviews by Jim Granath, internal structures parallel the trend of the chain. In contrast, the Peter Hennings, and Alexander A. Kitchka. folds and thrusts in the Songpan-Garzeˆ basin form a series of convex-to-the-south thrusts and folds (Figure 1) (Burchfiel et al., 1995; Harrowfield and Wilson, 2005). In this region, the Longmen Editor’s Note: Shan rises to more than 6 km (3.7 mi) above the Sichuan Basin, Color versions of figures may be seen in the forming one of the steepest mountain fronts along any margin of online version of this article. the Tibetan Plateau. Burchfiel (2004) provided the interpretation that the steep topographic front of the Longmen Shan results from resistance to eastward crustal flow because of a strong, less ductile crust underlying the adjacent Sichuan Basin (Lebedev and Nolet, 2003). However, the timing and geometry of the complex structures of the northeast-trending Longmen Shan fold-thrust belt remain controversial (Lu et al., 1989; Luo, 1991; Xu et al., 1992; Burchfiel et al., 1995; Liu et al., 1995; Chen and Wilson, 1996; Jia et al., 2003; Wallis et al., 2003; Roger et al., 2004). It is generally assumed that at least two major orogenic events occurred in the Longmen Shan belt since the Mesozoic: a Late Triassic compressional event (Indosinian orogeny) and a Cenozoic deformation related to the India–Asia collision (Dirks et al., 1994; Burch- fiel et al., 1995). Many geologists propose that the Longmen Shan

1426 Longmen Shan Fold-Thrust Belt Figure 1. Tectonic setting and generalized map of the Longmen Shan fold-thrust belt and western Sichuan Basin in central China. The locations of structural cross sections and seismic lines are indicated. QCF = Qinchuan fault zone; BCT = Beichuan thrust; MJT = Majiaoba thrust; WCT = Wenchuan thrust; WLT = Wulong thrust; LGT = Lingguan thrust.

Jia et al. 1427 belt in the Late Triassic evolved as a peripheral fore- foredeep history of the Upper Triassic foreland and the land system during the closure of the Songpan-Garzeˆ uppermost Cretaceous–Paleogene rejuvenated fore- ocean basin (Lu et al., 1989; Luo, 1991; Liu et al., 1994, land in the western Sichuan Basin. 1995).

Exploration History STRATIGRAPHY AND BASIN EVOLUTION

The western Sichuan Basin covering approximately The stratigraphic relationships between the Longmen 66,000 km2 (25,482 mi2) was the first natural-gas– Shan and the western Sichuan Basin are complicated producing area of China. In historical records of ancient by multiple phases of tectonism in the Late Triassic China, several shallow wells drilled in AD 76–147 at and Cenozoic. The chronostratigraphy in Figure 2 has Chengdu and Qionglai produced small quantities of been compiled from published data and proprietary natural gas that were used locally for boiling salt. Ex- information from recently drilled wells. In general, the ploration in this basin began with the drilling of wells western Sichuan Basin consists of a Proterozoic base- near oil seeps and seismic surveys in the 1950s. The ment of the Yangtze craton covered by a thin, incom- first commercial gas field (Zhongba gas field) near plete succession of Sinian (uppermost Proterozoic) Jiangyou was discovered at the PetroChina, Chuan 19 to Middle Triassic shallow-marine rocks interrupted well in 1971, which has produced 1.2 Â 1010 m3 (4.23 Â by Permian basic magmatism in the south and Upper 1011 ft3) gas. Since then, more than 300 exploration Triassic to Paleogene nonmarine sedimentary rocks. and production wells have been drilled, resulting in the discovery of more than 20 gas fields containing re- coverable gas reserves in excess of 2.7 Â 1011 m3 (9.5 Â Pre-Mesozoic Evolution 1012 ft3), including Zhongba (discovered in 1971), Da- xingchang (1977), Xiaoquan (1984), Pingluoba (1987), Pre-Sinian rocks are high-grade metamorphic igneous Hexingchang (1988), Fenggu (1989), and Xinchang rocks and low-grade metasedimentary rocks, which (1992) gas fields in the western Sichuan Basin (Fu et al., represent the Proterozoic basement of the Yangtze 2001; Li and Lu¨ , 2002; Wang et al., 2002). Recently, craton in the South China block. During the latest Pro- new exploration in this area focuses on the frontal terozoic (Sinian), the Yangtze craton underwent sig- zone of the Longmen Shan fold-thrust belt. nificant rifting, with deposition of very thick terrige- nous and volcanic deposits (Huang and Chen, 1987). CambrianandOrdovicianrocksweredepositedin Statement of Purpose shallow-marine and nonmarine environments, forming a thin cover of variable thickness over most of the In this article, we present structural interpretations of western Sichuan Basin. the Longmen Shan fold-thrust belt and its foreland in Rocks of Silurian, Devonian, and Carboniferous the western Sichuan Basin using recently released hy- ages are missing over a large area of the subsurface of drocarbon exploration data in the area. Because fault- the western Sichuan Basin. However, these rocks are related hydrocarbon migration is thought to be contem- well developed within the Longmen Shan fold-thrust poraneous with the deformation, a precise understanding belt, where they are reported to reach 2000–4000 m of the timing of trap structures is crucial. We use dif- (6600–13,100 ft) thick (Burchfiel et al., 1995; Chen ferent dating methods of thrust motion that include et al., 2005). The thickening of these rocks along the growth strata and unconformities to identify episodes Longmen Shan belt suggests that the western margin of growth of each of the major structures. We also pro- of the Yangtze craton had experienced an extensional vide a few examples of the structures from the frontal process during the Silurian–Carboniferous (Chen and zone of the Longmen Shan belt and comment on po- Wilson, 1996; Chen et al., 2005). tential hydrocarbon-trap configurations. One of the Within the western Sichuan Basin, Permian rocks major contributions of this article is the finding of are about 400–900 m (1300–2900 ft) thick and con- preexisting Paleozoic rift basins under Cenozoic thin- sist mainly of shallow-water limestone and dolomite. skinned thrust belts, which represent a new potential In the middle Permian, there was a major eruption of hydrocarbon play. Finally, we demonstrate a two-stage basalt (Emei Shan basalt) over a broad region of the

1428 Longmen Shan Fold-Thrust Belt Figure 2. Schematic stratigraphy and hydrocarbon potential of the western Sichuan Basin.

southern part of the basin (Luo, 1991; Luo and Long, of prominent coal deposits. These deposits are the 1992). The basalts reach more than 500 m (1600 ft) in first indication that convergent activity had begun thickness and were extruded in a terrestrial environment. within the Longmen Shan belt (Chen et al., 1994; Burchfiel et al., 1995; Zeng and Li, 1995; Yong et al., Mesozoic Evolution 2003). The Lower and Middle Jurassic rocks rest uncon- During the Mesozoic, a major change in depositional formably on deformed Upper Triassic and older rocks environment occurred, reflecting the deformation in only along the front zone of the northern Longmen the Longmen Shan belt. Lower and Middle Triassic Shan belt. Upper Jurassic and Lower Cretaceous sedi- rocks over the western Sichuan Basin are shallow ma- mentary rocks in the northern part of western Sichuan rine to locally nonmarine. In contrast, to the west in Basin thicken toward the north, with patterns of dis- the Songpan-Garzeˆ basin, the Middle Triassic rocks tribution that differ from Upper Triassic rocks, and in- are open marine to deep marine, with the beginning dicate the foredeep subsidence of this period along of flysch deposition in the Middle Triassic. During the east-west–trending frontal Micang Shan. Exposure the Late Triassic, the western Sichuan Basin became of Upper Cretaceous rocks is more restricted than that a flexurally loaded foredeep filled with several kilo- of the Lower Cretaceous units, and they are present meters of nonmarine clastic rocks with a development mainly in the southern part of the basin.

Jia et al. 1429 Cenozoic Evolution At the western end of this section (Figure 4), the fold and thrust structures of the northern Longmen Most of the Cenozoic rocks are restricted in the south- Shan belt are intersected by a series of major, deep- ern part of the basin and adjacent foothills of the Long- rooted, westward- and eastward-verging faults com- men Shan. In these areas, Paleogene rocks conformably monly associated with complex anticlinal structures that overlie Cretaceous rocks and consist of fluvial-lacustrine resemble a positive flower structure. Figure 5 shows red beds. Neogene rocks are uncommon within the the seismic interpretation of the western tectonic bound- basin, and exposures occur only along the easternmost ary that separates the Longmen Shan fold-thrust belt part of the southern Longmen Shan. They generally lie from the Songpan-Garzeˆ basin. These faults present a unconformably on folded Cretaceous rocks in small major fault zone, i.e., the Qingchuan fault zone (QCF) outcrops. The Quaternary strata are mainly subhor- (Figures 3, 5). The Qingchuan fault zone is character- izontal and unconformably deposited on older rocks. ized by brittle deformation with a variable thickness of gouge and brecciated rocks. Slickensided surfaces are abundant and contain oblique fault lineations that STRUCTURAL GEOMETRY AND show left-thrust slip. Microfabrics study (Wang et al., TIMING OF DEFORMATION 2000) indicates that the Qingchuan fault zone formed from a deep-seated ductile shear zone and experi- The Longmen Shan fold-thrust belt consists of numer- enced multiple tectonisms of Late Triassic metamor- ous thrust sheets and nappes that have been thrusted phism and Cenozoic brittle deformation at the very over the western side of the Yangtze craton. Several least. We believe that the deep-rooted, orogen-parallel eastward-verging, rootless thrust sheets and imbricates faults and associated en echelon anticlines can better of Cambrian–Triassic rocks have been recognized in be interpreted as a major northeast-trending, sinistral, the Longmen Shan belt (Xu et al., 1992; Chen and transpressional system. Wilson, 1996; Hu and Deng, 1996; Wu et al.,1999; Yu, To the east, the surface structure presents an anti- 2000; Jia et al., 2003). As seen from surface geology cline (Jiaoziding anticline) exposing a sequence of Si- alone (Figures 1, 3), this fold-thrust belt can be divided nian, Cambrian, Ordovician, and a thick section of Si- into two longitudinal segments: the north Longmen lurian rocks. Seismic data show that another thrust Shan and the south Longmen Shan. In our interpreta- sheet, consisting of deformed lower Paleozoic and per- tion, the structural geometry and ages of deformation haps older crystalline rocks, was developed under the have significant differences between the two segments. anticline. Exploration for oil and gas has been concentrated Farther to the east, the dominant feature is a large in the frontal zone of the Longmen Shan belt. Between syncline (Tangwangzhai syncline) that consists of thick 2000 and 2003, a large amount of seismic and drilling Silurian–Devonian rocks. Northwest-dipping thrusts data was acquired in this area. The complex structural cut its eastern limb. Rocks in the hanging wall of a styles in the frontal zone not only represent a formida- southeast-vergent major thrust (Figure 3; Majiaoba ble challenge to exploration, but also provide favorable thrust) are complexly deformed, metamorphosed Pa- trapping characteristics for hydrocarbon accumulations. leozoic rocks that are similar to those in the Songpan- Garzeˆ basin and are interpreted as part of the alloch- The North Longmen Shan thonous Paleozoic sequences, whereas footwall rocks are unmetamorphosed and belong to the autochthon. The northern Longmen Shan belt, extending for about Although both the autochthonous and allochthonous 200 km (124 mi) from the intersection of the Long- sections contain Sinian to Permian rocks, their strati- men Shan and the Micang Shan near Guanyuan to the graphic sections are quite different. Permian rocks dis- elbow of the easternmost orocline at Anxian, contains conformably overlie Ordovician rocks in the Yangtze several major thrust sheets and a blind frontal thrust autochthon, in contrast to a thick section of Silurian, zone (Figure 3). Most of the folds and thrust sheets Devonian, and Carboniferous rocks in the allochthon were emplaced toward the southeast. Across this zone, (Chen et al., 2005). a regional structural cross section has been constructed The easternmost anticline of this section, here (Figure 4) using available seismic reflection profiles named the Qinglinkou anticline, occurs as a fault- (e.g., seismic line L-55 across the north Longmen bend fold in the hanging wall of a blind frontal thrust Shan) and surface geological mapping. (Figure 4). The Qinglingkou anticline has a wide crest

1430 Longmen Shan Fold-Thrust Belt Figure 3. Surface geology of the northern Longmen Shan belt and adjacent areas and locations of seismic lines and structural cross sections. Also shown (rig symbols) are the Zhongba (ZHBA) and Kuangshanliang (KSL) gas fields. QCF = Qinchuan fault; BCT = Beichuan thrust; MJT = Majiaoba thrust. and gentle limbs. Cretaceous and Jurassic rocks in conformably overlie folded and thrusted Upper Trias- the forelimb of the anticline form a monocline with sic and Paleozoic rocks in the northeastern part of the southeast dips of 30–50j along the eastern margin of anticlines (Figure 3). Seismic and well data indicate the northern Longmen Shan belt. that the subsurface duplex structures are composed of three thrust sheets that wedge to the southeast under Lower Triassic evaporites (Figures 6, 7). Kuangshanliang The generative potential of Paleozoic source rocks The surface structure of Kuangshanliang (KSL) anti- in the Sichuan Basin has been recognized by Fu et al. cline is dominated by two large anticlines. Remnants (2001) and Wang et al. (2002). The hydrocarbons may of Lower Jurassic conglomerates and sandstones un- have been sourced from Paleozoic rocks (e.g., Cambrian

Jia et al. 1431 shales, Permian carbonates). The subsurface thrust duplex structures, with potential reservoirs in Permian dolomites, apparently represent the prospective structural traps sealed by the Lower Triassic evaporites and the uppermost thrust zone. The effectiveness of this trap structure clearly depends on good sealing proper- ties of both the overlying faults and the hanging-wall rocks. So far, only small, noncommercial accumulations of gaseous and liquid hydrocarbons and traces have been found in the area.

Zhongba The Zhongba gas field at Jiangyou is another example of the structural traps related to foreland thrusting (Figure 3). The cross section in Figure 8 through the PetroChina, Zhongba 5, 6, and 9 wells suggests that most southeast-verging thrusts splay from the Silurian decollement and fold the unconformity separating Up- per Triassic and Jurassic rocks (Figure 8). Thickness variations in strata that accumulated in the vicinity of the growing structures allow precise dating of individ- ual thrust movements (Medwedeff and Suppe, 1986; Medwedeff, 1989; Suppe et al., 1992). In Figure 9, onlapping seismic reflections are observed apparently in the Upper Triassic, which provide the evidence of a rotating fold limb. In the same structural position, the Upper Triassic is eroded by the base Jurassic unconformity, demonstrating additional limb rotation. The Upper Triassic growth strata are commonly thinner and are eroded over the fold crests (Figures 9, 10). The unconformable relations between Upper Tri- assic and Lower Jurassic rocks also determine the age of the early Mesozoic deformation, referred to in the Chinese literature as the Indosinian deformation stage. Here, the unconformity records only the time at which the thrust sheets reached this position and stopped moving in the Zhongba trap structure. The folding of Jurassic and Lower Cretaceous rocks and the erosional unconformity demonstrates that a younger, compressional, deformational event is present, but its age is poorly determined. Hydrocarbons of the Zhongba gas field occur in Triassic dolomites and siliciclastic rocks and are sealed by the unconformity of the base of the Jurassic sequence and the tectonically superimposed thrusts. The hydrocarbons that were generated from the Perm- Cross section A through the northern Longmen Shan belt. Location is shown inian Figures 1 and 3. source rocks migrated into reservoirs in the Mid- dle Triassic Leikoupo Formation and the Upper Tri- assic Xujiahe Formation (Fu et al., 2001; Wang et al.,

Figure 4. 2002).

1432 Longmen Shan Fold-Thrust Belt Figure 5. Interpretation of the western segment of seismic line L55 showing the flower structure caused by a sinistral transpression. For line location, see Figure 4.

North Longmen Shan Summary evidence suggests that the first episode of folding The timing of deformation in the northern Longmen occurred in the Late Triassic. The Jurassic and Cre- Shan belt is constrained by the age of the strata in- taceous rocks along the eastern side of the northern volved in the deformation, growth strata, and uncon- Longmen Shan belt were folded in a younger event formity analysis. Remnants of the Lower Jurassic rocks that caused additional folding and thrusting of the unconformably overlie folded and thrusted Triassic older structures, exhibiting clear patterns of super- andPaleozoicrocksintheeasternflankofthenorth- imposed deformation. Folds and faults of both events ern Longmen Shan belt (Figure 3). Onlapping growth are generally parallel; thus, it is difficult to separate strata are recognized only within Upper Triassic in structures originating from the different events in the forelimbs of folds (Figure 9). The stratigraphic the pre-Jurassic rocks. The age of the younger folding

Figure 6. Structural cross section 1 through the KSL 01 and 03 wells based on surface geology, seismic data, and dip and formation data from the wells. Line location is showninFigures1and3.

Jia et al. 1433 Figure 7. Seismic line CHXI- KSL-01-15 showing the duplex structure tested by the KSL 01 and 03 wells. Blue = base Jurassic; pink = base of Upper Triassic; orange (LT) = base Lower Triassic; white (UP) = base of Upper Permian; green (LP) = base Lower Permian. Faults (F) are shown in red. See online version for colors.

and thrusting event is also difficult to establish be- The South Longmen Shan cause Lower Jurassic to Lower Cretaceous strata form a continuous sequence, and there are no rocks in this The southern Longmen Shan belt can be characterized area younger than Early Cretaceous. In view of the by dominantly southeast-verging, basement-involved broader geodynamic setting (Dirks et al., 1994; Burch- folds and thrust faults, extending for about 300 km fiel et al., 1995; Tapponnier et al., 2001; Kirby et al., (186 mi) from the elbow of the eastern orocline at Anx- 2002; Fang et al., 2003), we infer that the younger de- ian to the northwestern Kangdian paleohigh (Figure 1). formational event is late Cenozoic in age and perhaps A compiled geologic, structural map and a regional continues to the present. structural cross section of the southern Longmen Shan

Figure 8. Structural cross section 2 through the ZB 5, 6, and 9 wells based on well data, seismic data, and surface geology. Line location is shown in Figures 1 and 3.

1434 Longmen Shan Fold-Thrust Belt Figure 9. Seismic line CHXI-ZB-02-18 showing the erosional unconformity at the base Jurassic and growth strata in Upper Triassic in the forelimb of the anticline. Arrows indicate the erosion of the top Triassic; triangles show onlapping reflections. For line location, see Figure 8.

belt are shown in Figures 11 and 12. This belt contains incomplete section of Paleozoic rocks is characteristic three major thrust sheets (Pengguan, Baoxing, and of the autochthonous sequence and is in contrast to the Wulong sheets) and an outer klippen belt (Figure 11). Wulong sheet, which contains thick Silurian, Devonian, The Baoxing and Pengguan sheets contain the Prote- and Carboniferous strata. We suggest that the Baoxing rozoic igneous and metamorphic basement and their and Pengguan sheets belong to a part of the autochtho- sedimentary cover. The cover sequence ranges from nous western margin of the Yangtze craton, whereas Sinian to Permian in age and is characterized by a lack the Wulong sheet belongs to the allochthon displaced of Devonian and Carboniferous strata. This thin and from the Songpan-Garzeˆ basin.

Figure 10. Seismic line CHXI-ZB-02-16 showing the erosional unconformity between Jurassic and Triassic in the backlimb of the anticline. For line location, see Figure 8.

Jia et al. 1435 Figure 11. Surface geological map of the southern Longmen Shan belt and locations of seismic lines and cross sections. Also shown (rig symbols) are the Pingluoba (PLB), Gaojiachang (GJC), and Wuzhongshan (WZS) gas fields. WCT = Wenchuan thrust; BCT = Beichuan thrust; WLT = Wulong thrust; LGT = Lingguan thrust.

To illustrate structural geometries of the southern the Wulong thrust (Figure 12). The Wulong thrust Longmen Shan belt, cross section B is constructed with may be an inverted preexisting normal fault (Chen the aid of seismic profiles in the eastern part of this et al., 2005). cross section and our field mapping in the western part In the central part of this section, the dominant (Figure 12). structural feature is a basement-cored anticlinorium, Thick, low-grade, metamorphic sediments of Or- the Baoxing anticline. Precambrian rocks of the anti- dovician, Silurian, and Devonian age occur at the west- cline are overlain unconformably by a thin and incom- ern margin of this section in the allochthonous thrust plete section of Paleozoic rocks. The rocks in the eastern sheets and are emplaced onto the western limb of the limb of the anticline are cut by west-northwest–dipping Baoxing anticline along a major southeast-verging thrust, thrusts with east-southeast displacement, whereas the

1436 Longmen Shan Fold-Thrust Belt rocks above the crest of the anticline generally are subhorizontal and dip gently west along its western limb. These geometric relations indicate that the Baoxing anticline initiated as a broad, flat-topped fault-bend fold (Suppe, 1983) and was subsequently cut by younger thrusts. To the east, several klippen lie above folded Upper Triassic or Jurassic strata to form the so-called Outer Klippen belt. The Outer Klippen belt is one of the most remarkable tectonic units and is present along the eastern margin of the southern Longmen Shan belt (Figure 11). It consists of small and large (up to several-kilometer-long) masses of allochthonous De- vonian to Permian and uncommon Silurian rocks. Most of the rocks are limestones and dolomites, but some sandstones and shales are also present. Rocks in the klippen are complexly faulted and folded, commonly recumbently folded. Thus, we infer that the klippen consist of allochthonous Paleozoic rocks, which were derived from the Wulong sheet and the eastern margin of the Songpan-Garzeˆ basin. Burchfiel et al. (1995) suggested that the emplacement time of the klippen is clearly Cenozoic. Although the kinematical process forming the klippen is still in controversy between the nappe structures (Xu et al., 1992) and grav- ity slides (Liu et al., 1994), this belt greatly charac- terizes the south Longmen Shan as different from the north segment. At the leading edge of the southern Longmen Shan belt along the Lingguan thrust, the Triassic sequence is thrusted over nonmarine red molasses of the uppermost Cretaceous–Paleogene foredeep (Figure 12). Far- ther east, thin-skinned structures with basal detach- mentareshownintheeasternpartofthissection (Figure 13). Folds in the Mesozoic–Cenozoic rocks affect rocks as young as Oligocene (Lushan Formation), and some of the Neogene conglomerate rocks (Dayi Conglomerate) unconformably overlie folded Creta- ceous and Paleogene rocks (Figure 11). These relations suggest that folding occurred before the deposition of the Neogene rocks. Farther to the east, the folded and thrusted structures can be traced into the western Si- chuan Basin, such as the Hanwangchang, Daxingchang, and Longquanshan anticlines (Figure 1). Growth strata were deposited during the main phase of fold deformation. This interval varies drastically in

Regional structural cross section B through the southern Longmen Shan belt. Location is shownboth on Figures 1 and 11. thickness and seismic character in very short distances across the fold belt. In the southern Long- men Shan belt, growth strata are recognized in the fold limbs of the Lianhuashan and Gaojiachang anti-

Figure 12. clines (Figures 14, 15). Because the age of the growth

Jia et al. 1437 strata is well known with magnetic-polarity stratigraphy (Zhuang et al., 1988; Enkin et al., 1991) and bio-

ceous; J = stratigraphic data (BGMRSP, 1991; Gu and Liu, 1997), we interpret the main growth interval to lie between the near-top Cretaceous and the Oligocene. Eastward, only poorly developed growth strata are observed in seismic profiles, including some unclear reflectors that onlap the forelimbs of the anticlines. Our interpretation is in agreement with palynological ages of synorogenic conglomerates in the footwalls of the leading thrust sheets of the southern Longmen Shan belt (BGMRSP, 1991; Gu and Liu, 1997). The time of emplacement of the thrust sheets in the southern Longmen Shan belt has not been well de- fined, except in its frontal zone, where Neogene rocks unconformably overlie deformed rocks and growth strata of the latest Cretaceous–Oligocene ages, as evidenced in seismic profiles. In contrast, the conformable rela- tionship between Upper Triassic and Lower Jurassic rocks in the eastern side of the southern Longmen Shan belt indicates that, unlike the northern Longmen Shan belt, deformation was weak or absent during the deposition of these rocks (Figure 16). Thus, no obvious and direct evidence of the Late Triassic deformation exists in this area. However, evidences for Late Triassic deformation to the west of the belt can be inferred. The thick section (>3500 m; >11,500 ft) of Upper Triassic sandstone present within the western Sichuan Basin is considered to be the foredeep deposits originating from deformation farther west in the Songpan- Garzeˆ basin (Harrowfield and Wilson, 2005).

RIFT BASINS UNDER THRUST STRUCTURES

Based on the restoration of balanced cross sections, Chen et al. (2005) discussed that the western margin of the Yangtze craton had undergone a Paleozoic extensional process, and several syndepositional normal faults were recognized within the Longmen Shan belt. Here, some preexisting Paleozoic rift basins have been identified under the Cenozoic thin-skinned thrust structures in the southern part of the western Sichuan Basin and provide the direct structural evidence that further the understanding of the early extensional history of the Longmen Shan and the western Sichuan Basin. The preexisting normal faults and Cenozoic thrust faults Seismic line LHS-02-12 showing the structure penetrated by the LHS 01 well. For line location, see Figures 11 and 12. E = base of Paleogene; K = base Creta were correlated between seismic lines to construct the structural map shown in Figure 17. In general, these Ce- nozoic fold-thrust structures trend northeast-southwest, Figure 13. base Jurassic; UT = base of Upper Triassic;parallel LT = base Lower Triassic. Faults (F) are shown in to red in the onlinethe version. Longmen Shan belt. In the interpreted

1438 Longmen Shan Fold-Thrust Belt Figure 14. Part of seismic line LHS-02-12 showing the growth strata in the forelimb of an anticline. For line location, see Figure 13. EL = base of Oligocene (Lushan Formation); EM = base of Eocene and Paleocene (Mingshan Formation); KG = base of uppermost Cretaceous (Guankou Formation); KJ = base of Lower Cre- taceous (Jiaguan Forma- tion); J = base Jurassic; TX = base of Upper Triassic (Xujiahe Forma- tion). Faults (F) are shown in red in the online version.

seismic reflection profiles (Figures 18, 19), two super- The geometry of the shallow structural level sug- posed structural levels can be recognized. A shallow gests that the thin-skinned fold and thrust belt is a structural level consists of thin-skinned Cenozoic thrust forward-propagating thrust system. The Lower Triassic structures involving Triassic to Paleogene strata. A deep evaporitic rocks of the Jialingjiang Formation and Lei- structural level contains Paleozoic extensional grabens koupo Formation act as an important decollement hori- below the near-horizontal Permian sequences. Some zon, separating shallow-level structural complexity from of the normal faults in these basins were inverted slightly simple extensional structures beneath. Thin-skinned in the Cenozoic as reverse faults. thrusting may occur in a foreland-directed sense at the

Figure 15. Seismic line GSZ-02-05 showing the growth strata in the forelimb of the wedge structure. For line location, see cross section 3 in Figure 11. EL = base of Oligocene (Lushan Formation); EM = base of Eocene and Paleocene (Mingshan Formation); KG = base of uppermost Cretaceous (Guankou Formation); KJ = base of Lower Cretaceous (Jia- guan Formation); J = base Jurassic. Faults (F) are shown in red in theonlineversion.

Jia et al. 1439 Figure 16. Seismic line WZS-02-20 showing the structure of Wuzhongshan anticline. For line location, see cross section 4 in Figures 1 and 11.

Hangwangchang gas field (Figure 19) or in a hinterland- (Figures 18, 19), such as the stratigraphic truncation directed backthrust, as illustrated by the Daxingchang or thickening across the fault. Bright reflectors charac- structure in Figure 18. teristic of the synextensional sequences display normal The graben-bounding faults below the Ceno- separation of up to approximately 1000 ms (1750 m zoic thin-skinned thrust structure are well expressed [5700 ft] for an interval velocity of 3500 m/s by geometric relationships in seismic reflection data [11,500 ft/s]).

Figure 17. Simplified structural map of Paleo- zoic rift basins below the Cenozoic thrust-fold belt, showing the Dax- ingchang (DXC) and Hanwangchang (HWC) gas fields (rig symbols), and exploration wells (dots). Gray (blue online) lines indicate normal faults below the thin- skinned structures.

1440 Longmen Shan Fold-Thrust Belt Figure 18. Seismic reflection profile PQ-92-D1 across the Daxingchang gas field (DXC) in the southern part of western Sichuan Basin (see Figures 1 and 17 for location of cross section 5), showing the rifted basins under the thin-skinned thrust belt. TWT = two-way traveltime. i tal. et Jia 1441 1442 oge hnFl-hutBelt Fold-Thrust Shan Longmen

Figure 19. Seismic reflection profile MB-92-D2 across the Hangwangchang gas field (HWC) in the southern part of western Sichuan Basin (see Figures 1 and 17 for location of cross section 6), showing the rifted basins under the thin-skinned thrust belt. TWT = two-way traveltime. Although the timing constraints of the synexten- rocks to the formation of a foredeep caused by tec- sional strata are not precise, the composite interpreta- tonic loading of thrust sheets of the Longmen Shan tions of the two main seismic profiles (Figures 18, 19) belt. Southeast convergence along the Longmen Shan and their longitudinal profiles across exploration wells belt must have been of sufficient magnitude to flex- suggest that the graben basins are developed under the urally load the foreland, producing the thick Upper near-horizontal Permian sequence, which is character- Triassic foredeep basin (Yong et al., 2003). This pe- ized by parallel reflectors showing no effects of normal riod of foredeep deposition corresponds well with the faulting. Thus, it represents a postextensional sequence. early deformation in the Longmen Shan belt (Jia et al., Two exploration wells, PetroChina, Dashen 01 and 2003). Hanwangchang 01, penetrated several meters into this Based on well data and interpreted seismic pro- Permian sequence and encountered sandstones and files, an isopach map (Figure 20) of the Upper Triassic shales. However, no core samples were recovered. The Xujiahe Formation was constructed by Southwest Oil base of the preextensional Sinian–Ordovician sequences and Gas Field Company of PetroChina (2001). The is denoted by very strong reflections characteristic of the Upper Triassic sequence forms a wedge of sedimentary western Sichuan Basin. Rocks of Silurian–Carboniferous rocks that thickens toward the Longmen Shan in the ages are missing, and Permian rocks unconformably western Sichuan Basin. The maximum observable thick- overlie the Sinian–Ordovician strata over a large area ness of this sequence is greater than 3000 m (9800 ft) of the subsurface. These relations imply that the in the west, gradually thinning to the southeast. Much graben basins may be filled with Devonian–Carbon- of the thick, Upper Triassic sequence is composed of iferous sequences. This extensional phase was probably fine-grained clastic strata with uncommon conglomer- associated with the development of the lower Paleo- ate; Burchfiel et al. (1995) suggests that the western zoic rifted margin in the western Yangtze craton prior edge of the foredeep must have been located west of the to the Longmen Shan compressional tectonism (Chen present exposures of Upper Triassic rocks. et al., 2005). Uppermost Cretaceous–Paleogene Foredeep

THE WESTERN SICHUAN FORELAND BASIN The uppermost Cretaceous–Paleogene foredeep ranges in width from 50 km (31 mi) in the northern part of The western Sichuan Basin is clearly distinguished by the foredeep basin to more than 200 km (124 mi) in Cenozoic deposits in its southern part. The remainder the southern part. The isopach map of the uppermost and larger part of the basin contains unfolded to weakly Cretaceous to Paleogene helps define the geometry folded Mesozoic strata at the surface (Figure 1). Con- of this foredeep (Figure 21). The depocenter was prob- temporaneous with thrusting was the development ably located at Ya-an, where the maximum observable of the western foreland Sichuan Basin. Sedimentary thickness is more than 2500 m (8200 ft) (Guo et al., features of the foredeep suggest that this basin exhibits 1996). Rock types vary greatly within the foredeep. a two-phase flexural subsidence history caused by tec- Conglomerate and sandstone of synorogenic molassic tonic loading of the Longmen Shan thrust and fold belt. character are common. These conglomerates tend to be The Upper Triassic foredeep extends along the entire most abundant along the western side of the foredeep, Longmen Shan belt, but the uppermost Cretaceous– but in some places like the eastern part of the basin, Paleogene foredeep is restricted in the southern part only fine-grained sediments constitute the bulk of the of the western Sichuan Basin. thin foredeep sequence. Structurally, the inner part of the foredeep is involved in the folding and thrusting of Upper Triassic Foredeep the thin-skinned thrust belt. These deformed foredeep rocks form northeast– and north-northeast–trending Within the western Sichuan Basin, the Mesozoic rocks anticlines. The geometry of these folds suggests that are generally horizontal or broadly folded, with limb they are fault-bend and fault-propagation folds that die dips of only a few degrees. We follow previous researches out into a subhorizontal decollement in the Lower Tri- (Cui et al., 1991; Luo and Long, 1992; Chen et al., 1994; assic rocks. The latest Cretaceous–Paleogene defor- Liu et al., 1994; Burchfiel et al., 1995; Jia et al., 2003; mation in the southern Longmen Shan belt clearly was Yong et al., 2003) and attribute the rapid thickening accompanied by flexural subsidence of the Yangtze of the succession of the Upper Triassic sedimentary craton to form a rejuvenated foreland basin.

Jia et al. 1443 Figure 20. Isopach map (in meters) of the Upper Triassic strata (Xujiahe Formation) in the western Sichuan Basin based on well data and interpreted seismic profiles.

IMPLICATIONS FOR HYDROCARBON EXPLORATION

The fold and thrust belts of the world have been explored for more than 100 yr. Large oil and gas fields have been found in some belts (e.g., Carpathians, Zag- ros, Andes, and Rocky Mountains). Recognition of the thin-skinned geometry of some thrust belts has been particularly important, not only greatly improv- ing the efficiency of exploration in the thrust belts, but also opening new and promising plays beneath some of these thin-skinned belts (Picha, 1996; Menese-Rocha and Yurewicz, 1999). A simple Lopatin diagram was constructed to model the post-Carboniferous burial and maturation history of the source rocks at the HWC 01 well (Figure 22). For more information on the rift basin, we extend this well 500 m (1600 ft) deep into the Carboniferous. This model shows that the Triassic entered the oil window for its maturation cycle by the Late Jurassic and went through the following age. Because the top Car- boniferous had already passed well into the gas window by the end of the Jurassic, gas is anticipated to be the primary hydrocarbon for the rift play. Figure 22 in- Figure 21. Isopach map (in meters) of the uppermost dicates that at maximum burial, the top Carboniferous Cretaceous–Paleogene strata in the western Sichuan Basin.

1444 Longmen Shan Fold-Thrust Belt Figure 22. Burial history reconstruction for the Hanwang- chang area of the Sichuan Basin based on the well HWC 01. A geothermal gradient of 2.25jC/ 100 m was used (Zhang et al., 1996). TTI = time-temperature index.

was still in the gas-generation window. In summary, the The rifted basins below the thin-skinned fold and source rocks built up in both upper Paleozoic and Tri- thrust structures provide a new significant hydrocarbon assic, having been generating hydrocarbons (oil or gas) play in the western Sichuan. These graben basins are continuously from the Late Triassic or earlier to today, filled with Devonian to Carboniferous continental mar- which provide a great exploration perspective in the gin sequences, probably containing high-quality source Sichuan Basin and its adjacent fold-thrust belts. rocks and contemporaneous reservoir sequences that The timing and structural styles presented here are still generating gas. The subsequent tectonic inver- provide important constraints for an improved assess- sion, although of small magnitude, has folded and tilted ment of hydrocarbon potential in the western Sichuan these strata to create some gentle anticlinal traps. These Basin. The frontal zone of the Longmen Shan belt with inversion anticlines tend to be large and structurally sim- prominent Permian and Triassic reservoirs and a variety ple, in contrast to more complex structures above. The of structural traps has been the focus of intensive hy- Lower Triassic evaporites may act as regional seals, pos- drocarbon exploration over the last decade. Known sibly retaining hydrocarbons generated in the Devonian– source rocks (Figure 2) are found in the Triassic and Carboniferous succession in these structures. Some of the Paleozoic strata, including the Lower Silurian marine deep, rift-related structures of the Arkoma Basin beneath shales, with up to 8% total organic carbon (TOC) and the edges of the Ouachita thrust belt have been suc- Upper Triassic (Xujiahe Formation) carbonaceous mud- cessfully explored, primarily for deep gas (Picha, 1996). stones with 4% TOC. Reservoirs include Permian dolomites, Middle Triassic limestones, and Upper Triassic and Middle Jurassic sandstones. These reservoirs each CONCLUSIONS have corresponding top seals provided by Lower Tri- assic evaporates, Upper Triassic coal seams, and Juras- 1. The complex structures of the northeast-trending sic silty mudstones. We suggest that the generation of Longmen Shan belt and its foreland in the western hydrocarbons occurred after the last phase of thrusting Sichuan Basin were formed by southeast-directed in the early Cenozoic, when most of the potential struc- thrusting. Structural styles illustrated by regional tural traps had been formed. However, the continuing structural cross sections suggest that prospective activity along some of the deep faults may have dis- trap geometries are mainly developed in the frontal rupted the trapping mechanism of deep structures in zone of the Longmen Shan fold-thrust belt and the the frontal zone. south part of the western Sichuan Basin.

Jia et al. 1445 2. The Longmen Shan belt can be divided into two Tectonic evolution of the NE margin of the Tibetan Plateau: Evidence from the central Longmen Mountains, Sichuan segments: the north Longmen Shan and the south Province, China: Journal of Southeast Asian Earth Sciences, Longmen Shan. In our interpretation, the structural v. 9, p. 181–192. geometry and ages of deformation have significant Enkin, R. J., Y. Chen, V. Courtillot, and J. Besse, 1991, A Cre- differences between the two segments. The north- taceous pole from south China, and the Mesozoic hairpin turn of the Eurasian apparent polar wander path: Journal of Geo- ern Longmen Shan experienced at least two major physical Research, v. 96, p. 4007–4027. periods of deformation in the Late Triassic and Fang, X., C. Garzione, R. Voo, J. Li, and M. Fan, 2003, Flexural again in the Cenozoic. The relationships of growth subsidence by 29 Ma on the NE edge of Tibet from the mag- netostratigraphiy of Linxia Basin, China: Earth and Planetary strata indicate that, unlike the northern Longmen Science Letters, v. 210, p. 545–560. Shan belt, its major period of deformation was Fu, X., W. Shu, R. Yi, and Y. Xiong, 2001, Exploration and de- during the latest Cretaceous–early Cenozoic at the velopment of nature gas in western Sichuan Basin: Wuhan, Publishing House of China Geology University, 177 p. frontal zone of the southern Longmen Shan belt. Gu, X., and X. Liu, 1997, Lithostratigraphy of the Sichuan Prov- 3. Preexisting Paleozoic rift basins are identified under ince: Wuhan, Publishing House of China Geology University, the Cenozoic thin-skinned thrust belts in the south- 417 p. ern part of the western Sichuan Basin. These graben Guo, Z., K. Deng, and Y. Han, 1996, The formation and development of Sichuan Basin (in Chinese): Beijing, Geological Pub- basins exhibit mild inversion and may be filled with lishing House, p. 48–138. Devonian–Carboniferous sequences, which repre- Harrowfield, M. J., and C. J. L. Wilson, 2005, Indosinian de- sent a new hydrocarbon play. formation of the Songpan Garzeˆ fold belt, northeast Tibetan Plateau: Journal of Structural Geology, v. 27, p. 101–117. 4. The pattern of synorogenic sediments exhibits a Hu, X., and J. Deng, 1996, Structure features of the nappe tectonic two-phase flexural-loading history in the western belt in the middle Longmen Mountains (in Chinese with En- Sichuan Basin and suggests that the Upper Triassic glish abstract): Journal of Chengdu College of Geology, v. 23, p. 101–106. foreland basin extends along the foredeep of the en- Huang, J., and B. Chen, 1987, The evolution of the Tethys in China tire Longmen Shan belt, whereas the uppermost and its adjacent regions (in Chinese with English abstract): Cretaceous–Paleogene rejuvenated foreland basin is Beijing, Geologic Publishing House, 109 p. restricted in the southern part of the western Si- Jia, D., Z. X. Chen, C. Jia, G. Wei, B. Li, Q. Zhang, D. Wei, and Y. Shen, 2003, Structural features of the Longmen Shan fold chuan Basin. and thrust belt and the western Sichuan foreland basin, central China (in Chinese with English abstract): Geological Journal of China Universities, v. 9, p. 462–469. REFERENCES CITED Kirby, E., P. W. Reiners, M. A. Krol, K. W. Whipple, K. V. Hodges, K. A. Farley, W. Tang, and Z. Chen, 2002, Late Cenozoic evolution of the eastern margin of the Tibetan Plateau: Inferences BGMRSP (Bureau of Geology and Mineral Resources of the Sichuan from 40Ar/39Ar and (U–Th)/He thermochronology: Tectonics, Province), 1991, Regional geology of Sichuan Province (in v. 21, no. 1, doi:10.1029/2000TC001246. Chinese with English abstract): Beijing, Geological Publishing Lebedev, S., and G. Nolet, 2003, Upper mantle beneath southeast House, 730 p. Asia from S velocity tomography: Journal of Geophysical Re- Burchfiel, B. C., 2004, 2003 presidential address: New technology, search, v. 108, no. B1, p. 2048– 2074, doi:10.1029/2000JB000073. new geological challenges: Geological Society of America Today, Li, G., and M. Lu¨ , 2002, Atlas of China’s petroliferous basins, 2d ed. v. 14, p. 4–9. (in Chinese with English abstract): Beijing, Petroleum Industry Burchfiel, B. C., Z. Chen, Y. Liu, and L. H. Royden, 1995, Tectonics Press, 492 p. of the Longmen Shan and adjacent regions, central China: Liu, H. F., H. S. Liang, L. G. Cai, and F. Shen, 1994, Structural International Geology Review, v. 37, p. 661–735. systems of The Longmenshan thrust belt and evolution of the Chen, S. F., and C. J. L. Wilson, 1996, Emplacement of the Long- foreland basin in western Sichuan Province, China (in Chinese men Shan thrust-nappe belt along the eastern margin of the with English abstract): Acta Geologica Sinica, v. 68, p. 100– Tibetan Plateau: Journal of Structure Geology, v. 18, p. 413– 117. 430. Liu, S. G., Z. L. Luo, S. L. Dai, D. Arne, and C. J. L. Wilson, 1995, Chen, S. F., C. J. L. Wilson, Z. Luo, and Q. Deng, 1994, The The uplift of Longmenshan thrust belt and subsidence of the evolution of the western Sichuan foreland basin, southwest- western Sichuan foreland basin (in Chinese with English ab- ern China: Journal of Southeast Asian Earth Sciences, v. 10, stract): Acta Geologica Sinica, v. 9, p. 205–213. p. 159–168. Lu, H. F., H. G. Dong, X. Y. Deng, P. J. Li, B. Q. Wu, and D. L. Chen, Z. X., D. Jia, Q. Zhang, G. Wei, B. Li, D. Wei, and Y. Shen, Pen, 1989, Types and origins of the thrusts and nappes in outer 2005, Balanced cross-section analysis of the fold-thrust belt of Longmenshan foreland basin (in Chinese with English ab- the Longmen Mountains (in Chinese with English abstract): stract): Journal of Nanjing University (Earth Sciences), v. 4, Acta Geologica Sinica, v. 79, p. 38–45. p. 32–41. Cui, B. Q., X. M. Long, and Y. L. Li, 1991, The subsidence of west Luo, Z. L., 1991, The dynamical model of the lithospheric evo- Sichuan depression and the rise of Longmenshan Mountains lution in Longmen Shan orogenic belt (in Chinese with English (in Chinese with English abstract): Journal of Chengdu Col- abstract): Journal of Chengdu College of Geology, v. 18, p. 1– lege of Geology, v. 18, p. 39–44. 7. Dirks, P., C. J. L. Wilson, S. Chen, Z. Luo, and S. Lin, 1994, Luo, Z. L., and X. M. Long, 1992, The uplift of Longmen Shan

1446 Longmen Shan Fold-Thrust Belt orogenic zone and subsidence of the western Sichuan foreland Suzuki, and H. Hyodo, 2003, Cenozoic and Mesozoic meta- basin (in Chinese with English abstract): Acta Geologica Si- morphism in the Longmenshan orogen: Implications for geo- chuan, v. 12, p. 1–17. dynamic models of eastern Tibet: Geology, v. 31, p. 745–748. Mattauer, M., P. Matte, J. Malavieille, P. Tapponnier, H. Maluski, Wang, Q. W., B. Liang, Q. X. Xie, and W. J. He, 2000, Research on Z. Xu, Y. Lu, and Y. Tang, 1985, Tectonics of the Qinling belt: microstructures and deformation conditions of the Qingchuan Build-up and evolution of eastern Asia: Nature, v. 317, p. 496– fault zone (in Chinese with English abstract): Journal of Min- 500. eral Petrology, v. 87, p. 87–90. Medwedeff, D. A., 1989, Growth fault-bend folding at southeast Wang, Z., W. Zhao, L. Zhang, and S. Wu, 2002, Tectonic se- Lost Hills, San Joaquin Valley, California: AAPG Bulletin, quences and nature-gas exploration in Sichuan Basin (in Chi- v. 73, p. 54–67. nese with English abstract): Beijing, Geological Publishing, Medwedeff, D. A., and J. Suppe, 1986, Growth fault-bend folding 287 p. precise determination of kinematics, timing and rates of fold- Wu, S., B. Zhao, and Z. H. Gou, M. S. Deng, J. H. Deng, and X. W. ing and faulting from syntectonic sediments (abs.): Geological Hu, 1999, The structure framework and its evolution in the Society of America Abstracts with Programs, v. 18, p. 692. middle-south section of Longmen Mountains (in Chinese with Menese-Rocha, J., and D. A. Yurewicz, 1999, Petroleum explora- English abstract): Journal of Mineralogy and Petrology, v. 19, tion and production in fold and thrust belts: Ideas from a Hed- p. 82–85. berg research symposium: AAPG Bulletin, v. 83, no. 6, p. 889– Xu, Z. Q., L. W. Hou, and Z. X. Wang, 1992, Orogenic processes 897. of the Songpan-Garzeˆ orogenic belt of China (in Chinese): Picha, F. J., 1996, Exploring for hydrocarbons under thrust belts— Beijing, Geological Publishing House, 190 p. A challenging new frontier in the Carpathians and elsewhere: Yin, A., and S. Nie, 1993, An indentation model for the North and AAPG Bulletin, v. 80, p. 1547–1564. South China collision and the development of the Tan-Lu and Roger, F., J. Malavieille, P. H. Leloup, S. Calassou, and Z. Xu, 2004, Honam fault systems, eastern Asia: Tectonics, v. 12, p. 801– Timing of granite emplacement and cooling in the Songpan- 813. Garzeˆ fold belt (eastern Tibet Plateau) with tectonic implica- Yong, L., P. A. Allen, A. L. Densmore, and X. Qiang, 2003, Evo- tions: Journal of Asian Earth Sciences, v. 22, p. 465–481. lution of the Longmen Shan foreland basin (western Sichun, Sengo¨r, A. M. C., and K. J. Hsu¨ , 1984, The Cimmerides of eastern China) during the Late Triassic Indosinian orogeny: Basin Re- Asia: History of the eastern end of Paleo-Tethys: Societe search, v. 15, p. 117–138. Geologique de France Memoires 147, p. 139–167. Yu, S. J., 2000, Geometrical characteristics and strain analysis of Southwest Oil and Gas Field Company of PetroChina, 2001, The nappe structures in the northern section of Longmenshan (in Late Triassic sedimentary features and its hydrocarbon po- Chinese with English abstract): Journal of Mineralogy and tential in the southwestern Sichuan Basin (in Chinese): The Petrology, v. 20, p. 67–74. project research report of Southwest Oil and Gas Field Com- Zeng, Y. F., and Y. Li, 1995, The formation and evolution of Long- pany of PetroChina Company Limited, 223 p. men Mountains foreland basin (in Chinese with English ab- Suppe, J., 1983, Geometry and kinematics of fault-bend folding: stract): Journal of Mineralogy and Petrology, v. 15, p. 40–49. American Journal of Science, v. 283, p. 684–721. Zhang, X., X. F. Hu, J. X. Shen, C. Z. Tian, L. Zhao, M. Liu, and Suppe, J., G. Chou, and S. Hook, 1992, Rates of folding and fault- X. W. Chen, 1996, Reversal construction of the Courier iso- ing determined from growth strata, in K. R. McClay, ed., thermal surface in the Sichuan Basin and its western earthquake Thrusts tectonics: London, United Kingdom, Chapman and Hall, region (in Chinese with English abstract): Acta Seismologica p. 105–121. Sinica, v. 18, p. 83–88. Tapponnier, P., Z. Xu, F. Roger, B. Meyer, N. Arnaud, G. Witt- Zhuang, Z. H., D. X. Tian, X. H. Ma, X. F. Ren, X. C. Jiang, and linger, and J. Yang, 2001, Oblique stepwise rise and growth of S. L. Xu, 1988, A paleomagnetic study along the Ya’an- the Tibet Plateau: Science, v. 294, p. 1671–1677. Tianquan Cretaceous–Eogene section in Sichuan Basin: Geo- Wallis, S., T. Tsujimori, M. Aoya, T. Kawakami, K. Terada, K. physical and Geochemical Exploration, v. 12, p. 224–228.

Jia et al. 1447