Early Mesozoic thrust tectonics of the northwest Zhejiang ...sourcedb.igg.cas.cn/cn/zjrck/200907/W020101130538853859963.pdf · The marine sedimentary environment did not change until

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ABSTRACT

The NW Zhejiang region of South China occupies a key tectonic position near the suture zone of the Yangtze and Cathaysian blocks and is of critical importance for the assembly of East Asia. Sedimentological and tectonic analyses indicate that the region had a SE-dipping paleoslope in the late Paleozoic to Early Triassic. A transitional sedimentary environment from deep sea to continental molasse is documented in the early Triassic–late Triassic interval. Associated structures are NW-vergent folds and thrusts that root southeastward beneath the high-grade Chen-cai metamorphic complex. The structural styles of this foreland fold-and-thrust belt are characterized by multifold duplexes and individual folds that are together zoned from SE to NW as follows: (1) core zone character-ized by shear folds and ductile thrusts; (2) SE belt with out-of-sequence thrusting of multi-fold duplexes and an average shortening of 50%; (3) central belt with duplexes, imbri-cate fans, and an average shortening of 40%; and (4) NW belt with Jura-type folds and a shortening of ~10%. A tectonic model for the foreland fold-and-thrust belt is discussed in relation to the early Mesozoic archipelago paleogeography of South China.

Keywords: NW Zhejiang region, turbidites, deep-water basin, multifold duplexes, struc-tural styles, early Mesozoic orogeny.

INTRODUCTION

The NW Zhejiang region includes part of northwestern Zhejiang Province and neigh-boring NE Jiangxi Province, China. It is situated along the southeastern margin of the

Yangtze block; its southeastern boundary, the Jiangshan-Shaoxing fault, separates it from the Cathaysian block to the southeast (Figs. 1 and 2). The region occupies a key tectonic position near the Jiangshan-Shaoxing suture zone of the Yangtze and Cathaysian blocks, which is the boundary between Tethyan and Pacifi c domains (Lingzhi et al., 1989; Jiliang, 1993; Chen et al., 1999; Ho et al., 2003). Pre-cambrian to late Mesozoic rocks in this region (BGMRZ, 1989) are far from the major late Mesozoic to Cenozoic strike-slip faults to the southwest (e.g., Fig. 1A, the sinistral Lishui-Haifeng and Changle-Nan’ao faults; Lo et al., 1993; Xu and Zhu, 1994; Liu and Shan, 1995; Tong and Tobisch, 1996; Wang and Lu, 2000; Li et al., 2003). Thus the region provides an ideal laboratory to study not only the early Mesozoic architecture of the South China Oro-genic Belt, but also the interaction between the Tethyan and Pacifi c paleobotanical domains, and the assembly of East Asia.

Despite its geodynamic importance, the tectonic evolution of the NW Zhejiang region remains poorly understood, and the late Paleo-zoic–early Mesozoic sedimentary and structural relations have been controversial (Sheng et al., 1985; Jiliang, 1993; Chen, 1999). Hsü et al. (1988) presented an early Mesozoic, Alpine-type, collisional orogenic model for the general structure of South China, but they provided no detailed, supportive sedimentary and structural evidence. Rowley et al. (1989), Chen (1999), and Li et al. (1997) criticized the early Mesozoic collisional model, Lingzhi et al. (1989) proposed an alternative early Paleozoic orogenic model, and Goodell et al. (1991) and Pirajno and Bagas (2002) attributed the Mesozoic structures and tectonism in South China to the subduction of the Pacifi c Plate to the southeast. Chen (1999) made a detailed structural study of a similar early Mesozoic orogen in Fujian Province, which is located SE of the NW Zhejiang region.

Here, we present a new detailed structural map and comprehensive sedimentary and tec-tonic analyses of the NW Zhejiang region, and emphasize their relation to the South China oro-genic controversy concerning the nature of the early Mesozoic tectonic events. We discuss the Mesozoic geology, structural style, and defor-mation history of the region. Our data provide important new information on the history and mechanisms of deformation along the southeast-ern margin of the Yangtze block, and we demon-strate a history of large-scale crustal imbrication associated with northwestward vergence. These relations enable us to interpret the late Paleozoic to early Mesozoic tectonic evolution in the region and to discuss the accretion of the early Mesozoic South China archipelago.

REGIONAL GEOLOGY AND PREVIOUS MODELS

It is generally accepted that the early Paleo-zoic geology of the study region is character-ized by a passive continental margin (BGMRZ, 1989; Hsü et al., 1990), the development of which followed rifting in the Neoproterozoic. The Precambrian basement consists of mafi c-intermediate volcanic rocks, molasse-type sand-stone and carbonates, together with ca. 866 Ma blueschist and ca. 1000 Ma ophiolite, which are associated with the breakup of Rodinia (Chen et al., 1991; Wang and Li, 2003; Li and Li, 2003; Shu et al., 1993). Thick clastics and carbon-ates were deposited in the Cambrian, and thick turbidites in the Ordovician. The sedimentary character of the Cambrian-Ordovician sedi-ments indicates that the passive margin had a slope toward the south (Yan, 1986).

Later, the environment changed from deep-sea marine to terrestrial. Devonian terrigenous rocks unconformably overlie early Paleozoic rocks (BGMRZ, 1989; Wang et al., 2002). In the Carboniferous, >200-m limestones were

Early Mesozoic thrust tectonics of the northwest Zhejiang region (Southeast China)

Wenjiao Xiao†

State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China

Haiqing HeResearch Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China

GSA Bulletin; July/August 2005; v. 117; no. 7/8; p. 945–961; doi: 10.1130/B25417.1; 17 fi gures; 3 tables.

†E-mail: [email protected].

WENJIAO XIAO and HAIQUING HE

946 Geological Society of America Bulletin, July/August 2005

deposited on this south-tilted continental margin. In the Permian, mainly shelf clastics, both marine and terrestrial, were deposited. Finally, Lower Triassic marine sandstones and limestones (BGMRZ, 1989) were deposited. The marine sedimentary environment did not change until the Late Triassic, when large volumes of continental sediment accumulated (Table 1).

Debate on the Paleozoic and early Mesozoic geology of South China has raged among the international geological community (e.g., Xie, 1964; Huang et al., 1980; Wang, 1986; Hsü, 1981; Hsü et al., 1988, 1989, 1990; Rowley et al., 1989; Jiliang, 1993; Shi et al., 1994; Char-vet et al., 1994; Chen, 1999; Chen et al., 1991; Sun et al., 2001; Li et al., 2003). Nowadays this debate is concentrated on two tectonic models: an early Paleozoic platform without Mesozoic orogenesis (Huang et al., 1980; Wang, 1986; Charvet et al., 1994; Chen, 1999) versus an early Mesozoic collisional orogeny (Hsü, 1981; Hsü et al., 1988, 1990). The focus of the debate lies on whether there was deposition of

deep-water sediments after the early Paleozoic, and on the nature of the fold-and-thrust belt (Ren et al., 1990; BGMRZ, 1989; Wang and Shu, 2001). Recently, considerable evidence for late Paleozoic to early Mesozoic geological events has accumulated (Jiliang, 1993; Sun et al., 1991; Xu et al., 1993; Zhao et al., 1995; Xiao et al., 2001), but the sedimentary envi-ronments and thrust tectonics in this period still remain poorly studied. The NW Zhejiang region offers an opportunity to address these important problems, which we present below.

SEDIMENTARY TRANSITION AND ASSOCIATED STRUCTURES

New data reported below show that the Permian and Triassic paleogeography was characterized by a south-tilted slope along the SE continental margin of the Yangtze block. In the following section, we describe the Upper Permian and the Lower Triassic turbidites and multistage molasse sediments in the region.

Deep-Water Turbidites: Late Permian to Early Triassic

Deep-water sediments include the Late Per-mian Dalong Formation and the Early Triassic Changxing and Zhengtang Formations, which were previously interpreted to have a shallow-water origin (BGMRZ, 1989).

The Permian Dalong Formation is composed of calcareous siltstone, silty marl, and mudstone, which have Bouma sequences (Xiao, 1995) with graded layers, parallel bedding, small-scale cross-bedding, sole casts, and ammonoid print marks. According to the synthetic analysis of bioecology and sedimentary features, the Dalong Formation consists of deep-water turbid-ites (Xiao, 1995).

The Early Triassic Changxing Formation consists of black and grayish black bioclastic limestones that also have Bouma sequences (Xiao, 1995) with graded bedding, parallel bedding, small-scale cross-bedding, and sole marks, with minor bioturbation structures at the top of the formation (He, 1995). There are no

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Figure 1. (A) Schematic map of China with emphasis on distribution of Yangtze and Cathaysian blocks. (B) Schematic tectonic map of South China showing major tectonic elements along southern boundary of Yangtze block. Paleozoic-Permian mélange is mainly from Fan et al. (1996) and Chen et al. (1998). Other resources of the discovery of Paleozoic fossils are mainly from He et al. (1996, 1999, 2000), Zhao et al. (1996, 1997), and Xue et al. (1996). Figures 2 and 7 are marked.

NW ZHEJIANG THRUST TECTONICS

Geological Society of America Bulletin, July/August 2005 947

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wave- and storm-generated structures. The biota include not only shallow-water–derived fossils that are believed to have been redeposited by turbidity currents, but also deep-water fossils, such as radiolarians, ammonoids, thin-shell brachiopods, and siliceous sponge-spicules. The Changxing Formation was deposited in a deep-water environment off a southeastward-inclined carbonate ramp.

The Early Triassic Zhengtang Formation includes carbonate turbidites and clastic turbi-dites (Xiao, 1995), which are mainly located in the southeast of the region (Xiao, 1995). The upper part consists of mudstones and siltstones, and the middle and lower parts of calcareous mudstones and siltstones intercalated with limestones (Fig. 3). Isoclinal folds and thrust faults at Zhengtang, Jiangshan, indicate that the sediments were deformed by intense folding and thrusting (Fig. 3). Some of the strata have been transposed and the depositional sequence lost. The distribution of the Bouma sequence assemblage (Fig. 3) shows that the distal tur-bidites of the Zhengtang Formation (mainly C, D, and E) are located in the southeast and the proximal turbidites (mainly A, B, C, and D) in the northwest. Together with the evidence of a southeastward paleocurrent (Table 2), this spa-tial distribution of distal and proximal turbidites suggests a SE-tilted slope setting.

All in all, the above sedimentary and struc-tural characteristics indicate a SE-deepening continental margin capped by Late Permian to Early Triassic turbidites that were deformed in a NW-SE–directed compressional event.

Molasse

Molasse successions constitute the Upper Tri-assic, Lower Jurassic, and Mid-Upper Jurassic. These molasse basin sediments contain several regional angular unconformities (Figs. 4–8), forming a thick multiple-phase molasse sedi-mentary package.

The Upper Triassic rocks are locally dis-tributed along the Jiangshan-Shaoxing fault in small basins in Jiangshan, Quxian, and Wuzhao (Figs. 1 and 2). The Upper and Central parts of the Late Triassic Wuzhao Formation consist predominantly of continental deposits, including conglomerates, grits, sandstones, and mudstones with coal. However, in the lower part of the Wuzhao Formation, there are some marine lime-stones with lamellibranch, e.g., Waagenoperna sp. (He, 1995). Thus the Late Triassic stratigra-phy records an important transition from marine to continental sedimentary environments. There is a regional angular unconformity between the Upper Triassic and its underlying formations (BGMRZ, 1989; Wang et al., 2002).

A cross section of the Wuzhao Formation (T3w) was constructed at Wuzhao (Fig. 4). This Formation forms a small basin, the SE border of which is overthrust by the Chencai Complex and Paleozoic rocks, whereas at the

NW boundary the Wuzhao Formation is thrust over Upper Jurassic sediments. The main struc-tures are open folds and reverse faults (Fig. 4), most of which dip to the southeast. Gravel in the Wuzhao Formation is predominately made of

TABLE 1. STRATIGRAPHIC COLUMN OF NW ZHEJIANG REGION SHOWING FORMATION NAMES AND AGES

NW SE

Cretaceous KJurassic JTriassic

Upper Triassic Wuzao Formation (T3w)Middle Triassic South China OrogenyLower Triassic Yinkeng Formation (T1y) Zhengtang Formation (T1z)

PermianUpper Permian Changxing Formation (P2c) Dalong Formation (P2d)Upper Permian Longtan Formation (P2l) Wulinshan Formation (P2w)Lower Permian Gufeng Formation (P1g) Dingjiashan Formation (P1d)Lower Permian Qixia Formation (P1c)

CarboniferousUpper Carboniferous Chuangshan Formation (C3c)Middle Carboniferous Huanglong Formation (C2h)Lower Carboniferous C1

Devonian DPre-Devonian PreD

Note: Modifi ed after BGMRZ (1989). Italics indicate the South China collisional orogeny.

0 100m

ABCD

ABCD

E

CDE

CDE

13 12 11

10 9

7 6 5 4 3 21

111213

1

23456789

10

8

Lithologies: Lower TriassicGray and brown calcareous siltstone and calcareous mudstone intercalated withargillaceous limestone Dark thin-bedded argillaceous siltstone intercalated with limestone and marlGray thin-bedded dolomitized limestone, argillaceous limestone, and calcareous mudstone Dark mudstone, thin-bedded siltstone, marl, and calcareous mudstone, with minor limestone Yellowish green, gray, and purple siltstone interbedded with silty mudstone Grayish-green and grayish-yellow siltstone interbedded with silty mudstone Dark thin-bedded silty mudstone interbedded with thin-bedded siltstone Gray and purple argillaceous calcareous siltstone and marl intercalated with imestone Yellowish green mudstone intercalated with thin-bedded siltstone Yellow and dark purple siltstone interbedded with grayish-green and dark purple silty mudstoneYellow thin-bedded siltstone interbedded with grayish-green silty mudstoneYellowish-green and grayish silty mudstone interbedded with siltstoneYellowish-green and grayish silty mudstone interbedded with thin-bedded siltstone

NNW SSE

Figure 3. Cross section of Lower Triassic Zhengtang Formation at Zhengtang village with detailed lithologies. A through E represent divisions of Bouma sequences. See Figure 2 for location.



limestone and metamorphic rocks, indicating a provenance from high-grade metamorphic rocks of the Chencai Complex, whose detailed lithology is described in the following section. Folds and thrusts cutting the Upper Triassic

basins mainly formed after the deposition of Upper Jurassic rocks, because Upper Triassic rocks are thrust over Upper Jurassic rocks. The fact that Upper Jurassic rocks are horizontal or only slightly deformed by minor folds and faults

indicates that thrust deformation probably ter-minated shortly after the Late Jurassic, although this timing requires confi rmation. Lower to mid-Jurassic sedimentary rocks unconformably seal thrusts in Paleozoic rocks south of Changshan (Fig. 5) and NE to Machefu (Fig. 6). All the above features suggest that the sedimentary basins were shortened by a contractional tec-tonic deformation. Because the stratigraphy of the Late Triassic records an important transition from marine to continental sedimentary envi-ronments, and because there is a regional angu-lar unconformity between the Upper Triassic and its underlying formations (BGMRZ, 1989; Wang et al., 2002), we interpret the Late Triassic environment as a foreland molasse basin with high-angle thrusts and folds.

Lower-Middle Jurassic Formations are mainly composed of conglomerate, pebble-bearing sandstone, siltstone, and mudstone with interbeds of carbonaceous shale and coal; these are interpreted to be stream and lacus-trine sediments. The Lower-Upper Jurassic comprises purple sandstone, shale, tuff con-glomerate, tuff, and interbedded green sand-stone and shale. The upper part of the Upper Jurassic molasse includes conglomerate, grit, sandstone, and mudstone together with tuff, calcareous siltstone, and limestone-lenticules. The sediments were overthrust by Paleozoic rocks in the southeast and thrust over the Paleozoic in the northwest (Figs. 6 and 7), but the folding was appreciably weaker than in the Upper Triassic and Jurassic sediments.

In general, these molasse sediments were either overthrust by Paleozoic rocks in the south-east and/or they overlie unconformably, or were thrust over, Paleozoic rocks in the northwest (Figs. 6 and 7). The fact that unconformities exist between the Upper Triassic, Lower-Middle Jurassic, and Upper Jurassic, which defi nes mul-tiple-phase thrusting, suggests that the Upper Triassic–Jurassic sediments were deposited in foreland basins formed during episodic Late Triassic–Jurassic northwestward compression. In the Changshan area, SE-dipping thrusts in Paleozoic rocks were sealed by Jurassic sedi-ments (Xiao, 1995). For instance, as illustrated in Figure 5, Thrust 1 was unconformably overlain by Lower to Mid-Jurassic sediments, which indicates that Thrust 1 was only active in pre-Jurassic time. But the eastern segment of Thrust 1 was thrust over Lower to Mid-Jurassic sediments, which suggests continuous north-westward thrusting in the post–Middle Jurassic along this thrust. Thrust 2, the eastern segment of which joins the eastern segment of Thrust 1, was unconformably overlain by Upper Jurassic sedi-ments to the southwest and thrust over Lower to Mid- Jurassic sediments to the northeast,

TABLE 2. SOME PALEOCURRENT DATA FOR THE LOWER TRIASSIC IN NW ZHEJIANG

Section Locality Number Occurrence† Structure‡ Paleocurrent(°)

Dazipu Dazipu ABCDEFABCD

140°–52°

140°–48°

F

GFCFF

G

140150180190135180146150140135

Zhengtang Zhengtang ABCDEFABCDEF

150°–60°

148°–55°

GFC

FF

CCG

143152190170150140150135170140156178

Tiandun Tiandun ABCDEFGABCDEFG

136°–44°

148°–52°

GGC

GF

GC

190136142170150166140150145150160170180150

†Refers to the beds on which sedimentary structures occur.‡F—fl ute cast; G—groove cast; C—cross-bedding.

336

0 200m

Upper Jurassic continental redbed, limestone,tuff, rhyolitic porphyry, and basalt (J3)

Upper Triassic sandstone,siltstone, mudstone and coal (Wuzao Formation: T3w)

High-grade metamorphic rock (Chencai Complex)

Late Triassic foreland molasse succession

T3wJ3

J3

ChencaiComplex

unconformity

Figure 4. Cross section of Upper Triassic Wuzao Formation (T3w) in NW Zhejiang region showing Chencai high-grade metamorphic complex thrust over Upper Triassic and Jurassic foreland sediments. See Figure 2 for location.



therefore indicating that Thrust 2 was mainly active between the Middle and Late Jurassic. According to similar crosscutting relationships, Thrusts 3 and 4 were sealed by latest Jurassic sediments and were slightly younger than Thrust 2. Therefore, the thrusts show a northwestward propagation from SE to NW.

Figure 6 demonstrates that intra-Silurian thrusts are unconformably overlain by Lower to Middle Jurassic rocks, which are in return deformed at the southeastern boundary by thrust imbricates of Neoproterozoic to Cam-brian-Ordovician strata. Upper Jurassic rocks seal these thrust systems and were translated to the southeast by thrust imbricates of Neopro-terozoic to Cambrian-Ordovician rocks. These relations in the Machefu area observation indicate that multiple phase thrust tectonics was closely associated with Jurassic molasse deposition.

A systematic analysis of the sedimentologi-cal and biogeographic features of Permian and Early Triassic sediments across the NW Zhe-jiang region indicates that the paleogeography

was characterized by a roughly SE-facing con-tinental slope on which shallow marine sedi-ments were deposited generally in the north-west and deep-sea sediments in the southwest (He, 1995). Insofar as the Cambrian-Ordovi-cian paleogeography also had a slope toward the south (Yan, 1986), we envisage a long-lived SE-facing continental slope. All the character-istics enumerated above suggest that deposi-tion in the NW Zhejiang region was typifi ed by sediments on a SE-facing continental margin where the sedimentary environment evolved from a shallow sea during most of the Permian to deep water through the Early Triassic. The region then changed abruptly into an environ-ment of molasse-type basins in the late Trias-sic that continued through Jurassic time. This transition of sedimentary environment from deep-sea basins to foreland molasse basins is of great signifi cance because northwestward- propagating thrusting and folding were closely associated with it, thus providing important information to constrain the early Mesozoic tectonic architecture in the region.

STRUCTURAL STYLE ZONATION

The NW Zhejiang foreland fold-and-thrust belt is divisible into four major tectonic zones based on their diagnostic structural styles and cross-sectional positions: Core Zone, South-east Zone, Central Zone, and Northwest Zone (Table 3 and Fig. 7).

Core Zone

The Core Zone lies structurally above the sedimentary sequences of the NW Zhejiang con-tinental margin and encompasses a wide variety of fault-bound tectonic assemblages ranging in age from Late Proterozoic or Paleozoic to early Mesozoic, although superimposed by strike-slip faulting (Kong et al., 1995). The bound-ary between the Core Zone and the Southeast Zone is defi ned by a steep, SE-dipping reverse fault with similar occurrences to the Jiangshan-Shaoxing fault. These structures are cut or obliquely truncated by late sinistral transcurrent faults with a similar trend to the Jiangshan-

ChangshanChangshanChangshan

0 K m 1 0

Upper Jurassic redbed, tuff, and basalt

Upper Jurassic rhyolitic porphyry, weldedtuff, and tuffaceous conglomerateUpper Jurassic redbed, limestone,tuff, rhyolitic porphyry, and basaltLower to Mid-Jurassic sandstone,siltstone, mudstone and shaleSinian to Carboniferous stata(fold-and-thrust belt)

Unconformity

Thrust

J1-21-2

J3

J3

J1-2

J1-21-2J1-2

J3

J3J3

J3J3

J3

Anticline

118 30'18 30'118 30'118 15'18 15'118 15'

118 30'18 30'118 30'

29 00'29 00'29 00'

29 00'29 00'29 00'

28 50'28 50'28 50' 28 50'28 50'28 50'

0 Km 4

J3 J3

Unconformity B

BB

A

1

2

3

34

2 1

4

NW SE

Z-C

Z-CZ-CZ-C

Z-CZ-CZ-C

Z-CZ-CZ-C

Z-CZ-CZ-CZ-CZ-CZ-C

Z-CZ-CZ-C

Z-CZ-CZ-C Z-CZ-CZ-C Z-CZ-CZ-C Z-CZ-CZ-C

Z-CZ-CZ-C

Figure 5. Schematic geologi-cal map of Changshan area showing multiphase Jurassic foreland basins. See Figure 2 for location. Z-C—Sinian-Car-boniferous.



Shaoxing fault. The Core Zone mainly consists of metamorphic rocks of the Chencai Complex: ultramafi c rocks, greenschists, quartz schists, amphibolites, and metagraywackes, which all occur as xenoliths in granodiorites and diorites that are metamorphosed and ductilely deformed by mylonite zones. These well-foliated rocks were repeatedly overturned by folding and NW-directed ductile thrusting.

As defi ned by Kong et al. (1995), the Chen-cai Complex forms a complicated large-scale antiform with a kilometer-scale wavelength; it contains internal imbricates. There are also some low-grade metamorphic schists and slates of marine volcanic and sedimentary origin (Shi et al., 1994; Kong et al., 1995). Both metavolca-nic and metasedimentary rocks are tectonically interleaved with ultramafi c rocks and with overlying synorogenic clastic molasse sedi-ments. From their petrology and geochemistry, Shui (1986) concluded that the volcanic rocks are calc-alkaline and formed in an island arc. The ultramafi c and mafi c rocks are the main components of a tectonic mélange (Shui, 1984; Shi et al., 1994; Kong et al., 1995), the age of formation of which is controversial. The locus of displacement of the metamorphic rocks of the NW Zhejiang region is the set of ductile shear zones in the Chencai Complex. Late defor-mation, mostly sinistral strike-slip faulting, reworked the complex, forming an extensive NE-SW–trending fault system.

The above structures defi ne a fundamental structural discontinuity in the NW Zhejiang region, which marks the boundary between the Yangtze and Cathaysian blocks. Deforma-tion throughout the complex is heterogeneous and characterized by narrow shear zones that enclose lens-shaped domains of foliated mafi c and ultramafi c rocks (Shui, 1984). The Jiangshan-Shaoxing fault is between 0.5 and 7 km wide and contains an ophiolitic mélange (Piraj no et al., 1997) that includes rootless blocks of peridotite, pyroxenite, gabbro, basalt, spilite, keratophyre, and amphibolite in a matrix of sheared sericite-chlorite-phengite schist (Xu et al., 1992; Pirajno et al., 1997). K-Ar dating of muscovite from within the Jiangshan-Shao-xing fault indicates that movement occurred at 355 Ma (Xu et al., 1992; Pirajno et al., 1997). As a result of continual crustal shortening, the Chencai Complex developed a layering usu-ally with felsic metamorphic rocks at the top, mafi c and ultramafi c rocks in the middle, and sedimentary and felsic rocks at the bottom. In most cases, the rocks were intensely deformed into complicated antiformal stacks. In view of the above features we interpret the Jiangshan-Shaoxing fault as a reworked suture zone; the age of suturing is controversial. Because it is

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Upp

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Low

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rdov

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ambr

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Mid

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Low

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all

Gra

nite

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0K

m4

0K

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Sect

ion

B

Sect

ion

A29

5

316

J1-2

J1-2

J1-2

J1-2

J1-2

J1-2

J1-2

J3J3J3

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

ZaZaZa

S1

S1

S1

S1

S1

S1

S1

O3

ZaZb

ZbZbZaZa

Cam

2C

am2

Cam

2

Cam

2C

am2

Cam

3C

am3

Cam

2

Cam

3

Cam

3C

am3

Cam

3

O2

O2

O2

O2

O2

O2

S1

S1

S1

S1

S1

S1

S1

S1

S1

S1

S1

S1

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

O3

J1-2

J1-2

J1-2

J1-2

J1-2

J1-2

J3 J3J3

J3 J3J3

J1-2

J1-2

J1-2

Sect

ion

ASe

ctio

n A

Sect

ion

BSe

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Sect

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A

Sect

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BU

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ity

Unc

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Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Pz1

Faul

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ssic

fore

land

mol

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bas

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ssic

fore

land

mol

asse

bas

ins

Za Zb ZbZbZb

O1

O1

O1

O1

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O1

O1

O1

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119

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119

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E

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0'30

00'

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30 1

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15'

30 1

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30 1

5'30

15'

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important for the tectonics of South China, we discuss this problem later.

Southeast Zone

The Southeast Zone is mainly composed of Sinian to Paleozoic and Early Triassic strata. The Bailongshan Thrust defi nes the NW bound-ary of this zone (Fig. 7). Composite duplexes with outcrop-scale fold-nappe horses are one of the striking structural styles in this zone. We use the term “multiduplex” to describe this kind of composite duplex stacked by out-of-sequence thrusts (Fermor and Price, 1987; Bradley and Bradley, 1994) and by outcrop-scale fold-nappe horses. These duplexes in NW Zhejiang share more in common with natural examples than with the idealized models of Boyer and Elliott (1982) or Mitra (1986). At Shangrao and Guangfeng in NE Jiangxi province, the struc-tural style is characterized by multiduplexes with out-of-sequence thrusts (Figs. 1 and 2). In SE Tiandun, Guangfeng, a multiduplex consists of out-of-sequence thrusts in Lower Triassic limestone (Fig. 8). Fold-nappe horses are mutu-ally imbricated together with a NW vergence. NW-directed thrusts form roof and fl oor thrusts

Jiangshan-ShaoxingFault

Bailongshan ThrustLizhu-Changshan Thrust

J1-2O1 O2 O3

Cb

CbCb

Cb CbCb

O2O2 O3O3O2

OOO O

CbCbCb P2

P2

P2 C2P1 P1 P1P2 P2 P1T1T1T1 J

O1 O2O2 O3 O3 Cb

Cb

CbO3O2 O2 O2O3 Cb

duplexes

duplex

D 1 S 1S 2 K1S 1S 2D2C1Cb

CbCb

O OOO J1-2

Southeast ZoneCentral Zone

Northwest Zone

Core Zone

SE

NW

A

A'

strike-slip reworkedsuture zone

0 5Km

0 5Km

Chencai complex

Granite

340

0 20m

roof thrust

floor thrust

Upper Permian clastics Lower Triassic limestone

P2 T1

Figure 7. Cross section showing structural styles and tectonic zoning of NW Zhejiang foreland fold-and-thrust belt. A and A′ have approxi-mately the same SE-NW cross-sectional position. K1—Lower Cretaceous; J1–2—Lower-Middle Jurassic; J—Jurassic; T1—Lower Trias-sic; P1—Lower Permian; P2—Upper Permian; C2—Middle Carboniferous; D2—Middle Devonian; D1—Lower Devonian; S1—Lower Silurian; S2—Upper Silurian; O3—Upper Ordovician; O2—Middle Ordovician; O1—Lower Ordovician; Cb—Cambrian. Straws repre-sent Precambrian basement. See text for discussion and Figures 1 and 2 for location.

Figure 8. Multiduplex structures at Zhengtang, Jiangshan. See Figure 2 for location.

TABLE 3. ZONATION OF NW ZHEJIANG FORELAND FOLD-AND-THRUST BELT

Structure Northwest Zone Central Zone Southeast Zone Core Zone

Folds Broad synclines with narrow anticlines

Open folds Isoclinal folds, recumbent folds Shear folds

Faults High-angle thrusts Low-angle thrusts Multiple roof and fl oor thrusts Ductile thrusts reverse faults

Structural styles

Jura Mountain–type folds Imbricate fans, duplexes

Duplexes, multiduplexes, sandwich structures

Shortening ~10% 20%–39% 40%–50%

Vergence Main transport direction NW



in these areas. Duplexes consisting of tight folds and closely spaced thrusts characterize this zone (Figs. 9, 10, 11, and 12). In the Jiangshan area Neoproterozoic to Paleozoic rocks make up a large duplex structure (Fig. 11A, B), in which the folds are mostly tight or chevron folds (Fig. 9A), and the fold-thrust assembly dem-onstrates multiduplexing (Figs. 9B, 13A, 13B), in which both thrust stacking and folding were important.

Closely spaced tight folds and stacked thrusts have placed mainly Precambrian basement over Upper Paleozoic strata. The SE part of the Southeast Zone has many basement-involved structures in which Precambrian basement was thrust northwestward over Paleozoic rocks (Fig. 11). The total shortening is estimated to be 40%–50%, and some multiduplexes show 60% shortening (Xiao, 1995). Closely associ-ated with these thrusts are synorogenic foreland

basins containing Upper Triassic, Jurassic, and Lower Cretaceous sediments.

In the southeast of the NW Zhejiang region, sedimentary structures in Lower Triassic tur-bidites illustrate a complicated assemblage created by imbrication of many normal and inverted stratigraphic sequences (Fig. 3), and inverted by tight folding. Some large-scale inverted tight folds with northwestward-closing hinges are associated with forelimb thrusts and backlimb thrusts in Permian and Lower Trias-sic rocks. These fold-thrust package assem-blies form horses in duplexes at various scales (Fig. 7). Siltstones, sandstones, and carbonate turbidites were structurally superimposed, forming duplexes and imbricate fan structures with roof or fl oor thrusts (Figs. 10–12).

Central Zone

The Central Zone is characterized by a large-scale imbricate fan composed of Cambrian to Ordovician sediments (Fig. 7). The main difference between this zone and the above-mentioned zones is that it contains neither large-scale duplexes nor typical isoclinal folds. The major structures within this zone are large-amplitude folds, in particular open folds related to fault ramps. The Lizhu-Changshan Thrust consists of several listric thrusts with such folds. Some parts of the leading edge of the thrust sheet are characterized by multiple imbricate faults; others are characterized by a single major thrust. The thrusts mainly juxtapose lower Paleozoic strata, but they also involve lower to Middle Jurassic molasse basin sediments.

In the center of the NW Zhejiang region, lower Paleozoic and Permian sediments form imbricate fans with a few hinterland-dipping duplexes. NE to Jiangshan Permian rocks form large-scale imbricate fans, and Upper Permian siltstones and Lower Permian limestones were deformed into various thrust sheets. Most of the folds are fault related, such as fault-bend types, and others are cylindrical. The interlimb angles of these folds are much larger than those in horses of the multiduplexes in the southeast of the study region.

Northwest Zone

The Northwest Zone is made up of Neopro-terozoic to Mid-Devonian passive continental margin sediments (Fig. 7). The Lizhu-Chang-shan Thrust defi nes the southeastern boundary of the zone (Fig. 2).

The typical structures are box folds similar to Jura-type folds (Fig. 7 and Table 3); broad synclines separated by narrow anticlines with scattered thrusts are predominant. In Meishan,

A

B

Roof thrust

Floor thrust

0cmcm

8080

0cmcm

2020

NWNW SESENW SE

NWNW SESENW SE

Figure 9. Photographs of duplexes in Tiandun. (A) and (B) both look NE. Hammer for scale is ~40 cm long and is marked by a box in Figure 10A. See Figure 2 for location.



Changxing, in northern Zhejiang, imbricate structures are common (Fig. 13), but some folds are not typical Jura type because they have a complex fold-thrust style (Fig. 14).

The average shortening in this zone is ~10%, which is a minimum because only line balanc-ing was possible (Xiao, 1995). The relatively simple structural style and low degree of short-ening that characterize the Northwest Zone suggest that it most likely behaved as a foreland during the Early Triassic compression.

TRANSPORT DIRECTION

Zoning from the internal core to the external foreland is the principal response to the tectonic vergence (e.g., Lowell, 1985). The relative dispo-sition of the major thrust styles of the fold-and-thrust belt is shown in Figure 7. As mentioned above, their outcrop arrangement is zoned and represents a decreasing degree of deformation across the region. The local transport of each thrust and individual thrust sheet or horse is

demonstrated by the geometric arrangement of minor structures with northwestward vergence (Figs. 7 and 15). The general pattern of displace-ment of the major thrusts shows a progressive transportation toward the foreland of the Yangtze block. The decrease in deformation intensity from the SE to the NW is also consistent with the fold-style variation from isoclinal folds in the SE (e.g., Fig. 8) to open folds in the NW (Figs. 2 and 7). A Coulomb wedge model is compatible with a tectonic transport direction from SE to NW

28 40'

28 50' N

28 40'

28 50'

118 40' 118 50'

118 40' E 118 50'

Stratigraphic boundary

Main thrustReverse fault

Proposed thrust

Upper +Mid-Carboniferous (C2+3)

Upper Carboniferous (C3)

Lower Carboniferous (C1)

Upper Permian (P2)

Lower Permian (P1)

Lower Triassic (T1)

Upper Jurassic (J3)

Upper Cretaceous to Cenozoic

Lower-Mid-Jurassic (J1-2)

Upper Ordovician (O3)

Lower to Mid-Ordovician(O1-2)

Upper Cambrian (Cam3)

Mid-Cambrian (Cam2)

Lower Cambrian (Cam1)

Cambrian (Cam)

Neoproterozoic

Granite

Chencai Complex

Ordovician (O)

Za

ZaZaZa

ZaZaZa

ZbZbZb

ZbZbZb

ZbZbZb

ZaZaZa

T1

P2

P2P2P2

P2C1

OO

C1

C2+3C2+3C2+3

Cam2Cam2Cam2

CamCamCam

CamCamCam

P1

P1

P1

P1C3

O3O3O3

O3O3O3

O3O3O3

O3O3O3

O3O3O3

O3O3O3

O3O3O3

O3O3O3

O1-2O1-2O1-2

O1-2O1-2O1-2

J1-2

T1T1T1

J3J3J3

Zb

0 5Km

Jiangshan-Shaoxing Fault S

ystem (JS-F)

Jiangshan-Shaoxing Fault S

ystem (JS-F)

B

B'B'

B' CB

C

D

D'D'

C'C'

JiangshanJiangshanJiangshan

P2P2P2

ChencaiComplex

P2P2P2P2P2P2

P1T1T1T1

T1

ZaZaZaZaZaZa

ZaZaZa

Cam1Cam1Cam1

J3C3C3C3

C1O

O1-2

J1-2ZbZbZbZbZbZb

Jiangshan duplex

P2P2P2 P2P2P2

O3O3O3O3O3O3

0 5Km

A

B

Core Zone

Southeast Zone

Centra

l Zon

e

BLS-

TBL

S-T

BLS-

T

BLS-T

JS-F

Core Zone

Southeast Zone

Central Zone

C' D D'

Figure 10. (A) Geological map of Jiangshan region in NW Zhejiang area showing duplexes. See Figure 2 for location. (B) Cross section in (A).



(Xiao, 1995). Northwestward fold-and-thrust nappes have been interpreted to have formed during impingement and southeastward under-plating of the SE passive continental margin of the Yangtze block beneath the Cathaysian block in Early Triassic time (e.g., Hsü, 1981; Hsü et al., 1988, 1989, 1990; Sun et al., 1991; Jiliang, 1993; Xiao et al., 2001).

The SE tectonic zones differ markedly from the widely accepted thin-skinned fold-and-thrust belt model in several important respects: (1) the absence of a stratigraphically controlled,

continuous décollement horizon; (2) the domi-nant thrusts below individual nappes of the overthrust complex do not cut down to a basal thrust; and (3) a basement-involved structure formed in the hinterland and out-of-sequence thrusts usually in the SE tectonic zones, in par-ticular during the late stage of shortening.

Our preliminary tectonic zonation based on an analysis of structural styles is broadly similar to that in the standard model of fold-and-thrust belts of Lowell (1985). Because of the presence of basement-involved structures, there might

be a Piedmont Zone in the fold-and-thrust belt, similar to that in the Appalachians (Solar and Brown, 2001). In summary, the zonation indicates that deformation decreased from the southeast to the northwest as a result of continu-ous northwestward shortening.

AGE OF CONTRACTION

The timing of the NW-vergent thrusting in the NW Zhejiang region can be constrained by stratigraphy and crosscutting relations. An earlier important event during the northwest-ward thrusting can be bracketed by the earliest molasse (T3) and the latest marine sediments (T1) involved in the foreland fold-thrust defor-mation. The folds and thrusts are unconformably overlain by Late Triassic molasse sediments as seen in the Hengshan and Shangrao areas (Fan et al., 1999; see Fig. 2 for location). This can be used to constrain an associated NW-vergent thrusting and folding that started or mainly occurred during the Middle Triassic. Moreover, the synorogenic character of the Upper Trias-sic deposits suggests that these compressional structures were active from the Late Triassic. The fact that Paleozoic to Early Triassic rocks are involved in the folds and thrusts shows that the deformation event may have started in the Jurassic. The Jurassic sediments unconformably overlie Triassic to Paleozoic sediments; angular unconformities are visible both at map and out-crop scales.

The fold-and-thrust belt structures are sealed by Cretaceous sediments. Deposits of Late Tri-assic age (the Wuzhao Formation) rest uncon-formably on top of some shortening structures (BGMRZ, 1989; Fan et al., 1999); Jurassic and Cretaceous sediments also rest unconformably on top of Upper Triassic sediments and on the contractional structures, and they are thrust by Upper Triassic thrust structures along their southeastern border. Because the multiple molasse sediments are mainly of Upper Trias-sic to Jurassic age, and because they are closely associated with NW-directed thrusts, the asso-ciated NW-vergent thrusting and folding may have continued to the Jurassic and even the Early Cretaceous.

TECTONIC EVOLUTION

Lower Paleozoic to Permian-Triassic Mélange

Hsü et al. (1989) proposed an early Mesozoic Alpine-type orogeny for South China. This has led to much controversy. The main controversy has been the existence of a late Paleozoic–early Mesozoic mélange. Although Hsü et al. (1989,

B

A

0m

2

0m

2

NWNW SESENW SE

NWNW SESENW SE

Figure 11. Photographs of imbricate fan structures and duplexes in Tiandun. (A) and (B) both look NE. Width of view of (A) is 10 m, and cliff in (B) is ~10 m high. See Figure 2 for location.



1990) suggested an early Mesozoic orogeny in South China, they did not present any evidence for the associated mélange or ophiolite, and therefore the early Mesozoic orogeny model encountered severe opposition (Rowley et al., 1989; Rodgers, 1989; Hsü et al., 1990; Li et al., 1997). Shu et al. (1993) discovered blueschists and reported a K-Ar age of 866 ± 14 Ma for the high-pressure metamorphism. However, K-Ar age dating on metamorphic basalts rang from 398 Ma, 380 Ma, 375 Ma, 295 Ma, and 292 Ma to 255 Ma (Zhao et al., 1996). 40Ar-39Ar dating for gabbroic blocks in a serpentinite matrix has yielded 266 ± 5 Ma and 232 ± 5 Ma (Zhao et al., 1997; He et al., 1999). Large-scale 1:50,000 mapping projects and associated investigations were launched in South China in the late 1990s, as a result of which Paleozoic fossils were discovered along the NE Jiangxi and Jiang-shan-Shaoxing faults (Fig. 1). Zhao et al. (1995, 1996) fi rst reported Carboniferous–Late Perm-ian radiolaria in formerly defi ned Precambrian rocks at Zhangshudun in Yiyang (see Fig. 1 for locality). They also reported late Paleozoic radi-olaria from Zhangshudun northeastward of Dex-ing, in a zone that is connected to the northeast with the ophiolitic mélange in Shexian where Paleozoic to Permian fossils were found (He et al., 1996; Chen et al., 1998, 1999). Wang et al. (1995) discovered Late Permian radiolaria, such as Pseudoalbaillella sp., Entactinia sp., and Latentifi stula sp. in a mélange near Dengshan, NE of Yiyang, close to the same locality where Zhao et al. (1996) reported Upper Paleozoic to Permian radiolaria in cherts within ophiolitic ultramafi c rocks, and from a geochemical study, Liao et al. (1998, 1999) showed that blocks of Paleozoic volcanic rocks have an island arc-backarc signature. Near Hengfeng, ~20 km east of Yiyang, Paleozoic fossils were found in so-called Precambrian rocks (Xue et al., 1996).

Although no late Paleozoic fossils have been reported in the ophiolitic slices along the Jiang-shan-Shaoxing fault, isotopic age dating has demonstrated the presence of Late Precambrian, Caledonian, Hercynian, and Indosinian rocks (Shu et al., 1993; Kong et al., 1995; Shu and Charvet, 1996; Li et al., 1997). Zhang et al. (1984) proposed that the Jiangshan-Shaoxing ophiolitic mélange zone formed in a subduc-tion zone during early Paleozoic time. However, from a locality south of Zhuji (Fig. 1B), Kong et al. (1995) reported four groups of zircon dates from the components in a mélange, which include some Precambrian and Paleozoic ages and a 242 ± 2 Ma U-Pb zircon age for a gneissic granodiorite that has an island arc geochemical affi nity. This implies that there may have been an island arc as young as Early Triassic along the Jiangshan-Shaoxing fault (Fig. 1), and this is

consistent with the main Permian–early Meso-zoic island arc in SE China, as suggested by Faure et al. (1996, p. 102). The variety of ages, if correct, might indicate the presence of a tectonic mélange. There is an urgent need for systematic zircon dating of these ophiolitic mélanges along the Jiangshan-Shaoxing fault. Although some

Precambrian rocks (Chen et al., 1991; Li et al., 1997; Li and Li, 2003) have been identifi ed in the mélanges, the youngest radiolarian fossil of Late Permian age and the Early Triassic island arc fragment with a zircon age of 242 Ma provide important time constraints for the fi nal formation age of the ophiolitic mélange.

NWNWSESE NWSE

Drag foldDrag foldAnticlinal axial traceAnticlinal axial trace

Drag foldAnticlinal axial trace

Drag foldDrag foldAnticlinal axial traceAnticlinal axial trace

Drag foldAnticlinal axial trace

0cmcm

4040

NWNWSESE NWSE

Drag foldDrag foldAnticlinal axial traceAnticlinal axial traceDrag foldAnticlinal axial trace

0cm

40

Figure 12. Photographs of imbricate fan structures in Tiandun. Look southwest. Hammer for scale is ~40 cm long. See Figure 2 for location.

Figure 13. Photographs of imbricate fan structures in Meishan of Changxing. View to south-west. Hammer for scale is ~40 cm long.



All these factors point to a continuous, Y-shaped Paleozoic-Permian ophiolitic mélange zone in SE China, which may have been cre-ated by thrusting and/or strike-slip faulting (Fig. 1B). Because some components of the ophiolitic mélanges have a back-arc geochemi-cal signature (Zhao et al., 1997; He et al., 1999; Liao et al., 1998, 1999), because some ophio-lites have Precambrian isotopic ages (Chen et al., 1991; Li et al., 1997; Li and Li, 2003), and

because the blueschists have an isotopic age of 866 Ma or a little younger (Shu et al., 1993), we propose that the mélange zones are remnants of marginal basins situated behind the Cathaysian arc that belonged to the early Mesozoic South China archipelago, similar to the present-day SW Pacifi c (Hall, 2002). The northwestward distribution of the Mesozoic foreland fold-and-thrust belt, the mélange and the arc in the NW Zhejiang region, as described in this study, may

demonstrate an early Mesozoic retroarc contrac-tion event in SE China. The early Mesozoic con-traction event took place in a backarc region.

Investigations of tectonostratigraphy (Goodell et al., 1991) and mineral deposits support a retroarc contraction event (Pirajno et al., 1997; Pirajno and Bagas, 2002). The extensive belt of granitic rocks in SE China has been interpreted as a continental margin magmatic arc (Jahn et al., 1976; Zhou and Li, 2000). The magmatic arc rocks of the Cathaysian block, structurally above the ophiolitic mélange, occupy a very limited area. The absence of preserved mag-matic arcs of appropriate age in an analogous setting, e.g., in the Coastal Belt of Newfound-land, can be explained by transform faulting, thrusting, or rifting after contraction (Mitchell and Garson, 1981).

Thrusting Phases

The relative spatial and temporal relation-ships between folds and faults in the fold-and-thrust belt indicate a two-stage history of the early Mesozoic shortening: an early stage of Alpine-style folding and thin-skinned thrust-ing above a local basal décollement, as shown in the Bailongshan Thrust décollement of Fig-ure 7, followed by a second stage of high-angle reverse faulting and out-of-sequence thrusting, which may have been related to the northwest-ward telescoping of the Chencai Complex and the accretion of the Cathaysian block to the Yangtze block (e.g., Jiliang, 1993; Hsü et al., 1990).

Early-stage shortening is characterized by low-angle thrusts and related folds. These struc-tures record the early thin-skinned history of deformation. Shortening associated with these structures migrated northwestward through the Southeast Zone into the foreland to the northwest. The NW-verging folds and related structures in the NW Zhejiang region developed during this early stage of thin-skinned thrust-ing. They are interpreted to have formed either as buckle folds in front of an advancing thrust belt or as fault propagation folds above a basal décollement (the Bailongshan décollement, Fig. 7). Near the Jiangshan-Shaoxing fault at Zhengtang in Jiangshan County of Zhejiang province, these early folds make up horses in duplexes with back-limb thrusts, suggesting top-to-the-NW displacement.

Late-stage shortening in the Southeast and Core zones in the NW Zhejiang region is recorded by thrusts that imbricate the meta-morphic Chencai Complex in the foreland belt and by out-of-sequence high-angle reverse faults that cut across folded thrusts in the Cen-tral zone and the Chencai Complex. This late-

0m

2

NWNW SESENW SE

Figure 14. Photographs of fold-and-thrust structures in Meishan of Changxing, looking NE. Cliff face is ~15 m high. See Figure 2 for location. Arrow indicates younging direction.

N

S-type foldZ-type fold

n = 33

N

thrust planen = 27

A BFigure 15. Lower hemisphere stereogram of fold axes (A) and thrust planes (B) in pres-ent geographic orientation. For S- and Z-type folds, see Hansen (1971) and Cowan and Brandon (1994).



stage shortening gave rise to folds with larger interlimb angles than those created in the early stage of thrusting and that were subsequently tightened. These tightened folds turned into overthrust fold-nappes and can be recognized in parts of the complicated antiformal stacks.

Late-stage thrusts truncate the folded early thrusts along the Jiangshan-Shaoxing fault and are therefore out-of-sequence thrusts with respect to the main foreland fold-and-thrust belt. These faults root southeastward beneath the Chencai Complex and are cut by postkine-matic intrusions of the late Mesozoic plutonic-volcanic suite in SE China. The development of thick-skinned, out-of-sequence thrusts and of basement-involved structures in the hinter-land signals a southeastward retromigration of the deformation front during the continuing northwestward vergence in a late stage of the shortening. This shift in the locus of thrusting resulted in northwestward telescoping and it may account for the complex stacking order and thermal history (Jiliang, 1993; Hu et al., 2000), which in turn means that horizontal compres-sion lasted to a late stage.

Tectonic Evolution

Our integrated observations lead to the follow-ing conclusions concerning the geological history of the NW Zhejiang fold-and-thrust belt.

In the late Paleozoic, South China was an archipelago in which the southern continental margin of the Yangtze block and the Lower Yangtze subblock were separated from the Cathaysian block to the southeast with the Jiangshan-Shaoxing and Yiyang-Shexian deep marginal seas located between them (Figs. 16 and 17). The term “Lower Yangtze subblock” is introduced because this shows a striking latitudinal difference in the late Paleozoic on the basis of paleomagnetic, paloegeographic, and stratigraphic studies (Chen et al., 1993; Shi et al., 1994; Yin et al., 1999). Late Permian to Early Triassic turbidites were deposited on the southern slope of the Lower Yangtze subblock. As the Cathaysian block gradually approached the Yangtze block and the Lower Yangtze sub-block to the NW in the Middle Triassic, the Cathaysian block collided with the former blocks, terminating the intervening Jiangshan-Shaoxing and Yiyang-Shexian deep marginal seas (Fig. 16B).

Within this convergent tectonic setting the SE continental margins of the Yangtze block and the Lower Yangtze subblock were trans-formed to retroarc foreland basins, and the prism of continental margin sediments was deformed into foreland fold-and-thrust belts. Through the Late Triassic and into the Jurassic,

intense northwestward thrusting produced the structural styles that vary from multidu-plex in the southeast to the fold zone in the northwest. Meanwhile, the Upper Triassic Wuzhao Formation and Jurassic and Lower Cretaceous molasse sediments were deposited

on the foreland fold-and-thrust belt and were weakly deformed. Later (probably in the Early Cretaceous) the region underwent postorogenic transcurrent deformation, as by then it was an integral part of South China (Jiliang, 1993; Schmid et al., 1999; Okada, 1999).

Equator

10 S

?

South China Archipelago

NW ZhejiangPz2-T1continental slope

Yangtze block

Yangtze block

Lower Yangtze

Lower Yangtze

sub-blocksub-blockLower Yangtze

sub-block

Jiangshan-Shaoxing deep sea

Yiya

ng-S

hexi

an

deep

sea

Cat

hays

ian

bloc

k

Cat

hays

ian

bloc

k

Sub

duct

ion

zone

Cross-sectionsin Fig. 16

SE

A Late Permian — Early Triassic

NW

Cathaysian block

Cathaysian block

Jiangshan-Shaoxingdeep sea

Jiangshan-Shaoxingmelange zone

Lower-Yangtze block

Lower-Yangtze block

Yangtze block

Yangtze block

Continentalslope

SE

B Middle Triassic — Late Jurassic

NW

? ?

Yiyang-Shexiandeep sea

Yiyang-Shexianmelange zone

Figure 17. Late Paleozoic to early Mesozoic South China archipelago in which NW Zhejiang region is represented by SE continental margin of Lower Yangtze block and Cathaysian block separated by Jiangshan-Shaoxing ocean. Lower Yangtze subblock refers to southeastern part of Yangtze block that is possibly separated small block near Yangtze block (modifi ed after Met-calfe, 1996; Yin et al., 1999; Xiao et al., 2001). Arrow in Yangtze block refers to present north.

Figure 16. Schematic sequential cross sections showing tectonic evolution of NW Zhejiang foreland fold-thrust belt, SE China. (A) Late Permian to Early Triassic. (B) Middle Triassic to Late Jurassic. See text for discussion.



DISCUSSION

Scale of the Early Mesozoic Extension in South China

Faure et al. (1996) and Lin et al. (2000, 2001) documented important extensional events of early Mesozoic age in South China. On the basis of K-Ar and Ar-Ar dating of muscovite and biotite and structural analysis from the Wugongshan Dome, Faure et al. (1996) con-cluded that there were two stages of extensional tectonics: Late Triassic and middle Cretaceous. Although we note that in their description there are “up-dip” kinematics of possible 225–235-Ma age along the northern limb of the Wugongshan Dome, which may mean thrusting in the Late Triassic, we agree with the general two-stage extensional scenario in the Wugongshan area. However, although we agree with the extensional tectonics associated with some domal structures, we do not see that there is enough evidence to extrapolate similar extensional events to the whole Yangtze block or to the whole of SE China. The main extensional domes mainly occur along the foreland of the Yangtze block, which is northwest or west of the NW Zhejiang fold-and-thrust belt (Fig. 1B).

Lin et al. (2001) proposed that domal structures such as Jiulingshan, Wugongshan, or Lushan are characterized by conspicuous extensional features that appear to be the result of continental convergence between the North and South China blocks. These kinds of domal structures with postcollisional or syncollisional signatures are common in ancient orogenic belts, for example the Kangmar dome and simi-lar domes along the India foreland in the Hima-layas (Burg et al., 1984; Schärer et al., 1986). Even if there were large-scale extension in Late Triassic time as suggested by Faure et al. (1996) and Lin et al. (2000), we still could not rule out that there was simultaneous contraction, which is well documented in the NW Zhejiang region. On the other hand, syncompressional extension is also not uncommon in orogenic belts, such as the South Tibetan Detachment System in the Himalayas (Grasemann et al., 1999; Robinson et al., 2001; Ding et al., 2001), the complex kinematic history involving both crustal short-ening and extension within the internal zones of the Alpine Orogen (Wheeler et al., 2001; Reddy et al., 2003), and the recently documented fact that the exhumation of metamorphic rocks in the core of the Alpine orogen was contempo-raneous with thrusting in the foreland (Reddy et al., 1999). Large-scale extension of South China was mainly in the Cretaceous (Wang et al., 1998; Lin et al., 2000, 2001; Faure et al., 1996; Zhou and Li, 2000; Jiliang, 1993; Xiao,

1995; Xiao et al., 2001). On the basis of these arguments, we suggest that the early Mesozoic thrust tectonics of the NW Zhejiang region has widespread signifi cance in South China.

Nature of the Mesozoic Tectonics in South China

On the basis of formerly defi ned late Precam-brian ophiolite rocks in South China, Faure et al. (1996) stated that there was no separation between the Yangtze and the Cathaysian blocks. But this idea is inconsistent with the interven-ing Paleozoic to Permian mélange zone, as discussed in the section above. We argue here that the Mid- to Late Triassic, or even the Early Jurassic tectonic events in NW Zhejiang were related to the amalgamation of the Cathaysian block (SE China block) and the Yangtze block (Fig. 17). This is in good agreement with paleomagnetic (Chen et al., 1992; Dobson et al., 1999) and petrological and tectonic (Zhong et al., 1998; Ma, 1998) data from South China, which all indicate there was tectonic separation between the Yangtze and Cathaysian blocks in the Early Triassic.

Paleomagnetic investigations in South China have shown that there has been no relative movement between the eastern (Cathaysian) and western subblocks of the South China blocks since the Cretaceous (Seguin and Zhai, 1992; Zhai et al., 1992; Gilder et al., 1993; Morinaga and Liu, 2004). However, Chen et al. (1993) systematically studied the different subblocks of South China and found four con-tinental fragments based on the geological and paleomagnetic evidence—the Yangtze, Xiang-gui, Cathaysia, and Donanya—with scattered paleopole positions and an apparent latitudinal discrepancy. This is important evidence for the archipelago paleogeography in the early Meso-zoic, and it is in good agreement with the paleo-magnetic work of Zhai et al. (1992), who pro-posed that, on the basis of paleomagnetic data, there was a deep sea between the Yangtze block and the Cathaysian block. Dobson and Heller (1992) discussed a postfolding Cretaceous remagnetization and a Jurassic paleomagnetic overprint in South China that may be associated with an important tectonic event. This was prob-ably related to the retroarc contraction event that is the main reason for the foreland fold-thrust deformation in the NW Zhejiang region as discussed in this paper. Gilder et al. (1995) preferred post-Triassic strike-slip faulting with considerable displacement and counterclock-wise rotation to explain the amalgamation of the Cathaysian block to the Yangtze block, a situa-tion very similar to the transcurrent or displaced terranes in western America. This paleomag-

netic evidence all points to an active southern margin along the South China blocks as shown in Figure 17, although the exact position of the magmatic front is not yet known.

Jahn et al. (1976) pointed out that the exten-sive belt of granitic rocks in SE China represents a continental margin magmatic arc within which a granitic belt of Mesozoic and possibly Paleo-zoic age was emplaced above a northwestward-dipping subduction zone. Faure et al. (1996) pro-posed a back-arc environment for the Permian to Cenozoic volcanic rocks associated with a similar northwestward-subduction zone in SE China. If the Cathaysian block was a magmatic arc related to northwestward subduction of the Pacifi c plate in the late Paleozoic and the Early Triassic, the mélange zone along the Jiangshan-Shaoxing fault could have been a marginal (back-arc) basin and the NW Zhejiang fold-and thrust belt, a backarc thrust belt. Mitchell and Garson (1981) pointed out that the 700-km width of the granitic belt in SE China and the presence to the west of eastward-dipping thrusts suggest that plutons in the western part of the belt, most of which are associated with tungsten and antimony min-eralization, may have been related to back-arc thrusting and folding. We agree with Zhou and Li (2000) that late Mesozoic and early Cenozoic volcanic rocks in SE China mainly indicate a backarc origin associated with a northwestward subduction of the Pacifi c plate. The posttectonic thrusting, particular in the Late Jurassic, partially resulted from back-arc thrusting associated with the subduction of the Pacifi c-Kula ridge SE of the South China continental margin (Goodell et al., 1991).

An alternative proposal for the early stage of this is that, instead of a backarc setting, all the thrust tectonics were related to the Triassic collision between the North China and Yangtze blocks and the subsequent exhumation of ultra-high-pressure rocks to the north (Faure et al., 1998; Schmid et al., 1999; Yan et al., 2003). The similar age of deformation and the similar regional strike of the main thrusts and folds of the Dabieshan foreland and the NW Zhejiang thrust belt seem to support this model. However, they are not consistent with the northwestward tectonic vergence in the eastern Yangtze fore-land fold-and-thrust belt as reported in this paper. We tentatively propose that these blocks, together with those in Indochina, may have constituted an archipelago in the late Paleozoic and early Mesozoic (Yin et al., 1999; Xiao et al., 2001). The combined effect resulting from the squeezing of these various terrenes and their intervening basins mainly in the early Mesozoic (including the collision between the North China and Yangtze blocks), and the N- or NW-dipping subduction of the Pacifi c plate in



the late Mesozoic and Cenozoic, should have contributed to the formation of the NW Zheji-ang thrust belt and thus to the tectonic evolution of this part of South China.

ACKNOWLEDGMENTS

The work presented here began while we were Ph.D. students under the supervision of S. Sun and J.L. Li. We thank them for sound scientifi c advice and numerous discussions over the years. W. Lin and L.S. Shu kindly provided some key papers on South China geology. We acknowledge K. Burke and K.J. Hsü for their revealing comments and suggestions on an early draft. Critical reviews and suggestions from B.F. Windley, M. Faure, J. Walker, and P. Copeland sig-nifi cantly improved the fi nal presentation. The study was fi nancially supported by the Chinese Academy of Sciences (KZCX2-SW-119), the Chinese MOST (2001CB409801), and the Chinese NSF (40172080). This is a contribution to IGCP 411.

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