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 He
Research Institute of Petroleum Exploration and Development, CNPC, Beijing 100083, China
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 Chencai 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 characterized by shear folds and ductile thrusts; (2) SE
belt with out-of-sequence thrusting of multifold duplexes and an average shortening of
50%; (3) central belt with duplexes, imbricate 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, structural styles, early Mesozoic orogeny.
INTRODUCTION
The NW Zhejiang region includes part of
northwestern Zhejiang Province and neighboring NE Jiangxi Province, China. It is
situated along the southeastern margin of the
†
E-mail: [email protected].
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
Pacific domains (Lingzhi et al., 1989; Jiliang,
1993; Chen et al., 1999; Ho et al., 2003). Precambrian 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 LishuiHaifeng 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 Orogenic Belt, but also the interaction between the
Tethyan and Pacific 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 Paleozoic–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, Alpinetype, 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 Pacific 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 tectonic analyses of the NW Zhejiang region, and
emphasize their relation to the South China orogenic controversy concerning the nature of the
early Mesozoic tectonic events. We discuss the
Mesozoic geology, structural style, and deformation history of the region. Our data provide
important new information on the history and
mechanisms of deformation along the southeastern margin of the Yangtze block, and we demonstrate 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 Paleozoic geology of the study region is characterized 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 maficintermediate volcanic rocks, molasse-type sandstone 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 carbonates were deposited in the Cambrian, and thick
turbidites in the Ordovician. The sedimentary
character of the Cambrian-Ordovician sediments indicates that the passive margin had a
slope toward the south (Yan, 1986).
Later, the environment changed from deepsea marine to terrestrial. Devonian terrigenous
rocks unconformably overlie early Paleozoic
rocks (BGMRZ, 1989; Wang et al., 2002). In
the Carboniferous, >200-m limestones were
GSA Bulletin; July/August 2005; v. 117; no. 7/8; p. 945–961; doi: 10.1130/B25417.1; 17 figures; 3 tables.
For permission to copy, contact [email protected]
© 2005 Geological Society of America
945
WENJIAO XIAO and HAIQUING HE
118 00'E
31 00'N
120 00'E
Mainly Cretaceous ductile detachment fault
Mainly Cretaceous brittle normal fault
Fig. 1B
ng
Ya
LishuiHaifeng
Fault
tze
Major thrust
fbol
orec
lka
nd
Lantian tectonic window
(Chen et al., 1998,
1999)
Jingdezhe
Nanchang
Zhuji
Fig. 7
NE
Boyang
Dexing
n Fault
gs
ian
J
Jiangshan
Zhangshudun
Yifeng
ngx
NE Jia
Wugongshan detachment system
(Faure et al.,1996)
i Fault
0
116 00'E
n
ysia
a
Cath
km
han
-Sh
in
aox
gF
aul
t
29 00'N
242 + 2 Ma (U-Pb)
island arc gneissic granodiorite
(Kong et al., 1995)
Fig. 2
Dengshan
Yiyang Hengfeng Shangrao
28 00'N
n
Wugongsha
Shexian
ng
xi
han
Lus
29 00'N
n
g sha
Jiulin
Ya
ng
Jia
tz
e
Lushan detachment system
(Lin et al., 2000)
Julingshan detachment system
(Lin et al., 2001)
Hangzhou
Shaoxing
Shaoxing
Lantian
Anticline
sia
ay
h
t
Changle-Nan'ao
Ca
Fault
A
Hangzhou
Strike-slip fault
Fa
ul
t
CHINA
Locality of Late Paleozoic radiolaria
Tectonic window of Cambrian-Ordovician
volcanocalstics and turbidites
Mainly metamorphic domains
k
bloc
Lower Paleozoic-Permian
ophiolitic melange
60
118 00'E
B
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.
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; Charvet 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
946
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 environments 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 Permian Dalong Formation and the Early Triassic
Changxing and Zhengtang Formations, which
were previously interpreted to have a shallowwater 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, smallscale 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 turbidites (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
Geological Society of America Bulletin, July/August 2005
Upper Permian
P2
T1
Fig. 5
Geological Society of America Bulletin, July/August 2005
D3
O
Z
J S
C
Z
Z
Z
Jiangshan
C-P
O1+2
Changshan
Z-C
S
C
O3
S
O
Pt
O
O
Z
C
J
C-P
O
O3
S
Z
Z
K
Z
J
JS-T
C
D3-C
S
S
J
O3
C-P
D
O3 S1+2
O3
O1+2
C1+2
S3
S
O3
O1+2
C3
O3
Fig. 4
J
C
120 E
S
S
O3
O1+2
Ophiolitic melange
Late Jurassic granitic rock
Proterozoic
Neoproterozoic
Neoprot.-Cambrian
JS-T: Jiangshan-Shaoxing Thrust
29 N
Z
C-P
C-P
S
S D3-C
J
J
C3
Pt
Z
Z-C
J
Z
C
J
C
T3
C
O3
O
O3 S
O1+2
J
O3
S
O
S
S
Z
C
S
J
J
S1+2
S3
C-P
31 N
Z
C
D
D3
S
km
30 N
Shaoxing
E
N
30
C-P
D3
Meishan
C-P
Shaoxing
0
S
W
C
C-P
S
T1
Fig. 14
C-P
S3
Hangzhou
C
S1+2
S
J
C
O
Z
C
Z
C
O3
Z
O
Fig. 6
C
C1+2
C3
119 E
Figure 2. Geological map of NW Zhejiang region outlining distribution of structural styles (modified after BGMRZ, 1989; Wang et al., 2002; Wu et al., 2002). Positions of
Figures 3–12 and 14, and localities mentioned in text are marked. L—Lower.
BLS-T: Bailongshan Thrust
119 E
Major thrust or fault
O
C3
Z
S
O3
J Z
O3
S
J
D3
C-P
D3
S
C
C1+2
Cambrian
Lower to Mid-Cambrian
Upper Cambrian
Ordovician
L. to Mid-Ordovician
Upper Ordovician
Quzhou
Z
C
Fig. 7
P2
C
C
T1
C
Z
O
O3
O
O1+2
O
Pt
C
C1+2
C3
O
O1+2
O3
LC-T: Lizhu-Changshan Thrust
Fig. 3 Fig
.1
0
K
C-P
O
D3-C
O3
Z
Z
Pt
Silurian
L. to Mid-Silurian
Upper Silurian
Devonian
Upper Devonian
Upper Dev. to Carb.
Thrust
Figs. 8,9,11,12
P2
LC-T
Guangfeng
T1
BLS-T
K
118 E
Z-C
C1+2
Z-C
Z-C
S
S1+2
Lower Triassic
T1
Carb. to Permian
D
S3
Upper Triassic
T3
C-P
D3
Jurassic
J
D3-C
Cretaceous
K
NW ZHEJIANG THRUST TECTONICS
947
WENJIAO XIAO and HAIQUING HE
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 deepwater environment off a southeastward-inclined
carbonate ramp.
The Early Triassic Zhengtang Formation
includes carbonate turbidites and clastic turbidites (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 turbidites 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 spatial distribution of distal and proximal turbidites
suggests a SE-tilted slope setting.
All in all, the above sedimentary and structural 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.
NW
Cretaceous
Jurassic
Triassic
Upper Triassic
Middle Triassic
Lower Triassic
Permian
Upper Permian
Upper Permian
Lower Permian
Lower Permian
Carboniferous
Upper Carboniferous
Middle Carboniferous
Lower Carboniferous
Devonian
Pre-Devonian
Molasse successions constitute the Upper Triassic, Lower Jurassic, and Mid-Upper Jurassic.
These molasse basin sediments contain several
regional angular unconformities (Figs. 4–8),
forming a thick multiple-phase molasse sedimentary package.
The Upper Triassic rocks are locally distributed 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 limestones with lamellibranch, e.g., Waagenoperna
sp. (He, 1995). Thus the Late Triassic stratigraphy 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).
SE
K
J
Wuzao Formation (T3w)
South China Orogeny
Yinkeng Formation (T1y)
Zhengtang Formation (T1z)
Changxing Formation (P2c)
Dalong Formation (P2d)
Longtan Formation (P2l)
Wulinshan Formation (P2w)
Gufeng Formation (P1g)
Dingjiashan Formation (P1d)
Qixia Formation (P1c)
Chuangshan Formation (C3c)
Huanglong Formation (C2h)
C1
D
PreD
Note: Modified after BGMRZ (1989). Italics indicate the South China collisional
orogeny.
NNW
SSE
10
13
12
9
8
11
E
D
D
C
B
A
C
B
A
7
0
6
m
1
5
4
3
2
E
D
E
D
C
C
100
Lithologies: Lower Triassic
1
Molasse
948
TABLE 1. STRATIGRAPHIC COLUMN OF NW ZHEJIANG REGION SHOWING
FORMATION NAMES AND AGES
2
Gray and brown calcareous siltstone and calcareous mudstone intercalated with
argillaceous limestone
Dark thin-bedded argillaceous siltstone intercalated with limestone and marl
3
Gray thin-bedded dolomitized limestone, argillaceous limestone, and calcareous mudstone
4
Dark mudstone, thin-bedded siltstone, marl, and calcareous mudstone, with minor limestone
5
Yellowish green, gray, and purple siltstone interbedded with silty mudstone
6
Grayish-green and grayish-yellow siltstone interbedded with silty mudstone
7
Dark thin-bedded silty mudstone interbedded with thin-bedded siltstone
8
Gray and purple argillaceous calcareous siltstone and marl intercalated with imestone
9
Yellowish green mudstone intercalated with thin-bedded siltstone
10
Yellow and dark purple siltstone interbedded with grayish-green and dark purple silty mudstone
11
Yellow thin-bedded siltstone interbedded with grayish-green silty mudstone
12
Yellowish-green and grayish silty mudstone interbedded with siltstone
13
Yellowish-green and grayish silty mudstone interbedded with thin-bedded siltstone
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.
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 structures are open folds and reverse faults (Fig. 4),
most of which dip to the southeast. Gravel in the
Wuzhao Formation is predominately made of
Geological Society of America Bulletin, July/August 2005
NW ZHEJIANG THRUST TECTONICS
TABLE 2. SOME PALEOCURRENT DATA FOR THE LOWER TRIASSIC
IN NW ZHEJIANG
Section
Locality
Dazipu
Dazipu
Zhengtang
Tiandun
Zhengtang
Tiandun
Number Occurrence† Structure‡ Paleocurrent
(°)
A
B
C
D
E
F
A
B
C
D
A
B
C
D
E
F
A
B
C
D
E
F
A
B
C
D
E
F
G
A
B
C
D
E
F
G
140°–52°
F
140°–48°
G
F
C
F
F
150°–60°
148°–55°
G
G
F
C
F
F
C
C
G
136°–44°
G
G
C
G
F
148°–52°
G
C
140
150
180
190
135
180
146
150
140
135
143
152
190
170
150
140
150
135
170
140
156
178
190
136
142
170
150
166
140
150
145
150
160
170
180
150
†
Refers to the beds on which sedimentary structures occur.
F—flute cast; G—groove cast; C—cross-bedding.
‡
336
Late Triassic foreland molasse succession
unconformity
J3
Chencai
Complex
T3w
J3
0
m
200
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)
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.
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 terminated shortly after the Late Jurassic, although
this timing requires confirmation. Lower to midJurassic 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 tectonic deformation. Because the stratigraphy of
the Late Triassic records an important transition
from marine to continental sedimentary environments, and because there is a regional angular 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, pebblebearing sandstone, siltstone, and mudstone
with interbeds of carbonaceous shale and coal;
these are interpreted to be stream and lacustrine sediments. The Lower-Upper Jurassic
comprises purple sandstone, shale, tuff conglomerate, tuff, and interbedded green sandstone 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 southeast 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 defines multiple-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 sediments (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 northwestward 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 sediments to the southwest and thrust over Lower
to Mid-Jurassic sediments to the northeast,
Geological Society of America Bulletin, July/August 2005
949
WENJIAO XIAO and HAIQUING HE
118
18 30'
4
Upper Jurassic redbed, tuff, and basalt
J3
Upper Jurassic rhyolitic porphyry, welded
tuff, and tuffaceous conglomerate
Upper Jurassic redbed, limestone,
tuff, rhyolitic porphyry, and basalt
Z-C
Lower to Mid-Jurassic sandstone,
siltstone, mudstone and shale
Sinian to Carboniferous stata
(fold-and-thrust belt)
29 00'
J3
Z-C
29 00'
J1-2
J3
Unconformity
Anticline
Changshan
Thrust
Figure 5. Schematic geological map of Changshan area
showing multiphase Jurassic
foreland basins. See Figure 2
for location. Z-C—Sinian-Carboniferous.
Z-C
Z-C
3
Z-C
2
Z-C
28 50'
118
18 15'
J3
1
J1-2
NW
0
B
Z-C
28 50'
0
Km
10
A
118
18 30'
Km
SE
4
Z-C
J3
Z-C
4
Z-C
J3
Unconformity
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 Cambrian-Ordovician strata. Upper Jurassic rocks
seal these thrust systems and were translated to
the southeast by thrust imbricates of Neoproterozoic 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 sedimentological and biogeographic features of Permian and
Early Triassic sediments across the NW Zhejiang region indicates that the paleogeography
950
3
Z-C
2
1
Z-C
was characterized by a roughly SE-facing continental slope on which shallow marine sediments were deposited generally in the northwest and deep-sea sediments in the southwest
(He, 1995). Insofar as the Cambrian-Ordovician paleogeography also had a slope toward
the south (Yan, 1986), we envisage a long-lived
SE-facing continental slope. All the characteristics enumerated above suggest that deposition in the NW Zhejiang region was typified 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 environment of molasse-type basins in the late Triassic that continued through Jurassic time. This
transition of sedimentary environment from
deep-sea basins to foreland molasse basins is
of great significance because northwestwardpropagating thrusting and folding were closely
associated with it, thus providing important
information to constrain the early Mesozoic
tectonic architecture in the region.
B
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, Southeast 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 continental 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 strikeslip faulting (Kong et al., 1995). The boundary between the Core Zone and the Southeast
Zone is defined by a steep, SE-dipping reverse
fault with similar occurrences to the JiangshanShaoxing fault. These structures are cut or
obliquely truncated by late sinistral transcurrent
faults with a similar trend to the Jiangshan-
Geological Society of America Bulletin, July/August 2005
Za
Granite
Upper Cambrian
Lower Ordovician
Zb
Cam3
Zb
Za
O1
O3
O2
O1
O2
0
Km
4
O3
J1-2
Machefu
J1-2
Section A
d
lan
e
r
S1
fo
c
i
ss
ra
u
J
S1
O3
O3
O3
O1
as
ol
m
b
se
Cam2
as
i ns
J3
Cam3 119 30' 30 00'
O3
S1
Cam2
J1-2
S1
30 15' N
Figure 6. Schematic geological map of Machefu area showing multiphase Jurassic foreland basins. See Figure 2 for location.
Unconformity
Fault
Majorr thrust older than J3,
barb on hanging-wall
Mid-Ordovician
Zb
Neoproterozoic
Lower Palaeozoic (Pz1)
Upper Ordovician
Pz1
Lower Silurian
Mid-Cambrian
Lower-Mid-Jurassic
Upper Jurassic
0
J3
S1
O3
Section B
J1-2
Unconformity
0
S1
S1
Thrust younger than J,
teeth on hanging wall
Stratigraphic boundary
4
Km
Pz1
Km
J1-2
Unconformity
30 15'
Section A
119 30' E
J3
Sec
tion
B
O3
O3
Lower Cambrian
Za
4
Pz1
Pz1
J1-2
Pz1
316
Pz1
Pz1
295
Pz1
NW ZHEJIANG THRUST TECTONICS
Shaoxing fault. The Core Zone mainly consists
of metamorphic rocks of the Chencai Complex:
ultramafic 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 NWdirected ductile thrusting.
As defined by Kong et al. (1995), the Chencai 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 metavolcanic and metasedimentary rocks are tectonically
interleaved with ultramafic rocks and with
overlying synorogenic clastic molasse sediments. From their petrology and geochemistry,
Shui (1986) concluded that the volcanic rocks
are calc-alkaline and formed in an island arc.
The ultramafic and mafic 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 deformation, mostly sinistral strike-slip faulting,
reworked the complex, forming an extensive
NE-SW–trending fault system.
The above structures define a fundamental
structural discontinuity in the NW Zhejiang
region, which marks the boundary between
the Yangtze and Cathaysian blocks. Deformation throughout the complex is heterogeneous
and characterized by narrow shear zones
that enclose lens-shaped domains of foliated
mafic and ultramafic rocks (Shui, 1984). The
Jiangshan-Shaoxing fault is between 0.5 and
7 km wide and contains an ophiolitic mélange
(Pirajno 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-Shaoxing 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 usually with felsic metamorphic rocks at the top,
mafic and ultramafic 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 JiangshanShaoxing fault as a reworked suture zone; the
age of suturing is controversial. Because it is
Geological Society of America Bulletin, July/August 2005
951
WENJIAO XIAO and HAIQUING HE
SE
A
strike-slip reworked
suture zone
Central Zone
Southeast Zone
Cb Cb O1 O 2 O 3 J1-2 O2 O3 O2 O 3
O
O2 Cb
O
Cb
Cb
Core Zone
Jiangshan-Shaoxing
Fault
Granite
duplexes
Cb
O
T1 P 2 T1 P1 C2P 2P 1
P 2 duplex
Cb
O
P1P 2
P 2 T1 P 1
Chencai complex
Cb
Cb
J
Lizhu-Changshan Thrust
0
5
Km
Bailongshan Thrust
NW
A'
Northwest Zone
O1 O 2
O3
O2
O3
Cb
O2
O3
O 2 Cb
O3
O2
Cb
D 1 S2 S1
Cb
S 1S 2 D 2C 1 Cb O O
O
O
Cb
0
Km
K1
J1-2
Cb
5
Figure 7. Cross section showing structural styles and tectonic zoning of NW Zhejiang foreland fold-and-thrust belt. A and A′ have approximately the same SE-NW cross-sectional position. K1—Lower Cretaceous; J1–2—Lower-Middle Jurassic; J—Jurassic; T1—Lower Triassic; 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 represent Precambrian basement. See text for discussion and Figures 1 and 2 for location.
340
important for the tectonics of South China, we
discuss this problem later.
roof thrust
Southeast Zone
floor thrust
The Southeast Zone is mainly composed of
Sinian to Paleozoic and Early Triassic strata.
The Bailongshan Thrust defines the NW boundary 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 foldnappe 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 structural 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 mutually imbricated together with a NW vergence.
NW-directed thrusts form roof and floor thrusts
952
P2
T1
0
m
Upper Permian clastics
20
Lower Triassic limestone
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 floor 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
Geological Society of America Bulletin, July/August 2005
NW ZHEJIANG THRUST TECTONICS
NW
SE
0
cm
80
basins containing Upper Triassic, Jurassic, and
Lower Cretaceous sediments.
In the southeast of the NW Zhejiang region,
sedimentary structures in Lower Triassic turbidites 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 Triassic rocks. These fold-thrust package assemblies 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 floor thrusts (Figs. 10–12).
Central Zone
A
NW
0
cm
SE
20
Roo
f
Floor thrust
thr ust
B
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.
The Central Zone is characterized by a largescale imbricate fan composed of Cambrian
to Ordovician sediments (Fig. 7). The main
difference between this zone and the abovementioned zones is that it contains neither
large-scale duplexes nor typical isoclinal folds.
The major structures within this zone are largeamplitude 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
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 demonstrates 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 associated with these thrusts are synorogenic foreland
The Northwest Zone is made up of Neoproterozoic to Mid-Devonian passive continental
margin sediments (Fig. 7). The Lizhu-Changshan Thrust defines 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,
Geological Society of America Bulletin, July/August 2005
953
WENJIAO XIAO and HAIQUING HE
A
Zb
118 40' E
118 50'
O3
Upper Cretaceous to Cenozoic
B
28 50' N
28 50'
Upper Jurassic (J3)
O3
Cam
Lower-Mid-Jurassic (J1-2)
Lower Triassic (T1)
Za
lZ
on
e
0
Lower Permian (P1)
O
Zb
O1-2
Ce
nt
ra
O3
Upper +Mid-Carboniferous (C2+3)
Za
Cam
O3
C1
P2
P1
BL
S -T
Zb
as
he
P2
P1
C
t
u
So
m(
ste
y
S
lt
Fau
g
n
i
ox
ha
S
ne
an
Zo
sh
e
g
r
n
Co
Ji a
Za
P1
O3
Jiangshan
C1
Cambrian (Cam)
Zb
Neoproterozoic
Za
Upper Carboniferous (C3)
Upper Cambrian (Cam3)
Lower Carboniferous (C1)
Mid-Cambrian (Cam2)
118 40'
Chencai Complex
Granite
Stratigraphic boundary
Reverse fault
Main thrust
Proposed thrust
28 40'
118 50'
Jiangshan duplex
B
Lower Cambrian (Cam1)
Lower to Mid-Ordovician
(O1-2)
Ordovician (O)
O3
D'
F)
J S-
Upper Ordovician (O3)
C2+3
28 40'
T1
J3
C'
P2
D
T1
P2
B'
O3
Cam2
e
on
Z
t
P1
C3
J1-2
O1-2
O3
Upper Permian (P2)
5
Km
B' C
C' D
D'
Core Zone
Chencai
Complex
J1-2
Zb
Zb
O C1
P1
P2
Za
Za
O3
C3
P2
Za
P2
J3
P2
T1
JS-F
O3
O1-2
B
T1
Cam1
Southeast Zone
BLS-T
Central Zone
0
Km
5
Figure 10. (A) Geological map of Jiangshan region in NW Zhejiang area showing duplexes. See Figure 2 for location. (B) Cross section in (A).
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 balancing was possible (Xiao, 1995). The relatively
simple structural style and low degree of shortening that characterize the Northwest Zone
suggest that it most likely behaved as a foreland
during the Early Triassic compression.
954
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 disposition of the major thrust styles of the fold-andthrust 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 displacement 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
Geological Society of America Bulletin, July/August 2005
NW ZHEJIANG THRUST TECTONICS
SE
NW
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 continuous northwestward shortening.
AGE OF CONTRACTION
0
m
2
A
SE
NW
0
m
2
B
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.
(Xiao, 1995). Northwestward fold-and-thrust
nappes have been interpreted to have formed
during impingement and southeastward underplating 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-andthrust belt model in several important respects:
(1) the absence of a stratigraphically controlled,
continuous décollement horizon; (2) the dominant 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 particular 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
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 northwestward thrusting can be bracketed by the earliest
molasse (T3) and the latest marine sediments
(T1) involved in the foreland fold-thrust deformation. 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 Triassic 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 outcrop scales.
The fold-and-thrust belt structures are sealed
by Cretaceous sediments. Deposits of Late Triassic age (the Wuzhao Formation) rest unconformably 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 Triassic to Jurassic age, and because they are closely
associated with NW-directed thrusts, the associated 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,
Geological Society of America Bulletin, July/August 2005
955
WENJIAO XIAO and HAIQUING HE
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 Jiangshan-Shaoxing faults (Fig. 1). Zhao et al. (1995,
1996) first reported Carboniferous–Late Permian radiolaria in formerly defined Precambrian
rocks at Zhangshudun in Yiyang (see Fig. 1 for
locality). They also reported late Paleozoic radiolaria from Zhangshudun northeastward of Dexing, 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
Latentifistula 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
ultramafic rocks, and from a geochemical study,
Liao et al. (1998, 1999) showed that blocks of
Paleozoic volcanic rocks have an island arcbackarc signature. Near Hengfeng, ~20 km east
of Yiyang, Paleozoic fossils were found in socalled Precambrian rocks (Xue et al., 1996).
Although no late Paleozoic fossils have been
reported in the ophiolitic slices along the Jiangshan-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 subduction 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
affinity. 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
956
SE
Drag fold
Anticlinal axial trace
NW
Drag fold
Anticlinal axial trace
0
40
cm
Figure 12. Photographs of imbricate fan structures in Tiandun. Look southwest. Hammer
for scale is ~40 cm long. See Figure 2 for location.
NW
SE
Drag fold
Anticlinal axial trace
0
40
cm
Figure 13. Photographs of imbricate fan structures in Meishan of Changxing. View to southwest. Hammer for scale is ~40 cm long.
consistent with the main Permian–early Mesozoic 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 identified 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 final
formation age of the ophiolitic mélange.
Geological Society of America Bulletin, July/August 2005
NW ZHEJIANG THRUST TECTONICS
SE
NW
demonstrate an early Mesozoic retroarc contraction event in SE China. The early Mesozoic contraction 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 magmatic arcs of appropriate age in an analogous
setting, e.g., in the Coastal Belt of Newfoundland, can be explained by transform faulting,
thrusting, or rifting after contraction (Mitchell
and Garson, 1981).
Thrusting Phases
0
m
2
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
N
n = 33
A
S-type fold
Z-type fold
n = 27
B
thrust plane
Figure 15. Lower hemisphere stereogram of fold axes (A) and thrust planes (B) in present geographic orientation. For S- and Z-type folds, see Hansen (1971) and Cowan and
Brandon (1994).
All these factors point to a continuous, Yshaped Paleozoic-Permian ophiolitic mélange
zone in SE China, which may have been created by thrusting and/or strike-slip faulting
(Fig. 1B). Because some components of the
ophiolitic mélanges have a back-arc geochemical signature (Zhao et al., 1997; He et al., 1999;
Liao et al., 1998, 1999), because some ophiolites 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 Pacific (Hall, 2002). The northwestward
distribution of the Mesozoic foreland fold-andthrust belt, the mélange and the arc in the NW
Zhejiang region, as described in this study, may
The relative spatial and temporal relationships between folds and faults in the fold-andthrust belt indicate a two-stage history of the
early Mesozoic shortening: an early stage of
Alpine-style folding and thin-skinned thrusting above a local basal décollement, as shown
in the Bailongshan Thrust décollement of Figure 7, followed by a second stage of high-angle
reverse faulting and out-of-sequence thrusting,
which may have been related to the northwestward 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 structures 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 thrusting. 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 metamorphic Chencai Complex in the foreland belt
and by out-of-sequence high-angle reverse
faults that cut across folded thrusts in the Central zone and the Chencai Complex. This late-
Geological Society of America Bulletin, July/August 2005
957
WENJIAO XIAO and HAIQUING HE
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 postkinematic intrusions of the late Mesozoic plutonicvolcanic suite in SE China. The development
of thick-skinned, out-of-sequence thrusts and
of basement-involved structures in the hinterland 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 compression lasted to a late stage.
A Late Permian — Early Triassic
Yiyang-Shexian
Jiangshan-Shaoxing
deep sea
deep sea
Lower-Yangtze
block
Yangtze block
Continental
Cathaysian block
slope
?
?
SE
NW
B Middle Triassic — Late Jurassic
NW Yiyang-Shexian
melange zone
Yangtze block
SE
Jiangshan-Shaoxing
melange zone
Cathaysian block
Lower-Yangtze block
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.
Tectonic Evolution
958
NW Zhejiang
Pz2-T1
continental slope
?
South China Archip
ela
go
ing d
Ca
th
ay
sia
nb
loc
k
10 S
ox
ha
n-S
ha
s
ng
Jia
eep sea
Cross-sections
in Fig. 16
zone
ck
blo
e
tz
ng
Ya
ction
Equator
Sub
du
gtze
r Yan
Lowe ck
blo
sub-
Yiy
de ang-S
ep
sea hexia
n
Our integrated observations lead to the following 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 subblock to the NW in the Middle Triassic, the
Cathaysian block collided with the former
blocks, terminating the intervening JiangshanShaoxing 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 transformed 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,
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 (modified after Metcalfe, 1996; Yin et al., 1999; Xiao et al., 2001). Arrow in Yangtze block refers to present north.
intense northwestward thrusting produced
the structural styles that vary from multiduplex 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).
Geological Society of America Bulletin, July/August 2005
NW ZHEJIANG THRUST TECTONICS
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 significance in South China.
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) concluded 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 similar domes along the India foreland in the Himalayas (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 shortening 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 contemporaneous 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,
Nature of the Mesozoic Tectonics in South
China
On the basis of formerly defined late Precambrian 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 intervening 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 continental fragments based on the geological and
paleomagnetic evidence—the Yangtze, Xianggui, Cathaysia, and Donanya—with scattered
paleopole positions and an apparent latitudinal
discrepancy. This is important evidence for the
archipelago paleogeography in the early Mesozoic, and it is in good agreement with the paleomagnetic work of Zhai et al. (1992), who proposed 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 probably related to the retroarc contraction event
that is the main reason for the foreland foldthrust 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 counterclockwise rotation to explain the amalgamation of the
Cathaysian block to the Yangtze block, a situation 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 extensive belt of granitic rocks in SE China represents
a continental margin magmatic arc within which
a granitic belt of Mesozoic and possibly Paleozoic age was emplaced above a northwestwarddipping subduction zone. Faure et al. (1996) proposed 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 Pacific 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 mineralization, 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 Pacific plate. The posttectonic
thrusting, particular in the Late Jurassic, partially
resulted from back-arc thrusting associated with
the subduction of the Pacific-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 ultrahigh-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 foreland 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 Pacific plate in
Geological Society of America Bulletin, July/August 2005
959
WENJIAO XIAO and HAIQUING HE
the late Mesozoic and Cenozoic, should have
contributed to the formation of the NW Zhejiang 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 scientific 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 significantly improved the final presentation. The study
was financially 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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