Download Tectonic controls on the late Miocene–Holocene volcanic eruptions

Journal of Asian Earth Sciences 30 (2007) 375–389
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Tectonic controls on the late Miocene–Holocene volcanic
eruptions of the Tengchong volcanic Weld along the southeastern
margin of the Tibetan plateau
Yu Wang a,¤, Xuemin Zhang b, Chaosong Jiang c, Haiquan Wei d, Jinglin Wan d
a
Geologic Laboratories Center and Department of Geology, China University of Geosciences, Beijing 100083, China
b
Institute of Geology, China Seismological Administration, Beijing 100029, China
c
Seismological Bureau of Yunnan Province, Kunmin 540000, China
d
Institute of Geology, China Seismological Administration, Beijing 100029, China
Received 23 January 2006; received in revised form 7 June 2006; accepted 6 November 2006
Abstract
The Tengchong volcanic Weld, located along the southeastern margin of the Tibetan plateau, experienced multiple eruption stages
since the late Miocene, including time intervals of »5.5–4.0 Ma, 3.9–0.9 Ma, 0.8–0.01 Ma, and younger than 0.01 Ma. These eruption stages
produced diVerent volcanic rocks, principally basaltic and basaltic-andesite series. At the same time or prior to volcanic eruptions in the
Tengchong volcanic Weld, NE–NNE-trending rift basins and NS-striking normal faults formed during the late Miocene–Pliocene. In
addition, rapid exhumation of the neighboring mountains occurred at »6–5 Ma constrained by apatite Wssion track dating and its thermochronological modeling. At present, the state of stress in the Tengchong volcanic Weld and its surroundings is NNE–NE-compression
and WNW–NW-extension based on seismic foci mechanisms. Petrologic and geochemical data indicate that the source of the Tengchong
volcanic rocks belongs to an intracontinental tectonic setting, but not a subduction or collision zone between Indian and Eurasian plates.
Since the late Miocene, the dextral strike-slip motion of the Sagaing fault induced E–W-extension. The Sagaing dextral strike-slip motion
might disturb the lower crust-upper mantle of the Tengchong block, resulting in the partial melting of the upper mantle which, in turn,
induced volcanic eruptions characterized by mature island-arc features.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Tengchong volcanic Weld; Pliocene–Holocene; Tectonic setting; Subduction and collision; Sagaing fault; Dextral strike-slip motion
1. Introduction
Widespread volcanic eruptions occurred in the Tibetan
plateau and its adjacent areas in Cenozoic time, including
along the Yalung–Zangpo suture zone, northern Tibet
and southeastern margin of the Tibetan plateau (Le Dain
et al., 1984; Stephenson and Marshall, 1984; WhitfordStark, 1987; Xizang Bureau of Geology and Mineral
Resources, 1993; Cong et al., 1994; Wang, 1999; Wei et al.,
2003). The late Miocene-Quaternary volcanic eruptions
are distributed along the Gangdese belt, north of the
*
Corresponding author. Tel.: +8610 82321028; fax: +8610 82321006.
E-mail address: [email protected] (Y. Wang).
1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2006.11.005
Yalung-Zangpo suture zone, the Kunlun Mountains belt,
the Burma arc, and the Tengchong area in Yunnan Province of China (Fig. 1). The Tengchong volcanic Weld is
located along the southeastern margin of the Tibetan plateau, at 24°40⬘–25°30⬘N, 98°15⬘–98°45⬘E, and since the
late Miocene time, volcanic eruptions have discontinuously occurred. At present, numerous active hot springs
along or around the volcanic clusters indicate sites of
active geothermal Welds (Wang and Huangfu, 2004) and
the potential for future eruptions.
Petrologic, geochemical, and regional tectonic data has
lead to diVerent interpretations of the tectonic setting of
the Tengchong volcanic Weld. Zhu et al. (1983) and Mu
et al. (1987) argued that the Tengchong volcanic eruptions
376
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
Fig. 1. Tectonic site of the Tengchong volcanic Weld along the southeastern margin of the Tibetan plateau. Location of Fig. 2 and sample YN3 are shown.
were derived from the subduction–collision zone between
Indian and Eurasian plates. Zhao and Chen (1992)
believed that it was a post-collision arc-volcanism or a
delayed arc-volcanism in the southeast margin of the
Tibetan plateau. Cong et al. (1994) proposed that the
Tengchong volcanic swarms were derived from a mature
island-arc area. Ji (1998) pointed out that during 5–8 Ma,
the lower lithosphere delaminated in the Tengchong area,
resulting in upwelling of the upper mantle. In contrast,
Chen et al. (2002) proposed that the magma source for
these volcanic rocks was enriched-mantle.
These interpretations are mutually contradictory.
Although the timing of volcanism in the Tengchong volcanic Weld was same as shoshonite eruption west of
the Kunlun Mountains belt in the northern Tibetan
plateau (Wang, 1999) and overlapping in age with eruptions of Burma volcanoes along the Sagaing fault, the
tectonic setting of the Tengchong volcanic Weld is not
clear. Is it related to the Sagaing dextral strikes-lip fault,
the subduction–collision tectonics between Indian and
Eurasian plates, or some intracontinental tectonic setting?
Based on structural mapping (1:50,000), basin sedimentary analysis, K–Ar data, geochemical data of volcanic
eruptions, eruptions sequences of volcanic rocks, seismic
foci mechanisms, and apatite Wssion-track dating and its
thermochronological modeling, we analyzed sequences of
the volcanic eruptions and characterized the relations
between volcanic eruptions and structural development of
the region. Also, we attempt to reconstruct the tectonic
setting of volcanic eruptions for the Tengchong volcanic
Weld since the late Miocene–Pliocene.
2. Geological background and features of the Tengchong
volcanic Weld
2.1. Geological background
The tectonic cross-section from west to east, displays a
series faults or suture zones, they are: Naga Hills subduction zone, Sagaing dextral fault, Myitkyina suture zone,
Burma gneissic belt, Yingjiang island-arc magmatic belt,
Tengchong volcanic area, Gaoligong metamorphic belt,
Baoshan Block, Red-River fault zone, and Yangtze Plate
(Fig. 1). The Tengchong volcanic Weld is located on the
western side of the uplifted Gaoligong metamorphic belt.
Along the western side of the Tengchong area, late Mesozoic to early Cenozoic island-arc granites and gneiss are
exposed between Burma and southwestern China (Yunnan
Bureau of Geology and Mineral Resources, 1979; Luo and
Hu, 1983; Ji, 1998; Jiang, 1998). The Tengchong volcanic
Weld is located within a N–S-trending fault zone. In this
area, island-arc granites intruded into Mesozoic and Cenozoic geologic units which represent the subduction of the
Indian plate under the Eurasian plate along Naga Hills and
Myitkyina-Mandalay suture zones (Ji, 1998). Sinistral
strike-slip motion accommodated southeast escape of the
Tibetan plateau between 27 and 15 Ma (Molnar and Tapponnier, 1978; Tapponnier et al., 1982; Zhong, 1997; Wang
and BurchWel, 1997; Ji, 1998; Socquet and Pubellier, 2005).
2.2. Geophysical features
The spatial distribution of earthquakes during 1998–
1999 shows that, earthquakes are mostly shallower than
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
15 km beneath the Tengchong area, but at depths of
»40 km to its west or east (China Earthquake Catalog).
The chromatography of this area shows that there are a
high velocity body at 10–15 km depth, a low velocity body
at 16–24 km and a high velocity body at 25–40 km (Qin
et al., 2000a; Wang et al., 2002; Wang and Huangfu, 2004).
The low velocity body is proposed to be a magma body or
partly-molten material (Qin et al., 2000b). In addition, the
depth to the top of the high conducting layer in the upper
mantle is shallow at the Tengchong volcanic Weld, only
about »64 km in depth (Sun et al., 1989). But at only
»50 km deep to the west, it changes abruptly on the both
sides of the Gaoligong mountains belt.
Wu et al. (2001) used receiver function to calculate the
velocity structure of the Yunnan Province. Their results
show that the thickness of the crust reduces gradually
from northwest to southeast, whose shape and scale coincide with Sichuan-Yunnan relatively rigid blocks. Moho
surface beneath the Tengchong area is about »38 km
deep, and to its east of the Baoshan block, the crustal
thickness is »40–45 km. Wang et al. (2002) studied the
velocity of Sichuan and Yunnan areas with tomography,
and determined that, compared with their surroundings,
the Tengchong volcanic Weld has not only a negative
anomaly in the uppermost mantle, but also a negative
anomaly in the upper crust and a positive anomaly in the
lower crust. Unlike its northwest, the Tengchong volcanic
Weld has no additional evidence of crustal thickening.
2.3. Types of volcanic rocks and geochemical characteristics
In the Tengchong volcanic Weld, major volcanic rocks
are basalt, dacite welded tuV, basaltic trachyandesite and
trachyandesite. All of them belong to a high-potassium
calc-alkaline volcanic suite. Volcanic eruptions in this
region can be roughly divided into at least four stages or
swarms: late Miocene–Pliocene basalt and olivine-basalt
volcanic rocks (5.5–4.0 and 3.8–0.9 Ma) and Pleistocene
acid rocks (0.8–0.1 Ma); late Pleistocene–Holocene basalts
and intermediate-acid rocks such as andesites (0.1–
0.01 Ma) (Mu et al., 1987; Ji, 1998; Li et al., 1999; Wang
et al., 1999).
From Pliocene–Pleistocene to Holocene, K2O content
increases in the volcanic rocks from 1.5% to 3.65%, but
MgO content decreases from 5.91% to 3.04% (Fan et al.,
1999). The Tengchong volcanic component has high
Al2O3 and K2O, but low TiO2 and LREE, similar to
island-arc volcanic rocks. On the diagram of K2O–SiO2, it
belongs to high-potassium basalt and andesite, as well, in
the Log –Log diagram, it drops into the island-arc
domain (Zhao and Chen, 1992). Nd–Sr isotopic and
micro-element analysis indicates that the main-series
rocks are sourced from metasomatic mantle eclogite and
pyrolite (Zhu et al., 1983). Basalts and andesitic basalts
are characterized by high 87Sr/86Sr ratio (0.7057–0.7081),
low Nd values (¡1.1 to ¡5.7), and particularly high
208
Pb/206Pb ratios (1.08–1.12) (Chen et al., 2002).
377
3. Constraining structures of the Tengchong volcanic Weld
On the eastern side of the Tengchong volcanic Weld, the
Gaoligong mountains belt was rapidly uplifted and
exhumed to its present elevations in the time interval of
»6–5 Ma, meanwhile, west-dipping and steep (>80°) normal faults were developed. In the Tengchong volcanic Weld,
the structural framework shows that its eastern and western
parts were uplifted, but the central region was depressed
and the elevation diVerence between uplift and depression
is more than 500 m.
3.1. Structural framework of the Tengchong volcanic Weld
and its evolution
In the Tengchong volcanic Weld, faults and volcanic clusters are distributed mainly within N–S, NE and NW trending zones. A structural framework, built up by the
extensional basins with NE–NNE trends together with
their marginal faults, indicates an “arc”-type feature
(Fig. 2). The north Xank of the “arc”-type structure trends
N330° and the south Xank trends NE40–50°. Some parts of
the “arc”-type structure were covered with Pleistocene volcanic rocks. NW–NNW-striking faults are characterized by
sinistral slip and transpression while NE–NEE-striking
faults are characterized by dextral slip and transtension
(Fig. 3).
Based on the correlation of faults, folds, sedimentary analysis and volcanic eruptions, three structural stages can be distinguished. The shear-extension or transtension deformation
started during late Miocene–Pliocene (»6–4 Ma). After the
formation of rift basins, the Wrst stage volcanic eruptions
occurred. After this event, during the Middle-Early Pleistocene, the NW–SE-direction extension resulted in normal
faults in this region; at the same time, eruption of the second
stage volcanic rocks occurred (0.8–0.1 Ma). Later, NNE–
SSW-striking dextral strike-slip and normal faulting controlled the formation of latest three volcanic clusters: Heikongshan, Dayingshan and Maanshan volcanoes.
BrieXy, in the Wrst structural stage, structures are focused
on the southeastern part of the Tengchong volcanic Weld
seen as the NE–SW-striking normal and dextral faults.
During this stage, NE-striking extensional faults controlled
extensional sedimentary basins. The third-stage faulting
formed during middle-late Pleistocene in the central Tengchong area, which controlled the distribution of the latest
volcanic eruptions. From the edges to the central part of
the volcanic area, the faults were developed from the early
Pliocene to Pleistocene, but in the central part, faults were
active from the Pleistocene to Holocene. During these
structural stages, volcanic eruptions and faulting are closely
related spatially and temporally.
3.2. Basins and related sedimentation
During Miocene–Pliocene time a series of rift basins
were formed in the Tengchong and surrounding areas
378
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
Fig. 2. Distribution of late Cenozoic basins and “arc”-shaped structures surrounding the Tengchong volcanic Weld (modiWed from Yunnan Bureau of
Geology and Mineral Resources, 1979; Liao and Guo, 1986; Jiang et al., 1998). Abbreviations are as follows: F1, Nujiang fault; F2, Longchuanjiang-Longling-Ruili fault; F3, Xiaolongchuan-Tengchong-Ruidian fault; F4, Lianghe-Guyong fault; F5, Wanding fault; B1, Jietou basin; B2, Luxi basin, B3,
Husha basin, B4, Longchuan basin, B5, Zhefang basin. Figs. 3–5 are shown.
(Fig. 2). In the Tengchong Basin, the basement is volcanic
rocks with ages of »5.5 Ma (ages based on K–Ar method
dating on volcanic rocks that crop out on the margins of
basins; Ji, 1998). The basins are constrained by the NE- and
NS-striking faults. The distributions of volcanic rocks and
eruption clusters imply that the volcanism in the Tengchong area has been related to local extensional stress since
the Pliocene.
Within these extensional basins, Upper Miocene Namulin Formation and Pliocene Mangbang Formation were
deposited, and then, folded. After folding, deposition
appears to have ceased. A couple of small oVset (<10 m)
faults in the central region of the Tengchong volcanic Weld
(i.e. the Liuhuangtang and Huangguaqing faults) are characterized by dextral strike-slip movement.
3.3. Distributions of the volcanic rocks and relationships to
structures
Volcanic eruption centers migrate from the margins
towards center of the basins during the Pliocene–Pleistocene (Figs. 4 and 5). On the both sides of the NS-striking
faults in the Tengchong volcanic Weld, the timing of volcanic eruptions is older than that of eruptions along or
within the NS-striking faults. Also, from south to north
along the volcanic zone, the age range of eruptions was
from »4 to »1.0 Ma (Mu et al., 1987). In its eastern and
western parts, volcanic eruptions were earlier, but in the
central part they were later. The youngest eruptions of the
andesites and trachyandesites was <10,000 yrs (Wang
et al., 1999).
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
379
Fig. 3. Structural diagram and distributions of latest volcanic eruptions in the Tengchong volcanic Weld.
3.4. Rapid exhumation of the Gaoligong mountains belt on
the eastern side of the Tengchong volcanic Weld
3.4.1. Apatite Wssion-track method
Apatite was dated by the Wssion-track method. Single
apatite crystals were separated from 3 to 4 kg rock samples
using conventional separation techniques. Approximately
300–500 crystals of apatite were picked. Analytical procedures followed the external detector method described by
Gleadow and Duddy (1981) and were completed in the
Neogeochronological Laboratory of Institute of Geology,
China Seismological Administration. The 0.05–0.3 mm apatite were Wxed in epoxy and ground and polished to expose
internal surfaces of the crystals, and then etched in 7%
HNO3 at 20 °C for 35 s to reveal 238U spontaneous Wssion
tracks. The surfaces of the crystals were then covered with a
white mica external detector. The crystals with an external
detector were irradiated in the Heavy Water Research
Reactor (HWRR) in the China Institute of Atomic Energy.
Induced tracks were revealed in muscovite external detectors by etching in 40% HF at 20 °C for 20 min. Grain
mounting, polishing, and etching were all done by standard
techniques using the external detector method which are
documented in Hurford and Green (1982). Fission tracks
and track-length were counted and measured under an
OLYMPUS microscope using a magniWcation of £1000
immersion objectives. Only those grains mounted in the
plane of the C-axis for both mineral types were counted.
International standard samples of Durango apatite
(31.4 Ma) and Fish Canyon TuV apatite (27.8 Ma) were
used in the Zeta calibration, and SRM612 uranium glass
was used as the standard glass. All samples were counted by
J. L. Wan with zeta values of 352 § 19. Using the formula
from Hurford and Green (1982), both pool and central ages
of the apatites were calculated, in which the error associated with this zeta factor has been propagated into the calculation of the grain and sample ages. Fission-track
apparent ages were determined for apatite from four samples. The analytical data are listed in Table 1. The annealing
temperature for Wssion-track of apatite is estimated to be
about 110 § 10°C (Green et al., 1985; Hurford, 1986), and
the ages are given with an error of §1. The single-grain
age distributions for two samples passed a P(x2) test
indicate a homogeneous age for each (Fig. 6).
3.4.2. Apatite Fission-track dating and track length-age
modeling
Three samples from the Gaoligong mountains belt have
similar apatite Wssion-track central ages in the range of
5.2 § 0.4–6.4 § 0.7 Ma (Fig. 6, Table 1), but a sample from
380
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
Fig. 4. Distributions of volcanic eruptions prior to the Pleistocene in the Tengchong volcanic Weld. In the Wgure, data represent K–Ar ages (Ma) of volcanic
rocks. Data are cited from Mu et al. (1987), Liu et al. (1992), Nakai et al. (1993) and Li et al. (1999).
the Baoshan block on the eastern side of the Gaoligong
mountains belt yields an apatite Wssion-track age of 44 § 3
Ma. The distribution of track lengths yields a mean length
of 12–14 m (Table 1).
Scattered ages from »5 to »44 Ma in samples from similar elevations but belonging to two tectonic units indicate a
long-lived evolution. To further interpret the signiWcance of
the data, three apatite Wssion track data sets were modeled
to quantify the timing and amount of cooling at speciWc
locations. Modeling followed the approach of Ketcham
et al. (1999, 2000). The results of thermal modeling of three
samples are presented in Fig. 7.
Apatite Wssion track ages, combined with modeled thermal histories from the Gaoligong mountains belt, reveal a
rapid cooling period since the end of Miocene to Pliocene,
but diVers from cooling process on the eastern side of the
Gaoligong mountains belt. Samples YN1-2 located in the
margin of the Tengchong Basin experienced a single thermal history during the late Miocene to Pliocene when they
cooled at » > 110–70 °C during <7–0 Ma. Sample YN3
located on the eastern side of the Gaoligong mountains
belt, experienced a single thermal history during >40–33 Ma
when they cooled at » > 110–70 °C with very rapid cooling
rates and followed by thermal stability during 30–0 Ma.
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
381
Fig. 5. Distributions of volcanic eruptions during the Pleistocene–Holocene in the Tengchong volcanic Weld. In the Wgure, data represent K–Ar ages (Ma)
of volcanic rocks. Data are cited from Mu et al. (1987), Liu et al. (1992), Nakai et al. (1993) and Li et al. (1999).
BrieXy, two distinct periods (»44–33 Ma, < » 7.0 Ma) of
accelerated cooling can be identiWed from modeled timetemperature cooling histories as shown above. These cooling events are the same time as yielded by Fan (2005) along
the southern extension of the Gaoligong mountains belt.
4. Tectonic stress and earthquake focal mechanisms
Earthquakes in Tengchong and its adjacent areas
(20–27°N, 90–100°E) since 1976 were collected from ISC
catalogue, and earthquake proWles with depth along longi-
tude (90–100°E) and latitude (20–26°N) were provided
(Figs. 8 and 9-1). Earthquakes deeper than 40 km were
mainly distributed from 93°E to 96°E, along the Burma
suture zone, that includes Naga Hills-Arakan Yoma suture
zone in the eastern margin of the Indian plate and Myitkyina-Mandalay suture zone. Between 92 and 94°E, thrust
belts have small oblique angles and extend to »100 km in
depth, but in the area of 95°E, the thrust belts are near vertical to »180 km in depth. The earthquake distributions
between 95 and 97°E in the thrust belts were sparse, maybe
because the main collision belt changed from nearly NS- to
1.4
1.1
1.0
1.2
1.6
1.1
14.5 § 0.2 (15)
14.9 § 0.6 (23)
14.9 § 0.1 (25)
Nc—number of apatite crystals analyzed.
ND, number of tracks counted in determining RhoD. Personal Wssion-track dating Zeta value is for apatite ZetaSRM612 D 352 § 19.
a
Rock type name for each sample.
b
Rhos, spontaneous track density; Ns, number of spontaneous tracks counted.
c
Rhoi, induced track density; Ni, number of induced tracks counted.
d
P(x2), probability of obtaining observed x2 value (Galbraith, 1981) for n degrees of freedom (n is number of crystals minus 1).
e
r, correlation coeYcient between individual crystal track counts.
f
RhoD, track density measured in external detector adjacent to glass dosimeter during irradiation.
g
from Fan (2005).
6.3 § 0.4
6.4 § 0.7
43.9 § 2.7
5.2 § 0.4
1.7 § 0.5
6.8 § 1.3
5.3 § 0.9
6.3 § 0.5
6.4 § 0.7
44.2 § 2.4
5.2 § 0.5
1.7 § 0.5
7.5 § 1.1
5.5 § 0.9
1.670(6681)
1.670(6681)
1.670(6681)
1.670(6681)
0.683(1703)
0.678(1691)
0.678(1691)
0.897
0.744
0.924
0.918
0.748
0.786
0.874
65.9
93.3
0.0
99.3
86.5
1.5
29.2
41.3
15.6
99.1
50.4
8.3
17.5
9.0
5.614(10666)
2.118(4025)
13.45(8878)
6.840(9439)
0.460(902)
0.964(1128)
0.497(1054)
1.221(232)
0.4632(88)
20.47(1351)
1.225(169)
0.0663(13)
0.607(71)
0.231(49)
Gneiss
Mylonite
Mylonite
Granite
Schist
Mylonite
Mylonite
YN1
YN2
YN3
YN4
X16g
X18g
X19g
21
21
21
24
21
16
22
Mean track length
(m § 1)(Nj)
Central age
Fission track age (Ma § 1)
Pooled age
RhoD (ND)f
£ 106 cm¡2
Re
P(x2)d %
U Concentration
(ppm)
Rhoi (Ni)c
£ 106 cm¡2
Rhos(Ns)b
£ 105 cm¡2
Nc
Rock
typea
Sample
Table 1
Apatite Wssion-track data and brief description for samples from Gaoligong mountains belt
13.4 § 0.2 (38)
12.3 § 0.8 (17)
13.1 § 0.6 (75)
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
Standard deviation
(m)
382
NNE- trending at 24°N. However, to the east of 98°E,
earthquakes occurred at 0–40 km in depth.
In the Tengchong volcanic Weld and its adjacent areas,
shallow source earthquakes were developed extensively
(Figs. 9-1 and 9-2). The Yunnan seismic network shows
that: (1) The activities of earthquakes in the Tengchong
volcanic Weld are weaker in frequency and intensity than its
surroundings; (2) There were no historical strong earthquakes and the largest earthquakes recorded are Ms 5.0–
5.8; (3) The microseismicity observations indicate that
microseismicity in the Tengchong volcanic Weld are at
depths of 1–6 km and are typical shallow source earthquakes, but those outside the Weld are always deeper than
20 km (Qin et al., 1996; Qin et al., 2000a).
Mid-source earthquakes occurring at 80–180 km depths
only developed in Burma along the western side of the Sagaing dextral strike-slip fault, which is located between 94–
95.5°E. The source of the earthquakes is also the position of
the Naga Hills suture zone, which was a subductional zone
during 27–20 Ma where the Indian plate subducted under
the Burma–China micro-plate (Stephenson and Marshall,
1984; Le Dain et al., 1984). Far from the subductional zone,
focal depths are shallow. In the Tengchong and related volcano areas, the focal depth of earthquakes is »30–35 km,
and no earthquake deeper than »40 km has been recorded.
In order to study the tectonic stress in the Tengchong
area, earthquake mechanisms of CMT solutions from
Harvard University were plotted (Fig. 9-2). The mechanism of shallow earthquakes shows that the main stress in
Burma and Tengchong area is NE–SW horizontal compression. Based on seismic foci mechanisms and depth
information of 660 earthquakes (Jiang et al., 1998), from
98° to 100°E, the tectonic stress is complicated, with main
stress of NNE and NE, some NNW and NW, fewer ENE
and ESE. But the main type of foci mechanism is shearing.
Within a belt trending 20–24°N, distributions of earthquakes are N–S-trending, and from 24°N on, the earthquakes are distributed as limited in a NNE-trending belt
(Figs. 8 and 10). The focal mechanism show the same
compressional stress. On the A–A⬘ (N–S) seismic proWle
(Fig. 10-a), the main compressional stress of 60% of the
earthquakes are focused along a NNE–NE-trend, and elevation angles of P-axis for 89% of the earthquakes are
<30°, which indicates that most of the earthquakes are
characterized by horizontal shearing. A few earthquakes
are characterized by oblique-slip displacement; these
earthquakes are primarily distributed along the Myitkyina-Mandalay suture. On the B–B⬘ (NE) seismic proWle
(Fig. 10-b), the main compressional stress of 58% of the
earthquakes are distributed along a NNE–NE-trend, and
83% of the earthquakes yield elevation angles of P-axis of
<30°. Most earthquakes, whose focal depths are shallower
than 80 km, are characterized by horizontal shearing,
while those earthquakes deeper than 80 km appear as
thrust oblique-slip displacement. This indicates that along
this section the regional tectonic stress Weld is created by a
NNE–NE-trending horizontal compressional stress, but
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
383
Fig. 6. Apatite Wssion-track data plots.
deep earthquakes show the function of compressional
stress with larger elevation angle. On the C–C⬘ (E–W)
seismic proWle (Fig. 10-c), the main compressional stress
of 64% of the earthquakes are focused along a NNE–NEdirection, and 87% of the earthquakes yield elevation
angles of P-axis of <30°, similar to A–A⬘ and B–B⬘ proWles. The main extensional stress axis of 55% earthquakes
is focused along the NNW–NW direction. The earthquakes shallower than 30 km are characterized by horizontal shearing displacement, and with the increasing
depth, earthquakes have larger strike-slip component. All
of the mechanisms of eight earthquakes deeper than
120 km express transpressional displacement. On the
NNW, NNE and almost NS-trending fault planes, the
dip-angle is larger than 60°, and appears as right-hand
horizontal shearing displacement. Overall, earthquakes at
shallow structural levels are characterized by horizontal
shearing displacement while at deep structural levels,
earthquakes are characterized by thrust oblique-slip displacement.
The earthquake depth distributions and focal mechanism in the Tengchong area are the same as those in
Burma (Molnar and Tapponnier, 1978; Le Dain et al.,
1984; Wang and Long, 2000). Based on the character of
earthquake mechanisms at diVerent structural depths, the
Indian plate may produce lateral compression-shearing
(transpression) and oblique thrust in the Burma suture at
the same time, so the compressional stress of earthquakes
384
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
Fig. 7. Thermal modeling of apatite Wssion-track length and age data Modeled time-temperature paths for three apatite samples, computed with AFTsolve
program by Rich Ketcham and Raymond Donelick (2001) (version 1.3.0). Initial track length of 14.5 m was used in constructing these models (Ketcham and
Donelick, 2001). Temperature ranges between 110 and 60°C delineate apatite partial annealing zone, deWned as temperature interval in which majority of track
length shortening take place. Additionally, several conditions are as follows: (1) the present day temperature is set to a constant value of 20 °C; (2) kinetic variable was Dpar D 1.50; (3) modeling scheme was Monte Carlo. We modeled 10,000 paths for each plot. Thick lines show best-Wt solutions obtained for these
model run, and dark-grey and light-grey colors show general-Wt solutions and good-Wt solutions obtained for same model run, respectively.
occurring in the crust are nearer to N–S-direction, and
with smaller elevation angles.
5.1. Magma source of volcanic eruptions in the Tengchong
volcanic Weld
5. Discussion of tectonic controls on the Tengchong volcanic
Weld
Calc-akaline basalt and shoshonite volcanic rocks are
typically associated with continental margin and orogenic
belt melting (Barr and Macdonald, 1981; Gill, 1981; Glazner and Bartley, 1994). Based on petrologic and geochemical data from the calc-alkaline basalt in the Tengchong
area, there have arisen diVerent viewpoints for their tectonic setting, including collisional zone between Indian and
Eurasian plates (Zhu et al., 1983; Mu et al., 1987; Jiang
et al., 1998), post-collision island-arc environment (Zhao
and Chen, 1992), or mature island-arc environment (Cong
et al., 1994). Zhu et al. (1983) suggested that a signiWcant
proportion of subducted oceanic materials exist within the
To characterize the tectonic setting of the Tengchong
volcanic Weld, we must address three questions: (1) How did
the tectonic setting constrain the locations of volcanic eruptions and result in volcanic materials similar to those at an
active continental margin? (2) Was there subduction of the
Indian plate under Burma or Tengchong block during the
volcanic eruptions? (3) Did northward penetration of the
India Plate into Asia and resultant shear deformation contribute to generation of magmas and volcanism?
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
385
Fig. 8. Tectonic sites of seismic proWles across the Tengchong volcanic Weld. Locations of A–A⬘, B–B⬘ and C–C⬘ in Fig. 10 are shown.
mantle beneath the Tengchong area, and in a back-arc
extensional environment, these mantle-derived magmas
were erupted on the surface along the extensional fractures.
However, the Tengchong block was in a continental
interior, far away from the suture zone, during the Miocene-Quaternary. Ji (1998) suggested that the late Miocene–
Pliocene volcanism in Tengchong area was related to
lithospheric denudation. Yet, Fan et al. (1999) conclude
that the origin of the volcanic magma was related to crust
and mantle interaction; these Pliocene–Pleistocene volcanic
rocks preserve evidence that crustal materials were assimilated into the magma (Fan et al., 1999).
The subduction of the Indian plate under the Eurasian
plate occurred before »27–15 Ma, yet the Tengchong and
Burma volcanoes erupted after »5.5 Ma. In the Gaoligong
metamorphic belt and Tengchong area, the sinistral strikeslip movement might also have been developed during »27-
15 Ma. Apatite Wssion track ages of »6–5 Ma indicate that
the Gaoligong metamorphic belt was rapidly exhumed,
which is somewhat earlier than eruptions within the Tengchong volcanic Weld, but the same age as formation of
extensive basins in the Tengchong area (Figs. 11 and 12).
During the middle-late Cenozoic time, in the Tengchong
area and its surroundings, the volcanic eruptions of the
Tengchong volcanic Weld had not been controlled by subduction or island-arc environment. The volcanic eruptions
in the region occurred with similar rare-element distribution models indicating that the diVerent stages of volcanic
eruptions have a similar origin, and were from the same
source (Fan et al., 1999; Chen et al., 2002). Pb, Sr, and Nd
isotope compositions prove that the source material of the
Tengchong volcanic rocks is EM II-type mantle, or old subduction and re-cycled mantle (Fan et al., 1999). This is similar to the Cenozoic volcanic rocks erupted in the northern
386
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
Longitude (˚)
90
0
92
94
96
98
100
Depth (km)
30
60
90
120
150
180
210
Longitude (˚)
22
24
26
Depth (km)
Depth (km)
Latitude (˚)
20
0
30
60
90
120
150
180
210
98
0
5
10
15
20
25
30
35
98.2
98.4
98.6
98.8
99
Fig. 9-1 and 9-2. 9-1 Seismic distributions in the Tengchong volcanic Weld and surrounding areas and 9-2 earthquakes beneath the Tengchong area. Location of seismic proWle is same as section C–C⬘ in Fig. 10.
Fig. 10. Seismic proWles in diVerent orientation (A–A⬘, B–B⬘ and C–C⬘) and their foci mechanism (simpliWed from Wang and Long, 1998).
Tibetan plateau and the Kunlun Mountains belt (Xizang
Bureau of Geology and Mineral Resources, 1993; Deng
and Sun, 1999; Lai, 1999; Wang, 1999).
The characteristics of the seismic source distributions
show that oblique subduction is focused on the upper of
100 km. The Tengchong volcanic Weld is far from Myitkyina-Mandalay suture zone (about 120 km), and 450 km
from Indian plate (Figs. 1 and 12). When the Tengchong
volcanic Weld began eruptions during the end of Miocene–
Pliocene, the Myitkyina-Mandalay suture had been closed.
So, the Tengchong volcanic rocks are the product of intracontinental magmatism, not a continental margin or collisional magmatism between Indian and Eurasian plates.
5.2. Sagaing dextral strike–slip fault and probable
constraints to the Tengchong volcanic Weld
Naga Hills subduction in the Burma arc initiated in the
Oligocene to Miocene and ceased during Miocene time
(Stephenson and Marshall, 1984). During subduction,
Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
387
Fig. 11. Composite diagram of structures, tectonic stress, volcanic eruptions and timing sequences of the Tengchong volcanic Weld along the southeastern
margin of the Tibetan plateau.
Fig. 12. Tectonic Model of origin and formation of the Tengchong volcanism along the southeastern margin of the Tibetan plateau.
volcanism was absent. Since the middle-late Miocene to
Pliocene, »460 km displacement accumulated along dextral strike-slip of the Sagaing fault. On both sides of the
Sagaing fault zone, the Burma and Tengchong volcanic
Welds developed at the same time and are characterized by
similar compositions (Zhu et al., 1983; Le Dain et al.,
1984; Stephenson and Marshall, 1984; Whitford-Stark,
1987). Besides the volcanic characteristics and regional
tectonic stress Weld, the seismic mechanism in the Burma
volcanic area appears as near N–S-direction compression
on P-axis, which is similar to that in the Tengchong area
(Fig. 9-1 and 9-2). Sagaing dextral strike-slip fault motion
also occurred at the same time as extension of the basins
in the Tengchong area. An east-dipping inclined zone of
intermediate depth earthquakes suggest that a slab of oceanic lithosphere was subducted to the east under the
Indoburma ranges (Le Dain et al., 1984). The Sagaing
fault probably accommodates most of the right-lateral
slip of India past Indochina, but large scale northward
movement of the Indian plate also causes internal deformation within Burma, Tailand and Yunnan Province in
China (Le Dain et al., 1984).
From the tectonic stress in the Tengchong volcanic
Weld, there is a NNE–NE direction horizontal shearing
and compression. Northward motion of the Indian plate
results in drag along its eastern margin resulting in the
transformation of the Myitkyina-Mandalay plate suture
to a transtension zone. This drag also results in the generation of the dextral Sagaing fault. The Sagaing dextral
strike-slip movement resulted in and has controlled the
formation and activation of the Tengchong volcanic eruptions since the late Miocene–Pliocene. This tectonic
regime also resulted in reversal of slip along W-dipping
thrusting faults that they exhibit normal slip and generation of new normal faults in the Tengchong volcanic Weld.
These normal faults and associated fractures provide
pathways for the magma upwelling and eruption on the
surface. Therefore, we conclude that shearing motion
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Y. Wang et al. / Journal of Asian Earth Sciences 30 (2007) 375–389
along the eastern margin of the Indian plate lead to the
volcanic activation.
Continental margin volcanic rocks did not form during
the middle-early Cenozoic subduction of the Indian plate
under the Eurasian plate. From the structures and tectonic stress changes and regional structural evolution
(Fig. 11), we suggest that the magma of the Tengchong
area was generated during the formation of a collisional
orogenic belt, but eruption of the magma was controlled
by structures developed during transtensional deformation (Fig. 12). The formation of the normal faults and
dextral strike-slip faults was a consequence of northward
motion of the Indian plate. The volcanic eruptions were
progressively younger from south to north, which seems
to imply that they are correlated to the northward motion
of the Sagaing fault.
6. Conclusions
In the Tengchong volcanic Weld, eruptions of the highK calc-alkaline basalt series and andesite series volcanic
rocks occurred during the late Miocene to Holocene. At
least four stages of volcanic eruptions have been classiWed
based on isotopic ages: the oldest stage is » 5.5–4.0 Ma,
the next stage is 3.9–0.9 Ma, the penultimate stage is 0.8–
0.1 Ma, and the youngest stage is <0.01 Ma. NS-NNEstriking with E- or WNW-dipping regional normal faults
and dextral strike-slip faults were developed. The Tengchong area and the Burma volcanic arc share similar petrological and geochemical characteristics, and eruption
time sequences. The geochemical and petrologic evidence
suggests that the magma was generated in the former subduction zone which developed before »27-15 Ma. Since
the late Miocene–Pleistocene, transtensional dextral
strike-slip motion along the Sagaing fault resulted in a
small component of EW-extension resulting in magma
eruption. These motions mainly resulted from the continuous northward penetration of India plate to Eurasia.
Based on the time of formation of faults and basins, analysis of the cooling history of the mountain belt, as well
calculations of seismic foci mechanism, the Sagaing fault
dextral strike-slip motion is a major factor that constrains
the deformation and volcanic eruptions in the Tengchong
volcanic Weld. The volcanic eruptions from the late Miocene were not in a continental margin but rather in an
intracontinental tectonic setting.
Acknowledgements
This research is supported by a Main Project (95-11-03)
from the China Seismological Bureau and the China
National Basic Research Program Project (2002CB412601).
Discussions with Profs. Wang, C.Y., Fan, Q.C. and Wang,
E.C. help to improve the manuscript. We are indebted to
Prof. J.L. Whitford-Stark and an anonymous reviewer for
their constructive comments and suggestions, and Dr.
JeVrey Lee for improvements to English.
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