Download Upper-mantle velocity structure beneath the North China Craton

Geophys. J. Int. (2009) 177, 1279–1283
doi: 10.1111/j.1365-246X.2009.04120.x
Upper-mantle velocity structure beneath the North China Craton:
implications for lithospheric thinning
Peifen Xu1 and Dapeng Zhao2
1 Key
Laboratory of the Study of Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, China
E-mail: [email protected]
2 Department of Geophysics, Tohoku University, Sendai 980-8578, Japan
SUMMARY
The presence of high-velocity lithospheric ‘keels’ in the upper mantle is a ubiquitous feature
of stable continental provinces. However, the upper-mantle structure of the reactivated craton,
whether all such high-velocity (high-V) keels are eventually removed and the mechanism of
removal, remain unclear. In this work, we determine 3-D seismic velocity structure beneath the
eastern part of the North China Craton and the surrounding regions, using a large number of
data from local, regional and teleseismic events. Our results show that the upper-mantle high-V
layer, which represents the lithosphere, is only 60–100 km thick beneath the Huabei Basin,
whereas it is at least 150 km thick under Liaodong. Over 100-km thick high-V anomalies
are visible atop the 410-km discontinuity and right beneath the lithospheric-thinning area. We
interpret the high-V anomalies as the remnants of the delaminated lithosphere, which caused
the lithospheric thinning under the eastern North China Craton. The most significant feature
of our tomographric results is that the lithospheric delamination occurred in localized areas
rather than the entire eastern North China Craton. It is an important observation that provides
geophysical constraints on the delamination mechanism.
Key words: Seismic tomography; Cratons; Dynamics of lithosphere and mantle; Asia.
I N T RO D U C T I O N
Cratons are relatively stable tectonic units on Earth. The cratons that formed during Archaean to early Proterozoic, such as
Kaappvaal in South Africa, Siberia in Russia and Slave in Canada,
have been stable for a long geological time. The continental keel under these cratons has a thickness of 250–300 km or greater (Polet &
Anderson 1995; Artemieva & Mooney 2001; James et al. 2001).
Under certain tectonic environments, however, old cratons can be
reactivated and their lithospheric keels can be removed, leading
to extensive magmatic activities. The eastern North China Craton
(NCC; Fig. 1) is the best example of an Archaean craton that has
lost its lithospheric keel (Fan & Menzies 1992; Menzies et al. 1993;
Carlson & Pearson 2005). In the last decade, many petrological and
geochemical studies have been made for the Palaeozoic kimberliteborne xenoliths and Cenozoic basalts and their mantle inclusions
(e.g. Fan & Menzies 1992; Menzies et al. 1993). These studies suggested that the Archaean lithospheric mantle with a thickness of at
least 220 km had existed under NCC till the Palaeozoic age, whereas
strong tectonic and magmatic activities occurred in the Mesozoic,
which reactivated the NCC. As a result, the lower part (over
100 km thick) of the lithosphere was removed and the lithospheric
thinning took place under the eastern NCC. Almost all the previous
results on the lithospheric thinning under the eastern NCC were
derived from petrologic and geochemical studies, and there has
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whether the eastern NCC lithospheric keel was totally removed, and
what mechanism caused the lithospheric thinning. Determination
of the detailed structure of the upper mantle under NCC is very
important for resolving these problems. In this work, we present
high-resolution tomographic images under the NCC, which indicate
that the lithospheric thinning might be caused by delamination and
the thermomechanic/chemical erosions associated with upwelling
of asthenospheric materials.
D ATA A N D M E T H O D
The seismic tomography method of Zhao et al. (1992, 1994) (see
Supporting Information, SI) was applied to determine the 3-D Pwave velocity structure of the crust and upper mantle under northeastern China (Fig. 1). We used 416 permanent seismic stations
installed in the study area during 1978–2006. In this study, we had
newly deployed 30 portable seismic stations with broadband seismometers during May 2004–July 2006. As shown in Fig. 1, 20
portable stations were located in the western part of the study area
(central Inner Mongolia District), and 10 stations were located in
the eastern part (eastern Liaoning and Jilin Provinces). We carefully
selected earthquakes, following the criteria described in SI. As a result, our data set includes 76 413 P-wave arrival times from 10 185
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GJI Tectonics and geodynamics
Accepted 2009 January 15. Received 2009 January 14; in original form 2008 October 30
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P. Xu and D. Zhao
Figure 1. Map showing the present study area and the distribution of seismic stations used. Red triangles denote the permanent stations of Chinese Seismic
Network, whereas black triangles denote portable stations that were deployed by this study. Three black lines show the locations of the cross-sections in
Fig. 3. The Archaean NCC is cut by two major geological and geophysical linear zones: the Tan–Lu fault zone (TLFZ) to the east and the Daxinganling–
Taihang gravity lineament (DTGL) to the west. Along the northern boundary of the NCC (NBNCC), there is a late Jurassic E–W trending fold-and-thrust belt
(Davis 2003).
earthquakes that occurred in the study area (Fig. 2a), and 107 847 P
arrival times from 6335 teleseismic events (Fig. 2b). All the selected
teleseismic events have epicentral distances of 30◦ –100◦ , each of
which has more than five P arrivals.
The initial 1-D velocity model is shown in Fig. S1 (SI). For
details of the model parameterization and data analysis, see SI.
Results of detailed resolution analyses (Fig. S3; SI) indicate that
our tomographic image has a lateral resolution of about 1◦ under
the entire study area, whereas the resolution is about 0.5◦ under the
central portion of the study area, where many seismic stations exist.
This resolution scale is much better than that of the previous regional
tomography under East Asia (Huang & Zhao 2006). Our extensive
synthetic tests show that both the high-velocity (high-V) anomalies
in the lithosphere and those existing above the 410-km discontinuity
and the low-velocity (low-V) asthenosphere were reliably resolved,
and there was no trade-off between them.
R E S U LT S
Fig. 3 shows three vertical cross-sections of the obtained tomographic image along profiles as indicated in Fig. 1. These
cross-sections show a similar pattern of velocity structure. Strong
heterogeneities are visible beneath both sides of Mt. Taihang. A
prominent feature is a high-V anomaly, shown by the dashed lines,
with relative velocity perturbations of +1.5–+3.0 per cent. Beneath
the eastern NCC, there is a ∼150–200 km thick low-V anomaly extending down to 300 km depth, which is located right beneath the
high-V anomaly.
Seismic lithosphere denotes a high seismic velocity layer above
the asthenosphere, which has a relatively low seismic velocity
(Anderson 1995). Our result shows that the thickness of the seismic
lithosphere is only 60–100 km under Huabei Basin (Figs 3a and b),
which is quite consistent with the results of seismic receiver functions (Chen et al. 2006), petrologic estimation (e.g. Fan & Menzies
1992; Menzies et al. 1993), geothermal (Hu et al. 2000) and geoelectric studies (Li 1996). It is considered that the lithosphere was
about 220 km thick before the Mesozoic (e.g. Fan & Menzies 1992;
Menzies et al. 1993), hence the lithosphere was thinned by as much
as ∼140 km. In addition, our tomographic results show that the
area with the thinnest lithosphere (< 60 km) is located in Jiaodong
Peninsula and central Bohai Sea (Fig. 3). Under those areas, low-V
asthenospheric materials ascend up and reach close to the Moho
discontinuity. In the surrounding areas, such as Mt. Taihang and
Mt. Yanshan, however, the lithosphere was not thinned very much.
The high-V lithosphere under Liaodong has a thickness of 150–
160 km (Fig. 3b), which is consistent with the magnetotelluric (MT)
sounding result (Li 1996).
A unique characteristic of Archaean cratons is that they are underlain by a high-V keel that extends down to depths of at least
200–350 km (e.g. Polet & Anderson 1995; Jordan 1975; James
et al. 2001). Our results show that the high-V lithosphere is still as
thick as ∼230 km under the Ordos plateau in western NCC (Figs 3a
and b), which exhibits features of a typical craton. The Ordos region
is characterized by very low seismicity, no magmatic activity and
very low heat flow, which are opposite to the features in the eastern
NCC.
It is interesting to see that pronounced high-V anomalies exist above the 410-km discontinuity under the eastern NCC from
Mt. Taihang, Huabei Basin to Bohai (Figs 3a and b), as well
as areas south of Mt. Yanshan (Fig. 3c). The high-V anomalies
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Figure 2. Epicentral distribution (red dots) of (a) 10 185 local and regional
earthquakes and (b) 6 335 teleseismic events used in this study. The white
box shows the present study area. All other labeling is the same as that in
Fig. 1.
have a thickness of ∼100–140 km and velocity increase of +1.5–
+2.5 per cent.
D I S C U S S I O N A N D C O N C LU S I O N S
The present results show that the lithosphere is very thin (<100 km)
beneath Huabei Basin, Jiaodong Peninsula and Bohai Bay, which
are located east of Mt. Taihang and south of Mt. Yanshan, whereas
the lithosphere is 100–150 km thick in the surrounding areas like
Mt. Taihang, Mt. Yanshan–Liaoxi, and Liaodong. Under the Ordos
Block, the lithosphere is as thick as ∼230 km. Within the NCC,
there is a big difference in the lithospheric thickness across Mt.
Taihang, which may reflect a deep dynamic process. In fact, during the period from the Late-Jurassic to Early-Cretaceous, the NCC
was controlled by not only the dynamic system of the Pacific plate
subduction but also the far-field tectonic effects produced by the
convergence and collision of the Siberian Plate from the north and
the Palaeo-Asian tectonic belt along the Mongol–Okhotsk Ocean,
as well as the SW compression resulting from collision between
the Lhasa Block and Qiangtang Block. The NCC was a centre,
and different plates converged towards it, forming multidirectional
compression and convergence (Dong et al. 2008). Late-Jurassic
multidirectional compression and intracontinental orogeny and
Early-Cretaceous large-scale lithospheric extension and thinning
(craton destruction) are considered to be the direct consequence of
this geodynamic setting (Dong et al. 2008).
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So far, many researchers have proposed different mechanisms
for the lithospheric thinning under NCC. Geochemically, a unique
type of Late Jurassic and Early Cretaceous igneous rocks exposed in this region have adakitic characteristics with high K 2 O,
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Sr/86 Sr > 0.704, ε Nd < 0, low heavy rare-earth elements (HREEs)
and negligible negative Eu anomalies, suggesting that lithospheric
delamination could have taken place in this area (Defant et al.
2002; Xu 2002; Davis 2003; Gao et al. 2004). The unique mafic
granulite and pyroxenite xenolith found in the Cenozoic alkaline basalts at Hannuoba (near Zhangjiakou in Fig. 1) have been
linked to basaltic underplating and granulite-facies metamorphism
(Xu 2006; Zhou et al. 2002; Liu et al. 2001). These results suggest that lithospheric delamination might have taken place, because as a result of accompanying removal of the lithospheric
mantle blob (lithospheric thinning), hot asthenospheric materials
will upwell into the space previously occupied by the delaminated blob, resulting in a regional horizontal mantle flow and eruption of asthenosphere-derived alkali basalts (Kay & Kay 1993).
Observations of shear wave splitting show northeast fast-velocity
directions in Liaodong and Jiaodong, whereas northwest fast directions prevail in Yanshan (Zhao & Zheng 2007). These observations
have been explained by a regional horizontal mantle flow that developed due to the upwelling asthenosphere above the delaminated
lithosphere (Zhao & Zheng 2007). Regional receiver-function analysis (Zheng et al. 2007) revealed a low-V zone, supporting the
hypothesis of asthenospheric ascension. According to the model
of delamination magmatism (Kay & Kay 1993), for regions where
compression has thickened the crust >50 km, basaltic composition rocks in the crust will undergo large density increases due to
‘eclogitic’ phase transitions and result in a crustal root. Under this
condition, not only does the shallowest lithospheric mantle furnish
the greatest negative buoyancy, but also that the lowermost part
of thickened (>50 km) crust, if it is basaltic, contributes negative
buoyancy as well. Therefore, the crustal root becomes denser than
the underlying mantle lithosphere and should delaminate with it.
Our present tomographic results support the lithospheric delamination mechanism. It is remarkable that the high-V anomaly atop
the 410-km discontinuity we imaged under the eastern NCC is located right beneath the areas where the lithospheric thickness is
less than ∼100 km (Figs 3a–c). The geographical coincidence may
reflect the tectonic processes such as lithospheric delamination. In
fact, previous seismic tomographic studies have revealed high-V
zones in the mantle transition zone under East Asia, which may
reflect the stagnant Pacific slab (Huang & Zhao 2006; Zhao et al.
2007) or detached lithospheric bodies (Seber et al. 1996; Calvert et
al. 2000).
An alternative interpretation is that the high-V anomaly atop
the 410-km discontinuity might reflect a subducted oceanic lithosphere. But why does the remnant of the delaminated lithosphere
stay atop the 410-km discontinuity? We still cannot explain this
well. The detached lithospheric mantle and lower crust may collapse down to the mantle transition zone, just like the subducting oceanic slabs stagnating above the 660-km discontinuity (Zhao
et al. 2007), because the olivine–spinel (410 km) and post-spinel to
perovskite plus magnesiowüstite (660 km) phase transitions act as
a barrier to short-wavelength convective flows (Tackley 1996). In
the present study, however, the stagnant Pacific slab exists beneath
the delaminated lithospheric materials, which may become a barrier and so cause the detached lithospheric materials stay above the
410-km discontinuity.
Note that our present tomographric results show that the lithospheric delamination may only take place in localized areas, rather
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Figure 3. Vertical cross-sections of P-wave velocity image along the lines showing in Fig. 1. Red and blue colours denote low and high velocities, respectively.
The velocity perturbation scale is shown at the bottom. Shallow and deep earthquakes within a 20 km width from each profile are shown as crosses and white
dots, respectively. The two dashed lines denote the 410 and 660 km discontinuities. The solid white line denotes the Moho discontinuity derived from deep
seismic soundings (Ma 1989; Jia et al. 2001).
than the entire eastern NCC. It is an important observation that
provides geophysical constraints on the delamination mechanism
and mantle dynamics under the NCC.
AC K N OW L E D G M E N T S
We are grateful to the critical and helpful review comments from
Profs. Thomas Hearn and David James. We thank Profs. F.T. Liu,
R.X. Zhu, K. Ye, M.G. Zhai, H.F. Zhang for their constructive suggestions and discussion and Dr M.F. Abdelwahed, Chuanjin Li and
Yongjun Sun for their help at the data processing stage. We are
grateful to the staff members of the Chinese Seismic Network, operated by China Seismological Bureau, for providing the valuable
data used in this study. We also thank the participants of the Broadband Seismic Array of Geological Survey of Canada and Laboratory
of IGGCAS. This research was supported by the Knowledge Inno-
vation Program of the Chinese Academy of Sciences (Grant No.
KZCX2-YW-103) and the National Natural Science Foundation of
China (Grants No. 40374035) to PX and a grant from Japan Society
for the Promotion of Science (Kiban A-17204037) to DZ. Figures
were made by using GMT (Wessel & Smith 1991).
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S U P P O RT I N G I N F O R M AT I O N
Additional Supporting Information may be found in the online version of the article:
Figure S1. Initial 1-D velocity model used in this study.
Figure S2. Configuration of the grid net adopted in this study.
Figure S3. Trade-off curve for the norm of velocity perturbations
and root mean square (rms) traveltime residual. Numbers beside the
gray circles denote the damping parameters adopted for the tomographic inversions. The black circle denotes the optimal damping
parameter for the final tomographic model.
Figure S4. Results of checkerboard resolution tests for P-wave
tomography at depths of 38 (a, d), 140 (b, e) and 420 km (c, f),
respectively. Open and solid circles denote low and high velocities,
respectively. The velocity perturbation scale is shown at the bottom.
The grid net in (a-c) is the same as that adopted for the inversion
of the real data (Fig. S2). The lateral grid interval is 1 degree
in (d-f).
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