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P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia

Jianshe Lei

Seismological Laboratory, Institute of Crustal Dynamics, China Earthquake Administration, Xisanqi, Haidian District, Beijing 100085, China, jshlei_cj@hotmail.com

This webpage is a synopsis of the paper: Lei. Jianshe & D. Zhao, P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia, Tectonophysics, 397, 281– 295, 2005.


Changbai volcano, also called Tianchi or Baitoushan volcano, is located in the Changbai Mountains, close to the boundary between NE China and North Korea (Figure 1a). Changbai volcano erupted in BC 1120, AD 1050, 1413, 1597, 1668, and 1702 (Simkin & Siebert, 1994; Liu, 2000). However, the origin of such intraplate volcanoes is still unclear. Some researchers suggested that Changbai volcano originated either from accretion processes at the craton margin since the Mesoproterozoic or as a result of subduction processes associated with the Pacific plate before the opening of the Japan Sea (e.g., Zhang & O’Reilly, 1997). Turcotte & Schubert (1982) assumed that it is a hotspot like Hawaii, while Tatsumi et al. (1990) considered it to be a kind of back-arc volcano. Global tomographic models are still too poorly resolved to show the detailed structure and reveal the origin of the Changbai intraplate volcano.

Many researchers have used geological, geochemical and geophysical approaches to study Changbai volcano (e.g., Zhang & Tang, 1983; Zindler & Hart, 1986; Zhao, 1991; Basu et al., 1991; Zhang & O’Reilly, 1997; Fan et al., 1999a, b; Tang et al., 1999; Wang et al., 2003). Magnetotelluric soundings show that low-resistivity anomalies exist beneath it in the crust (Tang et al., 1997, 2001). Seismic explosion experiments revealed low-velocity anomalies in the crust and upper mantle down to a depth of 40 km, suggesting the existence of magma chambers (Zhang et al., 2002).

With the recent installation of 19 portable seismic stations during seismic experiments in NE China (Wu & Hetland, 1999), a few studies investigated the structure under NE China. For example, receiver function techniques were applied to the data set recorded by the portable seismic network to study crustal structure and upper mantle discontinuities (Ai et al., 2003; Li & Yuan, 2003; Hetland et al., 2004). These studies showed that the crust is thicker and contains low-velocity anomalies beneath Changbai volcano, and that the subducted Pacific slab lies flat in the mantle transition zone there. It has also been suggested that pieces of slab material penetrated into the lower mantle under NE Asia (Ai et al., 2003). However, no tomographic study of this was made using the new data set. The availability of abundant teleseismic data recorded by the portable seismic network enabled us to determine the 3-D velocity structure beneath Changbai volcano and investigate the origin of the volcano.

Figure 1a shows the distribution of seismic stations used in this study. The portable seismic network contains 19 broad-band seismic stations equipped with Guralp 3T three-component digital seismometers (Wu & Hetland, 1999; Hetland et al., 2004). In this work, data from 3 permanent seismic stations, MDJ, HIA, and BJT were also used (Figure 1a). These stations are sited adjacent to the portable seismic network and have recorded data since 1986. The 19 temporary seismic stations were located around Changbai volcano and were in operation from late June to September 1998. Ten continued recording until April 1999 (Hetland et al., 2004). The geometry of the subducting Pacific slab as estimated from the deep seismicity by Gudmundsson & Sambridge (1998) is also shown in Figure 1a.

 

Figure 1: (a) Locations of 19 portable seismic stations (solid triangles) and 3 permanent stations (diamonds) used in this study. Open triangles denote intraplate volcanoes. CB, Changbai; LG, Longgang; XJD, Xianjindao; CUR, Ch’Uga-Ryong (Simkin & Siebert, 1994). The dotted lines show the depth contours of the Wadati-Benioff deep seismic zone (Gudmundsson & Sambridge, 1998). (b) Epicentral locations of the 68 teleseismic events (diamonds) used in this study. The triangle denotes the center of the NE China Seismic Network.

P-wave arrival times were hand-picked from high-quality original seismograms. The resulting data set contains 548 high-quality P arrival times from 68 teleseismic events (M > 4.8; Figure 1b). The events selected have a good azimuthal coverage except in the Pacific Ocean and Russia. Except for one event in Kamchatka that has an epicentral distance of about 25°, all events lie in the range 30-90° from the network.

The tomographic method of Zhao et al. (1994) was used to determine the 3-D P-wave velocity (Vp) structure beneath Changbai volcano. Our results (Figure 2) show a columnar low-velocity anomaly extending to 400 km depth with a P-wave velocity reduction of up to 3%. High-velocity anomalies are visible in the mantle transition zone, and deep-focus earthquakes occur at depths of 500-600 km beneath the region, suggesting that the subducting Pacific slab lies flat in the transition zone there, as imaged clearly by global tomography (Zhao, 2004; Lei & Zhao, 2006a). Such structural features are similar to those beneath Tengchong volcano in SW China (e.g., Lei et al., 2009a), but different from those under the Hainan, Hawaii and Iceland hotspots (e.g., Zhao, 2001a; Montelli et al., 2004; Lei & Zhao, 2006b; Lei et al., 2009b).

Figure 2: (a) North-south, and (b)  east-west vertical cross sections of P-wave velocity images. Red and blue colors denote low and high velocity anomalies, respectively. The velocity perturbation scale is shown below the cross sections. Black triangles in (a) and (b) denote volcanoes. White dots show earthquakes within 100 km of the profiles. The two dashed lines denote the 410- and 660-km discontinuities. The locations of the cross sections are shown in the insert map (c). Labels in (c) are the same as in Figure 1a.

These results indicate that the active intraplate volcanoes in NE Asia are not hotspots but a kind of back-arc volcano closely related to the subduction process of the Pacific slab. Low-velocity anomalies in back-arc regions are generally associated with back-arc magmatism and volcanism caused by the deep dehydration process of the subducting slab and convective circulation process of the mantle wedge (Zhao et al., 1997; Zhao, 2001b). These processes may lead to large-scale upwelling of asthenosphere under NE Asia and cause intraplate volcanism and continental rift systems in the region. Tatsumi et al. (1990) first invoked asthenospheric injection to explain the formation of Wudalianchi and Changbai volcanoes, but they did not consider the flat Pacific slab under the region because such a slab structure was unknown at that time. Here we modified the model of Tatsumi et al. (1990) to emphasize the role of the flat Pacific slab in the formation of intraplate volcanism in E Asia (Figure 3).

Figure 3: (a) Tectonic features on the surface in the NW Pacific and NE Asia. Black patches denote Cenozoic basalts. A, Baikal rift; B, Shanxi graben; C, Tancheng-Lujiang fault zone; D, Okinawa trough. (b) Schematic east-west vertical section showing upper mantle structure beneath NE Asia. The subducting Pacific slab becomes flat in the mantle transition zone. The deep dehydration process of the slab and convective circulation process in the mantle wedge cause upwellings of high-temperature asthenospheric materials, leading to the formation of the continental rift system as well as intraplate volcanoes in NE Asia (modified from Tatsumi et al., 1990).

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last updated 24th March, 2009
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