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Mt Paektu (Changbaishan in China) – Evidence for Partial Melt in the Crust Beneath Mt Paektu, a Supervolcano, North Korea and China

Mt Paektu is an enigmatic volcano on the border between the Democratic People’s Republic of Korea (DPRK) and China. According to Kyong-Song, R., et al. comparatively little is known about the magmatic evolution, geochronology and underlying structure of Mt Paektu, in spite of its being responsible for one of the largest eruptions in history. The results of receiver function from an unprecedented seismic deployment in the DPRK are presented in this paper. These are the first estimates of the crustal structure on the DPRK side of the volcano and indeed for anywhere beneath the DPRK. About 60 km from the volcano, the crust is 35 km thick and has a bulk of V_p/V_s­ of 1.76, which is similar to that of the Sino-Korean craton. The V_p/V_s­ ratio about 20 km from the volcano increases, reaching more than 1.87 directly beneath the volcano. A large region of the crust has been modified by magmatism that is associated with volcanism. It is suggested by such high values of V_p/V_s­ that partial melting is present in the crust beneath Mt Paektu. A potential source for magmas that were erupted over the last few thousand years is represented by this region of melt, and Kyong-Song et al. suggest it may be associated with an episode of volcanic unrest that was observed between 2002 and 2005

The Mt Paektu eruption in about 946 AD (Xu et al., 2013), is one of the largest in the historical record (Oppenheimer, 2011) with an estimated 24 km (dense rock equivalent) of rhyolite and trachyte magma erupted (Horn & Schmincke, 2000).  Attention was drawn to this enigmatic volcano from 2002-2005, a period of seismic unrest, by deformation of the ground and anomalies in fluid geochemistry (Xu et al., 2012; Wei, Liu & Gill, 2013). As a result there was a unique collaboration between scientists from the DPRK, UK, and the USA to try to understand the geological history and internal structure of the volcano (Stone, 2013). Receiver function (RF) estimates of the crustal structure beneath the volcano are presented in this paper. It is suggested by the receiver function that there are significant amounts of melt present in the crust beneath the volcano and a lateral extent of this melt of at least 20 km. According to Kyong-Song these are the first images of the structure beneath the DPRK side of the volcano and the first characterisation of the crustal structurer beneath any part of DPRK.

Geological background

The composition of Mt Paektu is a trachybasalt shield, that dates to about 2.8 – 1.5 Ma, a trachyte stratocone, dating to about 1.0-0.04 Ma, and a comendite ignimbrite (Holocene) above a granitic basement that dates to the Archaean and Mesozoic (Wang, Li, Wei & Shan, 2003; Wei et al., 2007). The origins of the volcano seem to be related to the subduction of the Pacific Plate below the Eurasian Plate, though the origins have remained enigmatic. It has been shown by many seismic tomographic models that there is a stagnant slab within the transition zone beneath northeast China (Zhao, 2004; Li & van der Hilst, 2010; Wei, Xu, Zhao & Shi, 2012; Tang et al, 2014) As well as results from the joint receiver function/surface wave inversions (Guo et al, 2015), these models reveal the presence of low upper-mantle seismic velocities linked to the presence of hot upper mantle that is beneath Mt. Paektu/Changbaishan. There are more than a single interpretation of these low velocities, one of which is that they represent water that was released from the slab within the transition zone and leads to a “big mantle wedge,” where there is upwelling above the slab (Zhao, 2004; Li & van der Hilst, 2010; Wei et al., 2012; Tang et al., 2014; Guo et al, 2015; Maruyama, 2009). More recent tomographic images (Tang et al., 2014) and measurement of discontinuity depths of the transition zone (Liu et al., 2015) that are based on seismic data from northeast China have been interpreted as indicating the presence of a gap in the stagnant slab. Hot subslab material that rises through this gap to the upper mantle is then said to explain the regional volcanism. All the observations are consistent with a source of partial melt in the mantle beneath Mt Paektu, though there is no consensus view on the origins of volcanism at this volcano.

Previous attempts to estimate the crustal structure beneath Mt Paektu have been restricted to the Chinese side of the volcano. It has been shown by controlled-source seismology there is a prevalence of low velocities in the lower crust beneath the volcano (Zhang et al, 2002; Song et al., 2007), though debate has continued in regards to the details of the structure. It is suggested by controlled-source data that the lowest velocities are located 30-60 km to the north (Song et al., 2007). A region of high conductivity in the lower crust directly beneath the volcano (Tang et al., 2001) has been revealed by magnetotelluric data and associated models, in a similar location to that of the low velocities that have been suggested by Zhang et al. (Zhang et al., 2002). Also, it is indicated by forward-modelled gravity data that low densities are present in this region (Choi, Oh & Götze, 2013). Finally, a few months of teleseismic observations that was recorded close to Mt Paektu, that were based of receiver frequencies, are consistent with a low-velocity crust (Hetland, Wu & Song, 2004). All of these studies interpreted the anomalous lower crust as being indicative of melt that could possibly be linked to the past eruptions as well as the recent unrest beneath the volcano.

Recent unrest

From 2002-2005, the unrest was characterised by significant seismicity. Earthquakes were independently located by teams from DPRK (21) and China (Xu et al., 2012), and the epicentres were shallower than 5 km. Another manifestation of unrest was ground deformation, which was measured by Global Positioning System and levelling on the Chinese side of the volcano. A Mogi source at depths of 2-6 km was used to model it (Xu et al., 2012). These observations were seen to be indicative of recharge of a shallow magma chamber. At hot springs, with high ³He/⁴He

 (R/R_a of 5.6), increases in He emissions were taken to suggest there was possible contribution from a deeper source, possibly in the mantle (Xu et al., 2012; Wei, Liu & Gill, 2013). These results suggest the existence of a complex magma storage region with shallow as well as deep crustal melt storage regions beneath Mt Paektu.

Sources & Further reading

1. Kyong-Song, R., et al. (2016). "Evidence for partial melt in the crust beneath Mt. Paektu (Changbaishan), Democratic People’s Republic of Korea and China." Science Advances 2(4).