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Australia: The Land Where Time Began |
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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 Vp/Vs of 1.76, which
is similar to that of the Sino-Korean craton. The Vp/Vs
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 Vp/Vs 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/Ra 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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |