Russian Journal of Earth Sciences
Vol. 6, No. 1, February 2004
The Sea of Okhotsk crust from deep seismic sounding data
V. B. Piip
Moscow State University, Moscow
A. G. Rodnikov
Geophysical Center, Russian Academy of Sciences, Moscow
The crustal structure of the Sea of Okhotsk was investigated by
way of reinterpreting the data obtained along 22 deep seismic
sounding (DSS) profiles. Eight profiles traversed the Sea of
Okhotsk. These seismic surveys were performed during the
International Geophysical Year in 1958-1959 by the researchers
from the Institute of the Physics of the Earth, USSR Academy of
Sciences. Seven profiles were surveyed in 1963-1964 near the
shores of Sakhalin Island by geophysicists from the Institute of
the Physics of the Earth (USSR Academy of Sciences) and from the
Sakhalin Multidisciplinary Research Institute. The new
interpretation of the seismic data confirmed the earlier ideas of
the reduced crustal thickness in the deep-sea basins of the Sea of
Okhotsk, such as, the Kuril Basin, the Deryugin and Tinro basins,
the sedimentary trough of the Tatar Strait, where the Moho surface
showed low boundary velocities of seismic waves, not higher than
7.6-7.8 km s-1. It appears that the sedimentary basins of the Sea
of Okhotsk reside above asthenospheric diapirs including magma
chambers. A system of rifts and spreading centers was mapped in
the northern and central parts of the Sea of Okhotsk, in the Tatar
Strait, and in the Kuril Basin. Paleosubduction zones that had
been active during the late Cretaceous and Early Paleogene time
and are marked by ophiolite belts at the present time have been
traced in the earth crust near the eastern shores of Sakhalin.
Remnants of paleosubduction zones were mapped using seismic data
in the Sea of Okhotsk along the Okhotsk-Chukotka volcanic belt,
which seem to be the fragments of a lithospheric plate that had
been going down under the active continental margin in Mesozoic
The crustal structure of the Sea of Okhotsk was investigated by
way of reinterpreting the data obtained along 15 deep seismic
sounding (DSS) profiles. Eight profiles, namely, 1M, 6M, 9M,
10M, 11M, 12M, 13M, and 14M, traversed the Sea of Okhotsk. These
seismic surveys were performed during the International
Geophysical Year in 1958-1959 by the researchers from the
Institute of the Physics of the Earth, USSR Academy of Sciences,
at the highest level of that time. Seven profiles, namely, 18,
19, 20, 27, 28, 29, and 30, were surveyed in 1963-1964 near the
shores of Sakhalin I. by geophysicists from the Institute of the
Physics of the Earth (USSR Academy of Sciences) and from the
Sakhalin\linebreak Multidisciplinary Research Institute. The location of
the profile lines is shown in Figure 1. The profiles varied from
250 to 750 km in length. The interpretation of the data,
performed at that time, served as a basis for the further
studies of the geologic structure of the region. The travel-time
curves of the first-arriving waves plotted from these profiles
were published in the papers of that time
[Galperin and Kosminskaya, 1964;
Zverev and Tulina, 1971].
We digitized these travel-time data and interpreted them using modern-technology
computer means, namely, a method of homogeneous functions
[Piip, 1991, 1997, 2001\link6].
Detailed seismic sections were obtained to
depths of 30-60 km. On the average, the new seismic sections
agreed with the old ones in terms of the main
boundaries and velocity values. The resulting seismic sections
and velocity maps showed, for the first time, the blocks of
down-going plates, the remnants of the subduction zones, rifts
in the deep basins, and a system of gently dipping and steep
The data were interpreted using a method of homogeneous
functions. This method of interpretation and\linebreak a "GODOGRAF''
computer program had been developed at the Moscow State University
[Piip, 1991, 2001\link6].
This method is based on the local approximation of real velocity fields using
homogeneous two-coordinate functions. This approach allows one to derive
automatically, using the travel-time curves of the first waves,
the 2-D heterogeneous velocity fields, determined at the nodes
of a rectilinear 250 by 100 network. The cross-sections are
displayed as a field of velocity contour lines with a constant
interval (usually\linebreak 0.1-0.2 km s-1 ). Seismic boundaries and faults
are clearly seen in the velocity field to be traced by the
interpreter. This representation of cross-sections allows
plot horizontal velocity maps at various depths, where several
seismic profiles area available. This method of interpretation
does not require any initially assumed model and, hence, the
resulting seismic sections can be regarded as objective ones. We
verified their reliability (1) by the coincidence of the
vertical velocity curves at the points of the profile
intersections (Figure 2b), (2) by the solution of a direct
problem (the rms deviation of the observed travel times from the
calculated ones was 0.3-0.4 seconds) (Figure 2a), (3) by the
agreement between the depths of the main discontinuity surfaces
using the results of the previous and new interpretation.
In this work we used the following designations. In the
case of the continental crust we used
V =6.0-7 km s-1 for the
upper crust, and
V =7.0-8.0 km s-1 for the lower crust. The
first, second, and third layers of the crust were labelled as I,
II, and III. The upper mantle was designated by an
M symbol. The
thin lines are the velocity contours obtained using a computer,
the dash lines show boundaries and the faults. The explanation of the colored
lines are given in the figure captions.
The Northern Part of the Sea of Okhotsk
The seismic sections along the 12M and 13M lines in the
northern part of the Sea of Okhotsk are shown in Figure 3. The
earth crust consists of two layers there. The central part of
this segment of the Sea of Okhotsk (west of Kamchatka) includes
a clearly expressed rift with a break in the
M depth changes from 25 to 35 km) and with an abrupt
velocity decline in the lower crust. The upper crust is broken
by faults. The sedimentary layer shows some velocity growth. In
the vicinity of the continent the depth of the
M surface is 25
km; the lower crust shows a high velocity gradient (oceanic
crust?) and descends stepwise under the continent. It appears
that the seismic data recorded the remnants of some ancient
subduction zones which had been active, as follows from the data reported in
[Abramovich et al., 2001;
in the Mesozoic era, when the oceanic crust had been
subsiding under the Okhotsk-Chukotka volcanic belt which had
been an island arc at that time.
The Central Part of the Sea of Okhotsk
Two profiles, 9M-9O and 14M, were surveyed across
central part of the Sea of Okhotsk, from Sakhalin I. and the
Deryugin Basin to the Kuril-Kamchatka Island Arc\linebreak (Figure 4). In
the central part of the Sea of Okhotsk the depth of the Moho
surface is greater than 30 km, which suggests, along with the dredging data
the development of the continental crust. The crust consists of two, upper
and lower, layers. The crust is broken by faults and rifts. The Moho
surface is uneven, and the Upper\linebreak Mantle shows regions with
seismic velocities, as high as 9 km s
-1. In the vicinity of the
Deryugin Basin, the 9M-9O profile showed a growth of the
crustal thickness at the expense of the sedimentary layer. As
profile 14M approached the Sakhalin Island, where ophiolites
have been traced along the eastern coast of the island, the Sea
of Okhotsk lithosphere was found to contain the remnants of the
ancient zones of subduction which had taken place in the Late
Cretaceous-Early Paleogene time. The structural features
interpreted as relict subduction zones are present in the Sea of
Okhotsk and under the Kamchatka Peninsula. These structural
features have roughly equal sizes and some peculiarities.
Examples of the latter are the development of listric faults
(roughly 15?), extending from the sedimentary layer into the
upper mantle, the faults observed in the Moho surface, thickness
variations in the upper crustal layer, and the presence of
high-velocity crustal blocks.
North Sakhalin and Deryugin Basin
North Sakhalin includes the North Sakhalin sedimentary
basin. Its basement is composed of Triassic-Early\linebreak Cretaceous
volcanogenic siliceous and occasional Late\linebreak Cretaceous
volcanogenic rocks. Its depth ranges from 5.0 to 12 km in some
downthrown blocks and from 1.5 to 3 km in the internal
The sedimentary basin is filled with Cenozoic deposits which had
accumulated in the lower part during the rift stage and later
during the post-rift stage of the basin evolution. The structure
of the Sakhalin crust was studied only using the data collected
along the 11M profile, extending from the Shantar Ils. to North
Sakhalin (Figure 5),
the data collected along profiles 18 and 20,
extending along the western and eastern shores of Sakhalin (Figure 6).
This region is underlain by continental crust, 30-40 km
thick. The heat flow values are intermediate. The asthenosphere
was recorded by electromagnetic data to be at a depth of about 70 km
[Rodnikov et al., 1996].
This basin is located above an ancient subduction zone which is represented now by
Shmidt Rise separating it from the Deryugin Basin, and composed
of ophiolites. It is supposed that the ophiolites mark the
position of an old seismic focal zone, that is, of the mesozoic
zone of the subduction of the Sea of Okhotsk oceanic crust under Sakhalin
This supposition has been confirmed by
the Late Cretaceous-Paleogene volcanic arc discovered in the
eastern areas of Sakhalin by V. M. Grannik and found by him to be
composed of the fragments of volcanic islands, inter-arc and
forearc basins, overlying in an allochthonous manner the
marginal-sea rocks in the eastern areas of central and North Sakhalin
The volcanic arc is composed of andesite, dacite, rhyolite and their calc-alkalic
tuffs. In Late Mesozoic time a back-arc trough composed of terrigenous,
siliceous, and carbonate rocks, with occasional interlayers of
calc-alkalic volcanic rocks, had existed in North Sakhalin.
The Deryugin Basin is located east of the old subduction
zone (Figure 5).
Its sedimentary cover consists of\linebreak Cenozoic mainly
deep-sea terrigenous and siliceous-\linebreak terrigenous deposits up to 12 km
Its recent tectonic activity is accentuated by heat-flow values as high
as 200 mW m
-1 and by seismic events restricted
mainly to the western side of the Deryugin Basin, where an old
seismic focal zone has been located. The thick sedimentary cover
of the basin rests on the uneven surface of the acoustic
basement with seismic velocities of 6.2-6.4 km s
-1. The Moho
surface showed low velocities (7.6 km s
The insignificant thickness of the basement ( < 10 km) is believed to
owe its origin to the processes of extension and subsequent
subsidence. The basement is supposed to be composed of Mesozoic
oceanic volcanogenic-siliceous and argillaceous deposits which
are exposed in Sakhalin and West Kamchatka\linebreak
[Rodnikov et al., 1996].
The seismic profiles available show the subduction zone as a series of
en-echelon arranged blocks of elevated-velocity
crustal and upper mantle rocks. The Deryugin Basin is
distinguished by elevated heat-flow values, a thin crust, and
seismic and hydrothermal activity. The new interpretation of
seismic data suggests a rift structure at the base of the basin
The basin is located above a mantle hot plume produced
by a partial-melting asthenospheric diapir, discovered in the
upper mantle at a depth of 25 km
[Rodnikov et al., 2001a, 2001b].
western side of the basin is bounded by an ophiolite belt of
ultrabasic igneous rocks, which seems to mark an old
(Cretaceous-Paleogene) earthquake focal zone between the
Deryugin Basin and East Sakhalin, which had been an island arc
with andesite igneous activity at that time (100-60 million
In terms of its crustal structure the Tatar Strait is a
rift filled with Late Cretaceous-Cenozoic sandy and
argillaceous deposits up to 12 km thick. It is bounded by the
Sikhote-Alin and West Sakhalin mountains in the west and east,
respectively. The basement is composed of Triassic-Early
Cretaceous and occasional Late Cretaceous terrigenous sandy and
clayey and volcanic siliceous rocks. Its depth ranges from 5 to
12 km. The four seismic sections obtained along the profiles
crossing the Tatar Strait showed many features in common (Figures 7 and 8).
All profiles showed a rift in the central part of the
Tatar Strait, which had not been recorded by the previous
seismic profiles. The rift is distinguished by lower velocities
in the lower crust and a series of faults dissecting the crust.
The modern seismic activity is proved by high heat flow values,
magmatic activity, and seismic events. As a result, the crust
has a low thickness (25 km), as compared with the surrounding
regions, the seismic velocities along the Moho discontinuity
being 7.4-7.6 km s
-1. The faults recorded by deep seismic
soundings (DSS) have been confirmed by geological evidence. A
good example is the West Sakhalin fault, bordering the Tatar
Strait in the east, where the Cenozoic deposits are dipping
steeply (50o-80o) to the west and are highly disturbed by normal
and reverse faults. The movements along the faults vary from
hundreds of meters to 4-5 km. Restricted to
the fault zone are Early and Late Miocene and Pliocene volcanic
centers. The faults show a high seismic activity and fluid penetration
[Rodnikov et al., 1996].
The calculation of deep temperatures revealed that the sedimentary trough
is underlain by a hot asthenospheric diapir which had been responsible for
crustal breaks, the formation of rifts at the base of the
trough, magmatic activity, and the heating of the sedimentary
layer. The asthenospheric diapir might have served as an
additional source of hydrocarbons and fluids responsible for
high hydrothermal activity
[Rodnikov et al., 2001b].
Kuril Deep-Sea Basin
Two parallel 1M-1O and 6M profiles cross the Kuril Basin
with a distance of roughly 200 km between them. The
seismic sections along them are presented in Figure 9. The depth
M discontinuity in the Kuril Basin is 12-15 km. The
M surface showed velocities of 7.6 to 7.8 km s
-1. The crust consists
of three layers there. The sedimentary layer with a maximum
thickness of 4 km showed velocities ranging from 2.0 to 5-6 km s-1.
The thickness of the upper layer of the consolidated
crust, varying from 6.0 to 6.8 km s-1 in seismic velocities, is 4 to 6 km.
The lower part of the crust with high seismic velocities
(7.0-7.6 km s-1 ) varies greatly in thickness (4 to 10 km) in the
margins of the basin.
The western segments of the profile seem to have recorded
the blocks of the subducted plate. They belong to a subduction
zone near the eastern coast of Sakhalin. In this zone the
oceanic crust of the Kuril Basin had been subducted under the
continental crust of Sakhalin during late Cretaceous-Early
Seismic section 19 (Figure 8) crossed the ophiolites,
representing the remnants of some old subduction zone in South
Sakhalin. The oceanic crust and mantle dip under Sakhalin to a
depth of 50 km. The oceanic plate dips roughly at an angle of
20o in a depth range of 30-50 km. The western segment profile
shows a thick continental crust and a mantle. The crust has been
estimated roughly to be 35 km there. The structure based on the
data of profile 19 has been verified by the seismic section
recorded along meridional profile 20 (Figure 6).
Profile 20 (Figure 6) crossed the subduction zone near the
Sakhalin coast along the strike of this zone. The main block of
the subduction zone, located under the central segment of the
profile, is broken by faults and seems to be composed of
ophiolites which outcrop in the eastern part of Sakhalin. An
accretionary prism is supposed to be located along the 290-330-km
segment of the profile. The subduction zone is distinguished
by seismic velocities as high as 9-10 km s-1 and by high velocity
gradients. This structure of the subduction zone there was
recorded confidently along five profiles (19, 20, 27, 1M, and
6M), where the position of the subduction zone coincides
everywhere with the edge of the kuril basin. This can be seen in
which shows a seismic velocity map for a depth of 12 km.
A rift or spreading zone has been mapped in the central
part of the Kuril Basin using the seismic data obtained along
profiles 1M and 6M. This structural feature has an excellent
expression in the upper sedimentary layers. The faults
responsible for its formation extend into the upper mantle,
where the zones of abnormally low velocities\linebreak (7.0-7.5 km s
-1 ) are
likely to mark an asthenospheric diapir including magma-formation
chambers. The fact that the upper mantle under the
Kuril Basin includes partial melting regions has been proved by
[Lyapishev et al., 1987].
These measurements located a layer with a specific conductivity of
0.3-0.5 S m
-1 and an integral conductivity of about 15000 S. The
origin of this layer is believed to have been associated with
partial melting, and its size is controlled by the size of the
basin. Another high-conductivity layer is believed to exist at a
> 100 km. The results of this study agree with the upper
mantle temperatures, as well as with seismic and other geophysical data
[Maruyama et al., 1997].
This basin is known to show high heat values. The highest mantle temperatures,
as high as
1200o, have been observed under the Kuril Basin at a depth of
about 25 km, marking a partial melting zone
[Smirnov and Sugrobov, 1980].
Rifts and basic magmatism in the floor of the
Kuril Basin mark the rises of the hot anomalous mantle.
Horizontal Velocity Maps
A horizontal velocity map is presented in Figure 10 for
depth of 12 km. This map was plotted using the data provided by
25 profiles from the region discussed. Because the velocity
contour lines are usually parallel to the seismic boundaries,
the horizontal velocity maps reflect the positions of deep
crustal structural features, the low-velocity areas showing the
basins, and the high-velocity areas, the uplifts. The maps also
show faults and folds.
The map presented in Figure 10 shows the structural features
of the upper crust (Figure 10). These are the accretionary prism,
the structural features of the third oceanic layer in the
Pacific, and the positions of the subduction and rift zones in
the Kuril Basin. This map also shows several blocks of the
Pacific plate, separated by transform faults (Figure 10). The
positions of these faults are known from the seismological and
magnetic data available. One can see that the active subduction
plate in the area of the Kuril Trench is broken into large
segments with are turned clockwise relative to the trench axis.
This suggests an oblique subduction.
Figure 11 shows a shadow of velocity map for a depth level of
One can see that the main structural elements and faults in
the Sea of Okhotsk region are expressed well in the velocity
field and have a NE-SW strike similar to that of the Pacific
structural features. The exception is the Deryugin Basin,
bordering Sakhalin in the east, where the isometric structural
features of the crust are preserved. Figure 12 shows a
horizontal velocity map for a depth of 20 km. The upper mantle
is shown by a red broken line at this depth west of the trench
and in the Kuril Basin. The down-going upper mantle in the
pacific region is broken into three large blocks. Under the
Kuril Basin the sheared upper mantle has the form of an arc with
a low-velocity region (a mantle plume) in the middle. The upper
mantle showed low velocities in some other sedimentary basins,
namely, in the Deryugin Basin and in the Tatar Strait trough.
The lower continental crust underlies the Sea of Okhotsk in its
northern and central parts, and the upper crust, in the north of
The Sea of Okhotsk lithosphere is broken into blocks, as
proved by the horizontal variation of the geological and
geophysical parameters of the crust. The blocks of different
ranks are separated by crustal faults, rifts, fold-and-nappe
systems, and subduction zones.
The new interpretation of the seismic data confirmed the
earlier ideas of the reduced crustal thickness in the deep-sea
basins of the Sea of Okhotsk, such as, the Kuril Basin, the
Deryugin and Tinro basins, the sedimentary trough of the Tatar
Strait, where the Moho surface showed low boundary velocities of
seismic waves, not higher than 7.6-7.8 km s-1. It appears that
the sedimentary basins of the Sea of Okhotsk reside above
asthenospheric diapirs including magma chambers.
A system of rifts and spreading centers was mapped in the
northern and central parts of the Sea of Okhotsk, in the Deryugin Basin, in the Tatar
Strait, and in the Kuril Basin.
Paleosubduction zones that had been active during the late
Cretaceous and Early Paleogene time and are marked by ophiolite
belts at the present time have been traced in the earth crust
near the eastern shores of Sakhalin.
Remnants of paleosubduction zones were mapped using seismic
data in the Sea of Okhotsk along the Okhotsk-Chukotka volcanic
belt, which seem to be the fragments of a lithospheric plate
that had been going down under the active continental margin in
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