Geotraverse across the Sikhote Alin-the Sea ...

Geophysical Center of the Russian Academy of Sciences, Moscow, Russia
World Data Center for Solid Earth Physics
The Geotraverse Project

The Japan Sea Geotraverse

Marine Geophysical Researches, 7, 1985, 379-387.

GEOTRAVERSE ACROSS THE SIKHOTE ALIN - THE SEA OF JAPAN
- THE HONSHU ISLAND - THE PACIFIC

A.G. RODNIKOV, A.G. GAINANOV, B.V. YERMAKOV, V.M. KOVYLIN,
V.A. SELIVERSTOV, YA.B. SMIRNOV, P.A. STROEV and YU.K. SHCHUKIN

Soviet Geophysical Committee, Molodezhnaya, 3, Moscow, GPS-1, U.S.S.R.

T. KATO and H. SHIMAMURA

Abstract

The paper presents the results of geological-geophysical research carried out during the Soviet-Japanese cooperative study of the structure and dynamics of the Earthēs crust and upper mantle in the transition zone from the Pacific Ocean to the Asian continent. The 300 km deep geological-geophysical section of the tectonosphere (geotraverse) has been compiled on the basis of combined interpretations of seismic, geological, petrographic, gravimetric, magnetometric, electromagnetic and heat flow measurements. Estimates were made of deep temperatures along the geotraverse and of the depths of the partial melting level that can be identified with the upper boundary of the asthenosphere.

Introduction

As part of the Soviet-Japanese Geophysical Program, a geotraverse was made across Hanka lake, Sikhote-Alin, the Sea of Japan, Honshu island (Oga Is., "green tuff" region Kitakami massif, Ojika peninsula), the Japanese trench and the Pacific Ocean

Deep Geological-Geophysical Cross-Section along the Japan Sea Geotraverse

A complex data interpretation technique was used to build a geological-geophysical- petrochemical model for the tectonosphere within the transition zone from the Asian continent to the Pacific. This complex approach to data interpretation permitted the modelling of the structure of the Earthēs crust and upper mantle, seismic foci, and the structure of the focal zone. Temperatures of depth were computed along the geotraverse and also the level of the partial melting zone which is usually related to the upper boundary of the asthenospheric layer.

The orientation for the geotraverse was conditioned by a large body of data obtained in this region during various geological-geophysical researches performed by the Soviet and Japanese scientists within the framework of the Geodynamic Project.

The present review is based on the book Rodnikov et al. ( 1982 ) . The pattern of the Earth's crust within Primorje follows the work of Argentov et al. ( 1976 ) . with B. V. Ermakov's geological interpretation. The structure of the crust and upper mantle of the Sea of Japan is based on the results of the Soviet and Japanese researches ( Kanamori, 1970 ; Kaseno, 1972 ; Murauchi et al., 1969 ; The crust and upper mantle ..., 1972 ; Vasilkovski, 1978 ). For modelling the Earth's crust and the upper mantle within Honshu I., data obtained by Japanese scientists were used ( The crust and upper mantle ..., 1972, 1973 ; Asada and Asano, 1972 ; Sugimura and Uyeda, 1973 ; Japanese Research Group, 1978 ) with additional data kindly presented by T. Kato ( Minato et al., 1978 ). The structure of the Earth's crust and upper mantle of the Pacific is shown after Asada and Shimamura ( 1976 ) . The position of the focal zone is determined according to Yoshii ( 1979 ) and Hasegawa et al. ( 1979 ) . The petrographic pattern of the upper mantle for Honshu I. and the Sea of Japan is described after Takahashi ( 1978 ) , and for Primorje - by the data of Soviet scientists and a synopsis made by V. A. Seliverstov. Profiles of the magnetic and gravity fields were drawn by A. G. Gainanov and P. A. Stroev ( Stroev and Maksimova, 1980 ; Tomoda, 1973 ) with data interpretation by Yoshi ( 1979 ) and Shevaldin ( 1978 ) . Deep temperatures were computed along the geotraverse by Ya. B. Smirnov. Results of the drilling from R/V "Glomar Challenger" were used to determine the composition of the sedimentary layer in the Sea of Japan and in the Pacific ( Heezen and MacGregor, 1973 ; Karig et al., 1975 ; Scientific Party, 1980 ). This comprehensive method of geophysical data interpretation shows the geological setting from a number of differing aspects, defines particular features of the deep structure of the tectonosphere within the transition zone and outlines basic trends in the interior processes at great depth.

Earth's Crust

The large-scale geostructures crossed by the geotraverse, the Sikhote-Alin, the deep basin of the Sea of Japan, the structures of the Honshu I. and of the Pacific differ in both the crust and upper mantle. The thickness of the crust within Primorje is 35-40 km, in the Sea of Japan it is 12-15 km, on the Honshu I. about 30 km and in the Pacific approximately 6-8 km. The Japanese deep basin is bounded by two continental blocks composed of rocks of the Precambrian age.

The most ancient formations in Primorje are the early Proterozoic metamorphic rocks uncovered in the Khanka massif (biotite and amphibolite gneisses, amphibolite rocks, biotite and diopside crystalline schists). The upper Proterozoic-early Cambrian formations metamorphosed in the green-schist facies from a terrigenous-carbonate-siliceous complex. These rocks seem to from the basement of the Primorje structures which can be divided into 4 or 5 layers according to the deep-seismic sounding data.

The presence of the Precambrian rocks on the Honshu Island is estimated to be quite probable. If this is so, the highly metamorphosed gneisses and straurolite kyanite schists developer in Abakuma, and the sillimanite gneisses, amphibolites and crystalline schists in the from of xenolith observed in the Mesozoic serpentinite of the Kitakami massif, are of the Precambrian age. It is probable that Silurian limestone composing corals were formed under shallow-water conditions thus transgressively overlapping the Precambrian rocks. Paleozoic volcanogenic-sedimentary rocks are metamorphosed into various crystalline schists of the green-schist facies. These rocks from the "granite" layer of the island. The "basaltic" layer seems to be composed of Precambrian metamorphic complexes including granulite, hornblendite gabbro and amphibolites.

Within the region of the geotraverse and magmatism shows the following features:

(i) In Primorje, the Sea of Japan and on Honshu Island, granitoids prevail among the intrusive rocks.

(ii) In Primorje, granitoids have high K ₂O content while within Japan with the exception of the ancient Paleozoic carboniferous granites of Higami, the rocks have natrium predominance.

The seismic cross-section of the Earth's crust in the region of "green tuffs" is characterized by low velocity of seismic waves is 7.6 km sec^-1. By contrast, under the Kitakami massif, velocities are more normal, i.e. 8.0-8.1 km sec^-1. This phenomenon seems to be related to active magmatic processes which occurred during the Cenozoic period within the region of "green tuffs" and to partial melting of the mantle.

Upper Mantle. Asthenosphere

Numerous research projects carried out by the Japanese scientists were based on the interpretation of the surface and body waves of the earthquakes and large events. These permitted the determination within the upper mantle under the Sea of Japan of a thick layer (over 100 km) with low velocities of P and S waves ( Kanamori, 1970 ; The crust ..., 1972 ). It was concluded that the upper mantle between the continent and the island arc has lower velocities of seismic waves (by about 0.3-0.4 km sec-1) as compared to the oceanic region. It also has a greater degree of energy absorption. In the upper mantle, within the transition zone from the Sea of Japan to the Pacific basin, the change in velocity is rapid, not gradual.

The Benioff zone (100-150 km wide) is accepted as the boundary between these structures. The great majority of earthquake foci are related to this zone.

The upper mantle structure in the north-western Pacific basin was studied by explosion seismology using the body waves registered by ocean bottom seismographs. The velocity of seismic waves is unusually high ( Asada and Shimamura, 1976 ). The low velocity layer (wave-guide) at the depth of 100 km has a thickness of 30-40 km and a velocity of 8.4-8.6 km sec-1 while under the Sea of Japan at the same level, wave velocity is only 7.7 km sec-1. Such a high velocity in the thin wave-guide of the upper mantle in the north-western Pacific basin is an important contrast in the physical properties of the mantle. It seems probable that "plastic" asthenosphere observed under the Sea of Japan is completely absent.

The extent of the tack asthenosphere under the Sea of Japan is supported by both geothermic and gravimetric observations. The computation was made for the 1200 ^o temperature layer because at greater temperatures the mechanism of heat transmission could be sharply changed as the result of fractional melting.

The 2000 ^o isotherm has a depth of about 100 km under the Sikhote-Alin where the pressure exceeds 30 kbar. Under the Sea of Japan and the western Honshu I. (region of "green tuffs") this isotherm ascends steeply to 40 km where the pressure is 11-17 kbar, within the Pacific the isotherm descends again to the depth of 100-120 km where the pressure increases accordingly to 30 kbar. The zone of partial melting is probably more distinct under the Sea of Japan where high temperature corresponds to low pressure. The zone of partial melting is confirmed by magnetotelluric research to be the zone of higher conductivity ( Vanyan et al., 1978 ).

This distribution correlates well with magmatism. In the last 20 m.y. magmatic processes have been active only within the region of the greatest uplift of the 1200 ^o isotherm, i.e. in the Sea of Japan and on the western Honshu I. In the Pacific (the north-werstern basin) magmatic activity (tholeiite basalts outflow) seemed to occur about 100 m.y. ago, that is during the Jurassic-Cretaceous period. In Primorje, at approximately the same period, there was a widespread phase of magmatism, predominantly of the acid type. In the Paleogene intrusion occurred in the from of various dyke complexes and subvolcanic bodies ranging from acid and alkaline to basic.

To construct the density section along the geotraverse, the gravity effect of the crustal layers was computed. By subtracting this gravity effect it was possible to model the anomalies relates to density variations in the upper mantle. A good correlation of the observed with the computed gravity anomaly was then obtained which supported the reduced thickness and density of the lithosphere and greater thickness of the asthenosphere under the Sea of Japan. This gravity interpretation agress well with the seismological data.

The upper mantle composition and distribution of mantle rocks along the geotraverse is shown in Fig. 1 on the basis of a synopsis of the petrographic analyses of xenoliths. Thus, it is inferred that within Primorje, the upper levels of the mantle are composed of lerzolites, spine lerzolites and pyroxenites with underlying garnet peridotites. The uppermost mantle of the Sea of Japan also contains spinel peridoties, spinel lerzolites and plagioclase peridotites, whereas the uppermost mantle of Honshu Island is composed of spinel lerzolites with a lesser amount of plagioclase lerzolites and olivine websterites.

The crust and upper mantle on Honshu I. under the Ichinomegata volcano is characterized by high water content and comparatively low temperature, in contrast to the region under the Sea of Japan, where the upper mantle and lower crust show a lack of water ( Takahashi, 1978 ).

Focal Zone

The cross-section of the focal zone constructed by Yoshii ( 1979 ) , Hasegawa et al. ( 1979 ) . shows the hypocenters of the earthquakes recorded from 1964 to 1973. It shows a seismically active layer with a 50 ^o dip and 400 km depth.

The seismicity has the following features:

(i) the majority of the foci and of the energy release falls within the first few tens of kilometers depth (to 50-70 km)

(ii) there is a distinct aseismic front in the upper mantle block northwest of the axial part of the focal zone which is probably a consequence of the difference in the physical-mechanical properties of the blocks over the focal zone

(iii) there are clear differences of focal mechanisms and stress-field between the upper region of crustal earthquakes (predomimamtly compressive) and the base of the focal zone

(iv) there is a double focal zone with compression and extension oriented along it.

Conclusion

Interpretation of geological, seismological, gravimetric magnetotelluric, petrographic, and geothermal data shows the presence of a thick asthenospheric layer in the upper mantle of the transition zone. In the adjacent Primorje and Pacific regions, low velocity layers are not distinct. In the northwestern Pacific basin the lithospheric thickness is 100 km, the mantle has higher density and there is on clear asthenosphere layer. There is a layer with somewhat lower seismic velocity ( 8.4 km sec^-1 ), however, this velocity is still considerably higher than that within the transition zone at the same level. The Pacific basin has low heat flow, and the majority of the magmatic activity can be dated as Mesozoic. The transition zone has relatively high heat flow which correlates well with the magmatism which occurred during the Cenozoic period.

Several epochs are distinguished by the predominance of compression and extension. The Late Paleozoic-Early Mesozoic is mostly characterised by faults and faults-fractures formed generally under extension conditions. Shifts, upthrusts, strike-slip faults are typical of the end of the Mesozoic and Paleogene. These structures combined with folded elements, which appeared in the end of the Mesozoic, confirm the leading role of compression stress. Compression stress changes to extension stress in the end of Genozoic when thrust faults again become the dominant type of faults.

This general pattern of deep structure within the geotraverse must be considered when analysing the evolution of the Earth's crust in this region. The upper mantle processes connected with the formation of the asthenospheric layer appear to have a close connection with the formation of the structures in the transition zone.

Acknowledgements

The authors are expressing their deep gratitude to Drs. S. Asano, T. Yoshii, and E. Takahashi from Earthquake Research Institute and Geological Institute of the Tokyo University for the scientific materials and participation in discussion.

Geophysical Center of the Russian Academy of Sciences, Moscow, Russia World Data Center for Solid Earth Physics The Geotraverse Project