RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES4002, doi:10.2205/2007ES000224, 2008
[72] Research in the deep structure of the Far East continental margins was conducted on the basis of combined interpretation of geological and geophysical data. To construct geodynamic models we used the results of geological, seismic, petrological, geothermal, magnetic, electromagnetic and gravimetric research. Geophysical fields, geological structure of sedimentary cover, the structure of the earth's crust and the upper mantle, location of deep faults, volcanoes and their magmatic sources, earthquake sources distribution, depths of occurrence of the asthenosphere and individual diapirs, paleo- and modern subduction zones, deep temperatures distribution are shown on geodynamic models.
[73] A feature of the deep structure of the transition zone from the Eurasian continent to the Pacific is the occurrence of asthenospheric layer in the upper mantle, from which anomalous mantle diapirs run off and the processes going on in them determine the formation of the earth's crust structures. The increase of asthenosphere thickness is revealed beneath all deep-sea basins of the transition zone from the Eurasian continent to the Pacific. Young and active spreading basins are areas of generation of new oceanic crust and the lithosphere; such basins correspond to the asthenosphere roof emergence immediately at the foot of the crust.
[74] The nature of continental margins geoid, apparently similar to the Earth on the whole, is determined by deep density heterogeneities, which may be either static or dynamic and are related to the mantle convection. Thus of structures of transition zone from Eurasian continent to the Pacific is characteristic linear relation between residual heights of geoid and the age of the beginning of back-arc areas formation. Since tectonic magmatic activity is caused by the asthenosphere conditions and its effects on the earth's crust (the higher is the level of asthenosphere bedding, the younger is tectonic magmatic activity), it is reasonable to relate geoid height variation to the features of asthenosphere structure.
[75] Correlation is noted between geological structures, tectonic and magmatic activity and the upper mantle structure. Thick and most pronounced asthenosphere corresponds to tectonically active areas like island arcs and rift structures of marginal seas. On the surface, asthenosphere rises correspond to rift structures and eruptions of mostly tholeiitic magmas. They are located in extension zones and are manifested against lithosphere thickness decrease and high heat flow. With the asthenosphere upwelling to the earth's crust, the lithosphere breaks, rift structures start to form, basalt (mostly tholeiitic) magma erupts, active hydrothermal processes with sulfide deposits formation go on and avalanche sedimentation is underway. Asthenosphere diapirs are channels by which hot fluids including hydrocarbons penetrate into sedimentary basins and other structures of the transition zone.
[76] The relation between heat flow, tectonic and magmatic activity is corroborated. It manifests itself in the heat flow increase with tectogenesis rejuvenation age. Increase in heat flow density is caused by the intrusion of asthenosphere diapirs into the lithosphere, which caused tectonic magmatic reworking of the crust and volcanism development. The higher is the level of asthenosphere position and the higher is the heat flow values, the younger is tholeiitic basalt eruption age. With the asthenosphere position level reaching 10-20 km, the lithosphere breaks, inter-arc troughs are formed and rift structures with tholeiitic basalt eruption are formed along their axial lines. Data on the geotraverse testify to the correlation between extension intensity and the structure of the formations under investigation in crustal and sub-crustal levels. Maximum intensity of extension (Kuril Basin) corresponds to continental crust rupture, marginal sea crust formation (associated with spreading) and asthenosphere upwelling up to near-surface level accompanied by maximum heat flow. Tatar rift, where only thinning of continental crust was underway, corresponds to asthenosphere diapir as well, but it is located at greater depths and heat flow is far less intense there. Extension structures in the geotraverse of the Okhotsk Sea region (Tatar Rift and Kuril Basin) are pull-apart basins; structural control caused by lithospheric plates interaction prevailed at their formation. Both structures were formed within continental crust and in the course of evolution they differed in the degree of extension: either with continental crust thinning or its rupture (with spreading) and marginal-sea crust formation. The similarity of rocks is in synchronous change, magmatism dynamics of the same type and similar structure of sub-crustal areas. Asthenosphere upwelling caused by lithosphere extension corresponds to both basins; in this case the level of asthenosphere diapir rise shows positive correlation with the degree of crustal extension. The latter feature determines magmatism dynamics: early stages of rift formation were accompanied by basalts associated with upper mantle areas that underwent hydrothermal changes, whereas maximum extension correlates with tholeiite of asthenosphere sources.
[77] The origin of volcanic rocks of the the Kuril Island Arc is related to subduction of oceanic lithosphere. Their magmatic sources are located in the above-subduction wedge within the upper mantle and locally in the asthenosphere.
[78] Thus the construction of active continental margin models on the basis of combined interpretation of geological and geophysical data allows us to reveal the epochs of highest rates of activation (including degassing) of asthenosphere diapirs and to study the geodynamic evolution of the processes going on in the transition zone from continent to the ocean.
Citation: 2008), The deep structure of active continental margins of the Far East (Russia), Russ. J. Earth Sci., 10, ES4002, doi:10.2205/2007ES000224.
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