RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES4002, doi:10.2205/2007ES000224, 2008

Tectonic and Magmatic Evolution of Cenozoic Extension Structures of Continental Margins

[65]  The origin of the Tatar and Kuril Basins crossed by the geotraverse is related to the general geodynamic settings that were formed there by the end of the Paleocene. Under the effect of Indo-Eurasian collision the continental framing of the western Pacific including the Amur and Okhotsk Sea micro plates underwent destruction by sublongitudinal zones of right-lateral shifts with the formation of pull-apart type basins of different intensity of extension, which for the major part falls on the Early and Middle Miocene [Filatova, 2006a]. Bounded by shifts the single pull-apart structure including the Japan and Tatar deep basins was a continent-marginal rift in the Eocene and in the first half of the Miocene, maximum extension resulted in the separation from Asian continent of Japan micro continent owing to spreading that appeared in the Japan Basin and marginal crust that formed there. In Tatar Rift, this episode corresponds to further thinning of the continental crust.

[66]  The Kuril Basin most likely has a similar nature. Its opening resulted from southward migration of eastern Hokkaido and the Kuril Island Arc along the Sakhalin-Hokkaido dextral strike-slip fault zone. Thus the origin of the Kuril Basin is related to shifts within continental crust and in this context it is similar to Japan pull-apart basin. With this model it appears reasonable that maximum extension of continental crust took place in southwestern area of the basin abutting structure-formation Sakhalin-Hokkaido shift zone and it was there that in the Early - beginning of the Late Miocene spreading went on with the formation of marine-margin crust, whereas northeastern "edge'' of the basin is underlain by extended continental crust though there, judging by isotopic parameters of rocks of Geophysicist seamount volcano, diffuse spreading processes may have taken place.

[67]  Data on the geotraverse testify to correlation between tension intensity and the composition of the structures under investigation on crustal and subcrustal levels. Maximum intensity of tension (Kuril Basin) corresponds to continental crust rupture, formation of marine-margin crust associated with spreading and asthenospheric upwelling up to near-surface levels accompanied by maximum heat flow. Tatar Rift where only continental crust thinning took place corresponds to asthenospheric diapir located at much greater depths; heat flow is far less intense there.

[68]  The same type of magmatism dynamics in extension zones emphasized above, which correlates with stages of basin formation and their deep structure, allowed us to reveal levels of magma formation and their changing with time. The most complete data on basins with different types of the crust suggest that the Eocene-Oligocene basalts of the initial stage of rifting with characteristics of calc-alkali series are related to activation of relict (Mesozoic) above-subduction sources of the lithosphere mantle. The stage of the Early-Middle Miocene maximum extension characterized by maximum asthenospheric upwelling corresponds to tholeiite of Pacific MORB type related to asthenospheric source. Therefore from the initial rifting stage to the maximum extension stage the change of sources takes place from the upper-mantle source to the asthenospheric one. At the same time, even marginal-sea lavas of maximum extension stage that are most close to Pacific MORB (in the Sea of Japan they are rocks of borehole 797 upper part) show signs of the upper continental mantle that underwent hydrothermal change (amphibolite-phlogopite content). Such mixture of isotopic and geochemical features of basalts of the major stage of basins formation testifies to the interaction of prevailing asthenospheric source and metasomatized lithospheric mantle. Alkaline basalts of post-rifting and post-spreading stage of the end of the Miocene - Holocene that is similar in isotopic-geochemical characteristics to OIB composition and of EMI sources and locally EMII sources may have been genetically related to lower-mantle matter by model although lithospheric source must not be ruled out either.

[69]  High heat flow, magmatic activity and sedimentary bed heating caused by asthenospheric upwelling became an additional source of hydrocarbon and fluidal flows fostering oil and gas fields formation in Tatar Rift sedimentary beds.

[70]  Thus extension structures on the Okhotsk Sea geotraverse (Tatar and Kuril structures) are pull-apart basins that started formation with predominant structural control caused by the interaction of lithospheric plates. The both structures appeared within continental crust and in the course of their evolution they differed in the degree of extension showing continental crust thinning or its rupture with spreading and the formation of marine-margin crust. The similarity of the Tatar and Kuril Basins is in synchronous change, magmatism dynamics of the same type and similar structure of subcrustal areas. Both basins correspond to asthenospheric upwelling caused by the lithosphere tension and in this case the level of asthenospheric diapir rise shows positive correlation with the degree of crustal extension. It is this feature that determines magmatism dynamics: early stages of rift formation were accompanied by basalts associated with areas of the upper mantle that underwent hydrothermal change, whereas maximum extension is correlated with tholeiite of asthenospheric sources [Filatova and Rodnikov, 2006b].

[71]  The composition of mantle fluids having a leading part in the formation of continental margin structures, sedimentary basins of marginal seas and island arcs is determined from gas-geochemical survey in recent rift structures and analysis of gas inclusions in the upper mantle rocks from kimberlite pipes. Thus in underwater mid-oceanic ridges and rifts of marginal seas the studies revealed high content of helium, hydrogen, methane and carbon dioxide [Craig et al., 1987; Hussong et al., 1981]. The studies of liquid and gaseous inclusions in diamonds and in kimberlite pipe rocks showed that besides the above-mentioned gases a high content of liquid hydrocarbons is noted as well [Zubkov, 2001]. Major components of mantle fluids are carbon dioxide, methane, hydrogen, fluorine, chlorine, selenium, arsenic, iridium, mercury, antimony and other elements. From data by A. F. Grachev [Grachev, 2002] one million km3 of lava contains no less than 1014 tons of methane and the same amount of carbon dioxide. Research conducted [Riabchikov et al., 2004] showed that in the eruption of basalt lavas of Siberia trappean province formed approximately 230 million years ago, more than 1013 tons of carbon dioxide was released in 1 million years. The emission of such amount of carbon dioxide in so short a period of geological time was a real course of disastrous global change in the boundary of the Permian and Triassic periods and corroboration of magma saturation with gaseous components.


RJES

Citation: Rodnikov, A. G., N. A. Sergeyeva, L. P. Zabarinskaya, N. I. Filatova, V. B. Piip, and V. A.  Rashidov (2008), The deep structure of active continental margins of the Far East (Russia), Russ. J. Earth Sci., 10, ES4002, doi:10.2205/2007ES000224.

Copyright 2008 by the Russian Journal of Earth Sciences

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