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The northern part of Sakhalin Island is an area of recent intensive tectonic movements and hydrothermal processes, as well as a place of accumulation of useful minerals. The deep structure of the lithosphere beneath the region of the Neftegorsk earthquake of May 27, 1995 in North Sakhalin, which killed residents and caused significant destruction, is examined in this paper. Our geodynamic model shows that North Sakhalin consists of the North Sakhalin Basin, Deryugin Basin and an ophiolite complex located between them. The Deryugin Basin was formed in place of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in the Late Cretaceous–Paleogene. The North Sakhalin Basin was formed on the side of the back-arc basin at that time. The ophiolite complex is fixed in the position of ancient subduction zone that was active in the Late Cretaceous–Paleogene. Approximately in the Miocene, the subduction of the Okhotsk lithosphere apparently ceased. The remains of the subduction zone in the form of an ophiolite complex have been identified from geological and geophysical data. On the surface, the subduction zone is manifested as deep faults stretched along Sakhalin. It is probable that the Neftegorsk earthquake was a result of activation of this ancient subduction zone.

This paper deals with geological aspects of the correlation between deep and surface structures of the island arcs, marginal seas and folded structures of the active margins of the Pacific. The asthenosphere in these regions is most clearly identified with the areas of partial melting where magma is generated. A thicker and more complete asthenosphere is confined to tectonically active regions. Within the interarc basins of island arcs, which are characterized by the outflows of present-day tholeiitic basalts, the anomalous mantle reaches up to the crust. In deep-water basins the asthenosphere lies at a depth ranging from 30 km in the Parece Vela basin to 50–80 km in other marginal seas. Accordingly, the eruptive tholeiites are of different ages ranging from Miocene to Paleogene and even Late Cretaceous. Marginal seas are characterized by a crustal structure similar to the oceanic one. The base of the deep basins comprises rift structures associated with upwelling of the asthenosphere causing extension stresses in the lithosphere and magmatic activity. Endogenic regimes can be distinguished by the distribution of the heat flow, which is high in tectonically active regions such as island arcs, marginal seas or young fold mountain belts and is low in the abyssal basins of the Pacific and over the territory of continental shields. The processes in the upper mantle, in particular in the asthenosphere, give rise to endogenic phenomena in the transition zones. The state of the asthenosphere is determined by partial melting, which also means a lowering of its density. A lower density of the substance in the asthenosphere leads to an increase in volume and the consequent rise and extension of the overlying lithosphere. The deep basins of the marginal seas and island arcs are formed. Eruptions of tholeiite basalts may take place along the rifts. The processes occurring within the zones where the asthenosphere has risen are accompanied by intense mineralization. The tectonics in the back-arc and interarc basins is similar to processes taking place in the mid-oceanic ridges where intensive hydrothermal activity is associated with the deposition of zinc, copper and ferrous Sulfides. Consequently, the ancient paleorift structures of transition zones could also be a source of useful minerals.