Gas Flows in the Sea of Okhotsk Resulting from Cretaceous-Cenozoic Tectonomagmatic Activity

A model of the geological evolution of the Sea of Okhotsk is presented based on data from radioisotope age determinations and the mineral and isotope-geochemical composition of Late Mesozoic–Cenozoic volcanic rocks. We discuss the probable interrelationship between gas geochemical manifestations of gas fluxes with anomalous methane concentrations and the formation of gas hydrates, along with the occurrence of volcanic processes in the Sea of Okhotsk, fault zones, and different geological structures of basement and sedimentary deposits, landslides, and earthquakes. As a result of the studies, the nature of each volcanic stage has been determined. These include the Late Cretaceous continental-marginal belt stage (calc-alkaline), the Eocene transform-marginal (adakite) stage, and the Pliocene-Pleistocene island arc stage in the southern part of the Sea of Okhotsk. The sources of magma generation have been recognized, namely, the lithosphere subcontinental, asthenosphere oceanic, and plume-oceanic (OIB). The change in geodynamic regimens was traced back from the Late Cretaceous subduction regimen to the Maastrichtian–Datian transform-marginal, which continued as far as the Pliocene and ended at the resumption of the Pliocene–Pleistocene subduction of the Pacific Plate beneath the Eurasian continent. This involved the processes of destruction of the subduction plate, lithosphere and asthenosphere diapirism, and lower mantle plume upwelling. In the periods of geodynamic, seismotectonic and volcanic activities, gas migrated along the fault zones from the depth to the surface, together with different volcanic substrates ascending through the upper and lower mantle. Gas contained CO2, CH4, H2, He, N2, O2, and superheated steam (Н2О). Gas plays an important dynamic and physicochemical role in the evolution of the Sea of Okhotsk. At the present stage, gas fluxes migrating from the depth to the surface manifest as gas bubbles penetrating from the bottom sediments into the water column, with some portion of gas penetrating into the atmosphere. In the areas with gas fluxes, the fields containing anomalous concentrations of hydrocarbons, gas hydrate, carbon dioxide, hydrogen, and helium result in the formation of gas hydrates and associations of authigenic minerals and various geochemical elements.