Geodynamic nature of the Okhotsk Sea lithosphere. An overview of seismic constraints

Abstract

A combined interpretation of the geological, seismological and active-source seismic data in the Okhotsk Sea (Sea of Okhotsk) region allows describing the structure and geodynamics of this transition zone between the continent and the ocean. The interpreted data on the crust and uppermost mantle structure of this region are based on the recent seismic profiles with detailed system of observation carried out during the last decades in the Okhotsk Sea. Large air guns and 119 ocean bottom stations were used to study the lithosphere structure of beneath the sea down to 70 km depth. Detailed P- and S-waves interpretation methods were applied for these profiles. The study shows the Okhotsk Sea crust to be of the continental type composed mainly of the felsic rocks. Only within the elongated basin along the Kuril island arc, the crust thickness is reduced and the velocities increased to the values typical for the oceanic crust. The new seismic data are also obtained on the upper mantle structure. The detailed mathematical modeling of the observed mantle waves revealed unusual type of the wide angle reflections recorded at the first arrivals which previously interpreted as refractions. As a result, two reflective boundaries M and M1 are revealed beneath the sea depression, with velocities of 7.9 and 8.0 km/s. Along the Sakhalin Island a deep mantle fault was traced down to the depth of 70 km. This deep fault is traced in the north-south direction far into the Pacific ocean and into the east Asia. The seismological data show the Okhotsk Sea to be surrounded by deep faults of global nature. In addition to the Kuril arc zone limiting the sea depression in the eastern part, the deep Magadan fault separates the depression from the continent. These data suggest the Okhotsk Sea region as a separate micro-plate of the continental type.