N. I. Filatova

The sea of okhotsk geotraverse : Tectonomagmatic evolution of cenozoic extension structures and implication for their deep structure

Issues of the origin of marginal seas and genesis of their igneous rocks remain controversial because of the insufficient amount of documentary information (small number of drilled holes, rare network of seismic profiles, and so on). Therefore, data on seismic profiles carried out within the framework of the Geotraverse International Project [1, 2] are of great significance. The geotraverse extends from the margin of Asia (Sikhote Alin) to the northwestern periphery of the Pacific plate (Fig. 1) and includes large Cenozoic extension structures, such as the Tatar continental rift and the South Kuril marginal basin. Comprehensive analysis of seismic data on deep zones of this profile coupled with the available information about the regional tectonic setting and magmatism have made it possible to detect a correlation between the Cenozoic geodynamic mode (correspondingly, tectonics and magmatism) at the crustal and deeper (lithospheric mantle and asthenosphere) levels. Ultimately, these data made it possible to refine the evolution model of marginal basins in the study region and provided new insights into the issue of passive and active rifting. The Tatar continental rift located in the western part of the geotraverse (Fig. 2) represents the northern termination of the Japan basin. Both structures are bounded by a convergent system of submeridional dextral strike-slip faults partly transformed into normal faults. The Tatar rift basement incorporates a thin continental crust with the velocity of seismic waves (hereafter, V ) ranging from 5.8 to 7.2 km/s [4, 7]. The continental crust began to break down after its disintegration in the terminal Paleocene‐Eocene into a system of narrow (5‐10 km) horsts and grabens [2, 4] and the accumulation of terrigenous sediments (up to 1.5 km thick) at the initial rifting stage. The next (Oligocene‐middle Miocene) stage of maximum extension of the Tatar rift is marked by the unconformably overlying transgressive sequence of deep-water clayey and siliceous‐clayey rocks (up to 4 km). The next (middle Miocene) regressive sequence is composed of coastal-marine sediments. In contrast to the almost undeformed cover of middle Miocene‐Holocene terrigenous rocks (1.5‐5 km thick) deposited after the rifting (hereafter, postrift complex), rocks deposited during rifting (rift complex) are characterized by significant dislocations. All of the rift and postrift complexes include basaltoid fields in some places. As will be shown below, these rock complexes have specific isotope‐geochemical parameters. The structure beneath the Tatar rift is characterized by drastic reduction of the continental crust (up to 25 km) and ascent of the asthenospheric layer up to the level of 50 km (Fig. 2). Therefore, this region is marked by high thermal flux (123‐132 mW/m 2 ). The temperature in the upper zone of the asthenospheric diapir is estimated at 1100 ° C [2‐4]. The diapir is bounded by narrow gradient stages [4] that extend to higher lithospheric levels and generally correspond to strike-slip faults, which serve as boundaries of the Tatar and Japan basins. The faults presumably extend to the mantle and asthenosphere.

Oceanic basins in prehistory of the evolution of the Arctic Ocean

During geodynamic reconstruction of the Late Mezozoic-Cenozoic evolution of the Arctic Ocean, a problem arises: did this ocean originate as a legacy structure of ancient basins, or did it evolve independently? Solution of this problem requires finding indicators of older oceanic basins within the limits of the Arctic Region. The Arctic Region has structural-material complexes of several ancient oceans, namely, Mesoproterozoic, Late Neoproterozoic, Paleozoic (Caledonian and Hercynian), Middle Paleozoic-Late Jurassic, and those of the Arctic Ocean, including the Late Jurassic-Early Cretaceous Canadian, the Late Cretaceous-Paleocene Podvodnikov-Makarov, and the Cenozoic Eurasian basins. The appearances of all these oceans were determined by a complex of global geodynamical factors, which were principally changed in time, and, as a result of this, location and configuration of newly opened oceans, as well as ones of adjacent continents, which varied from stage to stage. By the end of the Paleozoic, fragments of the crust corresponding to Precambrian and Caledonian oceans were transported during plate-tectonic motions from southern and near equatorial latitudes to moderately high and arctic ones, and, finally, became parts of the Pangea II supercontinent. The Arctic Ocean that appeared after the Pangea II breakup (being a part of the Atlantic Ocean) has no direct either genetic or spatial relation with more ancient oceans.

Tectonic–geodynamic settings of OIB-magmatism on the eastern Asian continental margin during the Cretaceous–Paleogene transition

At the Cretaceous–Paleogene transition, the convergent boundary between the Asian and Pacific plates was replaced by a transform boundary to determine destruction of the continental margin including the Okhotsk–Chukotka Cretaceous subduction-related belt along left-lateral strike-slip and downdip–strikeslip faults. The newly formed East Asian rift system (EARS) continues in the easterly direction the Mongol–Okhotsk zone of left-lateral strike-slip faults, a former transform boundary of the Asian continent. Basaltoids of the East Asian rift system that erupted through fractures onto the former active margin are similar intraplate OIB volcanics related to the lower mantle source. The specific feature of OIB-type magmatism in the system consists in its continental marginal position near the transform boundary.

Extent of the Middle Cretaceous Orogen in Eastern Asia and the Geodynamic Causes of Its Transformation

An extensive Middle Cretaceous orogenic belt of East Asia, which occupies the Okhotsk–Koryak–Western Kamchatka territory, encompasses thrust-and-fold structures. It formed over an interval of 130–110 Ma as a result of perioceanic accretion–obduction processes involving marine allochthonous complexes up to the Barremian inclusive, which experienced large amplitude displacements. Under the remote impact of subsequent (end of the Cretaceous and Cenozoic) perioceanic tectonic events, the primary structures of the Middle Cretaceous belt underwent compression-related superimposed deformations and fragmentation of allochthons with negligible displacement amplitudes. This led to the formation within the belt of a mosaic pattern of alternating fragments of neo-autochthons and allochthons bounded by young thrusts and strike–slip faults, often suggested as independent heterochronous terranes.

New data on correlation of the Middle Mesozoic oceanic and island arc rocks of Eastern Asia

Using a radiolarian method, correlation of stages and substages of the Middle Mesozoic submarine volcanic-siliceous sequences was carried out for the vast territory of Eastern Asia for the first time. The correlation of fragmented outcrops of genetically various siliceous-terrigenous-volcanic rocks showed a broad lateral distribution of sediments from the Norian to Hauterivian stages inclusive within the studied area. This work, based on a combination of paleontological, structural-tectonic, and lithological-petrological methods in contrast to the terrane approach, has demonstrated the possibility of age and facial characteristics of isolated outcrops of the Middle Mesozoic submarine rocks for the huge territory of Eastern Asia.