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We propose a scheme to subdivide the Samarka terrane, a Jurassic accretionary prism fragment, into tectonostratigraphic complexes. This subdivision provides a basis to study these formations and map them on a medium- to large-scale. Each complex corresponds to a certain stage in the accretionary prism formation. Thus, the complexes composed of subduction mélange and olistostromes (in our case, Ust-Zhuravlevka and Sebuchar complexes), can be correlated to episodes when the underthrusting of seamounts hampered subduction, as evidenced by seamount fragments contained in the complexes. Episodes of relatively quiet subduction have also been identified, resulting in complexes composed mainly of normally bedded terrigenous and biogenic formations (Tudovaka and Udeka and, partially, Ariadnoe complexes). Particularly considered is the Okrainka-Sergeevka allochthonous complex – a fragment of continental plate overhanging a subduction zone. It was included in the accretionary prism during gravitational sliding on the internal slope of the paleotrench. All volcanic rocks in the accretionary prism are allochthonous fragments of the accreted oceanic crust. The absence of the Jurassic-Berriasian volcanic belt related to this prism, as well as synchronous autochthonous volcanism, indicates that the Samarka terrane accretionary prism formed under conditions of flat-slab subduction, similar to modern examples along the Andean margin. [ABSTRACT FROM AUTHOR]

• U-Pb-Hf isotope data are reported for detrital zircons in the Mongol-Okhotsk belt. • The investigated metasedimentary formations form a set of thrust sheets. • The eastern Mongol–Okhotsk Belt hosts at least two types of accretionary complexes. • Two subduction zones with opposite polarities existed in the closing Mongol–Okhotsk Ocean. The Mongol–Okhotsk Belt is a major structural element of East Asia. However, a number of important aspects of its evolution remain unresolved, in particular, the timing and scenarios of accretion and collision. This paper presents first detrital zircon U–Pb–Hf and whole-rock Nd isotope data from metasedimentary rocks of the Selemdzha and Tokur terranes in the eastern Mongol–Okhotsk Belt. Our new data suggest that these terranes are not part of a single sedimentary sequence, as has been previously assumed, but likely form a set of thrust sheets. The eastern Mongol–Okhotsk Belt hosts at least two types of accretionary complexes. The first type includes the Talyma, Zlatoust, Sagur-1, Tokur, and Ekimchan formations, which contain abundant Paleoproterozoic and Archean zircons, as well as Paleozoic zircons with two-stage model ages (t Hf(C)) of >1.7 Ga. These formations are interpreted as fragments of a Paleozoic accretionary complex emplaced at the southern edge of the Siberian Craton. The second type includes the Afanas'ev and Sagur-2 formations, which lack Paleoproterozoic and Archean zircons but contain early Mesozoic zircons with t Hf(C) ages of <1.4 Ga. The Afanas'ev and Sagur-2 formations are probably fragments of an early Mesozoic accretionary complex formed at the northern edge of the Amur Block. Our new model focuses on stratigraphic architecture of the Selemdzha and Tokur terranes and implies that two subduction zones of opposite polarities existed during the suturing of the Mongol–Okhotsk Ocean. [ABSTRACT FROM AUTHOR]

New petrogeochemical data were used to determine the conditions of the formation and tectonic setting of volcanic rocks of the Lake Udyl' area. These rocks were compared with similar rocks in the region. The volcanic rocks of the Lake Udyl' area are subdivided into two genetic types: (1) within-plate oceanic volcanic rocks ascribed to the accretionary volcanogenic–siliceous complex (Tithonian–Valanginian–Hauterivian–Barremian) similar to accretionary complexes in other segments of the Kiselevka–Manoma Terrane, and (2) suprasubduction-zone volcanic rocks from the island-arc volcanogenic-terrigenous complex (Valanginian–Hauterivian–Aptian–Albian–Cenomanian). A synthesis of new geological and petrogeochemical data, as well as previous geochronological and paleomagnetic data indicates the co-occurrence of genetically diverse complexes in the Udyl' segment. [ABSTRACT FROM AUTHOR]

The Sikhote-Alin Orogen in the southeast of Russia is a collage of geological terranes of various age and tectonic origin, which formed along the East Asian continental margin as a result of the Jurassic–Early Cretaceous subduction of the Pacific oceanic plates. The Jurassic Samarka accretionary complex (AC) and the Early Cretaceous Zhuravlevka turbidite basin in the southern part of the orogen are considered indicators of subduction continental margin and transform plate boundary regimes, respectively. The regime conversion was assumed at the end of the Jurassic, when subduction stopped. Our biostratigraphic study of radiolarians from siliceous and fine-clastic sedimentary rocks has revealed the latest oceanic deposits and the youngest, Early Cretaceous fragment of the Samarka AC in its less studied northeastern part, which is ascribed to the Anuy tectono-stratigraphic element. The well-preserved radiolarians allow accurate dating of cherts, siliceous mudstone, and mudstone. This and other available biostratigraphic data allow the reinterpretation of the stratigraphy of the accreted oceanic sedimentary rocks. The refined stratigraphy is interpreted as a subsequent change in depositional settings on an oceanic plate, which moves to the convergent plate margin. Cherts accumulated in an oceanic pelagic zone from the Middle Triassic to the Late Jurassic (Early Oxfordian). Siliceous mudstone were deposited in a hemipelagic zone in the early Oxfordian–middle Tithonian. Mudstone and siltstones deposited on an external slope of a deep trench in the late Tithonian–Berriasian. The sandy deposits were deposited in an axial part of the trench in the early Valangian, which best corresponds to the timing of accretion. Thus recognized early Valanginian accretion episode shows that subduction underneath the continental margin lasted longer than previously suggested. The onset of the transform continental boundary regime occurred later, probably, in the late Valanginian. This further details the Mesozoic evolutionary scenario, which was previously proposed for the Sikhote-Alin segment of the East Asian continental margin. We also refine the tectonic zonation and evolution of the Samarka AC. [ABSTRACT FROM AUTHOR]