Tektono-Stratigraphy as a Reflection of Accretion Tectonics Processes (on an Example of the Nadankhada-Bikin Terrane of the Sikhote-Alin Jurassic Accretionary Prism, Russia Far East)

The Sikhote-Alin folded belt is one of the Asia eastern margin regions, where terranes of ancient accretionary prisms are widely distributed. Within this region there are fragments, as a minimum, of three such different-aged prisms Jurassic, Early Cretaceous and Middle Cretaceous (fig. 1). Accretionary prisms are complicate-dislocated sedimentary complexes that are formed at the basis of continental or island-arc slopes as a result of accretion of fragments of the sedimentary cover and morphologically positive structures of an oceanic plate during its subduction. The subduction of an oceanic plate under a continent or an island arc is commonly accompanied by intense deformation of the sedimentary cover deposits. At first, in the frontal part of the slope basis the upper part of a sedimentary cover (trench turbidite) under the action of “bulldozer effect” (off-scraping) repeatedly imbricate, forming a series of tectonic slices of terrigenous composition (fig. 2). Secondly, pelagic and hemipeladic sediments located below of an imbrication zone, plunging into a subduction zone rumple under the action of simple shear (effect of drag folding) into the differentamplitude and disharmonious overturned folds, which axial planes are inclined towards a trench. Crumpling of sediments continues until ultimate strength limit of rocks is reached and the ruptures are appearing. Further along these ruptures numerous underplating and doubling of a primary cut-section of an oceanic plate cover occur that results in formation of imbricate-underthrusted structure of the accreated rocks (Hashimoto & Kimura, 1999; Kimura, 1997; Kimura & Mukai, 1991; Moore & Byrne, 1987; Seely et al., 1974; Sokolov, 1992, 1997; Sokolov et al., 2001; etc.). For this reason the accretionary prisms are a tectonic package of repeatedly alternating tectonic slices consisting of marine (pelagic and hemipelagic deposits and also fragments of seamountns), oceanic-margin (turbidites), and chaotic (melange and olistostromes) formations. Structure of ancient accretionary prisms is even more complex because of a numerous post-accretion deformations such as thrusts and strike-slip faults.


Introduction
The Sikhote-Alin folded belt is one of the Asia eastern margin regions, where terranes of ancient accretionary prisms are widely distributed.Within this region there are fragments, as a minimum, of three such different-aged prisms -Jurassic, Early Cretaceous and Middle Cretaceous (fig.1).Accretionary prisms are complicate-dislocated sedimentary complexes that are formed at the basis of continental or island-arc slopes as a result of accretion of fragments of the sedimentary cover and morphologically positive structures of an oceanic plate during its subduction.The subduction of an oceanic plate under a continent or an island arc is commonly accompanied by intense deformation of the sedimentary cover deposits.At first, in the frontal part of the slope basis the upper part of a sedimentary cover (trench turbidite) under the action of "bulldozer effect" (off-scraping) repeatedly imbricate, forming a series of tectonic slices of terrigenous composition (fig.2).Secondly, pelagic and hemipeladic sediments located below of an imbrication zone, plunging into a subduction zone rumple under the action of simple shear (effect of drag folding) into the differentamplitude and disharmonious overturned folds, which axial planes are inclined towards a trench.Crumpling of sediments continues until ultimate strength limit of rocks is reached and the ruptures are appearing.Further along these ruptures numerous underplating and doubling of a primary cut-section of an oceanic plate cover occur that results in formation of imbricate-underthrusted structure of the accreated rocks (Hashimoto & Kimura, 1999;Kimura, 1997;Kimura & Mukai, 1991;Moore & Byrne, 1987;Seely et al., 1974;Sokolov, 1992Sokolov, , 1997;;Sokolov et al., 2001;etc.).For this reason the accretionary prisms are a tectonic package of repeatedly alternating tectonic slices consisting of marine (pelagic and hemipelagic deposits and also fragments of seamountns), oceanic-margin (turbidites), and chaotic (melange and olistostromes) formations.Structure of ancient accretionary prisms is even more complex because of a numerous post-accretion deformations such as thrusts and strike-slip faults.It is obvious, that the absence of normal sedimentary contacts between various lithological groups of rocks in such tectono-sedimentary complexes complicates the decision of a number of geological tasks such as study of stratigraphic sequence of deposits, clearing-up structure of an area of accreated formations distribution, its geological evolution, etc.Nevertheless, in some individual tectonic slices, the fragments of primary consecutive section of pelagic to hemipelagic or hemipelagic to oceanic-margin formations are observed.Numerous results of microfaunistic researches of such fragments (Kemkin, 1996;Kemkin & Golozubov, 1996;Kemkin & Kemkina, 1999;Kemkin & Khanchuk, 1992;Kemkin & Khanchuk, 1993;Kemkin & Philippov, 2002;Kemkin & Rudenko, 1998;Kemkin & Taketani, 2008;Kemkin et al., 1999;Kirillova, 2002;Philippov et al., 2000;Philippov et al., 2001;Philippov & Kemkin, 2007;Zyabrev, 1998;Zyabrev & Matsuoka, 1999, etc.) have allowed us to reconstruct primary cut-sections of accreated paleooceanic deposits.Their lowermost part is composed by pelagic cherts which are gradually replaced by hemipelagic cherty-clayey formations, and, further, by terrigenous rocks.Such sequences of deposits named by Oceanic Plate Stratigraphy sequences (Berger & Winterer, 1974;Isozaki et al., 1990;Wakita & Metcalfe, 2005, etc.) reflect the history of sedimentary process on an oceanic plate during its drift from the spreading zone to the subduction zone.Each lithological group of these sequences is very informative.For example, cherts characterize a history and features of pelagic sedimentation.Hemipelagic deposits (siliceous mudstones, mudstones and aleuroargillite) fix the moment of the approach of a foremost site of an oceanic plate to convergent border.The terrigenous rocks, which accumulation occurred in a trench, are the indicator of the time of beginning of immersing of an oceanic plate into subduction zone and, accordingly, of subsequent accretion of fragments of its sedimentary cover.Fig. 2. Model of an accretionary prism structure (After Hashimoto & Kimura, 1999) Knowing the age of rocks of such Oceanic Plate Stratigraphy sequences in various tectonic slices of the accretionary prism, it is possible to specify the time when individual oceanic fragments were accreted.These date give us the base to subdivide the accretionary prism to several tectono-stratigraphic units responding to certain stages of the prism formation.Subsequent mutual correlation and comparison of the allocated tectono-stratigraphic units allows to reconstruct a succession of the accretion of an oceanic plate fragments and to specify the prism structure as a whole.
Thus, a study of the accretionary prisms is very important as for specification of geological structure and evolution of regions composed of such prisms, as for elucidation of specific features of the accretion process in different areas of a convergent boundary, as well as for correlation of geological events at a junction zone of lithospheric plates, and for reconstruction of a geodynamic evolution of continental margins along which the accretionary prisms were formed.
One of these ancient prisms is a Jurassic accretionary prism, which was being formed during more than 70 m.y. by the consecutive accretion of different-aged and different-facies formations of Paleo-Pacific under the Paleo-Asian continent east margin.In the Sikhote-Alin region, the Jurassic prism is represented by four terranes: Samarka, Nadankhada-Bikin, Khabarovsk and Badzhal.
This article presents results of lithological-biostratigraphic studying of chert-terrigenous formations of the Nadankhada-Bikin terrane of the Sikhote-Alin Jurassic accretionary prism, which allow allocating in terrane structure three different-aged tectono-stratigraphic complexes reflecting successive stages of its formation.

Regional tectonic position and the previous researches
The Nadankhada-Bikin terrane is located in the lower reaches of the Ussuri River, in the area between the Black River and the Naolihe River mouths (fig.3).It extends for about 350 km in a northeast direction along the northwest edge of the Bureya-Jiamusi-Khanka superterrane and is about 60 km in width.The terrane is separated from the Bureya-Jiamusi-Khanka superterrane by Dahezhen fault in the west and by Misha-Fushung-Alchan fault in the south.The Lyaolihe and the Arsen'evsky faults separate it from the Khabarovsk and the Samarka terranes, respectively.The Ussuri River valley divides this terrane into two parts: Nadankhada, located in China, and Bikin, located in Russia.
The Bikin part of the terrane is composed mainly (fig.4 A) of cherts and terrigenous rocks and much less of volcanogenic formations that had formerly been referred to the Carboniferous-Permian Samurskaya Series (Bersenev, 1969), and later, after the findings of Triassic conodonts in the cherts, to the Triassic-Jurassic chert-terrigenous suite and Early Cretaceous volcanogenic-terrigenous Koultoukha suite (Liht, 1997).A structure of this area of the Sikhote-Alin was considered as a multiple sedimentary alternation of cherts, terrigenous rocks and rare layers of volcanites.However, it was established by the subsequent researches (Phillippov, 1990;Phillippov & Kemkin, 2004) that the cherts compose generally the stratum-like bodies occurring among terrigenous deposits and limited by faults.The results of microfaunistic study have shown that the age of the cherts ranges from Middle Triassic to Early Jurassic (in some cut-sections up to Middle Jurassic), and the age of the terrigenous rocks is Middle-Late Jurassic (up to Early Berriasian in some cut-sections).These data convincingly testify that repeated alternation of Triassic cherts and Jurassic terrigenous rocks can not be sedimentary.It is result of tectonic recurrence.As a whole, the structure of the Bikin part of the terrane represents a package of repeated alternation of tectonic slices of cherts and terrigenous rocks.The rocks composing this stack of slices are crumpled in the compressed differentamplitude and asymmetric folds of northeast (on some sites submeridional) strike.In the central and eastern parts of the Bikin area the axial planes of folds have northwest vergence with a mirror of folding inclined on the southeast (fig.4 B).In the western part, on the contrary, the folds axial planes are inclined on the southeast and the mirror of folding is gently immersing to the northwest (fig.4 C).
The inversion of the folds axial planes in the central and eastern parts is caused by the flexurelike bending of the Jurassic prism deposits during the left-lateral motion of the Bureya-Jiamusi-Khanka superterrane block along the Misha-Fushung-Alchan fault (Khanchuk, 1994).The similar situation takes place in the Nadankhada part of the terrane (so-called Nadankhada ridge), which occupies a territory between the Naolihe, Tsikhulinhe and Ussuri rivers (fig.5).Up to middle 1980, this territory was considered to be composed of mainly Upper Paleozoic volcanogenic-chert-terrigenous formations (Li et al., 1979).Later, during the joint Chinese-Japanese researches, it has been established that the cherts contain Triassic and the siliceous shales contain Middle Jurassic radiolarians (Kojima & Mizutani, 1987;Mizutani et al., 1986).
According to the subsequent works (Mizutani et al., 1989(Mizutani et al., , 1990;;Shao et al., 1990Shao et al., , 1992 etc.) etc.), geological structure of the Nadankhada ridge was defined as a complex alternation of tectonic slices of terrigenous rocks and cherts crumpled in the asymmetric and overturned folds of mostly northeast strike.In the southwestern part of the Nadankhada area, the northeast orientation of the folds axes is gradually changing to the submeridional and further to the northwestern and sublatitudinal one (fig.5).

Structure of the Nadankhada-Bikin terrane chert-terrigenous sequences and age of the chert-terrigenous formations
Deformations behavior of the Nadankhada-Bikin terrane deposits and spatial orientations of the major structural elements of rocks occurrence (i.e. a general dipping of layers, a vergence of the folds axial planes, a direction of the folding mirror inclination) are such that the tectonic slices of the lower structural level of the terrane are cropped out in its central and northeastern parts, and those of the upper level -in the southeastern, southwestern and western parts of the terrane (fig.6).In some tectonic slices, fragments of primary cut-section of the paleooceanic plate sedimentary cover are observed, in which cherts are sequentially and gradually replaced by the siliceous mudstones and then by the terrigenous rocks (fig.7).The results of lithological and biostratigraphic researches of such chert-terrigenous sequences fragments belonging to various structural levels of the terrane show some differences in their composition, structure and age.

Lower structural level
The most complete fragment of cut-section of the chert-terrigenous sequence has been investigated in the interstream area between the Ulitka and Zolotoy Klyuch Rivers (Phillippov & Kemkin, 2004).Here are observed (from the bottom to top): 1. Grey, light grey and pink-grey bedded cherts 60 m 2. Grey and light grey limestones and siliceous limestones 40 m 3. Grey and light grey bedded cherts 40 m 4. Grey and greenish-grey massive clayish cherts 24 m 5. Light greenish-grey slightly layered siliceous mudstones 12 m 6. Dark grey layered mudstones and silty mudstones 60 m 7. Dark grey siltstones with rare layers of fine-grained sandstones 20 m 8. Alternation of light grey fine-grained sandstones and dark grey siltstones, (rhythms from 20 to 40 cm).Sandstones contain layers of alkaline basalts and hyaloclastites (10-20 m, rarely up to 100 m).In the upper part of this pack, sandstones are replaced by chaotic formations represented by aleuro-psammites containing different-sized lumps, blocks and fragments of cherts and sandstones.more than 400 m According to the radiolarian and conodont fauna (Phillippov, 1990) the age of the cherts ranges from Middle Triassic (Anisian) to Middle Jurassic (Bathonian-Callovian).The limestones embedded in the middle part of the chert cut-section contain Late Carnian-Early Norian conodonts.

Middle structural level
Fragments of the chert-terrigenous sequence belonging to the middle structural level are exposed in the numerous outcrops in the interstream areas and banks of the Peshkova, Perepelinaya, Cheremshanka, Kamenushka, and Kedrovka Rivers (all are the right tributaries of the Ussuri River), as well in the road quarries.The reconstructed cut section looks as following: It should be noted, that the same age radiolarians (Middle Jurassic) have been found (Kojima & Mizutani, 1987) in siliceous mudstones (fig.10) on a left bank of the Ussuri River, in China (20 km to the west from Shichang village).But originally they were considered as Bathonian-Callovian in age.However, according to the specified data on a time intervals of the Jurassic radiolarian species distribution (Baumgartner et al., 1995), their age is Bajocian.These data indicate that fragments of the middle structural level are also extended in the Nadankhada part of the given terrane.

Upper structural level
A fragment of cut-section with gradual cherts-to-terrigenous rocks transition, belonging to the upper structural level, was investigated in detail in the Nadankhada part of the terrane (Yang & Mizutani, 1991;Yang et al., 1993).This cut-section locates about 6 km to the north from the Hongqiling Nongchang village, representing over 40 m exposure of cherts, clayish cherts and siliceous mudstones in which five consecutive radiolarian assemblages have been established (Yang et al., 1993).Four of them, extracted out of the cherts and clayish cherts, characterize their host rocks as being Middle Carnian-Early Pliensbachian in age.The fifth assemblage is related to the siliceous mudstones with Late Pliensbachian radiolarians.The terrigenous rocks associated with these cherty formations are Middle Jurassic in age.The silty mudstones and siltstones contain, correspondingly, Aalenian and Bathonian radiolarians (Yang et al., 1993).In the area of the terrigenous rocks distribution, the chaotic formations representing different-sized lumps, blocks and fragments of Carboniferous-Permian limestones, Triassic cherts, basalts, gabbro and serpentinous ultramafic rocks among the schistose aleuro-psammites are observed.Middle Jurassic radiolarians have been extracted from them too (Yang et al., 1993).
Fragments of the similar chert-terrigenous sequence have been also investigated in the Bikin part of the terrane, in the numerous bank outcrops of the middle reaches of the Khor River and its inflows (Phillippov, 1990).According to the microfaunistic data, the following cutsection was reconstructed: the lower part of this sequence (about 90 m thick) is composed of cherts that are changed by the clayish cherts up the sequence.The age of these rocks, according to the conodont fauna (Klets, 1995)

Discussion and conclusions
Biostratigraphic researches of the above mentioned cut-sections have revealed that the chertterrigenous deposits of the different structural levels of the Nadankhada-Bikin terrane are slightly differing in their ages (fig.11).
In particular, the age of the cherts-to-terrigenous rocks transitive siliceous mudstones varies from Pliensbachian to Oxfordian-Tithonian.It means that the chert-terrigenous rocks composing this terrane are fragments of the sedimentary cover of the different-aged sites of the paleooceanic plate (fig.12) or, in other words, they are the fragments of three differentaged Oceanic Plate Stratigraphy Sequences, which are characterized by the different time of accretion (the accretion starting time is correlated with the age of terrigenous rocks, accumulation of which occur in a trench).
Hence, in the terrane structure, it is possible to allocate as a minimum three successive tectono-stratigraphic complexes (fig.13) representing the primary cut-sections of the sedimentary cover from the different-aged sites (i.e.areas at different distances from the seafloor spreding center) of the Paleo-Pacific Ocean and reflecting a process of their consecutive accretion.Let's name them Ulitka (Triassic-Early Cretaceous), Ussuri (Triassic -Late Jurassic) and Khor-Hongqiling (Triassic -Middle Jurassic) Formations.
It should be noted that the age of rocks of the allocated Formations, as well the time of their accretion are gradually rejuvenate from the upper structural level to the lower one.As a whole, the Nadankhada-Bikin terrane structure is characterized by the inverted stratification of formations that make up the terrane.Compared to the upper structural level (the Khor-Hongqiling Formation), the lower level (the Ulitka Formation) is composed of relatively younger rocks, thus the rocks of the middle level (the Ussuri Formation) are of intermediate age.At the same time, a primary stratigraphic succession of deposits within each complex is normal (from older to younger bottoms up).Such structure of the Nadankhada-Bikin terrane completely corresponds to a structure of modern accretionary prisms forming at the basis of internal slopes of the modern convergent margins trenches, and is a result of a successive subduction and partial accretion of the oceanic plate sites of different age and lithology.During the subduction the most remote from the spreading center sites of the oceanic plate, its oldest part, are accreted first, with the following underplating of its younger fragments in future.As a result, a package of tectono-sedimentary complexes is formed.The true structure of the Nadankhada-Bikin terrane thus appears to be a regular recurrence of the strongly deformed fragments of the primary sedimentary cover of the paleooceanic plate sites situating at a different distance from the spreading center, rather than a chaotic repeated alternation of tectonic slices of different age and lithology.
Based on lithological and age characteristics of the rocks, the allocated tectono-stratigraphic complexes of the Nadankhada-Bikin terrane confidently correlate with the corresponding structural complexes of the Samarka terrane which is the most thoroughly investigated unit of the Sikhote-Alin Jurassic prism.For example, the Ulitka Formation that composes the lower structural level of the Nadankhada-Bikin terrane can be compared with the Katen Formation that is also the lowermost tectono-stratigraphic unit of the Samarka terrane.The Ussuri Formation, composing the terrane middle structural level, is correlated with the Breevka Formation, and the Khor-Hongqiling Formation is comparable with the Amba-Matay Formation which also contains chaotic formations with exotic blocks and fragments of Paleozoic limestone, cherts, basalts, and gabbro within.
It should be added that in the southwestern part of the Nadankhada ridge (in China), structurally above the slices of the chert-terrigenous rocks, there are tectonic slices of ophiolite (Dahezhen Formation).These widely distributed ophiolitic slices are made up of basalts in association with Carboniferous -Permian limestones, serpentinites and gabbro (Mizutani et al., 1989;Shao et al., 1992 etc.).Tectonic slices of same composition and age compose the uppermost structural level of the Sebuchar Formation of the Samarka terrane (Kemkin, 2006).These data suggest as a minimum one more tectono-stratigraphic complex in the Nadankhada-Bikin terrane structure (see fig. 10, it is allocated by a dashed line).
See legend on a Fig. 4 and 5.The comparative analysis of even-aged Formations of the Jurassic accretionary prism different terranes indicates that during the Jurassic the geodynamic mode along the Paleo-Asian continent eastern margin was invariable.

Fig. 5 .
Fig. 5.The scheme of geological structure of the terrane Nadankhada part (After Shao et al., 1992 with additions)

Fig. 6 .
Fig. 6.Block-diagram illustrating distribution of rocks of various structural levels of the Nadankhada-Bikin terrane folded formations on a denuded earth surface

Fig. 12 .
Fig. 12.The scheme illustrating spreding-subduction model of the Plate Tectonics and different-aged sites of drifting oceanic plate

Fig. 13 .
Fig. 13.Different-aged tectono-stratigraphic complexes of the Nadankhada-Bikin terrane , ranges from Anisian to Rhaetian, however, taking into account that they are gradually replaced by the siliceous mudstones containing Pliensbachian radiolarians, their upper age border is likely to be early Pliensbachian.The sequence upper part is composed of terrigenous rocks representing rhythmical alternation of siltstones and sandstones.Middle Jurassic Radiolarians such as Gongylothorax sakawaensis Yao, Eucyrtidiellum sp., Diacantocapsa normalis Yao, Protunuma turbo Matsuoka, Tricolocapsa tetragona Matsuoka, Tricolocapsa sp., Stichocapsa sp. and others have been determined in the siltstones layers.Upwards of the terrigenous rocks there is a section of chaotic formations (up to 200 m thick) with rock blocks and fragments represented by Carboniferous-Permian limestones, Triassic cherts, basalts and sandstones.Besides that there are also 10-40 m thick layers of basic volcanites, such as hyaloclastites, tuffs, basalts lavas and picritebasalts among the terrigenous rocks.