Dislocations of the cretaceous and cenozoic complexes of the northern part of the West Sakhalin Terrane

The contemporary structure of the West Sakhalin Terrane started to form in the Pleistocene and the process of its formation continues up to now in a setting of ENE (60°–90°) shortening. Evidence of the preceding NE (30°–45°) compression was revealed during the study. This compression prevailed in the Eocene–Pliocene. Under the settings of NE (30°–45°) compression, dextral displacements occurred along the West Sakhalin and Tym’–Poronai fault systems, bounding the West Sakhalin Terrane.     

Geochemical Features and Geodynamic Settings of the Formation of Cretaceous Terrigenous Rocks of the West Sakhalin Terrane

The geochemical composition of Cretaceous terrigenous rocks of the West Sakhalin terrane is analyzed and their paleogeodynamic interpretation is suggested. It is revealed that the rocks are characterized by the low maturity of clastic material. They contain fragments of both volcanomictic and sialic material and in composition correspond to graywackes, being petrogenic or “first cycle” rocks. The geochemical data were generalized and interpreted on the basis of their comparison with compositions of the present-day and ancient rocks formed in known geodynamic settings. The obtained results indicate that terrigenous rocks of the terrane were formed in a pull-apart basin at an active continental margin. The source area, which supplied clastic material in this sedimentation basin during the Berriasian–Danian, included a sialic land made up of granite-metamorphic and sedimentary rocks and the mature (deeply dissected) ensialic Moneron–Samarga island arc, which was accreted to the continental margin at the moment of basin initiation. Sediments were accumulated in general along the continent–ocean boundary against the background of large-scale sinistral transform strike-slip movements of the Izanagi Plate relative to the Eurasian continent.     

Material Composition,Provenances, and Geodynamic Settings of the Accumulation of Cretaceous Deposits in the West Sakhalin Terrane

Lithology and Mineral Resources - The results of studying the material composition of sandy rocks in Cretaceous deposits of the West Sakhalin Terrane are considered. Studies were carried out to...     

Paleogeodynamic Reconstruction of Cenozoic Sedimentation Settings in the West Sakhalin Terrane Based on the Material Composition of Terrigenous Rocks

Lithology and Mineral Resources - The results of studying the material composition of sandy rocks from Cenozoic deposits in the West Sakhalin Terrane are considered. It has been established that...     

Cretaceous complexes of the frontal zone of the Moneron-Samarga Island arc: Geochemical data on the basalts from the deep borehole on Moneron Island, the Sea of Japan

The variations of petrogenic oxides and trace elements have been studied in the Cretaceous volcanic rocks recovered by a deep borehole from the depth interval of 1253–4011 m on Moneron Island. The volcanic section is subdivided into two complexes: the Early Cretaceous and Late Cretaceous. The rocks of the Early Cretaceous Complex occur below 1500 m. Chemically, they belong to low-potassium island arc tholeiites, and their trace element distribution suggests their formation in a suprasubduction mantle wedge under the influence of water fluids that were subsequently released from subducted sediments and oceanic plate during the dehydration of subducted sedimentary rocks and oceanic basalts and, finally, mainly from basalts. The Early Cretaceous basalts from the borehole are interpreted as ascribing to the frontal part of the Moneron-Samarga island arc system. The volcanic rocks of the Late Cretaceous Complex are situated at depths above 1500 m. They also were formed in a suprasubduction setting, but already within the East Sikhote-Alin continental-margin volcanic belt that was initiated after the accretion of the Moneron-Samarga island arc system to the Asian continent. The island-arc section of the Moneron borehole contains basaltic andesite dikes, which are geochemically comparable with the Early-Middle Miocene volcanic rocks of Southwestern Sakhalin.     

Radiolarians and stratigraphy of Cenomanian-Coniacian deposits in the Crimean and West Sakhalin Mountains, Pt. 2: Comparative analysis

The work is aimed at summarizing data on Late Cretaceous radiolarians from the Crimean and West Sakhalin mountains. The Cenomanian-lower Coniacian stratigraphic interval of the Crimean Mountains yields 215 radiolarian species, and 80 concurrent species are known from the West Sakhalin Maintains. According to results of comparative analysis, there are 62 species in common from two regions, i.e., most of the species occurring in the West Sakhalin Mountains are also known from southerly regions, which is a good premise for correlation.     

Detailed seismic zoning of Sakhalin

Detailed seismic zoning of Sakhalin based on seismological, tectonic, geomorphological, hydrogeological, and other data is discussed. It is shown that strong crustal earthquakes occurred at the boundary between the Eurasian and Okhotsk plates and their recurrence in Central Sakhalin is equal to the duration of the tectonic cycle (75 years). This boundary in North Sakhalin is marked by the Upper-Piltun fault, which was the epicenter of the 1995 Neftegorsk earthquake with an intensity of 9. The analysis of paleosoils in the fault zone showed that such events repeat with an interval of 400 years. The development of large oil and gas reservoirs on the Sakhalin shelf will be accompanied by intensification of the seismicity, which can reach a magnitude of M = 6.0–6.5 in the Lunskoye field.     

The tectonics of Sakhalin

The controlling tectonic features of Sakhalin are the East Sakhalin horst, the northern fold belt, the western flank of the Tatar Strait geosyncline and the block-faulted area east and north of Aniwa Bay. The linear folds are associated with the Tatar Strait geosyncline. The block faulting is treated as Miocene, and earlier than the folding. — M. E. Burgunker.     

Radiolarians and stratigraphy of Cenomanian-Coniacian deposits in the Crimean and West Sakhalin Mountains, Pt. 1: Biostratigraphic subdivision and correlation

Radiolarian biostratigraphy suggested in this work for the Cenomanian-Coniacian deposits in the Crimean and West Sakhalin Mountains is enhanced to be of a higher resolution. The Cenomanian-Coniacian succession of the West Sakhalin Mountains is subdivided into nine instead of five biostratigraphic units formerly known in this region. Deposits of the Crimean Mountains are subdivided based on radiolarians within the lower Cenomanian-lower Coniacian stratigraphic interval (the original biostratigraphic chart corresponded here to the upper Cenomanian-middle Turonian interval only). The established biostratigraphic subdivisions are correlated with those of the Tethyan and Pacific scales.     

Early Paleozoic monzodiorite–granodiorite association in the northeastern flank of the South Mongolia–Khingan orogenic belt (Nora–Sukhotinsky Terrane): Age and tectonic setting

The paper presents the results of U–Pb geochronological and geochemical studies of the rocks of the monzodiorite–granodiorite association in the northeastern flank of the South Mongolia–Khingan orogenic belt, which composes a tectonic block among the provisionally Lower Paleozoic volcanosedimentary complexes of the Nora–Sukhotinsky Terrane. It is shown that the studied rocks have similar petrographic features (with the presence of transitional varieties) and form common trends in the petrographic diagrams. This suggests that they are members of a single magmatic association. The geochemical features of the monzodiorites, quartz monzodiorites, and granodiorites, in particular their enrichment in large ion lithophile elements (LILE) and depletion in some HFSE, indicate their similarity with island-arc magmatic rocks. The presence of monzonites and quartz monzonites in the studied monzodiorite–granodiorite association along with high K, Rb, Th, and Pb concentrations gives reasons to believe that it formed in active continental margin or ensialic island-arc environments. The granodiorites of the monzodiorite–granodiorite associations of the Nora–Sukhotinsky Terrane are dated at 440 ± 10 Ma and may be considered as a fragment of the early Silurian active continental margin or ensialic mature island arc in the structure of the South Mongolia–Khingan orogenic belt.     

Late Vendian postcollisional leucogranites of Yenisei Ridge

The Late Vendian (540–550 Ma) U–Pb zircon age of postcollisional granitoids in the Osinovka Massif was obtained for the first time. The Osinovka Massif is located in rocks of the island-arc complex of the Isakovka Terrane, in the northwestern part of the Sayany–Yenisei accretion belt. These events stand for the final stage of the Neoproterozoic history of the Yenisei Ridge, related to the completing accretion of the oceanic crust fragments and the beginning of the Caledonian orogenesis. The petrogeochemical composition and the Sm–Nd isotopic characteristics support the fact that the granitoid melt originated from a highly differentiated continental crust of the southwestern margin of the Siberian Craton. Hence, the granite-bearing Late Riphean island-arc complexes were thrust over the craton margin at a distance considerably exceeding the dimensions of the Osinovka Massif.     

Reconstruction of the Sakhalin marginal paleobasin by geological and petrochemical data

The structural and compositional volcanosedimentary complexes and igneous rocks of the Sakhalin marginal paleobasin as well as the geodynamic setting were described. The Sakhalin marginal paleobasin was formed in Sakhalin Island and the adjacent water areas at the end of the Early Cretaceous-start of the Late Cretaceous. The paleobasin was a part of the Kula Plate separated from the ocean along with spreading zones and oceanic volcanic islands by the Sea of Okhotsk microcontinent and the Jurassic-Early Cretaceous Sheltinga volcanic island arc. The petrochemical features and geodynamic setting of the igneous rock formation testify to the fact that magma-generating tectonomagmatic structures of the epioceanic Sakhalin marginal paleobasin continued functioning after its isolation under intensive terrigenous sedimentation. The Sakhalin marginal paleobasin had a heterogeneous basement composed of oceanic and continental crust blocks of the Earth. The paleobasin completed its development in the Paleogene.     

西秦岭楔的构造属性及其增生造山过程

西秦岭楔是叠置于早古生代造山作用基础上形成的并插入祁连和昆仑早古生代造山带内部的楔形地质体,以大面积出露三叠系并发育多条蛇绿混杂岩带、大型韧性剪切带、中生代火山-岩浆作用和斑岩-矽卡岩型矿床为典型特征,具有增生造山作用的典型特征。这些蛇绿混杂岩带和岛弧钙碱性火山-岩浆岩的形成时代均具有向南逐渐变年轻的空间演化特征,显示了特提斯洋演化过程中海沟具有向南撤退的基本特征。砂岩碎屑组成以及源区特征研究结果表明,西秦岭楔三叠系形成于活动大陆边缘,其碎屑沉积物来自于古特提斯洋北侧的增生杂岩及岛弧。丰富的岛弧钙碱性火山-岩浆岩和沉积组合以及赋存的斑岩-矽卡岩型矿床,均与东昆仑及南秦岭相一致,呈现出相似的岩石组合类型以及岩石地球化学和同位素地球化学特征。这些事实表明,三叠纪时期,东昆仑、西秦岭以及祁连造山带是一个有机整体,自西向东存在一条三叠纪增生岩浆弧。锆石Hf同位素及岩石地球化学成分结果则表明,该增生岩浆弧部分岩浆来自于俯冲增生杂岩的部分熔融。     

Cenozoic stratigraphy of the northern Sakhalin shelf

The analysis of diatom, palynofloral, and benthic foraminiferal assemblages made it possible to substantiate the age of Cenozoic sections recovered by wells on the northern and northeastern Sakhalin shelf. Biostratigraphic materials, lithological properties of stratigraphic units, and standard logs served as the basis for developing the first stratigraphic correlation scale of Cenozoic sequences on the Sakhalin shelf.     

On the stress state of the Sakhalin crust according to the data of drilling deep boreholes

The stress state of the sedimentary rocks in the oil-and-gas fields of Sakhalin is estimated and analyzed; data from exploratory boreholes in the eastern Pacific (San-Andreas fault zone) are considered. The vertical and limiting horizontal stresses are calculated for different depths. The maximum sublateral compression can exceed the vertical stress by a factor of 1.2–4 on average in northern and southern Sakhalin. It is shown that the limiting horizontal stress and the maximum shear stress grow as the depth increases.     

Seismic anisotropy distribution beneath southern Sakhalin

Shear wave splitting parameters from local deep-focus and crustal earthquakes beneath southern Sakhalin and northern Hokkaido have been measured. The study of the split shear wave amplitude, polarization, and splitting parameter distribution revealed their correlation with the geometry of the subsiding Pacific Plate and horizontal heterogeneity of the rheological properties and viscosity of the medium. Comparison of the observed data with those modeled in anisotropic media allows the mantle flow to be oriented NNW beneath southern Sakhalin and northern Hokkaido. Based on the split shear wave time delays, the degree of mantle anisotropy is estimated to be around 1–2% beneath southern Sakhalin and 1.5–2.5% beneath northern Hokkaido. A relatively high anisotropy (2–15%) from local crustal earthquakes is found beneath the Central Sakhalin Fault.     

Parallel Tethyan sutures in mainland Southeast Asia: New insights for Palaeo-Tethys closure and implications for the Indosinian orogeny

Two contrasting parallel tectonic sutures can be recognised through the Yunnan–Thailand region of mainland Southeast Asia; they are sutures of the Devonian–Triassic Palaeo-Tethys Ocean and a Permian back-arc basin. The Changning–Menglian and Inthanon suture zones are regarded as the Palaeo-Tethys Suture Zone. The Jinghong–Nan–Sra Kaeo suture is regarded as a closed back-arc basin. The Sukhothai Zone is no longer treated as a part of the Sibumasu Terrane, but is defined as the core part of the Permian island-arc system developed on the western margin of the Indochina Terrane. Two tectonic events are interpreted from the parallel sutures; a Late Permian collapse of the back-arc basin and a mid-Triassic collision of Sibumasu to the Sukhothai Arc of Indochina (= closure of the Palaeo-Tethys). The Early–early Middle Triassic thermotectonism of Vietnam as linked to the Indosinian orogeny by some authors is incompatible with the suggested timing of Sibumasu collision, but instead it is temporally closer to the back-arc compression of western Indochina.     

Tectonic setting and geochemical characterisation of Neoproterozoic volcanics and granitoids from the Adobha Belt, northern Eritrea

The Adobha Belt consists largely of metamorphosed volcanic and sedimentary sequences and syntectonic granitoids. Identification of the ancient tectonic setting of the belt has been attempted using trace element (including rare earth element) geochemical data on igneous suites. The Himbol volcanics of the Nakfa Terrane represent a calc-alkaline island-arc setting, while the Uogame basalts and associated gabbros characterise a tholeiitic, transitional MORB setting. The chemistry and petrographic features of the basalts and gabbros clearly suggest that they are parts of a dismembered ophiolite. Other authors' work shows that further to the west, the Hager Terrane belongs to a calc-alkaline island-arc setting. The Uogame ophiolitic suite thus portrays a MORB crust sandwiched between two island-arc terranes. Based on the fact that the MORB-type crust occurs within a high strain zone that extends for several hundreds of kilometres along strike, it is concluded here that it represents a terrane-bounding Neoproterozoic ophiolite belt. It is further suggested that this belt might outline a suture that signifies the collision between the island-arcs.     

Structure of terrestrial crust in southern Sakhalin and adjacent aquatoria

Recent deep seismic sounding and other data suggest presence of a deep rift which cuts across southern Sakhalin, Aniva Bay, La Perouse Strait, and Hokkaido Island, a suboceanic type of crust in the Sea of Okhotsk, and a continental one in Sakhalin. The density-velocities relationship determined here may account for the observed gravity anomalies as due primarily to variations in thickness of the crust (fig. 6), without resorting to other hypotheses.     

Recent crustal movements and the Moneron earthquake of 1971

From the results of high-precision levelling conducted in the southern part of Sakhalin Island, the dynamics of the earth's surface have been determined before the Moneron earthquake in 1971, during its numerous aftershocks and in the following periods. Great subsidence, of up to 7–8 cm, preceded the earthquake, then a 3-year period of steady state followed and at the end of this period the earthquake happened. After this event vertical movements show an inherited character related to the very recent geological structures. Horizontal displacements of the triangulation net, 50 km distant from the earthquake epicentre, were also noted; they had a northeast direction and an amplitude of 7 cm.