R Kulinich

Late cretaceous to early paleogene paleomagnetic results from Sikhote Alin, far eastern Russia: implications for deformation of East Asia

Abstract Welded tuffs in the Bogopol and Sijanov groups were sampled at 27 sites from 12 caldera formations in the Sikhote Alin mountain range around Kavalerovo (44.3°N, 135.0°E) for chronological and paleomagnetic studies. K Ar age dates show that the welded tuffs erupted between 66 Ma and 46 Ma. All sites yield reliable paleomagnetic directions, with unblocking temperatures higher than 560°C. The high-temperature component at 12 sites and the medium-temperature component at 3 sites in the Bogopol Group show reversed polarity ( D = 193.7°, I = −57.6°,α 95 = 8.1°). The high-temperature component at 11 sites in the Sijanov Group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north ( D = −2.9°, I = 59.6°,α 95 = 11.2°). The combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250.5°E, 84.1°N (A 95 = 8.8°), which falls near Cretaceous paleomagnetic poles from Outer Mongolia, Inner Mongolia, the North China Block and the South China Block. The Sikhote Alin area appears not to have been subjected to detectable motion with respect to East Asia since about 50 Ma. This implies that the Sikhote Alin area behaved as an integral part of East Asia during the opening of the Japan Sea at about 15 Ma. However, significant separation between the paleomagnetic poles of East Asia and Europe during the Jurassic-Paleogene implies a major relative movement between these two blocks since the Paleogene.

Structural and geological characteristics of a “seismic gap” in the central part of the Kuril Island Arc

The results of the cruise of R/V Akademik M.A. Lavrentiev conducted by the Pacific Institute of Oceanology, Far East Division of the Russian Academy of Sciences and the Shirshov Institute of Oceanology, Russian Academy of Sciences in August to September 2005 are considered. The aim of the works was to specify the tectonic structure, seismogenic potential, and tsunamigenic hazard of the central segment of the Kuril Island Arc. The complex studies involved single-channel seismic profiling, gravimetry, magnetometry, detailed bathymetry, dredge sampling of sea-floor rocks and sediments, and gas geochemistry. Geophysical and geological data are reported. It was demonstrated that the target area is an active tectonic destruction zone, the zone boundaries were outlined, and the main internal structural and compositional heterogeneities were identified.

P-wave velocity structure in the northern part of the central Japan Basin, Japan Sea with ocean bottom seismometers and airguns

In 1996, an airgun-ocean bottom seismometer survey was carried out in the northern part of the central Japan Basin. The crustal thickness in the central part is about 9 km, including a sedimentary layer with thickness of 1.5 km, and increases eastward. The obtained crustal structure is slightly different from those of typical ocean basins. The thickness and velocity of less than 6.5 km/s in the upper part of the crust do not correspond to that of a typical oceanic crust and the clear linear geomagnetic anomaly around this survey line has been unconfirmed. Although, this crust could be interpreted to be either anomalous thick oceanic crust formed at spreading centers influenced by a mantle plume or thinned continental crust at ocean-continental boundaries in passive margins, we prefer the latter as a conclusion, that is, it may be formed by thinning of a continental crust rather than by the melt of mantle plumes during the opening of the Japan Sea. In addition, the difference of the crustal structures in the study area and the northeastern Japan Basin where the crust is typical oceanic, indicates that the process of crustal formation may differ in the northern part of the central Japan Basin from in the northeastern Japan Basin.

Thicknesses and types of the crust beneath the Sea of Japan inferred from marine and satellite gravimetric investigations

The combined gravimetric, altimetric, and seismic data used for calculating the crust thickness beneath the Sea of Japan revealed its different types in this region: oceanic and continental with different degrees of destruction. It is shown that the crust thickness derived from satellite altimetry is well consistent with that calculated from the data obtained by on-board gravimetric observations.

Peculiarities of the fluid regime in the lithosphere of the junction zone between South Primorye and the Sea of Japan from the comprehensive geophysical data

New data on the fluid lithospheric regime in the junction zone of the continent and the marginal seas are presented. For the first time, this problem was solved by the comprehensive interpretation of the geophysical methods, including magnetotelluric and geomagnetic-variation sounding, unique investigations of the variations of the electric field along the Japan Sea Cable (JASC), structural-density modeling, thermometry, and thermodynamics. A joint analysis of the distribution of the electric conductivity, density, and temperature in the lithosphere made it possible to substantiate the participation of mantle fluids in the formation of lithotectonic complexes that are quite different beneath the continent and the marginal Sea of Japan.

Late Cretaceous paleomagnetic results from northeast Asian continental margin: The Sikhote Alin Mountain Range, eastern Russia

Welded tuffs in the Upper Cretaceous Kisin Group have been collected for paleomagnetic study from 24 sites in Sikhote Alin, eastern Russia. Normal paleomagnetic directions from 11 sites are regarded as primary directions, whereas reversed paleomagnetic directions from 13 sites are ascribed to secondary origin. The normal directions of the Kisin Group together with the characteristic directions of the Sijanov Group yield the Late Cretaceous characteristic direction of Sikhote Alin (D = 355.0°, I = 59.8° with α95 = 7.2°). A significant separation (15.4°±11.9°) of pole positions is observed between Sikhote Alin (85.9°N, 11.3°E with A95 = 10.1°) and the North China Block. Sikhote Alin has been subjected to counterclockwise rotation through 20.5°±17.7° with respect to North China Block. The rotation occurred during 66–51 Ma, and is ascribed to sinistral motion along the central Sikhote Alin fault.

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

Abstract We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D =345.8°, I =49.9°, α 95 =14.6° ( N =9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.

New Data on the Deep Structure of the Northern Part of the Gulf of Tonkin in the South China Sea Based on the Results of Magnetotelluric Soundings

We present a geoelectric model for the northern part of the Gulf of Tonkin in the South China Sea, which was developed for the first time based on the data of magnetotelluric studies carried out in 2012- 2013. The existence of a regionally developed aniso� tropic conducting fractured horizon and a steeply dip� ping electrically conductive fault zones is a distin� guishing property of this model, which has a layer� block structure. The nature of the zones of electric conductivity is related to the accumulation of a water� graphite mixture during longterm propagation of the flow of deep gases, which include H 2 and CH4. This allows us to consider the outcrops of the deep fluid sat� urated fractures as potential oil bearing regions. In the conditions when traditional oil and gas deposits are being exhausted, the search for new objects of accumulation of hydrocarbons has become one of the present pressing problems. In recent years, research into fissured formations in the upper level of the basement, which is a transition complex between the crystal basement and the sedimentary cover, has become widely developed (1). The rocks related to the transition basement are almost not changed by the processes of regional metamorphism. They are dislo� cated more greatly than the overlying columns of the sedimentary cover, transformed by recrystallization processes and catagenesis, and have a different struc� tural position. The oilandgas-bearing properties of such complexes have been established in different regions of the world: East Siberia (2), Venezuela, US, Libya, Egypt, and India (3). In 1988, a unique deposit was discovered in the granites of the Mesozoic base� ment on the shelf of South Vietnam (4). Deep geophysical processes that give us the possi� bility to make a primary estimate of the structural and material characteristics of the basement in the study regions play an important role in the study of these structures. The estimate includes the existence of fis� sured complexes and locations of permeable zones in the lithosphere that provide pathways for migration of deep fluid fluxes, which are sources of saturations of the fissured complexes with hydrocarbons. All this allows us to forecast the oilandgas-bearing potential of the study region. One of the geophysical methods used for the solution of these problems is magnetotel� luric sounding, which makes it possible to study the differentiation of electric resistivity of the tectono� sphere deep section depending on its structural mate� rial composition.

Geophysical Fields, Block Structure, and Seismic Activity of the Central Kuril Islands

Based on the combined analysis of the seismologic and other geophysical data (the gravity and magnetic fields and the acoustic basement topography), it was revealed that the largest earthquakes that occurred in the region of the Central Kuril Islands in 2006, 2007, and 2009 were related to the block structure of the Earth’s crust. The aftershocks of the largest earthquakes recorded in 2006 and 2007 became foreshocks for the earthquakes of 2007 and 2009, respectively. In total, the earthquakes that occurred in this area in the period of 2006–2012 are closely related to the tectonic destruction zone existing there, clarify its position, and support the previous concept of the superimposed and cross cutting relationships of this zone with respect to the Kuril Island Arc.

A comparative analysis of the seismic and density models for the Earth’s Crust of the Central Kurils

The seismic and density models for the Earth’s crust of the central region of the Kuril Island arc are compared. The seismic data are based on the results of deep seismic soundings that were performed here in the 1980s. The density models were constructed by using marine and satellite gravimetric data obtained in recent years. Comparison of the seismic and density models reveals their overall consistency, with a few discrepancies. The possible sources of these discrepancies are discussed.