Home – Home – Jornals – Russian Geology and Geophysics 2016 number 10

The Chulyshman Upland is a big mountain structure at the boundary between the Altai and West Sayan mountain ranges. It is composed mostly of metamorphic rocks of the greenschist, epidote-amphibolite, and amphibolite facies as well as several large granitoid massifs. The upland stretches for 150 km to the northwest and is ≤50 km in width. The main divides are flattened and are almost 3000 m (or, seldom, slightly more) in height. The southwestern and northeastern boundaries of the upland are formed by recent dextral strike-slip faults, and the northwestern and southeastern boundaries, by reverse faults. Neotectonic structures of lower rank divide the uplift into four mountain massifs similar in morphology and size; the mountain masses, in turn, are divided into altitude stages. Recent E-W trending faults are reverse, whereas N-S trending faults are normal. Horizontal displacements along the dextral strike-slip faults are >2000 m, while vertical displacements along the reverse and normal faults are within 150-500 m. There is no contemporary glaciation on the upland. In the Pleistocene, the upper stage of the relief was occupied by ice sheets, which left numerous traces of exaration. The middle stage was a transit zone for the outlet tongues of ice sheets; here, moraine deposits cover the bottoms and slopes of valleys. The lower stage was a zone of moraine accumulation in the Pleistocene. In the Holocene, neotectonic activity in the area was expressed as numerous earthquakes, which have left traces in the form of rock slides at the base of 20% of the extension of the sides of glacial valleys and seismogenic trenches 80 to 2300 m in length. Studies have revealed a pleistoseist zone (50°37′10″ N; 88°51′08″ E) from a recent (about 200-300 years ago) catastrophic earthquake.

The performed 40Ar/39Ar geochronological studies yielded the first reliable age of trachyandesites of the Mogot volcanic field (115 ± 3 Ma), which, together with the age of trachyandesites of the Bomnak volcanic field (117 ± 1 Ma), gives ground to recognize a new stage (117-115 Ma) of evolution of the Stanovoi volcanoplutonic belt superposed on the igneous and metamorphic complexes of the Dzhugdzhur-Stanovoi superterrane of the Central Asian Orogenic Belt and Stanovoi structural suture. The trachyandesites of the Mogot volcanic field are similar in geochemical features to adakites. Their parental melts resulted from the transformation of Precambrian continental crust under postcollisional extension after the formation of the Mongolo-Okhotsk orogen or under sliding along the boundary between the North Asian craton and the Amur microcontinent.

A Late Cretaceous dismembered ophiolite unit in the south of Kahramanmaraş belongs to the Peri-Arabian ophiolite belt in southern Turkey. The ophiloitic rocks include, from bottom to top, metamorphic sole (plagioclase-amphibole schist and plagioclase amphibolite), mantle tectonites (serpentinized dunite and harzburgite), ultramafic cumulates (mainly websterite), mafic cumulates (olivine gabbro, gabbro, and olivine gabbronorite), and isotropic gabbros (olivine gabbronorite). The whole-rock geochemistry of the cumulates suggests that they can mainly be classified as Low-Ti ophiolite and cumulate rocks derived from an island-arc tholeiitic magma source. Chondrite-normalized REE and N-MORB-normalized multielement patterns and tectonomagmatic discrimination diagrams show that fractional crystallization was important during the formation of the cumulate rocks. The presence of highly magnesian olivines (Fo_78.24-81.89), clinopyroxenes (Mg#_71.46-85.82), and orthopyroxenes (Mg#_62.63-87.18) as well as highly calcic plagioclases (An_81.88-97.40) in the mafic cumulates indicates a subduction-related tectonic environment. The geochemical, petrographical, and field data suggest that the ophiolitic rocks in the studied region formed in a suprasubduction zone environment in the southern branch of Neotethys in the Late Cretaceous.

The platinum-group minerals (PGM) in placer deposits provide important information on the types of their primary source rocks and ores and formation and alteration conditions. Different characteristics of minerals can be determined by a set of conventional and modern in situ analytical techniques (scanning electron microscopy (SEM) and electron probe microanalysis (EPMA)). A study of PGM from placers of southern Siberia (Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge) shows that the morphology and composition of PGM grains, the texture, morphology, and composition of silicate, oxide, and intermetallic microinclusions, and the type of mineral alteration can serve as efficient indicators of the primary sources of PGM. The widespread rock associations in the Kuznetsk Alatau, Gornaya Shoria, and Salair Ridge, the compositions of PGM and microinclusions in them, and the dominant mineral assemblages testify to several possible primary sources of PGE mineralization: (1) Uralian-Alaskan-type intrusions; (2) ophiolite associations, including those formed in a subduction zone; (3) ultramafic alkaline massifs; and, probably, (4) rocks of the picrite-basalt association. The preservation of poorly rounded and unrounded PGM grains in many of the studied placers of the Altai-Sayan Folded Area (ASFA) suggests a short transport from their primary source.

Hard fragments (crushed only by a hammer) of manganese deposits differing strongly in appearance from the regional ferromanganese crusts were sampled from depths of 3500-3200 m during the dredging of an unnamed seamount in the Central Basin of the Sea of Japan. Their surface has a black carbonaceous coating; after its removal, the crusts become steel-gray. The specific weight of these crust fragments is 3.35 g/cm3, whereas the regional crusts have a specific weight of <2 g/cm3. X-ray diffraction analysis showed that the sampled fragments consist of pure pyrolusite. There are also fragments of crusts formed by todorokite and birnessite. All pyrolusite samples have an abnormally high content of Mn (up to 63%). The Mn/Fe ratio reaches 9016. The conclusion is drawn that the manganese crusts formed on this seamount are of hydrothermal genesis.

The theory of resistivity logging (RL) originally stems from the Fock-Stefänescu forward problem solution for the stationary electric field E in a piecewise homogeneous isotropic medium, with a single boundary corresponding to the surface of a cylinder unlimited by height. The cylinder simulates a borehole filled with drilling mud of resistivity ρ = ρ_b, which penetrates a formation with resistivity of rocks ρ = ρ_r. The primary field E is produced by the charge of a current electrode A placed on the cylinder axis. In this paper the forward problem for the field E is investigated for the electrode A at an arbitrary point off the axis of a borehole embedded in a transversely isotropic formation, with an anisotropy axis parallel to the borehole axis. The solution of forvard problem is used in algorithms and respective software for processing resistivity logs affected by electrode eccentricity and formation anisotropy. The changes caused to apparent resistivity by the two effects are estimated in percent for axial and lateral electrode dispositions.

Three-dimensional inversion of magnetotelluric fields made by 3D block modeling of magnetotelluric (MT) fields of geoelectric models permitted construction of alternative models fitting real MT data. Based on their analysis, highly conductive zones have been identified at different depths of the crust. Their correct interpretation requires the use of reliable gravity, temperature, and seismic data. Sensitivity analysis of 3D MT model curves with respect to conductive crustal blocks has shown that it is advisable to use the maximum curves of the phase tensor to estimate the electrical conductivity of the crustal and mantle parts of the section. The information value of these curves is close to that of the maximum induction curve on mapping conductive blocks in the upper and middle crust. It is also shown that the real Wiese-Parkinson vectors provide high resolution on estimating the excess integralted conductivity of the crustal blocks. Therefore, magnetovariational sounding in the Northern Tien Shan should be continued.

Using an MST-05 NMR relaxometer, we determined the dependence of NMR parameters on the specific surface and resistivity of water-saturated shaly sandstone and siltstone core samples. The influence of the type and quantity of clay minerals was evaluated at residual water saturation, because the main contribution to the measured NMR signal in this case is made by clay content. Based on NMR relaxometry data, we obtained quantitative estimates of the specific surface with clay- and capillary-bound fluids. The surface relaxivity was estimated from thermal-desorption and NMR relaxometry data. The high degree of reliability of its values was confirmed by the agreement between the pore size distributions determined by NMR relaxometry and by capillarimetry and granulometry. We have established that the parameters of NMR spectra depend on the specific surface and resistivity, which are, in turn, a function of the surface properties of both clay and sandstone/siltstone particles.

Magnetotelluric (MT) responses are commonly interpreted assuming vertical plane-wave excitation. However, the primary electromagnetic (EM) field can be laterally inhomogeneous along the surface near auroral zones and its sources. It is suggested to use synchronous records of MT responses to an inhomogeneous source field with a dominant spatial harmonic. The applicability of this approach is checked using spatial filtering of synchronous wave data and a calculated directional pattern, which has its maximum along the wave vector of the dominant harmonic. The parameters of this harmonics are further estimated by optimization, with the EM field being presented as a sum of transverse electric (TE) and transverse magnetic (TM) modes. The TM mode, which forms in the presence of surface structures, is responsible for galvanic distortions. Inversion of the EM field components recorded synchronously at three or more sites gives the amplitudes of the two field modes and period-dependent apparent resistivities, while the static shift in the data becomes removed. The algorithm has been successfully tested on data from the BEAR project in the Fennoscandian Shield.