Vol 53, No 1 (2019)

The structure and peculiarities of the tectonic evolution of Precambrian terranes included in the structure of Paleozoides in different parts of the Central Asian orogenic belt are reviewed; types and comparative characteristics of Precambrian terranes are provided. We shed light on two types of Precambrian terrane structures: essentially juvenile Neoproterozoic crust (1); Mezo- and Early Neoproterozoic crust formed due to reworking of Early Precambrian formations (2). Terranes with juvenile Neoproterozioc crust, located in the Central and Eastern parts of the Central Asian orogenic belt, were generated in the oceanic sector of the Earth. Their formation was related to the Early- and Late Neoproterozoic cycles of tectogenesis up to 200 Ma each. Terranes with Mezo- and Early Neoproterozoic crust, found mainly in the west of the Central Asian orogenic belt, were generated in the continental sector of the Earth during the Neoproterozoic and their evolution occurred mainly in intracontinental environments. During the evolution of all of the considered terranes in an interval of 800‒700 Ma, an event associated with rift zone formation and intraplate magmatism was revealed: it coincided with the breakup of the supercontinent of Rodinia. The conducted research makes it possible to relate the formation history of the Precambrian terranes of the Central Asian orogenic belt to processes that took place at the margin of the Syberia-Tarim part of Rodinia and the adjacent sector of the paleocean.

The paper considers the origin of hydrocarbon accumulations within the Pre-Cenozoic basement of the Vietnam shelf. It is shown that the formation of hydrocarbon deposits is associated with protrusive granite massifs that underwent structural–tectonic reworking at the prototectonic and posthumous (postmagmatic) tectonic stage. Together, the posthumous structure-forming processes led to changes in the viscosity properties of rocks, their tectonic and material inhomogeneity, lamination, and, as a result, vertical and lateral spatial redistribution with the formation of granite protrusions. The mechanisms by which voids and oil and gas traps formed within protrusions are considered. A conclusion on the organic nature of petroleum in the basement of the Vietnam shelf is drawn, based on the similarity of the geochemical characteristics and biomarkers of the petroleum and organic parameters in the Oligocene–Miocene sedimentary cover and basement rocks. Possible mechanisms for hydrocarbon migration and accumulation in basement rocks are considered. It is confirmed that the formation of hydrocarbon deposits occurred due to lateral and downward migration of hydrocarbons via contact zones from Oligocene and Miocene oil source rocks into crystalline massifs: into voids and increased fracture zones in the protrusion body.

Vendian and Permian–Triassic plagiogranite magmatism is distinguished for the Ust-Belsky and Algansky terranes of West Koryak fold system. The U‒Pb zircon ages from Vendian and Permian–Triassic plagiogranites are 556 ± 3 Ma (SIMS), 538 ± 7 Ma (LA–ICP–MS) and 235 ± 2 Ma (SIMS), respectively. It is revealed that Vendian and Permian–Triassic plagiogranites are mainly low-K and low-Al. Sr‒Nd isotopy and rare earth element patterns make it possible to suggest their formation as partial melting of primarily mantle substrate or fractional crystallization of the basic magma. Vendian plagiogranites formed within active margin in ensimatic island arc simultaneously with deposition of the lower part of the volcano-sedimentary complex of Otrozhninskaya sheet. We suggest that the Permian–Triassic plagiogranites were formed within the limits of the Ust-Belsky segment of the Koni-Taigonos arc during partial melting of melanocratic ophiolite material built up as fragments in the accretionary structure of that arc or by fractional crystallization of basic magmas melted from a similar substrate.

Estimation of finite strain and microstructural analysis of deformed rocks are keys to better understanding deformational processes and related structures in a variety of scales started from microscopic fabric development to regional-scale structures. In the present work, we carried out the quantitative calculation of strain using the R_f/φ and Fry methods for quartz, feldspar and mafic grains (e.g. biotite and hornblende) from twenty two collected samples for granitic gneiss, amphibolite and hornblende schist samples from the Um Junud area situated in south Eastern Desert of Egypt. Forty four thin sections were prepared and measured by using finite strain methods. The strain data indicate high to moderate ranges of deformation of the amphibolite to granitic rocks. The axial ratios in the XZ section range from 1.74 to 4.37 and 1.50 to 4.46 for the R_f/φ and the Fry methods respectively. The finite strain direction for the long axes displays clustering along N to WNW trend, and shallow WNW plunging in the majority of the studied samples. The short axes are found to be subvertical associated with a subhorizontal foliation. It is concluded that finite strain is of the same order of magnitude for various lithologic units outcropping in the area, and that contacts were formed under semi-brittle to semi-ductile deformation conditions. Thus, the finite strain accumulated during superimposed deformation on a previously nappe structure assemblage, which pointed out that these contacts were created during the accumulation of finite strain. This result is inconsistent with the generally believed that nappe creation in orogens carried out by simple shear deformation.

The quality agents for P- and S-waves (Q_p and Q_s) were estimated for earthquakes that recorded by nine seismic stations (DHA, NUB, TR1, TR2, KAT, SH2, GRB, HRG and SFG) from the Egyptian National Seismological Network (ENSN). These stations are located at southern Sinai and northern Red Sea coast region. For Q-values estimation, one station spectral rate method was applied and the spectral amplitude rates have been calculated between 2‒20 Hz for this aim. The obtained Q-values prove that Q_p and Q_s increase as a follower of frequency according to the relation Q = Q₀f ⁿ. By averaging Q-values which were calculated at all nine stations the median attenuation relations for Q_p and Q_s were obtained as: Q_p = (13.15 ± 0.76) f ^0.95±0.19 and Q_s = (20.05± 0.79)f ^1.03±0.04. These relations are helpful for respect of origin parameters of earthquakes and emulation of earthquake strong ground motions. The Q_s/Q_p ratio for KAT station is less than 1 at lower frequencies, whereas at HRG and SH2 stations Q_s/Q_p ratios are greater than 1.