Vol 52, No 2 (2018)

The structural and geodynamic features of the Pamirs and the Hissar–Alay have been revealed based on geological and geophysical evidence supplemented by experimental data. It has been shown that both the Pamirs and the Hissar–Alay are geodynamic systems, the formation of which is related to interference of two geodynamic regimes: (i) global orogeny covering extensive territories of Eurasia and determining their similarity and (ii) regional regimes differing for the Pamirs and the Alay, which act independently within Central Asian and Apline–Himalayan mobile belts, respectively. The Pamirs do not act as an indentor during the formation of structure of the Hissar–Alay and areas to the north. It is stated that the Pamir–Alay segment of Asia is a reflection of the geodynamic countermotion setting (3D flow of mountain masses) of several distinct segments of the continental lithosphere, while the Pamirs are an intracontinental subduction domain at the surface, which represents a special tectonic–geodynamic type of structures.

The paper considers Cretaceous magmatism at the continental margin of the Arctic Region. It is shown that Cretaceous igneous rocks of this region are rather heterogeneous in age, composition, and geodynamic formation setting. This differentiates them from rocks of typical large igneous provinces (LIPs). Local areas of magmatic activity, their substantial remoteness them from one another, and significant distinctions in age, composition of rocks, and formation conditions prevent us from unreservedly combining all occurrences of Cretaceous magmatism at the continental margin of the Arctic Region into a common igneous province. The stage of tholeiitic magmatism in the Svalbard Archipelago, Franz Josef Land, Arctic Canada, and the Alpha–Mendeleev Rise, which can be considered an LIP, began in the Early Cretaceous and continued for a long time, at least until the Campanian. The magmatism apparently had a plume source and was caused by extension during opening of the Canada Basin. Tholeiitic magmatism gave way to the alkaline magmatism stage from the Campanian to the onset of the Paleocene, related to continental rifting at the initial stage of formation of Eurasian Basin in the Arctic Region. No convincing evidence for a genetic link between Early Cretaceous tholeiitic and Late Cretaceous alkaline magmatism is known at present, nor for the alkaline magmatism belonging to a plume source.

Data on the petrography, geochemistry, and isotopic geochronology of granites from the northern part of the Taimyr Peninsula are considered. The Early–Middle Carboniferous age of these rocks has been established (U–Pb, SIMS). Judging by the results of ⁴⁰Ar/³⁹Ar dating, the rocks underwent metamorphism in the Middle Permian. In geochemical and isotopic composition, the granitic rocks have much in common with evolved I-type granites. This makes it possible to specify a suprasubduction marginal continental formation setting. The existence of an active Carboniferous margin along the southern edge of the Kara Block (in presentday coordinates) corroborates the close relationship of the studied region with the continent of Baltia.

Neoproterozoic rocks, Oligocene to Neogene sediments and Tertiary Red Sea rift-related volcanics (Harrat) are three dominant major groups exposed in the Jeddah tectonic terrane in Western Arabia. The basement complex comprises amphibolites, schists, and older and younger granites unconformably overlain by a post-amalgamation volcanosedimentary sequence (Fatima Group) exhibiting post-accretionary thrusting and thrust-related structures. The older granites and/or the amphibolites and schists display mylonitization and shearing in some outcrops, and the observed kinematic indicators indicate dextral monoclinic symmetry along the impressive Wadi Fatima Shear Zone. Finite strain analysis of the mylonitized lithologies is used to interpret the deformation history of the Wadi Fatima Shear Zone. The measured finite strain data demonstrate that the amphibolites, schists, and older granites are mildly to moderately deformed, where XZ (axial ratios in XZ direction) vary from 2.76 to 4.22 and from 2.04 to 3.90 for the R_f/φ and Fry method respectively. The shortening axes (Z) have subvertical attitude and are associated with subhorizontal foliation. The data show oblate strain ellipsoids in the different rocks in the studied area and indication bulk flattening strain. We assume that the different rock types have similar deformation behavior. In the deformed granite, the strain data are identical in magnitude with those obtained in the Fatima Group volcanosedimentary sequence. Finite strain accumulated without any significant volume change contemporaneously with syn-accretionary transpressive structures. It is concluded that a simple-shear deformation with constant-volume plane strain exists, where displacement is strictly parallel to the shear plane. Furthermore, the contacts between various lithological units in the Wadi Fatima Shear Zone were formed under brittle to semi-ductile deformation conditions.

The Darreh Sary metapelitic rocks are located in the northeast of Zagros orogenic belt and Sanandaj-Sirjan structural zone. The lithological composition of these rocks includes slate, phyllite, muscovitebiotite schist, garnet schist, staurolite-garnet schist and staurolite schist. The shale is the protolith of these metamorphic rocks, which was originated from the continental island arc tectonic setting and has been subjected to processes of Zagros orogeny. The deformation mechanisms in these rocks include bulging recrystallization (BLG), subgrain rotation recrystallization (SGR) and grain boundary migration recrystallization (GBM), which are considered as the key to estimate the deformation temperature of the rocks. The estimated ranges of deformation temperature and depth in these rocks show the temperatures of 275–375, 375–500, and >500°C and the depths of 10 to 17 km. The observed structures in these rocks such as faults, fractures and folds, often with the NW-SE direction coordinate with the structural trends of Zagros orogenic belt structures. The S-C mylonite fabrics is observed in these rocks with other microstructures such as mica fish, σ fabric and garnet deformation indicate the dextral shear deformation movements of study area. Based on the obtained results of this research, the stages of tectonic evolution of Darreh Sary area were developed.