Nº 4 (2023)

This article presents results of the structural and morphological analysis of the fracture zones which are part of Doldrums Megatransform System (MTS), located in the northern part of the Equatorial Atlantic (6.5°–9° N) that include Vernadskiy and Bogdanov transform faults and the Doldrums and Pushcharovskiy megatransforms. Bathymetric map, based on the multibeam echo sounding data, collected during 45 cruise of the R/V Akademik Nikolaj Strakhov was used for this analysis. It was established that large-scale variations in the width of fracture zone valleys are determined by the distribution of stresses perpendicular to the fracture zone. In the areas with compressive stresses, the fracture zone valleys are narrower, and the in extension areas are wider. The difference in geodynamic settings within the MTS is due to the difference in spreading directions, which change from \(\perp \)89° to \(\perp \)93° when moving from south to north. The depth of fracture zone valleys consistently increases from the periphery of the MTS (Bogdanov and Doldrums faults) to the center (Pushcharovskiy fracture zone) in accordance with a decrease in the upper mantle temperature. In each fracture zone, the valley depth decreases from the rift- fracture zone intersections towards the center of the active part to a certain background depth. It is assumed that this phenomenon is the result of the uplift of the valley bottom, which occurred due to the decompaction of the lithosphere, caused by the serpentinization of ultramafic rocks. The violation of the revealed variations in the width and depth of fracture zone valley patterns occurs as a result of various ridges and uplifts formation in the fracture zone. In the axial zones of the active parts of the fracture zone valleys median ridges are widespread, extending parallel to the fracture zone and representing serpentinite diapirs squeezed out above the bottom surface. Transversal ridges which were formed 10‒11 million years ago as a result of the lithospheric plate edge flexural bending under extensional conditions are now located in the western passive parts on the southern sides of the of Doldrums and Pushcharovskiy fracture zone valleys. The transverse ridge on the northern side of the Vernadskiy fracture zone, which includes Mount Peyve, was formed between 3.65‒2.4 Ma. Due to the frequent jumps of the spreading axis in this region, it was divided into three segments. There are interfracture zone ridges in megatransforms, which in the active part consist of two fracture zone valleys. Time of their formation: in Pushcharovskiy megatransform ‒ 30‒32 million years ago and in Doldrums megatransform ‒ about 4 million years ago. Due to the curvilinearity of the outlines and under the pressure of moving lithospheric plates, the interfracture zone ridges experience longitudinal (along the fault) compressive and tensile stresses, which are compensated by vertical uplifts of their separate blocks and the formation of depressions, pull apart depressions, and spreading centers (the latter are only in Pushcharovskiy megatransform). Structure-forming processes that determine pattern and morphology of the fracture zones as a part of the MTS are related by their origin to the spreading and transform geodynamic systems.

The authors propose a typification of intra-oceanic island‒arc systems according to the geodynamics of their development in the oceanic space. The currently existing and reconstructed (represented by terranes on the margins of the continents) intraoceanic island-arc systems of the late Mesozoic-Cenozoic are subdivided into expansive, accretionary, and stationary types. Systems of the expansive type (Izu-Bonin–Marian and Lesser Antilles) grow both towards the subducting oceanic plate and towards the free oceanic space – their geodynamics is determined by processes in the oceanic plates. The mantle currents under the overhanging lithospheric plate are directed towards the subducting plate. Accretionary systems such as the Olyutor–East Kamchatka, Nemuro–Lesser Kuril, and Talkitna systems have completed their development as part of active continental margins. The paleotectonic reconstruction of such systems shows that these systems in the course of their development were reduced to relict terranes, tectonically aligned with continental margins. The geodynamics of intra-ocean systems of the accretion type also depends on processes in oceanic plates, but leads to the opposite result compared to expansive systems. This is due to the direction of mantle flows under the overhanging plate, which is opposite to the expansion type, i.e. coinciding in direction with the mantle flow under the absorbed plate. The stationary Aleutian island-arc system is intercontinental and its development in space, as well as the formation of internal structures (the Paleogene island arc of the Bowers Ridge), depended on the difference in the relative movement of the Eurasian and North American lithospheric plates. The most specific feature of this system is the absence of signs of back-arc basin opening, which invariably characterizes expansive and accretionary island-arc systems. It is assumed that this specific feature of the system may be related to the mantle flow under the overhanging slab, which has a transverse direction with respect to the direction of the subducting slab. The Aleutian system, from the moment of its formation, was and remained autochthonous in relation to the North American and Eurasian continents.

The article presents new data obtained as a result of field research in 2022 Sevan intermountain depression in Armenia. The emergence of the Sevan intermountain depression in the Miocene was associated with the development of the Sevan almond-shaped structure, bounded by the right-lateral Pambak‒Sevan‒Syunik fault zone in the northeast, the Garni zone in the south, and the Arpa‒Zangezur zone in the southwest. Within the Sevan almond-shaped structure, strike-slip structures of Small Sevan (western part of Lake Sevan) and Gavar almond-shaped structure, Gavar horst, a number of faults, as well as extension zones were formed, including the southern part of Greater Sevan (eastern part of Lake Sevan) and axial zone of the Geghama Range. The development of the Sevan intermountain depression continued in the Pliocene under the influence of the uplift of the Lesser Caucasus and the Armenian Highlands. We have summarized the available data on the geological structure and geodynamics of the Sevan intermountain depression, presented the obtained data on the stratigraphy of the Pliocene–Quaternary deposits and their position, and showed that during the Akchagyl transgression at the Pliocene–Pleistocene boundary, sedimentary accumulations did not occur in the Sevan intermountain depression.