The Tjörnes Transform Zone connects the Kolbeinsey spreading ridge and the Northern rift zone of Iceland. It includes two overlapping rift segments resulting from the rift zones propagating. Nowadays it includes several morphostructures that vary in their organization and dynamics, such as magmatic and amagmatic rifts, oblique fault zones, or tectonic-volcanic rises. They often overlap with each other promoting the development of block morphostructures of different scale. Using the morphometric analysis of present-day fault scraps we revealed the areas of different recent tectonic activity and faulting intensity, and explained geodynamic reasons of these differences. For instance, the most active morphostructures are spreading and rift segments of the western branch of the transform zone. The eastern branch has significantly weaker tectonic activity which is mainly due to the intensive volcanic processes. Nowadays the eastern branch of the transform zone continues its development that is expressed through the formation of block rises between the overlapping rift segments. The western branch has less complicated structure and gradually fading tectonic-magmatic activity. We also showed the functional changes of amagmatic rifts: at the early stages of transform zone development, they probably evolved as a continuation of adjacent spreading centers. Meanwhile now they have their own independent extension centers. All explored morphostructures influence each other in the process of overlapping that significantly impacts the endogenic relief formation, especially faulting, at different sections of the morphostructures.

A petrogeochemical study of basalts (lithophile elements and Sr-Nd-Pb isotopes, compositions of liquidus olivine and spinel) from the transition zone of the Southwest Indian Ridge in the area of the Du Toit and Andrew Bain faults revealed significant differences in their composition. Within the rift valley adjacent to the faults, tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are typical. Deep-type basalts (TOR-1) are present in the western side of the Andrew Bain Fault. The outpouring of these types of magmas reflects a possible change in geodynamic regime during this zone formation: from deeper and higher temperature melting to shallower ones (Sushchevskaya et al., 2022). Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the liquidus olivine compositions. The rift valley olivines are similar to typical Na-TOR olivines with a Mg content of Fo88–87, low Ni and elevated Mn. On the contrary, tholeiite olivines of the Andrew Bain Fault are enriched in Ni and depleted in Mn, which may indicate pyroxenite included in the primary melt formation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which are subsequently involved in melting during the spreading axes jumping. A similar process is typical for the region of the Bouvet Triple Junction, where a significant heterogeneity of the olivine composition in terms of trace-element contents was revealed. The isotope characteristics of the Andrew Bain Fault tholeiites differ in Pb and Sr radiogenic composition and are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but not from the Konrad and Af. Nikitin Rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), possibly with an admixture of mantle material with EMII characteristics (with elevated Rb/Sr).

Iceland is a unique example where the rift zone of the Mid-Atlantic Ridge emerges at the surface, and whose morphology and tectonic structure differ considerably from typical rift zones of mid-oceanic ridges. The morphology and geodynamics of the western branch of Icelandic rifts are largely controlled by the thermal influence of the Iceland plume that has created the North Atlantic large igneous province. The western branch of Icelandic rifts is characterized by ceasing tectonic and magmatic activity. Overlapping the Eastern Rift Zone, it forms the rotating Hreppar microplate block, which results in a northward decrease of its tectono-magmatic activity. Based on a morphometric analysis of fault scarps, we identified the degree of present-day activity for individual areas of volcanic systems, and determined its variation during Late Quaternary for some areas. The inferences drawn here demonstrate distinct differences in the present-day tectonic structure and dynamics of rift zones and individual volcanic systems within them. The southernmost, transtensional Reykjanes Rift Zone shows tectono-magmatic activity decreasing to the east, which is due to a lower influence exerted by the Reykjanes Ridge that is adjacent to it to the southwest. We observed its gradual eastward diminution, which is probably due to an analogous southward movement of the most active Eastern Rift Zone and to the formation of a new transtensional zone that combines the present-day Reykjanes Rift Zone and South Iceland Seismic Zone. The Western Rift Zone is developing independently of the Reykjanes Rift Zone, having a major extension center in the area of Lake Thingvallavatn. The Holocene manifestations of tectono-magmatic activity in its northern part, as is the case in the Central Rift Zone, are very weak, being mostly due to glacio-isostatic reactivation of older structures. The identified structural inhomogeneities can also be traced in the morphological aspect of rift zones. As an example, the Western and Central Rift Zones typically contain well-developed shield volcanoes that are largely composed of hyaloclastites, while individual lava shield edifices are observed within fissure swarms. In contrast to this, the Reykjanes Rift Zone is characterized by an absence of central volcanoes expressed in topography, and chains of small volcanic vents are observed within fissure swarms.

The African-Antarctic sector of the Southern Ocean is the least studied part of the World Ocean, the structure and evolution of the tectonosphere of which remains debatable. The complex history of the development of the region under study, accompanied by manifestations of intense magmatic and tectonic activity, has contributed to the formation of a number of large underwater ridges and rises. Identifying the features of the deep structure of the tectonosphere on the basis of the analysis of geophysical information and understanding the geodynamic nature of the morphostructures of the studied region is an urgent problem of marine geophysics and geodynamics. Structural analysis of the anomalous gravitational and magnetic fields of the studied region was carried out. Zoning scheme by the anomalous fields as well as a generalized scheme are constructed. The results of the sestudiesmadeit possible to identify heterogeneous blocks of the lithosphere, which for medon different spreading ridges and are separated by submarine rises.

Based on an analysis of global digital models, the distribution of fracture zones of the oceanic crust, global and regional models of evolution, and published geological and geophysical data, the structural features of the ocean floor in the junction area of the Indian and Atlantic oceans are considered. Tectonic zoning of the region’s crust has been carried out. Heterogeneous blocks of the lithosphere with crust formed on different spreading ridges, separated by structural boundaries, which are pseudofaults, fixing traces of propagating rift zones, jumping and cessation of spreading ridges are identified. The main stages in the evolution of the lithosphere associated with the activation of plume magmatism and kinematic reorganizations of plate boundaries are identified.

Abstract—The features of potential fields characterizing the Islas Orcadas and Meteor undersea rises located in the South Atlantic are considered. The rises are located at approximately the same distance to the west and to the east from the axis of the southern segment of the Mid-Atlantic Ridge. Density modeling along the profiles intersecting the rises was carried out. The analysis of potential fields and the results of density modeling shows a generally similar structure of the crust and lithosphere of the Islas Orcadas Rise and the Meteor Rise, which supports the common nature of their formation as a result of the splitting of the lithosphere of the small Agulhas plate due to kinematic restructuring, which led to the extinction of the Agulhas Ridge and formation of the southern segment of the Mid-Atlantic Ridge. However, some features in the density structure of these rises, both along their north-south trend where the influence of hot spots on the heating of the mantle is particularly evident, and along the conjugate profiles, indicate an asymmetric structure of the rises during the initial stage of their formation.

The tectonosphere and formation conditions of the Mozambique Ridge are considered. The Mozambique Ridge is located in the southwestern part of the Indian Ocean between the Natal and the Mozambique Mesozoic Basins. The way how the ridge formed remains debatable. The anomalous structure of the crust of the Mozambique Ridge can be explained either by underplaying - thickening of the oceanic crust from below due to magmatism, or by stretching and thinning of the continental crust. Based on gravitational and magnetic field anomalies, seismotomography and other geological and geophysical information, density modeling was carried out along four profiles. Physical modeling determined the conditions of formation of the Mozambique Ridge. It formed due to the splitting of the African-Antarctic continent, the presence of structural heterogeneities in the lithosphere of the African continent and the influence of the Karoo plume.

The results of density modeling of the tectonosphere structure in the Antarctic sector of the South Atlantic Ocean are presented. The modeling was carried out along the profiles stretching from the Falkland Plateau to the Mozambique Ridge and crossing a series of submarine rises and ridges separated by deep-sea basins. Studies show that the crust of this region has a heterogeneous structure and the crust underlying the rises is of a different structure, indicating different origins of the respective rises.

Petrogeochemical data on basalts (lithophile elements and Sr–Nd–Pb isotopes, liquidus olivine and spinel compositions) from the transition zone of the Southwest Indian Ridge (SWIR) in the Du Toit–Andrew Bain fault zone indicate significant differences in their composition. Tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are developed within the rift valley adjacent to the faults. Deep-type basalts (TOR-1) are present in the western wall of the Andrew Bain Fault. The outpouring of these magma types reflects a possible change in geodynamic setting during formation of this zone: from deep and high temperature to shallower magma generation conditions (Sushchevskaya et al., 2022). Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the composition of liquidus olivine. The olivines from rift valley are similar to the typical Na-TOR olivines with Mg number of Fo88-87, low Ni and elevated Mn contents. On the contrary, olivines in tholeiite from the Andrew Bain Fault Zone are enriched in Ni and depleted in Mn, which may indicate the involvement of pyroxenite in melt generation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which were subsequently involved in melting during the spreading axes jumping. A similar process is typical of the Bouvet Triple Junction, where the trace-element composition of olivine shows significant heterogeneity. The radiogenic composition of Pb and Sr of the Andrew Bain Fault tholeiites are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but differ from those of the Conrad and Af. Nikitin rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), which likely contains an admixture of EM-II component (with elevated Rb/Sr).

The tectonic zoning of the African-Antarctic sector of the Southern Ocean was carried out on the basis of cluster analysis of a large volume of heterogeneous geophysical and geological data. It is known that the degree of geological and geophysical exploration of this part of the ocean by marine surveys and drilling remains extremely low. The results of such an analysis make it possible in areas with weak geophysical knowledge to quickly draw up tectonic zoning schemes based on regional geophysical data that are freely available throughout the entire territory of the world ocean and use them as a basis for further interpretation. The following materials were used as input data or classification features: ocean floor topography, gravity anomalies in free air and in the Bouger reduction, ∆Ta anomalous magnetic field model, ocean floor age, geoid height data, and the SL2013sv seismotomography model. «K-means» algorithms and the «Gaussian mixtures» method used for clustering. Based on the results of the application of these algorithms, cluster models of the study region were obtained, in which the selected classes fully reflect the main tectonic elements.

The joint zone between subaquatic Reykjanes Ridge and Icelandic rift zones known as Reykjanes Rift Zone is a unique transform zone formed in conditions of interaction between the mantle plume and the mid-oceanic ridge. Its morphostructures include a row of en-echelon volcanic systems with oblique extension. The modern tectonic and volcanic relief was formed almost completely during Holocene: the previous landforms were reworked or destroyed by erosion of the Late Pleistocene glaciation. Morphology of the Reykjanes Rift Zone is transitional between the Reykjanes Ridge and the adjacent Western Rift Zone from the northeast. Unlike the Reykjanes Ridge, volcanic activity is not concentrated with in axial volcanic ridges, but is confined by eruptive centers represented by fissure volcanoes. Central volcanoes, in contrast to the Western Rift Zone, are not expressed in topography. Oppositely, tectonic activity increases with distance from the Reykjanes Ridge. Fault scarps reach their largest size within the Western Rift Zone, where Thingvellir graben is located. At the same time, the tectonic and volcanic landforms of the Reykjanes rift zone are in paragenetic association, appearing sequentially during the episodes of riftogenesis.

Joint zones of Reykjanes and Kolbeinsey spreading ridges with Iceland large igneous province considerably differ in structure from adjacent spreading segments despite of similar kinematics. Tjörnes transform zone is a complicated system. It comprises several volcanic and amagmatic structures. In contrast, Reykjanes rift zone has relatively simple structure and homogeneous type of tectonic and magmatic activity. The causes of those differences and their modern dynamics are not fully explained hitherto. Basing on morphometric analysis of normal fault scarps parameters it was concluded that such significant differences of transform zones are result of spatial and temporal stability of adjacent structures. In turn, the latter is controlled by periodic increase of Iceland plume magmatic activity. Rift structures development within transform zones directly correlates with their position over adjacent spreading segments and their magmatic conditions. Modern development of both transform zones is caused by Iceland rift zones instability and migration impacted by Iceland plume thermal pulses. Consequently, transform zones undergo kinematic changes. For Tjörnes transform zone it is expressed in its structure gradual simplification: the western branch and block structures cease their activity. In Reykjanes rift zone rift axis gradually migrates southwards that probably results in its intensive volcanism.

The kinematic reorganization of plate boundaries in the southeastern Antarctic part of the South Atlantic, expressed in the jump of the spreading axis of the Agulhas Ridge, led to a restructuring of the region’s structural plan. This resulted in the formation of the southern segment of the Mid-Atlantic Ridge (MAR), extinction of the previously active Agulhas spreading ridge, and formation of the Meteor and Islas Orcadas rises, marking the location of the MAR and located symmetrically about its axis. Based on the research results, the conditions for the jump in the spreading axis were identified and an experimental model was constructed for the formation of accompanying structures, in which an important role was played by the accretion of oceanic crust on the Agulhas Ridge and westward migration of the Falkland Plateau. This resulted in southward movement of the southern segment of the MAR, formation of the Malvinas microplate, and a jump in the axis of the Agulhas spreading ridge, which led to cessation of spreading on this ridge. An important role in this kinematic restructuring was played by activity of the Shona hotspot.

The Tjörnes Transform Zone (TZ) is on the northern margin of Iceland, mostly in the shelf zone. It connects the Kolbeinsey spreading ridge and the Northern Rift Zone (RZ) of Iceland. The zone is complexly structured and comprises several heterochronous sections that evolved in various geodynamic conditions. The geodynamic conditions of the formation and evolution of the Tjörnes TZ were reconstructed by means of physical modelling. The models measured the thickness of the model lithosphere, displacement, and the overlapping between the spreading segments. Results of the modelling show that the Tjörnes TZ formed successively, in two stages. The first stage was the formation of the general configuration of the area of interaction of the extension centres of the Kolbeinsey Ridge and the Northern RZ of Iceland in the form of a small overlap of the spreading centres with a rotating block between them. At the second stage, one of the formed transtensional fault zones between the spreading centres became the feeder channel for a magma pulse of the Iceland Plume, which led to the formation of the Grímsey Oblique Rift (OR). Standing alone is the Húsavík-Flatey fault zone that might have formed under the influence of two at a time spreading segments, with the domination of the more magmatically active Northern Rift Zone.

The problem of the interaction of spreading ridges with large igneous provinces is widely discussed in the scientific literature. The most striking and well-preserved example of such interaction in the geological past is the “collision” of the Southeast Indian spreading ridge and the Kerguelen plateau, which occurred about 43 Ma ago. The consequence of this event was the division of a single plateau into two parts, the Kerguelen Plateau and the Broken Ridge. The conditions for the formation of these structures were studied experimentally. This paper presents the physical model of the spreading ridge interaction with a large igneous province. The experimental results are in a good agreement with the geological and geophysical data and with the proposed model of the evolution of the region.

The features of the structure formation of the Earth’s crust at the early stage of formation of the Southeast Indian Ridge were studied. The process of formation was associated with the separation of Australia from Antarctica and the advance of the rift zone westward towards the ancient oceanic lithosphere and, then, towards the large igneous province of Kerguelen, formed by the activity of the plume of the same name. The separation of Australia and Antarctica spanned a long period of continental rifting ( 160‒80 Ma), which was followed by ultraslow spreading ( 80‒45 Ma), slow spreading ( 45‒40 Ma), and, finally, by recent stationary spreading at average rates (after 40 Ma). The advance of the rift zone towards the ancient oceanic lithosphere gave way to the accretion of young oceanic crust on the emerging Southeast Indian Mid-Oceanic Ridge. The early stages of development of the young spreading ridge are reflected in the modern structural plan of the study region. The advance of the rift zone from the continent into the boundaries of the ancient oceanic lithosphere led to the formation of the Naturalist Plateau and the Bruce Bank near the Antarctic margin. The break-up of the ancient oceanic lithosphere and the formation of the young crust on the Southeast Indian Ridge led to the formation of conjugated Diamantina and Labuan suture zones, fixing the position of the initial rifting. The transition from ultraslow spreading at the initial stage of oceanic crust formation to stationary spreading at average rates is clearly recorded in the change in the ruggedness of the accretionary relief. The Southeast Indian Mid-Oceanic Ridge collided as a result of its propagation westward with a large igneous province during the formation of the Kerguelen Plateau and separated the Broken Range from the plateau. The authors carried out physical modeling of the development conditions of rifting and spreading processes, as well as structure formation within the Southeast Indian Ridge.

The complex structure of the region has been formed as a result of ​kinematic reorganization of plate boundaries, accompanied by the extinction of old spreading centers, the formation of new ones, and plume magmatic activity in the South Atlantic southeastern part. These processes resulted in the formation of a system of ridges, rises, and plateaus with different morphologies and geochemical characteristics. The results of density modeling of the structure of the crust and tectonosphere along the profiles, which extend from the Falkland Plateau to the Mozambique Ridge and cross a series of rises and ridges divided by deep-water basins, showed that the rises have different crustal structures indicative of their different origins. The formation conditions of various submarine rises have been studied on the basis of physical modeling. The new experimental model of the formation of the lithosphere and submarine plateaus of the region is provided, in which the important role belongs to the breakup of the Agulhas large igneous province on the Agulhas Plateau and Northeast Georgia Rise, the accretion of the oceanic crust on the Agulhas spreading ridge, and a further jump of the Agulhas spreading ridge axis leading to the termination of spreading at this ridge and the formation of a southern segment of the Mid-Atlantic Ridge and conjugated Meteor and Islas Orcadas Rises. Spreading axis jumps, which are accompanied by periodic activity of hotspots, played an important role in the formation of the submarine rises of various genetic types that, in turn, determined the different structures of their crust.

The the southwestern part of the Indian Ocean, where the Mozambique and Madagascar ridges are located was studied. The ideas about the tectonic structure of these ridges are still under debate. The anomalous structure of the crust of these ridges can be explained either by underplating (thickening of the oceanic crust from below due to magmatism), or by stretching and thinning of the continental crust. Based on the data on anomalous gravitational and magnetic fields, seismotomography, and other geological and geophysical information, density modeling was performed along four profiles, which revealed differences in the crustal structure and evolution of the Mozambique and Madagascar ridges.

Tjörnes transform zone (TFZ) is complicated fracture zone in North Iceland connecting Kolbeinsey ridge and Northern rift zone of Iceland. It includes several different structures such as segmented oblique rift, amagmatic rifts and oblique slip fault zones. They developed consequently since ca. 9 Ma. The aim of this work is to determine current geodynamic activity and ratio of tectonic and magmatic activity of each structure and adjacent structures of Mid-Atlantic ridge (MAR) basing on normal faults morphometric parameters and to reconstruct evolution of TFZ by physical modelling. Morphometric analysis is based on multibeam bathymetry data of Marine and Freshwater Research Institute in Iceland and ArcticDEM digital elevation model. There were collected data on more than 900 normal faults on five parameters: heave, thrust, length, distance between faults and maximum profile curvature. They reflect recent rate of horizontal and vertical deformations and morphological age of the normal fault. Heave and distance ratio shows the relative intensity of tectonic and magmatic activity. The results show that structures have different level of recent tectonic activity and therefore, are on different stages of their evolution. In addition, they have various tectono-magmatic ratio that proceeds from their development stage, width of faulting zone and mantle structure. Physical modeling is based extending setting with mineral oil that have numerical resemblance with oceanic crust in density, shear modulus and thickness. Two-layered model have elastic bottom layer, brittle top one and local heating source corresponding to Icelandic plume impulses. Initial configuration reflects two spreading segments of MAR that develop transform zone in conditions of crust thinning in direction out of Icelandic plume center. In result of their interaction is generation of overlapping spreading centers. One of them became extinct and another one develops into transtensive transform zone, which corresponds to Husavik-Flatey oblique slip fracture zone (HFFZ) and adjacent amagmatic rift. Activation of local heating source rejuvenates extinct branch of the overlap and generates subparallel to extension direction rifting fractures reconstructing Grímsey oblique rift with high magmatic activity. HFFZ activity abruptly declines. In conclusion, consequent development, activation and decline of structures correctly correlate with results of morphometric analysis and reflect the development stages of each structure. The specific current structure of TFZ is determined by initial development of overlapping spreading centers and their control by Icelandic plume magmatic impulses.

Numerical simulations of the burial and thermal history of the Mannar Basin using the GALO system for basin modeling make it possible to evaluate the change in the temperature and organic matter maturation in the source rocks of the basin and trace the variations in the rock temperature and maturity with distance from the axis of the ancient rift. Our analysis shows that organic matter of Jurassic rocks recovered by Wells Pearl 1 and Dorado North, located in a flank of the rift, occurs within the oil window (0.50 ≤ Ro ≤ 1.30%), whereas the maturity of these rocks in Wells Pseudo Mannar Deep and Barracuda, which are close to the rift axis, considerably exceeds maturity within this window. The probable Jurassic source rocks generate mainly heavy and light oil and a little gas in areas far from the rift axis, but these rocks generate mainly gas in the areas near the axis. The modeling shows that the thermal activation of the lithosphere of this basin in the Upper Cretaceous had a noticeable effect on the temperature and maturity of the rocks in areas close to the ancient rift axis (Pseudo Mannar Deep and Barracuda wells), but its effect is markedly reduced in areas remote from the rift axis (Wells Pearl 1 and Dorado North).