Graben System Research Articles

Tectonic-Paleoseismological Characteristics and Quaternary Activity of Maymundağı Fault (Northern Acıgöl Graben)

The Aegean region and its graben system constitute one of Turkey’s most significant seismic zones. The faults within the Aegean graben generate numerous earthquakes, leading to various human and economic losses. To better understand the seismicity of western Anatolia, it is necessary to obtain concrete findings regarding the seismic history of earthquake-producing graben faults. This can be achieved through paleoseismological studies and other relevant disciplines. This study focuses on paleoseismological investigations along the northern boundary fault of the Acıgöl graben, located east of the Aegean graben system. The Maymundağı fault zone has been examined in two separate segments: east and west. The Dazkırı segment to the east shows evidence of movement dating back at least 10,000 years, with subsequent intensified activity observed later on the western Bozkurt segment. An earthquake occurred approximately 2370 years ago east of the Bozkurt segment, followed by movements migrating westward, resulting in earthquakes approximately 1322 and 598 years ago. Further analysis of the western segment indicates an average recurrence interval of 724 years for earthquakes, with a slip rate of 0.58 mm/year. Based on these findings, a future earthquake can be expected in this region around 2028–2129 AD.

Structural features and tectonic activity of the Weihe Fault, central China

Abstract The study of tectonic activity holds great significance in assessing historical and modern seismic risks. The Weihe Graben is a significant intracontinental graben system in China, with numerous active faults that have caused a series of earthquakes, including the Huaxian great earthquake (M ∼ 8.5) in 1556 with approximately 830,000 deaths. Despite the obvious spatial relationship between the Weihe fault (WF) and these earthquakes, detailed research on the fault is still lacking. Various techniques including remote sensing images, digital elevation model, shallow seismic lines, trench excavation, drilling sections, and optically stimulated luminescence dating, have been utilized to obtain the following results: (1) The WF passes through the scarp of the Weihe River terrace in the north of Xianyang and can be divided into east and west sections at Jinjia village; (2) The WF is a normal fault that strikes northeast and dips south with a dipping angle of 65°–75°; (3) The fault has been active during the Holocene; (4) The vertically offset and dating results suggest that the fault activity has gradually decreased since the late Pleistocene, and the vertical slip rate during the Holocene is only 0.04–0.13 mm/year.

River profile and relict landscape analysis reveal the Cenozoic geomorphic evolution of the Nihewan Basin in North China

The Nihewan Basin, renowned for the numerous world-class Paleolithic sites and diverse mammalian fauna preserved in its sedimentary sequence, is a key area for studying early human evolution in East Asia. Despite extensive studies on the fluvial-lacustrine basin sediments, the evolution and formation process and mechanism of the Nihewan Basin and Nihewan Paleolake remain unclear. Here we assess the spatial pattern and recent history of rock uplift of the Liuleng Shan, one of the main bedrock mountain ranges encompassing the Nihewan Basin, using river profile and topographic analysis. The transient topography of the Liuleng Shan is characterized by two generations of elevated non-lithological slope-break knickpoints that delimit the uplifted Dianziliang and Tangxian relict landscape surfaces, suggesting two episodes of accelerated rock uplift during the Cenozoic. We link the first phase of rapid rock uplift to the initiation of the central and northeastern portions of the North Liuleng Shan Fault (NLSF) during the opening of the Shanxi Graben System in the Late Miocene, implying the initiation of the Nihewan Basin at this time. We propose that the dextral transtensional southwestern portion of the NLSF formed and propagated northeastward at around 4.2 Ma, and was responsible for the second phase of surface uplift. This phase of deformation also terminated the Tangxian planation process and led to the further subsidence of the Nihewan Basin. Accordingly, drainage rearrangement and fault activity hydrologically closed the Nihewan Basin, resulting in the development of the Nihewan Paleolake. Our findings outline a timeline for the geomorphic evolution of the Nihewan Basin and Nihewan Paleolake, highlighting the dominant role of tectonics in their formation.

3D study of dyke-induced asymmetric graben: The 1971 Mt. Etna (Italy) case by structural data and numerical modelling

In this work, we have integrated field data and numerical models to characterise a unique dyke-induced graben system, exposed both in section and plan view, with an unexpected asymmetric fault geometry. This volcanotectonic feature is related to the 1971 eruption of Mt. Etna (southern Italy) and is located near the northern wall of the Valle del Bove, a huge depression carved into the eastern flank of the edifice. A new structural map and quantitative data were obtained from the analysis of aerial stereophotos collected before the onset of the 1971 eruption and after, high-resolution drone-derived models and field surveys carried out in the summer of 2022. In plan view, the graben is 2-km-long and its width ranges 27–143 m from the bottom to the upper part of the section view, with about 82 m of difference in elevation from top to bottom. Graben faults clearly show an asymmetric setting in terms of attitude, with one fault that dips 70° to the south, and the other one that dips 50° to the north. Vertical offset values are greater at higher elevations. We also ran a set of numerical models, aimed at investigating the distribution and orientation of stresses around the inferred dyke tip and in the host rock. The comparison between field data and numerical models suggests a key role of the inclined topography, as shown in section view, in determining the orientation of dyke-induced σ1 and σ3 in the host rock. This, in turn, controls the geometry of the graben faults, resulting in the observed asymmetric setting. Additionally, dyke-induced stress concentrations and vertical offset values support the hypothesis of a downward propagation of the graben faults, from the surface down to the dyke tip.

Geology Structural Analysis of the Tanjung Betung II Region, Bengkulu: Recognized through Geological Mapping and Lineament Analysis

The research area is located in the Tanjung Betung II area, North Kaur, Bengkulu. This research was conducted to determine the geological structure and its relationship with the graben system in the Bengkulu basin by identifying and measuring structural components recorded in rock units. In interpreting deformation mechanisms, this research utilizes surface data obtained from geological structure mapping and lineament analysis based on Digital Elevation Model (DEM) images. Field observations on outcrops have identified five types of structures that have a general northwest-southeast trend, hereinafter referred to as the Manau IX fault, Bunian fault, Tanjung Kurung fault, Aek Kule syncline and Padang Manis fault. This structure may be associated with a graben system that formed in the late phase of basin evolution in the Neogene. Importantly, the structures recorded in the outcrop sequence may be the result of tectonic deformation that occurred during the Late Neogene, coinciding with the start of the Barisan orogeny.

Strain partitioning, transfer and implications for the ongoing process of intracontinental graben formation in the northwestern margin of the Ordos block, China: insights from densified GNSS measurements

SUMMARY The northwestern margin of the Ordos block is structurally separated by the Yinchuan–Hetao graben system. As one of the most active intracontinental graben systems within the Eurasian continent, its kinematic pattern of crustal extension is crucial for unraveling the ongoing processes of intracontinental graben formation, while it remains unclear principally due to a lack of geological constraints on crustal deformation. We obtained and analysed a densified GNSS (Global Navigation Satellite System) velocity field in this region. Our results suggest that the western margin of the Hetao graben exhibits the NW-directed crustal extension (∼ 1.1 mm yr−1), which can be attributed to the conjugate transtension resulting from the left-lateral motion along the E–W-trending northern boundaries of the Alashan and Ordos blocks, as well as the right-lateral motion along the N–S-trending western margin of the Ordos block. Additionally, in response to the NE-directed extrusion of the Tibetan Plateau, the Alashan block undergoes approximately NE-directed contraction (4.9 ± 1.1 nanostrain yr−1) and NW-directed extrusion (2.8 ± 0.8 nanostrain yr−1), which vacates space for the crustal extension of the Yinchuan graben with a rate of 0.9 ± 0.1 mm yr−1. Although it is challenging to determine whether the left-lateral motion (approximately 1 mm yr−1) along the E–W-trending Hetao graben is the far-field effect of western Pacific subduction, the gradual decrease in right-lateral motion from the N–S-trending western margin of the Ordos block toward the north side of the Yinshan Orogen manifests the far-field effect of the Indo-Eurasian plate convergence extending into the Mongolian Plateau.

Developmental Characteristics and Genesis of Ground Fissures in Wangjiacun, Emei Plateau, Yuncheng Basin, China

The Yuncheng Basin is part of the Fenwei Graben System, which has developed ground fissure hazards that have caused serious damage to farmland, houses, and roads and have brought about huge economic losses. Located in Wanrong County on the Emei Plateau in the northwestern part of the Yuncheng Basin in China, the Wangjiacun ground fissure is a typical and special ground fissure developed in loess areas, and its formation is closely related to tectonic joints and the collapsibility of loess. In order to reveal the formation and genesis of the Wangjiacun ground fissure, the geological background, developmental characteristics, and genesis pattern of the Wangjiacun ground fissures were studied in detail. A total of three ground fissures have developed in this area: a linear fissure (f1) is distributed in an NNE-SSW direction, with a total length of 334 m; a circular fissure (f2) is located near the pool, with a total length of 720 m; f2-1, a linear fissure near f2, has a fissure length of 110 m and an NE orientation. This study shows that tectonic joints in loess areas are the main controlling factors of the linear fissure (f1); differential subsidence in the pool caused by collapsible loess is the main source of motivation for the formation of the circular fissures (f2, f2-1), and tensile stresses produced by the edges of subsidence funnels lead to the cracking of shallow rock and soil bodies to form ground fissures (f2, f2-1). This study enriches the theory of ground fissure genesis and is of great significance for disaster prevention and the mitigation of ground fissures in loess areas.

Tenfold lower late quaternary throw rates in central Tibet compared to southern Tibet reflect different extensional deformation mechanisms

Present-day tectonic deformation in central Tibet is characterized by a series of ∼NS-trending grabens which accommodate EW extension. Quantifying the geometry and kinematics of these grabens is essential to understand Cenozoic tectonic deformation and Tibetan Plateau evolution. Here, we focus on the Norma Co graben (NCG), i.e., the southern segment of the Shuanghu-Norma Co graben (SH-NCG) system, which is the most prominent graben system within the Qiangtang terrane in central Tibet. We study its tectonic and geomorphologic characteristics to determine the late Quaternary throw rates of the normal faults bounding the graben, based on high-resolution satellite images interpretation, field investigation, and cosmogenic 10Be dating (n = 23). Using terrestrial LiDAR, UAV, and kinematic GPS, we precisely measure vertical offsets (up to 15 m) of ∼90–120 ka-old alluvial surfaces, yielding a throw rate of 0.10(+0.04/-0.03) mm/yr. This rate is ten times lower than those published along other NS-trending grabens in southern Tibet, reflecting different deformation mechanisms, as previously suggested: grabens in eastern Qiangtang formed by rapid eastward block extrusion, those in western Qiangtang formed by distributed extension on numerous scattered normal faults, and those in southern Tibet result from divergent orthogonal thrusting along the curved Himalayan arc.

Study examining active buried faults using shallow seismic reflection and joint multi-drilling: A case from the Xinding Basin, Shanxi Graben system

Active buried faults are a type of potential seismic source that are typically very difficult to explore and study in detail due to their undetectability using traditional geological survey methods. A ring of active basins has currently formed around the stable Ordos Block. However, few studies have been conducted to determine the activity of buried faults in these basins. This study utilized the Xinding Basin of the northern Shanxi Rift System as the target area. Based on comprehensive evidence from shallow seismic exploration, joint geological multi-drilling, and Quaternary dating methods, a new buried active fault named the Xinzhou Fault (XF) was identified in the Dingxiang Graben, a subbasin of the Xinding Basin. The youngest faulted layer was dated to earlier than 144.84 ± 16.92ka and is likely at approximately 118.94 ± 15.2ka. The slip rate of the newly observed Xinzhou Fault could be limited to 0.08–0.12 mm/a since the Late Pleistocene. This fault that was the first NW-striking active fault in the Shanxi Rift System is likely a secondary tension fracture of the main NE-striking Holocene basin-controlling faults. Currently, a strong earthquake at M7.2 can be considered as the maximum seismic potential of the XF, thus posing a significant seismic hazard to the area of the Xinding Basin. The discovery of this NW-striking active fault likely presents a low seismic risk. However, the occurrence of a strong earthquake should not be ignored due to its high seismic hazard.

The Early Cenozoic Volcanism of the Northern Okhotsk Region System of Grabens and Faults

Abstract —We studied geochemical compositions of the early Paleocene basaltic and andesite dikes associated to linear zones of the Lankovo–Omolon shearing system (Northern Okhotsk region) and basalts of the Evdyreveem volcanic field associated to the Okhotsk–Penzhinsk fault system, and compared them to other synchronous manifestations of basic volcanism: andesibasalts and andesites of the studied earlier Garmanda field, as well as with the Late Cretaceous basalts of the Mygdykit Formation of the Northern Okhotsk region, roofing the Okhotsk–Chukotka volcanic belt. The isotopic composition of Sr and Nd in dikes, the distribution of major and trace elements with the ratios of noncoherent elements indicate the formation of volcanic bodies in the environment of continental margin rifting, which is confirmed by the combination of depleted, intraplate and above subduction geochemical features of their composition. Such behavior of the elements indicates multi-stage processes of the earlier Mesozoic supra-subduction fluid metasomatosis. Melting of an ancient buried Cretaceous slab may explain the appearance of such “above subduction” marks as the Nb–Ta negative anomalies in the studied basaltoids. Andesite dikes are characterized by higher isotope ratios of Nd and lower Sr, with lower absolute concentrations of trace elements and more pronounced anomalies on spider plots.

Sedimentology and chronostratigraphy of the Apt Basin, Southeastern France: lacustrine response to late Paleogene cooling and regional rifting

The Apt Basin, sub-basin of the wider Manosque Basin of southeastern France, contains deposits of Eocene-Oligocene lake systems that were part of a broader network of evaporative lakes and lagoons spread across the European Cenozoic rift system. The onset and mechanisms of subsidence in the rift system, the interconnectivity of these lakes, and their response to the transition into the Oligocene icehouse are poorly understood. This study aims to clarify these points by examining the stratigraphy and depositional environments in the Apt Basin. We correlate and date Eocene to lowermost Oligocene geological units using a basin-wide facies model combined with sedimentological and geochronological approaches. We show the existence of three lacustrine phases, each separated by complete lake drying events: (1) an Ypresian (?) − Lutetian lake-marsh system dominated by palustrine carbonates; (2) a Bartonian (?) − Priabonian siliciclastic fluvio-deltaic and saline lake system; and (3) a lower Rupelian saline carbonate lake system. The presence of a lake system during the Ypresian (?) − Lutetian suggests an onset of basin subsidence before most other basins of the rift system in southeastern France, and is associated with the late Pyrenean deformation phase. The initiation of the second lacustrine phase marks the beginning of E-W extension and the formation of the Apt Basin as an individual horst & graben system. Based on facies distribution, we demonstrate the hydrological isolation of the Apt Basin from other basins and the improbability of any marine connection during the first two phases. This isolation is less certain for the third phase. We show a close synchronicity between the second lake drying event and the Eocene-Oligocene Transition. The third lacustrine phase, dominated by carbonate production and low siliciclastic input, is interpreted as reflecting a long-term decrease in surface runoff associated with the fall into the Oligocene icehouse.

Age and generation conditions of quartz crystal deposits in the Subpolar Urals

Research subject. The Dodo and Puiva quartz crystal deposits in the Neroiskiy mineral province (Subpolar Urals) were studied. Although these deposits have received sufficient research attention, a number of issues remain to be elucidated, including the age of crystal-bearing mineralization and the age of host rocks. Aim. In this work, we aim to determine the age of the metasomatites that bear quartz crystal mineralization and to develop a model of the formation of the Dodo and Puiva deposits. Materials and Methods. 40Ar/39Ar dating was carried out according to the method described by A.V. Travin et al. The argon isotope composition was measured on a Micromass 5400 mass spectrometer (IGM SB RAS). Results. Microprobe studies of the mineralogy of the Dodo and Puiva quartz crystal deposits were carried out. The temperatures of meta-somtaite formation were determined to range within 339–364°C. Six main structural and morphological types of quartz veins were identified and described. The 40Ar/39Ar ages of mica of quartz crystal deposits and host metamorphic schists were determined. Reliable plateaus were obtained, the values of which ranged within 251–257 Ma and agreed with each other within the deviation (average for 4 dates is 255 ± 2 Ma). Conclusion. A new model for the formation of quartz crystal deposits in the Subpolar Urals is proposed. The obtained 40Ar/39Ar ages correspond to the period of the post-orogenic extension of the Urals. At that time, a system of meridional grabens and rifts was formed in the Urals and in the West Siberian Plate. At the same time, and as a result of the same stretching impulse, large blocks of the middle crust (granite, schist, etc.) were brought to the Earth’s surface in the Urals. During the rise of deep blocks, at a depth of 10 km, a sharp, approximately 3-fold drop in fluid pressures (from lithostatic to hydrostatic) and a strong decrease in temperatures due to adiabatic expansion and the throttling effect occurred. The obtained 40Ar/39Ar ages (255 Ma) reflect not only the age of the quartz crystal deposits of the Subpolar Urals, but also place the moment of rise of the entire Central Ural uplift above a depth of 10 km, i.e., to the level of the upper cortex.

Bayesian inference elucidates fault-system anatomy and resurgent earthquakes induced by continuing saltwater disposal

An earthquake sequence in western Canada exhibits resurgent aftershocks, possibly in response to persistent, post-mainshock saltwater disposal. Here, we reduce uncertainty in mainshock source parameters with joint inference of interferometric synthetic aperture radar and seismic waveform data, showing that the mainshock nucleated at about 5-km depth, propagating up-dip toward the injection source, and arresting at about 2-km depth. With precise hypocenter relocations and Bayesian inference, we reveal that four subparallel faults were reactivated, likely part of a regional, basement-rooted graben system. The reactivated faults appear to be truncated by a conjugate fault that is misoriented for slip in the present-day stress regime. The nearest saltwater disposal well targets a permeable Devonian reef in direct contact with Precambrian basement, atop a ridge-like uplift. Our observations show that a fault system can be activated more than a decade after saltwater disposal initiation, and continued disposal may lead to a resurgence of seismicity.

Fault Extension Characteristics of the Middle Section of Shanxi Graben System and the Seismogenic Environments of the Hongdong and Linfen Earthquakes

Fault Extension Characteristics of the Middle Section of Shanxi Graben System and the Seismogenic Environments of the Hongdong and Linfen Earthquakes

Mapping of the Structural Lineaments and Sedimentary Basement Relief Using Gravity Data to Guide Mineral Exploration in the Denizli Basin

The Aegean Graben System is a complex tectonic structure in Western Anatolia and the Denizli Graben is a member of this system that hosts many geothermal springs, ore deposits, and travertine areas. In this study, the gravity data were analyzed to determine the subsurface geological structures and the depth model of the basin. The Bouguer gravity anomaly has a NW–SE pattern that is consistent with the general trend of the Denizli Basin. The pre-Neogene basement depths range from 0.1 km to 2.3 km. The Denizli Basin is composed of the Çürüksu Basin and the Laodikia sub-basin. The basins have undulated structures with many depressions; the deepest depression region is in the northern part of the Çürüksu Basin, which is close to the Pamukkale Fault Zone. In addition, the new gravity lineament map was obtained by using new-generation edge detection techniques: the tilt angle of the horizontal gradient amplitude (TAHG), and fast sigmoid-edge detection (FSED) of gravity data. The new proposed lineament map shows that the Denizli Basin has complex structures consisting of NW–SE, E–W, and NE–SE trending lineaments, and the major NW–SE trending faults and NE–SW trending lineaments control the main structural configuration. The uplifts and depressions in the basin deposit and the intersection area of lineaments are promising prospective areas for mineral deposits and have energy resource potential.

Morphometric analysis of the North Liuleng Shan Fault in the northern Shanxi Graben System, China: Insights into active deformation pattern and fault evolution

The North Liuleng Shan Fault (NLSF) is one of the major active faults in the northern Shanxi Graben System (SGS) in North China. Although the fault has been investigated extensively in terms of late Quaternary activity, its active deformation pattern is still poorly characterized. In this study, we conducted a morphometric analysis of the fault using various geomorphic indices to assess the along-strike relative uplift rates, which include basin relief, slope, mountain front sinuosity (Smf), hypsometric integral (HI) and curves, valley floor width-to-height ratio (Vf), asymmetry factor (Af), basin elongation ratio (Re), and normalized channel steepness indices (ksn). Our findings indicate that the central portion of the fault has experienced higher relative uplift rates than the southwestern and northeastern portions, as demonstrated by higher basin-averaged slopes and ksn values, and lower Vf, Re, and Smf values. This spatial distribution pattern of relative uplift rates appears to correlate well with along-strike changes in the fault geometry and kinematics of the NLSF. We interpret the EW-striking, central portion of the NLSF as a releasing bend formed by the NE-striking, right-stepping, en-echelon southwestern and northeastern portions of the fault system, which are characterized by right-lateral oblique slip. Furthermore, the northeastward increase in HI values along with the regional basin evolution history suggest that the NLSF likely propagated from southwest to northeast. We propose that the southwestern portion of the fault most probably initiated earlier and that the central and northeastern portions formed later as extensional structures at the termini of the fault zone. Our findings contribute to a better understanding of the tectonics and genesis of the basin-and-range geomorphology in the northern SGS.

Geochemical and H–O–Sr–B isotope signatures of Yangyi geothermal fields: implications for the evolution of thermal fluids in fracture-controlled type geothermal system, Tibet, China

High-temperature hydrothermal systems are mainly distributed in the north–south graben systems of southern Tibet as an important part of the Mediterranean–Tethys Himalayan geothermal belt in mainland China. As the largest unit capacity and second stable operating geothermal power station in China, Yangyi is the fracture-controlled type geothermal field in the center of Yadong–Gulu Graben. In this paper, hydrogeological and hydrochemical characteristics, isotope composition (δD and δ18O, 87Sr/86Sr and δ11B) of borehole water, hot springs, and surface river samples were analyzed. From the conservative elements (such as Cl− and Li+) and δD and δ18O values, the geothermal water of the Yangyi high-temperature geothermal field is estimated to be of meteoric origin with the contributions of chemical components of the magmatic fluid, which is provided by partially molten granite as a shallow magmatic heat source. According to logging data, the geothermal gradient and terrestrial heat flow value of the Yangyi high-temperature geothermal field are 6.48 ℃/100 m and 158.37 mW m−2, respectively. Combining the hydrothermal tracer experiment, 87Sr/86Sr and δ11B ratios obtained with gradually decreasing reservoir temperatures from the Bujiemu stream geothermal zone to Qialagai stream geothermal zone, we suggested the deep geothermal waters were mixed with local cold groundwater and then flow northeastward, forming the shallow reservoir within the crushed zone and intersect spot of faults in the Himalayan granitoid. Furthermore, in the process of ascent, the geothermal water is enriched in K+, Na+, and HCO3− during the interaction with underlying Himalayan granitoid and pyroclastic rocks that occur as wall rocks. The detailed description and extensive discussion are of great significance for the further exploitation and utilization of north–south trending geothermal belts in Tibet.

U-Pb Zircon Geochronology of Detrital and Ash Fall Deposits of the Southern Paraná Basin: A Contribution for Provenance, Tectonic Evolution, and the Paleogeography of the SW Gondwana

Zircon U-Pb geochronology was applied to investigate the provenance, depositional ages, and paleogeography of the southwestern Gondwana in detrital and ash fall sediments from Carboniferous to Jurassic succession of the southern Paraná Basin. Four detrital age populations suggest provenance from local and distal sources located to the south, southeast, and southwest: (i) Archean to Paleoproterozoic zircons from the Rio de La Plata Craton, Nico Peres and Taquarembó terranes; (ii) Grenvillian zircons from the basement of the Gondwanides and Namaqua–Natal belts; (iii) Neoproterozoic grains from the Don Feliciano Belt; and (iv) Phanerozoic populations from Paleozoic orogenic belts and related foreland systems in Argentina, as well as eroded units of the Paraná Basin. The paleogeographic reconstruction indicates an evolution in three distinct stages: (1) a gulf open to the Panthalassa Ocean during the Carboniferous; (2) an epicontinental sea with the rise of the Gondwanides Orogeny during the Permian; and (3) continental deposits controlled by an intra-plate graben system during the Triassic. Permian–Triassic volcanogenic zircons provide constrained maximum depositional ages and attested persistent volcanism, related to the Choiyoi magmatism and effects of the climate change episodes. During the Triassic, the extensional graben system recorded the uplift of the basement through regional northwest and northeast fault systems, and the recycling of Permian zircons, modifying source-to-sink relationships.

History of the geological development of the Ukrainian sector of the Black Sea from the middle Early Cretaceous to the beginning of the Late Miocene

Regional seismic investigations have made it possible to obtain new knowledge on the geological history of the Ukrainian sector of the Black Sea during rifting from Albian to Cenomanian, post-rift subsidence (Turonian¾Maastrichtian and Paleocene¾Middle Eocene), tectonic compression at the end of the middle Eocene and post-rift subsidence interrupted by a series of short-lived, compressional events (late Eocene ¾ the beginning of the Early Miocene). Rifting occurred simultaneously in the entire area of study and formed three long rift basins, each of which consisted of a system of (half)grabens, separated from each other and their margins by faults with amplitudes of up to3 km. The intensity of Cretaceous rifting was significantly less than would be required to produce continental lithosphere break-up and oceanic crust formation, or through-going «oceanisation» of continental lithosphere. Sedimentation during the pre-Late Eocene post-rift phase took place in relatively shallow marine conditions. Eocene compression caused a strong deformation of the sedimentary cover, partial and complete inversion of rift faults and the formation of three largely separate sea basins, between which a large landmass arose. The primary area of deposition of sedimentary sequences was significantly reduced due to strong deformations caused by compressional phases in the Late Miocene. That which is now the deep Black Sea was a relatively shallow basin until the beginning or even the end of the Pleistocene. Only thereafter did the water depth increase rapidly to more than 2 km. Research results indicate that modern tectonic reconstructions of the Western Black Sea and Eastern Black Sea basins, which are based on assumptions about the formation of the (sub)oceanic crust in these basins and/or different times of their formation, look unreliable. It also follows that any view of back-arc basins as small oceanic basins is not universally applicable.

Deep structure and tectonics of the South Ukrainian monocline in the zone of juncture of the East European platform and the Scythian plate (according to gravimetric data)

In order to investigate the deep structure and tectonics of the central part of the South Ukrainian monocline (SUM), a detailed study of this structure was carried out using a network of gravimetric profiles. It is shown that on the part of the SUM, which belongs to the southern edge of the East European Platform (EEP), the basement is split into separate blocks with different characteristics of the consolidated crust. The crust of the Scythian Plate (SP) within the SUM has undergone considerable changes as a result of tectonic activation and significant complications of its structure. The distribution into the areas with different density characteristics of the basement of EEP and SP does not coincide, indicating that the development of the latter has not been fully inherited from the former. The obtained results, compared with data from other researchers in Prydobrydzha and Crimea, show that the northwestern shelf is an intermediate link between these terranes. A tectonic scheme of the research area based on the hypsometry of the surface of the second layer of the basement was built on the basis of the simulation. It was shown that the southern edge of the platform has undergone rather insignificant transformations, which are manifested in the emergence of an elongated intracrustal graben system along the EEP/SP boundary, divided by sublatitudinal faults into three blocks with different parameters. At the northern margin of the SP within the SUM, in addition to the grabens and horsts in the basement, there are sags and uplifts with boundary faults without vertical crustal displacements along them. Areas of the heterogeneous-heterochronous structure of the crust of the Baikal, Hercynian, and Kimmerian generations have been identified within SUM part of the SP. It has been determined that the Golitsyn fault system and the northern flank of the Karkinit trough were further restructuring during the Alpine tectogenesis phase. A variant of the Scythian Plate origin scenario based on the Mobilist ideas is proposed and substantiated by the results of the conducted studies and the available geological and geophysical information as a discussion.