Continental Rift Systems Research Articles

Shear‐Dominant Continental Rifting in Northern Ryukyu Revealed by Ambient Noise Tomography

AbstractIn this study, we reveal the deformational structure of the crust of the northern part of the Ryukyu Arc and Okinawa Trough using ambient noise tomography. Compared with southern Ryukyu, the northern segment exhibits a wide and shallow basin, a crust without localized thinning, slow extension rates, and highly arc‐oblique, right‐lateral retreat of the Ryukyu Arc. We present both isotropic and azimuthally anisotropic shear‐wave velocity models using data recorded by an ocean‐bottom seismometer array and nearby island stations. The isotropic model demonstrates a monotonic decrease in velocity from the backarc to the forearc, in accord with the accretionary‐prism origin of the latter. The resolved azimuthal anisotropy exhibits arc‐parallel and arc‐perpendicular fast shear‐wave polarization directions in the upper to mid‐crust and the lower crust to uppermost mantle in much of the arc and backarc, respectively. We interpret the arc‐parallel anisotropy as resulting from the anisotropic fabrics aligned by the vertical shearing imposed by the right‐lateral motion of the Ryukyu Arc. The underlying arc‐perpendicular anisotropy may be attributed to horizontal shearing driven by corner flow in the mantle wedge. We found arc‐perpendicular anisotropy in the forearc upper crust, which may reflect crack alignment caused by the collision of the Amami Plateau. The oblique arc retreat and the resolved deformation fabrics in the arc and backarc together attest to the shear‐dominant, transtensional nature of the northern Ryukyu continental rift system. Some of the features in northern Ryukyu may be better explained from the perspective of transtensional rifting.

The influence of the strength of pre-existing weak zones on rift geometry and strain localization

Continental rifts normally initiate within previously deformed lithosphere and thus their evolution and architecture can be largely controlled by inherited weak zones in the pre-rift crust. Here, we quantify the role of the strength and obliquity of pre-existing crustal-scale weak zones in the evolution of continental rift systems. We use a 3D numerical geodynamic model to assess strain localization, associated fault development, and rift segmentation during the early stages of tectonic extension. We find that both the strength and obliquity of the weak zones significantly influence the patterns of strain localization. A pre-existing very weak zone with low obliquity can promote the development of continuous and long-lived border faults parallel to the rift axis. Conversely, a comparatively strong weak zone with high obliquity leads to a staggered en-echelon rift geometry that lacks rectilinear laterally persistent strain localization. Furthermore, we find that rift obliquity and weak zone strength may modulate rift fault length, throw, and azimuth. These results provide new and compelling insights into the structure and evolution of natural active rifts that develop within orogenic basement terranes.

Miocene climate cooling and aridification of Antarctica may have enhanced syn-extensional magmatism in the western Ross Sea

Continental rift systems are commonly characterized by volcanism with parental basaltic magmas sourced from the mantle. Erosion of the rift shoulders and sedimentation in the adjacent basins can affect the stress and thermal fields at depth, thereby affecting partial mantle melting. However, the sensitivity of magmatic activity to such surface forcing is elusive. Geological observations from the western Ross Sea, Antarctica, suggest rift onset in the Cretaceous with a transition from wide-rifting to narrow-rifting at the boundary between the Antarctic craton and the Transantarctic Mountains. Miocene climate cooling during rifting in the western Ross Sea, in addition, leads to an abrupt decrease in sedimentation rate, synchronous to the emplacement of the McMurdo Volcanic Group. This represents the largest alkali province worldwide, extending both inland and offshore of Transantarctic Mountains and western Ross Sea, respectively. Here, we use coupled thermo-mechanical and landscape evolution numerical modeling to quantify melt production in slowly stretching rift basins due to changes in erosion/deposition rates. The model combines visco-elasto-plastic deformation of the lithosphere and underlying mantle during extension, partial rock melting, and linear hillslope diffusion of the surface topography. The parametric study covers a range of slow extension rates, crustal thicknesses, mantle potential temperatures and diffusion coefficients. Numerical simulations successfully reproduce the ∼150–200-km-wide extension of western Ross Sea and Miocene-to-present asthenospheric melt production (McMurdo Volcanic Group). Results further show that slow rifts magmatism is highly sensitive to sediment deposition within the basin, which inhibits mantle decompression melting and delays the crustal breakup. Regional climate-driven sedimentation rate changes are thus likely to have affected the syn-rift magmatic history of the western Ross Sea, Antarctica, supporting the relevance of interactions between surface and deep-seated processes across extensional settings.

Mineralogical and geochemical characterization of the Wadi Natash volcanic field (WNVF), Egypt: Alkaline magmatism in a Late Cretaceous continental rift system

Mineralogical and geochemical characterization of the Wadi Natash volcanic field (WNVF), Egypt: Alkaline magmatism in a Late Cretaceous continental rift system

Deciphering the interplay between tectonic and climatic forces on hydrologic connectivity in the evolving landscapes

The intricate interplay between climate and tectonics profoundly shapes landscapes over time frames surpassing 10 million years. Active tectonic processes and climatic shifts unsettle established drainage systems, instigating fragmentation or amalgamation of watersheds. These activities yield substantial transformation in surface hydrologic connectivity, thereby underlining the profound influence of these tectonic and climatic forces on the evolution of both landscape and hydrology. Such transformations within the hydrological landscape have direct implications for the evolution of aquatic species. As connections among aquatic habitats undergo reconfiguration, they incite shifts in species distribution and adaptive responses. These findings underscore the role of tectonics and climate in not only sculpting the physical landscape but also steering the course of biological evolution within these dynamically changing aquatic ecosystems relying on hydrologic connections. Despite the significance of these interactions, scholarly literature seldom examines alterations in hydrologic connectivity over tectonic, or orogen-scale, timescales. This study aims to bridge this gap, exploring changes in hydrologic connectivity over extended periods by simulating a continental rift system akin to the Rio Grande Rift, USA, subject to various tectonoclimatic scenarios. Multiple rift basins hosting large lakes, brought into existence by active tectonic extension, are further molded by tectonic extension and post-rift climatic changes. The study focuses on phenomena such as interbasin river breakthroughs and knickpoint generation, assembling a time-series of connectivity metrics based on stream network characteristics such as flow rate, flow distance, and captured drainage areas. We anticipate that the insights gleaned from this study will enhance our comprehension of the enduring impact of tectonic and climate processes on hydrologic connectivity and the subsequent evolution of aquatic species.

CO2 flux from the French Massif Central groundwaters: Modelling and quantitative estimation of the degassing process

Passive rift systems are often characterized by CO2 degassing, witnessed by the presence of mineral and thermal springs, bubbling pools, mofetes. Despite these field manifestations, the quantitative estimation of the CO2 budget released to the atmosphere from these geodynamic structures is not well constrained. Here, we examine the chemistry of 169 springs, the isotopic composition of the dissolved carbon (δ13CTDIC) of 33 springs and the dissolved gases composition of 6 springs from the French Massif Central, part of the European Cenozoic Rift System (ECRIS), in order to describe the CO2 degassing process and to compute the CO2 emission rate released from groundwaters at regional scale. Water-gas-rock models reveal that the separation of gas from the liquid phase occurs at P-T conditions between 10 bar-180 °C and 1 bar-10 °C. The carbon mass and isotopic balance of spring waters of the French Massif Central allow us to compute a total deeply-sourced CO2 emission rate of 1.52 ± 0.14 × 109 mol yr−1, suggesting that the CO2 release from passive rift systems is significant at global scale and should be considered in the present-day global Earth degassing budget. The comparison of our data to other continental rift systems shows a high variability of CO2 emission rates, highlighting that more detailed studies are needed to constrain the CO2 flux from this geodynamic setting that, at present, is likely underestimated.

Научная школа «Кайнозойский континентальный рифтогенез»: от основания к развитию (к 95-летию со дня рождения академика Н.А. Логачева)

The article, prepared in connection with the 95th anniversary of N.A. Logatchev's birth, highlights his role in the development of N.A. Florensov's ideas about Cenozoic continental rifting in the Baikal area. The author's vision of his path as a scientist is given, having established himself in his youth and uniting specialists of various profiles in his mature age into a new scientific direction "Cenozoic Continental Rifting", within the framework of which systematic knowledge about the Baikal and other Cenozoic continental rift systems was obtained.

Influence of Inherited Brittle Fabrics on Continental Rifting: Insights From Centrifuge Experimental Modeling and Application to the East African Rift System

AbstractThe presence of pre‐existing fabrics at all lithospheric scales has been proven to be of primary importance in controlling the evolution of continental rifts. Indeed, observations from natural examples show that even in conditions of orthogonal rifting, when extension should result in simple fault patterns dominated by normal faults orthogonal to extension vectors, inherited fabrics induce complex arrangements of differently‐oriented extension‐related structures. This paper explored the influence of inherited fabrics on rift‐related structures by using a series of analog models deformed in a centrifuge. The models reproduced a brittle‐ductile crustal system and considered the presence of pre‐existing discrete fabrics in the brittle crust in conditions of orthogonal narrow rifting. These fabrics were reproduced by cutting the brittle layer at different orientations with respect to the extension direction. Modeling shows pre‐existing fabrics have a significant influence on rift‐related faults, provided that the angle between inherited fabrics and the rift trend is less than 45°. In these conditions, fabrics cause prominent segmentation of rift‐related faults and induce the development of isolated depocenters. Pre‐existing fabrics strongly influence the geometry of extension‐related structures, resulting in curved fault patterns and en‐echelon arrangement of oblique faults. These findings provide insights into the development of continental rift systems in nature: our modeling shows indeed significant similarities (i.e., peculiar fault architecture and geometries) with the faults in different sectors of the East African Rift System (e.g., the Magadi and Bogoria basin, Kenya Rift), testifying that reactivation of inherited fabrics is a paramount process in shaping continental rifts.

Continental rift asymmetry and segmentation – contributions from the African plate

Prior to the mid-1980s, continental rifts were generally depicted as roughly symmetrical graben, with little complexity along the strike of the basin. In reality, most continental rifts originate as asymmetric, large-scale half graben that are strongly segmented along-strike. This basic architecture does not usually persist unaltered to the rift-to-drift transition but can commonly be observed in both failed continental rift systems and along the passive margins of oceanic plates where rifting succeeded. These geometric aspects of continental rifting have fundamental implications for both hydrocarbon exploration and understanding the causes and processes of continental rupture. Contributions to our understanding of rift asymmetry and segmentation have spanned the past seven decades and come from many settings around the world. However, Africa has played a key role in this undertaking, due to both the presence of well-exposed, active extensional basins in the Afro-Arabian rift system and numerous buried Mesozoic and early Cenozoic rift systems.The first well-documented examples of both rift asymmetry and along-strike segmentation occurred in the Gulf of Suez, northern Egypt, in the 1970s. These early ideas were greatly enhanced and expanded by later studies in both the Western and Eastern branches of the East Africa Rift, including field and refraction/reflection seismic work in Burundi, the Democratic Republic of the Congo, Ethiopia, Kenya, Malawi, Tanzania, and Uganda during the 1980s and 1990s. Asymmetry and segmentation were recognized to be fundamental characteristics of both magmatic and amagmatic rifts, although important differences regarding scale and temporal evolution are evident. At smaller scales, hard-linkage of growing and interacting extensional faults was another key component of subsurface interpretation in the Gulf of Suez by the mid-1970s. Later, outcrop studies in East Africa, supported by subsurface seismic and well data, led to greatly improved models of syn-tectonic sedimentation in step with the advances in understanding of structural geometries. Most recently, concepts derived from the continental margins of other plate settings have been incorporated with the earlier African observations to produce new models of the tectonic evolution of the Red Sea and Gulf of Aden, which in turn has then influenced our overall understanding of the progression from continental to oceanic rifting. This is an ongoing topic of research and discussion that will certainly proceed for many years and require the collaboration of both the international scientific community and local African earth science expertise.

Diffuse soil CO2 emissions at rift volcanoes: Structural controls and total budget of the Olkaria Volcanic Complex (Kenya) case study

Continental rift systems are first-order emitters of deep-sourced CO2 but their impact on the global budget is largely unknown. For instance, the estimate of the total amount of deep-sourced CO2 released by the East African Rift (EAR) is largely undocumented due to the small number of direct measurements available and the impracticability of remote sensing methods induced by atmospheric noise. This study explores the degassing budget and the structural control of soil CO2 emissions at an active rift volcano aiming to implement the database of available direct measurements of EAR's volcanoes. We investigated soil CO2 degassing at the Olkaria Volcanic Complex, a large, multicentred, geothermally exploited volcanic complex in the southern portion of the Kenyan Rift. A total of 1158 soil CO2 flux measurements were collected on fumarolic fields or crosscutting unaltered major faults. The contribution of multiple sources of CO2 (biogenic background vs magmatic/hydrothermal) was tested using both graphical statistical analysis and the carbon isotopic signature (13C-CO2). Soil CO2 fluxes reached up to ∼5500 gm-2d-1. The emission of deep-sourced CO2 coupled with the structurally controlled hydrothermal fluid circulation, sums up to ∼280 tCO2 released per day by the entire complex. Finally, we provide an estimate of the total budget for soil CO2 of the rift, summing up the contributions of all volcanoes in the EAR.

Spatio‐Temporal Variations in Sediment Delivery as a Response to Rapid Quaternary Climate Change in the Lake Malawi Rift, East Africa

AbstractThe interplay of rapid climate change and tectonics drives landscape development, sediment routing, and deposition in early‐stage continental rift systems. The Lake Malawi Rift, in the Western Branch of the East African Rift, is an archetype of a juvenile rift and an ideal natural laboratory for evaluating lacustrine source‐to‐sink systems on orbital or shorter timescales. We examine the interplay of these processes over the past 140 kyr using observations from nested seismic reflection data sets tied to scientific drill cores, which calibrate numerical forward models of this closed sedimentary system. Fault slip rates measured from seismic data drive tectonic displacements in the model. Satellite‐derived precipitation maps constrain modern precipitation and are scaled to previous hydrologic balance studies to reconstruct past climates. Our model reproduces known sediment thicknesses across the rift and accounts for 96% of the estimated siliciclastic sediment deposited over the past 140 kyr. The results demonstrate that the onset of arid climate conditions (140–95 kyr BP) causes extreme drainage adjustments downstream and the formation of mega‐catchments that flow axially into a shallow restricted paleo‐lake. Sedimentation rates during this time are twice the present values due to increased sediment focusing via these axial systems into a much smaller, hydrologically closed lake. As the climate became wetter (95–50 kyr BP), the lake rapidly expanded, decreasing both erosion and sedimentation rates across the rift. This closed‐loop approach allows us to evaluate the role of high‐frequency climate change in modulating basin physiography as well as sediment fluxes in juvenile rift systems.

Adaptive vertical-deformation field estimation and current tectonic deformation significance analysis in Shanxi rift valley

The Shanxi rift valley is a continental rift system that is characterized by mantle uplift, crustal thinning, and tectonic deformation. A continuous, vertical crustal-deformation field can elucidate the constraints for understanding the mechanisms currently driving the deformation of the rift. Accordingly, we acquired and processed data from 250 continuous and mobile GPS stations located in the Shanxi rift valley and its surrounding region. Accounting for the influence of fault activity, we established the vertical crustal-deformation field of the Shanxi rift valley as a continuous function of space, using an adaptive least-squares collocation method. The main research findings are stated as follows: 1) the adaptive least-squares model yielded a reliable interpolation prediction results with adequate robustness, even for relatively sparse actual observation data. 2) The current general deformation pattern of the Shanxi rift valley exhibits an uplift of the mountainous regions on both sides and subsidence of the central basin. The average uplift rate of the mountain area is 2–3 mm/a, and the subsidence rate of the basin is not uniform and is positively correlated with the spatial distribution of the Cenozoic sediment thickness. However, in certain areas, a high subsidence rate of 10–30 mm/a is associated with human activities such as groundwater exploitation. 3) In summary, the current vertical crustal deformation occurring in the Shanxi rift valley correlates with the pattern over a time scale of millions of years. Overall, it is controlled by regional geological structure pattern, and is influenced by nonstructural factors in the shallow crust, exhibiting both complex and orderly characteristics in its spatial distribution.

Mechanical strength and nature of lithospheric deformation beneath the rifted sedimentary basins of the east India passive margin

The study of gravity signatures corresponding to sediment load over the passive margins is helpful to constrain the mechanical properties of continental rift systems as well as the adjacent ocean basins. Seismic reflection, free-air gravity and well information from the East India Passive Margin (EIPM) are investigated through spectral approach and 2D process-oriented gravity modelling for quantifying effective elastic plate thickness (Te) to understand the rift mechanisms and the sedimentary basins evolution. The characteristics of lithosphere within the major structural domains along the margin are quite variable. A narrow necking zone (40 km) in the southern segment, and approximately a 70 km hyper-thinned and exhumed domains are inferred in the central segment, while a narrow hyper-thinned domain (30–35 km) is observed in the northern segment of the margin. The southern segment of EIPM, adjacent to Southern Granulite Terrain has low Te values ranging from 5 to 10 km. The segment in the central margin, offshore Dharwar Craton has moderate Te values of 15–20 km. And the northern segment adjacent to onshore Eastern Ghats Mobile Belt has relatively high mechanical strength with Te values ranging from 25 to 35 km. The pattern of Te values and extent of major structural domains along the margin indicates that the southern, central and northern segments evolved as three different modes of rifting such as transform, hyperextended and hypoextended, respectively. In the oceanic domain, the lithospheric strength broadly increases with age following the thermal induced cooling plate model (600 °C isotherm). The thick sedimentary basin of the Bay of Bengal had attained the high value of elastic thickness (Te > 45 km) in the last 23 Myrs. The significant rheological strengthening of the oceanic lithosphere may be attribute to the following reasons: 1) increase in shear-wave velocity anomaly at a depth of 150 km and upper mantle density, 2) change in plate curvature caused by excessive deposition of Bengal Fan sediments in the deep ocean basins and subduction of oceanic plate under the Eurasian plate near the Bengal Fan and 3) modification of stress state of the lithosphere as a convergent interaction of the Indian plate with SE Asia.

Provenance and Tectonic Setting Discrimination and Their Effects on the Pore-Scale Fluid Flow of Black Shale: Take the Micangshan Tectonic Belt, Central China, as an Example

The Lower Paleozoic reservoir in the Micangshan tectonic belt is a new shale gas exploration area with excellent potential. However, the black shale’s sedimentary environment and tectonic setting and their effects on pore-scale fluid flow have not been thoroughly elucidated and urgently require further research. A large amount of trace and rare earth element data of black shale in this area was tested to analyze the sediment’s provenance, tectonic setting, and coupling process, and nuclear magnetic resonance was applied for fluid flow ability determination. The characteristics of trace elements and rare earth elements in shale samples were drilled from the Upper Sinian Doushantuo Formation, the Lower Cambrian Niutitang Formation, and the Lower Silurian Longmaxi Formation in the Micangshan tectonic belt. The results reveal that the provenance of the shales was dominated by island arc volcanic rocks and some ancient fine-grained sediments. The provenances of shale in the Micangshan, Huijunba, and Zhenba areas may have a strong genetic relationship with the igneous rocks from the nearest Hannan massif. The provenance of the Niutitang shale in the Ningqiang area may comprise sediments derived from the Longmenshan continental rift system. The provenance of the shale tends to transition from the island arc to the continental margin from the depositional stage of the Doushantuo Formation to the Longmaxi Formation. The Micangshan tectonic belt inherited the tectonic evolution of the South China block, North China block, and Qinling, and those blocks provided the igneous provenance of the shale by island arcs, which were formed due to weathering caused by the process of subduction and collision orogeny. The movable fluid distribution of the Longmaxi Formation and Niutitang Formation is relatively uniform, and excessive pursuit of large amount of movable fluid will be counterproductive to the shale reservoir. The proportion of the cracks was generally less than 3% except for one sample from the Doushantuo Formation with 12.06%. Based on the centrifugation force in this research, the low fluid flow limits were less than 1.47, and there are strong positive relationships between movable fluid saturation and saturated porosity. In this study, the coupling relationship between the material source and pore scale flow is discussed for the first time, and the results can provide theoretical references for local provenance analysis and tectonic movement.

Magmatic cycles in Santos Basin (S.E. Brazil): Tectonic control in the temporal-spatial distribution and geophysical signature

The Santos Basin, in the offshore southeastern Brazilian margin, evolved from a continental rift system to a passive divergent margin during the Cretaceous as a onsequence of the Gondwana breakup and the formation of the proto-Atlantic Ocean. The basin exhibits several cycles of magmatism from Valanginian to Eocene times, with a regionalized distribution of igneous rocks. Using seismic, gravity, magnetic, and wellbore data, the magmatic cycles were investigated and connected with the basin's tectonic activity. Our study includes a regional analysis of the various magmatic cycles, with a focus on identifying the temporal variations and spatial differences observed in the basin's many tectonic compartments. The first magmatic cycle, named rift-onset, is observed in both coastal and offshore regions and includes magmatic rocks from ca.135 to ca. 132/130 Ma. Onshore, the rift-onset magmatic rocks are observed in the coastal dike swarm province and in the coastal alkaline province. Offshore, the riftonset is observed as a prolongation of the coastal dike swarms into the proximal basin and as flood basalts in the distal basin. The second cycle includes the synrift (ca.132-130 to 123 Ma) and postrift (ca.123 to 113-112 Ma) magmatic rocks. The separation of intrusive synrift and postrift rocks is challenging in the onshore fieldwork and in the seismic analyses offshore, which is aggravated by the basin's scarcity of geochronological dating. Therefore, these two events have been treated together when the separation is not clear. The third cycle comprises the drift (or post breakup) magmatic rocks, spanning from the Lower Cretaceous to the Tertiary (from ca. 110 to ca. 40 Ma), and is well documented in coastal and offshore domains. Drift magmatism seems to be controlled by tectonics, since magmatic bodies tend to concentrate along lines of crustal discontinuities. Magmatic cycles (rift-onset, syn/postrift, and drift) also display spatial differences, exhibiting variation in the magmatic volume and in the morphology of igneous rocks from the southern to the central/northern Santos Basin. The study of magmatism in the, Santos Basin offers an excellent opportunity to investigate the processes that occurredduring the evolution of a volcanic rifted margin.

Spatial distribution of 1.4-1.3 Ga LIPs and carbonatite-related REE deposits: Evidence for large-scale continental rifting in the Columbia (Nuna) supercontinent

Large-scale continental rifting and large igneous provinces (LIPs) have played important roles in the breakup and dispersal of supercontinents Pangea and Rodinia. Previous paleomagnetic and geological evidence suggests fragmentation of Columbia (or Nuna) supercontinent during 1.4-1.2 Ga. However, no large-scale continental rift system has been identified in Columbia during this period, although some continental rifting events between 1.6 Ga and 1.2 Ga have been reported. Here we provide a refined ca. 1.4 Ga paleogeographic reconstruction of Columbia based on previously published paleomagnetic data and geological evidence for connections between continental blocks, and consistent with spatial and temporal distributions of the 1.4-1.3 Ga LIPs and smaller intraplate mafic magmatic events interpreted as LIP fragments/remnants. Our results indicate a 1.4–1.3 Ga large-scale continental rift zone located along western Laurentia, western-northern Siberia, southeastern Baltica, western-northern West Africa, southwestern Amazonia, southern-eastern Congo/São Francisco, eastern Kalahari, northern North China and northern North Australia. This rift zone extends about 15,000 km across supercontinent Columbia, and is a main indicator and a proximal reason for its final breakup. The spatial distribution of the 1.4-1.3 Ga carbonatite-related REE deposits shows that this newly identified rift system also controlled Bayan Obo, the world's first and largest REE-Nb deposit in northern North China, and Mountain Pass, the world's second largest REE deposit in western North America. Given the scale of this rift system it can be expected to be the locus of additional carbonatite intrusions as well as other commodity types, and provides important constraints on mechanism and processes for final breakup and dispersal of Columbia supercontinent.

Western flanks of the Central Asian late paleozoic continental rift system and uranium mineralization

Introduction. Available information on the occurrences of high alkaline magmatic rocks fixing the position of the western flanks of the southern branches (Gobi Altai and Gobi Tien-Shan) of the Central Asian Late Paleozoic continental rift system in the territory of the North-West of Chingiz (the eastern part of Central Kazakhstan), the Kendyktas-Chu-Ili-Bet-Pak-Dal uranium-bearing province of the Southern Kazakhstan and the Chatkal-Naryn zone of Tien-Shan (Uzbekistan) is reviewed. Data on the confinement of uranium mineralization to these occurrences and its paragenetic relationship with the Late Paleozoic alkaline volcanic-plutonic association is provided.Aim. To study the uranium mineralization of Late Paleozoic alkaline magmatism to clarify metallogenic representations and justify prospects for the ore-bearingness of the western flanks of the southern branches of the Central Asian continental rift system.Materials and methods. The largest part of materials was obtained in the process of geological surveys and prospecting works of various scales in the regions of Central Kazakhstan in the period 1965–1985 with the participation of the author. Published materials concerning the subdivision of magmatic formations in the uranium ore fields of Southern Kazakhstan and the Chatkal-Naryn zone were also used.Results. Subvolcanic bodies of comendites and small masses of alkaline granites of the Late Paleozoic were identified across the area of uranium ore occurrences in the South-Western Chingiz and some deposits of South Kazakhstan confined to Devonian volcanic structures, which indicates their relationship with the southern branches of the Central Asian continental rift system. Rare-metalalbite, uranium-phosphorus and uranium-molybdenum formations are confined to alkaline rocks. Rare-metal mineralization is closely associated with alkaline rocks, while uranium mineralization occurred after the introduction of the most recent microgabbrodiorite and lamprophyre dikes. Similar correlations of rare-metal and uranium mineralization with alkaline granites and of microgabbrodiorite and lamprophyre dikes are observed in the uranium deposits of the Chatkal-Naryn zone located among the early Permian bimodal trachybasalt-trachyolite formation.Conclusion. The establishment of ore mineralization on the western flanks of the Gobi Altai and Gobi Tien-Shan branches in the territory of Southern and Central Kazakhstan allows these areas to be distinguished as metallogenic zones, promising in terms of rare-metal and uranium mineralization.

State of stress and stress rotations: Quantifying the role of surface topography and subsurface density contrasts in magmatic rift zones (Eastern Rift, Africa)

In rift settings, the crustal stress field is dominated by extension, which leads to rift-parallel topography and basin alignments. However in some continental rift systems, some observables of the orientation of principal stresses show substantial deviations from these patterns. Such stress field rotations are currently poorly understood and could reflect the critical role of rift magmatism in the creation of topography, the plate state-of-stress, and volcanic and tectonic processes. Yet the role of magma intrusions, crustal thinning, and rift basin and flank topography on rift zone stress field rotations remain poorly quantified. The seismically- and volcanically-active Magadi-Natron-Manyara region of the East African Rift shows a 60∘ local stress field rotation with respect to regional extension. Here, we test the hypothesis that such rotation is due to the cumulative effects of surface and subsurface loads (lateral subsurface density contrasts). We use analytical and calibrated numerical models of magmatic rift zones to simulate lithospheric deformation in the presence of magma bodies, crustal thinning, and topography to quantify their effect on intrusions and fault kinematics in a rift setting. Our 3D static models of a weakly extended rift suggest that surface topography influences shallow stress localization, whereas subsurface density contrasts play a larger role in lower crustal stress localization. Both patterns suggest a preferred region for melt storage beneath the rift valley. We show that the interaction between topography, crustal thinning, extension, and a pressurized magma reservoir could generate principal stress orientations consistent with the local stress rotation observed from earthquake focal mechanisms. Our results demonstrate how rift topography and the geometry of crustal thinning can guide magmatism and strain localization, highlighting the need for a three-dimensional treatment of rift kinematics.

Rifting of the Indian passive continental margin: Insights from the Langjiexue basalts in the central Tethyan Himalaya, southern Tibet

Abstract The Triassic tectonic setting of the Tethyan Himalaya in southern Tibet remains controversial and is key to revealing the mechanism of Neo-Tethys Ocean opening and the breakup history of Gondwana. This paper reports 227–225 Ma mafic volcanic rocks interbedded within the Tethyan Himalaya sequence in southern Tibet, which were formed in a typical passive continental margin setting. The basalts are tholeiitic with high TiO2 (3.20–3.38 wt%) and moderate MgO (4.05–5.40 wt%) contents and exhibit enrichment in light rare earth elements and weak negative Nb, Ta, Eu, and Ti anomalies. These geochemical compositions, combined with uniform whole-rock εNd(t) (+1.50 to +2.70) values, indicate that the magmas were derived by low-degree melting of a deep-seated garnet source that was heterogeneously modified by an oceanic island basalt-type of component. Given the other geological evidence, we propose that the magmatic evolution of the Tethyan Himalaya during the late Paleozoic–Mesozoic was generally a lithospheric thinning process, that is, it evolved from a fully developed continental rift system during the Late Permian to wane during the Triassic. This interpretation further supports the hypothesis that the Neo-Tethys Ocean opened in a passive pattern and that the breakup of Gondwana in the late Paleozoic was initiated by lithospheric thinning. Therefore, the subsequent magmatism was related to the passive upwelling of normal asthenospheric materials.

The Eocene armadillo Utaetus buccatus (Euphractinae) in the Guabirotuba Formation (Curitiba Basin) and carapace morphological implications

The Guabirotuba Formation belongs to a small sedimentary basin located in the southernmost portion of the Cenozoic continental rift system of southeastern Brazil. In this unit was recognized the Guabirotuba Fauna from Eocene, that includes several cingulates such as Machlydotherium, Proeocoleophorus, Parutaetus, Meteutatus, Utaetus and an indeterminated Astegotheriini. The genus Utaetus in the Guabirotuba Fauna is confirmed by presence of Utaetus buccatus based on the specimen MCN.P.1231 and reinforce the Barrancan Age for the Guabirotuba Formation. In this study we analyzed numerous osteoderms associated to an individual, representing most of the external morphological diversity among fixed, semi-movable and movable osteoderms. Morphological features are described for fixed and movable osteoderms as well as, for the first time, for the semi-movable osteoderms. Features of some semi-movable osteoderms indicate the presence of some kind of pectoral buckler on Utaetus buccatus. The known geographic distribution of the genus Utaetus includes Argentinian Patagonia, Northwest of Argentina, and South of Brazil. The geographic distribution of the species Utaetus buccatus, previously restricted to Argentinian Patagonia, is here expanded to lower latitudes during the middle Eocene, which allow to infer that the species distribution was wider than previously thought.