Search

The East African Rift System (EARS) provides an ideal natural laboratory for studying the mechanisms of tectonic plate breakup and continental drift, as well as a unique perspective for exploring the maturation process of continental rifting and its drivers. This study combines high-resolution satellite gravity data and seismic tomography model with an integrated geophysical approach to reveal the density structures in the upper mantle of the EARS. The results show that the northeastern to central Congo and Zimbabwe Craton exhibit significant high-density anomalies extending up to 250 km, which is indicative of a thicker and more intact lithosphere. In contrast, the Uganda, Tanzania, eastern and southern Congo, and Kaapvaal Craton show shallow high-density anomalies underlain by low-density anomalies that are clearly derived from the deeper mantle, indicating a thining of the lithosphere with some degree of possible melting at the base. The various rift segments of the EARS exhibit different rift morphologies. The Main Ethiopian Rift and the Kenya Rift of the Eastern Rift Branch show strong low-density anomalies, indicating intense melting, which is much stronger than that observed in the Western Rift Branch. However, the two rifts may have originated from the same mantle uplift in which the low-density anomalies of the Eastern and Western Rift Branches connected in the deep upper mantle. The lower portion of the Malawi Rift exhibits weaker low-denstiy anomalies, which can be observed to the south of the Malawi Rift, extending further south as a continuation of the EARS. Combining the results of previous kinetics simulations and our density perturbation results, it can be inferred that the Eastern Rift Branch is mainly affected by active rifting, while the Western Rift Branch is affected by both active and passive rifting. [ABSTRACT FROM AUTHOR]

The Shunbei 5 (SB5) strike-slip fault, situated in the central Tarim basin, is distinguished by its considerable length, significant variations in planar orientation, and intricate multi-stage tectonic evolution. This study delves into the geometric, kinematic, and dynamic features of both the southern and northern parts of the SB5 fault, utilizing the latest seismic data from the Fuman Oilfield, and examines the factors influencing the fault's planar deflection. The fault can be categorized into three structural deformation layers based on lithological features and fault features: the deep structural deformation layer below TЄ3), characterized by basement rifting and limited strike-slip activity; the middle structural deformation layer (TЄ3-TO3), marked by vigorous strike-slip movements and the dominance of flower structures; and the shallow structural deformation layer (TO3-TP), featuring echelon-type normal faults and boundary graben faults, specifically in the southern SB5 fault. The fault activity is more pronounced in its southern SB5 fault compared to the northern, with the weakest activity at the TЄ 3 interface and the peak at the TO3 interface. The southern SB5 fault transitions to sinistral slip at the TO3 interface, while the northern SB5 fault shifts from dextral to sinistral slip at the TC interface, highlighting variable slip directions across different interfaces. Rifts are extensively distributed within the Precambrian basement along the SB5 fault. The initial strike-slip fault rupture, which is primarily localized in these areas, exerts a significant influence on the formation of the S-shaped fault plane. This process involves four distinct evolution stages: the embryonic stage of strike-slip activity during the Middle- Late Cambrian; the intense strike-slip fault activity stage during the Middle-Late Ordovician; the reactivation stage of deep strike-slip fault in the Silurian; and the connection and reactivation stage during the Devonian-Carboniferous. [ABSTRACT FROM AUTHOR]

The East African Rift System (EARS) provides an opportunity to constrain the relationship between magmatism and plate thinning. During continental rifting, magmatism is often considered a derivative of strain accommodation—as the continental plate thins, decompression melting of the upper mantle occurs. The Turkana Depression preserves among the most extensive Cenozoic magmatic record in the rift. This magmatic record, which comprises distinct basaltic pulses followed by periods of relative magmatic quiescence, is perplexing given the lack of evidence for temporal heterogeneity in the thermo‐chemical state of the upper mantle, the nonexistence of lithospheric delamination related fast‐wave speed anomalies in the upper mantle, and the absence of evidence for sudden, accelerated divergence of Nubia and Somalia. We focus on the Pliocene Gombe Stratoid Series and show how lithospheric thinning may result in pulsed magma generation from a plume‐influenced mantle. By solving the 1D advection‐diffusion equation using rates of plate thinning broadly equivalent to those measured geodetically today we show that despite elevated mantle potential temperature, melt generation may not occur and thereby result in extended intervals of quiescence. By contrast, an increase in the rate of plate thinning can generate magma volumes that are on the order of that estimated for the parental magma of the Gombe Stratoid Series. The coincidence of large‐volume stratiform basalt events within the East African Rift shortly before the development of axial zones of tectonic‐magmatic activity suggests that the plate thinning needed to form these stratiform basalts may herald the onset of the localization of strain. Plain Language Summary: The magmatic record in the Turkana Depression—part of the East African Rift System—is characterized by pulses of basaltic activity that are followed by long periods of relative magmatic quiescence. This is a puzzling observation assuming that these magmas are generated by decompression melting of the upper mantle; there is no obvious changes in the rate of plate motion between Nubia and Somalia. This study presents new geochemical data on the final pulse of basaltic volcanism (during the Pliocene) and interprets these data in the context of a mantle melting model. We find that pulses of basaltic volcanism and intervening periods of quiescence could be simulated using different rates of thinning of the plate. We examine the consequences of a period of enhanced plate thinning in context of melt generation both below and within the plate. Key Points: Pulses of basaltic volcanism and intervening periods of quiescence could be simulated using different rates of thinning of the plate [ABSTRACT FROM AUTHOR]

Inherited rift topography controls the sediment routing, timing of sand supply, and sedimentary linkage of early post‐rift depocentres. Exhumed examples of early post‐rift turbidite systems are rare and previous studies have examined the evolution of individual depocentres; in contrast, the detailed evolution of early post‐rift turbidite systems across multiple depocentres has never been documented. Current fill‐and‐spill models do not detail the stratigraphic architecture and evolution of sedimentological characteristics of multiple intraslope fans developed across topography, including bed type distributions. Here, the evolution of three intraslope fans that developed across two early post‐rift depocentres is documented along an 18 km long transect in the southern Neuquén Basin, Argentina. The relative chronology of sand supply in depocentres is constrained with new U–Pb ages, and sediment source areas with provenance analysis. The early post‐rift intraslope fans record progradation of the system and progressive sedimentary linkage of post‐rift depocentres, transverse to local syn‐rift structures, with sediment routing subparallel to the cratonic basin margin. The large‐scale stratigraphic architecture of intraslope fans indicates an evolution as a fill‐and‐spill system, with initial confinement through flow stripping and overspill to spillover with erosion and bypass across a transverse topographic high separating the depocentres. Changes in early post‐rift intraslope fan characteristics, including thickness, sandstone content, lobe complex stacking patterns, stratal termination patterns and bed type distribution, record changing confinement through time within a depocentre, and spatially across depocentres. The strong spatial and vertical stratigraphic variability of transitional flow deposits and hybrid event beds reflects enhanced erosion, sediment bypass and flow transformation across transverse relief between the two depocentres during the spillover phase. These findings advance current understanding of early post‐rift turbidite systems and refine fill‐and‐spill models, which will help the prediction of spatial and vertical changes in rock quality and connectivity in subsurface hydrocarbon reservoirs and CO2 storage sites. [ABSTRACT FROM AUTHOR]

The morphology of the shield volcanoes on Bioko, a volcanic island in central Africa, is controlled both by tectonic and volcanic processes, but the complex interplay of these regional and local mechanisms is poorly understood. Using a TanDEM-X digital elevation model, we are able to create an inventory of 436 vents and monogenetic cones, and over 1330 structural elements and lineaments, and perform a comprehensive morphological and geospatial analysis. We provide detail on the general geomorphology of Bioko Island, and describe its flat top, apical graben-like structures, and the setting of the structural inventory created. Based on vent density and lineament mapping, we are able to identify volcanic rift zones that are governed by vent clustering and the asymmetry of associated monogenetic cones. Specifically, we find that eruption vents are not only clustered but aligned and follow the principal NE-SW axis, although we also highlight evidence for complex structures such as side-stepping alignments and en échelon patterns indicative of strike-slip contributions to the volcano-tectonic fabrics. We discuss possible volcano-tectonic and regional tectonic contributors, such as the Cameroon Volcanic Line and intersecting fracture zones, as well as gravity-tectonic processes dominant at Bioko Island. In this view, our results are relevant for understanding the past and recent volcanic activity and discuss the influence of regional and local volcano-tectonic architectures. [ABSTRACT FROM AUTHOR]

The article focuses on analyzing groundwater storage anomalies in Ethiopia's Rift Valley Basin using advanced techniques. Topics include downscaling Gravity Recovery and Climate Experiment (GRACE) satellite data with Artificial Neural Networks, assessing regional groundwater storage changes, and understanding the impacts of data scarcity on groundwater management.

Assimilation and prolonged suspension of crust-derived sulfide liquid in komatiites are essential to form Ni-rich mineralisation. Evaluating the spatial relationship between komatiite-hosted Ni mineralisation and crustal S sources may thus provide insights into mechanisms of transport, metal enrichment and deposition of assimilated sulfide liquid. This study applied facies analysis and S isotopes to sulfides in Ni-mineralised komatiites and stratigraphically underlying bimodal volcanic-volcaniclastic and sedimentary rocks, which formed during rifting in the Agnew-Wiluna Greenstone Belt, Western Australia. The results revealed a lateral variation from rift-distal sedimentary sulfides, through sulfidic BIF, to rift-proximal VMS-style sulfides, the latter of which was predominantly assimilated by komatiites. Both crustal and komatiite-hosted sulfides were overprinted by granite-related skarn alteration during later basin inversion. Spatial S isotopes correlation revealed that Ni mineralisation in komatiites predominantly formed < 5 km from their crustal S sources, excluding long lateral transport as the main metal enrichment mechanism. Rather, metal enrichment likely happened through multiple cycles of sulfide entrapment and entrainment in lava flow vortices that formed in the wake of topographic steps represented by syn-rift faults. These faults were the main loci for pre-existing crustal weaknesses, hydrothermal fluid circulation, and VMS-style sulfide deposition, which were subsequently utilised by komatiites for enhanced thermo-mechanical erosion and crustal sulfide assimilation. This study shows that proximity to the syn-rift faults was the dominant control on the formation of komatiite-hosted Ni–sulfide mineralisation, regardless of substrate lithology. The S isotope signatures of crustal sulfides may be used as a proxy to identify syn-rift faults in highly deformed terranes. [ABSTRACT FROM AUTHOR]

The effects of the composition and angle of the subducting slab and mantle wedge flow on tectonic and magmatic processes in island arcs and associated back-arcs are poorly understood. Here we analyse the ages and compositions of submarine lavas from the flanks and the floor of the back-arc Futuna Trough some 50 km east of Tanna Island in the New Hebrides arc front. Whereas >2.5 Ma-old back-arc lavas formed from an enriched mantle source strongly metasomatized by a slab component, the younger lavas show less slab input into a depleted mantle wedge. The input of the slab component decreased over the past 2.5 million years while the enriched mantle was replaced by depleted peridotite. The change of Futuna Trough lava compositions indicates rapid (10 s of km/million years) replacement of the mantle wedge by corner flow and slab steepening due to rollback, causing extensional stress and back-arc rifting in the past 2.5 million years. A compositional change in lavas from the Futuna trough at 2.5 Ma suggest a change in mantle source, attributed to slab rollback and the opening of the back-arc basin. [ABSTRACT FROM AUTHOR]

This study addresses the lithospheric structure of the West and Central African rift system (WCARS) and explores its origin and development in relation to the enigmatic Cameroon volcanic line (CVL). Based on a recent seismic tomography model, we subdivide the areas in tectonic domains. We perform integrated 3D geophysical and petrological forward modeling. By exploring the thickness and composition of different domains, we compare the model response to the observed topography and gravity anomalies, under consideration of the available seismic Moho depth points. Our model reveals three distinct domains within the study area: The WCARS is predominantly underlain by a Phanerozoic‐type lithospheric mantle, surrounded by the West African and the Congo Cratons, where the lithospheric mantle has a Proterozoic‐type signature. Between these domains, we identify a transition area where lithospheric thickness changes rapidly. Our preferred model shows significant variability of crustal thickness from 20 km in the rift area to 50 km beneath the cratons accompanied by thin lithosphere of 80 km in the rift area to thick lithosphere of up to 240 km beneath the cratons. The final model confirms that the WCARS' origin is passive, and suggests that the origin of the CVL, particularly its continental part, is the result of two tectonic events: (a) V‐shaped opening of the lithospheric mantle beneath the WCARS, resulting in (b) a strong variation of the lithosphere thickness at the transition between the rift zone and the northwestern part of the Congo craton. Plain Language Summary: In this study, we describe the current structure of the subsurface (from the surface to a depth of 300 km) in Central and Western Africa. The aim is to understand the formation of the Central African Rift zone during the opening of the Atlantic Ocean, and how this relates to the linear chain of volcanoes that cross Cameroon, known as the Cameroon Volcanic Line. To achieve these objectives, we divide the study area into tectonic domains reflecting their seismological signature, and then, establish a three‐dimensional representation of the subsurface structure, based on fitting topography and gravity data. Our model confirms the geological subdivision of the study area into three blocks corresponding to two cratons and a rift zone, with transitional areas between them. Our model is compatible with a passive origin of the rifts in the region. We propose that the origin of the volcanic line of Cameroon is related to magma ascent during the separation of the African and South American plate in connection with the opening of the Atlantic and channeled by the lithospheric architecture. Key Points: We present a new 3D model of the lithosphere for the West and Central African Rift System (WCARS)Our model confirms that the WCARS has a passive originOur model suggest that the origin of the Cameroon volcanic line is linked to the architecture of the WCARS and adjoining cratons [ABSTRACT FROM AUTHOR]

The dominant structural fabric in the Eastern Goldfields is the steep north- to north-northwest-trending S2 foliation that is axial planar to upright F2 folds, developed during intense horizontal east–west shortening (D2–4 event). These structures consistently overprint D1 structures, which comprise layer-parallel S1 foliation, extensional shears, thrusts and recumbent F1 folds. The D1 event represents a separate and distinct episode of deformation with a markedly different stress regime (dominant vertical σ1), compared with the stress regime during the D2–4 events (horizontal east–west oriented σ1). There is little evidence for pre-2670 Ma ductile deformation in the Eastern Goldfields, as: (1) the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences, with the first significant angular unconformities observed at the base of the post greenstone late basins; (2) a lack of schist or gneiss clasts in the late basins suggests that the surrounding uplifted sequence was largely undeformed; (3) the layer-parallel S1 foliation is observed throughout the 2720–2670 Ma greenstone sequence; and (4) the dominant low north- and south-plunges of F2 folds suggest that the sequence was predominantly flat-lying prior to strong east–west shortening (D2–4 events). The ca 2670–2655 Ma D1 event represents a period of sagduction that occurred immediately after cessation of rifting as a result of gravitational instability from deposition of dense, cold, greenstone sequences onto thinned, light, hot felsic crust. Late basins represent depocentres on the sinking greenstones during sagduction. Marked contrasts in the structural style of gold deposits, metallogeny and fluid sources, typically attributed to progressive deformation during orogenesis, could instead reflect temporally distinct mineralising events during changing tectonic regimes. In the East Yilgarn, an early plumbing system dominated by north-northwest-trending basin-controlling structures was likely established during rifting and focused fluids during multiple hydrothermal circulation events. ca 2720–2670 Ma greenstone sequences in the Eastern Goldfields were deposited in intracratonic rift basins. Rifting triggered an episode of sagduction that formed the early dome-and-basin geometry of the Eastern Goldfields and the onset of orogenesis at ca 2650 Ma (D2–4 events) modified the existing dome-and-basin geometry. Late basins represent depocentres on sinking greenstone sequences during sagduction. Gold mineralisation occurred throughout the changing tectonic regimes. [ABSTRACT FROM AUTHOR]

In plate kinematic reconstructions, the restoration of rifted margins and their fossil equivalents exposed in orogens remains challenging. Tight fit reconstructions rely on the mapping of margins rift domains, their restoration to their pre-rift crustal thickness, and the removal of the oceanic and exhumed mantle domains. At present-day margins, high-resolution wide-angle seismic imaging allows mapping and measurement of rift domains; however, restoring fossil margins is trickier because they are largely overprinted and partially lost during convergence. Here, we present a new kinematic model for the Mesozoic rifting along the Tethys–Atlantic junction, which relies on two assumptions: (1) the width of the fossil Alpine Tethys rift domains was comparable to that of their present-day analogs, and (2) the necking zones of the former tectonic plates can be mapped, dated and used as kinematic markers. This reproducible workflow allows us, for the first time, to restore the rifted margins of the Alpine Tethys. Our reconstruction shows: (1) a westward propagation of extension through the Ionian, Alpine Tethys and Pyrenean rift systems from the Triassic to the Cretaceous, (2) the segmentation of the Mesozoic Tethyan rifted margins by strike-slip corridors, (3) the opening of an oceanic gateway at 165 Ma as mantle was exhumed along the entire Alpine Tethys and (4) the subdivision of the Mesozoic oceanic domain into compartments that were later consumed during subduction. This new model is supported by published data from the Alps, the Ionian Sea, the Pyrenees and the southern North Atlantic. [ABSTRACT FROM AUTHOR]

The article critiques Álvaro et al.'s (2023) model of Avalonia's depositional history, arguing that it inaccurately applies a rift/half-graben model to a region better described by a strike-slip regime. It also discusses errors in terrane boundary definitions, stratigraphic correlations, and the regional extent of Avalonia, emphasizing the need for accurate bio- and chronostratigraphic correlations to reconstruct Avalonian and Caledonian Highlands deposition.

We investigate the upper-crustal structure of the Rukwa–Tanganyika rift zone in East Africa, where the Tanganyika rift interacts with the Rukwa and Mweru-Wantipa rift tips, evidenced by prominent fault scarps and seismicity across the rift interaction zones. We invert earthquake P-wave and S-wave travel times to produce 3D upper-crustal velocity models for the region and perform seismicity cluster analysis to understand strain accommodation in rift interaction zones and at the propagating rift tips. The resulting models reveal the occurrence of anomalously high Vp/Vs (primary-to-secondary wave velocity) ratios in the upper crust beneath the Rukwa and Mweru-Wantipa rift tips – regions with basement exposures and sparse rift sedimentation. We detect distinct earthquake families within the deeper clusters which exhibit a temporal evolution pattern characterized by an upward linear trend that suggests triggering caused by upward fluid migration and creep failure. A spatial transition from proximal tip zones dominated by a thinned crust and through-going crustal and upper-mantle seismicity to distal tip zones with a thick crust and dominantly upper-crustal seismicity indicates an along-axis variation in the controls on rift tip deformation. Overall, the collocation of basement faulting, crustal and upper-mantle seismicity, and high upper-crustal Vp/Vs ratios suggests a mechanically weakened crust at the rift tips, likely accommodated by brittle damage from crustal-bending strain and thermomechanical alteration via ascending fluids (mantle-sourced volatiles and hydrothermal fluids). These findings provide new insights into the physics of the propagation, linkage, and coalescence of continental rift tips – a necessary ingredient for initiating continental-breakup axes. [ABSTRACT FROM AUTHOR]

Historic volcanic activity in Tenerife was concentrated within two of the island's three dorsals and on the Teide-Pico Viejo complex located inside Las Cañadas caldera. Eruptions on the island are primarily characterised by basaltic and trachybasaltic fissural eruptions. However, the Teide-Pico Viejo complex also hosted explosive and effusive phonolitic eruptions. Our study focused on the analysis of the 2004-2005 unrest in Tenerife, which was characterised by an onset of a ground deformation pattern, heightened on-land seismic activity, changes in the chemical composition of fumaroles of the Teide composite volcano, an increase in diffusive emissions of carbon dioxide along the NW rift, and, significant gravity changes. We used the Envisat-ASAR satellite images from 2003 to 2010 to generate the Line-Of-Sight SBAS- DInSAR deformation time series to investigate the source responsible for ground deformation. We applied the Independent Component Analysis (ICA) to separate distinct ground deformation patterns. Specifically, we selected four components for the Independent Component Analysis decomposition: the first one mainly affects the stratovolcano's summit region of Teide and shows a circular symmetry; the second and third components are possibly related to the topography and atmospheric artifacts, while the fourth contains only a noisy signal. We employed a non-linear optimisation approach in a Finite Element modelling environment to determine the source geometry responsible for the first identified ICA pattern of ground deformation within Las Cañadas. Our results revealed the existence of an ellipsoidal ground deformation source oriented along the E-W axis, located beneath the Teide and Montaña Blanca volcanoes at 1,600 m a.s.l. This source became active during the seismic crisis of 2004-2005, which was associated with the degassing of a magma batch that intruded into the northwest rift of Tenerife island. We propose that the ground deformation observed at Teide volcano during the 2004-2005 crisis was related to hydrothermal activity within the volcano. [ABSTRACT FROM AUTHOR]

There is a growing consensus that Venus is seismically active, although its level of seismicity could be very different from that of Earth due to the lack of plate tectonics. Here, we estimate upper and lower bounds on the expected annual seismicity of Venus by scaling the seismicity of the Earth. We consider different scaling factors for different tectonic settings and account for the lower seismogenic thickness of Venus. We find that 95–296 venusquakes equal to or bigger than moment magnitude (Mw) 4 per year are expected for an inactive Venus, where the global seismicity rate is assumed to be similar to that of continental intraplate seismicity on Earth. For the active Venus scenarios, we assume that the coronae, fold belts, and rifts of Venus are currently seismically active. This results in 1,161–3,609 venusquakes ≥Mw4 annually as a realistic lower bound and 5,715–17,773 venusquakes ≥Mw4 per year as a maximum upper bound for an active Venus. Plain Language Summary: Venus could be seismically active at the moment, but it is uncertain how many earthquakes (or to use the proper term: venusquakes) there could be in a year. Here, we calculate the minimum and maximum number of venusquakes we could expect in a given year on Venus based on different assumptions. If we assume there is not much seismic activity on Venus (comparable to the interior of tectonic plates on Earth), we find that we could expect about a few hundred venusquakes per year with a magnitude bigger than or equal to 4. For an estimate of the maximum amount of venusquakes, we assume that Venus has regions with more seismic activity: the so‐called coronae, fold belts, and rifts. Depending on our assumptions, we then find that more than 17,000 venusquakes with a magnitude bigger than or equal to 4 could occur in a year. Key Points: An inactive Venus with global background seismicity like Earth's continental intraplate seismicity has a few hundred quakes ≥Mw4 per yearA lower bound on an active Venus where fold belts, coronae, and rifts are seismically active predicts a few thousand quakes ≥Mw4 annuallyThe upper bound for an active Venus results in thousands (∼5,000–18,000) venusquakes ≥Mw4 per year [ABSTRACT FROM AUTHOR]

Widespread volcanism has been known in the Sicilian Channel for a long time, even if some submarine volcanoes have only recently been discovered. Most of this volcanism formed along the NNE-trending transfer zone known as the Capo Granitola–Sciacca Fault Zone, while others, such as the islands of Pantelleria and Linosa, are associated with the continental rift zone that has developed since the early Pliocene in the central part of the Sicilian Channel through the formation of three deep tectonic troughs (Pantelleria, Linosa and Malta). However, the origin of a group of five volcanoes (here called "Tetide volcanic cluster") that form a NW-SE alignment on the eastern edge of the Adventure Plateau is not yet known. In this work, we hypothesize that this volcanic alignment may represent the remnants of a failed rift attempt that was unable to generate another tectonic trough in the Sicilian Channel. Based on seismic sections and gravimetric data, three phases in the formation of this volcanic alignment can be identified: (i) a major magmatic intrusion in the early Pliocene associated with a NW-SE normal fault that formed during the opening of the Pantelleria graben, leading to the uplift and deformation of the host sedimentary rocks; (ii) a late Pliocene-Quaternary tectono-magmatic quiescent phase; and (iii) a renewed magma intrusion through fissures or cracks that led to the formation of the volcanoes in the late Quaternary. This process was not able to cause significant extension and only limited volcanism, which is why the "Tetide volcanic cluster" is interpreted as the morphological expression of a failed rift. [ABSTRACT FROM AUTHOR]

An animated 100,000‐year‐interval tectonic reconstruction of the Woodlark Basin in the southwest Pacific illustrates how, at intermediate initial spreading rates, orogenic continents break up (dyke model), spreading segments nucleate, transform faults initiate and ocean basins evolve. We refine the location/timing of initial seafloor spreading and Euler poles of rotation back to 6.2 Ma. In the easternmost basin, where spreading younger than 2.6 Ma is not co‐polar with that to the west, we recognize the formation of a Ghizo microplate and Ranongga Transform Fault at ∼2.6 Ma and a 3‐degree rotational opening of the Itina Trough from 2.6 to 1.0 Ma. Allowing for that motion, we show that the 5.2–2.6 Ma seafloor in the easternmost basin formed co‐polar with that to the west. We also identify a ridge jump reorientation at ∼1.0 Ma that formed the NE‐trending Simbo Spreading Segment, whose neovolcanic zone includes Simbo Island and a submarine edifice to its south. Proposed deterministic models of ridge propagation (due to topographic gradients, mantle flow away from hotspots and/or changing plate motion) are not consistent with those observed; mantle chemical heterogeneities and melting anomalies are a potential cause that remains to be tested. We reconstruct the northern conjugate of the oldest extant oceanic crust and estimate the initiation of its subduction at ∼2.6 Ma, concomitant with observed changes in plate motion and segmentation. Where subducted, the young oceanic lithosphere between the conjugate rifted margins appears to be resorbed into the mantle, leaving a slab window where the Pacific subducted slab remains attached. Key Points: An animated tectonic reconstruction of the Woodlark Basin illustrates continental breakup and ocean basin formation and evolutionMost deterministic models of ridge propagation are not consistent with observations; mantle heterogeneities and melting anomalies may beThe Ghizo microplate and Ranongga Transform Fault formed ∼2.6 Ma; the Itina Trough rift graben rotated open by 3‐degrees from 2.6 to 1.0 Ma [ABSTRACT FROM AUTHOR]

Central Afar is shaped by the interaction between the Red Sea (RS) and Gulf of Aden (GoA) rifts. While there have been several studies conducted in the region, we know surprisingly little about the mechanism of connection between these two rift branches. Here we use high‐resolution 3D lithospheric scale geodynamic modeling to capture the evolution of linkage between the RS and GoA rifts in central Afar. Our results demonstrate that the two rifts initially overlap and interact across a broad zone of faulting and vertical axis block rotation. However, through time, rift overlap is abandoned in favor of direct linkage which generates a series of localized en‐echelon basins. The present‐day direct linkage between the two rifts is supported by geodetic observations. Our study reconciles previously proposed models for the RS and GoA rift connection by considering spatial and temporal evolution of the rifts. Plain Language Summary: Rifts are places where tectonic plates move away from each other. They normally start as short, isolated features, and then grow and connect together to eventually form oceans. Central Afar in East Africa is a great location to study how these rifts form and grow by connecting with other rifts. In this area, the Red Sea (RS) and Gulf of Aden (GoA) rifts interact, but it is not clear how this happened through time. Some studies suggest that the two rifts form an overlap zone where blocks within the overlap rotate, while others argue that the two rifts directly link and form a continuous rift zone. To resolve this debate, we conducted a high‐resolution computer simulation of the evolution of the RS and GoA rifts in central Afar. We compared our model results with earthquake positions and satellite data that constrain the present‐day motion of the plates. Our results demonstrate that the RS and GoA rifts first overlapped for a few millions of years, and then formed a direct linkage. Our study suggests that both conceptual models can be reconciled when we consider the temporal evolution of the two rifts through geological times. Key Points: Analyzing the spatiotemporal evolution of rifts can address the controversial issue on the connection between propagating riftsWe present results from InSAR and geodynamic models to describe the connection between Red Sea (RS) and Gulf of Aden (GoA) rifts in central AfarThe connection between the two rifts evolves from rift overlap to direct linkage elucidating the observed deformation in the region [ABSTRACT FROM AUTHOR]

West Africa continues to host a growing number of low and intermediate‐magnitude earthquakes (M2‐5) along its passive margins, and its continental interior. Earthquake activity in these regions raises the need to comprehend the causes and the tectonic controls of the seismicity. Unfortunately, such studies are rare. Here, we apply single‐station inversion techniques to constrain fourteen focal mechanisms, computed after compiling a set of high‐quality waveforms. We describe the connection between seismicity, the contemporary stress field, anthropogenic activity and Holocene fault scarps in the region. Our results indicate transpressive stresses acting on the inherited brittle structures in the passive margins. We also observe a compressive regime in the intracontinental failed rifts. We attribute the seismicity to the reactivation of "weak" faults in the Neoproterozoic and Mesozoic failed rifts, the passive transform structures, and the intracratonic Precambrian brittle shear zones. Plain Language Summary: Earthquakes have occurred in West Africa, in the interior and the edges of regions that host several brittle structures. The causes and the mechanisms of this seismicity have not been comprehensively investigated. Hence, the characteristics of the regional tectonics are not fully established. In this study, we resolve the properties of the source of earthquakes, and the regional stress field. Our description of the sources of earthquakes matches the geometry of inherited brittle shear zones and failed rifts. The results indicate a strong influence of transpressive stress transfers from the ocean to the passive margins of West Africa. In the continental interior, especially, the West African Belt, the stress is compressional in the E‐W direction, consistent with stress orientations in adjoining regions. An extensional earthquake is observed in Mali, within the Gourma failed rift. We find that the seismicity of West Africa and the variations of the stress can be explained by the combination of preexisting faults, plate forces, ocean‐continent stress transfer, and density heterogeneity across the geologic domains. Key Points: Improved constraints on stress regimes and stress orientations from inversion of new focal mechanisms of M < 5.5 earthquakesNew focal mechanisms obtained from classical and novel approach based on single station waveform inversionEarthquakes in West Africa are rupturing the passive margin faults, intracontinental failed rift faults, and brittle shear zones [ABSTRACT FROM AUTHOR]

The northern Red Sea (NRS) is considered an extended continental region that has resulted in a rift system. Gravity and bathymetry data were used to estimate the Moho depth and the elastic thickness Te of the lithosphere beneath the NRS region to characterize its flexural rigidity and understand its mechanical behavior. Focusing on the Mabahiss Deep in NRS, we analyzed the lithosphere's flexural rigidity. The observed long-wavelength positive Bouguer anomaly is attributed to crustal thinning and lithospheric mantle uplift. The crustal thickness varies from 28 km in coastal areas to 24 km beneath the axial rift, supporting a regional compensation model over the Airy model. Forward modeling suggests that the optimal model explaining the regional Bouguer anomaly is a flexural model with Te equal to 7 km, indicating a weak and irregular continental crust. The primary factor contributing to this weakness is heating activity. Given the weakened state of the crust and the ongoing extension in the region, the NRS rift could evolve into a rupture, potentially leading to the formation of oceanic crust. [ABSTRACT FROM AUTHOR]

In the central Tarim Basin, numerous hydrocarbon deposits were found along ultra-deep strike-slip faults, and its evolving progress and formation mechanism are research hotspots. The Paleozoic small-angle X-type strike-slip fault in the Tabei Uplift is the research subject in this article. Based on high-precision three-dimensional seismic data, three structural deformation layers were revealed: the rift system, weak strike-slip deformation and salt tectonics in the deep structural layer (Sinian-Middle Cambrian), the strong strike-slip deformation and karst-dissolution structure in the middle structural layer (Upper Cambrian-Middle Ordovician), and echelon normal faults in the shallow structural layer (Upper Ordovician-Carboniferous). The formation and evolution of strike-slip faults is jointly controlled by the distribution pattern of basement rift and the activities of surrounding orogenic belts, which can be divided into three stages. In the Middle to Late Cambrian, the initial subduction of the Paleo-Asian and Proto-Tethyan oceans precipitated the emergence of two sets of small-angle X-type strike-slip faults, striking NW and NE above the grooves of Precambrian rifts, influenced by local weak compressive stress. Affected by the closure of peripheral paleo-ocean, strike-slip faults deformed considerably in the Middle-Late Ordovician and were reactivated in the Silurian-Carboniferous, forming en-echelon normal faults in the shallow layer. The layered deformation structure of the strike-slip faults significantly affects the accumulation of hydrocarbons. The differential hydrocarbon enrichment of faults in the Tabei Uplift is collectively influenced by the distribution of source rocks and the migration of oil and gas. The topographical features of the Tabei Uplift, along with the distribution of strike-slip faults across tectonic units, have rendered the NE direction the preferential pathway for hydrocarbon migration. Additionally, impacted by the development of en echelon faults, the NE-trending faults offer superior conditions for hydrocarbon preservation and charging condition, compared to the NW-trending faults. [ABSTRACT FROM AUTHOR]

This paper is concerned with the distribution of heat flow in the Baikal Rift Zone, the Shanxi-Liaohe Rift, the Jordan Rift, and in the Orly (Storoya) Trough in the northern Svalbard plate, as well as in the rift zone of Iceland. One notes an asymmetry in heat flow about the rift axial line compared with adjacent areas upon the background of its higher value. The origin of this asymmetry is due, not only to differences in the permeability of the faults in the sides of the rift structure, but also to planet-wide factors, in particular, the Coriolis force. The north–south rift structures considered here are situated in the northern hemisphere; their eastern flanksshow increased heat flow compared with the western flanks, and this is in agreement with the Coriolis force vector in the Earth's northern hemisphere. The geothermal asymmetry in oceanic crustal divergent zones as noted previously is also seen in pull-apart structures of continental rift zones. [ABSTRACT FROM AUTHOR]

Although many collisional orogens form after subduction of oceanic lithosphere between two continents, some orogens result from strain localization within a continent via inversion of structures inherited from continental rifting. Intracontinental rift-inversion orogens exhibit a range of structural styles, but the underlying causes of such variability have not been extensively explored. We use numerical models of intracontinental rift inversion to investigate the impact of parameters including rift structure, rift duration, post-rift cooling, and convergence velocity on orogen structure. Our models reproduce the natural variability of rift-inversion orogens and can be categorized using three endmember styles: asymmetric underthrusting (AU), distributed thickening (DT), and localized polarity flip (PF). Inversion of narrow rifts tends to produce orogens with more localized deformation (styles AU and PF) than those resulting from wide rifts. However, multiple combinations of the parameters we investigated can produce the same structural style. Thus, our models indicate no unique relationship between orogenic structure and the conditions prior to and during inversion. Because the style of rift-inversion orogenesis is highly contingent upon the rift history prior to inversion, knowing the geologic history that preceded rift inversion is essential for translating orogenic structure into the processes that produced that structure. [ABSTRACT FROM AUTHOR]

Extension tectonics responsible for intracratonic rift basin formation are often the consequences of active or passive tectonic regimes. The present work puts forth a plume-related rifting mechanism for the creation and evolution of two Proterozoic sedimentary basins outlining the Bundelkhand Craton, namely the Bijawar and Vindhyan basins. Using global gravity data, a regional-scale study is performed over the region encompassing the southern boundary of the Bundelkhand Craton consisting of the Bijawar Basin, Vindhyan Basin, and Deccan basalt outcrops. The gravity highs in the central part of the complete Bouguer anomaly and the upward-continued regional anomaly, derived from global gravity grid data, suggest that the Vindhyan sedimentary basin overlies a deeper high-density crustal source. The deepest interface as obtained from the radially averaged power spectrum analysis is observed to occur at a depth of ∼30.3 km, indicating that the sources responsible for the observed gravity signatures occur at larger depths. The 3D inversion of complete Bouguer anomaly data based on Parker–Oldenburg's algorithm revealed the Moho depth of ∼32 km below the Vindhyan Basin, i.e., south of the craton. The 2D crustal models along two selected profiles showcase a thick underplated layer with a maximum thickness of ∼12 km beneath the southern part of the Bundelkhand Craton. The inferred large E–W-trending underplating and deciphered shallower Moho beneath the regions south of the exposed Bundelkhand Craton point to crustal thinning compensated for magmatic emplacement due to a Paleoproterozoic plume activity below the craton margin. [ABSTRACT FROM AUTHOR]

Afar is undergoing the final stages of continental rifting and hosts the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rifts. To better understand the nature of the crust and continental breakup in the region, we calculate teleseismic receiver functions across northeastern Afar and the Danakil microplate, using new data from a regional deployment in Eritrea. We estimate the Moho depth and bulk crustal VP/VS ratio using the H‐κ stacking method. The heterogeneity of our crustal thickness estimates (∼19–35 km) indicates that the Danakil microplate has undergone stretching and crustal thinning. By investigating the relationship between crustal thickness and topographic elevation, we estimate the regional crustal bulk density as ρc ≈ 2,850 ± 20 kg m−3, which is higher than expected, given the crustal thickness of the region. We show that topography is 1.5 ± 0.4 km higher than would be expected due to crustal isostasy alone. We propose that this topography is supported by the same hot mantle upwelling suggested to be responsible for the onset of rifting in East Africa. Uplift is generated due to the presence of a hot thermal anomaly beneath the plate and by thinning of the lithospheric mantle. Our results are consistent with a number of independent constraints on the thermal structure of the asthenospheric and lithospheric mantle. Evidence of melt within the crust is provided by anomalously high VP/VS ratios of >1.9, demonstrating that magma‐assisted extension continues to be important in the final stages of continental breakup. Plain Language Summary: Afar is an area of northern Ethiopia that extends into Eritrea. It hosts three tectonic plate boundaries that are pulling apart from one another (rifting) as continental breakup is occurring. These rifting processes have led to a complicated tectonic history; isolating a small microcontinent (the Danakil) and giving rise to volcanism across the region. To better understand the nature of the crust, we study seismic data to estimate the crustal thickness and the ratio of seismic wave speeds. Our results indicate that the crust shows substantial variation in thickness, meaning that the Danakil microplate has undergone crustal thinning. We use our results to determine that dynamic mantle processes are responsible for supporting the elevation of the region. We also show that partially molten rock (magma) is likely to be present in the crust beneath northeastern Afar and the Danakil microplate, which is evidence that magma assists with continental breakup. Key Points: Receiver functions produce the first estimates of Eritrean bulk crustal propertiesThe crust of Afar and the Danakil microplate is denser than global average and highly heterogeneousEvidence found for melt within crust and for support from hotter mantle propping up topography beneath Afar and the Danakil microplate [ABSTRACT FROM AUTHOR]

Understanding how the Miocene N–S‐trending rifts on the southern Tibetan Plateau formed is crucial for understanding the evolution of the plateau. Most competing models suggest that all the rifts developed uniformly, but there are differences in magmatism among them. We conducted ambient noise tomography based on a broadband seismic array deployed across the rifts. A mid‐crustal low‐Vs layer extends laterally beneath the Tangra Yum Co Rift and the Pumqu‐Xianza Rift, implying that E‒W stretching of the ductile middle crust segmented the brittle upper crust, causing rifting and subsequent magmatism. In contrast, the Yadong‐Gulu Rift, which experienced prerifting magmatism, is characterized by an isolated low‐Vs anomaly extending subvertically from the surface to the middle crust, indicating that the crust experienced magmatic intrusion and local disruption, which promoted the later formation of a large rift. The results suggest that the various rifts might have distinct formation modes. Plain Language Summary: The rifts in the southern Tibetan Plateau are closely related to the growth and evolution of the plateau. These rifts are often considered as a whole in studies on their developmental modes. However, different rifts exhibit variations in terms of the timing of magmatism (before or after rifting). This implies that the developmental modes of different rifts are likely to involve different crustal deformation processes. To further investigate the formation modes of different rifts, we used the ambient noise method to obtain the crustal shear wave velocity structure. The presence of a low‐velocity layer in the mid‐crust indicates a vertical layering of crustal strength, which reveals that the upper crustal segmentation together with ductile deformation of the weak mid‐crust contributed to the formation of some rifts. In contrast, the presence of an isolated low‐velocity anomaly extending to the mid‐crust implies that a disruption of the lateral crustal strength caused the formation of another rift. Our findings will change the traditional understanding of the formation of N–S‐trending rifts and the evolution of the Tibetan Plateau. Key Points: The crustal structure beneath the rifts in southern Tibet is revealed by ambient noise tomographyRifts with post‐ or pre‐rifting magmatism lie above a mid‐crustal low‐velocity layer or a subvertical low‐velocity anomaly, respectivelyThe intrinsic differences in crustal structure indicate that the rifts in southern Tibet formed via different modes [ABSTRACT FROM AUTHOR]

Compositions of island arc and back-arc basin basalts are often used to trace the recycling of subducted materials. However, the contribution of subducted components to the mantle source during initial arc rifting before back-arc basin spreading is not yet well constrained. The northernmost Mariana arc is ideal for studying this because the transition from rifting to back-arc spreading is happening here. Here we report major and trace element and Pb isotopic compositions of olivine-hosted melt inclusions from lavas erupted during initial rifting at 24°N (NSP-24) and compare them with those in active arc front at 21°N and mature back-arc basin at 18°N. NSP-24 high-K melt inclusions have highly radiogenic Pb compositions and are close to those of the HIMU end-member, suggesting the presence of this component in the magma source. The HIMU-like component may be stored in the over-riding plate and released into arc magma with rifting. HIMU-type seamounts may be subducted elsewhere beneath the Mariana arc, but obvious HIMU-type components appear only in the initial stages of arc rifting due to the low melting degree and being consumed during the process of back-arc spreading. HIMU-type seamounts may be subducted elsewhere beneath the Mariana arc, but obvious HIMU-type components appear only in the initial stages of arc rifting due to the low melting degree and being consumed during the process of back-arc spreading. [ABSTRACT FROM AUTHOR]

The distribution and timing of Neogene extensional structures along the offshore Tanzania margin and their influence on submarine sediment dispersal pathways remain poorly constrained. This knowledge gap limits understanding of the propagation of the East African Rift System (EARS) in the western Indian Ocean. In this study, we use 2D and 3D seismic reflection data to explore a portion of the upper slope region offshore the Rufiji River delta which led to the discovery of a new extensional structure. Horizon maps and seismic sections extracted from the 3D volume reveal that the slope was intersected by W‐E‐oriented turbidite channels during the Cenozoic until the early Pliocene (5.3 Ma). Since then, the opening of this graben, whose timing is also constrained by stratigraphic horizon flattening, has led to a southward reorientation of these channels, a pattern that persists today, as evidenced by the flow direction of the channels at the modern seafloor. 2D seismic profiles reaching depths of 10 s two‐way travel time (TWT) indicate that the formation of this graben is not related to the reactivation of Mesozoic structures. In detail, seismic data show that the acoustic basement is intersected by extensional faults, likely related to the Jurassic rift tectonics, which is reactivated during the middle Cretaceous forming a gentle monocline. The lack of deformation in the post‐Cretaceous suggests a period of tectonic quiescence which persists until the establishment of a new extensional regime responsible for the graben's opening, indicating a decoupling between Mesozoic and Neogene tectonics. Considering the similarity in kinematics, orientation and timing between the graben and other structures along the margin, onshore and offshore, we interpret this graben to be generated by a later tectonic phase of the EARS. These new results may indicate that tectonic stresses associated with the EARS migrated from the Tanzania craton, where the oldest rift structures are dated to ca. 25 Ma, to the western Indian Ocean, where the tectonic activity started during the middle‐late Miocene to Pliocene. [ABSTRACT FROM AUTHOR]

The thick late syn‐ to early post‐rift shallow water evaporites in the most distal part of wide rifted margins is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning. Elevation of the distal margin and water depth during deposition of the late syn‐rift evaporites in the central South Atlantic are not well constrained and remain to be quantified. We use forward 2‐D thermo‐mechanical modeling coupled with melt prediction and surface processes to assess the contribution of lithospheric and mantle processes on the distal margin topography and subsidence history during continental rifting. Models show that (a) counter‐flow of depleted lower lithospheric mantle during rifting explains the magma‐poor nature of these margins and (b) weak crust and syn‐rift sediment control the wide crustal necking and subsidence history of the distal margin. Integration of our modeling results with quantified geophysical and geological observations suggests that (a) base level was down to −600 m below present‐day global sea level (bsl) during distal margin formation in the Aptian before sag and evaporite deposition, (b) base level was about −300/−400 m bsl at the end of evaporite deposition, and (c) scenarios with a fixed shallow base level (−400 m bsl) or with an increasing base level from an initially deep position (−1,600 m bsl) during evaporite deposition can both fit the observed evaporite distribution. However, erosional features along the base of evaporites suggest a deep initial base level. Plain Language Summary: The opening of the Atlantic Ocean contributed to the fragmentation of the Pangaea supercontinent 100–200 millions years ago. Water depth in the basins formed during this process is not well constrained while it is fundamental to understand their environmental conditions. For instance, in the central South Atlantic that comprises the west African Gabon, Congo, and Angola margins and their conjugates along the Brazilian margin, a giant evaporite basin deposited during continental breakup in shallow water formed very far from the continent. In this study, we compiled data along these margins and used 2‐D numerical models to show that an initially deep basin isolated from global ocean can provide the conditions necessary for this giant evaporite basin to be deposited. Key Points: Weak lower crust and syn‐rift sedimentation rate are first‐order controls on the formation and geometry of wide distal rifted marginModeling suggests a base level ∼−600 m below present‐day global sea level (bsl) during the distal margin formation before evaporite depositionModeling suggests a base level of ∼−300/−400 m bsl at the end of evaporite deposition [ABSTRACT FROM AUTHOR]

Volcanic islands are often subject to flank instability, resulting from a combination of magmatic intrusions along rift zones and gravitational spreading causing extensional faulting at the surface. Here, we study the Koaʻe fault system (KFS), located south of the summit caldera of Kīlauea volcano in Hawaiʻi, one of the most active volcanoes on Earth, prone to active faulting, episodic dike intrusions, and flank instability. Two rift zones and the KFS are major structures controlling volcanic flank instability and magma propagation. Although several magmatic intrusions occurred over the KFS, the link between these faults, two nearby rift zones and the flank instability, is still poorly studied. To better characterize the KFS and its structural linkage with the surrounding fault and rift zones, we performed a detailed structural analysis of the extensional fault system, coupled with a helicopter photogrammetric survey, covering part of the south flank of Kīlauea. We generated a high-resolution DEM (~ 8 cm) and orthomosaic (~ 4 cm) to map the fracture field in detail. We also collected ~ 1000 ground structural measurements of extensional fractures during our three field missions (2019, 2022, and 2023). We observed many small, interconnected grabens, monoclines, rollover structures, and en-echelon fractures that were in part previously undocumented. We estimate the cumulative displacement rate across the KFS during the last 600 ~ 700 years and found a decrease toward the west of the horizontal component from 2 to 6 cm per year, consistent with GNSS data. Integrating morphology observations, fault mapping, and kinematic measurements, we propose a new kinematic model of the upper part of the Kīlauea's south flank, suggesting a clockwise rotation and a translation of a triangular wedge. This wedge is bordered by the extensional structures (ERZ, SWRZ, and the KFS), largely influenced by gravitational spreading. These findings illustrate a structural linkage between the two rift zones and the KFS, the latter being episodically affected by dike intrusions. [ABSTRACT FROM AUTHOR]

Slab tearing is widely reported in oceanic slabs; however, tearing in continental slabs is still not very well understood. Geophysical data have shown the existence of tearing of the Indian lithosphere underneath the Yadong-Gulu rift in southern Tibet. Along this rift, the Jiacun lamprophyres and the Yangying trachytes comprise the youngest alkaline volcanic rocks in the Himalayan-Tibetan orogen, and hence provide evidence for understanding the operation of continental slab tearing. Jiacun lamprophyres, with an age of 13 Ma as determined by Ar-Ar dating, are the only outcrop of alkaline volcanic rocks in the Himalayan block. Geochemical analysis indicates that they were derived from a peridotite source in the Indian lithospheric mantle near the spinel field. Yangying trachytes, dated at 8.81 ± 0.15 Ma by the U-Pb dating method, were derived from a pyroxenite melt in the Tibetan lithospheric mantle with a higher crustal influence. Both sites show high phlogopite and pyroxene temperatures, indicating a hot influx favoring the melting of these magmas, which is likely associated with the tearing of the Indian slab. Ages of this magmatism suggest that the activity along the rift lasted at least 4 m.y. and migrated from south to north. This shows that slab tearing can trigger over-thickened lithospheric melting in a collisional orogen. [ABSTRACT FROM AUTHOR]

The volcanic aquifer of Bilate River Basin (BRB) situated on the southwestern side of the Main Ethiopian Rift (MER) serves as the sole reservoir of water supply for urban and rural people found in the basin. This study investigates residence time and groundwater recharge process in the Bilate River Basin. For the first time, this research work delivers information on the resident time and groundwater recharge on the basis of multi-tracer approach (CFCs, Cl, δ18O and δD). The δ18O and δD values of groundwater are drawn along or close to local Addis Ababa Meteoric Water Line (AAMWL), indicating that groundwater recharge is of meteoric origin. CFC data show that the oldest component of groundwater was recharged before 1950, while the younger component recharged in different time as of 1950. The assigned apparent age for groundwater range from 24 to more than 50 years. Binary mixing model is used to estimate the age and fraction of the younger component. Mixing model suggest that groundwater consists young groundwater that mainly recharged the aquifer between the mid-1980s and mid-1990s. The proportion of young water in the groundwater shows decreasing trend from highland towards the rift indicating the main recharge area is the highland part. The chloride mass balance employed to Bilate River Basin calculated yearly average groundwater recharge to be 217 mm/y which accounted for 20% of the mean annual areal rainfall. The estimated recharge decrease from highland to the rift floor highlighting the importance of preferential flow recharge mechanism. The analysis of apparent age, young proportion and recharge rate, can aid in understanding complex groundwater flow system and residence time of groundwater in the complex hydrogeological setting of rift volcanic aquifer. [ABSTRACT FROM AUTHOR]

The Eastern Cuban block has experienced a complex tectonic history characterized by plate interactions, resulting in a diverse array of geological features observable in the offshore sedimentary record. We investigate the tectonic evolution of offshore Eastern Cuba, specifically in the Old Bahamas Channel and its surrounding areas, by integrating multi‐channel seismic (MCS) reflection and published geological data. Our analysis employs stratigraphic frameworks and MCS data to assess deformation and key geological events in the region. We highlight the complex tectonic history of the Eastern Cuban block, marked by significant geodynamic events, such as rifting, the subduction of the oceanic Proto‐Caribbean plate, and syn‐orogenic and post‐orogenic phases. The seismic units observed in the majority of the study area reveal the early evolution of the Northern Proto‐Caribbean margin, subsequently impacted by the Cuban orogeny and the reactivation of the Cuban Transform Fault zone corresponding to a former plate boundary. We propose estimated ages for the seismic sequences, correlating them with available well data from neighboring regions. This study offers valuable insights into the tectonic history and geological evolution of offshore Eastern Cuba, contributing to a more comprehensive understanding of the region's geodynamic development. Key Points: The Old Bahamas Channel offers insights into the Caribbean's early tectonic history dating back to the Pangea breakupBahamian carbonate banks formed as reefs on top of tilted blocks following the Pangea rift and were later affected by fault reactivationThe Eocene northern Caribbean plate boundary aligns with the Cuban Transform Fault, reactivated in the Cuban orogenic and post‐orogenic phases [ABSTRACT FROM AUTHOR]

The Beyşehir-Hoyran Nappes, one of the tectonostratigraphic units of the Taurides, are thought to be originated from the Izmir-Ankara Ocean (northern branch of Neotethys). In this study, Late Paleozoic rock units from the blocks of Beyşehir-Hoyran Nappes were studied in detail using foraminiferal assemblages in two different locations from the southwest of Karaman City (southern Turkey). In both places, blocks/slices and pebbles of various origins are embedded within a highly sheared matrix of Late Cretaceous Age, and the whole unit can be regarded as a sedimentary mélange. The ages of the blocks from the southwest of Karaman City range from the Late Serpukhovian (Late Mississippian) to Late Capitanian (Middle Permian) with some depositional breaks (e.g., Bashkirian, Kasimovian). Combined with the previous data from the Mersin Mélange, which also include the remnants of the Beyşehir-Hoyran Nappes, our new findings suggest that a shallowing-upward sequence, characterized by a shallow water environment with foraminifera-bearing limestones, was deposited over the Tournaisian pelagic sequence in the Beyşehir-Hoyran Nappes till the Early Moscovian (Early Middle Pennsylvanian). This shallowing-upward sequence in the Beyşehir-Hoyran Nappes could be related to the Late Paleozoic Glaciation on the Gondwana supercontinent (Glacial II), which resulted in a sea-level drop and deposition of platform carbonates during the Viséan–Early Moscovian (Middle Mississippian to Early Middle Pennsylvanian) time interval. The absence of the main part of the Middle-Upper Pennsylvanian deposits (continental phase during the Middle Moscovian–Middle Gzhelian) in the Beyşehir-Hoyran Nappes can be mainly attributed to the occurrence of a mantle plume and partially to the effect of Late Paleozoic Gondwanan Glaciation (Glacial III). Progressive uplifting by the buoyant mantle plume material has resulted in rifting at the center of the basin where the Beyşehir-Hoyran Nappes have deposited. The rifting process led to tectonic destabilization of the platform in the basin, causing accumulation of the Upper Gzhelian (uppermost Pennsylvanian) detrital limestone with broken and abraded foraminiferal shells. Following this, deep basinal conditions prevailed during the Late Asselian–Kungurian (Early Permian), as revealed in the Mersin Mélange, where radiolarian cherts are associated with continental within-plate lavas of extreme incompatible trace element enrichment. Similar processes were responsible for the continual deposition of detrital limestones in the same basin until the end of Late Capitanian (Middle Permian). Based on all these, the uplifting process followed by rift-related volcanic rocks and detrital limestones can be interpreted as the opening of the Izmir-Ankara Ocean (northern Neotethys). [ABSTRACT FROM AUTHOR]

The Meso-neoproterozoic Bayan Obo rift is located along the northern margin of the North China Craton, and was associated with the break-up of the Columbia supercontinent. During rift evolution, syn-sedimentary deformation occurred due to tectonic activity and earthquakes. Seismic events are recorded in the Jianshan Formation of the Bayan Obo Group, Inner Mongolia, as soft sediment deformation structures in the central Bayan Obo rift. Druse calcite crystals and collapse breccias in the Jianshan Formation may provide information on the rift evolution. The druse calcite crystals are idiomorphic-columnar in shape and associated with graphite, pyrite, and quartz. δ13C values of the graphite are −20‰, indicative of biogenic deoxygenation and formation in water. The druse calcite crystals are inorganic in origin and formed in water at a temperature of 55 °C, based on calcite δ13C and δ18O data. The calcite grew in paleo-caves containing fault breccias, with heat derived from faulting. As such, the druse calcite crystals are important evidence for seismic events. The collapse breccias (i. e., fault breccias) and other indicators of slip show that displacement occurred from NE to SW, which is different from the paleocurrent direction in the Jianshan Formation. The thickness of the collapse breccia is ∼200 m, which represents the height of the fault scarp. The strike of the fault scarp was NE-SW, based on the distribution of the collapse breccia. The Bayan Obo and Yanliao rifts experienced rapid NW-SE extension, and developed similar deformation structures at ca. 1.6 Ga related to break-up of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

The interactions among dip-slip and strike-slip faults are critical features in rift segmentation, including strain and slip transfer between faults of different rift segments. Here, we focused on the influence of factors such as fault and fracture geometries, kinematics, and local stress fields on the interaction and linkage of synchronous strike-slip and normal faults. Well-exposed faults along the tectonically active boundary between the central and northern Basin and Range provided for both reliable geometric data and consideration of rift segment development. We documented relative ages and distributions of Quaternary deposits, scarps, and geometries of three ~20-65-km-long Quaternary faults: the N-striking, normal Coyote Spring fault; the ENE-striking, left-lateral Kane Springs Wash fault; and the N-striking, normal Wildcat Wash fault. The normal faults bend to accommodate slip-type differences across linkage zones, with the strikeslip fault and local processes influencing interactions. Influenced by the local stress field of the Kane Springs Wash fault, the Coyote Spring fault bends SE as it approaches and links to the Kane Springs Wash fault. Influenced by the off-fault or process-zone fractures of the Kane Springs Wash fault, the Wildcat Wash fault bends NE and links with the Kane Springs Wash fault. The Kane Springs Wash fault continues beyond the normal fault terminations, suggesting slip transfer between them via the Kane Springs Wash fault. These relations and the ages of offset units suggest that activity on the faults was approximately synchronous despite slip-type differences. Consequently, in slip transfer, the local strike-slip stress environment and off-fault fractures influenced the geometry of the normal fault terminations; the strike-slip fault formed a boundary to dip-slip fault propagation; and this boundary facilitated kinematic and geodetic segmentation, forming a Basin and Range rift segment boundary. [ABSTRACT FROM AUTHOR]

The upper mantle and the transition zone of the Baikal rift zone (BRZ) are studied. The observations are analyzed from P-wave receiver functions. It is found that in the BRZ central and northeastern part, the P410s converted seismic phase is preceded by a precursory wave with negative polarity which is formed in the low S-wave velocity layer at a depth of 350–410 km. A similar precursory wave with low S-wave velocity and negative polarity is formed at a depth of 600–660 km. The low-velocity layers are interpreted as resulting from the hydration of wadsleyite and ringwoodite during the subduction of the Pacific lithosphere. A similar study of the mantle in Central Mongolia found no expected signs of hydration. The modeling of the lithosphere–asthenosphere system in Central Mongolia by joint inversion of the body wave receiver functions and surface wave dispersion curves reveals a very thin lithospheric lid beneath Khangai and a thick layered asthenosphere down to a depth of 200 km with a lithospheric inclusion between low-velocity layers. [ABSTRACT FROM AUTHOR]

The study of the volcano–sedimentary sequence in the lower part of the Zaonega formation in the Paleoproterozoic Onega structure (Karelian craton, Fennoscandian Shield) has shown that tuffaceous and high-silica rocks predominate in its composition. High-silica rocks (SiO2 up to 94 wt %) are depleted in all elements and probably represent chemogenic siliceous silts. Tuffaceous rocks vary broadly in composition and were formed during mixing of tuffogenous basalt material and high-silica chemogenic sediments. In terms of levels of contents and the distribution character of trace elements, tuffogenous rocks are close to N-MORB volcanic rocks. This rock association is typical of the early stages of continental rifting in the Phanerozoic and may indicate the formation of volcano–sedimentary complexes of the Zaonega formation in the environment of continental rifting. The tuffaceous rocks in the lower part of the Zaonega formation are geochemically identical to dolerite dikes and MORB-type volcanic rocks 2.10–2.14 Ga in age. Their formation was probably related to this episode of large-scale extension and thinning of the continental lithosphere of the Karelian craton in the Middle Paleoproterozoic. In this case, the age limit of the Zaonega and underlying Tulomozero formations should be somewhat older than the 2.06–2.10 Ga interval accepted in the modern regional stratigraphic schemes of the Paleoproterozoic Fennoscandian shield. [ABSTRACT FROM AUTHOR]

The tectonic evolution of the Lishui Sag in the East China Sea Shelf Basin significantly influences its sequence stratigraphy. By integrating seismic reflections, well logs and core data, this study examines the quantitative activity of the Lingfeng fault, sequence framework and sequence architecture in the Lishui Sag. The Paleocene succession is divided into one second-order sequence (SSQ1), three third-order sequences (SQ1–SQ3) and nine system tracts, representing a complete syn-rift evolution consisting of initial syn-rift, rift climax and late syn-rift stages. The results indicate that the differential activity of the Lingfeng fault shapes the basin structure and controls the subsidence centre, leading to prominent variations in the spatial distribution and filling of the sequence stratigraphy during different tectonic stages. The initial syn-rift sequence (SQ1), characterised by a low activity of the segmented Lingfeng fault, is primarily found in isolated small sub-sags. The rift climax sequence (SQ2), with increasing displacement and longer segmented fault zones, exhibits a wedge-shaped filling pattern with substantial topographic elevation differences. The late syn-rift sequence (SQ3) shows a gradual decrease in tectonic activity and uniform filling, with all system tracts within the sequences fully developed. The sequence architecture of the hanging wall in the Lishui Sag can be classified into fault-scarp and fault-slope type, whereas the sequence architecture of the hanging wall dip-slope can be classified into sedimentary slope-break and faulted slope-break. The Lingfeng fault controlled the tectonic evolution, sedimentary subsidence centre and sequence filling and architecture in the Paleocene rifted Lishui Sag. The Paleocene succession is divided into one second-order sequence (SSQ1), three third-order sequences (SQ1–SQ3) and nine system tracts, representing a complete syn-rift evolution consisting of initial syn-rift, rift climax and late syn-rift stages. The sequence architecture of the hangingwall can be classified into fault-scarp and fault-slope type, and hanging wall dip-slope sequence architecture can be divided into sedimentary slope-break and faulted slope-break types. [ABSTRACT FROM AUTHOR]

The identification of numerous quasilinear chains of earthquakes in the epicenter field of seismicity in the Baikal region and a study of these chains showed that the chains may include, not only chains of "migrating" earthquakes, but also those arising in a random space–time distribution of earthquakes. The present statistical study of the distribution of interepicenter distances showed that "migrating" chains may be formed within clusters of seismic events, and the distribution has been investigated. We note that chains in earthquake clusters can be identified based on the distribution of time, as well as from the distribution of distances. We have found the formation of earthquake chains in the areas of the following clusters of earthquakes: the Busingol earthquakes of 1976 and 1991; the South Baikal, the Kyakhta, and the Kichera earthquakes in the Tompuda sequence, the Oldongsin and Charuoda clusters. We showed that the chains were formed during the occurrence of these clusters. The areas of concentrated chains of clustered seismicity are found to have a relationship of the positions and directivity of the chains to the strike of fault zones, to near-fault cracks, and to the orientation of nodal planes at earthquake foci. [ABSTRACT FROM AUTHOR]

Three-dimensional magnetotelluric (MT) imaging in central Colorado reveals a set of north-striking high-conductivity tracks at lower-crustal (50-20 km) depths, with conductive finger-like structures rising off these tracks into the middle crust (20-5 km depth). We interpret these features to represent saline aqueous fluids and partial melt that are products of active extensional tectonomagmatism. These conductors are distributed over a wider region than the narrow corridor along which Rio Grande rift structures are traditionally mapped at the surface, and they consequently demarcate regions of the lower crust where accommodation of bulk extensional strain has concentrated conductive phases. Our observations reveal limitations in existing models of Rio Grande rift activity and may reflect unrecognized spatiotemporal variations in rift system evolution globally. [ABSTRACT FROM AUTHOR]

The dominant tectonic mode operating on early Earth (before ca. 2.5 Ga) remains elusive, with an increasing body of evidence suggesting that non-plate tectonic modes were likely more prevalent at that time. Thus, how plate tectonics evolved after that remains contentious. We performed two-dimensional numerical modeling of mantle convection at temperatures appropriate for the Hadean-Archean eons and show that subduction and rift systems may have spontaneously emerged on Earth from an earlier drip-and-rift-dominated tectonic mode in response to the secular cooling of the mantle. This cooling of the mantle was mediated by repeated events of rifting and dripping that likely occurred over a few hundred million years. As the mantle cooled, its effective viscosity and the thickness and strength of the lithosphere increased, which helped establish rigid plates and initiate plate tectonics on Earth. [ABSTRACT FROM AUTHOR]

Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the topmost mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface that develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling-hinge model of Buck (1988) and compare modelling results with published seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on published seismic reflection profiles can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Our modelling supports the hypothesis of Lymer et al. (2019) that the S reflector on the Galician margin is a sub-horizontal detachment generated by the in-sequence incremental addition of the isostatically rotated soles of block-bounding extensional faults. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking, and the development of new high-angle shortcut fault segments within the hanging wall. This results in the transfer and isostatic rotation of triangular pieces of hanging wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few kilometres in lateral extent and thickness. The initial geometry of basement extensional faults is a long-standing question. Our modelling results show that a sequence of extensional listric or planar faults with otherwise identical tectonic parameters produce very similar seabed bathymetric relief but distinct Moho and allochthon shapes. Our preferred interpretation of our modelling results and seismic observations is that faults are initially planar in geometry but are isostatically rotated and coalesce at depth to form the seismically observed sub-horizontal detachment in the topmost mantle. In-sequence extensional faulting of hyper-extended continental crust results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast, out-of-sequence faulting results in a transition to exhumed mantle with bathymetric relief. [ABSTRACT FROM AUTHOR]

In East Africa and the Arabian Peninsula, Mesozoic and/or Tertiary rift basins are well developed. These rift basins have proven their importance to economic development of the region through major oil discoveries including the Jobi-Rii field in the Albertine basin, Uganda, the Ngamia field in Lokichar Basin, Kenya and the oil fields of Yemen. The Main Ethiopian Rift (MER) developed during the Oligocene–Miocene time as part of the East African Rift System and is commonly divided into the Northern, Central and Southern parts. The subsurface rift architecture of the MER remains poorly understood due to lack detailed geophysical studies. This study aims to investigate the subsurface stratigraphic and structural setup of the Southern MER by using 2D high-resolution seismic reflection surveys integrated with nearby well data. Analysis of 2D-seismic reflection data in the Southern MER reveals four horizons from which three horizons show good correlations in Northern Abaya and Gelana basins. The basement structure delineated the main basins in the study area: Northern Abaya, Southern Abaya, Chamo and Gelana sub-basins. The sub-basins showed a typical rift-basin development: pre-rift, syn-rift and post-rift sedimentation and the faulted basement is dominantly oriented NE–SW parallel to the regional structural trend of the area. The deepest basement is greater than 3600 m in Northern Abaya basin followed by Gelana Basin which reaches to 3100 m. Southern Abaya and Chamo basins have shallow basement depths less than 2250 m. Interpretation of the interval velocity integrated with the geology and nearby well data helped to identify the different stratigraphic units ranging from Jurassic to Quaternary time span in the area. Possible source rocks of Late Jurassic to Upper Cretaceous marine shales and fluvial–lacustrine sediments of Lower Miocene thickness 1200 m and 1800 m are identified in Northern Abaya Basin, respectively, where the thicknesses of those units are 900 m and 1000 m in Galena Basin. We also infer from seismic interpretation that the upper Miocene sand/sandstone inferred on both sub-basins can be considered as a reservoir, whereas distinctive fault closures against the basement with horst and grabens or tilted half grabens and anticlinal structures can smoothly provide the hydrocarbon trapping mechanism. Numerous faults mapped on seismic sections play a major role for migration of hydrocarbon from source to reservoir rocks. The interpreted NE–SW major faults located on the western side of most of the seismic sections reactivated during the East African rifting at Neogene period produced an appreciable throw which can also provide migration pathways for the hydrocarbon. [ABSTRACT FROM AUTHOR]

Although many studies link mineral deposit formation to rifting and hydrothermal processes, we present a study that focuses on the relationship between crustal necking and mineral deposit formation. Necking corresponds to the timing, location, and process of rift localization and abrupt crustal and lithospheric thinning. Although necking is well identified and described from present-day rifted margins and has been modeled numerically, little is known about the necking process and its possible link to ore deposit formation. We present observations from the Mont-Blanc fossil detachment system, one of the few exposed examples of a necking detachment fault. We show that fluids flowed along the fault zone and leached metals (mainly Pb and Zn). This process was associated with the hydrothermal breakdown of feldspar and biotite at temperatures of 200 °C and salinities ranging from 5 to 20 eq. wt% with a H2O-NaCl (-KCl) composition. The resulting metal-rich fluids reacted with mainly carbonate-rich units to form Pb-Zn ore deposits in basement and sedimentary cover rocks. A direct link can, therefore, be demonstrated between fluid and reaction-assisted breakdown of silicates, metal transfer and trapping along detachment faults, and the overlying sedimentary rocks during necking. Similar ore deposits can be found throughout the inner External Crystalline Massif of the Western Alps, interpreted as the former necking domain of the Alpine Tethys. This leads to the suggestion that necking and Pb-Zn deposit formation may be closely linked, a hypothesis, if correct, that has the potential to predict additional Pb-Zn-Ba-F resources in rifts, rifted margins, and reactivated fossil rifted margins forming collisional mountain belts. [ABSTRACT FROM AUTHOR]

The Midcontinent Rift System of North America is one of the oldest continental rifts but rifting ceased before continental breakup. The southern segment of the Midcontinent Rift System lies in Kansas, USA, where the stratigraphic succession and rift evolution are largely unknown. This study analysed the rift basin infill in this part of the Midcontinent Rift System to propose a depositional model. The Precambrian rift succession was described in discontinuous cores drilled in the Texaco Noel Poersch#1 well in Washington County. Sixteen lithofacies were identified and grouped into four different facies associations (fluvial, aeolian, lacustrine and alluvial fan). Overall, the studied succession comprises continental deposits accumulated dominantly in alluvial and aeolian settings, with the intermittent development of lacustrine systems. The proposed depositional models for the available core intervals indicate cyclic patterns of overfilled and underfilled phases within the rift basin. These changes in the accommodation‐to‐supply ratio were controlled by tectonism and probably modulated by climate during evolution in the syn‐rift phase. This study advances our understanding of variations across the Midcontinent Rift System. [ABSTRACT FROM AUTHOR]

Granitic batholiths of the ∼500 Ma Ross Orogen in Antarctica are voluminous in scale, reflecting prolific magmatism along the active early Paleozoic convergent margin of Gondwana. New age and isotopic analysis of zircons from a large suite of Ross granitoids spanning >2,000 km along the orogen provide a wealth of geochronologic, tracer, and inheritance information, enabling us to investigate the pace of magmatism, along‐strike temporal and geochemical trends, magmatic sources, and tectonic modes of convergence. Because granitoids penetrate the crust of the earlier Neoproterozoic rift margin, they also provide insight into the age and composition of the largely ice‐covered East Antarctic craton. Zircon U‐Pb ages from these and other samples indicate that active Ross magmatism spanned 475–590 Ma, much longer than generally regarded. Most samples have heavy zircon δ18O values between 6.5 and 11.5‰ and initial εHf compositions between 0 and −15; their isotopic co‐variations are independent of age, as in other contemporary continental arcs, and reflect largely crustal melt sources. Samples near Shackleton Glacier have distinctly more mantle‐like isotope composition (i.e., radiogenic εHf and low δ18O) and separate two regions with distinctive isotopic properties and inheritance patterns—a more juvenile section of Mesoproterozoic crust underlying the southern TAM and an older, more evolved region of Paleoproterozoic and Archean crust in the central TAM. The isotopic discontinuity separating these regions indicates the presence of a cryptic crustal boundary of Grenvillian or younger age within the East Antarctic shield that may be traceable into the western Laurentian part of the Rodinia supercontinent. Plain Language Summary: Isotopic information obtained from granitic rocks formed during subduction‐zone convergence can help distinguish mantle and crustal sources of melting, evaluate the processes leading to chemical changes in the magmas, and reveal information about the age and composition of rocks in the upper plate. The Neoproterozoic to early Paleozoic Ross Orogen in Antarctica is a significant part of the active Gondwana‐margin convergent plate boundary stretching from present‐day eastern Australia to Argentina, but it is largely obscured by ice and younger sedimentary cover. The Ross Orogen hosts a prolific and widespread continental‐margin magmatic arc dominated by granitoid intrusions that serve as geochemical "samplers" of upper‐plate rocks with which they interact. We report U‐Pb ages and O‐Hf isotopic compositions in zircons from nearly 40 samples of Ross Orogen granitoids covering a large section of the modern Transantarctic Mountains that reveal patterns in magmatism best explained by variations in composition of the cratonic upper plate. They thus help to identify an otherwise hidden lithospheric boundary separating a region of juvenile Mesoproterozoic crust from evolved Paleoproterozoic and Archean crust that may represent a tectonic suture formed during Rodinia amalgamation. Key Points: Subduction‐related granites are effective geochemical samplers of the upper plate lithosphereU‐Pb, O and Hf isotope data from zircons in Ross Orogen granitoids reveal variations in age and magma sourcesContrasting magma compositions distinguish the East Antarctic upper‐plate lithosphere and reveal key boundaries related to Rodinia assembly [ABSTRACT FROM AUTHOR]

Core complex type structures are common in continental margins. Recent studies have proposed exhumation of deeper crustal and mantle levels during extension of the continental margin along the Northern South China Sea (NSCS) in a core complex mode. Here, we present high-resolution seismic profiles across different sections of the NSCS that allow us to observe two types of core complex systems. The Kaiping Rift and eastern Baiyun Rift are characterized by a sub-horizontal Moho and exhumation of ductile deeper crust beneath an extremely thinned brittle upper crust. The central Baiyun Rift in contrast is marked by homogeneous thinning of the entire crust that is floored by mantle unroofing with a concave-down Moho geometry. While the former type, referred to as crustal core complexes, coincides with relatively intense syn-rift magmatism, the latter, referred to as mantle core complexes, is marked by limited magmatism. Consistently, basin subsidence rates are lower over crustal core complexes than mantle core complexes. Compared to mantle core complexes at the magma-poor Iberian margin, the widely distributed crustal core complexes in the NSCS indicate higher geotherms and more intense magmatism during extension of the continental lithosphere. We propose that the continental core complexes observed at the NSCS represent a typical outcome of extension of a gravitationally unstable lithosphere. [ABSTRACT FROM AUTHOR]