Construction Of Boundaries Research Articles

Microseismicity Around Loki's Castle Hydrothermal Vent Field Reveals the Early Stages of Detachment Faulting at the Mohns‐Knipovich Ridge Intersection

AbstractAt slow to ultraslow spreading ridges, the limited melt supply results in tectonic accretion and the exhumation of mantle rocks. Melt supply is focused toward volcanic centers where magmatic accretion dominates. In areas where the ridges reorientate, both types of accretion can occur across the ridge axis with detachment faults developing on the inside corners and hydrothermal vent fields located in close proximity. Microseismicity studies improve the understanding of the tectonic processes at detachment faults and their interplay with hydrothermal vent systems, but are mostly limited to mature detachment faults or short deployment times. This study presents results from a ∼11 months ocean bottom seismometer deployment around the Loki's Castle hydrothermal vent field at the intersection of the slow to ultraslow spreading Mohns and Knipovich Ridge. We observe seismicity to be highly asymmetric with the majority of the plate divergence being accommodated by an emerging detachment fault at the inside corner of the intersection west of Loki's Castle. Seismic activity related to the detachment fault displays a distinct contrast, with continuous low‐magnitude events occurring at depth and episodic large‐magnitude events concentrated in clusters within the footwall. The detachment fault shows no significant roll‐over at shallow depths and the locus of spreading is located east of the detachment. These results suggest that the detachment fault west of Loki’s Castle is at an early development stage.

Evolution of volcanism and geodynamic settings in the Sakmara zone of the Southern Urals in the early Paleozoic

Research subject. Northern part of the Sakmara zone of the Southern Urals, represented by the Krakinsko-Mednogorsk paleovolcanic belt, including the Mednogorsk ore district and the Blavinsko-Komsomolskoye ore field. Aim. To reconstruct the paleogeodynamic settings during the Cambrian–Early Devonian in the Sakmara zone. Methods. An analysis of geological, petrogeochemical, structural-tectonical, and paleovolcanological data obtained by the authors and those reported in literature using conventional and new petrogeochemical diagrams. Results. The following stages of tectono-magmatic evolution in the Sakmara zone were distinguished: (1) Cambrian–Early Ordovician, continental rifting; (2) Early–Middle Ordovician, oceanic spreading; (3) Early Silurian, suboceanic rifting, serpentinite protrusions, edaphogenic breccias, siliceous rocks (starting a subduction zone); (4) Late Silurian–Early Devonian, island-arc basalt-rhyolite volcanism and sulfide deposits, volcanism of the shoshonitic and alkaline series, formation of the rear island arc. The Sakmara zone features no analogues of boninite volcanism (D1e2) typical of the West Magnitogorsk zone, which indicates the absence of signs of the volcanogenic strata of the Voznesensk–Prisakmar zone being pushed into the Sakmara structural zone. Conclusions. (1) In the Cambrian–Early Devonian, volcanogenic and volcanogenic-sedimentary formations and strata of the Sakmara zone formed a megacycle, starting with the processes of continental and oceanic rifting, which led to the subduction process and generation of the frontal and rear island arcs. (2) The volcanism in the Sakmara and Voznesensk–Prisakmar zones developed autonomously, without tectonic transfers from east to west. (3) The partial coincidence of the formation age of the Chanchar (D1lh–e1) complex of the Sakmara zone and the Turin complex (S2p–D1lh) of the Tagil zone suggests the presence of a reduced continuation of the western wing of the Tagil zone in the Sakmara zone.

Strain Localization at Volcanoes Undergoing Extension: Investigation of Long‐Term Deformation at Krafla and Askja Volcanic Systems in North Iceland

AbstractVolcanoes in extensional environments may show gradual subsidence over decades during quiescent periods, due to various processes such as magma withdrawal, cooling, contraction, plate spreading and viscoelastic response. If significant rheological anomalies reside in volcano roots, due to the presence of magma and hot rock, they can influence the style of deformation. We use Finite Element Method (FEM) models to explore how strain localization due to extension can lead to volcano deflation. We apply rheological models comprising an elastic layer overlying a viscoelastic domain and include local up‐doming regions of low viscosity material beneath volcanic centers. The models reveal a localized subsidence above the rheological anomaly, influenced by the tectonic extension, and by the up‐doming volume and its viscosity. The models suggest that plate divergence may account for 4–5 mm/yr of observed subsidence at Krafla and Askja volcanic systems (KVS and AVS, respectively) in North Iceland.

Oceanic geodiversity along back-arc spreading centers reveals analogies with mid-ocean ridges

Oceanic geodiversity provides essential information on the dynamics of the Earth. Here, we focus on the geodiversity of three oceanic back-arc spreading centers: the Mariana Spreading Center, the Central-Southern Lau Basin spreading centers, and the East Scotia Ridge. We defined a method to identify their axial zones, obtaining spreading center depths along the basins. Results improve global plate boundary models and morphology variations, revealing that the average depths along the Mariana, East Scotia, and Lau Basin spreading ridges are 4.5, 3.5, and 2 km, respectively. We also measured new spreading rates based on five magnetic profiles crossing the three back-arc spreading centers, contributing to plate kinematic models. Furthermore, we computed subduction rates, including hinge velocities along the Mariana, South Sandwich, and Tonga Subductions, to understand the existing interactions between the subducting slab hinge motion and the kinematics of their related back-arc spreading centers. Our bathymetric, magnetic, and kinematic data show several differences among the Mariana, the East Scotia, and the Lau spreading centers, stressing the oceanic geodiversity in a similar geodynamic context. Our results also suggest a strong correlation between axial depth and full spreading rates along the back-arc spreading centers, a geological correspondence that allows a similar description of these divergent plate boundaries within the mid-ocean ridge classification. Finally, we show how hinge kinematics affects the relationship between convergence along subduction zones and back-arc spreading rates. All our findings contribute to understand how the oceanic geodiversity is directly related to geodynamic processes, increasing the knowledge of global tectonics.

Late Cambrian-early Ordovician extensional magmatism, uplift and sedimentation of the N Gondwana margin: new field, geochemical and geochronological data from the peri-Gondwanan Central Sakarya Terrane, Sakarya Zone, NW Turkey

ABSTRACT The timing and mechanism of the crustal extension that caused blocks of continental crust to rift and drift northwards from the northern Gondwana margin during the Early Palaeozoic are currently debated. Here, we present new field, geochemical and U-Pb zircon data from peri-Gondwanan Central Sakarya terrane in NW Turkey to shed light on the timing and petrogenesis of extensional magmatism and subsequent passive margin development of the northern Gondwana margin. Our 1/25.000 scale mapping of a selected area (250 km2) has revealed a south-vergent thrust stack comprising three slices. The Middle Slice of the thrust stack consists of a gneiss-amphibolite complex that has a basement-cover type stratigraphy (before regional metamorphism). The basement unit consists of meta-granitoids, intruded by meta-syenite and banded amphibolites, with geochemical characteristics reflecting I&S-type, A-type and within plate alkaline/transitional-type magmatism, respectively. The cover meta-clastic sediments have amphibolite lenses with MORB and back-arc basin-type geochemistry. Meta-granitoid bodies yielded zircon U-Pb ages of 507 ± 16 Ma, 503 ± 12 Ma, and 488 ± 2.6 Ma (Late Cambrian-Early Ordovician), whereas the meta-syenites gave ages of 473.9 ± 5.7 Ma and 471.4 ± 2.5 Ma (late Early Ordovician); also, age of the banded amphibolite was dated as 473.3 ± 4 Ma. The youngest detrital zircon population in the pebbly quartzite sample within cover meta-sediments (458 ± 3 Ma) constrains the maximum depositional age to around early Late Ordovician. The late Early Ordovician bimodal magmatism, a key focus of our study, is interpreted to have formed in response to the Rheic Ocean rifting along the northern margin of Gondwana. The Late Cambrian granitic magmatism, a precursor to bimodal magmatism, can be explained by lithospheric thinning and upwelling of the asthenosphere to form anatectic silicic melts. The unconformity between the basement and cover units may represent the initiation of a new phase of extension at the N Gondwana margin that culminated in the northward drifting of the Central Sakarya and the opening of Palaeotethys, associated with back-arc basic magmatism in Late Ordovician-Silurian.

Plate tectonics through Earth’s history: constraints from the thermal evolution of Earth’s upper mantle

ABSTRACT Temporal changes in Earth’s tectonic style play a crucial role in understanding the planet’s evolution. Modern-style tectonics is characterized by the formation of basaltic crust at divergent plate boundaries and its subsequent recycling at subduction zones, accompanied by wedge mantle formation and arc magmatism. It is commonly believed that the secular cooling of the mantle modified the tectonic style from stagnant lid or heat pipe on an early hotter Earth to horizontal tectonics during Meso-Neoarchean. However, various field, petrographic, and geochemical studies suggest that the onset of plate tectonics ranges from Hadean to Neoproterozoic. In this study, we re-evaluate the primary magma temperature (mantle potential temperature, Tp) of the upper ambient mantle, spanning from Eoarchean to Neoproterozoic. We used basalts from several Archean and Proterozoic greenstone belts worldwide, along with Proterozoic ophiolites, to (re)calculate the Tp using the FRACTIONATE-PT method. We observed a Tp range of 1444–1639°C during the Archean and 1414–1611°C during the Proterozoic. These findings indicate a strong correlation with previously estimated Tp values obtained from PRIMELT3 method. This further highlights strong internal consistency among different methods and supports models of a hot ambient mantle during the Archean and Proterozoic. We further reviewed numerical models regarding the effect of mantle temperature on the viability of early Earth plate tectonics. Such model results, composition of Archean continental crust, recent crustal growth models, and field evidence are consistent with the operation of plate tectonics on an early hotter Earth. The transition from a hotter mantle to a colder one from Eo-Neoarchean resulted in thinner oceanic lithosphere and a less depleted lithospheric mantle. Subduction of this thinner oceanic lithosphere led to modern-style tectonics. The intensity and style of plate tectonics on the early Earth differed from modern tectonics, which emerged during the Neoarchean.

Evolution of rift faulting in incipient, magma-poor divergent plate boundaries: New insights from the Okavango-Makgadikgadi Rift Zone, Botswana

Continental rifts are thought to transition from a power-law fault length distribution during the juvenile stages of extension, to an exponential distribution during break-up and oceanic spreading. However, fault scaling relationships have rarely been quantified in natural incipient rifts, particularly in the absence of magmatic influence. We address this knowledge-gap in the incipient Okavango-Makgadikgadi Rift Zone, Northern Botswana, consisting of the amagmatic Okavango Rift Basin (ORB) and Makgadikgadi Rift Basin (MRB). We utilize high-resolution satellite topography data (12.5 m TanDEM-X and 30 m SRTM) and aeromagnetic data to image and map faults across the rift zones and generate a new robust fault map for the region. Further, we analyze the length-frequency distribution of faults using a two-sample Kolmogorov-Smirnov test. The results show that the Makgadikgadi Rift Basin exhibits an exponential distribution associated with a nucleating rift as it exhibits diffuse faulting and lacks a border-fault, whereas the Okavango Rift Basin exhibits a power-law distribution that is consistent with its relatively more evolved rift structure with established border faults. Thus, we propose that continental divergent plate boundaries commonly nucleate with an initial exponential distribution of fault lengths which subsequently transition into a power-law distribution as the rift evolves into its stretching phase, and may again transition into an exponential distribution as break-up initializes.

On the delayed expression of mantle inheritance–controlled strain localization during rifting

Abstract In this paper, we explore the relative impact mantle inheritance can have on the evolution of magma-poor rift systems during the early stages of rift localization. To this end, we revisit the tectonic history of the Mesozoic Northeast Atlantic and its North Sea subdomain, analyzing these observations through comparison with results from recent analogue tectonic modeling work. Our analysis suggests that initial broadly distributed rift basin formation may be controlled by crustal inheritance, whereas the subsequent localization of deformation along deeper mantle inheritance may have caused the overprinting of previous rift basin trends in the Northeast Atlantic. This overprinting became possible as soon as the thinning of the ductile crust during progressive rifting allowed for sufficient coupling between the mantle and the upper crust. Importantly, we suggest that no changes in plate motion direction due to large-scale reorganization of the plate tectonic system are needed for differently oriented basin trends overprinting each other to develop. With these insights, we propose an updated scenario for rift kinematics in the North Sea involving continuous E-W plate divergence and provide a framework to rethink the evolution of other rift systems around the world during their early stages of rift localization.

Seamless Map of Depth to the Moho Interface in the Afro‐Arabian Region Using Gravity Data Derived From EGM2008

AbstractThe Afro‐Arabian region is one of the few places on land, where rifting processes at divergent plate boundaries can be thoroughly investigated. One of the crucial factors in understanding rifting processes involves assessing the crustal thickness. In this study, gravity data from the Earth Gravitational Model 2008 is used to create a seamless map of the depth to the Moho interface. Unlike many previous investigations that focused on specific localized areas, within the region, results from the current study provide a comprehensive view. The depth obtained from the current investigation aligns well with findings from earlier studies, exhibiting a bias of 0.69 km and a standard deviation of 3.89 km. Within the region, maximum and minimum depths to the Moho interface are observed beneath the northwest Ethiopian Plateau and the Gulf of Aden Rift (GAR), respectively. Analyzing profiles across the Red Sea, Main Ethiopian, and GARs, the study concluded that the Southern Main Ethiopian Rift is in an earlier stage of the rifting process, while the GAR is at an advanced stage. Furthermore, the interpretation of the current findings led to the inference that there might exist two potential plume tails driving the rifting process in the East Africa Rift—one originating from the Afar region and the other from South Kenya. This inference primarily relies on the isostatic compensation stages observed in the various rift systems throughout the region.

Laptev Rift System Composite Tectono-Sedimentary Element, East Siberian Arctic

Abstract The Laptev Sea in the East Siberian Arctic is characterized by a system of extensional structures known as the Laptev Rift System (LRS), developed at the continental prolongation of an active divergent plate boundary - the Gakkel spreading ridge. The LRS hosts as much as 13–14 km of siliciclastic sedimentary strata of supposedly Late Cretaceous to Recent age. The strata accumulated atop of stretched and thinned continental crust that before the rifting onset was a complex tectonic juncture of Late Paleozoic/Early Mesozoic Taimyr and Late Mesozoic Verkhoyansk fold-and-thrust belts formed along the edges of the Siberian Palaeocontinent. In the context of this volume, the LRS is described as a composite tectono-sedimentary element that includes sedimentary rocks accumulated during pre-rift and syn-rift phases. The latter includes at least two syn-rift sedimentary cycles separated by a sedimentary succession formed during a stalled rift phase. In the absence of deep exploration wells, many aspects of the LRS geology and its petroleum prospects remain poorly constrained.

Tectonics and seismicity of the Gulf of Aden: Contributions of edge detection filters applied to potential field data

Several studies have applied edge detection filters based on potential field derivatives, ratios and statistical-based tools to evaluate structures and provide interpretations. These edge filters provide different responses that may lead to false interpretations, making the selection of an effective filter that can provide optimum response difficult for users. Evaluating the reliability of the edge filters and avoiding false interpretations of the geologic structures requires simultaneous evaluation of filters applied to the same set of theoretical models. By using 2.5D models of gravity and magnetic synthetic data, we evaluate the performance of 21 edge detection filters based on different scenarios. The outcome of the various tests shows that filters of the third class give best results, with reference to the second and first class, and present good to moderate resolution in gravity and magnetic models. This comparison provides a means of inferring the tectono-structural styles, having generated multiple results from different filters and screening for artefacts (i.e., false impressions). Thereafter, we applied the edge enhancement filters to the magnetic and free-air gravity data of Aden Rift. Boundaries corresponding to seafloor spreading, divergent plate and transform faults were identified. The boundaries correlate well with the source locations of earthquakes at shallow depths. In addition to the dominant extensional tectonic regime, we suggest that the local fault system influenced by the Afar plume provides insights to the complex interaction between magmatic, sedimentary, and tectonic forces, shaping the geology of the Arabian-Nubian triple junction and contributes to the dynamics of the Gulf of Aden. The occurrence of earthquake activities along normal and transform fault zones which correlates with identified structural boundaries resulting from the application of edge detection filters provides a novel application to the study of rifting processes by multiple observation techniques.

Rift obliquity in the Northern Volcanic Zone in Iceland using UAV-based structural data

Rift obliquity is known to be very frequent at divergent plate boundaries and shear components in the opening of fissures have been detected through seismicity in the monitoring of recent rifting events. However, the field structural evidence of obliquity in active rifts for Holocene events is still poorly studied, despite its relevance at both the lithospheric and upper-crust scales.We performed extensive high resolution UAV surveys on Holocene terrains in four areas along the active rift in the Northern Volcanic Zone of Iceland, to map structures and morphotectonic features in detail. We did a kinematic analysis and found an increase of the obliquity from N to S.We further discuss the origins of this obliquity and suggest that the combination of inherited structures reactivation, magmatic intrusions, and topography influence on the propagation of both fractures and magma intrusions can have an impact on the mismatch between rift opening direction and the far-field strain.We conclude that obliquity can be inferred for Holocene rifting events from surface structural data. It is thus fundamental to take into account the existing structural pattern and its (re)activation history in active rift zones to avoid missing out on possible preferential structure reactivation, magma propagation ways, and eruption sites during the monitoring of ongoing rifting events.

Pre-existing structural control on the recent Holuhraun eruptions along the Bárðarbunga spreading center, Iceland

The active rift zones in Iceland provide unique insight into the geodynamic processes of divergent plate boundaries. The geodynamics of Iceland are studied intensively, particularly, by geophysical methods sensitive to active and/or visible structures such as earthquake seismic and Synthetic Aperture Radar observations or aerial photographs. However, older and less active structures, that may exert a strong control on the presently active geodynamics, are often buried beneath recent volcanic or sedimentary deposits and are—due to their passive mode—overseen by the typical geophysical investigations. Aeromagnetic surveys provide spatial information about subsurface magnetization contrasts relating to both active and inactive structures. However, the aeromagnetic data in Iceland were collected in the 1970-80s and are relevant only to large-scale regional rift studies. With the availability of reliable drones and light-weight atomic scalar sensors, high-quality drone magnetic surveys can provide an unprecedented spatial resolution of both active and passive structures of rift systems as compared to conventional airborne surveys. Here, we present the results of a drone-towed magnetic scalar field and scalar gradiometry study of the north-northeast trending Bárðarbunga spreading center to the north of the Vatnajökull ice cap, Iceland. Our results provide new information about the structural complexity of rift zones with evidence of densely-spaced, conjugate and oblique faults throughout the area. Evidence is shown of a hitherto unknown and prominent east-northeast trending fault structure that coincides with the northern tip of the main eruption edifice of the 1797 and 2014-15 Holuhraun volcanic events. We suggest that this pre-existing structure controlled the locus of vertical magma migration during the two Holuhraun events.

Toroidal flow around the Tonga slab moved the Samoan plume during the Pliocene

Abstract Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ~2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2–3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries.

Trans‐Lithospheric Ascent Processes of the Deep‐Rooted Magma Plumbing System Underneath the Ultraslow‐Spreading SW Indian Ridge

AbstractProcesses of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum.

Gravity Data Enhancement Using the Exponential Transform of the Tilt Angle of the Horizontal Gradient

Detecting the boundaries of geologic structures is one of the main tasks in interpreting gravity anomalies. Many methods based on the derivatives of gravity anomalies have been introduced to map the source boundaries. The drawbacks of traditional methods are that the estimated boundaries are divergent or false boundaries appear in the output map. Here, we use the exponential transform of the tilt angle of the horizontal gradient to improve the edge detection results. The robustness of the presented method is illustrated using synthetic data and real examples from the Voisey’s Bay Ni-Cu-Co deposit (Canada) and the Tuan Giao (Vietnam). The findings show that the presented technique can produce more precise and clear boundaries.

Generation of Evolving Plate Boundaries and Toroidal Flow From Visco‐Plastic Damage‐Rheology Mantle Convection and Continents

AbstractEarth's style of planetary heat transport is characterized by plate tectonics which requires rock strength to be reduced plastically in order to break an otherwise stagnant lithospheric lid, and for rocks to have a memory of past deformation to account for strain localization and the hysteresis implied by geological sutures. Here, we explore ∼107 Rayleigh number, visco‐plastic, 3‐D global mantle convection with damage. We show that oceanic lithosphere‐only models generate strong toroidal‐poloidal power ratios and features such as a mix of long‐wavelength tectonic motions and smaller‐scale, back‐arc tectonics driven by downwellings. Undulating divergent plate boundaries can evolve to form overlapping spreading centers and microplates, promoted and perhaps stabilized by the effects of damage with long memory. The inclusion of continental rafts enhances heat flux variability and toroidal flow, including net rotation of the lithosphere, to a level seen in plate reconstructions for the Cenozoic. Both the super‐continental cycle and local rheological descriptions affect heat transport and tectonic deformation across a range of scales, and we showcase both general tectonic dynamics and regionally applied continental breakup scenarios. Our work points toward avenues for renewed analysis of the typical, mean behavior as well as the evolution of fluctuations in geological and model plate boundary evolution scenarios.

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.

The Development of Fault Networks at the Termination of Continental Transform Faults When Their Connecting Plate Boundary Is “Misaligned”

AbstractWe report the results of a series of scaled laboratory experiments that investigate the development of fault systems in plate boundary transition zones, where deformation is distributed between a continental transform fault (e.g., Alpine Fault, New Zealand; North Anatolian Fault (NAF), Turkey; San Andreas Fault (SAF), USA) and its connecting plate boundary. In these transition zones, continental transform faults are observed to branch into multiple subsidiary faults. Here we show that large‐scale transition zone fault networks comprise crustal‐scale Riedel shears that develop sequentially outwards and away from the parent transform fault. Such fault networks form within brittle upper crust that overlies a ductile lower crust that deforms by large‐scale distributed simple shear. We argue that large‐scale distributed deformation of the lower crust occurs when continental transform faults are “misaligned” relative to their connecting plate boundaries. This misalignment may occur: (a) where a transform fault does not directly connect with a convergent or divergent plate boundary, as in the northern termination of the Alpine Fault and the western termination of the NAF; and (b) where a significant bend in the transform fault occurs in the plate boundary transition zone, as in the southern termination of the SAF. The development of such plate boundary misalignments appears to occur when a plate boundary transition zone develops as a continental transform fault propagates toward a different type of “connecting” plate boundary.

A generalization of Jeffrey-Hamel problem to Reiner-Rivlin model for energy and thermodynamic analysis using Keller-Box computational framework

This study addresses the heat transport and irreversible mechanisms associated with energy and frictional losses in the Jeffery-Hamel flow of a non-Newtonian fluid adapting the Reiner-Rivlin model. The Reiner-Rivlin model is frequently used to tackle the impact of yield-stresses among the multiple rheological models suggested for constructing the flow of yield-stresses materials. The non-creeping flow between convergent and divergent plates (Jeffrey-Hamel flow), the viscoelastic characteristics of incompressible Reiner-Rivlin liquid are examined by adapting the Navier-Stokes flow. The flow of a Reiner-Rivlin fluid is caused by pressure differential at the inlet. The second law of thermodynamics ultimately provide entropy manifestation. Using a comprehensive Keller-Box approach, self-similar numerical solutions for the velocity and associated fields are provided. The distribution of velocities for fluid-specific characteristics reveals an impedance formed by interaction between wall and diverging flow orientations. Viscous dissipation has a notable impact on the irreversible process and subsequently enhance the transmission of heat. Irrespective of the conduit structure, the Eckert number raises both the centerline and wall temperatures. The wall temperature gradient show a dominant trend for diverging channel. Higher Brinkman number, particularly in the context of the Reiner-Rivlin fluid, render the effect of viscous dissipation on the development of entropy apparent.