Red Sea Rift Research Articles

Occurrence and genesis of the exposed Oligo-Miocene Ghar Formation in Kuwait, Arabian Gulf

A sequence dominated by fawn cross bedded sandstones that constitutes the basal part of the post Eocene Kuwait Group siliciclastics and has been assigned as Ghar Formation is well exposed within Bahrah coastal plain in the northeast of Kuwait. The geology of these exposures had been briefly described in the middle of the late century. The purpose of this paper is to assess in detail its sedimentology and specifically the diagenesis of its rocks. The exposures of this sequence have been investigated at several locations. A simplified geologic map was prepared and representative samples of the studied sequence were subjected to detailed petrographic examination and geochemical analysis including stable isotopes. Four main facies were recognized within this sequence; sandstones, palustrine dolomicrite, mudstone and carbonates. Structural, textural and geochemical characteristics of the sandstone facies indicated that the detrital constituents are mostly derived from the Arabian Shield to the west of the study area and transported for a long distance within relatively low to moderate relief terrains during tropical sub-humid to humid climate. The palustrine dolomicrite facies was precipitated within semi-closed lagoons that may be influenced by seawater intrusion. The occurrence of thin carbonate beds within this sequence suggests short lived marine incursions intermittent with the prevailed fluvial conditions. Sedimentation was episodic and significantly controlled by the regional tectonics particularly Red Sea rifting. It is, therefore, concluded that the studied sequence has been deposited by fluvial braided system close to a paleo-shore line. Due to its peculiar sedimentological and petrographic characteristics it is suggested to name it as Bahrah Formation. further detailed sedimentological and stratigraphic assessment of this Formation is recommended.

Стадийность геологического развития территории Южного Притиманья в позднем протерозое

Геологическая история Тиманской и Древнеуральской окраин Русской плиты в позднем протерозое (RF3V2) представлена с точки зрения стадийности их развития в рамках общей теории тектоники плит. Показана важность проблемы «тройных» точек, или точек тройного растяжения, в начале рифтинга. На основании проведенного стадийного анализа обосновываются следующие положения: 1) тесная связь территории Южного Притиманья и Тиманской подвижной системы в позднепротерозойской геологической истории; 2) опосредованное влияние Тиманской подвижной системы на стадии развития Полюдовского рифта; 3) более ранняя – позднерифейская (RF3), а не ранневендская (V1) – активизация Полюдовского рифта. Предположение о миграции тройной точки от Полюдовского рифта к КваркушскоКаменногорскому, наподобие точки тройного растяжения Красноморского рифта, остается проблематичным.

Rupture characteristics of a small-sized earthquake (MW 4.2), onshore the south Red Sea, Saudi Arabia

The present study is based on the use of Empirical Green's Function (EGF) deconvolution technique to retrieve the slip distribution of the 2014 Mw 4.2 Jizan earthquake, Saudi Arabia. Two datasets of complex Source Time Functions (STFs) were retrieved using two appropriate EGF events. We inverted the STF datasets to recover the slip distribution over both nodal planes, using the Bayesian modeling followed by a linear least-squares method. The inversion was performed assuming both planes as the fault plane and examined the goodness of fit for each nodal plane. Based on a series of finite-source inversions using different rupture velocities, we resolved the rupture velocity at 2.7–2.8 km/s and the fault plane of NNW trending; paralleling the Red Sea rift. Using the estimated rupture velocities and the preferred fault plane, we imaged quite similar slip models, exhibiting two slip patches located to the updip and downdip directions from the hypocentre. The spatiotemporal slip distributions revealed a complex rupture history of such small-sized earthquake is likely to that reported for large-sized earthquakes. A seismic moment of 2.8–3.2E+15 NM and a corresponding moment magnitude of 4.2-4-3 are inferred. The stress drops obtained from the slip distribution models were 2.2–2.5 MPa; indicating a typical value that characterized the plate-boundary earthquakes.

Imaging Lithospheric Discontinuities Beneath the Northern East African Rift Using S-to-P Receiver Functions.

Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S‐to‐P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere‐asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.

Origin and geotectonic evolution of Mir Tertiary basaltic andesite dykes, Western Desert, Egypt: Constraints from mineral and bulk‐rock chemistry

Compared with widespread Egyptian Tertiary basaltic rocks, Mir Tertiary basaltic dykes from the Western Desert of Egypt show distinct mineralogical and geochemical characteristics that provide insights into the nature of their mantle source and geotectonic evolution as well as melting and crystallization conditions. Their mineralogical features (i.e., presence of orthopyroxene and lack of olivine) distinguish them from the other Egyptian Tertiary basalts. They have basaltic andesite compositions (SiO2 = 53.55–55 wt.%, Na2O + K2O = 4.64–4.8 wt.%, MgO = 2.66–3.02 wt.%) with tholeiitic character. Temperature of crystallization ranges from 1,112°C to 1,140°C for plagioclase; from 1,085°C to 1,123°C for clinopyroxene; and from 1,034°C to 1,090°C for orthopyroxene. These results are in harmony with petrographic observations which suggest early crystallization of plagioclase, followed by simultaneous crystallization of plagioclase and clinopyroxene, and soon thereafter by orthopyroxene crystallization. Titanomagnetite–ilmenite intergrowths indicate oxygen fugacity (ƒO2) of −12.09 and −9.4 log units, suggesting crystallization at relatively oxidized conditions. Their temperature ranges from 959°C to 1,125°C, suggesting that the titanomagnetite joins the crystallized phases at early stages and continued until later stages. Mir basaltic andesite (MBA) has low Ni (15–27.9 ppm), Cr (19.8–48.5 ppm), and Sc (20–25 ppm) concentrations together with high FeOt/MgO ratios (3.18–3.58) relative to primitive basalts, arguing for their evolved nature. They have sub‐chondritic Nb/Ta ratios (15.5–17) similar to ocean–island basalts and higher than those of continental crust, ruling out a significant effect of crustal contamination. The high La/Nb (>1.4) with the negative Nb, Ta, and Ti anomalies on the primitive mantle‐normalized patterns most likely indicates that their primary magma was derived from partial melting of a mantle source previously metasomatized by influx of subduction‐related slab fluids/melts. Moreover, the super‐chondritic Zr/Hf ratios (39.55–55.21) and the negative K and Ti anomalies point to carbonate metasomatism of their mantle source. This inference is further supported by their high (K2O + Na2O)/TiO2 ratios (>1) which are comparable with experimentally produced carbonated peridotite melts. The high Zr/Y (average 9.75) and La/Yb ratios (8.6–10) of the MBA suggest that they were generated via low degrees of partial melting. Indeed, modelling calculations indicate generation by lower melting degrees (<10%) of a garnet‐bearing lherzolite mantle source. The melting region was at the garnet stability field suggesting a deeper origin (exceeds ~85 km) for MBA. Field and geochemical characteristics of the Egyptian Tertiary volcanism proved that it is spatially and temporally related to extension that has occurred during Red Sea rifting. Extension most likely due to the NE–SW tensional stresses which reactivated the old fractures and triggered mafic magma generation through passive upwelling of the asthenospheric mantle.

Scaling properties of normal faults in the central Afar, Ethiopia and Djibouti: Implications for strain partitioning during the final stages of continental breakup

The central Afar depression offers an opportunity for quantitative analysis of the final stages of continental breakup at a divergent triple junction. Here, Quaternary faulting dissects Plio-Pleistocene flood basalts, providing a regionally pervasive datum to quantify the regional finite strain and fault scaling laws. To this end, we mapped and measured more than 8500 normal faults using remote sensing products. Fault length generally follows a power law distribution, but exponential distributions are found in areas of magmatic extension. Fault throw and fault length scale following a power law relationship where T=0.031L0.96, similar to previous studies in Afar and elsewhere in the world. Combining the power law distribution with this throw scaling enables a comprehensive assessment of Quaternary finite strain throughout the region. Strain is greatest near Manda Inakir, Lake Asal, and the Dobe-Hanle accommodation zone. The Makarassou fault zone shows low strain, suggesting it does not kinematically link the Manda Inakir and Asal rift segments. The presence of moderate to high strain in the amagmatic portions of the central Afar rift suggests that those areas accommodate a significant proportion of plate motion. This may provide a future path for propagation between the Gulf of Aden Rift and the Red Sea Rift.

Two-stage Red Sea rifting inferred from mantle earthquakes in Neoproterozoic lithosphere

We use earthquake geothermometry, measured heat flow, and structural constraints from P-wave receiver functions to model the thermal evolution of the lithosphere beneath Harrat Lunayyir. We suggest that the lithosphere thinned to its present 60-km thickness in a second stage of lithospheric thinning at 15–12 Ma following initial Red Sea extension at ∼27 Ma. Harrat Lunayyir is an active volcanic field located in the Arabian Shield >150 km east of the Red Sea rift axis. In the lithospheric mantle beneath Harrat Lunayyir we locate 64 high-frequency earthquakes at depths of 42–48 km, all with mL≤2.5. These brittle-failure earthquakes must have nucleated at relatively low temperatures, based upon global maximum nucleation depths and temperature-dependent-deformation experimental results. Therefore, the mantle earthquakes show that the upper mantle lithosphere is not in thermal equilibrium with the shallow (60 km) underlying asthenosphere. Our thermal modeling indicates that the lithosphere beneath Harrat Lunayyir thinned to its current 60-km thickness at 12 ± 2 Ma, as constrained by thermal modeling of: (1) surface heat-flow; (2) the depth to the mid-crustal brittle–ductile transition, and (3) the depth to the upper-mantle brittle–ductile transition.

Geochemical evolution of geothermal fluids around the western Red Sea and East African Rift geothermal provinces

The geothermal provinces of the western Red Sea coast and East African countries have evolved synchronous with the evolution of the landmasses around the Red Sea. The Red Sea opening was initiated subsequent to the opening of the Aden Gulf that was triggered by the Afar plume. During the Late Oligocene, the Red Sea rift started extending from south towards north culminating into a failed arm near the Suez Gulf. The geothermal systems evolved due to shallow mantle below the Danakil depression and circulating seawater. Besides the volcanic flows, the regional sedimentary aquifers like the Wajid Sandstone and Amron limestone, that occur in Ethiopia, Egypt, Eritrea and Djibouti, that were deposited prior to the younger tectonic events that started at 31 Ma are the main geothermal reservoirs in these countries. In contrast, rain surface and evaporated lake water are the main sources for the Kenyan geothermal systems. Although these geothermal fluids show variation in their chemical components due to different recharging sources, all the geothermal systems are of high temperature, capable of generating electricity.

The modest seismicity of the northern Red Sea rift

The modest seismicity of the northern Red Sea rift

Seismicity During Continental Breakup in the Red Sea Rift of Northern Afar

AbstractContinental rifting is a fundamental component of plate tectonics. Recent studies have highlighted the importance of magmatic activity in accommodating extension during late‐stage rifting, yet the mechanisms by which crustal thinning occurs are less clear. The Red Sea rift in Northern Afar presents an opportunity to study the final stages of continental rifting as these active processes are exposed subaerially. Between February 2011 and February 2013 two seismic networks were installed in Ethiopia and Eritrea. We locate 4,951 earthquakes, classify them by frequency content, and calculate 31 focal mechanisms. Results show that seismicity is focused at the rift axis and the western marginal graben. Rift axis seismicity accounts for ∼64% of the seismic moment release and exhibits a swarm‐like behavior. In contrast, seismicity at the marginal graben is characterized by high‐frequency earthquakes that occur at a constant rate. Results suggest that the rift axis remains the primary locus of seismicity. Low‐frequency earthquakes, indicative of magmatic activity, highlight the presence of a magma complex ∼12 km beneath Alu‐Dalafilla at the rift axis. Seismicity at the marginal graben predominantly occurs on westward dipping, antithetic faults. Focal mechanisms show that this seismicity is accommodating E‐W extension. We suggest that the seismic activity at the marginal graben is either caused by upper crustal faulting accommodating enhanced crustal thinning beneath Northern Afar or as a result of flexural faulting between the rift and plateau. This seismicity is occurring in conjunction with magmatic extension at the rift axis, which accommodates the majority of long‐term extension.

Large salt accumulations as a consequence of hydrothermal processes associated with ‘Wilson cycles’: A review Part 1: Towards a new understanding

Abstract The formation of large salt deposits is observed especially in areas with a geological history of high tectonic activity. Over the last decade it has become a well-established fact that heavy brines form and solid salts precipitate, due to the thermodynamic and physico-chemical properties of seawater at high temperatures and pressures encountered within hydrothermal systems. This article reviews the modern theoretical and experimental research behind these findings, and also describes geological settings that most likely cause brine- and salt-forming hydrothermal processes to occur. This analysis has led to the identification of a set of specific conditions, properties, and processes (referred to as Conceptual elements) that are used to explain the often complex processes of brine behavior that leads to hydrothermal formation of solid salt. The main objective of this review is to present hydrothermal conditions known to occur during Wilson cycles: subduction, collision, and rifting, e.g., zones of repeated tectonic unrest, where brines (commonly derived from seawater) are concentrated into heavy brines and precipitate solid salts. The internal heat of the Earth and its interaction with deeply-circulating seawater in hydrothermal systems and also the immense recycling of crustal materials, including porous oceanic crust and serpentinite (hydrated) rocks via mantle processes may lead to the formation of salt accumulations. It is also acknowledged that such brines and solid salts may often be stored sub-surface for long time periods, extending from one Wilson cycle to another. Thus, on the basis of this analysis, it is cautiously suggested that large amounts of salts ‘hidden’ inside subduction zones may appear on the surface during subsequent rifting and oceanization phases. In Part 2 of this review, the Conceptual elements, which are described and discussed herein (Part 1) are applied to selected cases, including the Andean Mountains, the East African Rift, the Red Sea Rift, and other locations.

Birth of an ocean in the Red Sea: Oceanic-type basaltic melt intrusions precede continental rupture

The role of magmatism in continental rupture at the birth of a new ocean is poorly understood. Continental rupture can take place with voluminous volcanism, as in the southern Red Sea, or in a relatively non-volcanic mode, as in the northern Red Sea. We report geophysical, geochemical and geochronological evidence suggesting that continental rupture in the northern Red Sea is preceded by MORB-type basaltic melt intrusions that cooled at depth forming gabbros at progressively shallower crustal levels as rifting progressed towards continental separation. One of these gabbros yielded an 40Ar/39Ar age of 25±6Ma, suggesting intrusion during early rifting. A magma chamber we detected 3.5km below Thetis Deep, an axial segment of initial oceanic crust accretion, represents the final stage in this progression. Oceanic crust accretion in the Red Sea rift starts first at depth before continental rupture and initiation of sea floor spreading.

Oil shale resources in Egypt: the present status and future vision

In Egypt, organic-rich sediments in the Duwi and Dakhla Formations of the Campanian-Danian age are customarily assigned as “oil shale” that occupies the middle latitudes of the country but may extend southward to Kurkur Oases. This oil shale belt has a vast worldwide extension, and it is considered as major oil- and gas-prone source rock in many places, especially in the Middle East. The sedimentation of the oil shale was triggered by the major transgression event that occurred during the Late Cretaceous. The lithology, type of kerogen, organic richness, and thickness of these organic-rich sediments vary markedly both on lateral and vertical scales. In Quseir area, the in-place geological reserves, of oil shale of the 800-kcal/kg quality, is estimated to be more than 9 billion tons that can produce 5.48 bbls equivalent upon retorting. Very optimistic resources are expected in the unexplored Nile Valley region. The factor analysis of data rank representing 1176 core samples and analysis of 58 major and trace elements besides Rock-Eval analyses point to five main controlling factors that control deposition of oil shale. The terrestrial indicators Al2O3, TiO2, Fe2O3, and K2O and the marine indicators Ca and Sr are oppositely loaded in the first factor. The second factor expresses the reducing conditions that prevailed during the deposition of the organic-rich marine environments. The euxinity of the basin is recognized by the third factor where sulfide and vanadium seem to be mutual. The fourth factor expresses the role of dolomitization while the fifth factor points to the humble role of oxidation. Considering the metric core samples, the highest TOC content recorded in the borehole drilled in Abu Tartur plateau is 3.6%, but it is about 14% for Quseir area. Regarding the spot samples in Quseir area, the highest TOC measured about 24%. It is not only the low TOC in Abu Tartur but also the kerogen type that is of type II + III, mostly of terrestrial origin (gas-prone) and lithology dominated by argillites. Organic richness is remarkable in Quseir-Safaga area, where the average TOC of 160-m-thick sequence is about 5%, with kerogen of type I or mixed I + II, mostly of marine origin (oil prone). The Dakhla Formation (Maastrichtian-Danian) is the richest in organic matter while Quseir Formation (Campanian) has the least organic richness and lowest kerogen quality. Detailed investigation on biomarkers confirms the relations among transgression, organic richness, kerogen type, and anoxic conditions. The organic matter is immature as witnessed by the low S1 values (<5%, in average), the low T max (<430 °C), the low vitrinite reflectance (<0.4%), and biomarker signature. The variation in the S1 values between 1 and 9% is attributed to the influence of tectonics associating the Red Sea rift. In Quseir-Safaga area, there are particular prolific horizons of oil shale that seem to be visible for utilization by different technologies of combustion and retorting. The content of the heavy metals and uranium, as well as the spent, is a significant benefit. The faulting, dragging, and steep tilting of beds in the Quseir-Safaga area shall remain a serious challenge for extensive utilization of the estimated in-place geological reserves.

The major tectonic boundaries of the Northern Red Sea rift, Egypt derived from geophysical data analysis

AbstractIn the present study, we have attempted to map the plate boundary between Arabia and Africa at the Northern Red Sea rift region including the Suez rift, Gulf of Aqaba-Dead Sea transform and southeastern Mediterranean region by using gravity data analysis. In the boundary analysis method which was used; low-pass filtered gravity anomalies of the Northern Red Sea rift region were computed. Different crustal types and thicknesses, sediment thicknesses and different heat flow anomalies were evaluated. According to the results, there are six subzones (crustal blocks) separated from each other by tectonic plate boundaries and/or lineaments. It seems that these tectonic boundaries reveal complex structural lineaments, which are mostly influenced by a predominant set of NNW–SSE to NW–SE trending lineaments bordering the Red Sea and Suez rift regions. On the other side, the E–W and N–S to NNE–SSW trended lineaments bordering the South-eastern Mediterranean, Northern Sinai and Aqaba-Dead Sea transform regions, respectively. The analysis of the low pass filtered Bouguer anomaly maps reveals that the positive regional anomaly over both the Red Sea rift and South-eastern Mediterranean basin subzones are considered to be caused by the high density of the oceanic crust and/or the anomalous upper mantle structures beneath these regions whereas, the broad medium anomalies along the western half of Central Sinai with the Suez rift and the Eastern Desert subzones are attributed to low-density sediments of the Suez rift and/or the thick upper continental crustal thickness below these zones. There are observable negative anomalies over the Northern Arabia subzone, particularly in the areas covered by Cenozoic volcanics. These negative anomalies may be attributed to both the low densities of the surface volcanics and/or to a very thick upper continental crust. On the contrary, the negative anomaly which belongs to the Gulf of Aqaba-Dead Sea transform zone is due to crustal thickening (with limited heat flow values) below this region. Additionally in this study, the crustal thinning was investigated with heat flow, magnetic and free air gravity anomalies in the Northern Red Sea rift region. In fact, the crustal thinning of the study area was also proportional to the regions of observable high heat flow values. Finally, our results were found to be well correlated with the topography, free air, aeromagnetic and heat flow dataset profiles crossing most of the study area.

Provenance of coastal dune sands along Red Sea, Egypt

Texture, mineralogy, and major and trace element geochemistry of 26 coastal dune sand samples were studied to determine the provenance and tectonic environment of two dune fields close to the beaches of Safaga (SF) and Quseir (QS) at the Egyptian Red Sea coast. Onshore winds generate fine, moderate, moderately-well to well-sorted, coarse-skewed to near-symmetrical dune sands with mesokurtic distributions. Winds pick up and transport grains from nearby beach sands and alluvial deposits into a wide Red Sea coastal plain at the border of the beach. The mineralogical (Qt–Ft–Lt) and geochemical composition of the sands, indicate that SF and QS coastal dune sands are mature and influenced by quartz-rich sands. The average CIA values in SF and QS coastal dune sands are low relative to the range of the PAAS, suggesting an arid climate and a low intensity of chemical weathering. The SF and QS coastal dune sand samples are plotted in the recycled orogen and partly in craton interior fields suggesting recycled older sedimentary and partly metamorphic-plutonic sources. The high content of quartz with shell debris and carbonates in coastal dune sands support the recycled sedimentary beach and alluvial sand sources. The dominance of heavy minerals like amphiboles (hornblende) and biotite in the coastal dune sands also supports the effect of metamorphic-plutonic source rocks. The new tectonic discriminant-function diagrams suggest that the coastal dune sands were deposited in a passive margin of a synrift basin. The results provide a good evidence for the extension in the Red Sea rift system during Oligocene-post Pliocene, which is consistent with the general geology of Egypt.

Origin of an extensive network of non-tectonic synclines in Eocene limestones of the Western Desert, Egypt

Satellite images of the Western Desert of Egypt display conspicuous sinuous color patterning that previous workers have interpreted as erosional flutes formed by catastrophic flooding. Our work with high resolution satellite imagery shows that the patterning is not erosional but, rather, the result of a network of thousands of narrow synclines in the Eocene bedrock capping the Limestone Plateau. Synclines form as isolated, 200-400 meter-wide downwarps in otherwise flat-lying strata. Limb dips are shallow, and doubly plunging hinges form multiple basin closures along syncline lengths. Anticlines form “accidentally” in inter-syncline areas where two adjacent synclines lie close together. Synclines have two dominant orientations, WNW-ESE and NNW-SSE, parallel to two prominent joint and fault sets, and synclines branch, merge, and change orientation along their lengths. Synclines are all at the same scale with neither larger structures nor parasitic structures and are best described as non-tectonic sag synclines. An Egypt-wide inventory reveals that these synclines are both confined to Eocene limestones and developed, albeit it sporadically, over nearly 100,000 km2. The syncline network predates plateau gravels of the Katkut Formation, which have been interpreted as Oligocene or early Miocene in age, and the network is cut by faults related to Western Desert extension associated with Red Sea rifting. The mechanism that caused sag of overlying layers is not clear. Modern karst collapse, subsurface dissolution of evaporites, and collapse of paleokarst are all unlikely mechanisms given the timing of formation and the underlying stratigraphy. Silica diagenesis and downslope mobilization of underlying shales are possibilities, although uncertainty about the origin of silica in the limestones, plus the consistency of syncline orientations over large areas, make these models problematic. Hypogene karst, perhaps related to aggressive fluids associated with basaltic intrusions, may be the model most consistent with the admittedly limited data we currently have for the network.

Red Sea rift-related Quseir basalts, central Eastern Desert, Egypt: Petrogenesis and tectonic processes

Mineral and whole-rock chemistry of Red Sea rift-related Tertiary basalts from south Quseir city, central Eastern Desert of Egypt is presented to investigate their petrogenesis and relationship to tectonic processes. The south Quseir basalts (SQB) are classified as high-Ti (TiO2 >2 wt.%) subalkaline transitional lava emplaced in an anorogenic tectonic setting. Their Mg# varies from 48 to 53 indicating the evolved nature of the SQB. Pearce element ratios suggest that the SQB magmas evolved via fractional crystallization of olivine + clinopyroxene ± plagioclase, but the absence of Eu anomalies argues against significant plagioclase fractionation. Clinopyroxene compositions provide evidence for polybaric fractionation of the parental mafic magmas. Estimated temperatures of crystallization are 1015 to 1207 °C for clinopyroxene and 1076 to 1155 °C for plagioclase. These values are interpreted to result from early stage crystallization of clinopyroxene followed by concurrent crystallization of clinopyroxene and plagioclase. The incompatible trace element signatures of the SQB (La/Ba = 0.08–0.10 and La/Nb = 0.89–1.04) are comparable to those of ocean island basalts (OIB) generated from an asthenospheric mantle source unaffected by subduction components. Modeling calculations indicate that the SQB primary magmas were derived from ~4–5% partial melting of a garnet-bearing lherzolite mantle source. The NE Egyptian basaltic volcanism is spatially and temporally related to Red Sea rifting and to the local E–W striking faults, confirming a relationship to tectonic activity. Our results suggest that the extensional regime associated with Red Sea rifting controlled the generation of the Egyptian basalts, likely as a result of passive upwelling of asthenospheric mantle.

A new tectonic model for Abu-Dabbab seismogenic zone (Eastern Desert, Egypt): evidence from field-structural, EMR and seismic data

Abu-Dabbab area is the most active seismic zone in the central Eastern Desert of Egypt, where seismic activities are daily recorded. The reported earthquakes are microearthquakes of local magnitudes (ML < 2.0). A spatial distribution of these microearthquakes shows that the earthquakes of the area follow an ENE–WSW trending pattern, which is nearly perpendicular to the Red Sea Rift. Focal mechanisms of different fault styles were recognized with dominant normal faulting (with a strike-slip component) events characterized by focal depths greater than 7 km and reverse ones of shallower focal depths. Several lines of evidence indicating that the brittle-ductile transition zone underlies the Abu-Dabbab area occurs at a relatively shallow depth (10–12 km) and it is acting as a low-angle normal shear zone (LANF). Field-structural, EMR and seismic data (this study) reveal that the maximum compressive stress (σ1) in the area is perturbed from the regional NW–SE direction to ENE–WSW orientation. This stress rotation is evidently akin to the reactivation of the crustal scale Najd Fault System (NFS), where such reactivation is attributed to the ongoing activity/opening of the Red Sea. Our tectonic model proposes that the continuous activity on the brittle-ductile transition zone including the LANF led to stress localization, which triggering a brittle deformation in the upper crustal-levels and associated shallow dipping thrusts. Such bimodal tectonic model suggests that the deep earthquakes are owing to the tectonic movement on the LANF (transtension), whereas the shallow earthquakes are related to a brittle deformation inside the fault blocks of the upper crust (transpression). Deformation creep along this zone didn’t permit continuous accumulation of strain and hence reduce the possible occurrence of large earthquakes.

Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform, Red Sea, NW Saudi Arabia

Different diagenetic environments have been recognized in the Early Miocene carbonate platform of Musayr Formation in the Red Sea rift area. Early marine diagenesis includes micritisation that occurs as thin envelope around skeletal and non-skeletal grains in low-energy mud dominated facies and isopachous fibrous calcite in high-energy grain-dominated facies. Pervasive meteoric water diagenesis resulted in cementation of the carbonates by coarse-crystalline blocky-drusy calcite and meniscus cements. Depletion of oxygen (avg. –9.08‰), carbon (avg. –1.6‰) isotopes and trace elements concentrations (avg. values of Fe: 1387 ppm; Mn: 1444 ppm; Sr: 419 ppm; Na: 1194 ppm) in conjunction with negative correlation between Mn2+ and oxygen isotope data suggest variable degrees of fluid-rock interactions and pervasive meteoric diagenesis. The formation of meteoric diagenesis in the Musayr Formation can be explained by two subsequent mechanisms: (1) the presence of meteoric lenses during the time of deposition might have been associated with active freshwater input from the hinterland (NE) due to fall in the relative sea level; (2) later uplift episode during Plio-Pleistocene may have also contributed to the pervasive meteoric diagenetic alterations of the carbonates of the Musayr Formation. The first mechanism is supported by the cement stratigraphy where the blocky-drusy cements postdate the meniscus cement. The latter mechanism seems to have more pronounced effect on the alteration of Musayr carbonate sequence by observing the occurence of late cements such as blocky calcite in most of the samples. The impact of meteoric diagenesis on the studied samples suggest that dissolution is less severe than cementation, hence the visible porosity is very low. Understanding the timing of meteoric diagenesis provides useful information about the reservoir quality distribution in syn-rift carbonate sequences.

Thermochronometric evidence for diffuse extension and two-phase rifting within the Central Arabian Margin of the Red Sea Rift

Numerical time-temperature models derived from a 2D network of apatite and zircon (U-Th)/He ages reveal a three-stage thermotectonic history for the central Arabian rift flank (CARF) of the Red Sea Rift (RSR) system. The pre-rift Arabian-Nubian Shield existed as part of a passive Paleo-Tethyan margin until a widespread tectonic event ca. 350 Ma exhumed the proto-CARF to mid-to-upper crustal structural levels. After remaining thermally stable through the Mesozoic, the first phase of RSR extension began with a distinct rift pulse ca. 23 Ma when fault blocks across a 150 km-wide area were exhumed along a diffuse set of rift-parallel faults from an average pre-rift flank depth of 1.7 ± 0.8 km. This rift onset age is mirrored in thermochronometric and sequence stratigraphic analyses elsewhere along the Red Sea Nubian and Arabian margins, confirming that rifting occurred concomitantly along the full Red Sea-Gulf of Suez rift system. Diffuse lithospheric extension lasted for 8 million years before a second rift pulse ca. 15 Ma, coincident with regional stress realignment, transferred active faulting basinward towards the modern RSR axial trough. CARF time-temperature models indicate that the prevalent rift style during both RSR extensional phases was one of localized, structurally controlled block faulting and contemporaneous dike injection, not wholesale rift flank uplift.