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In the context of rift basin formation, structural inheritance describes the influence of pre-existing structures on new rift-related structures, including faults. Pre-existing structures in the crust or upper mantle can determine where rift basins form. As these basins evolve, pre-existing structures in the rocks underneath the newly formed basin can also interact with the far-field strain or stress field, leading to variations in the orientations and kinematics of individual rift-related faults. Given that continental rifts commonly form in pre-deformed lithosphere, structural inheritance is likely to be the norm, not the exception. As such, structural inheritance has implications for reconstructing the paleotectonic history of rifts, investigating seismic hazards, and understanding the fluid transport and storage capabilities of natural fracture systems in the context of geo-energy and ore deposits. The aim of this review is to provide a framework for recognising the various expressions of structural inheritance and their underlying mechanisms in amagmatic rift basins. We synthesise field and geophysical observations and the results of analogue and numerical models to: (1) demonstrate how different inheritance mechanisms (frictional reactivation vs. local re-orientation of the far-field strain and/or stress) can produce different geometric and kinematic relationships between pre-existing structures and rift-related faults; (2) classify these mechanisms in terms of hard-linked and soft-linked inheritance; and (3) show that different mechanisms can be activated at different depths by the same pre-existing structure. These insights can help us better interpret basement structures under the sedimentary cover of rift basins, which are not often well-imaged in geophysical data, and understand the multi-stage evolution of rift basins worldwide. [ABSTRACT FROM AUTHOR]

The Okavango Graben (OG) has been considered as the terminus of the southwestern branch of the East African Rift System (EARS) since the 1970s based on fault morphology and early seismic and geophysical data. Thus it has been assumed to be an incipient rifting zone, analogous to the early stage of mature rifts in the EARS. Recent geodetic data and geophysical studies in the area bring new insights into the local crust and lithosphere, mantle activity and fault activity. In this study, we computed the velocities for three permanent GPS stations surrounding the graben and undertook a review of the new geophysical data available for the area. The northern and southern blocks of the graben show an exclusively low strike-slip displacement rate of about 1mm/year, revealing the transtensional nature of this basin. The seismic record of central and southern Africa was found to be instrumentally biased for the events recorded before 2004 and the OG may not represent the most seismically active area in Botswana anymore. Moreover, no significant lithosphere and crustal thinning is found in the tectonic structure nor any strong negative Bouguer anomaly and surface heat flux. Thus the OG does not match the classical model for a rifting zone. We propose a new geodynamic model for the deformation observed west of the EARS based on accommodation of far-field deformation due to the differential extension rates of the EARS and the displacement of the Kalahari craton relative to the Nubian plate. [ABSTRACT FROM AUTHOR]

Petrological and geothermobarometric data from (garnet)-spinel peridotite and pyroxenite xenoliths in Quaternary alkali-basalts at Mega (Sidamo region, Ethiopia) provide evidence for refertilization and thinning of the lower subcontinental lithospheric mantle (SCLM) during Tertiary rifting events in the southern Main Ethiopian Rift (MER). Samples of lherzolites, harzburgites, and olivine websterites contain spinel-pyroxene symplectites that were formed by garnet breakdown reactions. P-T equilibration conditions, in part calculated using the reconstructed garnet composition (pyrope), indicate the evolution of peridotites from the P-T conditions from the garnet stability field (2.9–2.2 GPa; 945–1025 °C), through garnet-spinel transition (< 2.1–1.7 GPa at 906–1017 °C), to the final equilibration in the spinel stability field (1.3 GPa at 862–1056 °C), corresponding to a nearly adiabatic decompression path of about 50 km. Peridotite chemical compositions plot in an off-craton trend, with variable fertility degrees, only in part due to partial melting processes. Rocks show variable CaO contents (0.96–4.12 wt%), CaO/Al 2 O 3 ratios (1.42–4.46), and clinopyroxene contents (6–31 vol.%), reaching compositions more enriched than pyrolite. Clinopyroxene shows evident LREE, U, Th, and Sr enrichments, particularly in harzburgites. Geochemical data suggest heterogeneous refertilization of a refractory lower SCLM, induced by infiltration and migration through porous flow of silicate-carbonate melts. Metasomatic processes drove melt compositions towards low-viscosity highly-mobile C-O-H-rich melts, inducing cryptic and hydrous modal metasomatism in refractory peridotites. Heterogeneous refertilization of the SCLM beneath the southern MER seems to have been induced by upwelling of asthenospheric melts and/or by reactivation of ancient lithospheric metasomes. Rejuvenation processes could have rheologically weakened the base of SCLM through gravitational instabilities, and increased the extensional stresses, causing a destabilizing effect at the onset of the lithospheric thinning and Tertiary rifting. [ABSTRACT FROM AUTHOR]

The seismic effective quality factor ( Q C ) and its frequently dependences or the frequency parameter ( n ) and attenuation coefficient (δ) for the Earth’s crust and upper mantle of the North Tanzanian divergence zone (East African rift system) were estimated from an analysis of the earthquake coda waves recorded in the SEISMO-TANZ’07 French-Tanzanian seismic experiment. The Q C values increase and the n and δ values decrease with increasing frequency and length of the lapse time window. This behavior of the attenuation parameters may be evidence that the degree of heterogeneity of the lithosphere decreases with depth. Comparison of the depth variations in the attenuation coefficient δ and the frequency parameter n with the velocity structure of the region shows that there is a distinct change in the behavior of seismic wave attenuation at velocity discontinuities. The obtained attenuation parameters were compared with the same parameters obtained in our previous studies for other continental rift systems—the Baikal rift system (Eurasia) and the Basin and Range Province (North America). [ABSTRACT FROM AUTHOR]

The Western Branch of the East African Rift System has experienced multiple episodes of basin development and intraplate alkaline volcanism since the Jurassic, however the geodynamic processes and lithospheric evolution involved in this protracted geological history remains poorly defined. Here, we present Sm-Nd, Lu-Hf, and Rb-Sr isotopic data of igneous minerals for three magmatic episodes that coincide with basin reactivation in the Rukwa Rift of southwestern Tanzania, respectively represented by: (1) the Jurassic to Cretaceous Panda Hill carbonatite; (2) the late Oligocene phonolitic‑carbonatitic Nsungwe Formation tuffs; and (3) the Miocene–recent bi-modal volcanism of the Rungwe Volcanic Province. Of these, the Nsungwe Formation tuffs offer a discrete record of the early phases of lithospheric disturbance associated with modern rifting of the Western Branch. Alkaline magmas erupted prior to the Miocene are isotopically distinct (ɛNd +0.5 to +1.5 and 87Sr/86Sr 0.7041) from modern Rungwe lavas and likely originate from a lithospheric mantle that experienced enrichment during the Pan-African Orogeny (~550–700 Ma). Consistent with previous studies, our results indicate that Rungwe Volcanic Province magmas were generated from a mixed, isotopically-enriched component of the lithospheric mantle that may have developed during the ca. 1.0 Ga Irumide Orogeny. The observed change in the source region of rift lavas though time is consistent with progressing melting of a compositionally layered lithosphere. Combined with existing geophysical and sedimentological data, we hypothesise that the lithosphere beneath the Rukwa-Malawi-Usangu junction of the Western Branch experienced an episode of destabilisation and foundering (lithospheric drip), during the early stages of East African Rift development. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The evolution and characteristics of narrow continental rifting are illustrated in this paper through a review of recent lithospheric-scale analog models of continental extension compared with selected examples from the East African Rift System. Rift location is controlled by reactivation of lithospheric-scale pre-existing weaknesses; in these areas, the initial phases of rifting correspond to the activation of few, large-offset boundary faults that accommodate basin subsidence, which can be at places strongly asymmetric. The plan-view geometry of rift faults is primarily related to the relative orientation of the lithospheric weakness with respect to the extension direction: orthogonal rifting gives rise to long, extension-orthogonal boundary faults with associated pronounced subsidence, whereas oblique rifting results in a general en-echelon arrangement of faults and basins with less subsidence. Inherited fabrics having variable orientation with respect to the rift trend may control rift architecture at both regional and local scales. In these initial phases, widespread magmatism may encompass the rift, with volcanic activity localized along major boundary faults, transfer zones and limited portions of the rift shoulders (off-axis volcanism). Progressive extension leads to a change in deformation style from the few, large-offset boundary faults at the rift margins to dense fault swarms – with limited vertical motions – affecting the rift floor where the magmatic activity is concentrated. In these areas of focused tectono-magmatic activity (the so-called magmatic segments) the thinned lithosphere is strongly modified and weakened by the extensive magma intrusion, and extension is facilitated and accommodated by a combination of magmatic intrusion, dyking and faulting. The feedback between strain localization, magma injection and lithospheric weakening is self-reinforcing, facilitating the rupture of the continental lithosphere. At this stage, magmatic segments (as for instance in the Northern Main Ethiopian Rift) act as incipient slow-spreading mid-ocean ridges, developing within a lithosphere that is transitional between continental and oceanic. [Copyright &y& Elsevier]

Abstract: A new 3D spring lattice computer model has been developed and used to calculate the stress-field in the vicinity of a rift transfer zone. The numerical setup is based on the Rwenzori block, a transfer zone in the Western Branch of the East African Rift Valley. The study has two closely related, yet independent aims: primarily to gain insight into the pattern and the causes of the stress field in the Rwenzori area. The second aim is the evaluation of the model itself, based on a comparison of the model results with local geological structures. The simulations calculate the stress in the brittle part of the crust, at the topographic surface and at a depth of 10km. The model does therefore not rely on the rate of the continental extension, but only on the amount. The generated stress maps display the maximum and minimum principal normal stress. The stress is calculated for a total extension of 7.5km, a value based on common estimates for the local rift extension. The local stress field is created by the interplay of the plate driven far-field stress and the mechanics of the rift system. The propagation of the rift system leads thus to stress rotations and to characteristic stress patterns. An important mechanism is lithospheric bending caused by rift flank uplift, which reverses the far field stress in rift vicinity. The stress-field resulting from the simulations explains several previously unexplained structural features encountered in the Rwenzori block. [Copyright &y& Elsevier]

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3-10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7-3.6 Ma) tholeiitic basalts, and (3) Recent (1.9-0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and Sr/Sr (0.70395-0.70417) and less radiogenic Pb isotopic ratios (Pb/Pb = 18.12-18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core-mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source. [ABSTRACT FROM AUTHOR]

The Rwenzori mountains in western Uganda, with a maximum elevation of more than 5,000 m, are located within the Albertine rift valley. We have deployed a temporary seismic network on the Ugandan side of the mountain range to study the seismic velocity structure of the crust and upper mantle beneath this section of the rift. We present results from a receiver-function study revealing a simple crustal structure along the eastern rift flank with a more or less uniform crustal thickness of about 30 km. The complexity of inner-crustal structures increases drastically within the Rwenzori block. We apply different inversion techniques to obtain reliable results for the thickness of the crust. The observations expose a significantly thinner crust beneath the Rwenzori range with thickness values ranging from about 20-28 km beneath northern and central parts of the mountains. Our study therefore indicates the absence of a crustal root beneath the Rwenzori block. Beneath the Lake Edward and Lake George basins we detect the top of a layer of significantly reduced S-wave velocity at 15 km depth. This low-velocity layer may be attributed to the presence of partial melt beneath a region of recent volcanic activity. [ABSTRACT FROM AUTHOR]

Asthenosphere-lithosphere interactions modulated by surface processes generate outstanding topographies and sedimentary basins, but the nature of these interactions and the mechanisms through which they control the evolution of extensional tectonic settings are elusive. Basal lithospheric shearing due to plume-related mantle flow leads to extensional lithospheric rupturing and associated magmatism, rock exhumation, and topographic uplift away from the plume axis by a distance inversely correlated to the lithospheric elastic thickness. When moisturized air encounters a topographic barrier, it rises, decompresses, and saturates, leading to enhanced erosion on the windward side of the uplifted terrain. Orographic precipitation and asymmetric erosional unloading facilitate strain localization and lithospheric rupturing on the wetter and more eroded side of an extensional system. This simple analytical model is validated against thermo-mechanical numerical experiments where a rheologically stratified lithosphere above an asthenospheric plume is subject to fluvial erosion proportional to stream power during extension. Our modeling results are consistent with Paleogene mantle upwelling and flood basalts in Ethiopia synchronous to distal initiation of lithospheric stretching/rupturing in the Gulf of Aden, which progressively propagates into the Red Sea. The present-day asymmetric topography and extensional structures in the Main Ethiopian Rift may also be an effect of a Neogene-to-present orographic erosional gradient. Although inherently related to the lithosphere rheology, the evolution of continental rifts appears even more conditioned by the mantle and surface dynamics than previously thought. • Asthenospheric flow and orography condition extensional lithospheric rupturing. • Basal lithospheric shearing forces extensional rupturing off the mantle plume axis. • Orography forces extensional rupturing on the more eroded side of a rifting system. • Modeling results explain several observed features of the East African Rift System. [ABSTRACT FROM AUTHOR]

Despite the foundational role East Africa has played in the advancement of our understanding of continental rifting, there remains substantial ambiguity as to how magmatism has evolved during progressive rift development. This contribution is a comparative study that explores the temporal development of magmatism in both rifts, and within the interposing Turkana Depression. Notwithstanding the independent evolution of the Main Ethiopian Rift and Kenya Rift prior to their Quaternary linkage, magmatic events within them show remarkable parallelism. Following an initial pulse of basaltic magmatism ca. 20 Ma, more evolved compositions (flood phonolites in Kenya, rhyolites in Ethiopia) dominated the landscape until ca. 12 Ma. From ca. 12 Ma to 9 Ma, a renewed phase of widespread basaltic volcanism (Mid-Miocene Resurgence Phase) impacted the entire region from the Afar margin to Kenya, though activity in the south slightly predates equivalent basaltic events to the north. Following this widespread basaltic event, silicic magmatism again dominated the now nascent rifts until a renewed phase of basaltic activity commenced ca. 4 to 1.6 Ma termed the 'Stratoid Phase'. Following the termination of the Stratoid Phase, the modern expression of volcanism in both rifts has been dominated by central silicic volcanoes. Magmatic activity within the Turkana Depression broadly parallels that in both rifts – basaltic events are evident at ca. 20 Ma, 12 Ma, and 4 Ma, however the silicic interludes that are evident within the rifts, are generally absent. Moreover, from 4 Ma to present, there is a progressive decrease in magmatic activity from an initial fissural basalt, to a series of large shield volcanoes, to modern belts of cinder cones. The results of this synthesis suggest widespread pulsed basaltic events precede silicic flare-ups throughout the East African Rift System, and the lack of such flare-ups in Turkana may relate to the Mesozoic rifting that modified the continental lithosphere in that region. • First magmatic correlation of Ethiopian and Kenyan sectors of the East African Rift • Three cycles of magmatic activity identified throughout the East African Rift. • These magmatic cycles correlate with phases of extension. • Results show that magmatism is pulsed in East Africa. • Intimate relationship between rift development and eruptions [ABSTRACT FROM AUTHOR]

Magmatic activity in the Tanzania Craton-influenced regions of the East African Rift System (EARS) exhibits clear evidence of derivation from a variety of metasomatic sources. These metasomatic sources, which may span a range of ages, have likely been preserved within the lithospheric mantle. The influence of these metasomes on the composition of the rift magmas represents the single greatest heterogeneity in the composition of erupted lavas within the EARS. The earliest widespread magmatic event in the craton-influenced regions of the EARS south of Lake Turkana is the Samburu Phase (ca. 20–17 Ma), which manifests as alkaline volcanics derived from the lithospheric mantle. This phase represents the initial destabilization of the continental lithosphere as a precursor of rift development. The Flood Phonolite Phase (ca. 16–12 Ma) was most pronounced in regions south of those impacted by the prior Samburu Phase, and likely represent the continued southward expansion of lithospheric destabilization. The Mid-Miocene Resurgence Phase (ca. 12–9 Ma) is poorly represented in the Southern Kenya Rift, though magmatic activity is recorded in the Western Branch of the EARS at Kivu during this time. In contrast, the Early Rift Development Phase (ca. 9–4 Ma) is particularly well-represented in the Kenya Rift. Large-scale magmatic events include: commencement of activity in the Northern Tanzania Divergence (NTD) and Mt. Kenya, Rungwe Volcanic Province, and the Southern Kenya Rift. The Stratoid Phase (ca. 4 Ma – 0.5 Ma) commences with significant magmatic events at ca. 3 to 2.5 Ma within the Southern Kenya Rift and NTD. This phase manifests as basalts and trachytes that flood the newly formed graben in the Southern Kenya Rift. This phase also correlates with the main phase of magmatism in the NTD, the commencement of volcanism at Virunga, and the intermediate shield phase in the Rungwe Volcanic Province. The Axial Phase (0.5 Ma to present) is defined by narrow axial grabens in the Kenya Rift with central volcanoes and interposing basaltic cinder cones. In the less mature regions of the EARS, alkaline volcanoes dominate. Three distinct pulses of magmatism and extension initiated in Turkana commencing at ca. 23 Ma, 12 Ma, and 4 Ma; the consistent temporal lag in magmatic activity in the Southern EARS following each pulse reveals the southward expansion of these extensional events. Lavas at any given location within the rift exhibit a consistent temporal sequence characterized by a shift from Type II lavas (lithospheric mantle derived) to Type IV lavas (hybrid lithospheric mantle – sub-lithosphere derived) to Type III lavas (sub-lithosphere derived), requiring a progressive change in the reservoirs contributing to rift magmatism from lithospheric to sub-lithospheric. When examined in aggregate, these data paint a picture of progressive rift development and attendant lithospheric thinning, mediated by pulses of extension originating in Turkana. • There is a common cycle of extension and magmatism in the East African Rift System. • Initial lithospheric destabilization predates the surface manifestation of rifting. • The composition of the erupted lavas changes over time. • Initial sources of magmatism are easily fusible domains in the lithospheric mantle. • The source of later magmatism transitions to sub-lithospheric sources. [ABSTRACT FROM AUTHOR]