Gezahegn Yirgu

Timing of the Ethiopian flood basalt event and implications for plume birth and global change

Continental flood basalts are often considered as fossil evidence of mantle plume heads impinging on the lithosphere, and have been related to continental breakup. Many of these flood basalts erupted within a short time span—of the order of 1 Myr—and were apparently synchronous with crises in global climate and with mass extinctions. Here we present geochronological (40Ar/39Ar) and magnetostratigraphic results for the Ethiopian traps, one of the last remaining flood basalts for which few such data were available. The bulk of the traps, which have been inferred to mark the appearance of the Ethiopian-Afar plume head at the Earths surface, erupted approximately 30 Myr ago, over a period of 1 Myr or less. This was about the time of a change to a colder and drier global climate, a major continental ice-sheet advance in Antarctica, the largest Tertiary sea-level drop and significant extinctions.

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the ∼60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of ∼2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975–84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia–Arabia plate boundary.

ISOTOPIC AND TRACE ELEMENT SIGNATURES OF ETHIOPIAN FLOOD BASALTS : EVIDENCE FOR PLUME-LITHOSPHERE INTERACTIONS

Abstract Trace element and radiogenic isotope data have been measured on Oligocene flood basalts from the northwestern Ethiopian plateau. Our aim was to investigate and identify the nature of mantle and crustal sources involved in the genesis of this huge volume of pre-rift basalts to constrain the interaction between the Afar mantle plume and the lithosphere at the onset of continental break-up. The three magma types previously identified on this plateau display contrasting geochemical signatures. The Low-Ti magma type (LT) basalts display a strong and variably developed lithospheric signature characterized by relative depletions in Nb, Ta, Th, and Rb and peaks at Ba and Pb compared to oceanic basalts. The High-Ti magma type basalts (HT2) display much more homogeneous compositions and have ocean island basalt-like trace element signatures, whereas HT1 basalts exhibit intermediate compositions between those of the two other groups. In contrast to the wide range of trace element compositions, Sr, Nd, and Pb isotope ratios display limited variations (87Sr/86Sr = 0.70304–0.70429; 143Nd/144Nd = 0.51271–0.51298; 206Pb/204Pb = 18.00–18.86). Correlations among isotopic and trace element ratios provide evidence for the involvement of various mantle and crustal components in the petrogenesis of these flood basalts. Two distinct mantle components are involved in the genesis of the LT and HT2 extreme magma types. The HT2 basalts were derived from an ocean island basalt-like mantle component (87Sr/86Sr ∼ 0.704; 143Nd/144Nd ∼ 0.51295; 206Pb/204Pb ∼ 18.8) that corresponds to the initial material of the Afar mantle plume. By contrast, the LT basalts result from the melting of a more depleted mantle component (87Sr/86Sr ∼ 0.7033; 143Nd/144Nd ∼ 0.5130; 206Pb/204Pb ∼ 18.6), either intrinsic to the plume itself or entrained in the Afar plume head during its ascent. Correlations of incompatible trace element and isotopic ratios with differentiation indices indicate that the more or less pronounced lithospheric signature of the Ethiopian flood basalts was acquired by crustal contamination of the magmas during their variable residence time in the lower and upper crust. The effects of crustal contamination are much more evident in the LT basalts because of their much less enriched initial characteristics.

The northwestern Ethiopian Plateau flood basalts: Classification and spatial distribution of magma types

Abstract The extensive, complex, continental flood basalt (CFB) province which occurs in Ethiopia and Yemen consists of Oligocene prerift volcanism related to the Africa–Arabia continental break-up. Basalts from the northwestern Ethiopian Plateau exhibit a particularly large range of compositions and, for the first time in the Afro-Arabian CFB province, low-Ti basalts have been encountered. Major and some trace element data have been used to identify distinct geochemical groups and evaluate the role of differentiation processes. Three magma types have been distinguished: two high-Ti groups (HT1 and HT2) and one low-Ti group (LT). The transitional to tholeiitic LT suite exhibits low TiO2 (1–2.6%), Fe2O3* (10.5–14.8%), CaO/Al2O3 (0.4–0.75), Nb/La (0.55–0.85) and high SiO2 (47–51%). In contrast, the HT2 suite exhibits high TiO2 (2.6–5%), Fe2O3* (13.1–14.7%), CaO/Al2O3 (0.9–1.43), Nb/La (1.1–1.4) and low SiO2 (44–48.3%). The HT1 series is intermediate between the LT and HT2 groups. These three groups of lavas originated from different parental magmas. They display distinct differentiation trends, either controlled by the removal of a shallow level gabbroic (Pl+Ol+Cpx) assemblage (LT and HT1 suites) or by deeper Ol+Cpx fractionation (HT2 suite). Most of this thick continental flood lava pile was emplaced over a short time interval (about 1–2 Ma). The three contrasted magma types do not reflect a temporal evolution of their sources but rather a strong spatial control. Indeed, the northwestern Plateau may be subdivided into two different subprovinces as all the low-Ti basalts are located in the northern part of the plateau, and the high-Ti basalts are exposed in the eastern and southern parts. The LT and HT1 basalts display compositional ranges similar to those of the low- and high-Ti groups from other main CFB provinces (e.g. Parana, Deccan, Karoo, Siberia, …). However, the HT2 group exhibits extreme OIB-like compositions. This unusual geochemical signature suggests the involvement of deep mantle in the genesis of the HT2 magmas. The LT compositions rather reflect the participation of the continental lithosphere, through mantle derived melts and/or crustal contamination.

Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia

The process of strain localization as rifting proceeds to continental breakup is readily observed along the Oligocene-Recent southern Red Sea rift, yet much of the Red Sea margin in Ethiopia remains unmapped. Rifting initiated above or near a mantle plume, which is marked by the Eo-Oligocene Ethiopia-Yemen fl ood basalt province. Objectives of this fi eld, remote sensing, and geochronology study are to establish a structural and stratigraphic framework for the southernmost Red Sea passive margin using new and existing 40 Ar/ 39 Ar age data along 6 transects. We present new sketch geological maps and cross sections to document the timing of extension in relation to magmatism and its variation along strike. These new data are integrated with plate kinematic, geological, and geophysical data to present a model for evolution of the southern Red Sea margin. Faults commonly marked by eruptive centers initiated between 29 and 26 Ma, coincident with rifting in the Gulf of Aden. The Red Sea rift terminated at 10°N until linkage of the Main Ethiopian rift and southern Red Sea occurred at ca. 11 Ma. Rifting progressed in three distinct stages; each new phase saw a marked change in the style of volcanism and a narrowing of the locus of extension. Stage 1 rhyolites were emplaced from 29 to 26 Ma in basins bounded by a steep border fault system. Between 25 and 20 Ma, strain localized to narrow zones of basaltic fi ssural eruptions and minor faulting. Stage 2 faults and eruptive centers are located ~50 km to the east of the border faults, and they comprise fl ows spanning at least 16‐7 Ma. After ca. 7 Ma, the locus of strain again migrated eastward (Stage 3). Strain in Stage 3 was largely accommodated by dike injection. Plate reconstructions predict high stretching factors (β ~3) in the southern Red Sea, suggesting that Stages 2 and 3 mark the onset of formation of crust transitional between oceanic and continental.

MAGNETOSTRATIGRAPHY AND TIMING OF THE OLIGOCENE ETHIOPIAN TRAPS

A combined paleomagnetic, 40Ar/39Ar and geochemical investigation has been conducted in two type sections (65 sites) of the NW Ethiopian plateau volcanic pile. The main 2 km-thick complete section of the flood basalts at Lima-Limo contains a succession of only three magnetic chrons. The central normal chron appears to correspond to Chron C11n, according to the 40Ar/39Ar ages, which cluster around 30 Ma. Two alternative magnetostratigraphic interpretations both indicate a duration on the order of or less than 1 Myr. A detailed major element study of the section shows remarkable magma homogeneity, with two cycles of differentiation. Using correlation with other sections, we propose that the emplacement of the bulk of the Ethiopian igneous province (with an estimated volume on the order of 106 km3) in about 1 Myr or less should be linked with the Oi2 global cooling event, which occurs in Chron C11r. Further independent support for this identification comes from the finding of prominent tephra horizons in central Indian Ocean sediments from leg 115, with a biostratigraphic age coinciding with the Oi2 event. The 30 Ma African pole produced by this study (77°N, 208°E, A95=3.7°, N=53) is far-sided by 6°±6° with respect to the reference synthetic pole for Africa and yields a large paleosecular variation.

Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts

Abstract The Ethiopian continental flood basalt (CFB) province (∼30 Ma, > 3 × 105 km3) was formed as the result of the impingement of the Afar mantle plume beneath the Ethiopian lithosphere. This province includes major sequences of rhyolitic ignimbrites generally found on top of the flood basalt sequence. Their volume is estimated to be at least 6 × 104km3, which represents 20% of that of the trap basalts. Their phenocryst assemblage (alkali feldspar, quartz, aegyrine-augite, ilmenite ± Ti-magnetite, richterite, and eckermanite) suggests temperatures in the range of 740 to 900°C. Four units were recognized in the field (Wegel Tena, Jima, Lima Limo, and Debre Birhan areas), each with its own geochemical specificity. Zr/Nb ratios remain constant between basalt and rhyolite in each area, and rhyolites associated with high-Ti or low-Ti basalts are, respectively, enriched or depleted in titanium. Their trace element and isotope (Sr, Nd, O) signatures (high 143Nd/144Nd and low 87Sr/86Sr ratios, compared to those of rhyolites from other CFB provinces) are clearly different from those of typical crustal melts and indicate that the Ethiopian rhyolites are among the most isotopically primitive rhyolites. Their major and trace element patterns suggest that they are likely to be derived from fractional crystallization of basaltic magmas similar in composition to the exposed flood basalts with only limited crustal contribution. Since Ethiopian high-Ti basalts have been shown to form from melting of a mantle plume, it is likely that Ethiopian ignimbrites, at least those that are Ti-rich, also incorporated material from the deep mantle. Rb-Sr isochrons on whole rocks and mineral separates (30.1 ± 0.4 Ma for Wegel Tena and 30.5 ± 0.4 Ma for Jima ignimbrites) show that most of the silicic volcanism occurred within 1.4 × 1015 mol) and Cl (6.4 × 1015 mol) into the atmosphere over a short time span, with the global cooling event at 30.3 Ma suggests that this volcanism might have accelerated the climate change that was already underway.

Timing of East African Rift development in southern Ethiopia: Implication for mantle plume activity and evolution of topography

Accurate determination of rifting chronology and associated uplift is crucial to understanding the evolution of the East African Rift System (EARS) and for identifying the significance of mantle plumes during continental breakup. This investigation of rift-related cooling along a major fault scarp in southern Ethiopia, using (U-Th)/He thermochronometry, shows that rifting started not before 20 Ma. Therefore, there is an absence of significant rift activity synchronous with the earliest volcanics of the EARS, which are Eocene in age. In contrast, this initial magmatic episode, which preceded the main flood basalts and rifting events by 15–20 Ma, is attributed to convective instabilities above the rising Afar mantle plume. A detailed spatial and temporal quantification of uplift and denudation along this rift shoulder shows that rift development in southern Ethiopia has been continuous since initiation in the Miocene. This direct evidence of denudation is inconsistent with the hypothesis that massive Plio-Pleistocene rifting and associated uplift occurred in this part of the EARS and could have triggered recent aridification. To the contrary, our study rather supports a major contribution of plume-related doming for creation of topographical barriers in the Ethiopian province.

Heads and tails: 30 million years of the Afar plume

Abstract Primitive recent mafic lavas from the Main Ethiopian Rift provide insight into the structure, composition and long-term history of the Afar plume. Modern rift basalts are mildly alkalic in composition, and were derived by moderate degrees of melting of fertile peridotite at depths corresponding to the base of the modern lithosphere (c.100 km). They are typically more silica-undersaturated than Oligocene lavas from the Ethiopia-Yemen continental flood basalt province, indicating derivation by generally smaller degrees of melting than were prevalent during the onset of plume head activity in this region. Major and trace element differences between the Oligocene and modern suites can be interpreted in terms of melting processes, including melt-induced binary mixing of melts from the Afar plume and those from three mantle end-member compositions (the convecting upper mantle and two enriched mantle sources). The Afar plume composition itself has remained essentially constant over the past 30 million years, indicating that the plume is a long-lived feature of the mantle. The geochemical and isotopic compositions of mafic lavas derived from the Afar plume support a modified single plume model in which multiple plume stems rise from a common large plume originating at great depth in the mantle (i.e. the South African superplume).

Melting during late-stage rifting in Afar is hot and deep

Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin.