J. Varet

Volcanism of Afar: Small-scale plate tectonics implications

Structures analogous to oceanic spreading ridges, transform faults, and “leaky” fracture zones have been identified in Afar. They were formed during the past 3 to 4 m.y. and permit identification of the present plate boundaries within Afar and determination of the amount of spreading in this interval. At least two microplates are required in the zone of the junction of the African, Arabian, and Somalian plates. Variations of the spreading rate along single ridges or from one ridge to another, ridge jumping, migration of spreading with time, and counterclockwise rotation of the microplates are inferred by comparing volcanological, chronological, and geochemical data with aeromagnetic data. Deformation affects both accreting and transform plate boundaries within zones similar in width to the microplates, whose interiors are also systematically affected by regional faulting and locally by volcanism and transverse faulting. This indicates that although plate tectonics can explain most of the Afar features it fails when applied at such a small scale.

Evolution of the Danakil Depression (Afar, Ethiopia) in Light of Radiometric Age Determinations

Fourteen new K/Ar and fission-track ages obtained on volcanic rocks from the Danakil depression are presented. Together with previous data, they allow a preliminary reconstruction of the volcanic evolution of the northern Afar (Danakil) rift. Independent age determinations indicate that the Danakil rift began to form in the lower Miocene (23-25 m.y. B.P.). Since then and up to the present day, volcanism has been almost continuous producing rock associations of contrasting origin according to the tectonic nature of the areas involved. The development of axial volcanic ranges of subcrustal origin, marking the evolution from a continental to an oceanic rift by means of separation of the sialic crust probably took place only very recently (less than 1.2 m.y. B.P.).

Transverse tectonics during the split of a continent: Data from the afar rift☆

Abstract The northern Afar Rift is a region where the earliest stages of the splitting of a continent and creation of new oceanic crust can be observed. New oceanic-type crust is at present being emplaced along the axis of the rift. The axis is offset at several places; offsets of the western margin of the rift (scarp of the Ethiopian Plateau) correspond to the axial offsets. The main tectonic extensional features of the region are parallel to this axis; however, tectonic structures which are transversal relative to this axis were also observed at the offsets and elsewhere. These transversal structures are characterized by 1. (a) alkalibasalt volcanism which contrasts with the tholeiitic or intermediate basaltic volcanism of the axial structures 2. (b) upward intrusion of mantle material, as shown by geophysical data and by the presence of mantle-derived peridotite inclusions in the basalts from the transversal structures. The transversal tectonic structures developed by rejuvenation of older, pre-rift structures. These characteristics are similar to features observed in the equatorial Atlantic, where the large fracture zones, which probably also developed along ancient tectonic structures, are characterized by alkali-basalt volcanism, and by upbulging and upward intrusion of the mantle. The development of axial offsets and transverse tectonic structures both in the Afar Rift and in the early stages of the Atlantic Rift can be explained by essentially the same model.

Surface features associated with transform faults: A comparison between observed examples and an experimental model

Abstract In several tectonic provinces where active ridge segments are offset, transform faults are expected but not observed. This paper discusses the evolution of the surface expression of some transform faults with the help of a few geological examples and a simple experimental clay model in which the importance of en-echelon fault systems is assessed. We conclude that the azimuth of observed fault traces may not coincide with the direction of movement, but be oblique to it. Thus we must be cautious when using a fieldobserved fault direction to infer a transform-fault direction for use in plate-tectonics models. This study also suggests the scale at which the assumption of rigid plates fails.

Géologie de l’Afar Septentrional

RésuméTectonique — Du 15° au 13° parallele, l’apex septentrional de l’Afar est conditionné par des fissures de tension ouvertes et des failles normales de direction NNW. Un graben à fosse centrale étroite est nettement visible sur plusieurs dizaines de km au NNW du lac Giulietti. Ensuite il est caché sous les appareils volcaniques de la Chaíne de l’Erta-Alc qui jalonnent l’emplacement de son fossé central. Au-delà, dans la Plaine de Sel, la trace de ce fosse est marquée par l’alignement d’accidents divers (dómes de sel, sources carboniques, cratères d’explosion). Au nord de la Plaine de Sel enfin, la Chaine de l’Alid se trouve exactement dans l’alignement du graben.Par contre aucun linéament confirmant ni l’existence de la Wonji Fault Belt ni celle de décrochements horizontaux n’a pu être observé sur le terrain. L’hypothèse selon laquelle l’Escarpement Ethiopien serait le résultat d’une flexure plutôt que d’une fracturation semble contredite par les faits, ainsi que celle qui considére la Dépression Danakil comme résultant du simple basculement vers l’Ouest d’un bloc faillé le long de sa bordure occidentale seulement.L’Afar Septentrional est une structure en graben à fosse centrale étroite, semblable à celui de la Mer Rouge, et relayerait ce dernier « en échelon ». Volcanologie — Le volcanisme de cette région peut provisoirement être subdivisé en 3 grandes unités:1) Au Nord, la chaine de l’Alid l.s., allongée sur une soixantaine de km vers le SSE, 2) au centre, la chaine d’Erta’Ale l.s., de même direction, séparée de la précédente par l’étendue de la Plaine de Sel mais reliée à elle par une succession d’accidents subvolcaniques marquant l’axe de cette Plaine, 3) au Sud, un alignement géographique, à cheval sur le 13° parallèle: le Barawli Franca et l’Afdera, strato-volcan complexe; l’Alayta, puissant bouclier embryonnaire; le complexe volcanique Pierre Pruvost, comportant des coulées basaltiques, un strato-volcan à caldera et ignimbrites associées et un groupe de dômes rhyolitiques actifs.La partie méridionale de la chaíne de l’Erta’Ale l.s. montre un volcanisme basaltique fissural; vers le Nord, le volcanisme fissural fait place à des volcans boucliers; enfin la partie septentrionale porte, sur les basaltes fissuraux de base, des appareils plus évolués (strato-volcans sur horst) avec émissions tardives de laves rhyolitiques.Tous les volcans de la chaîne sont actifs. Pétrographie et magmatologie — Les échantillons analysés révèlent l’existence de 5 types de roches volcaniques dans la zone étudiée:basaltes; basaltes à andésine; trachytes sombres; trachytes sursaturés; rhyolites sodiques.Dans la Chaîne Centrale, les roches acides apparaîssent dans la moitié septentrionale; leur proportion augmente vers le Nord, mais demeure toujours infime par rapport aux basaltes.Les études faites à ce jour ont permis de déceler l’existence d’une série évolutive à caractère alcalin:1)l’activité fissurale originelle émet des basaltes alcalins à olivine.2)le stade suivant est caractérisé par d’abondants basaltes à andesine.3)un dernier stade engendre soit des trachytes fémiques sombres, et (ou) des rhyolites sodiques. Les relations existant entre, d’une part la série basaltes-trachytes et d’autre part la série trachytes sursaturés-rhyolites sodiques, constituent l’un des problèmes principaux de la magmatologie de l’Afar septentrional.AbstractTectonics — Between 15° and 13° N, Afar northern apex’ tectonics are determined essentially by sets of fractures with a NNW trend. This faulting is made of open tension fissures and normal faults, that form a graben with a narrow central trench. This trench is clearly visible over approx. 30 km NNW from lake Giulietti. It is then hidden below volcanic piles of the Erta Ale Range that mark the central trench. Further north the graben is concealed below very thick (several thousand m) evaporite deposits of the Salt Plain; but the central trench is still marked there by a line of varied accidents such as salt domes (including the potash dome of Dalol), phreatic explosion craters, and CO2 charged springs. North of the Salt Plain, the NNW trend is marked by the Mara’a-Alid Range.As a set-off, no lineaments have been observed in the field that could back the hypothesis of a N-S active zone (Wonji Fault Belt) being the most important feature of Afar Depression; this is definitively not the case, at least north of latitude 13°. Neither transverse transcurrent nor transform faults have been found.The assertion the huge scarp bordering Afar to the West being only an erosional feature superimposed over a large downwarp of the Ethiopian Plateau stratas is contradicted by several facts (actual normal faults, magnetic data, volcanoes close to the scarp, etc.).The conclusion of this first approach is that Northern Afar is a graben structure «en échelon» with the main Red Sea Rift, taking the place of the latter exactly at the latitude where it dies out. According to us, northern Afar is definitively a Red Sea structure and not, as previously proposed, a funnel shaped widening of the Main Ethiopian (Est African) Rilt. Volcanology — The volcanism of the Erta Ale Range is typically fissural in its southermost third; northwards shield volcanoes appear in the central part of the range; eventually, strato volcanoes, with trachytic and rhyolitic lavas, are heaped over the fissure basalts in the northern third of the chain. All the 7 volcanoes of the Range are active (either eruptive or fumarolic activity).The volcanoes over the 13th parallel are, from East to West, Barawli-Franca, a complexe acid center; Afdera, a dormant strato volcano; Alayta a big half fissure, half shield volcano; and Pierre Pruvost complex, with basaltic lava fields, a caldera strato volcano with related ignimbritic sheets, and a big cluster of active rhyolitic domes with rhyolitic lava flows. Peirology and magmatology — Samples analysed to date show five types of rocks outcropping in the surveyed area:1) basalts; 2) andesine basalts; 3) dark trachytes; 4) oversatured trachytes; 5) soda rhyolites and pantellerites.In the Erta Ale range, acid rocks appear in the northern half and their quantity, relative to basic ones, increases northwards though remaining always quite subordinate. These acid rocks are always emitted by the central crater itself or through a nearby point. In the complex volcanoes of the 13th parallel lineament, trachytes and rhyolites are more generally concentrated on one side of the related basic strato-volcano.Current studies allow to detect the existence of an evolutive series of alkaline character:1) initial fissure activity emits olivine alkali basalts; 2) a second stage is characterized by abundant andesine basalts; 3) a third stage generates either dark, femic trachytes, and (or) soda rhyolites.The inter-relationships between the basalt-dark trachytes series on the one hand and the oversatured trachytes — soda rhyolites series on the other hand, is one of the main problems of the northern Alar magmatology.

Pétrologie de la Série Volcanique de l’île de Fayal (Açores)

AbstractFour volcanic units have been distinguished on the islanf of Fayal. In order of decreasing age, these are:the eastern rift, and products of the activity preceding the collapse of the caldera visible at the summit of the stratovolcano, characterized by an alkaline series: basalt-hawaiite-mugearite-trachyte;the products of the explosive and postcaldera activity where only evolved lavas occur (benmoreites and trachytes);the recent basaltic activity of the Horta region;the western fissural activity — recent and historical. The two last units are characterized by exclusively basaltic, frequently picritic, eruptions.The lava groups cannot be distinguished by chemical criteria and have thus been treated as a single suite.Ninety samples have been analysed by X-ray fluorescence, and the mineralogy of 6 representative specimens has been determined by microprobe. The data were used to work out the evolution of the lava. The series is shown to have been produced by crystal fractionation under moderate water pressure from an alkali basalt. Moderate fractionation of amphibole during the last stages allow the liquids to remain weakly undersaturated from initial basalts until final trachytes.Mineralogical and chemical diversity between the most evolved lavas, benmoreites and trachytes, is an evidence of the strong influence ofpH2O and/orfO2 on the composition of such residua.ResumeQuatre unités volcaniques sont distinguées dans l’île de Fayal:le rift Est, et l’activité anté caldeira du stratovolcan, caractérisé par une série alcaline: basalte-hawaite-mugéatite-trachyte.L’activité explosive et post caldeira, caractérisée par des laves évoluées (benmorites et trachytes).L’activité basaltique récente de la région de Horta.L’activité fissurale ouest, récente et historique. Ceux deux dernières unités sont caractérisées par des éruptions exclusivement basaltiques, souvent picritiques. Ces ensembles de laves constituent des séries magmatiques que des critères chimiques ne permettent pas de distinguer. L’analyse d’une vingtaine d’échantillons, par fluorescence X, et l’étude minéralogique par microanalyse électronique de six roches représentant les types pétrographiques principaux, permet de préciser le processus de l’évolution de ces laves: cristallisation fractionnée sous pression modérée de H2O à partir d’un magma alcalin, faible fractionnement d’amphibole en fin de série permettant que les liquides demeurent faiblement sous-saturés depuis les basaltes jusqu’aux trachytes.Une certaine dispersion minéralogique et chimique au niveau des termes les plus évolués: benmoreïtes et trachytes, souligne la grande sensibilité de ces produits de fin de différentiation à l’influence depH2O oufO2.