Djibouti Country Geology
Djibouti nation has been fairly well mapped on a regional scale. Djibouti is located at the junction of the three rifts – the East African rift, the Gulf of Aden rift and the Red Sea rift (Figs.2a & b). The opening of these trans-crustal rifts in three radiating alignments created a complex array of faults in the Djiboutian landmass. The region is in an early stage of continental drift and the faults are still active, opening @ 10-15 mm/year. The complexly faulted triangular land mass at the junction of the 3 major rifts is known as ‘Afar Triangle’ or ‘Afar Depression’ that exposes the new crust developed by rifting and eruption of oceanic-type basalts derived from the Earth’s Mantle.
Djibouti is almost entirely occupied by volcanic flows and Pleistocene cover sediments, barring a patch of Mesozoic sedimentary rocks southeast of Ali Sabieh (Fig.1a). The western half of Djibouti exposes large tracts of stratiform basalts with minor rhyolitic flows locally covered by younger Pleistocene sedimentary rocks. The eastern half is underlained by intimately associated older bimodal volcanics comprising rhyolites and basalts, locally inter-bedded with limestone, marl and gritty sandstones. There are a few small patches of recent volcanic flows east of Dikhil, north and west of Dorra and Lake Asal.
The volcanic activity in this part of Africa started 27-26 Ma ago by the emplacement of the Adolei basalts, followed by the eruption of Ali Adde rhyolitic series of flows between 25 and 19 Ma. The Mabla alkaline to peralkaline rhyolites and basalts were then emplaced between 15 & 10 Ma. Thereafter, the volcanic eruptions were essentially basaltic with local and minor felsic differentiates, which include Dalha and Somali series (9-4 Ma), the Stratoide series (4-1 Ma), the Tadjoura series which corresponds to the opening of Tadjoura Gulf and the Recent Moussa Ali strato-volcano series. At present the active volcanic centres are located at Asal rift and the Manda Inakir towards north. Based on the availability of large amount of geochronological data the following stratigraphic sequence has been worked out –
Pleistocene Sedimentary cover rocks
Recent volcanic series (Moussa Ali strato-volcano) < 1.0 Ma
Stratiform Rhyolites 1.0 Ma
Stratiform Basalts (Tadjoura series) 3.4 – 1.0 Ma
Gulf Basalts (Stratoide series) 4.0 – 3.4 Ma
Dalha and Somali Basalt series 9.0 – 4.0 Ma
Mabla Rhyolite/ Basalt series 15.0 –10.0 Ma
Ali Adde Rhyolitic series 25.0 -19.0 Ma
Adolei or Ancient Basalts 25 Ma
By virtue of its location over the regional ‘Great African Rift’ system considered to be the most active tectonic region, major part of Djibouti is traversed by a large number of transcrustal faults and fracture, creating several half-grabens and rift valleys.
The physiographic expression of hill ranges, particularly in Afar Depression around the Lake Asal is defined by steep NE dipping & NW-SE trending faults, traversing the shallow SW dipping sequence of basaltic flows. Structurally both the explored blocks are located close to major rift valleys, occupied by Tertiary to Recent multiple volcanic flows.
Hesdaba Prospect is located close to the northwestern edge of the Map showing areas of stretching and oceanic crust and areas exposing flood basalts that preceded rifting. Areas un-shaded or covered by flood basalts represent normal continental crust. As the crust is pulled apart, thinned crust with a complex mixture of continental and volcanic rock is generated. Eventually thinning of crust leads to eruption of oceanic-type basalts signaling the formation of new ocean crust.
The regional faults bounding the rifts; the Pliocene Flood Basalts and the segments of Quaternary magmatic segments. The map links the intrusion of Asal-Ghoubbet dykes with the outpouring of lavas from Ardakouba Volcano of 1978.
Gaggade Graben, hosted along a few sub-parallel fault bounded half-grabens, whereas the Asaleyta is located along the faults bounding southwestern margin of the active Lake-Asal Rift. Though both blocks exhibit evidences of hot spring and hydrothermal activity in the presence of patches of silica sinters and bladed calcite-quartz veining, Asaleyta Prospect is also marked by the presence of late stage fumarolic activity. Evidence of hydrothermal activity is apparent in intense leaching and de-lithification of rocks along the mineralized wall rocks, particularly in northwestern part of mineralized zones in Hesdaba Prospect. Blading of calcite (Fig 13c) is characteristic of rapid dissolution and precipitation of calcium carbonate from quick depressurizing supersaturated fluids as they ascend to the surface.
In Hesdaba block, there are several sub-parallel steep northeasterly dipping faults, rendering the stratiform volcanics (mainly vesicular basalt with minor rhyolites and pyroclastics) tilted towards southwestwards. Since these NW-SE trending faults exhibit their northeastern sides as downthrown blocks, they are sympathetic to Gaggade Graben bounding faults. The shallow southwesterly tilting of intervening blocks between the steep northeasterly dipping faults constitute half-grabens of Hesdaba Prospect.
These faults have acted as channel-ways for ascending of hydrothermal / magmatic fluids and interaction with the meteoric waters for deposition of silica and carbonate gels with the metals. The ascending hot fluids have also pervaded through the porous wall rocks around the faults, partially altering, indurating (by the process of silicification) and dissipating mineralization (sulphidization) in them.
The presence of silica sinters – being direct evidence of hot-spring activity is recorded at a few places in southeastern parts, despite erosion and weathering over the long periods. Presence of hydrothermal breccias and shatter-cone structures, permeated by carbonates or cryptocrystalline silica gels are commonly found associated with mineralized chalcedony veins, which indicate reactivation of faults and multi-phase hydrothermal activity.
The Asaleyta prospect is located along the northeastern slope of the major ridge bounding Lac-Asal rift zone has evidences of fumarolic activity along the contact of mineralized pyroclastics with the overlying bed of silica sinter, wherein the rocks are heated-up, bleached in different hues and become friable and are found to be releasing hot fumes into the atmosphere. This NW-SE trending line of fumarolic activity should mark a major fault, which extends on either side. The steep dips of mineralized pyroclastics also show variation from place to place, possibly due to reactivation of faults and local disturbances.
In addition to above, there is a wide zone of typical silica sinters towards southwestern part of Asaleyta block. These silica sinters locally carry parallel zones of geodes, which possibly represent spherulites (soap-bubble textured) grown near hot hydrothermal upwelling centers. Most of them are rounded to sub-rounded nodular Geode-like features, filled with drusy quartz, carbonates or rare zeolites. Since the areas southwest and northwest of the delineated Asaleyta prospect are yet to be mapped systematically, further work is necessary to delineate the structural details of the entire Asaleyta block.