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DYNAMIQUE DU RIFTING CONTINENTAL DE 30 Ma A L'ACTUEL DANS LA PARTIE SUD EST DU TRIANGLE AFAR. TECTONIQUE ET MAGMATISME DU RIFT DE TADJOURA ET DES DOMAINES DANAKIL ET D'ALI SABIEH, REPUBLIQUE DE DJIBOUTI.

Abstract : The results of our study about (i) the recent basaltic series in the Tadjoura Gulf and (ii) the oldest rift series in the Ali Sabieh and Danakil area supply new constraints about the recent-active kinematical history of the Tadjoura rift, and those previously recorded by the marginal zones of the Afar depression. Hypotheses are also attempted at a wider scale about the kinematic development of he Afar extensional system over the past 30 Ma.
- The structural analysis of the recent basaltic series in the TG allows us to regard the overall structure of the Tadjoura rift as a typical half-graben facing to the S, developed over older trap complexes, and accommodating a bulk extension of about 25-30%. The progressive younging of faulting towards the inner trough of the half-graben is the expression of focussing of strain with time along the axial part of the rift. From the geometrical and statistical analyses of onshore fault networks, a marked westerly increase of strain is documented along the southern fault belt, and is mechanically assigned to the frontal locking of axial fault tip propagation against the Arta submeridian fault zone. According to our alternative kinematic model, transfer of extension from the Tadjoura rift into the Ghoubbet-Asal trough operated via a rift jump on both sides of the Arta discontinuity (hard linkage), unstead of being accommodated by newly-formed en echelon fault structures (soft linkage).
- The structural and geochemical-radiometrical studies of older synrift volcanics and their Mesozoic substratum in the Ali Sabieh range shows the flowing facts as :
. Initial volcanics of the Ali Sabieh mafic complex were emplaced in the time range 26-19 Ma, i .e. 6 Ma later than the Ethiopian and Yemen trap series which are thus totally missing in the Ali Sabieh intermediate region.
. The intrusive part of the magmatic complex is interpreted as a mafic laccolith causing the antiformal structure of the Mesozoic cover during its uprise through shallow crustal level. Over the rising antiform, the arched Jurassico-Cretaceous sedimentary cover underwent fragmentation into detached xenoliths which later collapsed into the magma, as roof pendants typical of a block-stoping process.
. The ali Sabieh antiform involved up to the 15-11 Ma Mablas acidic series, whereas it is sealed by the trap basalts of the (7.2-3.0 Ma) Somali and (8.6-3.8 Ma) Dalha series.
- The causative mechanisms responsible for the edification of the Danakil range to the N are much younger, since they partly involved the Stratoid Basalts, and they are likely related to Tadjoura and/or Asal rift tectonics.
- Our results have some implications on the dynamical history of the Afar extensional system, as a whole, and new hypotheses are tentatively proposed to a number of key-issues dealing with : (i) the sens of propagation of the active rift axes extending throughout the Afar depression, as well as their belonging to either the Gulf of Aden or Red Sea spreading systems, (ii) the spatial distribution of the initial trap volcanism emplaced at 30 Ma over two disconnected magmatic sub-provinces, separated by a submeridian amagmatic crustal domain involving the Danakil and Ali Sabieh domains, and (iii) the evolution of the Danakil domain, as well as the nature of its eastern margin which is regarded as one of the fundamental discontinuity of the entire Afar framework, on either sides of which mechanisms of crustal extension vary dramatically, as being exclusively accommodated by either magmatic accretion to the W (Afar), or by tectonic faulting and associated basin development to the E (Red Sea).
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https://tel.archives-ouvertes.fr/tel-00312800
Contributor : Mohamed Ahmed Daoud <>
Submitted on : Tuesday, August 26, 2008 - 3:18:40 PM
Last modification on : Wednesday, October 14, 2020 - 3:52:15 AM
Long-term archiving on: : Thursday, June 3, 2010 - 5:38:58 PM

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Mohamed Ahmed Daoud. DYNAMIQUE DU RIFTING CONTINENTAL DE 30 Ma A L'ACTUEL DANS LA PARTIE SUD EST DU TRIANGLE AFAR. TECTONIQUE ET MAGMATISME DU RIFT DE TADJOURA ET DES DOMAINES DANAKIL ET D'ALI SABIEH, REPUBLIQUE DE DJIBOUTI.. Géologie appliquée. Université de Bretagne occidentale - Brest, 2008. Français. ⟨tel-00312800⟩

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