|
Alberto Pizzi, University of Chieti (Italy)
Mauro Coltorti, University of Siena (Italy)
Giuseppe Pomposo, University of Chieti (Italy)
Bekele Abebe, University of Addis Ababa (Ethiopia)
Leonardo Disperati, University of Siena (Italy)
Dario Firuzabadì, University of Siena (Italy)
Laura Pontarelli, University of Chieti (Italy)
Giorgio Sacchi, University of Siena (Italy)
Riccardo Salvini, University of Siena (Italy)
|
|
An integrated structural and geomorphologic study along the Somalian Plateau escarpment at the southern Afar margin (Dire Dawa area, Ethiopia) provides insights into the rift geometry, timing and evolution of this sector of the East Africa Rift system.
Rift architecture is characterized by faulted blocks tilting away from the Afar Depression and has been previously described as a classic example of "domino-style faulting". Faulted monoclines composed of footwall anticline-hanging-wall syncline pairs, adjacent to many of the block-bounding and intra-block faults, testify to a fault-propagation folding in an extensional regime. Kinematic indicators measured along the fault planes commonly show a slight left lateral component providing a ca. NNW-SSE direction of extension, most probably as a consequence of the propagation of the Aden rift towards the Afar.
The reconstruction of landscape evolution provided the possibility of recognising two major periods of faulting. In fact, a large pediment was modelled across the faulted blocks along the Rift margin. In the foothill it is covered by the basalts of the Afar stratoid series, dated to the Early-Middle Pleistocene. However, from a morphostratigraphic point of view, the pediment could be much younger. In fact, it is the only feature that predates the late Pleistocene pediment and overlying deposits, later affected by the deepening of the drainage network and the deposition of Holocene calcareous tufa. Nowadays the older pediment is heavely dissected and preserved at the top of the blocks, more than one hundred metres above the valley bottom in places. Some of the faults have been severely reactivated with the creation of long fault escarpments with bottle necked hanging valleys. Close to the main fault escarpment its remains are locally tilted counterslope up to 5° suggesting a minimum new phase of rotation of 10°. Other information on the long scale evolution can be obtained by the analysis of the drainage network that along the rift margin developed after the modelling of the older pediment. It was mostly influenced by the main E-W trending extensional faults and by the NNW-SSE system of fractures and doleritic dikes emplaced together with the Oligocene Trap Basalts.
An even older drainage network has been recognised along the watershed where wide paleovalleys cut for over 1400 m the top of the Trap Basalts, testifying a pre Rift drainage oriented towards the Somalian lowlands. They most probably originated as a consequence of the upwarping that followed the emplacement of the Trap Basalts, and evolved as superimposed rivers. The northward drainage is therefore the result of river reversion as a consequence of the down-faulting associated to the rift activation and evolution. The evolution of the drainage suggests that the emplacement of the Trap occurred at a low elevation and was followed by upwarping and much later by rifting processes and down-faulting.
|
