New geodetic measurements in central Afar constraining the Arabia-Somalia-Nubia triple junction kinematics

Monday, 15 December 2014
Cecile Doubre1, Aline Deprez2, Frederic Masson3, Anne Socquet4, Elias Lewi5, Raphael Grandin6, Eric Calais7, Tim J Wright8, Rebecca O Bendick9, Carolina Pagli8, Gilles Peltzer10, Jean-Bernard de Chabalier6 and Saad Ibrahim Ahmed11, (1)University of Strasbourg, Strasbourg Cedex, France, (2)EOST École et Observatoire des Sciences de la Terre, Strasbourg, France, (3)Univ Strasbourg, Strasbourg Cedex, France, (4)ISTerre Institute of Earth Sciences, Saint Martin d'Hères, France, (5)Addis Ababa University, IGSSA, Addis Ababa, Ethiopia, (6)Institut de Physique du Globe de Paris, Paris, France, (7)Ecole Normale Supérieure Paris, Paris, France, (8)University of Leeds, Leeds, United Kingdom, (9)University of Montana, Geosciences, Missoula, MT, United States, (10)University of California Los Angeles, Los Angeles, CA, United States, (11)Geophysical Observatory of Arta, Arta, Djibouti
The Afar Depression is an extraordinary submerged laboratory where the crustal mechanisms involved in the active rifting process can be studied. But the crustal movements at the regional scale are complicated by being the locus of the meeting of three divergent plate boundaries: the oceanic spreading ridges of the Red Sea and the Aden Ridge and the intra-continental East-African Rift (EAR). We present here the first GPS measurements conducted in a new network in Central Afar, complementing existing networks in Eritrea, around the Manda-Harraro 2005-2010 active segment, in the Northern part of the EAR and in Djibouti. Even if InSAR data were appropriate for mapping the deformation field, the results are difficult to interpret for analyzing the regional kinematics because of the atmospheric conditions, the lack of complete data catalogue, the acquisition configuration and the small velocity variations. Therefore, our measurements in the new sites are crucial to obtain an accurate velocity field over the whole depression, and focus specifically on the spatial organization of the deformation to characterize the tripe junction. These first results show that a small part of the motion of the Somalia plate with respect to the Nubia plate or the Arabia plate (2-3 mm/yr) occurs south of the Tadjura Gulf and East of the Adda-do segment in Southern Afar. The complex kinematic pattern involves a clockwise rotation of this Southeastern part of the Afar rift and can be related to the significant seismic activity regularly recorded in the region of Jigjiga (northern Somalia-Ethiopia border). The western continuation of the Aden Ridge into Afar extends West of the Asal rift segment and does not reach the young active segment of Manda-Inakir (MI). A slow gradient of velocity is observed across the Dobi Graben and across the large systems of faults between Lake Abhe and the MI rift segment. A striking change of the velocity direction occurs in the region of Assaïta, west of Lake Abhe, suggesting that this area represents the most probable location for the triple junction.