Continental rift formation and transition to ocean sea floor spreading: A case study of the Afar triple junction

Abstract

Lithosphere extension, thinning and breakup are fundamental processes in geodynamics. During rift development, both the lithosphere and the mantle are involved in a coupled system, in which the main mechanisms and the forces associated with them often vary during the rift evolution. Furthermore, the presence of lithospheric heterogeneities and mantle plumes may determine variations in rifting location and style. These features occur in Afar Rift, a region characterized by the transition from continental rifting features to seafloor spreading westwards. The Afar Rift represents the triple junction among the Main Ethiopian Rift, the Gulf of Aden and the Red Sea, separating the Nubia, Arabia and Somalia plates. The presence of a mantle plume has long been recognized in the region as an important factor influencing the tectonic activity and the extensive volcanism. The work presented in this thesis aimed to provide new insights in the role of lithospheric structure and mantle plumes on the development and evolution of rifting areas, with particular regard to the Afar Rift region. The quantitative analysis of the area has been carried out by constructing numerical models where different physical interacting mechanisms are taken into account. Thesemodelsexamine the evolution pattern of a continental lithosphere subject to extension, where the lithosphere is characterized by the presence of heterogeneities and impinged by a mantle plume. In addition, procedures to simulate temperature-induced metamorphism, crustal anatexis and partial melting ofmantle materials have been incorporated, with resultingrocks rheological variations.The aim of this model is to assess the importance of single factors that may influence theevolution of rifting areas, and particularly Afar Rift. The results highlight the importance of lithospheric structure in the first phases of rifting development, whereas further stages may be more effectively influenced by the presence of mantle plumes. The inherited basement structure may have exerted a primary control on initial rift location in the Afar and Main Ethiopian Rift regions, even in presence of a melt-intruded and weakened lithosphere. The observations on volcanic activity in Afar highlight a beginning of volcanism in the Nubian Plateau region, followed by a migration toward the rift axis area, fitting the model results. However, the observed southeastward migration of volcanism is better explained by a variable topography of the lithosphere-asthenosphere boundary, with a thicker lithosphere in the Nubian region and a thinner lithosphere in the Somalian area. The effect of lithosphere heterogeneities on the rift axis localization occurs in combination with the melt-induced weakening of the crust and the lithosphere, explaining the increased seismic activity on the rift areas. However, the widespread acidic volcanism in Afar can be explained better by processes of fractioned crystallization of mafic, underplated melts, further contaminated by crust-derived melts. At present day, the highest mantle thermal anomaly is located in the N-MER and may determine a further progression of the extensional tectonics towards continental breakup and seafloor spreading conditions. Thus, a further evolution of the area into conditions of seafloor spreading may be expected, similarly to the Danakil area’s current evolution.