Vent distribution and crustal thickness in stretched continental crust: The case of the Afar Depression (Ethiopia)

The spatial clustering of vents in basalticvolcanic fi elds within a stretched continentalcrust is here used as a proxy for crustalthickness. Basaltic monogenetic vents showself-similar clustering with a power-law distributiondefi ned by the correlation exponent(D) computed in a range with lowerand upper cutoffs. The upper cutoff for thefractal clustering of vents yields the thicknessof the crust. The spatial distribution ofvents is analyzed in the Afar Depression (thenorthern termination of the East African Riftsystem in Africa), where the continental crusthas thinned considerably. More than 1700vents were identifi ed and mapped in the Afarregion through the use of Landsat ETM+(enhanced thematic mapper) satellite imagemosaics. Vents cluster in seven main groupscorresponding to the principal structuralfeatures of the Afar Depression. The mappedvents are generally younger than 2 Ma, andmost are Holocene age. The Afar vents showself-similar clustering (D = 1.39 ± 0.02) in the~2–23 km range. The upper cutoff of ~23 kmmatches well the thickness of the crust in theAfar region as derived from seismic and gravitydata (~25 km). The distribution of vents inthe Afar Depression is compared with that ofvents in the northern Main Ethiopian Rift