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

Abstract

The spatial clustering of vents in basaltic volcanic fields within a stretched continental crust is here used as a proxy for crustal thickness. Basaltic monogenetic vents show self-similar clustering with a power-law distribution defined by the correlation exponent (D) computed in a range with lower and upper cutoffs. The upper cutoff for the fractal clustering of vents yields the thickness of the crust. The spatial distribution of vents is analyzed in the Afar Depression (the northern termination of the East African Rift system in Africa), where the continental crust has thinned considerably. More than 1700 vents were identified and mapped in the Afar region through the use of Landsat ETM+ (enhanced thematic mapper) satellite image mosaics. Vents cluster in seven main groups corresponding to the principal structural features of the Afar Depression. The mapped vents are generally younger than 2 Ma, and most are Holocene age. The Afar vents show self-similar clustering (D = 1.39 ± 0.02) in the 2–23 km range. The upper cutoff of 23 km matches well the thickness of the crust in the Afar region as derived from seismic and gravity data (25 km). The distribution of vents in the Afar Depression is compared with that of vents in the northern Main Ethiopian Rift.