Tensional fissures and crustal extension rates in the northern part of the Main Ethiopian Rift

Abstract

This paper describes a swarm of tensional fissures located at the northern end of the Main Ethiopian Rift, and attempts to relate their dimensions and age to the overall rate of Rift widening. The fissures dissect an otherwise undisturbed welded tuff formation associated with the caldera complex of Mount Fantale. This tuff was dated by the fission track method to 168,000 ± 38,000 years. Examination of a comparable single fissure dissecting welded tuff and radiocarbon dated sediments at nearby K’one caldera complex suggests that the fissuring episode occurred within the last 7000 years. Detailed measurement of the total extension across the fissure swarm, taken in conjunction with these ages, indicates an average rate of widening of 0.01 cm per year across the 20 km width of the welded tuff. The overall extension rate of the rift at this latitude as determined by plate tectonic modelling is of the order of 0.5 cm per year, and by geodetic measurements between 0.1 and 0.45 cm per year. This marked discrepancy reflects the different scales of time and distance over which the various means of measurement operate. It indicates that the extension process is very variable over both time and space, and that surface evidence of extension may not be immediately apparent.