Strain rate and bimodal volcanism in the continental rift: Debre Zeyt volcanic field, northern MER, Ethiopia

Abstract

Bimodal volcanism, which is typical of the continental rift, is supposed to be mainly dependent on strain-rate setting. High strain rates enhance the coupling between brittle and ductile crust and create a high density of the fractures used by basaltic magmas to erupt. Decoupling of brittle and ductile layers is generated by low strain rates with low fracture density, whereby the magma can be emplaced in the middle and upper crusts and can successively differentiate to produce evolved products. Analogue experiments have been performed to simulate the extension of continental crusts with underplated magmas at different strain rates. The spatial distribution of the faults is examined for high and low strain rates. A high strain rate is characterised by high fracture density (about 2.25 faults/cm) and low fracture spacing (about 0.4 cm), whereas low strain rate has a low fracture density (about 1 fault/cm) and high fracture spacing (about 0.9 cm). The model was tested in the Debre Zeyt area, where a sequence of alternating basaltic and silicic products has been erupted since the late Miocene. In particular, the spatial distribution and self-similar clustering of the Pliocene silicic domes and Quaternary-Recent basaltic cones and maars were investigated. The cones are more anti-clustered ( D f:1.54) than the domes ( D f:1.35), implying a denser fracture distribution for basalts. The more homogeneous distribution for cones (basalt) than for domes (silicic) is interpreted as being linked primarily to different strain rates, as suggested by the results of the analogue experiments. The data obtained for the Debre Zeyt area corroborate the hypothesis that bimodality of magnetism is primarily dependent on strain rate.