Central volcanoes and caldera collapses in the late Miocene – Late Pleistocene Tibesti Volcanic Province, northwest Chad

Abstract

The Tibesti Volcanic Province (TVP) in northwest Chad represents the second largest of the five Gharyan–Tibesti volcanic provinces and covers an area around 29,000 km². The other four provinces are in Libya, but all five provinces are from late Miocene to Quaternary and may have a common mantle source. The TVP, however, differs from the other four as regards volcano-tectonic processes, eruption style, and production of volcanic materials. The volcanic products of the TVP were erupted from the end of Miocene to late Pleistocene, range from basaltic to acidic, and suggest a double magma source – a shallow chamber fed by a deeper and larger reservoir. More specifically, field observations and numerical modelling results suggest that the basaltic magmas forming scoria cones, primarily at the periphery of the TVP, came from a deeper magma reservoir in the lower crust while the rhyolite and ignimbrites were fed by a shallow crustal magma chamber. By contrast, the volcanic products of the four volcanic provinces in Libya are primarily basalts and fed directly from single deep reservoirs. In the period from 8 Ma to 7−5 Ma, the evolution of the TVP was characterised by the formation of central volcanoes. Subsequently, in the period from 7−5 Ma to 0.43 Ma, the TVP was subject to caldera collapses that produced large-volume ignimbrites (>100 km³). Here we present numerical models with plausible loading conditions to understand better the volcano-tectonic evolution of the TVP and the spatial and temporal distribution of its major volcanic units. Our results suggest that the normal local stress field encouraged the formation of a central-volcano edifice (a volcanic cone) fed by radial dykes and inclined sheets and mostly erupting small volumes. By contrast, stress fields generated as a result of small crustal uplift or doming, due to the accumulation of magma in a deep-seated reservoir, triggered the formation of a ring-fault and the injection of a ring-dyke above the lateral margins of a shallow crustal magma chamber. Subsequently, the piston-like caldera subsidence helped to squeeze magma out of the shallow chamber resulting in large eruptions.