The role of tephrachronology in recognizing synchronous caldera-forming events at the Quaternary volcanoes Longonot and Suswa, south Kenya Rift

This paper is part of the special publication No.161, Volcanoes in the Quaternary (eds: C.R. Firth and W.J. McGuire). The Quaternary volcanoes Longonot and Suswa, located in the south Kenya Rift, each have a caldera formed by incremental collapse accompanied by airfall, pyroclastic flow and pyroclastic surge activity. To constrain the relative ages of caldera formation at these two closely spaced centres, syn-caldera deposits are correlated using tephrachronology. Volcanic products from Longonot and Suswa display contrasting Nb/Zr ratios which provide a unique signature for each centre. Early syn-caldera pyroclastics at Suswa contain airfall pumice beds which have Longonot Nb/Zr signatures. Variation of FeO(T) and Al ₂O ₃ with Zr is particularly effective at discriminating the various Longonot units in two component space, and has been combined with volcanological evidence to identify the equivalent Longonot deposits as either early syn-caldera or late syn-caldera. In both cases, the correlation indicates synchronous initiation of caldera formation. This suggests that caldera formation is a regional rather than local event in the southern part of the rift, highlighting the potential role of regional tectonics as a controlling mechanism. Major alignments of eruptive vents, fissures and the caldera collapse centre at each volcano are parallel to rift faults, suggesting that rift floor tension fractures or normal faults are utilized as conduits for magma transport. Synchronous caldera formation at Longonot and Suswa may, therefore, be linked to lateral magma withdrawal along rift floor fractures and decomposition of the shallow-level magma chambers during a period of rift floor extension.