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The Kencherra Ignimbrite (Central Main Ethiopian Rift): a case study for assessing emplacement mechanisms of high-grade, welded ignimbrites

Raffaello Cioni1 , Zara Franceschini1, Alex Scarani2, Alessandro Vona2, Stéphane Scaillet3, Federico Sani1, Giacomo Corti4, Bruno Scaillet3, Gaëlle Prouteau3, Abate Assan Melaku^5 ^

  • Affiliations: 1Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Firenze, Italy; 2Dipartimento di Scienze, Università degli Studi Roma Tre, Roma, Italy; 3Institut des Sciences de la Terre d'Orléans, CNRS, Orléans, France; 4Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, Firenze, Italy; 5School of Earth Sciences, Addis Ababa University, Addis Ababa, Ethiopia 

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 189

  • Programme No: 3.9.20

  • Theme 3 > Session 9

Abstract

Having been never directly observed, processes controlling eruption and emplacement of high-grade, densely welded ignimbrites still present many open questions. We focus here on the Lower Pleistocene rhyolitic "Kencherra Ignimbrite", an important marker bed within the volcanic stratigraphy of the Central Main Ethiopian Rift. The ignimbrite crops out along the eastern margin faults of the rift, covering a minimum area of 1000 km2 with a nearly constant thickness of 10-20 m. Internal stratigraphy is defined by a characteristic sequence of facies, with basal and top vitrophyres enclosing the main body of the ignimbrite. This presents a pervasive, sub-horizontal foliation defined by laterally discontinuous parting planes never showing clear rheomorphic deformation, and possibly results from progressive aggradation and strain from a sustained current. Microstructural data, microscopic observations, density measurements, rheological experimental data, geochemical and geochronological analyses were collected to shed new light on its peculiar emplacement processes. The analysis of strain indicators and the collected rheological data are used to discuss in detail the main mechanisms controlling the emplacement of the current and the shearing imparted by the overriding particulate flow. This ignimbrite represents an interesting and peculiar case to study the syn-eruptive sequence of agglutination, deposition and deformation which characterizes the emplacement of high-grade ignimbrite deposits.