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Vertical growth of caldera-forming crust: a case study of 50 ka Maninjau ignimbrite, Sumatran Fault Zone (SFZ), Indonesia

Takeshi Hasegawa1, Yasuaki Kaneda1, Indranova Suhendro2, Shohei Shibata1, Gabriela Nogo Retnaningtyas Bunga Naen3,4, Chie Kusu1

  • Affiliations: 1Department of Earth Sciences, Ibaraki University, Mito, Ibaraki, Japan 2Department of Environmental Geography, Universitas Gadjah Mada, Yogyakarta, Indonesia 3Research Center for Geological Disaster, National Research and Innovation Agency of Republic Indonesia (BRIN), Jakarta, Indonesia 4Center for Regional Revitalization in Research and Education, Akita University, Akita, Akita, Japan

  • Presentation type: Poster

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

  • Poster Board Number: 31

  • Programme No: 1.7.18

  • Theme 1 > Session 7

Abstract

Caldera-forming eruptions have significant social impact. It also has considerable impact on crustal evolution. We conducted geologic and petrologic study for 50 ka Maninjau caldera-forming eruption deposits (VEI=7), the Great Sumatran fault zone, Indonesia, to reveal large silicic magma processes extending throughout the crust. Maninjau ignimbrite consists of three eruptive units showing distinct 14C ages and paleomagnetic directions, suggesting each eruption occurred spasmodically with a time gap of a few hundred years. Crystal-poor rhyolite, represented by white pumice, was likely derived from ephemeral eruptible magma episodically (three times) extracted from deeper crystal mush zone. A minor amount of crystal-rich andesite (gray pumice) can be interpreted as dispersed fragments from the mush.  Estimated depths from "extraction" and "storage" pressures by using rhyolite-MELTS for crystal-poor rhyolite are 1.6--7.5 km and 1.7--21.7 km, respectively. Both storage/extraction depths become shallower and wider in later units. The shallower magma system at later stage might have been situated on top of former, melt-depleted mush zone compacted by subsidence of roof block(s) with segregation/evacuation of large amount of silicic magma. The depleted and compacted mush zone after former eruption might remain high heat transfer for maintaining next partial melting at shallower level, resulting in thickening of mush zone. Temporal shallowing of storage/extraction depths is also observed at Taupo Volcanic Zone, New Zealand, which may have related to tectonic controls by crustal extension. We need additional work (e.g. thermal modeling, tectonic stress field simulation) to fully explain the vertical growth of caldera-forming crust.