From prodigious volcanic degassing to caldera subsidence and quiescence at Ambrym (Vanuatu): the influence of regional tectonics

Tara Shreve,Nicolas Henriot,Dan Tari,Pierre Lebellegard,Esline Garaebiti,Raphaël Grandin,Francisco Delgado,Valérie Ballu,Marie Boichu,Yves Moussallam,Bernard Pelletier,Martin Vallée,Frédérique Leclerc,Sandrine Cevuard

doi:10.1038/s41598-019-55141-7

Abstract

Eruptive activity shapes volcanic edifices. The formation of broad caldera depressions is often associated with major collapse events, emplacing conspicuous pyroclastic deposits. However, caldera subsidence may also proceed silently by magma withdrawal at depth, more difficult to detect. Ambrym, a basaltic volcanic island, hosts a 12-km wide caldera and several intensely-degassing lava lakes confined to intra-caldera cones. Using satellite remote sensing of deformation, gas emissions and thermal anomalies, combined with seismicity and ground observations, we show that in December 2018 an intra-caldera eruption at Ambrym preceded normal faulting with >2 m of associated uplift along the eastern rift zone and 2.5 m of caldera-wide subsidence. Deformation was caused by lateral migration of >0.4 cubic kilometers of magma into the rift zone, extinguishing the lava lakes, and feeding a submarine eruption in the rift edge. Recurring rifting episodes, favored by stress induced by the D’Entrecasteaux Ridge collision against the New Hebrides arc, lead to progressive subsidence of Ambrym’s caldera and concurrent draining of the lava lakes. Although counterintuitive, convergent margin systems can induce rift zone volcanism and subsequent caldera subsidence.

Highlights

Onset of caldera subsidence is dated around 2000 BP, based on two 14C dates of charcoal embedded in debris flows on the caldera rim and flank[11]
On 14 December 2018, a volcano-seismic crisis begins at Ambrym when 8 M < 3 seismic events are detected inside the caldera between 13h00 and 20h00 UTC (Fig. 2b)
Joint analysis of remote sensing and seismicity demonstrates that the condition initiating the rift intrusion in December 2018 was the creation of an open fracture at Lewolembwi, connecting the caldera’s localized magma supply to the rift zone

Summary

Introduction

A basaltic volcanic island, hosts a 12-km wide caldera and several intensely-degassing lava lakes confined to intra-caldera cones. Using satellite remote sensing of deformation, gas emissions and thermal anomalies, combined with seismicity and ground observations, we show that in December 2018 an intra-caldera eruption at Ambrym preceded normal faulting with >2 m of associated uplift along the eastern rift zone and 2.5 m of caldera-wide subsidence. Broad caldera formation (>10 km in diameter) is often attributed to ignimbrite-forming, explosive eruptions[1]. These eruptions are believed to be fed from a central reservoir situated beneath the caldera, and magma transported to the coast by lateral dikes[26]. Whether the broad caldera of Ambrym should be interpreted as a relict structure, or should be considered an active fault system, remains an open question. The relationship between the evolution of Ambrym’s caldera, the rift zone’s persistence, and the complex tectonic setting of the New Hebrides is yet to be explored

Methods

Results

Conclusion