The interplay between collapse structures, hydrothermal systems, and magma intrusions: the case of the central area of Piton de la Fournaise volcano

Abstract

International audienceWe explore the possible relationships between a structural heterogeneity, the hydrothermal system, and the intrusive activity at Piton de la Fournaise volcano. Geological and geophysical data show that as the result of repeated collapses (the last one in 2007), a cylinder of faulted, fractured, and crumbled rocks must exist between the surface and the top of a magma reservoir at about sea level. This structure constitutes a major geological heterogeneity. An obvious spatial correlation exists between this column of fractured and brecciated rock and the location of (1) most of the seismic activity, (2) a low-resistivity dome, (3) a huge self-potential anomaly, (4) thermal evidence of hydrothermal activity, and (5) the root of magma intrusions. The dominant factors that make this structural heterogeneity a trap for the activity are probably its higher permeability and its weaker mechanical strength. Evidence exists for the presence of an active hydrothermal system confined in this permeable zone. The long-term stability of the activated zone above sea level and the similarity of the pre-eruptive crises, in spite of the inferred large perturbation of the magmatic system in 1998, suggest a common triggering mechanism for all the eruptions since at least the first data recorded by the observatory in 1980. This mechanism can be purely magmatic, resulting from the pressurization of a reservoir, but we also propose that the hydrothermal system may play a role in the development of volcanic instabilities. A qualitative model is proposed to explain the triggering of magma intrusions by hydrothermal processes, and its speculative aspects are discussed. This work represents a first attempt to integrate the structural and dynamic information in a unified framework at Piton de la Fournaise

Internal structure and building of basaltic shield volcanoes: the example of the Piton de La Fournaise terminal cone (La Réunion)

“…On the uppermost part of the terminal cone, only signs of superficial fluid circulations along vertical discontinuities are visible. The hydrothermal system suggested by Lénat et al (2012), from a combination of geological and geophysical data, is confined deeper in the faulted and crumbled rock cylinder below the summit, as attested to by geo-electrical studies. From 300 m depth to the limited depth of investigation (∼1,000 m), Lénat et al (2000) observed an electrical conductor below the summit interpreted as the altered hydrothermal zone (resistivity values<20 Ωm below ∼500 m depth and <250 Ωm from ∼500 m up to ∼300 m depth), whereas the overburden (now exposed in the outcrops) is composed of resistive layers, interpreted as a water-sparse region (1,000 to 3,000 Ωm and>8,000 Ωm for the shallowest 100 m).…”

“…As proposed by Lénat et al (2012), the hydrothermal system may play a role in the development of magma injections and volcanic instabilities and thus in the constructive and destructive phases of basaltic shields. During the most recent Piton de la Fournaise caldera collapses (notably in 2007), and as observed during the Miyakejima caldera collapse in 2000, no major explosive, phreatic or phreatomagmatic, activity and thus no strong magma-water interaction seemed to occur.…”

A new comprehensive classification of the Piton de la Fournaise activity spanning the 1985–2010 period. Search and analysis of short-term precursors from a broad-band seismological station

“…According to this hypothesis, possible seasonality can be proposed for the Piton de la Fournaise activity, linked to the annual hydrological cycle. Moreover, the inflation/deflation evolution of the volcano edifice could also be affected by the shallow hydrothermal system (Lénat et al, 2011), which could also play a role in the evolution of the stress field.…”

Synthesis of global satellite observations of magmatic and volcanic deformation: implications for volcano monitoring & the lateral extent of magmatic domains

“…Long-lived hotspot eruptions at Kilauea, Hawai'i (e.g., Poland et al, 2008) and Piton de la Fournaise, Réunion (e.g., Peltier et al, 2010), as well as repeated episodes of intrusion in the Galapagos (e.g., Bagnardi et al, 2013) dominate the small number of systems worldwide where multiple cycles of eruption have been observed geodetically (although this also includes non-hotpot volcanoes such as Okmok, Aleutians, Biggs et al, 2010a). As well as cycles of pre-eruptive uplift followed by co-eruptive subsidence, such long-lived eruptions have also provided evidence for endogenous growth, change in intrusion location in response to varying stress fields (e.g., Bagnardi et al, 2013) and cycles of feedback related to topographic structures (e.g., Jónsson, 2009;Lénat et al, 2012).…”