Evidence of sheared sills related to flank destabilization in a basaltic volcano

Abstract

International audiencePiton des Neiges basaltic volcano (La Réunion) has been deeply dissected by erosion, exposing large volumes of debris avalanche deposits. To shed light on the factors that led to volcano flank destabilizations, we studied the structure, the crystallographic and magnetic fabrics of the substratum of a debris avalanche unit. This substratum is a complex of N50 seaward-dipping sills that has been exposed by the avalanche. Structural observations show that the sill plane in contact with the avalanche is one of the latest intrusions in the sill complex. In this uppermost sill, the anisotropy of magnetic susceptibility (AMS) is correlated to the crystallographic preferred orientation of magmatic silicate minerals, allowing us to use AMS as a proxy to infer the magmatic flow. The AMS fabric across the intrusion is strongly asymmetric, which reveals that the contact sill was emplaced with a normal shear displacement of its hanging wall. The shear displacement and the magma flow in the intrusion are both directed toward the NNE, i.e. toward the sea, which is also the direction of the slope and of the debris avalanche runout. Because all the sills in the intrusion complex have a similar dip and dip direction, it is likely that several of them also underwent a cointrusive slip toward the NNE. We conclude that this cointrusive normal slip, repeated over many intrusions of the sill complex, increased the flank instability of the volcano. This incre-mental instability may have ended up into the observed debris avalanche deposit. At Piton de la Fournaise, the active volcano of La Réunion, sill intrusion and cointrusive flank displacement have been inferred from geophys-ical studies for the April 2007 eruption. By providing direct evidence of sheared sills, our study substantiates the idea that repeated sill intrusions may eventually trigger flank destabilizations in basaltic volcanoes

Shear Versus Tensile Failure Mechanisms Induced by Sill Intrusions: Implications for Emplacement of Conical and Saucer‐Shaped Intrusions

“…Shear damage associated with the emplacement of sills and laccoliths has been observed in the field, both in sedimentary settings (e.g., de Saint‐Blanquat et al, ; Pollard, ; Spacapan et al, ; Wilson et al, ) and in basaltic volcanoes (e.g., Berthod et al, ; Got et al, ). In addition, laboratory models of sill emplacement in the Coulomb brittle crust strongly suggest that plastic shear damage is likely controlling the emplacement of magma (Galland, ; Galland et al, ; Galland et al, ; Montanari et al, ; Guldstrand et al, ; Schmiedel, Galland, & Breitkreuz, ).…”

Landslide Processes Involved in Volcano Dismantling From Past to Present: The Remarkable Open‐Air Laboratory of the Cirque de Salazie (Reunion Island)

“…The Black Breccia is characterized by a clast‐supported gravel facies with an irregular clay matrix (Arnaud, 2005). This unit results from flank failures of the edifice during its early stage of activity (>2 Ma; Berthod et al., 2016), with propagation toward the north (Berthod et al., 2016; Chaput, 2013; Famin & Michon, 2010; Famin et al., 2016).…”

Inversion of coeval shear and normal stress of Piton de la Fournaise flank displacement

“…Instead, we propose that the posteruptive eastern flank displacement is due to the compaction and ongoing slow slip on a shallow detachment fault.An alternative process for flank destabilization has been indicated by field investigations at Piton des Neiges, the extinct volcano of La Réunion Island (Figure 1a). The existence of a large discontinuity associated with a stack of low-dip intrusions presenting ductile and brittle deformation has been interpreted as a detachment plane [Famin and Michon, 2010;Berthod et al, 2016]. Injections of magma along this plane of weakness can trigger cointrusive lateral displacement of the volcano flanks, in the form of sheared sills.…”