Sheared sheet intrusions as mechanism for lateral flank displacement on basaltic volcanoes: Applications to Réunion Island volcanoes

Abstract

Field work carried out on the Piton des Neiges volcano (Réunion Island) suggests that the injection of magma along detachments could trigger flank failure by conjugate opening and shear displacement. We use 3-D numerical models to compare the ability of purely opened sheet intrusions, sheared sheet intrusions, and normal faults to induce flank displacement on basaltic volcanoes. We assume that shear stress change on fractures results from stress anisotropy of the host rock under gravity. Exploring a large range of stress anisotropies, fracture dips, and fracture depth over length ratios, we determine that the amount of shear displacement is independent of the proximity to the ground surface. Sheared sheet intrusions are the most efficient slip medium on volcanoes. Consequently, the largest flank displacement is induced by the longest, deepest sheared intrusion dipping closest to 45°in a host rock with the highest stress anisotropy. Using our model in a forward way, we provide shear and normal displacements for buried fractures. Applying the model to a pile of sills at the Piton des Neiges volcano, we determine that the mean shear displacement caused by each intrusion was 3.7 m, leading to a total of a 180-260 m of lateral displacement for the 50 m high pile of sills. Using our model in an inverse way, we formulate a decision tree to determine some fracture characteristics and the host rock stress anisotropy from ratios of maximum surface displacements. This procedure provides a priori models, which can be used to bound the parameter space before it is explored through a formal inversion. Applying the decision tree to the 1.4 m coeruptive flank displacement recorded at Piton de la Fournaise in 2007, we find that it probably originated from a shallow eastward dipping subhorizontal normal fault.

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

“…Thus, the rake is fixed at 90°based on the eastward GPS displacement recorded between 2 and 10 April at the only station (FERg) located to the east of the summit [Peltier et al, 2009a]. We also use a modified computation where fracture interpenetration is not permitted, a condition enforced through the use of Lagrange multipliers as explained in Cayol et al [2014]…”

“…Because we questioned whether the obtained closure was really required to explain the surface displacement, inversions preventing fracture interpenetration were also conducted [Cayol et al, 2014]. A demonstrably less good fit was obtained (RMSE = 3.52 and AIC = 2889, inversion termed "No interp" in Table 4), which indicates that closure of the fracture is the most likely model.…”

“…resulting fracture is submitted not only to overpressure but also to tangential stress. When the volcano stress field resulting from gravity is extensive, simultaneous cointrusive flank displacement and fracture opening are induced [Cayol et al, 2014;Chaput et al, 2014a]. Following this approach, Chaput et al [2014b] proposed a new model for flank failure of basaltic volcanoes in which cycles of stress permutations explain the variety of stress fields, as inferred from detailed microstructural studies at Piton des Neiges.…”

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

“…Since then, the role of sheared sill intrusions as a trigger of flank instability has been tested numerically. Using the observed geometry of the Piton des Neiges sill zone, Cayol et al (2014) computed that the injection of sills in a volcanic edifice undergoing extension would generate an averaged shear displacement of 3.7 m per sill, or a total lateral displacement of 180-260 m for the sill zone. In addition, (Chaput et al, 2014a) showed that the injection of a sill could be sufficient to activate a detachment such as the one found at Piton des Neiges.…”

“…Each sill would activate slip on the detachment, progressively increasing flank instability and eventually leading to flank failure. This model has been tested by numerical simulations, which show that sill intrusions in a volcanic edifice under extension may indeed yield lateral flank displacements, and may also activate a pre-existing detachment (Cayol et al, 2014;Chaput et al, 2014a). However, there is to date no observational constraints that sill intrusions cause a shear displacement during their emplacement.…”