TSK 11 Göttingen 2006 Letourneur & Gudmundsson
 Asymmetrical deformation
 of the Piton de la Fournaise
 (Réunion Island) summit
 cone Poster
 Ludovic Letourneur1
 Agust Gudmundsson1
 Piton de la Fournaise (Réunion Island)
 is an active basaltic shield volcano in the
 south-western part of the Indian Ocean.
 The activity consists essentially of lava
 being issued from two rift zones close to
 the summit cone. The summit cone has
 been monitored since 1980 by the OVPF
 (Volcanological Observatory of Piton de
 la Fournaise). Geodetic data and radar
 interferometry show a systematic asym-
 metric pattern of deformation associ-
 ated with all the N–S trending eruptive
 fissures (Briole et al. 1998, Sigmundsson
 et al. 1999).
 The asymmetric deformation has been
 interpreted in various ways. The two
 main models proposed are (1) east-
 ward dip of the dykes, based on in-
 version of data provided by radar in-
 terferometry (Sigmundsson et al 1999),
 and (2) the existence of a free bound-
 ary in the east part of the volcano. The
 second model allows the accommoda-
 tion of dykes by eastward displacements,
 whilst the western part of the volcano
 is supported by the existing Piton des
 Neiges (Lénat et al., 1989). However,
 no displacements occur along the east-
 ern coastline of Piton de la Fournaise.
 The only parts of the volcano affected
 by deformation are the rift zones and
 the summit cone. Moreover, geodetic
 data provided by the new real-time GPS
 network show that there is an asymmet-
 ric deformation of the summit cone prior
 1 Department of Structural Geology and Geo-
 dynamics, Geoscience Centre, University of
 Göttingen, Germany
 to eruption (Staudacher 2005). The de-
 formation itself seems to be related to
 pressure increase in the magma cham-
 ber. Absence of permanent displace-
 ments during periods of rest (such as
 1992–1998) (Briole et al. 1998) sup-
 port this link between magmatic pres-
 sure and deformation.
 Here we complement previous deforma-
 tion studies by the results of several nu-
 merical models that aim at understand-
 ing the internal processes resulting in
 deformation of the volcano. We inves-
 tigate the influence of different parame-
 ters such as the topography and the lat-
 eral mechanical heterogeneity revealed
 by gravimetric data and seismic data.
 We are able to obtain an asymmetric
 deformation as resulting from (1) the
 asymmetric topography of the volcano,
 and (2) the existence of a pluton under
 the Plaine des Sables area. The asym-
 metric topography can explain the ma-
 jor part of the asymmetric deformation,
 whilst the existence of pluton beneath
 the Plaine des Sables area arrests the
 propagation of deformation in the west-
 ern part of the volcano.
 Our models explain the large features
 of the asymmetric deformation observed
 during eruptions at Piton de la Four-
 naise Volcano. Vertical and lateral het-
 erogeneities are very important for un-
 derstanding the deformation. In the
 case of Piton de la Fournaise Volcano,
 absence of deformation on the other
 parts of the volcano, especially the
 lower eastern part, whilst the upper
 part shows much deformation, implies
 that stresses concentrate primarily in
 the eastern flank.
 1
Letourneur & Gudmundsson TSK 11 Göttingen 2006
 Figure 1: Numerical model showing the displacement triggered by the injection of an
 overpressured dyke (1MPa) in the summit cone of an asymmetric volcano. Unit = 1 km
 References
 Briole P, Bachèlery P, McGuire B, Moss J,
 Ruegg J-C & Soubrault P (1998) Deforma-
 tion of Piton de la Fournaise: Evolution of
 the monitoring techniques and knowledge ac-
 quired in the last five years. In: Casale R,
 et al. (eds) The European Laboratory Vol-
 canoes, Proceedings of the Second Workshop
 Santorini, Greece 2 to 4 May 1996, pp 467–
 474
 Lénat J-F, Bachèlery P, Bonneville A & Hirn
 A (1989) The beginning of the 1985-1987
 eruptive cycle at Piton de la Fournaise (La
 Réunion); new insights in the magmatic
 and volcano-tectonic systems. J Volcanol
 Geotherm Res 36, 209–232
 Sigmundsson F, Durand P & Massonnet D
 (1999) Opening of an eruptive fracture and
 seaward displacement at Piton de la Four-
 naise volcano measured by RADARSAT
 satellite radar interferometry. Geophys Res
 Lett 26, 533–536
 Staudacher T (2005) Bulletin de l’observatoire
 volcanologique du Piton de la Fournaise, 23
 September 2005
 2