Résumé
L’étude combinée d’une scéne Spot et d’un modèle numérique de terrain a été réalisée sur les îles de Raiateu et Tahaa (archipel de la Société, Polynésie française) Afin de déterminer la géométrie et la structure de l’édifice volcanique et de cartographier les différentes coulées basaltiques et trachytiques. La difficulté majeure de l’interprétation d’une image Spot en domaine tropical océanique réside dans l’existence d’une couverture végétale importante qui masque tout affleurement sur le terrain et rend la cartographie par photographie aérienne délicate. L’utilisation d’images multispectrales comprenant un canal dans l’infrarouge sensible aux changements de végétation permet de contourner cette difficulté. En effet, la distribution et le type de végétation sont contrôlés par plusieurs paramètres dont la nature du sol. L’interprétation des images brutes et de différents rapports de canaux n’a malheureusement pas permis d’aboutir à des résultats significatifs à l’échelle globale des îles étudiées. Ceci est dû essentiellement au fait que, dans cet écosystème particulier que constitue une île intra-océanique, la distribution de la végétation est essentiellement contrôlée par l’altitude et non pas par la nature du substratum sur lequel elle se développe. L’exploitation du modèle numérique de terrain de l’île de Raiatea, réalisée à partir de carte topographique numérique a été beaucoup plus fructueuse. En effet, l’interprétation de ce type de topographie a permis de déterminer la géométrie de l’édifice volcanique qui résulte, à la fois, de l’emboîtement de deux caldeiras effondrées et du glissement du flanc oriental. Cette déstabilisation mixte est analogue à celle décrite au niveau d’autres édifices, comme à la Réunion ou à Hawaii.
Abstract
A combined analysis of SPOT imagery and a Digital Elevation Model (DEM) was carried out,for the islands of Raiatea and Tahaa, in the Society Islands of French Polynesia, to determine the geometry and structure of the volcano and to map the pattern of‘ the basaltic and trachytic lava flows. Previous studies on Raiatea have suggested different shapes ,for the volcanic caldera. Deneufbourg (1965) proposed a simple large caldera located in the southern part of the island, while Brousse and Berger (1985) described two ringed calderas. Recently, Blais et al. (1997) suggested that the eastern flank of the caldera corresponds to gravitational sliding toward the sea. The main technical problem in this study arises ,from the outcrop condition because of the highly dense vegetation that masks the rocks both in the.field and on satellite images. To overcome this difficulty, we used multispectral SPOT image, which includes an infrared channel sensitive to vegetation changes that may be related to differences in lithology. An analysis of the different untreated channels reveals a great amount of detail on the coral reef and its associated lagoon. However, the islands show a homogeneous radiometric pattern with low contrasts. Only some small sedimentury plains and Very recent flows can be extracted. For the areas covered by dense vegetation, a very low contrast can be detected that coincides with the topographic contour level. This is interpreted as resulting from a vegetation distribution that is guided by elevation and not by lithology. A more specific processing was performed to highlight these low contrasts due to vegetation by calculating index ratios between the different channels (Pouget et al., 1988). This processing exhibits the main facies observed on the untreated images, but no significant changes that could be interpreted as resulting ,from changes in lithology. The caldera structure and the,faulting pattern cannot be revealed by such a processing technique. Thus, information contained in the topography was digitized and transformed into a digital elevation model ,from the classical topographie map. This analysis reveals two topographie rings in the southern part of Raiatea: the larger one,forms the main topographie feature, whereas the smaller one is located inside the larger one and is largely discontinuous. These ring struc- tures displuy asymmetrical profiles with their steeper slopes oriented toward the central part of the island. The discontinuity of the small inner ring can be attributed to a late-stage event. The outer ring is largely dissected by broad deep valleys, while the inner corona is mainly affected on its eastern ,flank by narrow valleys. This difference in the erosion rate can be interpreted as resulting,from a difference in age: the inner ,feature being younger (SO less eroded) than the outer one The eastern flank of Raiatea is extensively cut by valleys trending N 50° E. Blais et al. (1997) have pointed out that the valley located to the east of the Faaroa trough is controlled by a set of N 50” E-trending faults. These faults overprint both ring structures located in the south of the island, indicating that they correspond to a late event. We suggest that the present-day structural pattern of Raiatea arises from three events: a main gravitational collapse forming the Faaroa trough, then a second collapse generating the smaller inner caldera, and,finally, N 50° E-trending faults affecting the eastern part of the island. This evolution of deformation through time seems to be characteristic of’ volcanic islands located within plates.
Dernière mise à jour le 02.07.2015