AN APPROACH USING SAR ERS IMAGES TO RELATE EXTENSION FRACTURES TO VOLCANIC VENTS - EXAMPLES FROM ICELAND AND MADAGASCAR

A new approach to the study of geometric relationships between tectoni c structures and volcanic vents is presented. It is based on the mappi ng of tectonic and volcanic features using high-resolution imagery, in cluding Synthetic Aperture Radar (SAR) images of the European Remote S ensing satellite number one (ERS-1). We have evidenced new faults: (1) N-striking normal faults in the Husavik fault zone of the North Atlan tic ridge, and (2) NW-striking faults belonging to the Husavik transfo rm fault zone, associated to drag folds which testify to a dextral str ike-slip movement component. We show that most of the volcanic edifice s are located on extension fractures responsible for fissure eruptions , volcanic linear clusters and elongated volcanoes. This relationship is studied first in Iceland where the geodynamics is well known, using SAR ERS-1 imagery. Applied to the Montagne d'Ambre, in northern Madag ascar, using aerial photographs, this approach makes it possible to id entify faults and the regional Plio-Quaternary direction of extension, i.e. N65 degrees E to N85 degrees E. Volcanoes forming linear cluster s, together with elongated volcanoes, are interpreted to be located on extension fractures. S- or Z-shape of the linear clusters allowed us to determine the tips of extension fractures, and consequently the ori entation of the regional tension. Alignments of extension fractures, d epending on their S- or Z-shape, permitted us to infer conjugated faul ts.