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 tectonic structures and volcanic vents is presented. It is based on the mapping of tectonic and volcanic features using high-resolution imagery, including Synthetic Aperture Radar (SAR) images of the European Remote Sensing satellite number one (ERS-1). We have evidenced new faults: (1) N-striking normal faults in the Husavik fault zone of the North Atlantic ridge, and (2) NW-striking faults belonging to the Husavik transform fault zone, associated to drag folds which testify to a dextral strike-slip movement component. We show that most of the volcanic edifices 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 Madagascar, using aerial photographs, this approach makes it possible to identify faults and the regional Plio-Quaternary direction of extension, i.e. N65 degrees E to N85 degrees E. Volcanoes forming linear clusters, 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 orientation of the regional tension. Alignments of extension fractures, depending on their S- or Z-shape, permitted us to infer conjugated faults.