DETAILED STUDY OF 3 CONTIGUOUS SEGMENTS OF THE MID-ATLANTIC RIDGE, SOUTH OF THE AZORES (37-DEGREES N TO 38-DEGREES-30' N), USING ACOUSTIC IMAGING COUPLED WITH SUBMERSIBLE OBSERVATIONS

Using a new tool of seafloor characterisation (sonar images from FARA- SIGMA cruise; Needham et al., 1992), coupled with submersible observat ions (DIVA1 cruise) we compare, at different scales of observation, th ree contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37 degrees N and 38 degrees 30' N. The two n orthernmost segments ('38 degrees 20' N' and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the th ird segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and '38 degrees 20' N' segments, volcanic constructional activity can obliterate, during periods of high magmati c supply, the morphology inherited from tectonic activity. The dive re sults constrain the recent evolution of each segment and show that a t emporal variability in volcanic dynamics exists. On the three segments , outcrops of eruptive lavas alternate with large areas of explosive v olcanic ejecta. This cycle in volcanic activity is influenced by chang es in water depth, both spatially (i.e. between segments) and temporal ly (i.e. for the same segment through time). Each segment has known a specific history in its accretionary processes with a succession of te ctonic and volcanic predominance and changes in its Volcanic phases be tween volcanic ejecta and effusive dynamics. The hydrothermal activity is focused at the central part of each segment and is controlled by t he presence of fresh lava and major tectonic features.