Abstract

In the past five years we have completed systematic seabed and water column surveys of two key sections of the Reykjanes Ridge and the Mid-Atlantic Ridge, between 36° and 38° North, close to the Iceland and Azores hot-spots, respectively. These surveys have provided a data set which provides a unique opportunity to address the relationship between the incidence and location of hydrothermal venting relative to volcanism/tectonism and ridge segmentation. Along the Reykjanes Ridge, running SW from Iceland, deep-tow (TOBI) sidescan sonar and swath bathymetry data have indicated a pattern dominated by volcanism in the form of axial volcanic ridges with only minor tectonic fracturing. A water column survey along this section of ridge crest, using conventional profiling techniques, indicated no evidence for high-temperature hydrothermal activity except at 63°N where the Reykjanes Ridge intercepts the bathymetric platform (≤500m water depth) which surrounds Iceland. Along the Mid-Atlantic Ridge between 36–38°N, running SW from the Azores archipelago, TOBI and swath bathymetry data have imaged a ridge with spreading rate and obliquity of spreading motion similar to the Reykjanes Ridge. But this ridge-section is dominated by tectonism rather than volcanism, the axis being partitioned into a series of short (20–55 km length) segments of spreading axis, each systematically displaced from its neighbours by right-lateral non-transform offsets of 15–50 km. Hydrothermal activity (as detected by a combination of deep-tow instruments and conventional profiling) is much more common along this latter section of spreading centre: evidence for discrete sources of hydrothermal discharge was obtained on fourteen different occasions, in seven separate segments of ridge crest. Sources of hydrothermal activity, including the major Rainbow hydrothermal field, were observed to coincide consistently with ridge-offset intersections suggesting a strong link between tectonic segmentation and the occurrence of high temperature venting.