Non-transform offsets along the Mid-Atlantic Ridge south of the Azores (38degrees N-34 degrees N): ultramafic exposures and hosting of hydrothermal vents

Tell contiguous non-transform offsets (NTOs) along the Mid-Atlantic Ridge ( MAR) south of the Azores (between 38 degrees N and 35 degrees 40'N) have be en studied in detail using swath bathymetric, acoustic backscatter and deep -tow high-resolution sidescan sonar (TOBI) data. In contrast with discontin uities studied elsewhere at slow-spreading ridges, these left-lateral NTOs are consistently broader and larger, with complex structural fabrics accomm odating the offset. They are characterized by a range of elevated and fault ed massifs detached from their segment flanks, with an irregular acoustic b ackscatter pattern. Some of these massifs have been explored and sampled re cently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks, and sometimes associated with high-te mperature hydrothermal venting. Water column surveys adjacent to these mass ifs show high CH4 and low TDM (total dissolvable manganese) concentrations, possibly resulting from the process of serpentinization of ultramafic rock s. The correlation between the shallow dome-like shaped massifs and the hig h concentrations of CH4 (associated with low levels of Mn) is of particular interest to predict the outcrop of ultramafic rocks within the NTOs where no geological data are available. The exposure of the ultramafic massifs wi thin the NTOs is favored by low magmatic supply and low-angle detachment fa ulting occurring at segment ends. The pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurren ce of high-temperature vent sites. (C) 2000 Elsevier Science B.V, All right s reserved.