In recent times, numerous hydrothermal vent fields have been discovered in the Atlantic Ocean. Along the Northern Mid-Atlantic ridge, hydrothermal fields are located on different parts of axial rift valley or on non-transform offsets and fracture zones. Extensive data from modern geological studies (in particular multibeam bathymetry and high-resolution side-scan and towed camera images) can be used to characterize perspective locations and geomorphological boundaries of hydrothermal areas on the Northern Mid-Atlantic ridge. However, new discoveries have changed our thinking about the possible limits of hydrothermal venting.
In general, the structure of rift valley along the Northern Mid-Atlantic ridge can be divided for specific morphological complexes of features according their origin and geodynamic position of seafloor massive sulfides.
Published descriptions of hydrothermal fields (e.g. Shilov et al., 2012; Cherkashov et al., 2010, 2008; Bel'tenev et al., 2006, 2004; Fouquet et al., 1997) suggest a higher density of possible locations in the rift valley of the Mid-Atlantic Ridge. Furthermore, some geomorphic features have a strong association with hydrothermal vents so that areas of particular activity can be defined (O’Brien et al., 2009). Target coverage areas for future prospecting include axial volcanic ridges, slopes of rift valley and various uplifted blocks and transit areas.
Various geodynamic processes form the distinctive configuration of rift valley: not only volcanism, but also regional and local tectonic and gravitational processes. Sedimentation and metamorphic processes can contribute to formation of polymetallic deposits, however there will not be discussed further here.
Most parts of volcanic features with hydrothermal fields restricted to two areas: well-known sector near Azores Archipelago (36 - 38ºN) and recently discovered 20ºN sector. Hydrothermal fields from the first group are related to volcanic features from the Azores Triple Junction, but these volcanoes were characterized by different morphology: from broken neck of Menez Gwen field to triangle of volcanoes of Lucky Strike field. Similar hydrothermal province was discovered in the South Atlantic Ocean at 4º 08′S (Haase et al., 2007).
Recent findings by Russian geologists have extended the hydrothermal area of 20ºN that was previously known only with hydrothermal field of huge volcanic neck of Puy des Folles. Three newest hydrothermal fields (Zenith-Victory,Yubileynoe and Surprise) were discovered here on the upper part of basalt-hosted slopes of rift valley.
The southernmost and oldest basalt-hosted hydrothermal field of the Northern Mid-Atlantic ridge is Krasnov (16º 38′N), located on the small dam-shaped peninsula between volcanic neck and eastern slope of rift valley.
Hydrothermal venting driven by high rates of serpentinization may occur episodically as new reaction surfaces are made available by tectonic activity or crack propagation (Lowell and Rona, 2002).
Formation of oceanic core complexes (OCC’s) has a considerable geomorphological significance for hydrothermal activity. Now corrugated surfaces of OCC’s are well known, but the relationship between hydrothermalism and morphology should be described more clearly. Modern multibeam bathymetric and side-scan data with available geological models provide an opportunity to subdivide OCC’s for further prospecting and tectonic analyses. Inner corner massifs (Lost City), assymetric twin OCC’s (TAG), elongated domal OCC’s (Semyenov-Irinovskoye), dissected OCC’s (Logatchev), irregular non-domal OCC’s (Ashadze), small contour OCC’s (without hydrothermal activity) and their giant combinations have different morphological peculiarities and different trends to the changes of morphology in their recent past.
Absolute age of seafloor massive sulfides is a more meaningful parameter from a geological point of view, mainly because it can be estimated for field occurrences with much greater accuracy than any geodynamic conditions. There are still a small number of dates to understand whole picture of sulfides-forming processes but the general examination of available 230Th/U-ages has shown different limits from short-lived activity (1 – 20 kyrs, Broken Spur, Snake Pit, Puy des Folles) to long-lived hydrothermal systems (>165 kyrs, Petersburgskoe).These 230Th/U-ages contain information not only about relationships among duration of the hydrothermal activity and possible sizes of associated polymetallic deposits (Cherkashov et al., 2013) but also about their distribution.
Hydrothermal fields can be divided into young (short-lived axial), typical (~60 kyrs, on the slopes) and old (long-lived) types. The temporal limit of hydrothermal activity seems to extend no less than 170 kyrs.
Apparently, the boundary of young hydrothermal fields lies above the foot of the axial valley. Typical fields closely follow the elements of the rift valley slopes from the foot to the top. The link between the distribution of old fields and segmentation of the rift valley is not clear. The off‐axis terrain can influence to the both hydrothermal precipitation and near bottom currents.