A. O. Mazarovich, S. Yu. Sokolov, N. N. Turko, and K. O. Dobrolyubova
Geological Institute, Russian Academy of Sciences
The Mid-Atlantic Ridge (MAR) between the Equator and 15o N was thoroughly studied during several expeditions. A significant contribution to understanding the inner structure of this Atlantic segment was made by Russian and Russian-Italian expeditions aboard the R/V "Akademik Nkolai Strakhov.'' At the same time, one of the least studied areas is that between 5o and 7o40' N. Below, it will be referred to as the Sierra Leone Fracture Zone, after its only seafloor feature with a conventional geographical name.
The area of the Sierra Leone Fracture Zone is characterized by lower seismicity [CNSS..., 1997] (Figure 3). Information on the rocks composing this MAR segment was previously obtained from the material dredged in 1968 [Bonatti et al., 1968], which indicates that the ultramafic rocks are strongly tectonized.
The MAR segment between 5o and 7o18'N was studied very poorly. All information on it was based on data obtained with outdated echosounders and navigation systems, three sites of rock sampling and satellite-borne observations.
The considerations and facts presented above led to the organization of Cruise 22 of the R/V "Akademik Nikolai Strakhov'' in this segment in June, 2000 (cruise head A. A. Peyve, research supervisor Academician Yu. M. Pushcharovskii). The idea of organization of the works was formulated by A. O. Mazarovich. The seafloor bathymetric surveying was conducted with a Simrad EM12S multibeam echosounder under supervision of N. N. Turko and A. O. Mazarovich. The primary bathymetric data were treated by N. N. Turko, K. O. Dobrolyubova, and S. Yu. Sokolov (Geological Institute, Russian Academy of Sciences). The data and materials were collected with the participation of A. A. Peyve, S. G. Skolotnev, Yu. N. Reznitsyn (Geological Institute, Russian Academy of Sciences), M. Ligi, D. Penitenti, S. Carluccio, V. Ferrante, A. Blasi, and A. Cipriani (Istituto di Geologia Marina). The most valuable contribution to the successful accomplishment of the works was made by Prof. Marco Ligi, who supervised the technical conditions of the echosounder and the GPS navigation system during the Russian part of the cruise and measured the sound velocity in water.
Before proceeding to the analysis of the structure of the area of the Sierra Leone Fracture Zone, it is necessary to emphasize some points concerning the position of the fracture zone itself and the terminology.
On the map published in the geological-geophysical atlas of the Atlantic Ocean [Mezhdunarodnyi..., 1989-1990, pp. 34-35], this name is borne by a trough at 7o20'N, south of which, at 7oN, a small nameless fault is located. At the same time, on the orographic scheme [ibid., p. 32], the Sierra Leone Fracture Zone is shown at 5o N. On a map sheet of "The General Bathymetric Chart of the Oceans'' [General..., 1982], which is recognized as the official reference source of the position of seafloor features and their geographic names, the fracture zone runs at 6o10'N. According to "The Dictionary of Geographical Names of Ocean Bottom Features'' [Agapova et al., 1993, p. 125], the Sierra Leone Fracture Zone is located in the quadrangle with coordinates 6o00'N, 37o00'W and 7o00'N, 27o00'W. Exactly the same coordinates are cited in gazetteers [Gazetteer of undersea..., 1997; Gazetteer of geographical..., 2001]. Because of this, the term of "the Sierra Leone Fracture Zone'' is applied to a small-displacement feature located at approximately 6o00'N. Below, we will return to the position of this transform feature.
We propose to name the fault at 7o10'N for Aleksei Alekseevich Bogdanov, a prominent Soviet tectonist (see http://atlantic.tv-sign.ru/names/russian/names_r.html for details). This proposal, along with others below, were submitted to the GEBCO Subcommittee for Names and Nomenclature for approval.
Here we discuss the areas referred to in the literature in English as "ridge-transform intersection'' (see, for example, [Karson and Dick, 1983]) and "discontinuity.'' However, the rift usually does not intersect a transform fault, and in the Russian literature the area is known as "the junction zone between the transform and the rift valley'' [Stroenie..., 1991], "junctions between transform faults and rifts'' [Kaz'min and Borisova, 1992], and "junction between rift and transform depressions'' [Agapova, 1993]. We believe that the phrase "junction between the rift and the active portion of the transform fault'' can most adequately convey the meaning. For brevity, below we will use the term "junction.''
In describing a variety of anomalous zones within the axial portion of the ridge, the term "discontinuity'' is used [MacDonald et al., 1987]. In essence, these are areas of changes or breaks in the strikes of the rift mountains or rift valleys, i.e., their "discontinuity,'' and, hence, this term will be used below.
Multibeam mapping with 100% area coverage confirmed predicted topography evidence that a clearly pronounced rift valley lies between 5o and 7o10'N (Figure 4) and is characterized by numerous changes in its strike, a transform fault is situated at 7o10'N, and the segment includes a number of discontinuities.
The axial MAR segment between the Bogdanov and Strakhov fracture zones (Figure 4) is subdivided into the following segments (from north to south): 6o54' -7o10'N, 6o12' -6o54'N, 6o12' -5o42'N, and 5o42' -5o00'N, which differ in topography and the strikes of their principal morphological features.
The junction between the active portion of the Bogdanov Fracture Zone and the rift valley is a seafloor area (Figure 5) with pronounced high inside and outside corners, a nodal basin, a rift valley, and a fault trough.
The rift valley immediately south of the fracture zone is 3.2 km wide and extends along 33o54'W. West of it, a high outside corner is located (Figure 4); we propose to name it Mazarovich Seamount, after the founder of the Geological Department of Moscow State University (see http://atlantic.tv-sign.ru/names/russian/names_r.html for details). The high outside corner consists of two parts: eastern, oriented roughly eastward and extending for approximately 20 km, and western, trending roughly from north to south.
The eastern part of the high outside corner is a massive seamount, whose northern face is complicated by two scarps and has a minimum depth of 1635 m. The overall topographic magnitude relative to the seafloor attains almost 3700 m. Judging from the linear configuration of the southern boundary of the high outside corner, it is of fault nature. The rest of the high outside corner is a continuous ridge trending in a general south-north direction; its orientation near the fracture zone is 45o, south of it the ridge turns 5o, still farther to the south its trend is 347o, and, finally, 325o. The minimum depth is 1307 m. The width of the ridge varies from 2500 to 3800 m. To the east of it, another spur (about 6 km wide) branches off the massive portion of the high outside corner and trends nearly parallel to the former ridge, roughly along 34o07'W.
The isometric seamount between the ridges (at 6o57.7'N, 34o06.8'W) has a basement diameter of approximately 1900 m, its top is located at a depth of 3055 m, and the depths of the basement are 3269 m in the north and 3393 m in the south. Hence, the maximum elevation of the seamount, which can be interpreted as a central volcano, is 338 m. The southern face of the high outside corner bears other edifices of the same type, a fact demonstrating the possibility of active volcanic processes outside the rift zone.
The sharp terminations of the ridges, their bends, and the position of individual ridges and elongated depressions make it possible to distinguish a series of northwest-trending zones, which are most probably of tectonic nature. Some of these zones are also marked by Bouguer gravity anomalies (Figure 8). Since eight of the ten transform faults [Sokolov, 1999] are practically not pronounced in the form of Bouguer anomalies compared to free-air gravity anomalies (a fact testifying that most of the faults are crustal structures), the correspondence of northwest-trending faults to Bouguer anomalies indicates that their depths probably exceed the crustal thickness.
Farther to the east, a high outside corner is located (Figures 4, 5). We propose to name it "Saveliev Seamount,'' after a geologist of the Geological Institute, Russian Academy of Sciences (see http://atlantic.tv-sign.ru/names/russian/names_r.html for details). The mount is elongated in a general south-north direction. Its western slope is complicated with a narrow (300-800 m) valley, which branches off the main rift.
A ridge running farther to the north is 20 km long and intersects the trough of the Bogdanov Fracture Zone at 33o55 ' W (Figure 9). This seafloor feature coincides with a zone of elevated seismicity (Figure 9) and Bouguer anomalies (Figure 8). The ridge comprises several central volcanoes a few dozen meters high, and its crest zone has a wall crowned with three volcanoes (perhaps, an extrusive zone). East of this area (Figure 5), there is a number of structurally similar ridges, which also turn to the west. Judging from their morphology, they are of similar genesis.
The east-west-trending depression located south of all of the aforementioned seafloor features was traced on both the eastern and western sides of the rift and serves as the boundary of this MAR segment. The depression is characterized by a complicated topography (Figures 4, 5), has walls with scarps at angles of a few degrees (Figure 6), and is mostly separated from the rift valley by north-trending elevations.
This part of the rift valley (Figure 10) is separated from its northern portion by a steep en-echelon slope (swell) of northwestern strike, 7-8 km wide, approximately 6.5 km long, and up to 800 m high.
The segment includes a depression 40 km long, 8-12 km wide, and up to 4800 m deep. Its deepest portion is located in the north, in the close vicinity to the swell. The depression (striking 351o ) is intersected by a ridge (striking 356o ) in the central and southern portions. The ridge has an average width of 1100-1800 m and elevations of 200-300 m. At 6o32 ' N, the ridge widens, and a narrow spur (trending 326o ) diverges from it. East of them, the deepest portion of the rift valley is located, which is expressed as a narrow trough, doubly displaced in the southeastern direction.
The eastern wall of the depression is a cliff 300-400 m high. The ridge running east of it is characterized by minimum depths of 2144-2300 m; at 6o32'N it is sharply displaced for 4600 m. This suggests the existence of a fault at this latitude, which seems to have a sinistral nature, extends further eastward, and is identified based on the bends of two narrow ridges with minimum depths of 2151 and 2267 m. Farther eastward, narrow ridges (trending 350o ) occur, which have steep western slopes and southern portions curved to the southwest. This structural situation is typical of dextral displacements. No faults were detected in the western wall of the rift valley.
A ridge running along 33o55'W trends for 54 km and has a width of 9 km. Its steep eastern slope, which faces the rift valley, has a height of approximately 1200 m.
To the east of this area, an elevation (1500 m across and approximately 160-170 m high) and an interrfift seamount (with a minimum depth of 1902 m) are situated. The local topographic magnitude attains 2448 m. The seamount is proposed to be named "Leonov Seamount,'' after Georgii Pavlovich Leonov, a professor of the Geological Department of Moscow State University (see http://atlantic.tv-sign.ru/names/russian/names_r.html for details).
Northeast of Leonov Seamount, a depression was mapped, which trends for 15 km and has a width of 1700-6300 m. It is characterized by steep northern and eastern walls up to 600-650 m high.
In the south, the Markov Deep is bounded by a 9 by 10.3-km swell, whose minimum depth is 2822 m. It corresponds to a positive gravity anomaly, which is commensurable with that of Leonov Seamount. The depression behind the seamount has isometric outlines and depths up to 4674 m. Southward, it grades to a narrow valley of complicated configuration, which separates two nearly parallel narrow (2-3 km) ridges with depths of 2685 and 2641 m.
The southernmost (within our study area) portion of the rift valley was mapped only partly (Figure 11). Its western portion is a depression elongated in a roughly northern direction ( 347o ) with depths of 4000-4200 m, which is divided by ridges (perhaps of volcanic genesis, trending 343o ) with a minimum depth of 3452 m.
The western segment is bounded by a mountain massif, which is composed of a series of subparallel ranges. Near 5o06 ' N, they change their trend from northern to northwestern.
The southernmost segment is located between the Strakhov and 5o05'N fracture zones. The rift valley trends in a general south-north direction, has a width of about 6 miles, is rectilinear in plan view, and is bounded by rift mountain ridges up to 12 miles wide, with the long ridges complicated by east-west trending saddles. The material dredged from this area contained only variably altered basalts. This segment of rift valley is marked by numerous earthquake epicenters.
The northernmost portion of the segment is partly covered by mapping. Changes in the structure of the valley begin near 5o N, where long interrift ridges appear, which can be interpreted as neovolcanic zones. The south-north trending rift mountains start to curve to the northwest near 5oN. At the same time, predicted topography results indicate that the rift valley retains its general trend up to 5o30'N. It is also pertinent to note that the seismic activity drastically decreases.
The second segment is located between 5o30' and 6o15'N. The rift zone has a general strike at 320o and consists of three isolated depressions of complicated configuration, one of which is the deepest among Atlantic rifts. The western wall is composed of narrow long ridges.
The third segment is a topographically clearly pronounced rift valley with neovolcanic ridges, from which only basalts were recovered. Much strongly tectonized ultramafic rocks and gabbro were lifted from the rift walls [Peyve, 2000].
The analysis of the GEBCO bathymetric map led Mazarovich  to hypothesize that the Central Atlantic is crossed by a fault array striking along azimuths from 310o to 330o within both the Mid-Atlantic Ridge and the abyssal basins. Altimetric data and new evidence obtained during cruises of the R/V "Akademik Nikolai Strakhov'' confirm this notion. The faults intersect the rift valleys (for example, north of the Marathon Fracture Zone, between the Bogdanov and Sierra Leone fracture zones) with developing oceanic crust, as well as the active parts of the transform faults (for example, the San Paulu Fracture Zone). Where intersected by these faults, the rift valleys tend to "adapt'' to their northwestern directions. The effect of the faults is perceptible even in the rift-valley floor, except the neovolcanic zones and interrift basins. Obviously, the faults developed in a stress field, which encompassed the whole Central Atlantic, and their genesis could not be directly related to the evolution of the spreading system. There seem to have been a superposition of two mechanisms: a change in the geoid's shape and the effect of variations of the Earth's inertia axes on masses on the spheroid surface, with these processes occurring simultaneously with the overall extension of the Atlantic. The superposition of distinct stress types ensured the normal-fault character of the faulting.
Another distinctive feature of the rift near the Sierra Leone Fracture Zone is its active northward propagation at a high extension velocity. This also follows from data on Bouguer anomalies (Figure 8). Hence, the local geodynamic situation is currently notably changing. It can be theorized that the "break-up'' of the rift much farther northward should result in the "isolation'' of the active portion of the Bogdanov Fracture Zone, with the main transformation area displaced northward, to the 7o20'N Fracture Zone, and a significant part of the passive part becoming active.
Now, an expedition aboard the R/V "Akademic Ioffe'' works in the area of the Sierra Leone Fracture Zone. Hopefully, this expedition will uncover new data on this very complicated part of the Central Atlantic.
Agapova, G. V., Features of the morphology of the offset of the Strakhov Fracture Zone, (in Russian with an English abstract), Okeanologiya, 33, (2), 263-268, 1993.
Agapova, G. V., N. V. Vinogradova, and I. P. Kashnikova, Dictionary of geographical names of ocean bottom features, (in Russian and English), Geol. Inst., Rus. Acad. Sci., Moscow, 1993.
Bonatti, E., Ultramafic rocks from the Mid-Atlantic Ridge, Nature, 218, (5152), 363-364, 1968.
CNSS Earthquake Composite Catalogue, June 1997, (http://quake.geo.berkeley.edu/cnss/).
Equatorial segment of the Mid-Atlantic Ridge. Initial results of the geological and geophysical investigation under the EQUARIDGE program, Cruises of the R/V "Akademik Nikolaj Strakhov'' in 1987, 1990, and 1991, Intergovernmental Oceanographic Commission, Technical Series, no. 46, UNESCO, 1997.
Gazetteer of undersea feature names, Monaco: IHO IOC, 2nd Edition, 1997.
Gazetteer of geographical names of undersea features shown (or which may be added) on the GEBCO and on the IHO small-scale international chart series(1: 2 000 000 and smaller), Monaco: Int. Hydrogr. Bureau, 2nd Edition, 2001.
GEBCO-97 digital Atlas CD, IHO, Natural Environmentat Research Council.
General bathymetric chart of the oceans (GEBCO) 1: 10 000 000, Ottawa: Canad. Hydrogr. Serv., 1982.
Karson, J. A., and H. J. B. Dick, Tectonics of ridge-transform intersections in the Kane Fracture Zone, Mar. Geophys. Res., 6, 51-98, 1983.
Kaz'min, V. G. and I. A. Borisova, On the mechanism of the origin of rift valleys in slow-spreading ridges, (in Russian), Geotectonika, (4), 113-124, 1992.
MacDonald, K. C., J.-C. Sempere, P. J. Fox, and R. Tyce, Tectonic evolution of ridge-axis discontinuities by the meeting, linking or self-decapitation of neighboring ridge segments, Geology, 15, 993-997, 1987.
Marine trackline geophysical data CD, NOAA Product #G01321-CDR-A0001.
Mazarovich, A. O., The tectonic convergence of transform faults in the Equatorial Atlantic, (in Russian with an English abstract and in English), Dokl. Ross. Akad. Nauk, 335, (1), 70-73, 1994.
Mazarovich, A. O., Geologicheskoe stroenie Tsentral'noi Atlantiki: razlomy, vulkanicheskie sooruzheniya i deformatsii ojkeanskogo dna, (Geology of the Central Atlantic: faults, volcanic edifices, and seafloor deformations) (in Russian), Nauchnyi Mir, Moscow, 2000.
Mezhdunarodnyi geologo-geofizicheskii atlas Atlanticheskogo Okeana (Internationsal geological-geophysical atlas of the Atlantic Ocean), Mezhpravit. Okeanograf. Komiss. Min. Geol. SSSR, Akad. Nauk SSSR, Glav. Upr. Geodez. Kartograf., Moscow, 1989-1990.
Peyve, A., E. Bonatti, D. Brunelli, et al., New data on some major MAR structures: preliminary results of the R/V "Akademik Nikolaj Strakhov'' 22 cruise, InterRidge News, 9, (2), p. 28, 2000.
Sandwell, D. T., and W. H. F. Smith, Marine gravity anomaly from Geosat and ERS-1 satellite altimetry, J. Geophys. Res., 102, (B5), 10,039-10,054, 1997.
Smith, W. H. F., and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science, 277, (5334), Sept. 26, 1997.
Sokolov, S. Yu., Bouguer anomalies calculated from altimetric and bathymetric data on a 5 ' by 5 ' grid, in Geologo-geofizicheskii atlas tsentral'noi chasti Atlanticheskogo okeana. Tom 1: Obshchie geofizicheskie i geologicheskie dannye (Geological-geophysical atlas of the Central Atlantic Ocean. Vol. 1: General geophysical and geological data) (in Russian), Moscow: Geol. Inst., Ross. Akad. Sci., Moscow, 1999 (http://atlantic.tv-sign.ru/).
Stroenie razloma Doldrams, Tsentral'naya Atlantika (Inner structure of the Doldrums Fracture Zone, Central Atlantic) (in Russian), Tr. Geol. Inst., Ross. Akad. Sci., issue 459, Moscow, 1991.