N. N. Turko

New data on the structure of the Vitoria-Trindade seamount chain (western Brazil basin, South Atlantic)

In 2008, during cruise 24 of the R/V Akademik Vavilov, much of our research work was focused on the central segment (Jaseur and Davis seamounts, Dogaressa Bank) of the Vitoria-Trindade seamount chain (west of the Brazil basin) extending along 20.5° S. Work was conducted to survey the upper part of the sedimentary cover and to perform subbottom profiling. The samples dredged on the seamount slopes are represented by volcanites and Fe-Mn crusts.

New Data on the Geological Structure of the Junction between the Cape Verde Seamount and the Cape Verde Basin, Central Atlantic

The regions of conjugation of continental rise with abyssal oceanic basins at the margins of the Atlantic— a transitional zone between continental and oceanic lithospheres—are still poorly studied in geological terms. In the course of expeditions conducted by the Geological Institute (Moscow), the structure of this zone was studied at the continental slope of Africa, south of the Cape Verde Islands. In this area, the continental rise widens sharply making up a near-latitudinal promontory that divides the abyssal Cape Verde Basin in the south and the Canary Basin in the north. The study area is situated in the pinchout area of the system of transform fracture zones (TFZ) located south of the Fifteen Twenty TFZ [1, 2]. The near-latitudinal linear ridges and troughs on the bottom of the Cape Verde Basin are the eastern flanks of the Vema, Doldrums, Arkhangelsky, and Vernadsky TFZs of the Mid-Atlantic Ridge. Near the continental slope of Africa, these TFZs are cut off by the WNW-trending escarpment (Fig. 1). The bathymetric survey of a local area in the deepwater Cape Verde Basin that adjoins the southern margin of the Cape Verde Seamount (Figs. 1, 3) was carried out during Cruise 22 of the R/V Akademik Nikolai Strakhov in 2000. We have established an azimuthal unconformity between near-latitudinal depressions and ridges that extend from MAR, on the one hand, and the WNW-trending transversal Cabo Verde Escarpment, on the other hand [3]. The seafloor in the studied test area is complicated by volcanic edifices and by the anomalously deep (>6000 m) Strakhov Basin trending in the NW direction discordantly relative to other structural units. The previously unknown Neva deepwater channel was also found (Fig. 3). This paper has been prepared on the basis of the data collected during Cruise 16 of the R/V Akademik Ioffe in 2004. During this cruise, the sedimentary cover was studied with continuous seismic profiling (CSP). The structure of the upper part of the sedimentary cover and the bottom topography was investigated with a Parasound acoustic profilograph along a profile between 11.52 ° N × 22.67 ° W and 10.13 ° N × 24.07 ° W (Fig. 1). The structure of the upper part of the sedimentary cover in the Neva Channel and the Strakhov Basin was studied with the same method. The bedrock samples and cores of bottom sediments were recovered in the same place. Structure of the sedimentary cover (based on CSP data). The CSP profile across the junction of continen

Deformations and Manifestations of Degassing in the Sedimentary Cover of the Equatorial Segment of the West Atlantic: Implications for Lithospheric Geodynamics

Manifestations of fluids and deformations in the sedimentary cover, which are both factors of brightening (blanking anomalies) in seismoacoustic records, in the equatorial segment of the Atlantic coincide with the sublatitudinal zones of the activated passive parts of transform faults and with zones of lower gravity anomalies and higher values of remnant magnetization, which form as a result of serpentinization. The cause-and-effect sequence of intraplate phenomena includes: the contrasting geodynamic state → horizontal movements that form macrofractures → water supply to the upper mantle → serpentinization of rocks in the upper mantle → deformations associated with vertical uplift of basement and sedimentary cover blocks, coupled with fluid generation → and fluid accumulation in the sedimentary cover, accompanied by the formation of anomalies in seismoacoustic records. Based on the seismic data, we have identified imbricate-thrust deformations, diapir structures, stamp folds, and positive and negative flower structures, indicating the presence of strike-slip faults in the passive parts of transform faults. The general spatial distribution of deformation structures shows their concentration in cold mantle zones. Correlative comparison of the structural characteristics of deformations shows the direct relationship between the heights of structures and the development of serpentinization processes. As per the age of the basement, deformations range from 27–38 to 43–53 Ma; a quite thick sedimentary cover makes it possible to reveal them based on the characteristic types of seismoacoustic records. The formation of the Antilles arc ca. 10 Ma ago affected the equatorial segment of the Atlantic; it formed kink bands where lithospheric blocks underwent displacements with counterclockwise rotations, deformations related to compression and vertical uplift of crustal fragments, and local extension that favored degassing of endogenous fluids. Sublatitudinally oriented imbricate-thrust deformations with different vergences indicate irregularity and alternating strike-slip directions as blocks between fractures were laterally influenced.

Hydrothermal Mineralization in the Sierra Leone Fracture Zone (Central Atlantic Ocean)

The analysis of data on the location of hydrothermal fields, seismicity, and satellite altimetry evidences that in mid-ocean ridges with low spreading velocity hydrothermal fields tend to be grouped in areas with generally low seismic activity and at intersections of discontinuities and rift zones. Based on this assumption, the Sierra Leone Fracture Zone was studied in 2000 during cruise 22 of the R/V Akademik Nikolaj Strakhov.The study of gabbrodolerite and dolerite showed that sulfide ore minerals in them were formed both by hydrothermal and magmatic processes. The analysis of melt inclusions demonstrated that magmatic complexes formed from a high-temperature (1210–1255°C) low-potassium melt of the N-MORB type. Investigations of fluid inclusions revealed that gabbro and dolerite formed under the influence of an active hydrothermal system at a temperature of 205–226°C. Thus, the Sierra Leone Fracture Zone is considered to be perspective for the discovery of a new hydrothermal field.

Accretion of crust in the axial zone of the Mid-Atlantic Ridge south of the Martin Vas Fracture Zone, South Atlantic

New data are obtained on the structure, evolution, and origin of zones of nontransform offsets of adjacent segments in the Mid-Atlantic Ridge (MAR), which, in contrast to transform fracture zones, so far are studied insufficiently. The effects of deep mantle plumes developing off the crest of the MAR on the processes occurring in the spreading zone are revealed. These results are obtained from the geological investigation of the crest of the MAR between 19.8 ° and 21° S, where bottom sampling, bathymetric survey, and magnetic measurements have been carried out previously. Two segments of the rift valley displaced by 10 km relative to each other along a nontransform offset are revealed. A volcanic center of a spreading cell, which has been active over the last 2 Ma, is located in the northern part of the southern segment and distinguished by a decreased depth of the rift valley and increased thickness of the crust. Magnesian, slightly evolved basalts of the N-MORB type are detected in this center, whereas evolved and high-Fe basalts are found beyond it. The variation in the composition of the basalts indicates that the volcanic center is related to the upwelling of the asthenospheric mantle, which spread along and across the spreading ridge. In the lithosphere, the melt migrated off the volcanic center along the rift valley. In the northern segment, a vigorous volcanic center arose 2.5 Ma ago near its southern end; at present, the volcanic activity has ceased. As a result of the volcanic activity, an oval rise composed of enriched T-MORB-type basalts was formed at the western flank of the crest zone. The isotopic signatures show that the primary melts are derivatives of the chemically heterogeneous mantle. The mixing of material of the depleted mantle with the mantle material pertaining either to the Saint Helena or the Tristan da Cunha plumes is suggested; the mixture of all three sources cannot be ruled out. The conclusion is drawn that the mantle material of the Saint Helena plume was supplied to the melting zone beneath the axial rift near the oval rise along a linear permeable zone in the mantle extending at an azimuth of 225° SW. The blocks of mantle material that got to the convecting mantle from the Tristan da Cunha plume at the stage of supercontinent breakup were involved in melting as well. The nontransform offset between the two segments arose on the place of a previously existing transform fracture zone about 5 Ma ago. The nontransform offset developed in the regime of oblique spreading at the progressive propagation of the southern segment to the north. The zone of nontransform offset is characterized by recent volcanic activity. Over the last 2 Ma, spreading of the studied MAR segment was asymmetric, faster in the western direction. The rates of westward and eastward half-spreading in the northern segment are estimated at 1.88 and 1.60 cm/yr, respectively.