V. N. Efimov

The structure of the Knipovich-Mohns junction (North Atlantic)

In 2007, the Geological Institute of the Russian Academy of Sciences (GIN RAS) carried out investigations in the North Atlantic, in the southern part of the Knipovich Ridge extending for 600 km from the Mohns spreading ridge to the Molloy fault zone (chief of the cruise A.V. Zaionchek). The investigations were conducted according to the Program of the RAS Presidium entitled “Basic Problems of Oceanology: World Ocean Physics, Geology, Biology, Ecology” (Project “Regularities of the Structure and Formation of the Oceanic Crust in Characteristic Regions of the Atlantic Ocean: Tectonics, Magmatism, Composition and Genesis of Fe‐Mn Deposits,” supervisor Academician Yu.M. Pushcharovskii). The problem facing the expedition was to study the geological structure of the Knipovich‐Mohns junction. With the help of the R/V Akademik Nikolaj Strakhov (Cruise 25), there were conducted complex areal, medium-scale, regional geological study of the selected object, which involved echo-sounding with SeaBat 7150 multibeam sounder, continuous seismic profiling (CSP), high-frequency sounding with the Edgetech 3300 profiler, and bottom dredging (Fig. 1). Within the region of 74 ° N, south of the Greenland Fault Zone (FZ) extending southeastward, the Mohns spreading ridge passes into the north‐south-trending Knipovich Ridge. The peculiarity of both ridges is that these are unified extensive spreading structures not broken into segments by transform faults. The ridges differ in the time and conditions of formation. From the beginning of formation, regular and steady growth of the oceanic crust in the rift zone was characteristic of the Mohns Ridge, which is marked by the symmetrical and natural position of linear magnetic anomalies relative to the rift valley axis [1, 2]. The Knipovich Ridge began forming under unsteady geodynamic conditions, which was reflected in the disordered position and fragmentation of magnetic anomalies. The region of the Mohns and Knipovich junction attracts the attention of researchers in that this is a unique area where one spreading ridge passes into another with rift valley structures gradually bending by 40 ° without apparent transform faults serving as accommodation zones for stresses generated in the course of plate motions. Hence, the geodynamics of structures in this key region has been the subject of investigations.

Structure and composition of the sedimentary cover in the Knipovich Rift valley and Molloy Deep (Norwegian-Greenland basin)

The multidisciplinary approach is used to analyze the structure of the sedimentary cover in the northern Knipovich Rift valley, Molloy Fracture Zone and synonymous basin, Svyatogor and Hovgard rises, Gorynych Hills, Litvin and Pogrebitskii seamounts, and western slope of the Spitsbergen Archipelago studied in Cruise 24 of the R/V Akademik Nikolaj Strakhov. Materials of the bathymetric survey with multibeam echo sounder, as well as continuous seismic and vertical acoustic profiling, revealed two main (NNW- and NNE-trending) systems of fractures in the neotectonic structure of the region. It was established that a system of NNE-oriented fractures, linear zones of the dominant development of keyboard deformations included, is consistent with the strike of magnetic anomalies reconstructed for this region. Tectonic aspects of the Knipovich Rift and prospects of its further development are considered. Based on the wave field pattern of continuous seismic profiling (CSP) records, four seismocomplexes indicating contrasting sedimentation settings and intense tectonic processes at different formation stages of the northern Norwegian-Greenland Sea are conditionally defined in the sedimentary cover of the study region. It was established the Molloy Fracture Zone is responsible for a system of horizontal reflectors of acoustically transparent structureless light spots (“blankings”) in the upper well-stratified part of the sedimentary section, which are characteristic of areas with ascending pore fluids. The micropaleontological study (palynomorphs of higher plants, dinocysts, planktonic foraminifers, and diatoms) revealed the presence of Miocene assemblages in sediments. Benthic foraminifers include late Paleocene-middle Eocene assemblages. The composition of rock-forming components demonstrates a directed succession of mineral-terrigenous associations from the feldspar-quartz type to mesomictic quartz-graywacke type.

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

Structure of the transition zone from the barents sea shelf to the Knipovich Ridge Northward from Medvezhii Island (Preliminary results of the 26th cruis of R/V Akademik Nikolaj Strakhov

265 The works of the 26th cruis of R/V Academik Nikolaj Strakhov scientific research vessel showed that the Barents Sea shelf located north of Medvezhii island experiences lift and erosion delivering terrige� nous material for fan systems from the shelf edge to the Knipovich Ridge. Bottom relief indicates the presence of tectonic fracturing in the region of intensive iso� statitical process in a continent–ocean transition zone and traces of movements of icebergs formed at differ� ent times. The survey in the zone of the Knipovich Ridge from 74°40 ′ to 75°20′ N indicates intensifica� tion of neotectonic processes on the eastern side of the rift valley and increase of their amplitude to 1.5 km. The nature of deformations of the sedimentary cover testifies to the continuation of Riedel fractures under sediments on the eastern side and confirms the right

Spatial instability of the rift in the St. Paul multifault transform fracture system, Atlantic Ocean

The structure of the acoustic basement of the eastern part of the St. Paul multifault transform fracture system hosts rift paleovalleys and a paleonodal depression that mismatch the position of the currently active zones. This displacement zone, which is composed of five fault troughs, is unstable in terms of the position of the rift segments, which jumped according to redistribution of stresses. The St. Paul system is characterized by straightening of the transform transition between two remote segments of the Mid-Atlantic Ridge (MAR). The eastern part of the system contains anomalous bright-spot-like reflectors on the flattened basement, which is a result of atypical magmatism, that forms the standard ridge relief of the acoustic basement. Deformations of the acoustic basement have a presedimentation character. The present-day deformations with lower amplitude in comparison to the basement are accompanied by acoustic brightening of the sedimentary sequence. The axial Bouguer anomalies in the east of the system continue to the north for 120 km from the active segments of the St. Paul system. Currently seismically active segments of the spreading system are characterized by increasing amplitudes of the E–W displacement along the fault troughs. Cross-correlation of the lengths of the active structural elements of the MAR zone (segments of the ridge and transform fracture zones of displacement) indicates that, statistically, the multifault transform fracture system is a specific type of oceanic strike-slip faults.

Geological-geophysical studies in the northern Barents Sea and on the continental shelf of the Arctic Ocean during Cruise 25 of the R/V Akademik Nikolay Strakhov

ISSN 1028-334X, Doklady Earth Sciences, 2009, Vol. 427, No. 5, pp. 740–745.