Tectonic displacements of the Nansen basin sedimentary cover: causes and consequences

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Abstract

It is established that faults in the sedimentary cover of the Nansen basin and seismic anomalies of the “flat spot” type associated with the methane accumulation are grouped into three spatial combinations: 1 – synchronized faults and spots, 2 – spots without faults, 3 – faults without spots. They are distributed mainly between linear magnetic anomalies C20 and C12 over negative variations of the lithosphere density with depths up to ~25–30 km and lateral periodicity ~50 km. The genesis of combination 1 is provided by serpentinization of upper mantle rocks in the presence of water that has depth penetration through previously formed tectonic displacements, an increase in the rock volume and local rise of crystalline blocks, leading to the formation of faults of thrust kinematics, crossing the entire sedimentary cover from the acoustic basement to the ocean bottom. Combination 2 consists in the predominance of “flat spots” also of fluid genesis in the absence of faults, which, with a rare seismic observations, may be missed and not appear in the plane of the sections. Combination 3 consists in the presence of faults without “flat spots” with a spatial step of ~ 10 km above the highs of the acoustic basement. In the Bouguer anomalies this combination is manifested over ~80 km depression of ~25 mGal depth, comparable to the gravity depth under the axis of the Gakkel ridge. This is not due to the linear structure of the ridge, but, perhaps, to a single upper mantle plume. From this follows the mechanism of faults formation above it, associated not with serpentinization, but with the rise of the plume body to the surface. Physical modeling of the structure formation during the slowdown of the spreading rate, which took place in the range C20–C12, showed that the amplitude of the relief drops increases greatly. Comparison with a real acoustic basement shows the similarity of its relief in the corresponding time intervals of spreading slowdown with the areas of relief change in the physical model. The increase in the amplitudes of the basement highs is most likely due to the formation of faults that provide the circulation of water necessary for serpentinization.

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About the authors

S. Yu. Sokolov

Geological Institute Russian Academy of Sciences

Author for correspondence.
Email: sysokolov@yandex.ru
Russian Federation, Moscow

G. D. Agranov

Geological Institute Russian Academy of Sciences; Lomonosov Moscow State University

Email: sysokolov@yandex.ru

Earth Science Museum

Russian Federation, Moscow; Moscow

V. A. Kulikov

Lomonosov Moscow State University

Email: sysokolov@yandex.ru

Geological Faculty

Russian Federation, Moscow

A. V. Zayonchek

Geological Institute Russian Academy of Sciences

Email: sysokolov@yandex.ru
Russian Federation, Moscow

A. L. Grokholsky

Lomonosov Moscow State University

Email: sysokolov@yandex.ru

Earth Science Museum

Russian Federation, Moscow

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Supplementary files

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2. Fig. 1. The study of seismic exploration under the Arctic 2011 project [1, 19] in the western part of the Russian sector of the Nansen basin. The axes of linear magnetic anomalies are given according to [16]. The red line shows the position of the fragment of the seismic section in Fig. 2.

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3. Fig. 2. Fragment of the Arctic‑2011–06 section. The yellow arrows show the position of the protrusions of the acoustic foundation under the faults in the sedimentary cover. The blue arrow shows a wide ledge of the foundation under a flat spot with deflection of reflectors in the sedimentary cover. The position of the fragment is shown in Fig. 1.

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4. Fig. 3. Spatial distribution of faults identified by seismic data and the “flat spot” anomaly according to [7]. The Buge anomalies were calculated according to the Arctic Gravity Project [12] on a 2500 m grid grid using the IBCAO v.3 relief [14]. The red lines show the profiles along which the inversion of the gravitational field was calculated (Fig. 4). The profiles are drawn along the position of the seismic sections (Fig. 1) with continuation to the shelf and to the intersection with the Gakkel ridge. The numbers indicate the spatial combinations of anomalies and faults observed in the sections: 1 – faults in combination with flat spots; 2 – flat spots without faults; 3 – faults without flat spots.

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5. Fig. 4. Calculation of the gravity field inversion along the Arctic seismic sections‑2011 3, 4, 5, 6 ( fig. 1) according to the extended profiles, the position of which is indicated in Fig. 3. The calculation was performed in the ZondGM2D software [5]. The sections of density variations show intersections with linear magnetic anomalies according to [16], faults and flat spots. The red lines show the positions of the section fragment in Fig. 2 and [7, Fig. 4]. The numbers 1, 2 and 3 indicate combinations of anomalies and faults on the map Fig. 3. The gray rectangles show the areas between LMA C20 and C12.

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6. Fig. 5. Features of structure formation on the southern flank of the Gakkel ridge with a two‑stage deceleration of the spreading rate according to physical modeling data (experiment No. 2737). a – step–by‑step photographs of the structures of the simulated rift in the direction of stretching orthogonal to the primary weakened zone; b ‒ schematic section of the formed relief along line I–II with indication of velocities in the physical model; c ‑ comparison of the relief section with the results of identification of magnetic anomalies for the Arctic‑2011-3 profile, the relief of the foundation and the determination of spreading rates according to the data [4].

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