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Interpretation of MCS, HRS and bathymetry data allow to determine
geodynamics of the area of junction of Gakkel Ridge and Laptev Sea
margin. MAGE seismic line near 79 40’ N shows rift mountains and the
subsidence of the rift valley compensated by recent sedimentation. Basal
(Paleocene-Eocene ?) sequences are uplifted by magmatic dome of Gakkel
Ridge towards the center of the oceanic basin
(PICTURE).
Near the top of Gakkel Ridge these units are eroded and overlaid by
Oligocene sequence. The central part of Gakkel Ridge dome-like rise is
disturbed by graben-rift, and the major sedimentary (Paleocene-Pliocene)
section is displaced along normal faults.
Southwards the continental rise was less impacted by structural prolongation
of mid-ocean ridge. In this area uplift of sedimentary units is
characterized by minor amplitudes and it occurs only adjacent to rift
mountains completely overlaid by the sedimentary cover. To the south
from 79 N linear localization of the rift zone disappears in bathymetry
as well as in seismic records and earthquakes epicenters distribution.
Abyssal plane of Eurasian basin adjacent to the Laptev Sea shelf shows
asymmetric composition in modern Sea bottom topography [Naryshkin,
1987]. As well as in deposits distribution and depths of the
basement surface [Poselov et al., 1998].
Such asymmetry is proved also by different character of gravity field in
Nansen and Amundsen basins. Oceanic basement varies according seismic
records by the relief of its top: it is dissected by numerous fractures
in Nansen basin while in Amundsen basin it is more smooth. Stratigraphic
range of the sedimentary cover is also variable: Upper Cretaceous (?) –
Cenozoic in Nansen basin and Oligocene (?) – Holocene in Amundsen basin.
Simultaneous sedimentary sequences one characterized by variable seismic
patterns and age correlation appears to be very difficult. It is
impossible to explain this facts by different rates of sea floor
spreading [Sekretov, 1998] because
newly obtained seismic data [Poselov et al,
1998] shows that zone of spreading in Eurasian basin is
restricted by comparatively narrow area of the Gakkel Ridge rift zone.
Thus, differences in composition of deep sea Nansen and Amundsen basins
demonstrate independent styles of their evolution while Gakkel Ridge
obviously sometimes had been hinge boundary zone.
Unique composition of the rift valley is explained by long epoch of
sedimentation under stable tectonic conditions before magmatic dome
forming and further and further penetration of mid-ocean ridge. Adjacent
to the Laptev Sea continental margin deep sea Eurasian basin is filled
by the thick sedimentary cover (from 4 to 8 km) with very gentle angles
of dip of separate sequences not disturbed by faults
[Gusev et al, 2002].
This section is common for the whole basin and its slopes: this
phenomena demonstrates contradiction with Paleocene-Miocene (?)
horizontal movements. Thus, long epoch of tectonic stability in the
recent rift zone of Gakkel Ridge was ended in the second half of
Pliocene (?). Amplitude of Pliocene crust expansion in the rift valley
could he evaluated according general horizontal values of normal faults
displacements of subsidented blocks: these values are more than first
kilometers. General subsidence of the dome top rift zone and rift valley
farming occurred comparatively recently which is proved by equal
thickness of pre-Pliocene sedimentary units on rift shoulders and
subsidented block as well as by the presence of normal faults and
strike-slips displacements compensating distribution of sediments in the
rift zone during its crucial subsidence.
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