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Abstrakt

Seismic refraction studies on Central Spitsbergen have shown that there is the fault systems with north-south strike directions, which divide the crust into western, central and eastern blocks. Thickness of the crust in this area varies from 35 to 40 km. Interpretation and modelling of seismic refraction data indicate that the Moho boundary beneath the Central Spitsbergen Basin is a complicated transition zone between crust and upper mantle with the thickness of about 5 km.
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Abstrakt

This paper presents a study of the seismic P−wave velocity and density structure of the lithosphere−asthenosphere system along a 800 km long transect extending from the actively spreading Knipovich Ridge, across southern Spitsbergen to the Kong Karls Land Volcanic Province. The 2D seismic and density model documents 6–8 km thick oceanic crust formed at the Knipovich Ridge, a distinct continent−ocean−boundary (COB), the eastern boundary of the dominantly sheared Hornsund Fault Zone, and the eastern boundary of the Early Cenozoic West Spitsbergen Fold−and−Thrust Belt. The crustal continent−ocean transitional zone has significant excess of density (more than 0.1 g/cm 3 in average), characteristic for mafic/ultramafic and high−grade metamorphic rocks. The main Caledonian suture zone between Laurentia and Barentsia is interpreted based on variations in crustal thickness, velocities and densities. A high velocity body in the lower crust is preferably interpreted in terms of Early Cretaceous magmatism channelled from an Arctic source southwards along the proto−Hornsund zone of weakness. The continental upper mantle expresses high velocities (8.24 km/s) and densities (3.2 g/cm 3 ), which may be interpreted in terms of low heat−flow and composition dominated by dunites. The lower velocities (7.85 km/s) and densities (3.1 g/cm 3 ) observed in the oceanic lithosphere suggest composition dominated by primitive peridotites. The model of mantle allows for successful direct description of subcrustal masses distribution compensating isostatically uneven crustal load. The estimated low value of correlation between density and velocity in the mantle 0.12 kg × s × m −4 suggests that horizontal density differences between oceanic and continental mantle would be dominated by compositional changes.
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Abstrakt

During the Polish Antarctic Geodynamic Expeditions, 1979-91, a wide geophysical and geological programme was performed in the transition zone between the Drake and South Shetland microplates and the Antarctic Plate, in West Antarctica. In the Bransfield Strait area, and along passive continental margin of the Antarctic Peninsula, 20 deep seismic sounding profiles were made. The interpretation yielded two - dimensional models of the crust and lithosphere down to 80 km depth. In the coastal area between the Palmer Archipelago and the Adelaide Island, the Earth's crust has a typical continental structure. Its thickness varies from 36 to 42 km in the coastal area, decreasing to about 25-28 km toward Pacific Ocean. In the surrounding of Bransfield Strait, the Moho boundary depth ranges from 10 km beneath the South Shetland Trench to 40 km beneath Antarctic Peninsula. The crustal structure beneath the Bransfield Strait trough is highly anomalous. Presence of a high-velocity body, with longitudinal seismic wave velocities Vp > 7,0 km/s, was detected there in the 6-32 km depth range. This inhomogeneity was interpreted as an intrusion, coinciding with the Deception-Bridgeman volcanic line. In the transition zone from the Drake Passage to the South Shetland Islands, a seismic boundary in the lower lithosphere occurs at a depth ranging from 35 to 80 km. The dip of both the Moho and this boundary is approximately 25° towards the southeast, indicating the direction of subduction of the Drake Plate lithosphere under the Antarctic Plate. Basing on the results of four Polish Geodynamic Expeditions, the map of crustal thickness in West Antarctica is presented.
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Abstrakt

Three Polish Antarctic Geodynamical Expeditions in 1979/80, 1984/85 and 1987/88 undertook seismic investigations in West Antarctica. Seismic measurements, including multichannel seismic reflection and deep seismic soundings, were carried out in the region of the west coast of the Antarctic Peninsula, between Antarctic Sound and Adelaide Island, Bransfield Strait, South Shetland Islands and South Shetland Trench along several lines with a total length of about 5000 km. Selected crustal sections and one and two-dimensional models of the crust for this area are discussed in detail. The thickness of the crust ranges from 30-33 km in the South Shetland Islands to 38—45 km near the coast of the Antarctic Peninsula. The crustal structure beneath the through of Bransfield Strait is highly anomalous; a seismic discontinuity with velocities of 7.0—7.2 km/s was found at a depth of 10 to 15 km, and a second discontinuity with velocities of about 7.6 km/s was found at a depth of 20—25 km. A seismic inhomogeneity along the Deception-Penguin-Bridgeman volcanic line has also been found. A scheme for the geotectonic division and a geodynamical model of the area are discussed. On the base of all experimental seismic data, it will be possible to construct a continuous geotraverse from Elephant Island, across Bransfield Strait, up to Adelaide Island with a total length of about 1100 km. Crustal section and seismic models along the northern segment of the geotraverse from the King George Island to the Palmer Archipelago are discussed in detail here.
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Abstrakt

The lithospheric transect South Shetland Islands (SSI) — Antarctic Peninsula (AP) includes: the Shetland Trench (subductional) and the adjacent portion of the SE Pacific oceanic crust; the South Shetland Microplate (younger magmatic arc superimposed on continental crust); the Bransfield Rift and Platform (younger back-arc basin); the Trinity Horst (older magmatic arc superimposed on continental crust); the Gustav Rift (Late Cenozoic) and James Ross Platform (older back-arc basin). Deep seismic sounding allowed to trace the Moho discontinuity at about 30 km under South Shetlands and at 38—42 km in the northern part of Antarctic Peninsula (Trinity Horst), under typical continental crust. Modified crust was recognized under Bransfield Strait. Geological interpretation based on deep seismic refraction and multichannel reflection soundings, and surface geological data, is presented.
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Abstrakt

During the Polish Antarctic Geodynamical Expeditions in 1979-91, deep seismic sounding measurements were performed in the transition zone between the Drake and South Shetland Microplates and the Antarctic Plate in West Antarctica. For the Bransfield Strait area, the seismic records of five land stations in South Shetland Islands and two stations at the Antarctic Peninsula were used. The interpretation yielded two—dimensional models of the crust and lithosphere down to 80 km depth. In the uppermost crust, the unconsolidated and poorly consolidated young sediments with velocities of 1.9 — 2.9 km/s cover the layers 4.0—4.2 and 5.6—5.9 km/s. The crustal structure beneath the trough of Bransfield Strait is highly anomalous. The presence of a high velocity body, with longitudinal seismic wave velocities vp > 7.0 km/s, was detected in the 6 — 30 km depth range. This inhomogeneity was interpreted as an intrusion, coinciding with the Deception—Bridgeman volcanic line. For the uppermost crust, a qualitative comparison was made between the results from the reflection profiles (GUN) and deep seismic sounding profiles (DSS). In the study area, the Moho boundary depth ranges from 10 km beneath the South Shetland Trench to 40 km under the Antarctic Peninsula. In the transition zone from the Drake Passage to the South Shetland Islands, a seismic boundary in the lower lithosphere occurs at a depth ranging from 35 to 80 km. The dip of both the Moho and this boundary is approximately 25°, and indicates the direction of subduction of the Drake Plate lithosphere under the Antarctic Plate. The results obtained were compared with earlier results of seismic, gravity and magnetic surveys in West Antarctica. A scheme of geotectonic division and a geodynamical model of the zone of subduction of the Drake Plate under the Antarctic Plate is compared with subduction zones in other areas of the circum-Pacific belt.
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Abstrakt

In marine seismic wide−angle profiling the recorded wave field is dominated by waves propagating in the water. These strong direct and multiple water waves are generally treated as noise, and considerable processing efforts are employed in order minimize their influences. In this paper we demonstrate how the water arrivals can be used to determine the water velocity beneath the seismic wide−angle profile acquired in the Northern Atlantic. The pattern of water multiples generated by air−guns and recorded by Ocean Bottom Seismometers (OBS) changes with ocean depth and allows determination of 2D model of velocity. Along the profile, the water velocity is found to change from about 1450 to approximately 1490 m/s. In the uppermost 400 m the velocities are in the range of 1455–1475 m/s, corresponding to the oceanic thermocline. In the deep ocean there is a velocity decrease with depth, and a minimum velocity of about 1450 m/s is reached at about 1.5 km depth. Be − low that, the velocity increases to about 1495 m/s at approximately 2.5 km depth. Our model compares well with estimates from CTD (Conductivity, Temperature, Depth) data collected nearby, suggesting that the modelling of water multiples from OBS data might be − come an important oceanographic tool.
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Abstrakt

Deep seismic sounding measurements were performed in the continent-ocean transition zone of north-western Spitsbergen , during the expedition ARKTIS XV/2 of the RV Polarstern and the Polish ship Eltanin in 1999. Profile AWI-99200 is 430 km long and runs from the Molloy Deep in the Northern Atlantic to Nordaustlandet in north-eastern Svalbard . Profile AWI-99400 is 360 km long and runs from the Hovgĺrd Ridge to Billefjorden. Seismic energy (airgun and TNT shots) was recorded by land (onshore) seismic stations (REF) and ocean bottom seismometers (OBS) and hydrophone systems (OBH). Good quality refracted and reflected P waves were recorded along the two profiles providing an excellent data base for a detailed seismic modelling along the profile tracks. Clear seismic records from airgun shots were obtained up to distances of 200 km at land stations and 50 km at OBSs. TNT explosions were recorded even up to distances of 300 km . A minimum depth of about 6 km of the Moho discontinuity was found east of the Molloy Deep. Here, the upper mantle exhibits P-wave velocity of about 7.9 km/s, and the crustal thickness does not exceed 4 km . The continent–ocean transition zone to the east is characterised by a complex seismic structure. The zone is covered by deep sedimentary basins. The Moho interface dips down to 28 km beneath the continental part of the 99200 profile, and down to 32 km beneath the 99400 profile. The P-wave velocity below the Moho increases up to 8.15 km/s. The continental crust consists of two or three crystalline layers. There is a lowermost crustal continental layer, in the 99400 profile’s model, with the P-wave velocity in order of 7 km/s, which does not exist in the continental crust along the 99200 profile. Additionally, along the 99200 profile, we have found two reflectors in the lower lithosphere at depths of 14–42 and 40–50 km dipping eastward, with P-wave velocity contrasts of about 0.2 km/s. The characteristics of the region bears a shear-rift tectonic setting. The continent–ocean transition zone along the 99200 profile is mostly dominated by extension, so the last stage of the development of the margin can be classified as rifting. The uplifted Moho boundary close to the Molloy Deep can be interpreted as a south-western end of the Molloy Ridge. The margin in the 99400 profile area is of transform character.
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