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Abstract

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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Abstract

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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