SEISMIC STRUCTURE OF THE LITHOSPHERE ACROSS THE ZONE OF SUBDUCTED DRAKE PLATE UNDER THE ANTARCTIC PLATE, WEST ANTARCTICA

During the third Polish Antarctic Geodynamical Expedition in 1987-88, deep seismic sounding measurements were performed in the transition zo ne between the Drake plate and the Antarctic plate in West Antarctica. 30 shots were fired in the sea along profile DSS-17 of 310 km length. The interpretation was made with the use of seismic records of four l and stations in the South Shetland Islands and Antarctic Peninsula. Th e interpretation yielded a 2-D model of the lithosphere down to 80 km depth. The crustal structure beneath the trough of Bransfield Strait i s highly anomalous. The presence of a high-velocity body, with longitu dinal seismic wave velocities nu(p)>7.0 km s-1, was detected in the 6- 30 km depth range. This inhomogeneity was interpreted as an intrusion, coinciding with the Deception-Penguin-Bridgeman volcanic line. The Mo ho boundary depth ranges from 10 km in the South Shetland Trench area to 40 km under the Antarctic Peninsula. In the transition zone from th e Drake Passage to the South Shetland Islands, a seismic boundary in t he lower lithosphere occurs at a depth ranging from 35 to 80 km. The d ip of both the Moho and this boundary is approximately 25-degrees, and indicates the direction of subduction of the lithosphere of the Drake plate under the Antarctic plate. The results obtained were compared w ith earlier results of seismic, gravity and magnetic surveys in West A ntarctica. A scheme of geotectonic division and a geodynamical model o f 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.