Tectonic evolution of a restricted ocean basin: the Powell Basin (Northeastern Antarctic Peninsula)

摘要

The Powell Basin is one of the few present-day examples of a small isolated ocean basin largely surrounded by blocks of continental crust. The continental blocks in this basin result from the fragmentation of the northern Antarctic Peninsula. This basin was created by the eastward motion of the South Orkney microcontinent relative to the Antarctic Peninsula. The axial rift, identified by multichannel seismic profiles obtained during the HESANT 92/93 cruise, and the gravimetric anomalies of the basin plain, together with the transcurrent faults along the northern and southern margins, indicate a predominant WSW-ENE trend of basin extension. The South Orkney microcontinent was incorporated into the Antarctic Plate during the Miocene as a consequence of the end of basin spreading. The eastern and western margins are conjugate and have an intermediate crust in the region of transition to the basin plain. The differences in the basement structure and the architecture of the depositional units suggest that the extensional process was asymmetrical. The southern transtensive margin and the northern transcurrent margin are rectilinear and steep, without any intermediate crust in the narrow fault zone between the base of the continentalblocks slope and the oceanic crust. The multichannel seismic profiles across the central sector of the basin reveal a spreading axis with a double ridge and a central depression filled with sediments. The geometry of the reflectors in this depression indicates that the ponded deposits belong to the early stages of oceaniccrust accretion. This structure is similar to the overlapping spreading centres observed in fast-spreading oceanic axes, where the spreading axis has relay and overlapping segments.