Estructura de la Cuenca Powell y su significado en la evolución cenozoica del extremo nororiental de la Península Antártica

Abstract

The Powell Basin is a small oceanic basin nearly totally surrounded by continental crust. Analysis of magnetic, multibeam echo sounding and multichannel seismic profiles obtained during the HESANT 92-93 cruise with the B/O Hespérides, in addition to the GEOSAT free air anomaly map, has allowed us to study its margins and basin plain. An arched oceanic spreading axis, with a mean NNW-SSE trend, subparallel to the eastern and western margins, is identified in the oceanic crust. This axis is formed by several overlapping ridges that are covered by a 1.5 s (TWT) sequence of post-drift sediments. The basin margins and axis features suggest that the Powell Basin opened in an ENE-WSW direction. The northern and southern margins are straight, with steep slopes. While only a single ENE-WSW large slope is found in the northern margin, orthogonal to the spreading axis, the southern margin has E-W direction and a staircase profile with perched basins. This geometry suggests that the northern margin is transcurrent, while the southern one is transtensional. The eastern and western margins are arched, with intermediate crusts, and may be interpreted as conjugate passive margins. However, the eastern margin is more starved, and has steeper slopes and more intense recent faulting than the western margin. This variation in features suggests that the rifting probably was asymmetric, with a basal detachment dipping towards the eastern margin. ODP, heat flow and ocean basement depth data suggest that the drift period of Powell Basin occurred between the late Eocene and Lower Miocene. Rifting may have started at the end of the Cretaceous. The end of the oceanic spreading after the Lower Miocene and the absence of seismic activity in the margins suggest that the basin is now mainly undergoing a thermal subsidence period. However, the recent activity in the faults of the eastern margin may suggest the existence of a short recent reactivation stage. The development of Powell Basin is contemporaneous to the spreading of the Scotia Plate. The origin of the Powell Basin is related to the formation of the Scotia Arc. The oceanic spreading inside the arc dispersed the fragments of continental crust that once formed the connection of South America and the Antarctic Peninsula in the Eocene. The development of small oceanic basins, during short periods, may have been favoured by the existence of transcurrent boundaries between major plates with oceanic and continental crusts. The displacement of continental fragments along the plate boundaries can generate fast evolving oceanic basins such as the Powell Basin.