TECTONIC EVOLUTION OF A RESTRICTED OCEAN-BASIN - THE POWELL BASIN (NORTHEASTERN ANTARCTIC PENINSULA)

The Powell Basin is one of the few present-day examples of a small iso lated ocean basin largely surrounded by blocks of continental crust. T he continental blocks in this basin result from the fragmentation of t he northern Antarctic Peninsula. This basin was created by the eastwar d 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 n orthern 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 ha ve an intermediate crust in the region of transition to the basin plai n. The differences in the basement structure and the architecture of t he depositional units suggest that the extensional process was asymmet rical. The southern transtensive margin and the northern transcurrent margin are rectilinear and steep, without any intermediate crust in th e narrow fault zone between the base of the continental-blocks slope a nd the oceanic crust. The multichannel seismic profiles across the cen tral sector of the basin reveal a spreading axis with a double ridge a nd a central depression filled with sediments. The geometry of the ref lectors in this depression indicates that the ponded deposits belong t o the early stages of oceanic-crust accretion. This structure is simil ar to the overlapping spreading centres observed in fast-spreading oce anic axes, where the spreading axis has relay and overlapping segments . The depositional units of the margins and basin plain have been grou ped into four depositional sequences, comprising the classic stages in the formation of an ocean basin: pre-rift (S1), syn-rift (S2), syn-dr ift (S3), and post-drift (S4). The pre-rift sequence has deformed refl ectors and is observed in the southern and eastern margins. The syn-ri ft sequence, tectonically disrupted, fills depressions bounded by faul ts and is well-developed in the eastern margin where it is truncated b y an erosive surface identified as the break-up unconformity. The syn- drift sequence is wedge-shaped in the basin, thickening towards the ma rgins and having onlap relations on the flanks of the spreading ridge. The post-drift sequence is the thickest unit and is characterised by a cyclic pattern of alternating packages of high-amplitude reflectors, very continuous, and low-amplitude reflectors. Towards the western an d eastern margins, the same sequence has channel-levee complexes and c hannelised, wedged bodies attributed to turbiditic deposits of submari ne fans derived from canyons located in the slope and outer shelf. The cyclic nature of this sequence is probably related to advancing and r eceding grounded ice sheets in the continental shelf since the latest Miocene.