Ona Basin is a small intra-oceanic basin located in the southwestern corner of the Scotia Sea. This region is crucial for an understanding of the early phases of opening of Drake Passage, since it may contain the oldest oceanic crust of the entire western Scotia Sea, where conflicting age differences from Eocene to Oligocene have been proposed to date. The precise timing of the gateway opening between the Pacific and Atlantic oceans, moreover, has significant paleoceanographic and global implications. Two sub-basins are identified in this region, the eastern and western Ona basins, separated by the submarine relief of the Ona High. A dense geophysical data set collected during the last two decades is analyzed here. The data include multichannel seismic reflection profiles, and magnetic and gravimetric data. The oceanic basement is highly deformed by normal, reverse and transcurrent faults, as well as affected by deep intrusions from the mantle. The initial extension and continental thinning, with subsequent oceanic spreading, were followed by compression and thrusting. Several elongated troughs, bounded by faults, depict a thick sequence of depositional units in the basin. Eight seismic units are identified in a deep trough of the eastern Ona Basin. The deposits reach a thickness of 5 km, a consistent value not previously reported from the Scotia Sea. A body of chaotic seismic facies is also observed above the thinned continental crust of the Ona High. Magnetic seafloor anomalies older than C10 (~ 28.5 Ma) may be present in the region. The anomalies could include up to chron C12r (~ 32 Ma), although their identification is difficult, since the amplitude is subdued and the original oceanic crust was highly deformed by later faulting and thrusting. The magnetic anomaly distribution is not congruent with seafloor spreading from a single ridge. The basin plain is tilted and subducted southwestward below the South Shetland Islands Block, particularly in the western part, where an accretionary prism is identified. Such tectonics, locally affecting up to the most recent deposits, imply that a portion of the primitive oceanic crust is absent. Based on the stratigraphy of the deposits and the magnetic anomalies, an age of 44. Ma is postulated for the initiation of oceanic spreading in the eastern Ona basin, while spreading in the western Ona Basin would have occurred during the early Oligocene. The tectonics, depositional units and the age of the oceanic crust provide additional evidence regarding the Eocene opening of Drake Passage. The initial tectonic fragmentation of the South America-Antarctic Bridge, followed by oceanic spreading, was characterized by jumping of the spreading centers. An Eocene spreading center in the eastern Ona Basin was the precursor of the Scotia Sea. A model comprising four tectonic evolutionary phases is proposed: Phase I, Pacific subduction - Paleocene to middle Eocene; Phase II, eastern Ona back-arc spreading - middle to late Eocene; Phase III, ridge jumping and western Ona back-arc spreading - early Oligocene; and Phase IV, ridge jumping and West Scotia Ridge spreading - early Oligocene to late Miocene. The development of shallow gateways allowed for an initial connection between the Pacific and Atlantic oceans and, hence, initiated the thermal isolation of Antarctica during the middle and late Eocene. Deep gateways that enhanced the full isolation of Antarctica developed in Drake Passage from the Eocene/Oligocene transition onward. A significant correlation is observed between the tectonics, stratigraphic units and major climate events, thereby indicating the influence of the local tectonic and paleoceanographic events of the Southern Ocean on global evolution.
- Programområde 5: Natur og klima