Morphology, seismic structure and tectonic development of the Powell Basin, Antarctica

The Powell Basin, located near the Scotia-Antarctica strike-slip plate boundary, is bounded on three sides by continental crustal blocks, and it separates the South Orkney microcontinent from the northeastern Antarctic Peninsula. On the basis of the entire geophysical data set available, the main structural and geological elements of the basin are identified. The northern domain is characterized by an oceanic-like environment, with a ridge axis in the centre of the basin. Two conjugate deep, restricted sedimentary basins are located at the base of the northeastern and northwestern margins, and filled by thick sedimentary sequences. The southern domain is mainly floored by transitional crust and presents rift-related basement graben. The margins surrounding the basin are different, with the northern and southern margins mainly transcurrent, whereas most of the eastern and western margins show passive margin characteristics. The sedimentary sequences filling the basin have been correlated to the main tectonic processes responsible for Powell Basin opening, and partially dated using marine magnetic anomaly identifications. Three main evolutionary phases are recognized that explain the geological setting of the Powell Basin. An early phase of separation between the Antarctic Peninsula and the South Orkney microcontinent produced stretching and progressive continental lithospheric thinning, with the creation of restricted pull-apart basins (proto-Powell phase). This phase was followed by a northward movement, possibly accomplished with a clockwise rotation, of the South Orkney microcontinent (drift phase), with consequent oceanic crustal accretion in the northern sector of the basin, and development of rift-related structures in the southern sector (phase spanning 27-18 Ma, as determined from the marine magnetic anomalies). A successive clockwise rotation of the South Orkney microcontinent determined the complete development of the northern sector of the basin, with spreading and final structuration of the margins encircling the basin plain.