The opening of Powell Basin was part of the regional response to N55°W relative plate motion of South America away from Antarctica, which led to the formation of Drake Passage during the Eocene and Oligocene. Restoration of microplates around the basin using gridded magnetic anomalies from its margins illustrates the pre-break-up continuity of the Pacific Margin Anomaly magnetic high associated with a Mesozoic arc-batholith. Newly compiled magnetic anomaly data over the Powell Basin show subdued linear seafloor spreading type anomalies. These are used, together with marginal and regional geology, to constrain the opening history of the basin. Magnetic reversal modelling suggests that slow spreading in Powell Basin probably occurred between 29.7 Ma and 21.8 Ma, following rifting of Mesozoic continental crust with associated break-up volcanism. A simple, two-phase model for the rotation of the South Orkney Microcontinent away from the Antarctic Peninsula accounts for the pattern of magnetic reversals recorded in Powell Basin, and for the structure of its margins. Crown Copyright © 2002 Elsevier Science B.V. All rights reserved.