Tectonic evolution of the Antarctic-Phoenix plate system since 15 Ma

Joint inversion of magnetic isochron and fracture zone data from the extinct Antarctic-Phoenix spreading system in SW Drake Passage yields seven new finite reconstruction poles. The inversion results are very well constrained for such a short length of plate boundary. Although this is partly because the finite poles are located close to the reconstructed region, the optimum use of fracture zone identifications from satellite-derived free-air gravity data is also important-as the stability of stage poles throughout the short intervals in the model affirms. The model results describe a well-organised spreading system since magnetic anomaly chron C5AD (similar to15 Ma) in which the Phoenix plate rotated about stage poles nearby to the southwest. Stage pole locations are broadly consistent with a hypothesis of pivoting subduction as the driving force of Phoenix plate movement, and there is some evidence in the progression of stage poles for late stage movement of the subduction pivot in response to the changing azimuth of the subduction zone at which the Phoenix plate was being consumed. The model kinematics alone provide no unequivocal support for previous interpretations of disruption of the subducted part of the Phoenix plate. The very latest stages of spreading saw falling spreading rates between magnetic anomaly chrons C4 (similar to8.1 Ma) and C2A (similar to3.3 Ma) when the Antarctic-Phoenix Ridge became extinct. This is consistent with an increase in shear stress across the plate bounding Shackleton Fracture Zone due to a plate reorganisation in the neighbouring Scotia Sea following the cessation of spreading on the West Scotia Ridge. (C) 2003 Elsevier B.V. All rights reserved.