ACTIVE MARGIN PROCESSES ALONG THE ANTARCTIC PENINSULA

The Antarctic Peninsula has a remarkable record of active margin processes, which include subduction with progressive ridge-trench collisions, margin segmentation by major fracture zones, rifting in a hybrid back-arc and sheared plate margin context, fore-arc basin development and glacial-marine controlled trench fill processes. Several facets of these active margin processes both of internal (crustal dynamic) and external origin (climate-controlled) have been documented by a geophysical survey during the Antarktis VI/2 cruise of R.V. Polarstern (October-December 1987). Reflection seismic profiles have been shot over the rift basin of Bransfield Strait, over an elongated sediment-filled trough interpreted as a fore-arc basin, over accretional and progradational slopes, over recent and ancient trench environments and over the facing oceanic domain. In this oceanic domain, different fracture zones have highly contrasting morphological and geophysical expressions. The subduction of a fracture zone like Hero F.Z., characterized by a significant relief possibly related to the presence of buoyant (serpentinite) ridges, may have been a factor of subduction termination for the last segment of the Aluk (Drake) plate; it may also have played a role in the separation of a blueschist-bearing fragment (Smith Island) from the base of the accretionary plate margin and in its lift to the surface. The magnetic anomaly pattern of the oceanic slabs facing the northwestern Peninsula margin shows evidence of an intriguing spreading acceleration, which apparently preceded ridge-trench collision. The same anomaly pattern provides a clue to the stratigraphic interpretation of the oceanic sediment cover and of the frontal part of the prograding, now passive margin south of the South Shetland Island Arc. An apparently broken and tilted oceanic plate fragment, squeezed between the South Shetland Trench and Shackleton Fracture Zone, may argue for the role of transpression associated with the oblique convergence of the Antarctic and Scotia plates.