The magmatic arc of the Antarctic Peninsula displays a wide range of features related to extensional tectonics at a convergent margin. Cenozoic extensional features are concentrated in a linear belt on the western side of the peninsula. The West Coast Magnetic Anomaly (WCMA) indicates the presence of a composite batholith 90 km wide and 1 500 km long, part of which may have been emplaced during an initial splitting of the continental arc. Horst and graben structures subsequently developed in segments bounded by continental fracture zones reflecting those of the oceanic lithosphere. This suggests the influence of processes associated with the late stages of subduction. The arrival of segments of the ocean ridge at the trench may have resulted in the extension and uplift of the adjacent portion of the continental plate. The final set of these interactions began at about 7 Ma opposite the northern Antarctic Peninsula and coincided with the dramatic slowing of spreading at the surviving ocean ridge and the development of continental alkaline volcanism. Continued extension in response to the tendency of the subducted plate to continue sinking led to total failure of the continental lithosphere and the opening of Bransfield Strait, a marginal basin floored by oceanic crust.