SUBDUCTION INFLUENCE ON MAGMA SUPPLY AT THE EAST-SCOTIA RIDGE

Despite a spreading rate of 65-70 km Ma(-1), the East Scotia Ridge has , along most of its length, a form typically associated with slower ra tes of sea floor spreading. This may be a consequence of cooler than n ormal mantle upwelling, which could be a feature of back-are spreading . At the northern end of the ridge, recently acquired sonar data show a complex, rapidly evolving pattern of extension within 100 km of the South Sandwich Trench. New ridge segments appear to be nucleating at o r near the boundary between the South American and Scotia Sea plates a nd propagating southwards, supplanting older segments. The most promin ent of these, north of 56 degrees 30'S, has been propagating at a rate of approximately 60 km Ma(-1) for at least 1 Ma, and displays a morph ology unique on this plate boundary. A 40 km long axial high exists at the centre of this segment, forming one of the shallowest sections of the East Scotia Ridge. Beneath it, seismic reflection profiles reveal an axial magma chamber, or AMC, reflector, similar to those observed beneath the East Pacific Rise and Valu Fa Ridge. Simple calculations i ndicate the existence here of a narrow (<1 km wide) body of melt at a depth of approximately 3 km beneath the sea floor. From the topographi c and seismic data, we deduce that a localised mantle melting anomaly lies beneath this segment. Rates of spreading in the east Scotia Sea s how Little variation along axis. Hence, the changes in melt supply are related to the unique tectonic setting, in which the South American p late is tearing to the east, perhaps allowing mantle flow around the e nd of the subducting slab. Volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melt ing observed. A southward mantle flow may have existed beneath the axi s of the East Scotia Ridge throughout its history. (C) 1997 Elsevier S cience B.V.