Red barbs identify accreting margins, where the forearc is growing seaward by offscraping of trench-floor deposits. Yellow barbs are non-accreting margins along the base of with no modern growth of sedimentary prisms. Each year a total of about 1 cubic kilometer of upper plate material returns to the mantle. Map of the Central American convergent margin, showing the location of volcanoes and DSDP site 495 and ODP sites 844 and 1039. Filled symbols are samples from along the volcanic front.
Open symbols are back-arc samples.
Any triangle, formed by mixing between the two sediments and any of the three depleted sources, includes most but not all the lavas.