Balance of tectonically accreted and subducted sediment at the Chile Triple Junction

An active oceanic spreading ridge is being subducted beneath the South Amer ican continent at the Chile Triple Junction. Mass balance estimations to ch aracterize temporal and spatial variations in the frontal accretion, or und erplating and subduction of sediments since the Late Miocene, were made usi ng seismic and drill-hole data. At 200 km north of the triple junction, alm ost 80% of the sediment on the downgoing Nazca plate are subducted. Sedimen t subduction rate decreases towards the triple junction because of a low in sedimentation rates as the flank of the spreading ridge approaches the tre nch. At the triple junction, the forearc is almost completely destroyed by spreading ridge collision and subduction erosion. Less than 12% of the avai lable sedimentary input is accreted. South of the triple junction, where th e spreading ridge passed 6 Ma ago, a large fraction (> 60%) of the sediment on the incoming Antarctic plate has been scraped off and was frontally acc reted to the Chile forearc. Spreading ridge subduction leaves a distinctive geological fingerprint, and has a large impact on the mass balance of the subduction zone. However, the high rates of change in the process may make this fingerprint hard to detect in fossil convergent orogens. In the ridge collision zone the sediment supplied to the trench, and the amount of sedim ent subducted, show strong and distinctive variations on a 1- to 5-million- year time scale. On a 10-million-year time scale, sediment subduction to th e Earth's mantle is reduced by spreading ridge collision, caused by the nee d of the overriding forearc to regain a low angle of taper by frontal accre tion.