SOME CONSTRAINTS ON MATERIAL INPUT ALONG THE SOUTH AMERICA CONVERGENT MARGIN

The South America subduction zone provides an optimum setting for study of convergent margin tectonics. The trench is segmented by the subduction of four large topographic structures: the actively spreading Chile Rise, and the Juan Fernández, Nazca and Carnegie hot spot ridges. In addition, extreme climatic changes along the continent range from tropical low latitudes to the hyperarid Atacama Desert and the glacial southern latitudes. This results in a significantly variable sediment supply to the trench. Trench sediment distribution is also disrupted by high-relief on the oceanic plate. Plate convergence is normal along much of Chile but strongly oblique along Tierra del Fuego. Multichannel seismic reflection profiles and multibeam bathymetry display the variability in tectonic processes along the entire Andean margin as sediment supply, convergence vectors and topography of the underthrusting plate change. Where the trench contains thick turbidite infill, typically 50-60-km-wide sediment prisms have developed. However these prisms can be rapidly removed by high-relief features of the subducting plate. Where topographic barriers inhibit along-the-trench transport of turbidites, trench-sediment infill abruptly decreases thickness to less than 1 km and is confined to a narrow zone in the trench axis (e.g. north of Valparaiso to Nazca Ridge and central Ecuador). All this sediment is subducted; the margin extends by normal faulting and collapses due to basal tectonic erosion. The transition from accretion to tectonic erosion occurs over short distances (a few tens of km) along the trench. In turbidite-starved segments (e.g. northern Chile), slope debris ~1 km thick accumulates at the base of the slope and is subsequently subducted. Tectonic erosion along the base of the upper plate causes pronounced steepening across the slope associated to pervasive extension that continues into the emerged coastal area. The volume of subducted material probably varies little along much of the subduction zone. Where large topographic features on the oceanic plate subduct an anomalously large volume of clastic material reaches the subduction zone in a short time. The input varies from slope debris of upper-plate fragments and material removed by basal erosion, to turbidites derived from Andian rocks.