Extensional rupture of oceanic crust in the Chile Trench

Abstract

Extensive new bathymetric and seismic reflection coverage of the Chile Trench between 23 and 34° S allows a more detailed study of the tectonics and sedimentation of this feature. Using morphology, structure, and sediment distribution, the trench and outer continental margin can be divided into three provinces (Northern, 23–27° S; Central, 27–33° S; and Southern, 33–34° S) separated by distinct tectonic transition zones at 27 and 33° S. Large horst and graben blocks are the dominant structural features on the trench outer slope, with fault offsets of 500–1000 m into layer 2. Grabens generally range in width from 4 to 8 km, and extend up to 100 km parallel to the trench axis. Faulting is most pronounced in the deeper northern parts of the trench, but also occurs in the basement beneath thick axial sediments in the south. Sediment distribution along the axis is controlled by coastal climate and turbidity current transport northward along the trench axis. Over a kilometer of turbidites nearly fills the trench south of 33° S. Axial deposits are less thick but continuous from 33 to 27° S, with a patchy distribution of sediment ponds from 27 to 23° S. Transport of sediment is northward down the axis from the supply regions to the south. Faulting of the oceanic crust on the trench outer slope can be related to extensional stress created by downbending of the Nazca Plate prior to subduction. Horst and graben faulting patterns are similar to those predicted by Hafner (1951) for a bending slab. Horizontal turbidite layering surrounding these basaltic blocks in the trench axis indicates that the normal faulting occurs before the oceanic plate reaches the axis. A combination of sparse sediment cover and large fault offsets in the northern Chile Trench may result in tectonic erosion of the continental margin.