Description: A joint interpretation of swath bathymetric, seismic refraction, wide-angle reflection, and multichannel seismic data was used to derive a detailed tomographic image of the Nazca-South America subduction zone system offshore southern Arauco peninsula, Chile at similar to 38 degrees S. Here, the trench basin is filled with up to 2.2 km of sediments, and the Mocha Fracture Zone (FZ) is obliquely subducting underneath the South American plate. The velocity model derived from the tomographic inversion consists of a similar to 7-km-thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities >8.4 km s(-1). In the trench-outer rise area, the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults, horst-and-graben structures, and a reduction of both crustal and mantle velocities. These slow velocities are interpreted in terms of extensional bending-related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere. The incoming Mocha FZ coincides with an area of even slower velocities and thinning of the oceanic crust (10-15% thinning), suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone. Slow mantle velocities occur down to a maximum depth of 6-8 km into the upper mantle, where mantle temperatures are estimated to be 400-430 degrees C. In the overriding plate, the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients, resulting in a seismic segmentation of the marine fore arc. The margin is composed of three main domains: (1) a similar to 20 km wide frontal prism below the continental slope with Vp 〈= 3.5 km s(-1), (2) a similar to 50 km area with Vp = 4.5-5.5 km s(-1), interpreted as a paleoaccretionary complex, and (3) the seaward edge of the Paleozoic continental framework with Vp >= 6.0 km s(-1). Frontal prism velocities are noticeably lower than those found in the northern erosional Chile margin, confirming recent accretionary processes in south central Chile.

Description: The Chile subduction zone, spanning more than 3500 km, provides a unique setting for studying, along a single plate boundary, the factors that govern tectonic processes at convergent margins. At large scale, the Chile trench is segmented by the subduction of the Chile Rise, an active spreading center, and by the Juan Fernández hot spot ridge. In addition, the extreme climatic change from the Atacama Desert in the north to the glacially influenced southern latitudes produces a dramatic variability in the volume of sediment supplied to the trench. The distribution of sediment along the trench is further influenced by the high relief gradients of the segmented oceanic lithosphere. We interpret new and reprocessed multichannel seismic reflection profiles, and multibeam bathymetric data, to study the variability in tectonic processes along the entire convergent margin. In central and south Chile, where the trench contains thick turbidite infill, accretionary prisms, some 50–60 km wide, have developed. These prisms, however, are ephemeral and can be rapidly removed by high-relief, morphological features on the incoming oceanic plate. Where topographic barriers inhibit the transport of turbidites along the trench, sediment infill abruptly decreases to less than 1 km thick and is confined to a narrow zone at the trench axis. There, all sediment is subducted; the margin is extending by normal faulting and collapsing 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 the turbidite-starved northern Chile trench, ~1 km of slope debris reaches the trench and is subsequently subducted. There, tectonic erosion is causing pronounced steepening of the margin, associated pervasive extension across the slope and into the emerged coastal area, and consequent collapse of the overriding plate. The volume of subducting material varies little along much of the margin. However, the composition of the material varies from slope debris of upper-plate fragments and material removed from the upper plate by basal erosion, to turbidites derived from the Andes.