Subduction erosion along the North Chile margin

Swathbathymetry off Antofagasta reveals many tectonic features of the north Chile Trench. Since sediment on the oceanic plate is only similar to 100 m thick, and since the continental slope is essentially eroded crystalline c rust, basement rock is commonly near the sea floor. Therefore, high resolut ion bathymetry is a good indicator of basement structure. The narrow shelf and upper slope are characterized by canyons crossed locally by extensional faults. Mejillones Ridge is positioned anomalously seaward of the coastal Cordillera and its emergent crest forms the Mejillones Peninsula. The flank s of this 100 km long uplifted feature are characterized by many normal fau lts. Normal faults partition the middle slope into blocks obscured by a dow nslope mobile blanket of debris thick enough to mute all but the major base ment structure. In the middle slope, coherent detached blocks disintegrate rapidly by mass wasting. The lower slope is slide debris pushed into low ri dges and troughs by the well developed subducting horst and graben topograp hy of the oceanic plate. Detritus and sediment moves downslope into graben, levels the subducting plate, and is pushed up over small ridges. Sediment and debris piled against the base of the slope as the plates converge is un derthrust with the subducting plate. Compressional features are confined to the lower quarter of the slope and are isolated from extension of the midd le and upper slope. (C) 1999 Elsevier Science Ltd. All rights reserved.