J. Bourgois

SUBDUCTION EROSION RELATED TO SPREADING-RIDGE SUBDUCTION: TAITAO PENINSULA(CHILE MARGIN TRIPLE JUNCTION AREA)

During the Pliocene, subduction of the Chile ridge beneath the South American margin was coeval with the emplacement of the Cabo Raper pluton located at the seaward edge of the Taitao peninsula. The chemical characteristics of the Cabo Raper pluton combined with the available tectonic data allow us to reconstruct the paleogeometry of the Chile margin 3–4.2 m.y. ago. When compared to the modern configuration, the volume of material removed by subduction erosion can be estimated quantitatively. From 3–4.2 to 1.5–1.6 Ma, 625 km 3 of rock were removed along each kilometre of margin along the Taitao peninsula transect. This leads to a conservative subduction-erosion rate of 231–443 km 3 ċkm −1 ċm.y. −1 , significantly higher than those calculated along the Japan and Peru convergent margins. It is proposed that subduction of the Chile ridge contributes fluids in addition to the subducted sediment, resulting in a higher rate of subduction erosion.

Large Neogene subsidence event along the Middle America Trench off Mexico (18°N–19°N): Evidence from submersible observations

Exposures of pre-Eocene plutonic rocks located along the inner slope of the Middle America Trench were sampled during deep-sea dives off Manzanillo (lat 18°N–19°N) at depths ranging from 3950 m to 2823 m. Fission-track data show a rapid cooling of these plutonic rocks during the Paleocene. A subaerial unconformity indicates that the plutonic rocks were exposed after Paleocene time. The presence of upper middle bathyal upper Miocene–lower Pliocene marine sediments above the unconformity documents a Neogene subsidence event at a mean rate of 0.35 mm/yr. This subsidence indicates subduction erosion along this segment of the margin.

Tectonic history of the northern Peru convergent margin during the past 400 ka

The late Pleistocene tectonic history of the northern Peru convergent margin can be retraced using data collected during deep-sea dives of the submersible Nautile combined with a recent Hydrosweep survey conducted off Peru at lat 5°-6°S by the R/V Sonne. During the past 400 ka, a broad rollover fold formed in the middle slope area, in association with a major seaward-dipping detachment fault. A catastrophic debris avalanche occurred as the result of oversteepening of the landward flank of the rollover fold. The gravity failure of the slope, previously recognized by SeaBeam mapping, occurred at 13.8 ±2.7 ka and produced a destructive tsunami.