Abstract

We predict the shape of the outer rise along the Peruvian Trench (6°-21°S) using flexural and gravity modelling in order to study the elastic properties of the subducting/oceanic Nazca Plate (NP). We include in our modelling the hotspot swell topography of the Nazca Ridge (NR) in the ridge-trench collision zone (~15°S). Results show an anomalous overthickening of the oceanic crust beneath the NR (from ~6 to 20 km), which is capable to produce most of the swell topography (500–700 km wide and up to 2.2 km high). The swell was formed likely under isostatic conditions (Te ~ 0 km) by the interaction of the NP with a hotspot-spreading center system. Despite the high buoyancy forces of the NR (0.5–4.0 × 1013 N/m) associated to the anomalous thick crust, the 45–50 Ma oceanic NP approaching the Peruvian Trench presents a well pronounced fore-bulge morphology with similar elastic thicknesses (Te = 35 ± 5 km) to those found for cold and old oceanic plate in the western Pacific. Consistently, our results do not show evidence for plate weakening (reduced Te values) in the NR-trench collision zone. We obtain similar results (Te = 40 ± 10 km) north of the NR implying that the oceanic NP is strong prior to subduction along most of the Peruvian convergent margin. This is consistent with the lack of great outer rise events (Mw ≥ 6) and the absence of reduced uppermost mantle velocities offshore Peru suggesting the presence of a poorly hydrated and rigid lithospheric mantle.