Varying Rates and Modes of Subduction Erosion Along the Peruvian Margin

At least half of the world's active margin length now is attributed to the erosive type with regard to their mass transfer modes. However, the mechanisms, loci and rates of subduction erosion still are not fully understood. Among factors contributing to subduction erosion, subduction of asperities, roughness of the downgoing plate, and rheological properties of the overriding plate are thought to be of major importance in determining its styles and amounts. The Peruvian margin, the southern portion of which has experienced collision of the Nazca Ridge, is an exceptionally suitable location to study subduction erosion. Here it is possible to compare portions of a margin that have either been affected or not affected, respectively, by the subduction of major asperities. Swath bathymetry data acquired during RV Sonne cruise SO146 reveals high, but regionally different roughness of the almost sediment free Nazca plate. RMS roughness, local dip, curvedness, and void volume, i.e. the volume between peaks and valleys potentially filled with eroded material, decrease from north to south. Recently acquired wide angle data now together with information on long-term subsidence from ODP Leg 112 Site 683 enable to estimate the rates of subduction erosion across the Peruvian margin at 9\deg S, a region which was not affected by ridge subduction. Rates are 24 to 30 km³ km^-1 myr^-1 since the middle Miocene, 15 km³ km¹ myr^-1 since about 40 Ma and 6.5 km³ km¹ myr^-1 for the interval 40 to 13 Ma. These rates are considerably smaller than published long-term rates (since 40 Ma) estimated further to the south (12\deg S) or short-term (since the Pliocene) erosion rates estimated further north at about 7\deg S. However, at 9\deg S, the void volume is not much less than the eroded volume, whereas at 12\deg S, the eroded volume is about 3 times the void volume. This comparison reveals that different mechanisms of subduction erosion, which are suggested to be attributed to differences in the rheology and strength of the overriding South American plate, take place along the Peruvian margin.