Along the continental margin of northern Peru, Sea Beam bathymetry and seismic reflection records reveal features from mass wasting in the middle and lower slope areas. A curved scarp cuts the middle slope and marks a slip surface seaward of which a 20 by 33 km block was displaced 800 m and back rotated 5¿ as it moved downslope. The front of that block is itself marked by a 1-km-high curved scar where the block failed and created a ~30-km debris flow avalanche. The avalanche morphology of closed highs and lows without directional fabric covers the lower slope and trench axis seaward of the detached block. If the slip was catastrophic, a local ~50-m-high tsunami was generated. This example documents the type of slope failure commonly inferred as a source of destructive tsunamis. Since the block was detached along faults that displace beds 3--5 km deep , it represents more than a superficial slide where sediment was locally oversteepened. The detached block includes rocks that were part of the continental margin since at least Eocene time in front of which a 15-km-wide accretionary complex subsequently developed. Apparently, only low levels of horizontal compression could be tranasmitted into the upper plate because of weak coupling across the Benioff zone. This may have permitted detachment and mass movement of the block despite the long history of plate convergence here. Tsunamogenic slides and mass wasting at trench depths are difficult to detect without modern high-resolution techniques. ¿ American Geophysical Union 1989