Tectonic effects of a subducting aseismic ridge: The subduction of the Nazca Ridge at the Peru Trench

A 1987 survey of the offshore Peru forearc using the SeaMARC II seafloor mapping system reveals that subduction of the Nazca Ridge has resulted in uplift of the lowermost forearc by as much as 1500 m. This uplift is seen in the varied depths of two forearc terraces opposite the subducting ridge. Uplift of the forearc has caused fracturing, minor surficial slumping, and increased erosion through small canyons and gullies. Oblique trending linear features on the forearc may be faults with a strike-slip component of motion caused by the oblique subduction of the Nazca Ridge. The trench in the zone of ridge subduction is nearly linear, with no re-entrant in the forearc due to subduction of the Nazca Ridge. Compressional deformation of the forearc due to subduction of the ridge is relatively minor, suggesting that the gently sloping Nazca Ridge is able to slide beneath the forearc without significantly deforming it. The structure of the forearc is similar to that revealed by other SeaMARC II surveys to the north, consisting of: 1) a narrow zone (10 to 15 km across) of accreted material making up the lower forearc; 2) a chaotic middle forearc; 3) outcropping consolidated material and draping sediment on the upper forearc; and 4) the smooth, sedimented forearc shelf.The subducting Nazca plate and the Nazca Ridge are fractured by subduction-induced faults with offsets of up to 500 m. Normal faulting is dominant and begins about 50 km from the trench axis, increasing in frequency and offset toward the trench. These faults are predominantly trench-parallel. Reverse faults become more common in the deepest portion of the trench and often form at slight angles to the trench axis.Intrusive and extrusive volcanic areas on the Nazca plate appear to have formed well after the seafloor was created at the ridge crest. Many of the areas show evidence of current scour and are cut by faulting, however, indicating that they formed before the seafloor entered the zone of subduction-induced faulting.