Studies of Ocean Drilling Program Leg 112 cores and geophysical data define early Eocene-Quaternary normally faulted, continental sediment sequences across the Peru margin to the middle slope, and a 15-km-wide late Miocene-Quaternary accretionary complex beneath the lower slope. The Eocene shelf and upper-slope sea floor were eroded during the Oligocene, and in middle Miocene the seaward part had subsided to lower slope depths. This subsidence and the missing Paleogene continental slope indicate tectonic erosion during normal plate convergence. Subduction of the Nazca Ridge locally accelerated tectonic erosion, leaving a scarp but no associated compressional structure nearby. Late Miocene sediment accreted against the erosional scarp. Local tectonic mechanisms of forearc basin subsidence are suggested by the rapid subsidence of a late Miocene forearc basin and stability of the adjacent one despite similar histories of plate convergence.

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