Diagnostic species of fossil diatoms (genus Bogorovia, Triceratium, Thalassiosira), and molluscans (genus Pitar, Cucullaea, Peruchilus) demonstrate the presence of a late Oligocene-early Miocene sedimentary unit in the Pisco Basin, south-central Peru. The transgressive basal facies of this unit unconformably overlap older rocks, upward the unit shows subsident shelf conditions. This transgression took place after a compressional tectonic event which had affected the region by late Oligocene. These data, and data from other places in western South America, indicate that the late Oligocene-early Miocene was a transgressive period along this continental margin. These phenomena appear associated to 1) a global change in plate kinematics reflected by an increase of the convergence rate between the Nazca and South American plates; and 2) a global rise of oceanic temperatures; factors which could have favoured a sea-level rise. The amplitude of this transgression is variable along the margin which shows segments with different rates of subsidence; the northern and southern Andean segments display a mainly subsident character, whereas the central Andean margin exhibits a dominant tendency to uplift. Our data confirm that the central Peru segment is characterized by subsidence and constitutes an exception within the central Andean forearc. This characteristic is thought to be related to the presence of dense mantle material within the central-Peru forearc crust. -Authors