Marine seismic and drilling data from the Andean margin of Peru and northern Chile show trench retreat and tectonic erosion since at least 20 Ma. However, the onshore coastal zone has undergone modest regional uplift since ca. 2 Ma. This argues against subduction erosion of the margin by steady-state retreat, such as recognized in Tonga or South Sandwich. Instead we identify a new style of slow tectonic erosion. Uplifted forearc sediments indicate underplating of subducted material beneath the base of the coastal forearc, synchronous with tectonic erosion under the trenchward part of the forearc. These observations require slow rates of forearc crust subduction in the region during this time period, averaging no more than 13 km3/m.y./km since 20 Ma as the forearc wedge taper increased. Isostatic volume estimates suggest that as much as 85% of subducted material was underplated under the forearc since 330 ka and does not contribute to crustal growth under the Andes. The switch in tectonic style parallels that in the southern Andes, but cannot be triggered by increased sediment flux to the trench, as proposed for that area.