Geochronology of pediment surfaces in southern Peru: Implications for Quaternary deformation of the Andean forearc

Abstract

The geomorphology of the Andean forearc has historically been viewed as an old remnant of a late Miocene planar landscape with no significant active structures accommodating Quaternary deformation. However, the well-preserved sequence of planation surfaces and strath terraces developed within the forearc of southern Peru provide evidence of recent uplift along the western margin. Previously, the abandonment of these surfaces was attributed to uplift of late Miocene age, however, abrupt changes in topography and drainage incision within the pediment surfaces indicate recent deformation. We use in situ produced 10Be to determine the exposure ages of these abandoned surfaces in order to derive the spatial and temporal extent of this tectonic activity. Our new results, in contrast to previous work, yield pediment surface ages of ~ 119 ka, ~ 203 ka, ~ 278 ka, ~ 549 ka and ~ 1003 ka indicating a youthful morphology. Additionally, these surfaces are affected by steeply dipping active faults producing localized deformation, which can be quantified. Incision rates, based on the exposure ages of abandoned strath surfaces are on the order of 0.04–0.3 mm/yr. The data set presented here not only suggests significant active deformation within the forearc, but also highlights a sharp contrast between the style of deformation observed on the eastern and western margins of the Andes. While the general notion is that active deformation is localized in the Subandean fold and thrust belt, our data support an emerging view where active deformation is occurring in the western margin as well.