Deformation and Sediment Provenance of the Forearc Basin in the Central Andes of Southern Peru

Abstract The linkages between the development of the forearc regions and the accompanying retroarc in contractional mountain belts remain unclear. In the central Andes of southern Peru, new results constrain the timing of deformation and the provenance of forearc basin fill during Cenozoic mountain building. The trenchward‐verging Aplao thrust fault demonstrates multiple phases of fault slip. New maximum depositional ages (MDA) from detrital zircon U‐Pb geochronology samples within growth strata constrain Eocene‐Oligocene shortening, but the previous phase remains broadly bracketed between the Late Cretaceous to Eocene. The trenchward‐verging Toran reverse fault juxtaposes Jurassic marine strata, previously mapped as Precambrian basement, above Precambrian gneisses and schists. This younger‐on‐older relationship is interpreted as a Jurassic normal fault that was positively inverted during Andean orogenesis. The most recent phase of mid‐Miocene shortening is documented by dated tuffs and MDA in folded forearc basin fill. However, earlier positive inversion is detected based on new mapping and kinematic modeling, highlighting multiple phases of motion on the Toran fault. Sediment provenance modeling from forearc basin fill reveals upsection unroofing of the Western Cordillera and recycling of forearc basin fill, with minimal contribution from distal Altiplano or Eastern Cordillera (EC) sources. These results highlight the lack of large magnitude crustal shortening accommodated within the forearc, and multiple phases of shortening in this region prior to a shift to extension during Miocene development of the Western Andean Escarpment. Finally, shortening in the forearc was coeval with EC shortening at this latitude, suggesting orogen‐wide shortening spanning the Eocene to Miocene flat‐slab to normal‐slab transition.