From steady state to climate-driven denudation across the Central Andes in SE Peru

Abstract

To better constrain the orogenic growth of the Andean chain, we investigated the time-Temperature paths of bedrocks from the two morpho-structural highs of the Central Andes that are separated by the vanishing Altiplano, i.e. the Eastern and Western Cordilleras of SE Peru. The Western Cordillera is a volcanic to volcano detrital chain that developed ~40-35 Ma ago and is characterized by a 4000m high mean altitude whose origin is poorly constrained. Fission-Track data on apatite and zircon crystals extracted from an Eocene pluton yield ages comprised between 24 and 14 Ma, and 38 and 30 Ma respectively. One of the noteworthy aspects of the data is that analyses reveal a steady-state phase of exhumation from the late Eocene to at least the middle Miocene (38-14 Ma) with no disruption of the exhumation path since 38 Ma either by sedimentary burial and/or rapid exhumation. The uplift of the Western Cordillera was thus probably steady since, avoiding the deposition of foreland basin sequences as in the Altiplano region. Further east, Apatite Fission-Track ages are much younger and range between 7.6 and 2.5 Ma for the Eastern Cordillera and between 11.2 and 1.5 Ma for the Sub Andean Zone. Age-altitude relationshipssuggest that denudation increased from a more quiescent Late Miocene period to a high rate of 0.9 km/my for the Pliocene. Such abrupt change is supported by a net in sediment accumulation rates in the Andean Amazon Basin but as far as offshore the Amazon fan. A global climate change is usually invoked for high Pliocene rates; however it post-dated a documented period of surface uplift in the Eastern Andes.