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

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

Landscape evolution of the Apurimac river drainage basin, southern Peru

The northernmost part of the Altiplano in Southern Peru is drained by the river Apurimac and its tributaries. The Altiplano is a region that covers most of the eastern part of the Cordillera Occidental and is bounded in the East by the Cordillera Oriental. The Apurirnac River Drainage Basin (ARDB) extends roughly between 13 °S and l5°S over 50000 km 2. It mainly occupies the northeastern flank of the Cordillera Occidental and a negligible part of the southwestern flank of the Cordillera Oriental. At the latitude of the Abancay deflection, i.e. 13.5 S; 72.7 W, the Apurirnac River abandons the Cordillera Occidental. On its way to the Amazon Basin in the North the Apurimac River drains the Cordillera Oriental and the Sub-Andean Zone (SAZ). The incision by the rivers of the ARDB has created differences in relief of more than 2000 m that contributed to the denudation and exhumation, hence the evolution ofthe landscape. The Altiplano sedimentary basin contains an 8 kilometers thick succession of Cenozoic sediments, which it received from the surrounding highs in the West and the Easl Its structural framework has been formed by the various tectonic events that struck the Andes and the various pulses of magmatic activity that took place during the Cenozoic. In the western part of the Altiplano Miocene volcanics domínate, whereas in the northeastern part, in the Cusco region, clastic deposits do so. In the region of Abancay, a batholite got emplaced during the mid Tertiary, that extends over few 1000 km2

Steady-state exhumation pattern in the Central Andes – SE Peru

The Western Cordillera of SE Peru is part of the Central Andes and is situated to the west of the Eastern Andes from which it is separated by the northern termination of the Altiplano – the Inter-Andean Valley. It is a volcanic–volcano-detrital chain that developed in the Palaeogene, and is characterized by a 4000 m-high mean altitude whose origin is poorly constrained. We selected a vertical profile in the region of Abancay to trace the record the evolving uplift and erosion history of the Andean orogen. Fission-track data on both apatite and zircon crystals were completed on plutonic rocks of the Tertiary Andahuaylas–Yauri batholith. Ages ranged between 24 and 14 Ma, and 38 and 30 Ma, respectively. Thermal modelling was completed for the whole profile and does not, like age–altitude relationships, show evidence of any clear disruption of the exhumation paths since 38 Ma either by sedimentary burial and/or rapid exhumation. One of the noteworthy aspects of the data is that exhumation was steady at a rate of 0.17 km Ma-1 from the late Eocene until at least the middle Miocene (38–14 Ma). The uplift of the Western Cordillera was thus probably steady for this period with sedimentary deposition restricted to the present-day Altiplano and Inter-Andean Valley regions