The Machu Picchu: Witness of Incision-Driven Exhumation into the Core of the Abancay Deflexion (Southern Peru)?

The topography of the Andes results from the interplay between climate, tectonics, and surface processes. Located at the northern edge of the Peruvian Altiplano, the Abancay Deflexion (AD; 12-14°S) marks the along-strike segmentation of the Eastern Andes. Indeed, this huge curved transition zone spreading over 200 km of distance constitutes a unique and exceptional anomaly at the orogen's scale as it literally articulates the narrow Northern Andes and the wide deep-rooted Central Andes ( 250 km-wide - 54 km crustal-depth vs. 450 km-wide - 70 km crustal-depth respectively). In this remote part of the Eastern Cordillera, no data allowed to constrain the latest building phase of the Andes since 20 Ma. This region is deeply incised, free of Mio-Pliocene volcanics or of prominent seismic activity. The Urubamba River eroded through one of the numerous plutons of the area and formed a 1.5 km-deep narrow valley in the surroundings of the Machu Picchu. Focusing into the core of this unexplored deflexion, we report new in-situ apatite (U-Th)/He and fission track data from a 1 km-long vertical profile located along the Inca trail pathway to Machu Picchu. Time-temperature inversions (QTQt) reveal a long duration quiescence period between 20 and 5.3 Ma followed by a drastic continuous cooling phase until now. Firstly, we did not identify the expected pulse of Miocene exhumation into the core of the AD but rather a slow and continuous one. Secondly, we identified a late exhumation phase of the Machu Picchu batholith and in-extenso of the core of the AD. As it is hard to directly link a propagation of the deformation front in the sub-Andes to a strong deformation/uplift leading to a drastic incision in the core of the AD, we propose that rapid incision presumably related to a brutal drainage network capture and/or a global climate change which started at the Mio-Pliocene boundary without a local tectonic implication. However we cannot for now exclude contemporaneous large-scale exhumation mechanisms. Few syntaxes have so far been identified in orogens (Himalaya, Alaska). As syntaxes display a set of unusual characteristics, we here identify for the first time a potential syntax in the Andes, both by the rapid exhumation with young cooling ages and by the strong "bull's-eye" spatial location of exhumation in a region of high relief and topography.