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Abstract:
To this date, the question of why and how a plateau-type orogen formed with massive crustal thickening at the leading edge of
western South America remains one of the hotly debated issues in geodynamics. During the Cenozoic, the Altiplano and Puna plateau
of the Central Andes developed during continuous subduction of the oceanic Nazca plate in a convergent continental margin
setting – a situation that is unique along the 60 000 km of convergent margins around the globe. The key challenge is to understand
why a first-order mechanical instability of the later plateau extent developed along the central portion of the leading edge
of South America only, as well as why and how this feature developed only during the Cenozoic, although the cycle of Andean
subduction had been ongoing since at least the Jurassic. Although the widespread presence of partial melts or metamorphic
fluids at mid-crustal level has been suggested to indicate upper plate weakening from heating and partial melting, it is recently
found that upper plate strain weakening at lithospheric scale plays a significantly larger role. This first order control is tuned by
factors affecting the strength balance between the upper plate lithosphere and the plate interface of the Nazca and South American
plates such as variations in trenchward sediment flux affecting plate interface coupling and slab rollback or the role of inherited
structures. Late initiation of orogeny in the Eocene, however, and its sustained action over tens of million years is now found
to be related to the penetration of the slab into the lower mantle around 50 Ma ago, producing a slowdown of the lateral slab
migration (‚slab anchoring’), and dragging the upper plate against the subduction zone by large-scale return flow. The combination
of these parameters was highly uncommon during the Phanerozoic leading to very few plateau style orogens at convergent
margins like the Cenozoic Central Andes in South America or the Laramide North American Cordillera.
