We use a Landscape Evolution Model (FastScape S2S) to explore the impact of inherited topography in the foreland domain of a rising mountain range on its stratigraphic architecture and sediment accumulation history, inspired by the northern Pyrenean foreland. We simulate an uplifting half mountain range, its foreland basin and forebulge, and beyond, an open marine domain. We ran models with 4 different initial reliefs in the foreland domain: an initially flat foreland domain at sea-level, an elevated flat continental foreland (+300 m), a pre-existing 1 km-deep and 100 km-wide bathymetry at the location of the future foreland basin associated with a forebulge domain either at sea-level or elevated at +300m. All models show a prograding mega-sequence associated with building of mountain topography and development of the flexural foreland basin and forebulge, coalescence of alluvial fans at the foot of the range, progressive continentalization of the foreland domain, and burial of the forebulge. An initially elevated foreland domain ultimately produces a thinner foreland basin while an initially deep foreland basin produces a thicker one. After 10-13 Myr, the initial relief of foreland domain is smoothed out and the landscape does not exhibit a record of pre-existing relief. In contrast, the stratigraphic architecture of the foreland basin allows to trace inherited relief with deep marine sediments in the initially deep foreland basin, marine sediments onlapping and then burying the forebulge initially at sea-level, and continental sediments onlapping and burying the initially elevated foreland domain. We compare these interpretations to the Pyrenean retro-foreland.

Located at the northern tip of the Altiplano, the Abancay Deflection marks abruptly the latitudinal segmentation of the Central Andes spreading over the Altiplano to the south and the Eastern Cordillera northward. The striking contrast in terms of morphology between the low-relief Altiplano and the high-jagged Eastern Cordillera makes this area a privileged place to determine spatio-temporal variations in surface and/or rock uplift and discuss the latest phase of the formation of the Central Andes. Here, we aim to quantify exhumation and uplift patterns in the Abancay Deflection since 40 Ma, and present new apatite (U-Th)/He and fission-track data from five altitudinal profiles and additional individual samples. Age-Elevation relationships and thermal modeling both evidence that the Abancay Deflection experienced a moderate, spatially-uniform and steady exhumation at 0.2±0.1 km/m.y. between 40 Ma and ~5 Ma implying common large-scale exhumation mechanisms. From ~5 Ma, while the northern part of the Eastern Cordillera and the Altiplano registered similar ongoing slow exhumation, the southern part of the Eastern Cordillera experienced one order-of-magnitude of exhumation acceleration (1.2±0.4 km/m.y). This differential exhumation since ~5 Ma implies active tectonics, river capture and incision affecting the southern Eastern Cordillera. 3D thermo-kinematic modeling favors a tectonic decoupling between the Altiplano and the Eastern Cordillera through backthrusting activity of the Apurimac fault. We speculate that the Abancay Deflection, with its “bulls-eye” structure and significant exhumation rate since 5 Ma, may represent an Andean proto-syntaxis, similar to the syntaxes described in the Himalaya or Alaska.