Differential exhumation of the Eastern Cordillera in the Central Andes:
Evidence for south-verging backthrusting (Abancay Deflection, Peru)
Abstract
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.