Conrado Rubén Gianni

The southern Central Andes forearc preserves extensive low-relief wave-cut platforms and fluvial terraces that record long-term margin uplift, yet the timing and driving mechanisms are still debated. Here we present eleven new in situ ¹⁰Be exposure ages from high fluvial terraces and four ages from a lower terrace, combined with geomorphic analyses across eight adjacent catchments, to reassess the age and tectonic significance of the degradational surfaces (pediplain) present between 29.5 and 32.5°S. Erosion-corrected exposure ages indicate that the high terrace formed during the Early Pleistocene, while a lower terrace records incision at Middle Pleistocene. Longitudinal terrace–channel profiles reveal systematically increasing relief toward the coast that terminates near the surface projection of the 50–60 km slab-depth contour, coincident with the downdip limit of megathrust domain-C earthquakes. This spatial relationship supports a regionally coherent uplift signal produced by the cumulative effect of deep coseismic deformation. In peninsular settings, notably the Altos de Talinay, this long-wavelength signal is overprinted by short-wavelength uplift consistent with localized underplating. Our results demonstrate that the high fluvial terraces and the shore wave-cut platform constitute a single, regionally continuous geomorphic marker recording an Early Pleistocene forearc uplift phase extending from ~16° to 42°S. This orogen-scale emergence implies a subtle but widespread change in subduction dynamics during the last Early Pleistocene, the causes of which are not clearly understood.

Flat subduction, an intermittent phenomenon along active margins, arises from well-known causes, yet the mechanisms driving its expansion remain poorly understood. The prevailing view suggests that trenchward continental motion drives slab overthrusting, causing the flat slab to expand oceanward. Here, we explore an alternative mechanism: underthrusting of the subducting plate through forward propagation of the flat-slab hinge. We directly evaluate both hypotheses through a kinematic analysis of trench and flat-slab motions using a global flat subduction database cast into multiple absolute plate motion models. Our results indicate that flat-slab expansion reflects distinct end-member processes, with forward propagation emerging as the dominant mode. We present a framework for flat-slab propagation that emphasizes the dynamic interaction between lower-plate motion and slab pull from adjacent subduction zones, an interplay validated through numerical modeling. These findings challenge conventional assumptions and underscore the need to reconsider the role of lower-plate kinematics in flat-slab dynamics.