Cenozoic cooling and exhumation history of Southern Ecuador: The role of plate-boundary reorganizations and inboard tectonic conditions

The cooling and exhumation history of orogens along subduction systems provides unique constraints on regional tectonic evolution, reflecting the interplay between crustal deformation and plate convergence dynamics. Southern Ecuador lies within a transitional zone between the northern and central Andes, where contrasting tectonic histories have generated variations in inherited crustal architecture that influence how regional plate-boundary forcing is expressed in the upper plate. We present new apatite fission-track thermochronology data from seventeen plutonic rock samples across five crustal blocks in southern Ecuador: Western Cordillera, Eastern Cordillera, Intermontane basins, Celica-Lancones, and Amotape-Tahuin. Our results show that cooling ages are internally consistent within each block but differ across them, revealing a pattern of diachronous exhumation in most blocks. Some blocks instead record pronounced post-magmatic cooling, suggesting shallow pluton emplacement. Thermal history modeling, based on good-fit time–temperature paths indicates that regional exhumation initiated in the middle Eocene and persisted thereafter. The cooling trajectories group into three distinct, albeit partially overlapping, intervals: ∼45–38 Ma (middle - late Eocene), ∼40–30 Ma (late Eocene – early Oligocene), and ∼ 33–25 Ma (Oligocene). We interpret this protracted and spatially variable exhumation as reflecting changes in the boundary conditions of the subduction system, including a transition from oblique to more orthogonal convergence and increases in convergence rates during the Cenozoic. However, the observed spatial variability is more directly linked to the progressive re-activation of major fault systems and, locally, to magmatic activity.