STORIA TERMOTETTONICA DELLA PARTE PIÙ MERIDIONALE DELLE ANDES SETTENTRIONALI

The southernmost segment of the Andes, which encompasses southern Ecuador and northern Peru, is a geologically critical part of the Andean belt. This region marks a significant change in the mountain chain's orientation from northeast to northwest and lies adjacent to areas in the north that have experienced substantial oceanic terrane accretion. Consequently, the region has undergone prolonged and complex geodynamic evolution from the Paleozoic to the Cenozoic, making it a pivotal area for comprehending mountain-building processes and magmatism at convergent plate boundaries. This study uses U-Pb geochronology of detrital and bedrock zircon, as well as whole-rock and zircon geochemistry, Hf and O isotope analyses, apatite fission track dating, and numerical thermo-tectonic modeling with Pecube and 4DTherm to elucidate the thermo-tectonic evolution of this area. Provenance analysis reveals that the Neoproterozoic to Triassic history of the region aligns more closely with northern Gondwanan signatures, such as the Brasiliano/Pan-African orogenies and the opening of the Rheic Ocean, than with classic Andean signals, like the Pampean and Famatinian orogenies. These findings suggest that, during this period, the area experienced crustal reworking processes with limited mantle input. Therefore, the region functioned more as a zone of crustal refinement than of new crust formation. The results also indicate that the region experienced a predominantly extensional regime during the Mesozoic era, accompanied by an increase in mantle-derived contributions to magmas and a decrease in crustal signatures. By ∼110 Ma, slab rollback produced the migration of the arc to the west, which led to the development of an oceanic-type basement during the middle to late Cretaceous, which was coeval with the opening of the Celica-Lancones Basin. The Cenozoic evolution is characterized by sustained magmatic activity between ~60 and 20 Ma and a gradual shift in magma sources. Two-dimensional thermal history modeling reveals fault-controlled exhumation in southern Ecuador with three distinct cooling episodes: Middle Eocene, Late Eocene–Early Oligocene, and Late Oligocene–Early Miocene, varying across different crustal domains. Coeval magmatic and exhumation-related cooling is also evident. 4DTherm modeling indicates plutons with overlapping U-Pb and AFT ages were emplaced at ~3000–3700 m, showing a stark contrast between rapid post-magmatic (577–613 °C/Ma) and slower exhumation-related cooling (3.6–7.4 °C/Ma).