RELATIONSHIPS BETWEEN TECTONIC SHORTENING AND SLAB SHALLOWING: CHILEAN-ARGENTINE ANDES (25°-35°S)

Compressional deformation along the eastern edge of the Mesozoic intra-arc to back arc basins along the Andes of Chile and Argentina from 25-35°S shows a polyphasic history. The structural analysis of this tectonic boundary zone reveals complex variations in structural style which appears to be controlled by the geometry of pre-existing basins. Deformation involves tectonic inversion or the Mesozoic basins, formation of hybrid thin - thick skinned trust belts and out of sequence thrusting and/or reactivation of previously formed structures. Here we compare the evolution of Sierra Exploradora-Maricunga region, in the southern Central Andes (26°°S), with the Mercedario- La Ramada (32° S) in the modern Pampean flat slab zone and the Yeguas Muertas - Nieves Negras region (34°S) along the transition zone between the flat-slab and the steep slab zone of south central Chile. Main events in those three regions are: -Exploradora - Pedernales region (25°-26°S): 1) Tectonic inversion of Early Mesozoic rift basins and formation of the Pedernales foreland basin (ca 85 Ma), 2) Generation of an hybrid thin and thick-skinned thrust belt and rupture of the foreland basin in the early Paleocene (65-60 Ma), 3) Oblique reactivation of previous faults, out of sequence thrusting and regional uplift (Eocene-Early Oligocene, 45–37 Ma) -Mercedario - La Ramada region (32°-33°S): 1) Early Miocene thin skinned thrusting and formation of the Manantiales foreland basin (20 Ma),2) Tectonic inversion of the (Triassic) Mercedario rift basins and associated thick skinned thrusting (ca 14 Ma), 3) Out of sequence, thin skinned, thrusting (ca 9 Ma) -Yeguas Muertas - Nieves Negras region (33°-35°S): 1) Inversion of the Mesozoic and Early Cenozoic extensional basins and formation of the Alto Tunuyán Foreland Basin (20-14 Ma), 2) Hybrid thin and thick-skinned thrusting (Aconcagua fold and thrust belt 15-9 Ma), 3) Out of sequence thrusting (9-4 Ma). The onset of shortening along the eastern edge of the Andean Mesozoic basins between 32° to 35°S in the early Miocene seems to be related to the beginning of slab-shallowing, while north of that latitude main shortening episodes are essentially Late Cretaceous to Paleogene although a “flat slab” regime have been proposed for the Eocene. A causal link seems to exist between changes of slab dip and major shortening episodes in this segment of the Andes.