LATE OLIGOCENE-EARLY MIOCENE EXTENSION IN THE CENTRAL ANDES: INSIGHTS FROM NUMERICAL MODELING.

LUCAS FENNELL; MARIUS WALTER; JAVIER QUINTEROS; ANDRÉS FOLGUERA

Simposio; Primer Simposio de Tectónica Sudamericana; 2016

Subduction-type orogens such as the Andes are usually conceived as being constructed entirely through aseries of periods of uplift and widening due to crustal thickening and migration of the orogenic front.However, the history of the Andean range is quite more complex, starting in Late Cretaceous times, whenthe South American plate started its westward movement towards the proto-Pacific plate, overriding thetrench and inducing contractional deformation in the South American western margin. Most works havebeen dedicated to study the building of the Andes through the analysis of its contractional phases,emphasysing on the onset of deformation, rate of uplift and structural style. Nevertheless, extensionaldeformation has also been present in the Andes along its history, but the processes that have beenresponsible for it remain yet unclear. In Late Oligocene to Early Miocene times, the extensional regimereached a regional scale, so geodynamic processes have been proposed as the best fitting explanation,being the steepening of the slab angle and increase in the convergence rate the predominant. Over the lastdecades, numerical models started to test different variables involved in subduction processes, but most ofthem were focused on the contractional processes, being the extensional deformation less assessed. Thiswork aims to understand, through numerical modeling, the causes of a brief period of extensional regimein the Andes, occurred between Late Oligocene and Early Miocene times. Using an Andean subductionreference model, we analyze the consequences of the penetration of the oceanic slab into the mantletransition zone, triggering the slab pull force. In the model, the strength of this force produces a bendingof the slab and the retreat of the trench hinge away from the upper plate, along with an abrupt incrementin convergence velocity. The void produced by the bending of the slab is filled by an influx of hotasthenospheric material into the mantle wedge, producing a thermal anomaly beneath the lower crust. Onthe other hand, the retreat of the hinge is translated as stretching in the upper crust, generating widespreadextension and the creation of a basin. As a final hypothesis, the role of the slab pull force as theresponsible for the Farallón plate break-up into Nazca and Cocos plates at ca. 23 Ma is discussed.