Tertiary tectonics of the Patagonian Andes retro-arc area between 38º15’ and 40ºS latitude

EZEQUIEL GARCÍA MORABITO; HANS-JÜRGEN GÖTZE; VÍCTOR A. RAMOS

TECTONOPHYSICS

ELSEVIER SCIENCE BV

Año: 2010

0040-1951

 The structure and tectonics of the retro-arc area of the Andes between 38º30’ and 40ºS remains poorly understood because of the scarcity of previous structural studies. This paper analyzes the styles, distribution and timing of the deformation in the eastern flank of the Andes between these latitudes, with focus in the Tertiary tectonics of the area.  This Andean section can be divided into an internal and an external domain based on its distinctive structural evolution. The boundary between them coincides with the Bio Bio and Aluminé valleys, which separates sectors of the fold-and-thrust-belt with opposite vergence and different structural styles. Structures developed in the external domain during the Andean cycle are associated with a west-verging fold-and-thrust-belt which was controlled geometrically by preexisting normal faults of the Mesozoic rift phase. We also differentiate three segments along the external domain of the inner retro-arc zone at these latitudes. Each segment is associated with a Tertiary depocenter related to the western border of a structural high (Copahue Pino – Hachado Block and Cordilleras de Catan Lil and Chachil) which constitutes an outstanding feature within the study area. The recognition and interpretation of the main structures and the spatial and temporal distribution of the Tertiary sequences allowed us to establish a tectonic model in which the uplift of this NNW-trending block during upper Miocene times, originated a series of intermontane depocenters well represented between 38 and 40ºS. As a result of that, syntectonic and synorogenic deposits accumulated in a compressive regime. These depocenters can be integrated in a narrow deposition zone associated with the western slope of the Copahue – Pino Hachado Block and its southward prolongation in the Cordillera de Catan Lil and Cordillera del Chachil.  On the basis of tectonostratigraphic controls we define the last Andean contractional phase of the inner retro-arc area at these latitudes, between 11 and 5.67 Ma for the northern segment, between 10 and 6.2 Ma for the central segment and between 13 to 8 and 4.3 Ma for the southern segment. After this short-lived compressional stage, deformation migrated to the west associated with the onset of the Liquiñe-Ofqui strike-slip fault zone along the intra-arc zone.