Pliocene to quaternary back-arc extension in the Andes at 35º - 37º30´s.

FOLGUERA A.; RAMOS V. A; ZAPATA, T; SPAGNUOLO, M; MIRANDA. F

Barcelona, España.

Simposio; 6th International Symposium on Andean Geodynamics; 2005

  The Southern Central Andes are mainly the result of shortening imposed during the last 20 to 12 Ma (Ramos et al., 2004). However, field studies reveal that this picture is more complex than previously thought. The retro-arc area between 37º and 40ºS is flanked by recent extensional basins (Muñoz and Stern, 1988) associated with lithospheric attenuation (Kind et al., 2002) and profuse volcanic activity, active since at least Early Pliocene times (Ramos and Folguera, 1999; 2005; García Morabito et al., 2003; Melnick, et al., 2003). Three main depocenters are controlled by extensional structures superimposed to the inner sectors of the Andean fold and thrust belt (Agrio and Chos Malal fold and thrust belts) formed mainly during Middle to Late Cretaceous times with mild reactivations in Eocene and Late Miocene periods (Ramos, 1998): the Loncopué, Bío-Bío Aluminé and Lago del Laja troughs (Fig. 1). The origin of this phase of extension at 37º-40ºS seems to be linked to the steepening of the subducted oceanic slab (Muñoz and Stern, 1988; Stern, 2004; Ramos and Folguera, 2005), evidenced by westward moving and narrowing of the arc front during the last 5 Ma.  Contrastingly, north of 37ºS the inner retro-arc is being uplifted by dextral-transpressional strike-slip faults, the northernmost-known extreme of the intra-arc Liquiñe-Ofqui fault system (Lavenu and Cembrano, 1999) developed in the eastern side of the Andes (Folguera et al., 2004). The prolongation of these faults into the Argentinian side has been recognized as the Antiñir-Copahue fault system (ACFS). It is responsible of the closure of the northern part of the Loncopué trough (Fig. 1). As a result of that a very recent fold and thrust belt (Guañacos fold and thrust belt) is rapidly growing in an out-of-sequence order respect to the external fold and thrust belt, whose orogenic front is the ACFS (Folguera et al., 2005). This segmentation in the retro-arc seems to be related to the intersection of the long Liquiñe-Ofqui fault system (38º-46ºS) across the Main Andes, at the latitude of the Copahue volcano (Fig. 1). A new Plio-Pleistocene extensional retro-arc basin, the Las Loicas trough, is not involved in the young Guañacos fold and thrust belt, and it is located immediately to the east (35º-37º30´S). Las Loicas trough is the northernmost-known expression of this broad area of extension at the retro-arc (Figs. 1 and 2), which includes the Loncopué, Bío-Bío Aluminé and Lago del Laja troughs, and the most prominent basin developed between 5 and 3 Ma ago.