Extension and basin formation in the southern Andes caused by increased convergence rate: A mid‐Cenozoic trigger for the Andes

Abstract

Abstract. The southern Andes between 33 ø and 45øS latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well-described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18 ø-33øS), leading to the speculation that extension may have characterized the late Oligocene-early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains.

Nuevos antecedentes estratigráficos y geocronológicos para el Meso-Cenozoico de la Cordillera Principal de Chile entre 32° y 32°30’S: Implicancias estructurales y paleogeográficas

“…Estos estudios han permitido asignar las rocas de la Formación Abanico (Fig. 2) al Eoceno Superior a Mioceno Inferior, bien desarrollada en la cordillera Principal, entre los valles del Maipo, frente a Santiago, y del Maule, frente a Talca (Aguirre, 1960;Klohn, 1960;Vergara y Drake, 1978;Thiele, 1980;Charrier, 1981;Vergara et al, 1999;Astaburuaga et al, 2012), y concluir que se habrían acumulado en 'cuencas de intraarco', en ambiente extensional con volcanismo activo y una geoquímica que no representaría un arco volcánico típico (Muñoz et al, 2006;Jordan et al, 2001;Charrier et al, 2002Charrier et al, , 2007. La compleja distribución de los depósitos, la geoquímica y la contracción tectónica que reactivó algunas fallas que originalmente tuvieron movimientos normales, han permitido concluir que la 'cuenca de Abanico' habría sido invertida en un proceso compresivo entre 21 y 16 Ma (ver, por ejemplo, Charrier et al, 2002Charrier et al, , 2005Charrier et al, , 2007.…”

Resistivity distribution from mid-crustal conductor to near-surface across the 1200 km long Liquiñe-Ofqui Fault System, southern Chile

“…This plutonic belt was emplaced between 39 • -47 • S during two phases in Late Cretaceous and Miocene times mainly along the trace of LOFS (Pankhurst et al 1999). North of the volcanic chain the batholith is slowly replaced by Eocene-Miocene volcano-sedimentary units known as the Cura-Mallín formation (Jordan et al 2001). In this time period extensional tectonics caused the formation of a sequence of sub-basins in south central Chile between 33 • -39 • S. Sedimentary thickness of up to 3000 km is documented (Radic 2010), filled with non-marine sediments intercalated with pyroclastic rocks and lava flows mainly of andesitic composition (Niemeyer & Muñoz 1983;Suarez & Emparan 1995).…”

Variable structural controls through time in the Southern Central Andes (~36ºS)

“…The Palaoco Formation occupies the area interposed between the orogenic front and the foreland area. To the west, the Cura Mallín basin, a coeval sequence to these successions (Suárez and Emparán, 1995;Jordan et al, 2001;Radic et al, 2002;Radic, 2010) has been related to extensional relaxation of the hinterland zone, at the western slope of the Andes, beyond the scope of this work.…”