Pliocene extensional tectonics in the Eastern Central Patagonian Cordillera: geochronological constraints and new field evidence

Abstract

International audienceRecent field work and review of radiometric data obtained from Neogene lavas and plutonic rocks exposed in the Eastern Central Patagonian Cordillera (46–48ºS), which overlie subducted segments of the South Chile Ridge, suggest important Late Miocene to Pleistocene morphological changes in relation to base level variations and/or tectonic events. We present new field observations from a region south of Lago General Carrera-Buenos Aires, between the main Cordillera and the Meseta del Lago Buenos Aires, demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5–4 Ma and after 3 Ma. We also show that the 12 Ma basaltic flows of the Meseta del Lago Buenos Aires (~2000 m a.s.l.) have been subjected to deep incision, with younger lavas dated at 1.2 Ma partially filling the valleys. These incisions are thought to reflect progressive eastward tilting of the entire meseta. Our new observations, together with additional features from Central Patagonia, strongly suggest that tectonic events led to a regional widespread morphological change after 5 Ma. The coincidence in time and space between the subduction of segments of the South Chile Ridge at 6 and 3 Ma causing opening of a slab window, and strong base level variations in the studied area, suggests a cause-and-effect relationship. In Central Patagonia, compressional tectonics ended well before extensional events reported here. Causes of uplift and further extension are probably completely disconnected. The uplift is purely tectonic in origin and occurred prior to the subduction of the South Chile Ridge. Extension should be a consequence of this subduction

Subduction Zone Geodynamics

“…An extensional or transtensional tectonic event occurred after 5 Ma, reactivating ENE and NNW-trending lineaments Scalabrino et al, 2007). Ramos, 1989;Coutand et al, 1999;Diraison et al, 2000;Lagabrielle et al, 2004Lagabrielle et al, , 2007Ghiglione and Ramos, 2005 and Scalabrino et al, (Fig. 6) as well as on the edges of the Cenozoic inverted pop-down Cosmelli basin.…”

Repeated eastward shifts of arc magmatism in the Southern Andes: A revision to the long-term pattern of Andean uplift and magmatism

“…(C) Quaternary within-plate volcanic plateaux are developed when the Chilean ridge is subducted and relatively older oceanic lithosphere sinks in the Chilean trench potentially triggering a slab steepening setting. An extensional setting coexisted with the early within-plate products, as described for the lago Buenos Aires area (after Lagabrielle et al, 2007). and progressively older and colder oceanic crust would have produced a slab steepening setting that triggered asthenospheric injection, mafic intrusions and localized foreland extension.…”

“…(B) Synchronous foreland basins, less than 18 Ma Río Grande basin, Patagonia, Ñirihuao, Collón Cura and Santa Cuz Formations (Marshall et al, 1981;Marshall and Salinas, 1990;Soria, 1984;Mazzoni and Benvenuto, 1990;Yrigoyen, 1994;Silvestro et al, 2005;Silvestro and Atencio, 2010), and Late Miocene contractional structures in the foreland area. (C) Pliocene to Quaternary plateau basalts in Patagonia associated with extensión (Muñoz and Stern, 1988;Bermúdez and Delpino, 1989;Stern, 1989;Bermúdez et al, 1993;Hildreth et al, 1999;Lagabrielle et al, 2007;Folguera et al, 2009;Ramos and Folguera, 2011).…”

Uplift of quaternary shorelines in eastern Patagonia: Darwin revisited

“…This slab window triggered a new widespread episode of back-arc magmatism in southern Patagonia. Plateau-lavas related to the opening of this slab window seal the eastern thrust front of the Patagonian Andes, implying that the compressive tectonic activity in the Andes of Southern Patagonia essentially ended before Late Miocene (Ramos, 1989;Coutand et al, 1999;Lagabrielle et al, 2004Lagabrielle et al, , 2007. Guillaume et al (2009) argue that the opening of the slab window also resulted in a long-wavelength (N1000 km) dynamic uplift of Southern Patagonia.…”