Late Oligocene?Early Miocene submarine volcanism and deep-marine sedimentation in an extensional basin of Southern Chile (~44°?46°S).

ENCINAS ALFONSO; ANDRES FOLGUERA; VERÓNICA OLIVEROS; LIZET DE GIROLAMO DEL MAURO; FRANCISCA TAPIA; RICARDO RIFFO; FRANCISCO HERVÉ; KENNETH L. FINGER; VICTOR A. VALENCIA; GUIDO GIANNI; ALVAREZ ORLANDO

Santa Rosa, La Pampa

Congreso; VII Congreso Latinoamericano de Sedimentología y XV Reunión Argentina de Sedimentología. Sesión SE2: Sedimentación en costas, plataforma y talud durante el Cuaternario.; 2016

Asociación Argentina de Sedimentología

The East Pacific or Chilean margin is the archetypal example of an ocean-continent convergent system dominated by compression and active mountain building because of the strong mechanical coupling between the upper and the lower plates. The Andean Cordillera, however, shows evidence of alternating phases of compressional and extensional deformation. Volcano-sedimentary marine strata in the Aysén region of southern Chile (~44°?46°S) were investigated to better understand the causes of extensional tectonics and crustal thinning that occurred in the Andean orogeny as these deposits constitute the only reliable record of submarine supra-subduction volcanism during the Cenozoic in southern South America. We integrated information from sedimentology, ichnology, petrography, geochemistry, structural geology, foraminiferal micropaleontology, and U-Pb geochronology and we obtained the following results.1) U-Pb zircon ages from sedimentary and volcanic rocks of the Traiguén Formation at its type locality indicate ages of ~23?26 Ma (Late Oligocene?earliest Miocene).2) The sedimentary rocks of the Traiguén Formation consist principally on rythmically interbedded sandstone and siltstone that form partial Bouma cycles and are interpreted as classical turbidites. Also characteristic of this unit are chaotic intraformational breccia and synsedimentary folded strata interpreted as slump deposits.3) We found specimens of two benthic foraminifera agglutinant genera named Ammodiscus sp. and Bathysiphon sp. These, especially when represented by such large specimens, are most characteristic of upper-middle bathyal to lower-bathyal water depths.4) The volcanic rocks of the Traiguén Formation are porphyritic basalts and exhibit features that are commonly associated with subaqueous volcanism such as pillow structures and hyaloclastites. The geochemistry of the basalts suggest an extensional tectonic regime. This notion is also supported by the presence of normal faults and synextensional strata in the Traiguén Formation.The results of our study indicate that the volcano-sedimentary Traiguén Formation was deposited in a deep-marine extensional basin during the Late Oligocene?earliest Miocene. The geochemical and petrographic analyses suggest that the basalts of this unit represent the activity of subduction-related volcanism over a thinned crust. Correlation of the Traiguén Formation with volcanic and sedimentary rocks deposited in extensional basins widely distributed in Chile and Argentina between ~33? 46ºS indicates that the Traiguén basin was likely formed as a consequence of a regional episode of extension and widespread volcanism during the Late Oligocene?Early Miocene. This event has been attributed to a transient period of slab rollback and intensified asthenospheric wedge circulation resulting from an increase in trench-normal convergence rate at ca. 26?28 My. Progressive extension and crustal thinning after the formation of the Traiguén basin allowed a generalized marine transgression of Pacific and Atlantic origin in Patagonia during the Early Miocene. Compressive tectonics starting around ~16 Ma resulted in the inversion of the Traiguén basin concurrent with the uplift of the North Patagonian Andes. The Traiguén Formation constitutes the only reliable record of submarine supra-suduction volcanism during the Cenozoic in southern South America and confirms the importance of extensional tectonics along the Chilean margin during the Late Oligocene?Early Miocene.