EVOLUTION OF THE ANDEAN OROGENIC FRONT SOUTH OF BARILOCHE AREA, ÑIRIHUAU BASIN AND EL MAITÉN BELT (~42º S)

RAMOS, M.E.; ORTS, D.L.; CALATAYUD, F.; FOLGUERA, A.; RAMOS, V.A.

Heidelberg

Simposio; 22nd Colloquium on Latin American Earth Sciences; 2011

A field study of the Ñirihuau basin, nearby the El Maitén range, between the 42º00´s and the 42º20´s, has revealed new stratigraphic relationships. The structural analysis of this Andean segment, allowed a new interpretation of the tectonic setting of these Patagonian foothills. the stratigraphic sequence consists of four units: the base is represented by the oligo¬cene ventana Formation, formed mainly by andesitic volcanic and pyroclastic rocks. the sand¬stones of the Ñirihuau Formation of ~22-17 Ma rest in general conformably on the previously described volcanic piles (González Bonorino, 1973). Paredes et al. (2009) differentiated a series of lithofacies associations in stratigraphical order in the northern part of this basin. We identified a series of sections, circumscribed to the southern part of the basin that can be compared to the ones described to the north. These sequences are exhumed at the eastern slope of the El Maitén range corresponding to the Cushamen fold and thrust belt. in the western sector an east-verging thick-skinned structure is dismembered by a series of synthetic-to the main thrust front structures, affected by a series of west-verging backthrusts developed in the back limb. in the eastern sector, four sets of progressive unconformities from the base to the top of the Ñirihuau Formation and the base of the Collón Curá Formation were recognized. These find¬ings imply that this basement structure was created at the time of both Ñirihuau and Collón Curá sedimentation at the foothills and therefore that these constitute remnants of a proximal foreland basin. these progressive unconformities are typically found at the fold and thrust belt top wedge, implying that the early to late Miocene orogenic front was located in the El Maitén range. Based on these findings, four pulses of contractional deformation are proposed for the fold and thrust belt at these latitudes coetaneous to the sedimentation of the Ñirihuau and the base of the Collón Cura Formations (>22 to ~15 Ma), implying a mechanism of subsidence associated with orogenic loading of the El Maiten range.