Lithology and structure of the igneous-metamorphic basament of the SW margin of the North Patagonian Massif to the west of Rio Chico (Cushamen, Cubut, 42° 10´S - 70° 30´W)

Abstract

The pre-Triassic igneous-metamorphic basement of the SW border of the North-Patagonian Massif in Cushamen region is composed of metasedimentites, orthogneisses, foliated tonalites, foliated alkaline granites, leucogranites and diorites. Leucogranites and diorites present weak ductile deformation which allows us to considerer them as late to post-kinematics intrusions (post-regional foliation). The mineral metamorphic associations correspond to medium grade metamorphism (high grade amphibolitic facies) and a more extended low grade metamorphism (greenschist facies, biotite zone). Discrete sector associated to late deformations has low to very low grade. Metasedimentary rocks and orthogneisses have a penetrative foliation regionally crenulated. Two orthogonal directions of boudinage and folds axis indicate a strong flattening during ductile progressive syn-metamorphic deformation. NW to W-NW dominant regional foliation had an important control on the late-kinematics intrusions in the basement and on the Paleogene intrusions of the Huitrera Formation. The ductile and ductile-fragile basement structures and fabrics were formed in a stress field in which the plane that contain the maximum shortening axes (Z) has a 035°-215° strike.