Matrix deformation in a basement-involved fold-and-thrust-belt: A case study in the central Andes, Malargüe (Argentina)

An analysis of the anisotropy of magnetic susceptibility (AMS) is used to investigate the matrix strain record of weakly deformed clastic rocks, including sandstones and shales, along a cross-section of the Malargüe fold-and-thrust belt (MFTB, Argentina). We collected 339 oriented cores from the Tres Esquinas Sandstones, the red beds of the Tordillo and Neuquén formations, as well as from Shales/Silts of the Mendoza Group. Overall, ~90% of the magnetic fabrics are well defined and are related to the strain imprint of the layer parallel shortening. Among the tectonic-related fabrics, 40% show the development of a magnetic lineation at right angle of the shortening (type II), 38% display an apparent loss of magnetic foliation (type III) and 21% develops a magnetic foliation at right angle of the shortening (type IV). There is no lithological control on magnetic fabric typology despite contrasting ferromagnetic mineralogy between the red sandstones and dark shales. Thin-section observations highlight the good match between the orientation of both the susceptibility ellipsoid and microstructure orientation, which enables us to infer the preferred orientations of the petrofabric (i.e. finite strain directions). In this way, two trends can be recognized in the study area: shortening directions trending N110 in the north and trending N80 in the south. In addition, the spatial distribution of magnetic fabric typology and burial data suggests an atypical pattern of deformation from the foreland to hinterland. Such a pattern contrasts with previous studies in fold thrust belt settings, suggesting that, at the matrix-scale, the deformation (even in a pre-folding setting) is compartmentalized and controlled by the occurrence of deep-seated thrusts localized along inherited structures. From the analysis of cross-sections, we can distinguish several domains, each of them reflecting distinct states of matrix deformation.