Structural style of the Malargüe fold-and-thrust belt at the Rio Diamante area, Mendoza province, Argentina.

TURIENZO MARTÍN; DIMIERI LUIS

Glasgow

Workshop; Tectonic Studies Group Annual meeting; 2007

Tectonic Studies Group de la Geological Society of London

The Malargüe fold-and-thrust belt is a thick-skinned one developed in Miocene-Pliocene times during the Andean orogeny, in Mendoza province, Argentina. It is well known that the tectonics of this mountain chain involves both basement and cover rocks. The former are mainly plutonic and volcanic rocks formed during the Gondwanic magmatic cycle (Carboniferous-Triassic). These rocks formed the basement of Mesozoic marine and nonmarine sediments in the Neuquén basin. Large amounts of Cenozoic sintectonic materials fill the foreland basins related to the Andean orogeny. In order to understand the structural behaviour of all these rocks we made a detailed mapping (1:50.000) in the Rio Diamante area, approximately located between 34º 30? and 34º 45? south latitude and 69º30? and 69º 50? west longitude. In this sector of almost 1200 km2 (35 km x 34 km) we constructed three regional cross-sections that allowed us to recognize the geometry of the structures and so to propose a probable structural evolution of this Andean region. From a regional point of view, the main structures are two uplifted basement blocks that occur at both the west and east sectors of the study area surrounding a central region where thin-skinned deformation prevails. Although the basement rocks at the western block are not apparent, its uplift is evidenced by the occurrence of Jurassic marine rocks at very high topographic levels. The eastern block that belongs to the Cordillera Frontal morphostructural unit is a huge range of basement rocks overthrusted to the east and its south edge is located in the Rio Diamante?s valley. An interesting feature is the common development of backthrust systems at the toes of these basement blocks. Everywhere basement rocks appear at the surface it is possible to observe these backthrusts related with them. The folds formed by these west-directed thrusts have a particular geometry including slightly tilted, long backlimbs, steeply-dipping, short forelimbs and widely spaced backthrusts, which cannot be explained with certainty by classical models. Geometric and kinematic studies, supported by seismic and well information, allow interpreting a structural style characterized by large basement wedges, that may be related to low or medium angle thrust faults. These wedges propagates into the sedimentary cover along favourable horizons (commonly gypsum), transferring the shortening from the basement to the cover suggesting a close spatial and temporal relationship between basement and cover deformation. Some workers suggest that tectonic inversion was important in this region. However we think that this mechanism doesn?t explain the amount of shortening observed in this area and thus it can?t be considered likely in the orogenic building of this part of the Andes. In the thin-skinned zone there is a distinct behaviour according to the variations of thickness and composition of the rocks, related to changes in the environment from a deeper to shallow portion of the basin. While at the western zone the abundance of shales and salt horizons facilitate the formation of tight folding, generally interpreted as fault-propagation folds, the more competent units placed at the eastern zone, mainly composed by calcareous strata, are deformed into duplex structures and imbricated fans. Toward the south, these thrust systems overthrust cretaceous rocks upon Miocene synorogenic units.