Tectonic repetitions of the Early Cretaceous Agrio Formation in the Chos Malal fold-and-thrust belt, Neuquén basin, Argentina: geometry, kinematics and structural implications for Andean building

TURIENZO MARTÍN; SÁNCHEZ NATALIA; DIMIERI LUIS; LEBINSON FERNANDO; ARAUJO VANESA

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 p. 1 - 19

0895-9811

The Neuquén basin, developed in a backarc setting at the central-west of Argentina, contains more than 6000 meters of Mesozoic marine and continental sedimentary rocks that were deformed during the Andean orogeny leading to several thick-skinned fold-and-thrust belts. The Early Cretaceous Agrio Formation is mostly composed by a thick marine succession predominantly of deep-water black shales in which highlights a thin fluvial-aeolian sandy interval named Avilé Member. This unit constitutes an excellent structural marker and is one of the most important hydrocarbon reservoirs of the Neuquén basin. At the Chos Malal fold-and-thrust belt, the strong mechanical anisotropy given by the contrasting lithology of the Avilé Member within the Agrio Formation favored the location of detachments along the shales and ramps affecting the sandstones during the Andean compression. Detailed field mapping at the Chacay Melehue area allow us to recognize tectonic repetitions of the Avilé Member, which in the simplest case forms imbrications whereas in other places constitutes a more complex combination of imbrications, including fault-bend folding that duplicates stratigraphic sequences and fault-propagation folding that deforms more intensely the duplicated units. Along three structural cross-sections we illustrate the geometry of these tectonic repetitions of the Agrio Formation, which in the northern area have an eastward-vergence and in the central and southern regions show a clear westward-vergence. A tear fault along the arroyo Chacay Melehue could explain this vergence change. Forward modelling of the structures at the central cross-section, where a backthrust system produced imbrication, duplication and folding of the Agrio Formation, allows us to propose a balanced kinematic reconstruction of this complex structure and to compare the features produced at different stages of the deformation sequence with field observations. Our kinematic interpretation shows that the tectonic repetitions of the Agrio Formation involve 3 km of shortening above a basal detachment within the lowermost black shales. Based on a regional balanced cross-section constructed from the basement-cored Cordillera del Viento anticlinorium toward the east, across the thin-skinned sector of the Chos Malal FTB, it is possible to connect the backthrust system with east-vergent fault-bend folds that involve the stratigraphic units below the Agrio Formation. Finally, we propose a regional structural model considering the Cordillera del Viento as a basement wedge related to a low angle Andean thrust that is inserted into the sedimentary cover producing structures of different order, which evidence a strong relationship between thick and thin-skinned structures during the Andean orogeny.