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 vol. 53 p. 1 - 19

0895-9811

The Neuquén Basin, developed in a retroarc setting in the central-west of Argentina, contains more than6000 m of Mesozoic marine and continental sedimentary rocks. These rocks were deformed during theAndean orogeny leading to several thick and thin-skinned fold-and-thrust belts. The Early CretaceousAgrio Formation is composed by a thick marine succession predominantly of black shales in whichhighlights a thin fluvial-aeolian sandy interval named Avilé Member. The Avilé Member, one of the mostimportant hydrocarbon reservoirs of the Neuquén Basin, constitutes an excellent structural marker. Atthe Chos Malal fold-and-thrust belt, the strong mechanical anisotropy given by the contrasting lithologyof the Avilé Member within the Agrio Formation favored the location of detachments along the shalesand ramps affecting the sandstones during the Andean compression. Detailed field mapping at theChacay Melehue area allowed us to recognize tectonic repetitions of the Avilé Member, which formimbrications in the simplest case whereas in other places constitute a more complex combination ofimbrications, including fault-bend folding that duplicates stratigraphic sequences and fault-propagationfolding that deforms more intensely the duplicated units. Along three structural cross-sections weillustrate the geometry of these tectonic repetitions of the Agrio Formation, which in the northern areahave an eastward-vergence and in the central and southern regions show a clear westward-vergence. Atear fault along the arroyo Chacay Melehue could explain this vergence change. Forward modeling of thestructures at the central cross-section, where a backthrust system produced imbrication, duplication andfolding of the Agrio Formation, allows us to propose a balanced kinematic reconstruction of this complexstructure and to compare the features produced at different stages of the deformation sequence withfield observations. Our kinematic interpretation shows that the tectonic repetitions of the Agrio Formationinvolve 3 km of shortening above a basal detachment within the lowermost black shales. Basedon a regional balanced cross-section constructed from the basement-cored Cordillera del Viento anticlinoriumtoward the east, across the thin-skinned sector of the Chos Malal FTB, it is possible to connectthe backthrust system with east-vergent fault-bend folds that involve the stratigraphic units below theAgrio Formation. Finally, we propose a regional structural model considering the Cordillera del Viento asa basement wedge related to a low angle Andean thrust that is inserted into the sedimentary coverproducing structures of different order, which evidence a strong relationship between thick and thinskinnedstructures during the Andean orogeny.