The structure and evolution of magmatic complexes in fold-and-thrust belts: a case study of Cerro Negro, Neuquén Province, Argentina

Abstract

In contrast to the classical concept of magma ascent in extensional settings, recent studies show that volcanism also occurs in compressional settings. The nature of the interplay between magmatism and tectonics in fold-and-thrust belts however, remains a major question, notably in active margins. The mechanisms of magma transport in such settings and whether magmatism affects tectonic deformation need to be addressed. Therefore, we carried out detailed structural mapping and sampling of the Cerro Negro intrusive complex, at Tricao Malal, Neuquén Province, Argentina. This intrusive system belongs to a magmatic province in the Agrio fold-and-thrust belt, located between 37°S and 38°S in the Argentinean foothills of the Andes. The fold-and-thrust belt has resulted from intense E-W shortening, and contains tight folds and thrusts, trending N-S. The intrusive complex crops out as a network of sills and dykes around a main intrusion, all of which are of andesitic composition.

The Cerro Negro plumbing system is well exposed so that the relations between the intrusions and the tectonic structures can be studied. We have identified at least two geometries of intrusion: sills that have been folded in an open anticline, and subvertical dykes that strike N-S, i.e. perpendicular to the shortening. According to field observations, the main intrusive body and the dykes have formed in a central anticline, the dykes being close to the hinge, suggesting that there is a structural control on magma emplacement. Furthermore, the dykes locally crosscut the folded sills. No deformation has been observed in the dykes, possibly due to their location close to the anticlinal hinge. This suggests that sills predate or are coeval with deformation, whereas dykes postdate deformation. From the structural and temporal relationships between the anticline and the dykes we infer that local stresses controlled the formation of the dykes during outer-arc stretching. This illustrates how tectonic deformation may control magma emplacement. Conversely, the traces of the main tectonic structures curve around the intrusive complex, suggesting that the latter influenced the tectonic deformation. New U-Pb data for zircons for both dykes and sills yield ages of 11.0-11.5 Ma, indicating that the lifetime of the magmatic system was less than 0.5 m.y. Absolute dating confirms that the dykes were emplaced during a time of active shortening. This study contributes the first robust evidence of substantial regional shortening at least until 11 Ma in the area.