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

International audienceIn 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 an intrusive complex: the Cerro Negro of Tricao Malal, Neuquén Province, Argentina. This intrusive system belongs to a magmatic province that intruded into the intensely deformed 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 plumbing system of Cerro Negro is well exposed so that the structural relations between the intrusions and the tectonic structures can be studied. We have identified at least two generations of intrusions: two thick sills that predate or are coeval with deformation, and numerous sub-vertical dykes that strike N-S, i.e. perpendicular to the shortening. We observed that the main intrusive body and the dykes have formed in a central anticline, the dykes being close to the hinge. Furthermore, the dykes crosscut folded sills, postdating all visible 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

Combustibles sólidos (asfaltita). XVIII Congreso Geológico Argentino, Neuquén, Relatorio

International audienceWhere the Neuquén Basin abuts the Andes, hundreds of veins of solid hydrocarbon (asphaltite) crop out. Many of these veins were mined in the past. By consensus, the bitumen has resulted from maturation of organic-rich shale, especially the Vaca Muerta Fm. of late Jurassic age. To explain the maturation, recent authors have invoked regional subsidence, whereas early investigators invoked magmatic activity. In Neuquén Province, the veins are mostly sub-vertical dykes. They tend to be straight and continuous, crosscutting regional structures and all strata, from Jurassic to Palaeocene. Almost all of the dykes lie within 50 km of Tromen volcano, although two are close to Auca Mahuida. On both volcanoes, volcanic products are of late Pliocene to Pleistocene age. Although regionally the bitumen dykes track the current direction of tectonic compression (ENE), locally they radiate outward from the volcanoes. The thicknesses are greatest where the host rocks are the most resistant to fracturing. Many of the dykes occur near reactivated basement faults, especially at the foot of Tromen. Here the bitumen is highgrade, whereas elsewhere it tends to be lower-grade. Also near basement faults, bitumen dykes pass upward into surface caprocks of hydrothermal calcrete. Some dykes or their wallrocks contain hydrothermal minerals. A few contain fragments of Vaca Muerta shale. We infer forceful expulsion of source rock. Finally, some dykes splay upward near the current land surface. We conclude that the bitumen dykes of Neuquén province formed during Pliocene to Pleistocene volcanic activity

The Vaca Muerta transgression (Upper Jurassic), Neuquén Basin, Argentina: Insights into the evolution and timing of aeolian-marine transitions

Considering the evolution of aeolian to marine transitions for the geological record, either catastrophic or gradual transgressive scenarios showing high or low rates of coastal migration have been proposed. A critical evaluation of modern analogues suggests that a catastrophic transgression shares many characteristics with Holocene transgressions, yet they are caused by different rates of sea-level rise. The present study provides insights into the evolution of aeolian to marine transitions in order to discuss these alternative scenarios of sea-level rise. For this purpose, a sedimentological and ichnological analysis was carried out on ten stratigraphic sections of the Picún Leufú area, Argentina. There, marine deposits of the Vaca Muerta Formation accumulated over the aeolian deposits of the Quebrada del Sapo Formation during the early Tithonian. The sedimentary evolution of the transition can be summarized in: (i) a shutdown of aeolian dune field deposition, generating a planation surface in somewhat elevated areas and reworked megadunes in lowlands; (ii) beach sedimentation caused by episodic marine flooding that contributed to megadune reworking; and (iii) deposition in an embayed marginal-marine setting at the coast, recorded by bay margin bindstone, proximal bay and distal bay sedimentation. This transition indicates very rapid coastline migration and a condensed Transgressive Systems Tract succession throughout the study area. Rates of sea-level rise similar to Holocene ones (millimetres to centimetres per year) may have produced the transition between the Quebrada del Sapo and Vaca Muerta formations. The Late Jurassic represents a non-glacial time, and the global sea-level maximum highstand pre-dated the Vaca Muerta transgressive event. Thus, part of the sea-level rise has to be attributed to tectonic/thermal subsidence and compaction of underlying strata, which may have generated these atypical rapid rates of sea-level rise