The structure of the northern Agrio fold and thrust belt (37°30’ S), Neuquén Basin, Argentina

Abstract

The Agrio fold and thrust belt is a thick-skinned orogenic belt developed since Late Cretaceous in response to the convergence between the Nazca and South American plates. The integration of new structural field data and seismic line interpretation allowed us to create two balanced cross-sections, which help to analyse the geometry of both thick and thin-skinned structures, to calculate the tectonic shortenings and finally to discuss the main mechanisms that produced this fold and thrust belt. The predominantly NNW-SSE structures show varying wavelengths, and can be classified into kilometer-scale first order basement involved structures and smaller second, third and fourth order fault-related folds in cover rocks with shallower detachments. Thick-skinned structures comprise fault-bend folds moving into the sedimentary cover, mainly along Late Jurassic evaporites, which form basement wedges that transfer the deformation to the foreland. Thus, shortenings in both basement and cover rocks must be similar and consequently, by measuring the contraction accounted for thin-skinned structures, is possible to propose a suitable model for the thick skinned deformation. The balanced cross-sections indicate shortenings of 11.2 km (18%) for the northern section and 10.9 km (17.3%) for the southern section. These values are different from the shortenings established by previous works in the region, reflecting differences in the assumed model to explain the basement-involved structures. According to our interpretation, the structural evolution of this fold and thrust belt was controlled by major basement-involved thrust systems with subordinate influence of inversion along pre-existing normal faults during the Andean compression.

Structural Evolution of the Northern Agrio Fold and Trust Belt (∼37°30′S), Neuquén Basin, Argentina, Derived From Low‐Temperature Multi‐Thermochronometry

“…Cobbold and Rossello (2003) obtained an 40 Ar‐ 39 Ar age of 39.7 ± 0.2 Ma for a sill at Cerro Mayal, and noted that these rocks intruded pre‐existing folded Mesozoic formations. The Cerro Caycayén has a 40 K‐ 40 Ar on amphibole age of 44.7 ± 2.2 Ma (Llambías & Rapela, 1989), and these rocks were intruded along a backthrust located in the forelimb of the Rahueco anticline (Lebinson et al., 2018). Field relationships consistently indicate that those igneous rocks were intruded along pre‐existing structures, suggesting that deformation might predate magmatic activity.…”

“…These contratractional structures involve both Paleozoic to Triassic basement rocks and Jurassic to Cretaceous sedimentary sequences of the Neuquén Basin. While the basement is not directly exposed along the Agrio FTB, thick‐skinned structures have been identified using seismic data and field mapping (e.g., Lebinson et al., 2018; Turienzo et al., 2022; Zamora Valcarce et al., 2009; Zapata et al., 2002). These structures have given rise to large‐scale folds with wavelengths exceeding approximately 8 km.…”

“… (a) Structural cross‐section through the northern Agrio FTB (∼37°30S) adapted from Lebinson et al. (2015, 2018, 2020). The colored stars indicate the positions and numbering of the studied samples (b–d) Panoramic photographs of three fault‐related folds from which samples (1, 7, and 11) were obtained (see locations in Figure 3).…”

“…This investigation combines detailed structural analysis with inverse modeling of new lowtemperature multi-thermochronological data (AHe, ZHe, and AFT). Using recent detailed structural studies in this region (e.g., Aramendia et al, 2019;Lebinson et al, 2015Lebinson et al, , 2018Lebinson et al, , 2020Turienzo et al, 2022), we have constructed a regional balanced cross-section that illustrates the structural architecture of the FTB and facilitates the calculation of vertical displacement along major thick-skinned structures. In addition, we collected 13 sandstone samples along a ∼64 km-long E-W trending transect across the FTB (Figure 1c), comprising two from Upper Jurassic sandstones, nine from Lower Cretaceous sandstones, and two from Miocene conglomerates.…”

Mechanical Influence of Inherited Folds in Thrust Development: A Case Study from the Variscan Fold-and-Thrust Belt in SW Sardinia (Italy)

“…In the Sicilian-Maghrebian fold-and-thrust belt, according to [114], a large hinterland verging displacement was allowed by a buttressing related to an irregular shape of the continental margin or to anomalous thick sedimentary sequences. Additionally, in the Southern-Central Andes (e.g., [115,116], the buttressing is produced by the impeding of foreland verging displacement caused by progressive fault-related folding. In the Cantabrian zone [117,118], which for its Variscan age, structural level, and lithology involved is very similar with the study area, the development of fore-and back-thrusting was not influenced by pre-existing folding but mainly by the mechanical anisotropy in the stratigraphy [119].…”