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Defining the structural style of fold–thrust belts and understanding the controlling factors are necessary steps towards prediction of their long-term and short-term dynamics, including seismic hazard, and to assess their potential in terms of hydrocarbon exploration. While the thin-skinned structural style has long been a fashionable view for outer parts of orogens worldwide, a wealth of new geological and geophysical studies has pointed out that a description in terms of thick-skinned deformation is, in many cases, more appropriate. This paper aims at providing a review of what we know about basement-involved shortening in foreland fold–thrust belts on the basis of the examination of selected Cenozoic orogens. After describing how structural interpretations of fold–thrust belts have evolved through time, this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics, and emphasizes the key control exerted by lithosphere rheology, including structural and thermal inheritance, and local/regional boundary conditions on the occurrence of thick-skinned tectonics in the outer parts of orogens.

This paper focuses on one orogenic belt that formed during the Rinconada phase on the final stage of the Famatinian orogeny, between 445 and 410 Ma, which is well exposed at Sierra de Ramaditas and neighbouring ranges in western Argentina. The Ramaditas Complex is formed by metasedimentary and meta-ultrabasic rocks and amphibolites. This complex forms the upper nappe of a thrust stack resulting from westward thrusting. Deformation consists of an early high-temperature S1 foliation (stromatic migmatites), coeval with thrusting and metamorphism. Metamorphism attained peak P-T conditions of 6.0-6.9 kbar and 795-810 °C, at c. 440 Ma, i.e. coincident with the Rinconada orogenic phase. The lower unit and intermediate nappes crop out in the nearby sierras of Maz and Espinal and underwent low- to medium-grade Silurian metamorphism, respectively, together with the upper nappe, defining an inverted Barrovian-type metamorphism with T decreasing and P increasing downwards across the thrust stack (i.e. westward). We argue that the Rinconada orogenic phase developed near the continental margin of SW Gondwana, during a magmatic lull following accretion of the Precordillera terrane to the continental margin at c. 470 Ma. The active margin jumped to the west after accretion, and flat-slab subduction resumed in the early Silurian, provoking thrusting and imbrication of nappe stack under the still hot root (800-900 °C) of the older Famatinian magmatic arc. This 'hot-iron' process explains both the inverted Barrovian-type metamorphism and the missing overburden of 21 to 24 km implied by the P estimate.

Chromite from Los Congos and Los Guanacos in the Eastern Pampean Ranges of Cordoba (Argentinian Central Andes) shows homogenous and exsolution textures. The composition of the exsolved phases in chromite approaches the end-members of spinel (MgAl2O4; Spl) and magnetite (Fe2+Fe23+O4; Mag) that define the corners of the spinel prism at relatively constant Cr3+/R3+ ratio (where R3+ is Cr+Al+Fe3+). The exsolution of these phases from the original chromite is estimated to have accounted at ≥600 °C on the basis of the major element compositions of chromite with homogenous and exsolution textures that are in equilibrium with forsterite-rich olivine (Fo95). The relatively large size of the exsolved phases in chromite (up to ca. 200 µm) provided, for the first time, the ability to conduct in situ analysis with laser ablation-inductively coupled plasma-mass spectrometry for a suite of minor and trace elements to constrain their crystal-crystal partition coefficient between the spinel-rich and magnetite-rich phases (DiSpl/Mag). Minor and trace elements listed in increasing order of compatibility with the spinel-rich phase are Ti, Sc, Ni, V, Ge, Mn, Cu, Sn, Co, Ga, and Zn. DiSpl/Mag values span more than an order of magnitude, from DTiSpl/Mag = 0.30 ± 0.06 to DZnSpl/Mag = 5.48 ± 0.63. Our results are in remarkable agreement with data available for exsolutions of spinel-rich and magnetite-rich phases in other chromite from nature, despite their different Cr3+/R3+ ratio. The estimated crystal-crystal partitioning coefficients reflect the effect that crystal-chemistry of the exsolved phases from chromite imposes on all investigated elements, excepting Cu and Sc (and only slightly for Mn). The observed preferential partitioning of Ti and Sc into the magnetite-rich phase is consistent with high-temperature chromite/melt experiments and suggests a significant dependence on Fe3+ substitution in the spinel structure. A compositional effect of major elements on Ga, Co, and Zn is observed in the exsolved phases from chromite but not in the experiments; this might be due to crystal-chemistry differences along the MgFe-1-Al2Fe-23+ exchange vector, which is poorly covered experimentally. This inference is supported by the strong covariance of Ga, Co, and Zn observed only in chromite from layered intrusions where this exchange vector is important. A systematic increase of Zn and Co coupled with a net decrease in Ga during hydrous metamorphism of chromitite bodies cannot be explained exclusively by compositional changes of major elements in the chromite (which are enriched in the magnetite component). The most likely explanation is that the contents of minor and trace elements in chromite from metamorphosed chromitites are controlled by interactions with metamorphic fluids involved in the formation of chlorite.

The Pampean Orogen in the Eastern Sierras Pampeanas contains two paired magmatic belts, an eastern calc-alkaline magmatic belt and a western peraluminous granite/high-grade metasedimentary belt. The relationship between the two belts and their relative timing are constrained through new U-Pb zircon ages on granodiorites, monzogranites, and associated volcanic rocks from Sierra Norte and the easternmost Sierras de Cordoba. These ages indicate that calc-alkaline arc magmatism was active over at least a 30-m.yr. period from 555 to 525 Ma, terminating at the same time that peraluminous magmatism and associated high-grade metamorphism began in the adjoining metasedimentary belt (525-515 Ma). These temporal relationships and the metamorphic characteristics of the two belts appear to be in conflict with previously proposed models for the Pampean Orogeny as a continental-collision event, but they are consistent with models that propose eastward-dipping subduction of oceanic crust initiated at ca. 555 Ma, followed by ridge-trench collision at ca. 525 Ma. Similar-aged belts of arc-related and peraluminous magmatism occur elsewhere along the paleo-Pacific margin of Gondwana, suggesting that similar processes of subduction and noncollisional peraluminous magmatism occurred along much of the Gondwana margin in late Neoproterozoic to Cambrian time.

The Sierras Pampeanas consist of basement uplifts that are mainly controlled by W-verging crustal thrusts. One of these, the Sierra Pie de Palo, is controlled by a blind thrust whose kinematics is debated. We analysed the reverse faults located along the eastern side of the range and show that these are characterized by small displacements (few hundreds of metres) of the Neogene and Quaternary deposits controlled by the inherited basement metamorphic foliation and shear zones. At the sierra scale, the inherited foliation pattern appears already folded, before the Cenozoic shortening, forming a series of antiforms and synforms with various wavelengths. We propose that this structure is amplified during the Plio-Quaternary shortening, resulting in a large asymmetrical basement-cored anticline where foliation-parallel slip can act as folding mechanism. The overall kinematics is consistent with an E-verging fault-propagation fold; this proposed kinematics is an alternative to the frequently suggested W-verging fault-bend fold model. Finally, we propose a new, alternative and speculative crustal and lithospheric structure for this region consisting of two E-verging basement thrusts coeval to W-verging Pampean thrusts, both rooting below the Precordillera, above an eclogitized lower crust.

The Sierras Pampeanas of Córdoba are the easternmost uplifted blocks caused by Andean foreland deformation, over 700 km from the Chile trench. This deformation started at c. 340 Ma through basement faults, thrusts and reactivation of normal faults of the Cretaceous rift during the opening of the Atlantic Ocean. Other older faults, major oblique lineaments, were also reactivated. Thermochronological and geothermobarometric data indicate that some topographic relief could have been Palaeozoic–Mesozoic relicts and not only produced by the Andean orogeny. Faults are partially controlled by the early Cambrian S2 metamorphic foliation, coincident with the curved fault traces at map scale. During Pliocene time, two deformation phases post-dating Miocene–Pliocene magmatism are recognized. Shallow seismicity data (c. 25 km depth) indicate that the Sierras de Córdoba accommodate Quaternary displacement. Magnetotelluric studies detect the interface between the Pampia terrane and the Río de la Plata craton. The role of the oblique lineaments in the nucleation and development of the Tertiary faulting has been little considered; they could be correlated with an old pan-Gondwanan trend. During the Cretaceous period these lineaments worked in a transtensive way, producing the uplift of high-grade rocks and segmentation of the mountain chain favouring the diachronous uplift along the ranges. Recently, both the brittle–ductile transition at c. 23 km depth and the crustal thickness have been determined by seismicity analysis. The oblique lineaments displace normally the Mohorovicic discontinuity. Main basement thrusts were probably rooted in the suture between the Pampia terrane and the Río de la Plata craton.

In the southern Central Andes, the Andean foreland was deformed due to Neogene shallowing of the Nazca slab beneath the South America plate. In this 27–33ºS Pampean flat-slab segment, the N-trending Argentine Precordillera transpressional fold-and-thrust belt and the Sierras Pampeanas broken foreland developed as a consequence of inward migration of the orogenic front. At 28ºS, a NNE-trending westward-dipping, thick Neogene synorogenic sequence is exposed in the Sierra de los Colorados, which shares deformation features of the Precordillera and the Sierras Pampeanas. Integration of new structural and kinematic data and available structural, kinematic, geophysical and palaeomagnetic information allows consideration of the Sierra de los Colorados area as part of the northern sector of the Precordillera during the middle Neogene. At c. 9 Ma, basement block exhumation started with the uplift of the Sierra de Umango-Espinal that was triggered by deformation along the NE-trending Tucumán oblique belt. This stage marked the beginning of compartmentalization of the incipiently deformed Vinchina foreland. Since c. 6.8–6.1 Ma, basement block uplift linked to the Miranda–Chepes and Valle Fértil NNW-trending sinistral transpressional belts, as well as kinking of the Neogene sequence by localized WNW-striking cross-strike structures, resulted in multiple segmentation that produced a complex mosaic of basement-block pieces. The overprint of these regional, basement-involved, oblique, brittle–ductile transpressional and cross-strike megazones could be related to high interplate coupling. Localized mechanical and rheological changes introduced by magmatism favoured this thick-skinned deformation overprint.

The Sierra de Pie de Palo, in the Argentinean Sierras Pampeanas (Andean foreland), consists of a Mesoproterozoic basement and an Ediacaran - upper Cambrian sedimentary cover that underwent folding, thrusting and metamorphism during the Ordovician Famatinian orogeny. Mafic rocks and granitoids of the easternmost Sierra de Pie de Palo provide information about the magmatic activity at the proto-Andean margin of Gondwana during late Cambrian - Early Ordovician time. Magmatic activity began in the Sierra de Pie de Palo as dykes, sills and small intrusions of tholeiitic gabbros between 490 and 470 Ma, before shortening and regional metamorphism. Variable mantle sources (Nd depleted mantle age, TDM between 1.7 and 1.3 Ga) were involved in the mafic magmatism. Nd-isotope signatures were probably inherited from a Mesoproterozoic subcontinental mantle. Mafic magmatism was coincident with collapse of a Cambrian carbonate-siliciclastic platform that extended along SW Gondwana, and was probably coeval with the beginning of subduction. After mafic magmatism, peraluminous granitoids were emplaced in the Sierra de Pie de Palo along ductile shear zones during a contractional tectonic phase, coeval with moderate to high P/T metamorphism, and with the Cordilleran-type magmatic arc that resulted from a flare-up at c. 470 Ma. Granitoids resulted mainly from partial melting of metasedimentary rocks, although some hybridization with juvenile magmas and/or rocks cannot be ruled out. The evidence shown here further implies that the Pie de Palo block was part of the continental upper plate during the Famatinian subduction, and not an exotic block that collided with the Gondwana margin.

The Early Palaeozoic stratigraphy and tectonic history of the Eastern Sierras Pampeanas of central Argentina are complicated by metamorphism and deformation resulting from the Pampean (545-510 Ma) and Famatinian (490-440 Ma) orogenies. We report U-Pb sensitive high-resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at Quebrada de La Cebila (c. 28°45'S, 66°25'W): the Ambato and the La Cebila metamorphic complexes. The Ambato zircons record age peaks corresponding to Pampean (530±10 Ma), Brasiliano (c. 570 and c. 640 Ma), Grenville (c. 950 to c. 1025 Ma) and minor Neoarchaean ages. Similar peaks are also apparent in the La Cebila sample but it additionally contains Palaeoproterozoic zircons (c. 2.1 Ga) corresponding to the age of the Rio de la Plata craton, from which they are considered to have been sourced. Our interpretation is that the protolith of the Ambato complex was deposited prior to juxtaposition with the craton and is older than the Early Ordovician La Cebila metamorphic complex. We infer that the craton reached its current relative position in the Mid- to Late Cambrian, after the main Pampean tectonothermal event (530-520 Ma) and before deposition of the La Cebila protolith and the Achavil Formation (Sierra de Famatina), which contain comparable detrital zircon populations.

The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U-Pb ion microprobe analysis of detrital zircons and Sm-Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. TDM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic (c. 1.8-1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Rio de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa-Antofalla massif.