Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW–SE, parallel to the fold axes. The Kmin axes are orientated in the NE–SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300–290 Ma (Sauce Grande to Bonete Formation deposition) and 290–276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.


Geological setting
The Ventana Ranges comprises a fold and thrust belt with sigmoidal shape and a northeast vergence in present geographical coordinates 17,37,38 . They are composed by rocks from Late Precambrian in the west to Permian successions in the east (Fig. 1). The stratigraphic sequence is divided into three main stratigraphic units: Curamalal, Ventana and Pillahuincó groups, which show important differences in metamorphism degree and style of the deformation between them 9 (Fig. 1). The older lithologies belong to the Curamalal and Ventana Groups that are situated on the western sector ( Fig. 1), and show a lower greenschist metamorphism degree 11,27 . The Pillahuincó Group is the youngest group, situated on the eastern sector, and the rocks are at diagenetic grade 17,27,39 . Cenozoic deposits unconformably overlie the mentioned units (Fig. 1).
The Pillahuincó Group 9 outcrops at the east of the Ventana Ranges ( Fig. 1), and is divided in four formations, named from the base to the top: Sauce Grande, Piedra Azul, Bonete and Tunas (Fig. 2). Regional strike of fold axes is NW-SE. At the base of the sequence, folding tends to be cylindrical with shorter wavelength and dipping limbs, while towards the top of the sequence expands and show smoother wavelengths 40 . Cleavage planes trend NW-SE and are nearly vertical, dipping toward the west at the base of the sequence and mostly east on Bonete and Tunas formations (Figs. 2 and 3).
Paleomagnetic studies in the Tunas Formation (top of the Pillahuincó Group) indicate that the magnetizations are syntectonic, with the main grouping of the characteristic remanent magnetization reached at 32% of unfolding at the base of the succession 26 while at the top of the succession is needed a 90% of unfolding to reach it 25 . This behavior evidences a decrease of the tectonic deformation from the base toward the top of the sequence, consistent with the structural field observations ( 40 and other authors cited in) and AMS results 30,31 .
A Permian volcanism event is preserved the sedimentary record of the Tunas Formation 41,42 (Fig. 1). Provenance and age data provided by Alessandretti et al. 43 also indicate the presence of an active pyroclastic activity during Early Permian of Gondwana, which is interpreted as part of distal equivalent of the early episodes of the Choiyoi volcanism, located to the west.
Anisotropy of magnetic susceptibility (AMS) and compaction studies based on petrography in samples of Tunas Formation show a syntectonic deformation, during the Permian, that decreases in intensity toward the foreland located at the east 30,31,32 .
On the Arroyo Atravesado section (Fig. 1B), the Pillahuincó Group is exposed from west to east, base to top, including Sauce Grande, Piedra Azul, Bonete and the transition to the Tunas formation ( Fig. 2A). The differences between the formations are based on subtle changes such us the absence of clast in the Piedra Azul respect Sauce Grande or the presence of white motes in the Bonete Formation respect to Piedra Azul. The section is composed by a group of folds with the fold axes trending northwest-southeast. The general dip values of the bedding planes decrease toward the east, where the youngest strata crop out ( Fig. 2A,B). No evidence of thrusting in the surveyed sequence is visible 38 . The Sauce Grande Formation 9 has a thickness of 1100 m and it  www.nature.com/scientificreports/ is composed of diamictite deposits [44][45][46] , sandstones and a smaller proportion of mudrocks 47 . The palinological content indicates a Pennsylvanian-Cisuralian age 48,49 . The lithic grains are well rounded and have different sizes, between 2 mm and 5 cm, and different composition, as quarzites, granitic rocks, carbonates and mudrocks. The matrix is composed of dark gray siltstone with spaced cleavage. Samples of grains from diamictites and the fine-grain matrix were taken. Above the Sauce Grande Formation, in transitional contact, is the Piedra Azul Formation, with 300 m thickness 9,50 . It is composed of mudrocks, heterolites and minor dark gray fine sandstones, deposited in a marine environment 51,52 . This unit has a continuous cleavage and intense foliation. The Bonete Formation 9 lies conformably above the Piedra Azul Formation with a thickness of 400 m. It is composed of fine green arkosic sandstones, with whitish specks, intercalated with dark gray mudrocks 9 . Remains of plants that belong to the Glossopteris Zone indicate an Early Permian age 53 . The sandstones have inverse and cross-bedding stratification and parallel and cross-ripple lamination.
Tunas Formation 9 which is composed of fine to medium sand grade clastic sedimentites of green, gray, red and yellow colors, with parallel and cross-bedding stratification, and ripple lamination. The sandstones are intercalated with siltstones of red and green colors 52,54,55 , with plants imprints of Glossopteris and Lycopsids, and poor bivalve remains 9,53,56,57 . There are also some tuff levels intercalated in the Tunas Formation sequence 41,58 . Radiometric isotopic dating obtained from tuff outcrops at the upper part of the sequence are 280.8 ± 1.9 Ma 42 and 284 ± 15 Ma 43 , indicating an Early Permian age. In addition, Arzadún et al. 41 attained SHRIMP zircon ages of 291.7 ± 2.9 Ma and 295.5 ± 8.0 Ma (Asselian-Sakmarian) from outcrops at the base of the sequence (Ruta 76 locality in Arzadún et al. 30,31,41 ) and subsurface. Andreis et al. 55   and JS4 were selected ( Fig. 2A). Petrographic analysis shows poor textural and mineralogical maturity, with rounded and sub-rounded grains. Contacts between grains are matrix-supported and the average grain sizes is 500 µm, with maximum of 4 cm and minimum of 100 µm. The grains are mono and polycrystalline quartz, feldspars (microcline and plagioclase), different percent of lithic fragments of metamorphic rocks, mudrocks and minor quantities of volcanic rocks, opaque minerals, carbonate and some mafic minerals. The matrix is silt-size and it is composed of quartz, illite, chlorite, epidote and opaque minerals. Diagenetic illite and epidote are present, superimposed to the grains. The samples belong to the sites JS1 and JS4 classify as lithic diamictites while samples belong to JS3 classify as quarzitic diamictites (Fig. 2C). The samples of Sauce Grande Formation do not have a clear diagenetic foliation plane developed. A spaced foliation orientated 130° N (NW-SE), with a broad development depending on grain size variation of diamictite fabric were observed. Disjunctive roughshaped cleavage domains are represented by phyllosilicates and opaque minerals, spaced between 1 and 30% and gradational transition between them and microlithons 60 (Fig. 3A, Table 1). Locally, scarce microfractures are present (grain breakage) aligned to foliation planes (Fig. 3A). Two samples of Piedra Azul Formation siltstones, located at the sites JP3 and JP6 were selected ( Fig. 2A). Petrographic analysis indicates moderate sorting and moderate mineralogical and textural maturity, with angularshape grains. The contacts between grains are mainly straight to concave-convex and the average size of the grains ranges from 100 to 200 µm, with maximum of 300 µm and minimum of 30 µm. The grains are monocrystalline quartz, potassium feldspar, plagioclase and muscovite. The matrix is composed of quartz and sericite. Diagenetic muscovite and sericite superimposed to the grains and diagenetic epidote were recognized. The samples classify www.nature.com/scientificreports/ as feldspathic siltstones (Fig. 2C). In the sample of Piedra Azul Formation, from site JP3 ( Fig. 2A), a diagenetic foliation related to burial, S 0 , orientated 130° N (NW-SE) is evidenced by oxides, micas and pressure dissolution surfaces. This penetrative plane shows smooth and space cleavage domains, with parallel to anastomosing relationship with microlithons 60 (Fig. 3B). At 120°N (northwest-southeast), there is 50 µm thickness quartz filled microfracture with diffuse limits. Process of pressure dissolution related to foliation planes resulted in sigmoidal geometry of the vein (Fig. 3B), indicating that the vein formed before the foliation. Another weak foliation plane S 1 orientated nearly N-S that it is represented by micas and opaque minerals (Fig. 3C). The sample from site JP6 have a smooth, spaced, parallel to slightly anastomosed foliation, with discrete transition between domains and microlithons, orientated at 120° N. Microlithons are composed by grains of quartz and feldspar that depending of its orientation exhibit flattening, pressure shadows, subgrains and pressure dissolution (Fig. 3D, Table 1). Micas, recrystallized quartz, opaque minerals, and epidote compose the folia. Four samples of the Bonete Formation, located at the sites JB3, JB7, JB8 and JB9 were selected ( Fig. 2A). Petrographic analysis indicates a good selection with sub-rounded grains. The mineralogical and textural maturity is moderate. The contacts between grains are mainly sutured, with some straight to concave-convex contacts. The average size of the grains is 120 µm, with maximum of 200 µm and minimum of 30 µm. The grains are quartz, potassic feldspar, plagioclase and muscovite, with minor lithic fragments. The matrix is composed of quartz, illite, chlorite and montmorillonite and the cement by opaque minerals, carbonate and diagenetic epidote. Diagenetic chlorite and illite are present. Samples classify as feldspathic sandstones (Fig. 2C). In some sectors there are also wackes, with poor sorting, matrix supported grains and similar composition than sandstones. The sample of Bonete Formation, from the site JB3 (Fig. 2), have a primary foliation S 0 orientated at 125° N (NW-SE). It is evidenced by flattening of the quartz and feldspar grains in microlithons, mica and opaque minerals that constitutes cleavage domains (Fig. 4A). Foliation observed is spaced, between rough and smooth, and sub-parallel. Transition between the cleavage domains and microlithons is discrete 60 (Fig. 4A). Pressure shadows and subgrains in quartz grains and feldspar (plagioclase) grains are present (Fig. 4B). There is a second foliation S 1 , 150° N (NW-SE), defined by planes of fluid migration and opaque minerals and micas presence (Fig. 4A, Table 1). The foliation on sample from the site JB7 (S 0 ) is evidenced by grains flattening, mica and opaque minerals orientated 130° N (NE-SW) ( Table 1). Foliation is spaced, between rough and smooth and sub-parallel. Transition between cleavage Table 1. Outcrop data for each formation of the Arroyo Atravesado locality of 103 specimens and its anisotropy of magnetic susceptibility parameters. N AMS : number of data (all data were accepted in the analysis). B. Plane: bedding strike (0°-360°) and dip (90° clockwise, from given strike, 0-90°). Cleavage: direction (0°-360°) and dip (90° clockwise, from given strike, 0°-90°). S 0 and S 1 foliations measured in thin Sects. (0°-360°). K max -K int-K min average orientation of the AMS axes and its confidence ellipses e 1 , e 2 and e 3 61 , K m = (K max + K int + K min )/3: average susceptibility volume in SI units; P j av: average anisotropy degree (P = K max / K min : anisotropy degree according to Nagata 62 ; T: average shape parameter of Jelinek 61 ; L = K max /K int : Average lineation; F = K int /K min : Average foliation (Flinn 63 ). See also figure 2A.  www.nature.com/scientificreports/ domains and microlithons is discrete. Quartz grains present ondulose extinction and subgrains development; feldspars show deformed twins (Fig. 4C). No S 1 was observed in the site JB7. Three samples of Tunas Formation sandstones, located at the sites JT1, JT2 and JT3 ( Fig. 2A; spanning the meters) were selected. Petrographic analysis indicates moderate to good sorting with sub-rounded grains. The mineralogical and textural maturity is moderate. Contacts between grains are straight to sutured, and the average size of the grains range between 250 and 30 µm, with maximum of 900 µm and minimum of 100 µm. The grains are monocrystalline and polycrystalline quartz, plagioclase, potassium feldspar, lithic fragments of volcanic, granitic and clay rocks, muscovite and epidote. The matrix is composed of sericite, quartz and epidote. There are opaque minerals, silica and epidote as diagenetic cement and there is diagenetic sericite and chlorite. The opaque mineral mainly consists of hematite, that Andreis and Cladera 54 give a detrital origin. Samples classify as feldspar-lithic and quartzitic sandstone (Fig. 2C).
The samples of Tunas Formation, from sites JT1, JT2 and JT3, show primary foliation parallel to S 0 45° N (NE-SW) (Fig. 4D). This is evidenced by flattening of the quartz and feldspar grains in microlithons and mica and opaque minerals in cleavage domains. Foliation observed is spaced, between rough and smooth, and subparallel 60 . Transition between the cleavage domains and microlithons is discrete (Fig. 4D). Pressure shadows and subgrains in quartz grains and feldspar (plagioclase) grains with deformation are present (Fig. 4E, Table 1). There is a second foliation S 1 orientated 130° N (NW-SE), defined by planes of fluid migration, micas presence and opaque minerals (Fig. 4D). Vein development locally show dynamic recrystallization process with strong grain boundary migration in original siliceous cement phase (Fig. 4F).
Magnetic mineralogy. Diamagnetic and paramagnetic minerals were microscopically recognized in all samples of the Pillahuincó Group, which includes quartz, feldspar, carbonate and phyllosilicates (muscovite, illite, sericite and chlorite). In the samples of the Tunas Formation, the predominant magnetic mineral is hematite (antiferromagnetic mineral), recognized macroscopically and microscopically as detrital grains, cement, nodules and concretions with a detrital and early diagenetic origin 47,54 . The presence of this mineral it is also confirmed by X-ray diffraction analysis 30,31,64 . The average susceptibility measured in this formation is less than 5 × 10 -5 SI (Table 1; these low values of K mean are mainly consistent with contribution of hematite 65 . In the other formations, the average susceptibility measured is between 1.7 × 10 -4 and 3.2 × 10 -4 SI, due to the presence of magnetite (Table 1).
Tomezzoli 25 , obtained natural remanent magnetization intensities between 0.5 and 90 mA m -1 , in the samples from the Tunas Formation itself, with similar behavior during progressive thermal demagnetization. They were stable during experimental heating, with high magnetic coercivity and unblocking temperatures between 630° and 680 °C, suggesting that the magnetization is carried by hematite. The demagnetization by alternating field was not effective due to the high magnetic coercivity of hematite. Normalized isothermal remanent magnetization (IRM) was performed on five samples: JS102c (Sauce Grande Fm), JP601b (Piedra Azul Fm), JB701b (Bonete Fm), JT206b (Tunas Fm in the Arroyo Atravesado section) and CT633b (Tunas Fm in the San Carlos locality, see Fig. 1) (Fig. 5A). The modeling of coercivity spectra show components with low coercivity on samples JS102c, JP601 and JB701b, and high coercivity on samples JT206b and CT633b (Fig. 5B) 66 . The samples seem to be saturated up to 2 T or less, except those from Tunas Formation (JT206b and CT633b). This behavior is proper of ferromagnetic minerals, probably magnetite, that changes to the Tunas Formation, where the antiferromagnetics minerals (such hematite or non-stoichiometric hematite) begins to manifest. The presence of hematite was also recognized macroscopically and microscopically as detrital grains, cement, nodules and concretions with a detrital and early diagenetic origin 47 , and it was confirmed by X-ray diffraction analysis 30,31,64 . AMS data. All formations belonging to the Pillahuincó Group, that crop out on the El Atravesado section, show consistent AMS data with well-defined confidence ellipses (Figs. 1 and 2; Table 1). The AMS ellipsoids have maximum axes (K max ) in northwest-southeast position; this orientation is parallel to the axes of the folds (Fig. 6A,B; Table 1). Sauce Grande Formation presents oblate ellipsoids, with minimum axes (K min ) grouped in the first quadrant, almost horizontal, suggesting a flattening of the fabric with tectonic control. Moving stratigraphically upwards into Piedra Azul, Bonete and also the base of the Tunas Formation (three sites: JT1, JT2 and JT3 transitional to the Tunas itself described in Arzadun et al. 2016), the ellipsoids tend to change to prolate shapes with a persistent K max in northwest-southeast position, while the K min axes grouped in the first and third quadrant, tends to move toward the vertical (center of the stereographic network), showing a transition to a dominant sedimentary fabric (Fig. 6). However, a few meters above the base of the Tunas Formation, in the Ruta 76 and San Carlos localities (Fig. 1B), the K min axes, oriented NE-SW, lies again close to the horizontal with oblate ellipsoids, suggesting an overlap of tectonic fabric over the sedimentary fabric 30,31 (Fig. 7). Moving inside the Tunas Formation itself, upwards stratigraphically, in the localities situated to the east, the K min axes moved gradually toward the vertical again, with prolate ellipsoid shapes at first and then with oblate shapes, suggesting again a transitional fabric from tectonic to sedimentary to the top of the sequence 30,31,32 . The dominantly and more clear sedimentary control is in the Gonzales Chaves locality, situated at the Claromecó Basin center (Fig. 1A), where the K min are grouped in the vertical position, perpendicular to bedding planes (Figs. 7 and 8) 30,31,67 .
The anisotropy degree (P j ) shows a general decrease towards the younger formations and toward the east, with average values from 5% in Sauce Grande Formation to 3% in Tunas Formation. The Piedra Azul Formation has a higher degree of anisotropy than the Sauce Grande Formation, with maximum values of 8%, despite being above in the sequence, probably as a consequence of the finer-grained lithologies presumably more sensitive to the deformation [66][67][68][69][70] (Fig. 6C; Table 1). Toward the base of the Tunas Formation itself (Ruta 76, San Carlos and Golpe de Agua localities; Arzadún et al. 30,31 ; Fig. 7 (Figs. 6 and 7). The shape parameter (T) shows average values larger than zero (T > 0) in the Sauce Grande Formation, indicating oblate shapes of tectonic origin (K min in the horizontal, Fig. 6) that changed to T average values minor than zero (T < 0), toward Piedra Azul and Bonete formations indicating prolate shapes that suggest a transition to sedimentary origin (K min moving to the vertical; Fig. 6 and Table 1). In the base of Tunas Formation itself 30,31 , T values change again to oblate shapes (T > 0) of tectonic origin (K min in the horizontal), that moves to prolate and oblate shapes (transitional to sedimentary fabrics) to the top of the sequence (Fig. 7) 30,31 . Some changes in the www.nature.com/scientificreports/ AMS patters were observe in the subsurface 32 confirming that the deformation degree was gradually attenuated upwards in the sequence, to the younger strata and toward the foreland Claromecó basin (Fig. 1).

Discussion
Along and across the Pillahuincó Group (Upper Paleozoic in the south west of the Gondwana margin) there are evident changes in the outcrops features, micro-tectonic characteristics, types of magnetizations, magnetic mineralogy and AMS signature, from oblate (tectonic) to prolate ellipsoids (transition to sedimentary fabric), towards the top of the sequence located to the eastern and center of the basin 30,31,32 . These changes are also evident in the values of the shape parameter (T), anisotropy degree (P j ), foliation (F) and lineation (L) depending on location in the stratigraphic sequence and related with the shortening. Even when those parameters are sensitive to mineralogical changes and tectonic strain ( [68][69][70][71] , between others), should be noted that along the Pillahuincó Group the changes are correlated with the stratigraphic position, independently of the lithology since the lithological differences between the formations are subtle. In the localities situated westward, at the base of the sequence, with major tectonic deformation, the ellipsoids tend to have oblate shapes, changing upwards and eastern positions, to-prolate shapes, and toward the base of the Tunas Formation they have again oblate shapes (Figs. 6, 8A). Previous studies show a similar pattern along the Tunas Formation itself sequence (Fig. 7) 30,31 . In the outcrops, at the base (see Ruta 76 and San Carlos localities in 30,31 ), the ellipsoids tend to have oblate-prolate shapes (maximum effort σ 1 = K min in the horizontal showing tectonic fabric) and upwards they tend to have prolate to oblate shapes (maximum effort σ 1 = K min in the vertical showing sedimentary fabric). In subsurface, in the foreland Claromecó Basin (Fig. 1), with almost horizontal beds, the spatial distribution of the ellipsoid axis and AMS parameters tend to exhibit equivalent changes from prolate to oblate shapes ellipsoids Febbo et al. 32 (Figs. 7, 8A). These results and those obtained by Arzadún et al. 30,31 show a clear pattern that is similar to the theoretical models of Saint-Bezar et al. 34 , Parés and van der Pluijm 35 and Weil and Yonkee 36 for weakly to strongly deformed sedimentary rocks in fold and thrust belts. In these models, there are also changes of the AMS response, from oblate shapes in the more tectonically deformed zones to prolate-triaxial and then to  (Fig. 1), from de base to the top: AMS ellipsoids with structural correction, setting the bedding planes in the horizontal position, and ratio between the degree of anisotropy (P j ) and the shape parameter (T), with the average values in red 30,31 , data processed with the Anisoft 4.2 software).  (Fig. 8B,C). In all the formations of the Pillahuincó Group the K max axes trend northwest-southeast, parallel to the fold axes and to the primary foliation S 0 , clusters parallel to the intersection of the LPS fabric with bedding, and tend to be constant in all places (Figs. 6, 7). The structure correction in all formations shows a persistence of the K max axis orientation, suggesting a tectonic origin of its behavior with a maximum compressive stress (σ 1 ) perpendicular to this axis (Figs. 6, 7). The orientation of the poles of the weak secondary foliation S 1 coincide with the K min , indicating that is related to the shortening. At the base of the sedimentary log, in the westernmost and most deformed localities, the K min axes are almost horizontal, trending southwest-northeast, perpendicular or scatter away from the bedding poles, showing a transition to a tectonic fabric with a maximum compressive stress (σ 1 ) in the southwest-northeast direction (Fig. 6), indicating moderate LPS. In contrast, towards the easternmost localities, to the top of the stratigraphic sequence (Bonete Formation and base of Tunas Formation), the K min axes tends to be oriented vertically, showing a transition to a sedimentary fabric and indicating minor LPS (Fig. 6). The microtectonic reveals a primary foliation S 0 orientated northwest-southeast. The orientation of the S 0 is coincident with the bedding plane measured in the field, so it is considered as a primary foliation. The secondary foliation S 1 is penetrative in some samples of the Sauce Grande and Piedra Azul formations, while in the Bonete and Tunas formations is smooth and more spaced, indicating less deformation toward the youngest units. The poles of the S 1 foliation are coincident with the K min axes of AMS, related with the maximum effort.
Moving towards the Tunas Formation itself, in localities of the base 30,31 , the K min are in a horizontal position again, parallel to the maximum shortening direction with oblate to prolate ellipsoids shape, while at the top of  30,31 . The arrow below indicates the direction of the increase of the shortening, as the deformation decrease at the top of the sequence. SD, in red: spasmodic deformation (peaks of greater intensity of deformation). (C) Comparison with conceptual models [34][35][36] . There are changes in the shape of the ellipsoids, from triaxial to prolate and oblate indicating a migration from tectonic to sedimentary fabric in all the cases. www.nature.com/scientificreports/ this formation the K min tends to be near vertical positions with prolate to oblate shape (Figs. 7, 8). In the easternmost locality, in Gonzales Chaves 30,31 and in the subsurface 32 , situated in the center of the Claromecó Basin ( Fig. 1), the K min is vertical (Fig. 7). According to Parés 72 and Weil and Yonkee 36 , this is caused by overburden synchronously with the deformation during the deposition of the sediments (stages D to C of Weil and Yonkee 36 model Fig. 8C). This is consistent with the decrease of the deformation degree towards the eastern localities.
The changes of the AMS parameters in the sequence of the Arroyo Atravesado locality are in concordance with the characteristics observed microscopically. The foliation in samples of the Sauce Grande and Piedra Azul formations is penetrative, and in Bonete and Tunas formations the foliation is smooth and more spaced (Figs. 3,  4). In addition, these differences are agreed with the different geometry of the folds that is clearly visible in the outcrops along the Pillahuincó Group (Fig. 2). There are also two different types of magnetizations obtained previously in the Tunas Formation 25,26 , from which two different paleomagnetic pole (PPs) positions were calculated: Tunas I PP with 291 Ma (U/Pb ages in 41 , and Tunas II PP with 281 Ma (U/Pb age in López Gamundi et al. 42 and Arzadún et al. 41 ) (Fig. 8).
The differences in the ages of the rocks, the AMS pattern, the presence of syn-tectonic magnetizations, different percentages of unfolding and the changes in the type of foliation demonstrates that the tectonic shortening diminishes towards the top and during a relatively short period, towards the eastern foreland between the Early and Late Permian. Despite this, the K max axes remain with constant orientation in the geographic coordinates in all the localities, that means that σ 1 remains constant from the southwest, at least during that period of time (Figs. 6, 7).
Similar differences in the AMS patterns were obtained from nearby areas as the Carapacha Basin 33 (location in the Fig. 1) and in the Sierra Chica locality, belonging to the Choiyoi magmatic Province 73 (location in Fig. 1), which ages are closer to Tunas Formation (260.8 ± 3.2 Ma to 269.0 ± 3.2 Ma) (Fig. 9). These localities based on the AMS and paleomagnetic results, also show clear tectonic features at the base of the succession, which are attenuated to the top.
According to Tomezzoli 23,25,40,74 , the deformation on the southwestern Gondwana continent margin began during the Late Devonian 77 , and is related with the collision of microplates as Chilenia from the west and Patagonia from the southwest-CHI-PA microplate. These collisions give place to the Chañic (Acadic) orogenic phase ocurred during the upper Devonian 78 . The associated deformation continued until the Late Paleozoic, and is related to the post-collisional San Rafael (Hercinic or Gondwanic) an-orogenic phase in the Late Carboniferous to Middle Permian 78 . The Permian deformation is the consequence of translations movements of the tectonic plates to equatorial positions 76 , which re-organized and adjusted all the plates previously accreted to Gondwana (southern plates) and to Laurentia (northern plates) during the Permian, to configure Pangea during the Triassic 79 . This translation and deformation are reflected in the cusp observed in the apparent polar wander path of South America (Fig. 9) 74 and Gondwana 75 during the upper Paleozoic. During the Pillahuinco Group deposition, the geological evidences as macroscopic features of the outcrops, changes in the paleocurrent directions, environmental continentalization, change in the vergence direction to the southwest instead of the northeast, AMS patterns, paleomagnetic poles, minelalogical and microscopic texture chances of the rocks, among other aspects, accompany the Upper Paleozoic palaeogeographical reorganizations of Gondwana (Fig. 9), with two different threshold in the evolution of the basin. The first one between the Sauce Grande and Bonete Formations (nearly 300-290 Ma), and the second one insight the Tunas Formation (290-276 Ma), keeping constant the migration of the orogenic front migration towards the foreland basin during the Carboniferous-Permian.

Conclusions
As indicated by the AMS ellipsoids, AMS parameters and micro-tectonic analyses, the intensity of the deformation decreases inside the Pillahuincó Group from the Sauce Grande to Bonete and the base of the Tunas Formations (this work), and increases again in the Tunas Formation itself 30,31 .
The AMS results in the Pillahuincó Group (Fig. 8) show K max axes in NW-SE positions, parallel to the axes of the folds and a subhorizontal K min in the base of the sequence, oriented SW-NE indicating a tectonic fabric compatible with the SW-NE regional shortening. To the top of the sequence, the K min move perpendicular to bedding planes indicating the transition to sedimentary fabric. It is interesting to note that above the analyzed column, at the base of the Tunas Formation itself 30,31 , a reactivation of the deformation is found and the K min axis grouped again in the horizontal, indicating a new cycle of greater orogenic activity in the basin during the lower Permian, but always maintaining the SW-NE shortening direction. The K min axes chance from horizontal to vertical from the base to the top of the sequence accompanied with a decrease of the anisotropy degree, indicating an attenuation of the deformation to the younger strata, located at the east of the basin, The provided results represent a further evidence of a migration of the orogenic front towards the foreland basin, acting spasmodically in cycles of higher and lower intensity.
The reactivation of the deformation at the base of Tunas Formation coincides with a latitudinal displacement of Gondwana (from the South) and Laurentia (from the North) continents towards the Equator, between the Lower (nearly 300-290 Ma) and the Upper Permian (290-276 Ma) [75][76][77] . This is clearly reflected in the cusps that present the apparent polar wander path of South America (Fig. 8) for those times.

Methods
In order to determine the magnitude and the directions of the deformation stress two different techniques were used: petrographic analysis and anisotropy of magnetic susceptibility (AMS; Tarling and Hrouda 65 ). The petrography deals with the interpretation of small-scale features in rocks that yield abundant information on the history and type of deformation 60 . It is possible by this method to describe and measure different characteristics as cleavage and lineation in some minerals, lattice-preferred orientations, deformation mechanisms and www.nature.com/scientificreports/ kinematic indicators 60,80 . Moreover, the anisotropy of magnetic susceptibility (AMS) is an effective technique used to measure the primary or tectonic petrofabric of the rocks 81 . The method is based on measuring the intensity of magnetization and the direction of magnetic minerals in the rock 82 . Some selected samples were analyzed by petrography to make microtectonic determinations, using thin polished sections with a Nikon eclipse 50i POL microscope. AMS data were measured in 103 samples, previously sampled for paleomagnetic studies 25,26 . They come from 22 sites occupying different stratigraphic positions of the Pillahuincó Group on the Arroyo Atravesado locality (Fig. 1B, Table 1), 4 sites belonging to the Sauce Grande Formation (19 samples), 6 sites belonging to the Piedra Azul Formation (29 samples), 9 sites belonging to the Bonete Formation (43 samples) and 3 sites belonging to the Tunas Formation (15 samples). In these samples, the principal axes of the AMS ellipsoids (K max , K min and K int ), the shape parameters (T) 61 and the degree of anisotropy (P) were determined ( Fig. 2 and Table 1). This procedure was carried out with a Kappabridge MFK-1A Figure 9. Apparent polar wander path of Gondwana for the Late Paleozoic proposed by Tomezzoli 74 and Gallo et al. 75 with the respective ASM patterns (with bedding correction). The abrupt changes in the trajectory of the curve is accompanied by events of greater deformation, evidenced by geological changes in the outcrops, micro-tectonic characteristics, types of magnetizations and different AMS signatures. The shortening direction indicates that the maximum stress along this part of the Gondwana come from the southwest and stay stable during the Lower Carboniferous up to the Permian. These paleogeographic changes associated with that deformation are registered in the cusp that presents the apparent polar wander path of Gondwana during that period 76 . The deformation propagated diachronous eastwards to the foreland, displaying signs first of a decrease and a subsequent pick up of intensity to decays again, suggesting cycles of higher and lower deformation intensity. The first one between the Sauce Grande to Bonete Formations (nearly 300-290 Ma) and the second one insight Tunas Formation (290-276 Ma). The map was made in Gmap (http:// www. earth dynam ics. org/ earth histo ry/ gmap), and edited in Inkscape (https:// inksc ape. org/ es/). www.nature.com/scientificreports/ equipment. The obtained data were analyzed by Anisoft 5.1.08. The isothermal remanent magnetization (IRM) was induced using an ASC Model IM-10-30 Impulse Magnetizer successively with a of 3 T coil. The IRM was measured using an AGICO JR-6A Dual Speed Spinner Magnetometer. Modeling of coercivity spectra was performed using a fitting program 66 .

Data availability
The data and materials can be requested to the authors, all data are available in its database.