Tectonic Insight in the Southwest Gondwana Boundary Based on Anisotropy of Magnetic Susceptibility

The anisotropy of magnetic susceptibility (AMS) is an effective tool to measure the rock petrofabric and it allow analyzing the tectonic stress. The southwest boundary of Gondwana in South America is the counter-part of the Cape fold belt of South Africa and its geological evolution is still a subject of debate. Samples of different localities of this sector were analyzed with the AMS technique, from Buenos Aires, La Pampa and Mendoza province. For rocks of Permian age, there is a clear regional magnetic signature indicating a NW-SE elongation direction and a NE-SW shortening. The ASM patterns obtained in the oldest rocks are complex, probably as the result of stress interference in the magnitudes, space and time with different pulses of the orogenic activity developed from the Middle Devonian to the Permian. In the southwest of Gondwana, small continental plates were accreted to the main continent mass during the Middle Devonian. The Permian deformation has been interpreted as the consequence of a paleogeographic re-organization of Gondwana that moves to lowest latitudes to makes the Pangea continent during the Triassic. This younger deformation evidences an orogenic front migration and attenuation to the foreland basin.


Introduction
The anisotropy of magnetic susceptibility (AMS) is an effective tool to measure the rock petrofabric, both of primary and secondary origin with the potential to identify the orientations of the principal axes of finite strain to which the rocks were subjected [1,2].The base of the method is the measure of the orientation and intensity of the magnetic susceptibility (K) of a rock sample whose intensity is the sum of the diamagnetic, paramagnetic, antiferromagnetic and ferromagnetic responses of constituent minerals [3].The diamagnetic response is typically very weak, and opposes the applied field; this response is typically contributed by iron-free silicates such as quartz and feldspar.The paramagnetic contribution is carried by Fe-bearing silicates [4], which in magmatic rocks includes the phyllosilicates (biotite, chlorite and Fe-muscovite), Fe-Ti oxides, amphiboles, pyroxene, cordierite, garnet and tourmaline.The antiferromagnetic contribution, typically provided by hematite and goethite, is negligible in practice.The ferromagnetic contribution is commonly a major component and it is associated with magnetite and occasionally with sulfides such as pyrrhotite [3,5].
In deformed rocks, strain may control the orientation of ferromagnetic minerals.The AMS response in such rocks typically reflects the preferred crystallographic orientation of ironbearing silicates and the shape and distribution of magnetite or hematite grains.
To determine the AMS, the response of a rock when it is affected by a weak magnetic field is measured, related to the acquired magnetization degree with the applied magnetic field, evidenced in the formula K = M/H, where K is the magnetic susceptibility, M is the acquired magnetization and H is the magnetic field applied.
The magnetic behavior of the rocks in a weak field of constant magnitude depends of some factors such as the preferential crystallographic orientation, the shape of the mineral fabric, the composition, the distribution and size of the mineral grains and the microfractures [6].Therefore, a substance is isotropy, when the induced magnetization in a symmetric specimen, has the same intensity independent to the direction in which the field is applied.Instead, in the magnetic anisotropic rocks, the induced magnetization depends of the orientation of the sample in the magnetic field.
AMS measurement can be expressed by a second-rank tensor where eigenvectors/eigenvalues are geometrically represented as an ellipsoid (Figure 1a), with three perpendicular axis: K max , K int and K min , (K 1 , K 2 and K 3 ).The highest magnetic intensity is induced throughout the K max major axis and the weakest intensity is induced according to the K min minor axis; therefore, in an isotropic substance, the three axis are equal and the ellipsoid is in fact a sphere (Figure 1a).The shapes and orientations of these ellipsoids can be related to magmatic flow in igneous rocks and kinematics of deformation in metamorphic rocks [7].Commonly, the flattening of the AMS ellipsoid (plane perpendicular to K min ) is parallel to the foliation in the rock, and the elongation of the ellipsoid (K max ) is parallel to the fabric lineation.
The parameters to consider are the anisotropy degree (P), the ellipsoid shape (T) and the spatial orientation of the main axis.Their most important properties are: • The susceptibility ellipsoid tends to be coaxial with the total deformation ellipsoid with a correspondence to each principal axis (Figure 1b).
• The susceptibility ellipsoid tends to be coaxial with the petrofabric; the K min axis is perpendicular to the foliation, to the bedding plane in sedimentary rocks, plane poles of magmatic foliation or foliation poles or cleavage in rocks deformed in solid stage.
• If the magnetic fabric is only depositional origin, the angle between the K min and the pole of the bedding plane is small.
• K max is parallel to the lineation that could be tectonic origin, the direction of a magmatic flow or could be dominated by the paleocurrents direction.
• K min is denominated by "magnetic foliation pole" and K max "magnetic lineation".
• The ellipsoid shape is directly related with the rock fabric.In some rock types, there is a quantitative relation between L (lineation) and F (foliation), or any parameter that involve the relative length of the susceptibility axes and the intensity of the linear or planar orientations, respectively.
• In the case of the deformation in solid stage, there is a direct relation between the AMS and the deformation.A quantitative application of the AMS in this cases, it is only possible when a calibration is carry out with the stress and the original anisotropy of the rock, in other way, a qualitative ratio is only possible, as the rocks with major anisotropy are the more deformed.
• The ASM measurements are not affected by the natural or artificial remnant magnetizations.
• The simpler and quick way of visualize the obtained data about the directions of the main susceptibility axes is through a stereographic net.The most common is to use the inferior hemisphere and to plot the data of K max , K int and K min with different symbols, so it is  possible to distinguish the ellipsoid shape and to easily compare with structural data: cleavage, fractures, etc., and with the origin of the fabric: sedimentary, tectonic or sedimentary/ tectonic composite (Figure 1c, d).On the other hand, the shape parameter (T) represents the ellipsoid shape: if the ellipsoid has oblate shape, then T > 0 and if it is prolate then T < 0 [7].
The AMS technique was applied in different localities along the southwest boundary of Gondwana (Figure 2), in diverse lithologies that ranges from Cambrian to Permian-Triassic ages, to study the kinematic history of this area and to distinguish multiple tectonic events that account for the fabrics of the outcropping rocks.

Geological setting
The southwest boundary of Gondwana in South America is the counter-part of the Cape fold belt of South Africa [8].It is extended from Sierras Australes-Claromecó Basin in the Buenos Aires province to the San Rafael Block in the Mendoza province (Figure 2).This fold and thrust belt [9], known as Gondwanides [8], was subject to deformation during the Paleozoic.Ramos (1984) propose [10] an ocean closed by subduction toward the northern boundary of Patagonia at the southern margin of Gondwana, and generated a collision during the late Paleozoic.However, the geological evolution of this region is still a subject of debate and doubts remain about the origin of Patagonia and the age of the main deformation and if it is related or not with this collision.Now there are new proposals that include collisional and intra-continental deformation mechanisms (Ref.[11]).According to some researchers, the deformation occurs in one phase during the late Permian-Triassic, instead other authors propose that it began in the late Devonian-early Carboniferous and continued up to the Permian [12].
With the aim of regionally analyze the magnitude of the deformation regionally, the stress directions that acted in the different geological moments and the time-space relations between the different localities of the Gondwanides, AMS studies were making along its margin.Rocks of different lithologies and ages ranging from Cambrian to Permian-Triassic were studied in different localities along this belt (Figure 2), from the Sierras Australes-Claromecó Basin, Chadileuvú Block (Carapacha Basin, Los Viejos Hill and Sierra Chica) and San Rafael Block (Agua Escondida).

Sierras Australes-Claromecó Basin, Buenos Aires province
The Sierras Australes are at the southwest of the Buenos Aires province and they represent the outcropping part of the Claromecó Basin (Figure 2).They are a fold and thrust belt [9] with a general northwest-southeast strike and a northwest vergence.The outcropping rocks are Pre-Cambrian to Permian, with the oldest units at the west and the most modern at the east.
The Cerro Colorado granite represents the basement of the Sierras Australes, and it is situated at 40 km to the west.It has a penetrative cleavage and a gneiss-milonitic structure [13].
The sedimentary rocks of the Sierras Australes is classified into three main orographic units: Curamalal, Ventana and Pillahuincó groups that have an important difference in the metamorphism degree and in the style of the deformation [21].In the quartzites of the Curamalal, Bravard and Ventana groups, situated in the western sector (base of the sequence), there is a lower greenschist metamorphism [22,23].While in the Pillahuincó group, situated in the eastern sector (top of the sequence), there is a medium to high diagenesis degree [22,24,25].Cenozoic deposits cover in discordance these units.
The west sector presents more deformed strata in the Sierras de Curamalal, Bravard and Ventana [21], while in the eastern sector, in the Sierras de las Tunas and Pillahuincó outcrops the Pillahuincó group, with a characteristic more open folding.Here the regional strike of the axes of the folds is northwest-southeast.However, there are visible differences between the base and the top inside the Pillahuincó Group.At the base, the folds tend to be cylindrical with shorter wavelength and more defined flanks, while toward the top of the sequence they tend to expand and smooth their wavelengths [26].The cleavage planes are nearly vertical, dipping toward the west at the bottom of the sequence and mostly east on the Bonete and Tunas formations (Figure 3).
The analyzed lithologies correspond to Cerro Colorado, López Lecube, to the Lolén Formation [21], and the Pillahuincó Group [21] in the Sierras Australes area, and a little outcrop that correspond to the Tunas Formation situated at the Claromecó Basin, at the east of Sierras Australes.
The Lolén Formation, of Devonian age [27], is at the top of the Ventana Group.It has micaceous sandstones, phyllites and shales with lenticular beds of fine conglomerate, with a strong cleavage in northwest-southeast direction.
The Pillhuincó Group is composed from base to top by the Sauce Grande, Piedra Azul, Bonete and Tunas formations.The carboniferous Sauce Grande Formation has diamictites and sandstones.The Piedra Azul Formation has mudrocks.The Bonete Formation, of Lower Permian age, has fine sandstones with white spots, interbedded with gray mudrocks.The Tunas Formation, of middle Permian age, has fine to medium sandstones interbedded with green and red mudrocks.There are some small outcrops of the Tunas Formation in the Claromecó Basin, close to the González Chávez locality.The Pillahuincó Group has the Carboniferous-Permian Glossopteris flora [28], and the Tunas Formation has zircon shrimp data of 291-280 Ma [29][30][31].
Paleomagnetic studies in the Tunas Formation indicate that the magnetizations are syntectonic, with shortening values of 32% at the base (to the west) and 90% at the top (to the east).This evidences a decrease in the deformation toward the top of the sequence and is consistent with the structural field observations [26].Furthermore, anisotropy of magnetic susceptibility and compaction studies on Tunas Formation also show a decrease of the deformation toward the foreland [32].[11]).Notice that the western sector presents more deformed strata than the eastern sector, see shortening values.
The tecto-sedimentary [33] and paleomagnetic evidences [26], indicate that the deformation in the Tunas Formation occurs at the same time that the sedimentary deposition and it was related with the San Rafael orogenic phase defined by Azcuy and Caminos [34].[35,36].
In addition, there are sedimentary rocks outcrops belonging to the Carapacha Basin.
Los Viejos Hill is located at the southeast of La Pampa province (Figure 2).It belongs to the igneous-metamorphic basement considered by Linares et al.Linares et al. [37] as the southward prolongation of the Sierras Pampeanas geological province.It is part of a ductile deformation zone in low metamorphic degree with northeast vergence [38].There are different deformation degrees, from little foliate granitic gneiss to mylonitic gneiss, with ages from 466.4 to 261 Ma, obtained by K-Ar and Rb-Sr dating in biotite and muscovite [35].
Sierra Chica is located at the center of La Pampa province (Figure 2).It is a volcanic rock outcrop belonging to the Choiyoi magmatic province.The Choiyoi Group in La Pampa is located in a tectonically stable environment adjacent to an active continental margin [39].
According to Quenardelle and Llambías [40], the sequence is composed of different units.The lowermost unit, at the north, consists of trachyandesitic pyroclastic flows of unknown thickness and extent.The other ones are rhyolitic units, divided into the lower unit, composed of well-bedded, thin pyroclastic units interbedded with thin fall deposit units, and the upper unit is composed of coarse bedded, thick pyroclastic layers with rheomorphic structures [40].The Sierra Chica sequence is consistent with an eruption in an extensional tectonic regime immediately subsequent to a subduction-related compressional regime [40].Rapela et al. [41] obtained an Rb-Sr whole-rock isochron age of 240 AE 2 Ma and Domeier et al. [42] obtained U-Pb ages of 263 AE 1.6 Ma.
The Carapacha Basin is a continental half-graben, located at the southern of La Pampa province, central Argentina (Figure 2; [43]).The basin filling is up to 630 m thick and it is entirely composed of clastic deposits of the Carapacha Formation, of Permian age.Red and gray arkosic or lithic sandstones, mudstones and scarce conglomerates compose the Carapacha Formation.It is divided into two members: the lower Calencó Member and the upper Urre-Lauquen Member [43].The formation has yielded a typical Permian Glossopteris macroflora [44,45].The rocks of the upper Carapacha Formation along Río Curacó are gently folded, strike-slip, normal and reverse faults and extensional veins are also present.The structure and weak deformation of the upper Carapacha Formation was interpreted as reflecting left-lateral strike-slip deformation under a transpressive regime that it is associated with cessation of sedimentation in the basin [46].The upper part of the formation is intruded by an andesite assigned to El Centinela Formation, maybe associated with the Permian-Triassic volcanic rocks of the Choiyoi Magmatic Province.

San Rafael Block, Mendoza province
The Agua Escondida area is located at the southeast of the Mendoza province and it is situated at the south of the San Rafael Block (Figure 2).In this sector, the Piedras de Afilar Formation outcrops that is a pre-carboniferous granitic basement of 418.2 AE 3.1 Ma [47].It is covered by carboniferous, siliciclastic sediments of the Agua Escondida Formation [48] and is intruded by the igneous rocks of the Permian Choiyoi magmatic Group.The Agua Escondida Formation [48] is composed by siliciclastic sediments of carboniferous-permian age due their flora remains deposited over the basement.In this sector, the Choiyoi magmatic Group is mainly composed by a metasilicic lower section with dacitic intrusive, andesitic lavas and tuffs.The silicic upper section has lavas, tuffs, ignimbrites and breccias of rhyolitic composition and granites that intrude the Agua Escondida Formation and the pre-carboniferous basement [49,50].The completely Paleozoic rocks are cover by plio-pleistocene basalts and recent sediments.

Methodology
Each sampled site consists at least of 4-5 hand samples or 6-7 cores obtained by portable drilling and orientated by magnetic (Brunton) and sun compass (Azimut 0-360 and dip 0-90 ).
Kappabridge MFK-1A and KLY-2 (by Geofyzika Brno) were utilized to measure the AMS directions, at the Paleomagnetism Laboratory "Daniel A. Valencio" (IGEBA) of the Geologic Sciences Department of the Buenos Aires University (UBA) and at Colgate University, respectively.Previously to measure, it is necessary to ingress the field sample orientation because the AMS direction results are in specimen coordinates (without field correction) and in geographic coordinate (with field correction).The results were analyzed with the program Anisoft 4.2 (provided by Geofyzika Brno) to obtain directional results of AMS scalar axes represented in the ellipsoids and their statistic parameters, in situ and with structure correction, and the AMS degree (P) and the shape parameter (T) values.It is possible to make automatically the structure correction with the program, taking the bedding planes to the horizontal.The T and P parameter values were plotted in the diagram of Jelinek [51].
The AMS data obtained, were integrated with paleomagnetic and field structural data of each locality, to improve the interpretations.

Results of AMS in the southwest Gondwana boundary
Here the results of ASM from different localities along the Southwest Gondwana margin, from the Sierras Australes-Claromecó Basin to the San Rafael Block, are presented.Rocks of different lithologies and ages ranging from Late Devonian to Permian-Triassic were studied.Each sampled locality has its own magnetic signature, but it is possible to identify a pattern related to the age.

Sierras Australes-Claromecó Basin
In this area were studied the Cerro Colorado granite, the López Lecube sienite, the Lolén Formation and the Pillahuincó Group, and a little outcrop in the González Chávez locality of possible upper Permain-Triassic age [18,32,52].
In all the analyzed localities, the AMS data show good internal consistence in each sample site and between them (Figure 4), and is almost possible to correlate the structural characteristics with the AMS patterns.
In López Lecube (Upper Permian), the magnetization is stable in all specimens with a reverse polarity [18], characteristic of the Kiaman superchron.In some specimens, the anisotropy of magnetic susceptibility was measured (Figure 4).They present prolate magnetic fabric, with the K max axis parallel to the magmatic mineral lineation (Figure 4).This AMS spatial distribution axes were related with the magmatic conditions during the emplacement of the magma.However, the AMS fabric from Cerro Colorado (Cambrian) is oblate, with a tectonic signature instead of magmatic, in the distribution of the AMS axes with a northwest-southwest direction of the shortening (K min in the northeast; Figure 4).
The specimens of the Lolén Formation (Devonian) show the K max and K int axes contained in the cleavage plane and aligned with the strike of the structure in the northwest-southeast direction.The K max tends to lie close to the horizontal.The K min axes are always in the northeast direction close to the horizontal and parallel to the pole of the cleavages planes.The anisotropy degree (Pj) is low, minor than 10% and the susceptibility is minor than 2.52E À4 (SI), so the paramagnetic minerals control the magnetic fabric [7].From the disposition of the AMS axes, its ratio with the cleavage planes and the ratio between the Pj and T parameters is deduced that the fabric is typically oblate of tectonic origin [53].
In the Pillahuincó Group, the Sauce Grande Formation presents oblate ellipsoids (T < 0), with K min grouped in the first quadrant, almost horizontal, suggesting a flattening of the fabric with tectonic control.The Piedra Azul and Bonete formations show ellipsoids with prolate shapes (T > 0) and K min axes in the first and third quadrant grouped toward the center of the stereographic net.The Tunas Formation has oblate ellipsoids with K min axes tending to the vertical through the horizontal, suggesting a transition between tectonic to sedimentary fabric (Figure 4; [52]).The trend toward a fabric with a dominantly sedimentary control is clearly seen in the Gonzalez Chavez locality located at the Claromecó Basin (Figure 4).
The degree of anisotropy (P) shows a general decrease toward the younger formations and toward the east, with maximum values ranging from 11% in the Lolén Formation to 4.4% in Tunas Formation (Figure 5).
Figure 5. Ratio between the shape parameter (T) and the anisotropy degree (Pj) of the different localities, when T < 0 the ellipsoid is prolate and when T > 0 is oblate.

Chadileuvú Block, La Pampa province
In Los Viejos Hill, an AMS systematic sampling was made in a grid perpendicular to the main structures of the body [54].Unexpectedly, it was found that the K min poles deviate from the poles of the main foliation S 1 by about 25 (Figure 4).Similarly, but not so noticeably, the K max poles trend northeast/southwest, with a near-horizontal plunge, whereas the principal lineation L 1 plunges gently southwest.This unusual situation is attributed to the existence of superimposed fabrics arising from S and C structures.The distribution of the paramagnetic and ferromagnetic minerals in "S-C" plane structures interferes in the expected anisotropy pattern, indicating the presence of a cryptic foliation.From the petrographic point of view [38] and the AMS studies, it is possible to infer that the zone was affected by different deformation episodes, evidencing by the presence of a secondary foliation S 2 subordinate to the S 1 .
In all localities, the AMS signatures are predominantly triaxial, with well-defined axial groups and relatively small uncertainty ovals about means.Most of the sites exhibit oblate tendencies, and only a few are prolate (Figure 5).The T versus Pj diagram reveals an anisotropy degree mainly below 3%, with an ellipsoid more oblate than prolate (Figure 5).
A systematic sampling was made in localities situated at the southeast and south of the Carapacha Basin, to make paleomagnetic and AMS studies [55].The paleopolar positions of both Carapacha Formation members are different [55].According the AMS studies, the tectonic signature of the two members is different.The Calencó Member presents minor deformation than the Curacó Member (Figure 4).In the Curacó River samples, the K max is more variable, not related with the folding axes except locally, and so it is probable that the primary sedimentary fabrics are more preserved; instead of in the Curacó River locality the K max poles seem to be tectonically controlled because they have the same direction than the folding axes and the K min are bimodal distributed as a typically prolate ellipsoid with a main maximum stress with a southwest-northeast dominant direction (Figure 4).The different paleomagnetic positions calculated in both members are concordant with the lithological, structural, biostratigraphic and AMS data differences.The AMS axial ratios of Carapacha are mainly characteristic of triaxial ellipsoids.The Pj versus T diagram reveal an anisotropy degree mainly below 4%, with ellipsoids more oblate than prolate (Figure 5).
The isolated magnetization of the Sierra Chica locality is syntectonic [56].There are two different magnetic signatures: one in the base units and other one in the top units (Figure 4).These differences were interpreted as a tectonic discordance between both.The K min is situated in a gird with south-southwest-north-northeast direction from horizontal (base) to vertical positions (top) (Figure 4), evidencing that the maximum stress direction is southwestnortheast.The anisotropy degree is Sierra Chica is low, minor than 11% and the magnetic susceptibility is low, minor than 2.5E À4 (SI), indicating that it is carried by paramagnetic minerals.The Pj versus T parameters indicate a that predominate the oblate fabrics (Figure 5).

San Rafael block
The AMS pattern in the Agua Escondida area is complex.In the Piedras de Afilar Granite, of devonian age, the K min axes have a north-northwest direction while the K max are in east-west direction, indicating a secondary fabric and evidencing a deformation with a north-south compression direction.Nevertheless, in the La Menta Granite (388.4AE 3.1 Ma, [57]), that is part of the Piedras de Afilar Formation, the K min has a west-southwest direction and the K max a north-south direction, showing a secondary fabric and evidencing a compression direction from the west.
In the Permian Cavado Granite, the main AMS axes seem no follow a common spatial pattern; the magnetic fabrics seem to be primary and related to local effects of the emplacement conditions.The dikes that intrude the Cavado granite have a triaxial fabric, with the K max in the third quadrant and close to the vertical and subparallel to the diaclases plane.The K min tend to be in the northeast and the K int in the southeast.From the statistic parameter analyses, the fabric is oblate.The anisotropy degree is 4% and the medium susceptibility is minor than 1E À4 (SI) ( [58]; Figure 5).
The sedimentary rocks from the Agua Escondida Formation (Upper Carboniferous) have K max orientated to the northeast and contained in the bedding planes with northwest-southeast strike (Figure 4; [58]), while K min is parallel to the pole of this plane.This arrangement of the axes does not respond to a "pure sedimentary fabric" (Figure 4), but on the contrary it would indicate interference of signatures.
The fabrics of the dikes and ignimbrites of Choiyoi are similar to the Piedras de Afilar and La Menta granites; they are secondary and produced by a deformation stress with east-west and north-south compression directions, following complex patterns of previous stress in the area.
The plio-pleistocene basalts have K max -orientated northwest and close to the horizontal, indicating a primary fabric and a probable fluidity of the mineral components in a southwestnorthwest direction.The anisotropy degree is minor than 13% in all the analyzed sites, and the fabric is in general oblate, although in each individual site there are oblate, prolate and triaxial shapes, depending of the lithology (Figure 5; [58]).
A rhyolitic dike of the Choiyoi Group has a K max in northeast-southwest direction and contained in the diaclase planes.The K min and K int are in the second and fourth quadrant like a wreath.The fabric is prolate, the anisotropy degree is minor than 5% and the medium susceptibility is 43.5 E À4 (SI) (Figure 5; [58]).

Comparison with fold and thrust belt models
There are conceptual models of the AMS patterns proposed by Saint-Bezar et al. [59], Parés and Van Der Pluijm [6] and Weil and Yonkee [60], where weakly to strongly deformed sedimentary rocks in fold and thrust belts changed their AMS response (Figure 6).The AMS ellipsoids that have oblate shapes in the more tectonically deformed zones change to prolatetriaxial shapes and then to oblate shapes [6,59,60] (Figure 6).These changes indicate composite sedimentary/tectonic fabrics with layer parallel shortening (LPS).A secondary LPS [60] control the lineation (K max ), which indicates the maximum elongation direction.
The AMS patterns of the Sierra de la Ventana fold and thrust belt follow this model [32,52].In this area, the ratio between T and Pj parameters change with location in the stratigraphic succession and with shortening.In the western localities, at the base of the stratigraphic succession, with major tectonic deformation and syn-tectonic magnetizations (32% of unfolding), the ellipsoids tend to have prolate-oblate shapes.Toward the top of the sequence, with minor deformation and syn-tectonic magnetization (90% of unfolding), ellipsoids tend to have triaxial and prolate shapes.
In the foreland basin, in the Claromecó Basin, with bed-parallel sedimentary fabric, the ellipsoids tend to have triaxial-oblate shapes (Figure 5).
All along the south margin of Gondwana, in the carboniferous-permian localities, the K max axes trend northwest-southeast, parallel to the fold axes, clusters parallel to the intersection of the LPS fabric with bedding and tend to be constant in all sampled localities (Figure 4).The foliation (K min ) indicates that the shortening is related to the primary bedding.At the base of the sedimentary column, at the western most and also more 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 (σ1) parallel to that direction, indicating moderate LPS.In contrast, toward the foreland, to the eastern most localities, to the top of the stratigraphic succession, the K min axes tend to orient vertically, showing a transition to a sedimentary fabric and indicating minor LPS (Figures 4, 6).

Conclusions
Different rock types with different ages, from Devonian to Permian, were studied by AMS methodology, along the southwest margin of Gondwana, from the Sierras Australes-Claromecó Basin in the Buenos Aires province to Agua Escondida in the Mendoza province.Each sampled locality has its own magnetic signature according to their lithological and ages intrinsic characteristics.Nevertheless, in most samples there is a gradual tectonic development that overprinted differences of AMS patterns into and between the studied localities.In the Permian age rocks, as Pillahuincó Group, Carapacha Basin and Sierra Chica, there is a clear regional magnetic imprint that indicate northwest-southeast elongation directions (K max ) and therefore a northeast-southwest compression (K min ).This Permian deformation is linked to the San Rafael orogenic phase.There is also a movement of the K min from the horizontal, in the western sites at the base in the respective sequences, to the vertical in the eastern sites situated at the respective tops, and a change of the respective shape parameter.Thus, indicate a transition from a magnetic fabric with a clear tectonic imprint to transitional to sedimentary fabrics, similar to the conceptual models proposed by several authors in thrust and fold belts ( [60], between others).The younger localities as Gonzales Chavez and Lopez Lecube, practically have no signature of deformation.These are evidencing pulses in the deformation intensity that diminish toward the east and an advance of the orogenic front toward the foreland basin, with the main stress from the southwest.This deformation coincides with an abrupt curvature in the apparent polar wander of Gondwana in the Upper Paleozoic ( [61,62]; Figure 7).
Nevertheless, in the patterns of the old rocks (Cerro de los Viejos, Cerro Colorado, Lolén Formation and Agua Escondida), the signature is no constant and the main stresses could be placed from the southwest, from the west or from the south.They have complex patterns that are related to the overlapping of different orogenic phases or to local lithological control of each locality.
In the Cavado granite and its dikes, of the Choiyoi Group, the fabrics are triaxial without axes situation common pattern, so the emplacement of these bodies seem to correspond to local effects.In the case of other dikes and the Choiyoi ignimbrite, the fabrics are similar, but they do not seem to have a clear relation with the diaclase planes.Probably the emplacement of these bodies corresponds to complex stress patterns previously installed in the region.All these data indicate that the deformation in the area came from the southwest and it attenuate during the Middle Permian, evidencing an orogenic front migration to the foreland basin at the northeast, with deformation re-activation during the Permian, indicating tectonic activity in Gondwana.Tomezzoli [12] interpreted that the Permian deformation, with a main stress from the southwest, is the consequence of a paleogeographic re-organization of Gondwana that moves to lowest latitudes to makes the Pangea continent during the Triassic.In the southwest of Gondwana, small continental plates were accreted to the main center of the continent during the Medium Devonian (Figure 7).This devonian deformation known as Cháñica orogenic phase in Argentina [34] have been related by Tomezzoli [12] with the Chilenia and Patagonia collision with Gondwana from the west or southwest respectively.Based on that, Tomezzoli [12] proposed the possibility that Chilenia and Patagonia where the same allochthonous plate.
The AMS data allowed interpreting, by the analysis of different localities, the regional tectonic of the southwest margin of Gondwana.It is clear that, in the cases where there is an important tectonic imprint, the technique of the AMS is very useful for the interpretation of the efforts magnitude and direction, and their variability.

Figure 1 .
Figure 1.(a) Magnetic susceptibility ellipsoid, with three orthogonal axes that correspond to the maximum, intermediate and minimum [7].The space orientation is defined in a coordinate system (x,y,z).(b) Ratio between the anisotropy, deformation and stress ellipsoids.K1: Maximum anisotropy axis; K3: Minimum anisotropy axis; τ1: Maximum deformation axis; τ3: Minimum deformation axis; σ1: Maximum stress axis; σ3: Minimum stress axis.(c) Directional data plotted in a stereographic net of triaxial, prolate and oblate ellipsoid [7].(d) Stereographic net models of sedimentary fabric, sedimentary/tectonic composite fabric and tectonic fabric.

Figure 2 .
Figure 2. Location of the different localities and basin studied in the margin of Gondwana by the AMS technique.The Gondwanides fold and thrust belt is in purple, sierras Australes is in green, Claromecó Basin in yellow, Carapacha Basin in orange, San Rafael block in pink, and the Choiyoi magmatic province in light blue.

Figure 3 .
Figure 3. Structural cross-section of the sierras Australes: (a) general and, (b) detailed (taken from[11]).Notice that the western sector presents more deformed strata than the eastern sector, see shortening values.

Figure 4 .
Figure 4. AMS ellipsoids results of the different sectors of the southwest margin of Gondwana, with the K max , K int and K min axes and N the number of specimens.The stereographic nets are in geographic coordinate system and equal-area projection.

Figure 6 .
Figure 6.Conceptual model for evolution of AMS fabrics in weakly to strongly deformed sedimentary rocks in foldthrust belts [6, 59, 60].Idealized changes in AMS ellipsoid directions (relative to horizontal bedding) from a dominant bed-parallel sedimentary fabric (stage A), to mixed layer-parallel shortening (LPS) and sedimentary fabrics (weak LPS in stage B and moderate LPS in stage C), to strong tectonic fabric (stage D, with K max either parallel to structural trend, D1, or down the dip of cleavage, D2).Changes in ellipsoid shape parameter (T).

Figure 7 .
Figure 7. Apparent polar wander path of the south west Gondwana proposed by Tomezzoli[62] constructed from the paleomagnetic poles (PPs) selected from South America between the carboniferous and Triassic.Botton, plate accretion during Permian times.
Tectonic Insight in the Southwest Gondwana Boundary Based on Anisotropy of Magnetic Susceptibility http://dx.doi.org/10.5772/intechopen.72825 .2. Chadileuvú Block, La Pampa province Several localities that belong to the Chadileuvú Block were measured with the AMS technique: Los Viejos Hill, Sierra Chica and the Carapacha Formation.The igneous-metamorphic basement of the area includes Upper Cambrian to Lower Devonian metamorphic rocks (Las Piedras Metamorphic Complex, Paso del Bote Formation, El Carancho Igneous Complex and La Horqueta Formation), granitoids (Pichi Mahuida Group) and Late Paleozoic granite orthogneisses (Los Viejos Hill Complex) that outcrop in southeastern La Pampa province