4.2. Chadileuvú Block, La Pampa province

direction. The Kmax tends to lie close to the horizontal. The Kmin 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<sup>4</sup> (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

In the Pillahuincó Group, the Sauce Grande Formation presents oblate ellipsoids (T < 0), with Kmin 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 Kmin axes in the first and third quadrant grouped toward the center of the stereographic net. The Tunas Formation has oblate ellipsoids with Kmin 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

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

Figure 5. Ratio between the shape parameter (T) and the anisotropy degree (Pj) of the different localities, when T < 0 the

seen in the Gonzalez Chavez locality located at the Claromecó Basin (Figure 4).

deduced that the fabric is typically oblate of tectonic origin [53].

Tunas Formation (Figure 5).

22 Tectonics - Problems of Regional Settings

ellipsoid is prolate and when T > 0 is oblate.

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 Kmin poles deviate from the poles of the main foliation S1 by about 25 (Figure 4). Similarly, but not so noticeably, the Kmax poles trend northeast/southwest, with a near-horizontal plunge, whereas the principal lineation L1 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 S2 subordinate to the S1.

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 Kmax 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 Kmax poles seem to be tectonically controlled because they have the same direction than the folding axes and the Kmin 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 Kmin 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<sup>4</sup> (SI), indicating that it is carried by paramagnetic minerals. The Pj versus T parameters indicate a that predominate the oblate fabrics (Figure 5).

#### 4.3. San Rafael block

The AMS pattern in the Agua Escondida area is complex. In the Piedras de Afilar Granite, of devonian age, the Kmin axes have a north-northwest direction while the Kmax 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 (388.4 3.1 Ma, [57]), that is part of the Piedras de Afilar Formation, the Kmin has a west-southwest direction and the Kmax 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 Kmax in the third quadrant and close to the vertical and subparallel to the diaclases plane. The Kmin tend to be in the northeast and the Kint 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<sup>4</sup> (SI) ([58]; Figure 5).

The sedimentary rocks from the Agua Escondida Formation (Upper Carboniferous) have Kmax orientated to the northeast and contained in the bedding planes with northwest-southeast strike (Figure 4; [58]), while Kmin 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.

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

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 Kmax either parallel to structural trend, D1,

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All along the south margin of Gondwana, in the carboniferous-permian localities, the Kmax 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 (Kmin) 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 Kmin 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 Kmin axes tend to orient vertically, showing a transition to a sedimentary fabric and indicating minor LPS (Figures 4, 6).

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 (Kmax) and therefore a northeast-southwest compression (Kmin). This Permian deformation is linked to the

tend to have triaxial-oblate shapes (Figure 5).

or down the dip of cleavage, D2). Changes in ellipsoid shape parameter (T).

5. Conclusions

The plio-pleistocene basalts have Kmax-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 Kmax in northeast-southwest direction and contained in the diaclase planes. The Kmin and Kint 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<sup>4</sup> (SI) (Figure 5; [58]).

## 4.4. 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 (Kmax), 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

Tectonic Insight in the Southwest Gondwana Boundary Based on Anisotropy of Magnetic Susceptibility http://dx.doi.org/10.5772/intechopen.72825 25

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 Kmax either parallel to structural trend, D1, or down the dip of cleavage, D2). Changes in ellipsoid shape parameter (T).

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 Kmax 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 (Kmin) 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 Kmin 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 Kmin axes tend to orient vertically, showing a transition to a sedimentary fabric and indicating minor LPS (Figures 4, 6).

#### 5. Conclusions

compression direction. Nevertheless, in the La Menta Granite (388.4 3.1 Ma, [57]), that is part of the Piedras de Afilar Formation, the Kmin has a west-southwest direction and the Kmax a north-south direction, showing a secondary fabric and evidencing a compression direction

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 Kmax in the third quadrant and close to the vertical and subparallel to the diaclases plane. The Kmin tend to be in the northeast and the Kint in the southeast. From the statistic parameter analyses, the fabric is oblate. The anisotropy degree is 4% and the medium susceptibility is minor than

The sedimentary rocks from the Agua Escondida Formation (Upper Carboniferous) have Kmax orientated to the northeast and contained in the bedding planes with northwest-southeast strike (Figure 4; [58]), while Kmin 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

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 Kmax-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

A rhyolitic dike of the Choiyoi Group has a Kmax in northeast-southwest direction and contained in the diaclase planes. The Kmin and Kint are in the second and fourth quadrant like a wreath. The fabric is prolate, the anisotropy degree is minor than 5% and the medium

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]

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

control the lineation (Kmax), which indicates the maximum elongation direction.

from the west.

(SI) ([58]; Figure 5).

24 Tectonics - Problems of Regional Settings

indicate interference of signatures.

susceptibility is 43.5 E<sup>4</sup>

shapes, depending of the lithology (Figure 5; [58]).

4.4. Comparison with fold and thrust belt models

(SI) (Figure 5; [58]).

1E<sup>4</sup>

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 (Kmax) and therefore a northeast-southwest compression (Kmin). This Permian deformation is linked to the San Rafael orogenic phase. There is also a movement of the Kmin 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).

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] pro-

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27

posed the possibility that Chilenia and Patagonia where the same allochthonous plate.

magnitude and direction, and their variability.

Arzadún Guadalupe1,2\*, Tomezzoli Renata Nela2,3, Tickyj Hugo4

\*Address all correspondence to: guadalupe.arzadun@gmail.com

1 Laboratorio de Termocronología (LaTe Andes), Vaqueros, Salta, Argentina

Modelado Geológico (LAMOGE), Universidad de Buenos Aires, Argentina

2 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina 3 Departamento de Geología, Facultad de Ciencias Exactas y Naturales, Instituto de

4 Departamento de Geología, FCEyN, Universidad Nacional de La Pampa, Argentina 5 Departamento de Geología, Facultad de Ciencias Exactas y Naturales, Laboratorio de

Geociencias Básicas y Aplicadas de Buenos Aires (IGEBA), Laboratorio de Paleomagnetismo

[1] Borradaile GJ. Magnetic susceptibility, petrofrabrics and strain. Tectonophysics. 1988;156:1-20

[2] Graham JW. Magnetic susceptibility, an unexploited element of petrofabric. Geological

[3] Gleizes G, Nédélec A, Bouchez JL, Autran A, Rochette P. Magnetic susceptibility of the mount Louis-Andorra ilmenite type granite (Pyrenees): A new tool for the petrographic

Cristallini Ernesto Osvaldo1,2,5 and Gallo Leandro Cesar<sup>2</sup>

D.A. Valencio, Universidad de Buenos Aires, Argentina

Society of American Bulletin. 1954;65:1257-1258

Author details

References

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

,

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.

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.

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.
