4.1. Sierras Australes–Claromecó Basin

assigned to El Centinela Formation, maybe associated with the Permian-Triassic volcanic rocks

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 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.

Each sampled site consists at least of 4–5 hand samples or 6–7 cores obtained by portable drilling

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

The AMS data obtained, were integrated with paleomagnetic and field structural data of each

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

and orientated by magnetic (Brunton) and sun compass (Azimut 0–360 and dip 0–90).

P parameter values were plotted in the diagram of Jelinek [51].

4. Results of AMS in the southwest Gondwana boundary

locality, to improve the interpretations.

of the Choiyoi Magmatic Province.

20 Tectonics - Problems of Regional Settings

3. Methodology

2.3. San Rafael Block, Mendoza province

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 Kmax 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 (Kmin in the northeast; Figure 4).

The specimens of the Lolén Formation (Devonian) show the Kmax and Kint axes contained in the cleavage plane and aligned with the strike of the structure in the northwest-southeast

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

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

4.2. 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 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

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

http://dx.doi.org/10.5772/intechopen.72825

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episodes, evidencing by the presence of a secondary foliation S2 subordinate to the S1.

mainly below 3%, with an ellipsoid more oblate than prolate (Figure 5).

4%, with ellipsoids more oblate than prolate (Figure 5).

susceptibility is low, minor than 2.5E<sup>4</sup>

4.3. San Rafael block

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

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

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

minerals. The Pj versus T parameters indicate a that predominate the oblate fabrics (Figure 5).

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

(SI), indicating that it is carried by paramagnetic

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 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.
