2.2. Chadileuvú Block, La Pampa province

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

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

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

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

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 defor-

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.

formations (Figure 3).

18 Tectonics - Problems of Regional Settings

cleavage in northwest-southeast direction.

mation toward the foreland [32].

Australes.

[29–31].

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 [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 2 Ma and Domeier et al. [42] obtained U-Pb ages of 263 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.

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.

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

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

21

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

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

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

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.

4.1. Sierras Australes–Claromecó Basin

with the AMS patterns.

possible upper Permain-Triassic age [18, 32, 52].

of the shortening (Kmin in the northeast; Figure 4).

#### 2.3. 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 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.
