2.1. Sierras Australes-Claromecó Basin, Buenos Aires province

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

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

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.

belt [9], known as Gondwanides [8], was subject to deformation during the Paleozoic.

account for the fabrics of the outcropping rocks.

16 Tectonics - Problems of Regional Settings

2. Geological setting

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]. Different authors calculated age data: Rb/Sr 427-392 Ma [14], Rb/Sr 487 15 [15], Rb/Sr 381 9 Ma [13], U/Pb 531 4 Ma [16] and U/Pb 523 4 Ma [17]. The López Lecube sienite is situated at 80 km at the west of Sierras Australes, and correspond to another intrusive body with apparently no deformation [18]. It was interpreted as post-tectonic origin because of its age data: U/Pb de 258 2 Ma [19] and 251.5 3.0 Ma [20], related with the Gondwanic magmatic cycle of La Pampa and Mendoza provinces (Figure 2).

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 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

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

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

19

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].

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

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

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

related with the San Rafael orogenic phase defined by Azcuy and Caminos [34].

In addition, there are sedimentary rocks outcrops belonging to the Carapacha Basin.

466.4 to 261 Ma, obtained by K-Ar and Rb-Sr dating in biotite and muscovite [35].

2.2. Chadileuvú Block, La Pampa province

U-Pb ages of 263 1.6 Ma.

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 [29–31].

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].

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.

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].
