Tectonic insight based on anisotropy of magnetic susceptibility and compaction studies in the Sierras Australes thrust and fold belt (southwest Gondwana boundary, Argentina)

Abstract

The Sierras Australes fold and thrust belt (Buenos Aires Province, Argentina) was in the southwestern Gondwanaland margin during the Paleozoic. The Tunas Formation (Permian) is exposed along the eastern part of it and continues eastward beneath the Claromecó Basin. Anisotropy of magnetic susceptibility (AMS) and compaction studies are described and compared with previous paleomagnetic studies with the aim of determining direction and magnitude of the main stresses acting during the sedimentation of the Tunas Formation. The anisotropy ellipsoids are triaxial with oblate or prolate shapes, reflecting different stages of layer parallel shortening during the evolution of the basin. Kmax axes trend NW‐SE, parallel to the fold axes, while Kmin move from a horizontal (base) to a vertical orientation at the top of the succession, showing a change from a tectonic to almost a sedimentary fabric. The magnitude of anisotropy and compaction degree decreases toward the top of the succession. The AMS results are consistent with the outcrop structural observations and the compaction and paleomagnetic data. Regional pattern indicates a compression from the SW along this part of Gondwana, with a migration of the orogenic front and attenuation toward the NE in the foreland basin during the Upper Paleozoic. This deformation, locally assigned to the San Rafael noncollisional orogenic phase, is the result of the latitudinal movements toward the Equator of Gondwana (southern plates) and Laurentia (northern plates) during the Permian. This movement is the result of a rearrangement of the microplates that collided with Gondwana during the Late Devonian, to configure Pangea during the Triassic.

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

“…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]. 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].…”

“…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 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].…”

A pure dipole analysis of the Gondwana apparent polar wander path: Paleogeographic implications in the evolution of Pangea

“…The exclusive reversed polarity magnetization does not extend beyond the Kiaman upper limit (269 Ma). Differences in the type and age of the magnetization between the lower and upper Tunas Formation, and hence the two different paleomagnetic pole positions, are in accordance with the geological and AMS differences between the base and the top of the succession [ Arzadún et al ., ]. The same applies to San Roberto and Curacó PPs from the Permian Carapacha Basin [ Tomezzoli et al ., , Table 1].…”

Shrimp zircon geochronology constrains on Permian pyroclastic levels, Claromecó Basin, South West margin of Gondwana, Argentina

“…Ramos (1984), Von Gosen et al (1991, and Cristallini (1998, 2004) consider the Sierras Australes as a thrust and fold belt. The vergence of the system is northeastern, decreasing the intensity of the deformation from west to east (Harrington, 1947;Tomezzoli, 2001;Arzadún et al, 2016) (Figure 2).…”