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.