María Silvia Japas

Palaeomagnetic data from the Precordillera fold and thrust belt constraining Neogene foreland evolution of the Pampean flat-slab segment (Central Andes, Argentina)

Abstract Available and new palaeomagnetic data reveal transpressional deformation in the Argentine Precordillera fold and thrust belt contemporaneous with Juan Fernández ridge subduction. Localized changes in the orientation of palaeomagnetic directions indicate a vertical axis rotation pattern linked to local, oblique brittle-ductile shear zones that overprint the regional structure. The nearly homogeneous clockwise block rotation pattern from Western-Central Precordillera shows localized rotation nulls along NNW-trending left-lateral oblique belts, revealing that overprinting anticlockwise tectonic rotations could have balanced previous clockwise rotations. Conversely, clockwise rotations in Eastern Precordillera are only localized in the vicinity of these NNW-trending structures, and are controlled by rigid block rotations linked to basement-involved deformation.These results combined with available regional palaeomagnetic data in the forearc would indicate a regional bending of the upper plate margin between 27° S and 33° S that seems to be related to the subduction of the Juan Fernández aseismic ridge.

Basement-involved deformation overprinting thin-skinned deformation in the Pampean flat-slab segment of the southern Central Andes, Argentina

In the southern Central Andes, the Andean foreland was deformed due to Neogene shallowing of the Nazca slab beneath the South America plate. In this 27–33oS Pampean flat-slab segment, the N-trending Argentine Precordillera transpressional fold-and-thrust belt and the Sierras Pampeanas broken foreland developed as a consequence of inward migration of the orogenic front. At 28oS, a NNE-trending westward-dipping, thick Neogene synorogenic sequence is exposed in the Sierra de los Colorados, which shares deformation features of the Precordillera and the Sierras Pampeanas. Integration of new structural and kinematic data and available structural, kinematic, geophysical and palaeomagnetic information allows consideration of the Sierra de los Colorados area as part of the northern sector of the Precordillera during the middle Neogene. At c . 9 Ma, basement block exhumation started with the uplift of the Sierra de Umango-Espinal that was triggered by deformation along the NE-trending Tucuman oblique belt. This stage marked the beginning of compartmentalization of the incipiently deformed Vinchina foreland. Since c . 6.8–6.1 Ma, basement block uplift linked to the Miranda–Chepes and Valle Fertil NNW-trending sinistral transpressional belts, as well as kinking of the Neogene sequence by localized WNW-striking cross-strike structures, resulted in multiple segmentation that produced a complex mosaic of basement-block pieces. The overprint of these regional, basement-involved, oblique, brittle–ductile transpressional and cross-strike megazones could be related to high interplate coupling. Localized mechanical and rheological changes introduced by magmatism favoured this thick-skinned deformation overprint.

Cross-strike structures controlling magmatism emplacement in a flat-slab setting (Precordillera, Central Andes of Argentina)

Abstract Detailed structural, kinematic and geophysical data on the foreland of the Pampean flat-slab segment (Hualilán area, Andean Precordillera of Argentina) have shown that cross-strike structures have had an important role in the evolution of this Andean segment since Miocene times. These structures represent pre-existing crustal fabrics reactivated during the Andean orogeny and could have controlled the emplacement of the Miocene arc-magmatism migrating into foreland due to the flattening of the slab. Likewise, kinematic results obtained for these structures support a similar stress frame to that obtained elsewhere in the Precordillera but showing different motions as a consequence of their high obliquity to the orogen trend. Moreover, they record a reorientation of kinematic axes during Late Miocene–Pliocene times.