Vertical axis rotations across the Puna plateau (Northwestern Argentina) from paleomagnetic analysis of Cretaceous and Cenozoic rocks

Between 10°S and 30°S, the Central Andes are marked by both a major topographic anomaly, the Altiplano-Puna plateau, and a westward concave geometry whose origin remains controversial. The accurate shape is accompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the Central Andes paleomagnetic studies have demonstrated counterclockwise rotations on the northern limb of the arc (throughout Peru, northernmost Chile, and Northern Bolivia) and clockwise rotations on the southern limb (throughout Southern Bolivia, Northwestern Argentina, and Northern Chile). To fill a gap in data from Northern Argentina and to contribute to the ongoing debate on the orgin of rotations in the Central Andes, we have undertaken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into seven localities). The samples are from sediments and lava flows of Cretacous to Tertiary age located in intermontane basins of the Puna plateau in Northwestern Argentina. Vertical axis rotations calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of clockwise rotations associated with the Southern Central Andes. However, significant variations in the amount of local tectonics occur from one locality to another, suggesting that they are, at least in part, influenced by local tectonics. As most faults in the Puna plateau have reverse dip-slip components, we infer that the observed differential rotations between blocks are due to scissoring motions on thrust faults. Whether or not this mechanism has operated across the entire area of thickened crust in the Central Andes remains to be demonstrated. Even if such faulting has locally influenced rotations, Cenzoic oroclinal bending is a likely cause of the remarkable pattern of vertical axis rotations across the Central Andes. (Résumé d'auteur