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

Between 10 degrees S and 30 degrees S, the central Andes are marked by both a major topographic anomaly, the Altiplano-Puna plateau, and a westward co ncave geometry whose origin remains controversial. The arcuate shape is acc ompanied by a remarkable pattern of rotations about vertical axes. Indeed, in the central Andes paleomagnetic studies have demonstrated counterclockwi se rotations on the northern limb of the are (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 ongo ing debate on the origin of rotations in the central Andes, we have underta ken a paleomagnetic study of 373 cores, taken at 29 sites (grouped into sev en localities). The samples are from sediments and lava flows of Cretaceous to Tertiary age located in intermontane basins of the Puna plateau in nort hwestern Argentina. Vertical axis rotations, calculated from paleomagnetic declinations, are clockwise for all localities and confirm the pattern of c lockwise rotations associated with the southern central Andes. However, sig nificant variations in the amount of rotation occur from one locality to an other, suggesting that they are, at least in part, influenced by local tect onics. As most faults in the Puna plateau have reverse dip-slip components, we infer that the observed differential rotations between blocks are due t o scissoring motions on thrust faults. Whether or not this mechanism has op erated across the entire area of thickened crust in the central Andes remai ns to be demontrated. Even if such faulting has locally influenced rotation s, Cenozoic oroclinal bending is a likely cause of the remarkable pattern o f vertical axis rotations across the central Andes.