Constraining the timing of southern Central Andes vertical axis rotations

PREZZI, C., CAFFE, P.J.

Perugia, Italia

Congreso; IUGG XXIV General Assembly; 2007

IUGG

A pattern of vertical axis tectonic rotations has been paleomagnetically identified along the Central Andes. While such rotations are counterclockwise in central-northern Bolivia, Per and extreme northern Chile, they are clockwise in southern Bolivia, northern Chile and northwestern Argentina. Various models have been proposed to explain this pattern and the geodynamic evolution of the Central Andes, but the driving mechanism of these rotations remains controversial. Constraining the spatial variability and the timing of the rotations may contribute to a better understanding of their origin. Our study complements information from previous research, improving the knowledge of tectonic rotations in the region of the northern Argentine Puna and the southern Bolivian Altiplano. In Casa Colorada (22.32 S66.29W) 51 cores were drilled from the Pea Colorada Formation. Another 47 cores were collected from Peña ColoradaFormation in Paicone (22.20 S66.40W). Peña Colorada Formation corresponds to Eocene to lower Miocene thick fluviatile redbed deposits. The ~17 Ma Casa Colorada dacite dome complex and the basal member of Tiomayo Formation (~16 Ma), which unconformably overlie redbeds from Peña Colorada Formation in Casa Colorada area, were paleomagnetically studied previously. In our previous study, we found that such units do not show significant vertical axis rotations. Furthermore, Upper Miocene (10 Ma and younger) ignimbrites, which overlie these middle Miocene rocks, are unrotated. In the Bolivian Altiplano, close to the international boundary (22.12 S66.40W), 23 cores were drilled from the Esmoruco Formation. The 17 Ma Torrelaire tuffs overlie the redbeds from Esmoruco Formation. Preliminary results of our paleomagnetic study suggest that the sampled zones underwent 13-30 clockwise vertical axis tectonic rotations. Considering the previous studies mentioned above, these rotations should have occurred before approximately 16-17 Ma, imposing a new time constraint to the rotational history of this portion of the Altiplano-Puna. The main fault in the study area is the westvergingSan Vicente Thrust. On the basis of a local unconformity observed in the northern Puna, it wassuggested that the motion of this fault stopped between 18 and 15 Ma. Then, major activity in the San Vicente Thrust pre-dates Casa Colorada dacite dome complex, the basal member of Tiomayo Formation and Upper Miocene ignimbrites, which do not show significant vertical axis rotations. On the other hand, the clockwise rotated Peña Colorada and Esmoruco Formations should have been affected by the San Vicente Thrust motion. This analysis would suggest the existence of a probable, direct relationship between timing of significant rotations and timing of local deformation in the southern Central Andes.