Paleomagnetic data from the Precordillera of northern Chile: A multiphase rotation history related to a multiphase deformational history

Abstract

One of the most conspicuous features of the Andean chain is the change in its trajectory from NW-SE to N-S at 18°S known as the Bolivian Orocline. Although the Central Andes rotation pattern (CARP) agrees roughly with the geometry of the orocline, large variations and greater than expected rotation magnitudes cannot be explained by a progressive oroclinal bending history alone. In this work, 621 oriented samples from Mesozoic and Cenozoic rocks in three different areas in the Precordillera of northern Chile, north of Calama, were analyzed. The samples were subjected to different demagnetization processes to isolate the characteristic magnetic remanence and calculate the tectonic rotations. By sampling pretectonic, syntectonic and post-tectonic units, the rotation history of different structural blocks was determined and related to specific deformational events. The results obtained from Lower Cretaceous units intruded or overlain by Upper Cretaceous units not rotated or recording smaller rotations indicate the existence of 20° clockwise and 20° counterclockwise rotated sites related to the Late Cretaceous Peruvian orogenic event. Data obtained from Upper Cretaceous-lowermost Paleocene rocks show areas slightly clockwise rotated (10?15°) and not rotated during the early Paleocene ?K-T" deformation event. Remanence data from Upper Cretaceous-Eocene units across the area indicate as much as 20° clockwise rotation in the hanging wall of the Eocene Chintoraste-Quetena-Rabonas-Jaspe Reverse Fault System, whereas the footwall was unaffected. This Eocene rotation occurred during the early stages of the Eocene-Oligocene Incaic orogenic phase, prior to emplacement of the 40?37 Ma Fortuna Granodiorite Complex. Current models propose that rotations in CARP were entirely Cenozoic and largely accomplished in a single Paleogene event. Our data indicate that each of the Peruvian, K-T and Incaic deformation events contributed to CARP, with each event generating a non-uniform pattern of rotations which was superimposed on prior rotations in a non-uniform manner.