On the mechanism of crustal block rotations in the central Andes

The results of 62 paleomagnetic studies of Jurassic and younger rock units from the Andes of Argentina, Bolivia, Chile and Peru are examined for indications of the timing and mechanism of crustal block rotations. Rotations and their 95% confidence intervals are recalculated in pole space, using a new South American reference APW path. As observed by many investigators, the Arica deflection (the change in topographic and tectonic trends located near Arica, northernmost Chile) divides the data into a counterclockwise-rotated set north of the deflection and a clockwise-rotated set to the south. Rotations range from 75 degrees counterclockwise to nearly 50 degrees clockwise, and show substantial differences between neighboring crustal blocks. South of Arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin, but no such correlation is observed north of Arica. Rotation increases with age both north and south of Arica for rock units of less than or equal to 90 Ma. For older rock units no correlation between age and amount of rotation exists. Unlike the western margin of North America, no significant margin-parallel displacements accompany the rotations. Several models for rotation are examined. It is concluded that a combination of late Cenozoic oroclinal bending and post-mid-Cretaceous local block rotations in response to shear-driven sub-crustal ductile flow can explain most of the paleomagnetic and geologic observations. (C) 1998 Elsevier Science B.V. All rights reserved.