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 indic ations of the timing and mechanism of crustal block rotations. Rotations an d their 95% confidence intervals are recalculated in pole space, using a ne w South American reference APW path. As observed by many investigators, the Arica deflection (the change in topographic and tectonic trends located ne ar Arica, northernmost Chile) divides the data into a counterclockwise-rota ted set north of the deflection and a clockwise-rotated set to the south. R otations range from 75 degrees counterclockwise to nearly 50 degrees clockw ise, and show substantial differences between neighboring crustal blocks. S outh of Arica there is a weak indication of a decrease in amount of rotatio n 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 f or rock units of less than or equal to 90 Ma. For older rock units no corre lation between age and amount of rotation exists. Unlike the western margin of North America, no significant margin-parallel displacements accompany t he rotations. Several models for rotation are examined. It is concluded tha t 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 E lsevier Science B.V. All rights reserved.