On the mechanism of crustal block rotations in the Central Andes — Comment
In this paper Beck (1998)reviews the database of palaeomagnetically observed rotations in the Andean margin of South America, in particular with respect to the well known change in sense of rotation linked to the Arica deflection (Bolivian Orocline) in the orogen. The paper argues firstly for the latitudinal distribution of rotations supporting an interpretation that distributed shear combined with oroclinal bending is the best hypothesis to explain the observed pattern of rotations and secondly that this rotation/deformation is time-dependent. While we essentially agree with the former conclusion we would define it more tightly in that the over-arching control on the pattern of rotations is a combination of differential shortening, as opposed to oroclinal bending, along the orogen coupled with distributed shear across the orogen. Our main concern with this paper is that it fails to provide a convincing demonstration that there is a time-dependent element to the magnitude of rotations. Primarily we will argue that it is far from clear from the palaeomagnetic data presented that there is a case for a continuous rate of rotation applicable to the southern limb of the orocline, at least. Not withstanding this we would also argue that geologically the deformation cannot and should not be described by a continuous deformation rate over time, as postulated. We also make specific points about aspects of the paper dealing with the Atacama Fault Zone and its relation to the observed rotations.