CRUSTAL ROTATION IN THE ANDEAN FOREARC OF NORTHERN CHILE: MECHANISMS AND TIMING

The pattern of paleomagnetically detected rotations in the central Andes, usually referred to as the Central Andean Rotation Pattern and extending between the latitudes of ~15 and 30°S, has been greatly amplified over the last 10-15 years by numerous, modern, well constrained studies. As a consequence, collectively, we are better able to understand the scale and timing of this essentially cryptic part of the overall deformation pattern in much more detail. Young rotations (<20Ma) are in essence relatively small (<10-15°) and occur widely within the retro arc but are relatively rare in the forearc region. They show obvious spatial variation in relation to quantifiable variation in shortening estimates and arguably to ongoing deformation as seen by GPS measurements. Here we would imply that their local variation and strong coincidence with young structures, regardless of the age of the formation sampled, implies relatively high level detachment. In contrast rotations in the forearc region of northern Chile (23-30°S) are large (typically >30°), remarkably consistent over a wide area of the forearc (23-27.5°S) but show a marked but progressive graduated decrease to the south (30°S). The eastern margin of this domain of consistent rotations is difficult to define as it is either overprinted by variable rotations along the Domeyko Fault System (Taylor et al. 2005) or marked by these variable rotations (Arriagada et al. 2006). The principal differences between these two interpretations relates to timing and the significance of the Domeyko Fault System. These models will be considered further but one key aspect is that they both agree on the relatively uniform rotation throughout much of the forearc. One particular consequence of this uniform and widespread rotation is that it would seem to imply deeper detachment than the more recent Andean mountain building rotations. Possible evidence of detachment levels is reviewed and presented along with possible driving mechanisms for the rotations.