HR: 1340h
AN: GP43A-0844    [Abstracts]
TI: Temporal and Spatial Constraints on Multi-Phase Crustal Rotation in the Forearc of Northern Chile
AU: * Dashwood, B
EM: bdashwood@plymouth.ac.uk
AF: University of Plymouth, School of Earth, Ocean and Environmental Sciences, Drake Circus, Plymouth, Pl4 8AA United Kingdom
AU: Taylor, G K
EM: GTaylor@plymouth.ac.uk
AF: University of Plymouth, School of Earth, Ocean and Environmental Sciences, Drake Circus, Plymouth, Pl4 8AA United Kingdom
AB: The forearc of northern Chile between ~23-29$^{o}$S records some of the largest paleomagnetically detected crustal rotations reported to date in the Central Andes. In contrast to much of the rest of the Central Andes rotations appear to pre-date the main uplift and shortening of the Andean plateau between 25 Ma and the present time. We report new studies in which we have endeavoured to investigate the scale of the rotated area and timing of the rotation in the forearc area between 27-30$^{o}$S. Several authors have documented clockwise rotations in Mesozoic to Eocene units of up to 55$^{o}$ which, previously, appeared to decrease very sharply from about 30$^{o}$ of rotation at 28$^{o}$S to near zero at ~30$^{o}$S near La Serena. We present new data from over 120 sites from a range of Mesozoic to Eocene units in both the Coastal Cordillera and Precordillera. New data from two Paleocene plutons in the Tres Cruces area (29$^{o}$S) combined with existing information from contemporary plutons (66-62Ma) from as far north as Inca De Oro (26$^{o}$S) show the rotation to decrease smoothly suggesting a continuum in the deformation gradient controlling the rotations between these latitudes. These data also suggest that there was a distinct, if small ~10$^{o}$, rotation in Cretaceous times. In order to better constrain the age of the main rotation we also present new data from Triassic to Eocene units in the La Guardia area, east of the city of Copiap\'{o} (27$^{o}$S), in which we are able to demonstrate a variation in rotation during the period 60-40 Ma. In total these data strongly suggest to us that the large rotations of this region vary relatively uniformly and slowly with distance N-S and that a substantial part of this rotation pre-dates both the Andean orogeny and also the Incaic Orogeny of this part of the Central Andes. We suggest that the bulk of rotation was associated with the period of maximum obliquity of convergence between the Nazca and South American plates between 50-40 Ma. In addition, in the older rocks, of the Coastal Cordillera there is a small late Early Cretaceous component of rotation.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8150 Plate boundary--general (3040)
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting