MAJOR CRUSTAL ROTATIONS IN THE ANDEAN MARGIN - PALEOMAGNETIC RESULTS FROM THE COASTAL CORDILLERA OF NORTHERN CHILE

Paleomagnetic analyses of Mesozoic lavas and dike swarms from the nort hern Chilean Coastal Cordillera, between 25.4 degrees S and 26.4 degre es S, reveal a clockwise rotation of about 42 degrees. Magnetizations from lava flows of andesitic-basaltic composition of the Middle Jurass ic La Negra Formation pass both fold and reversal tests and are interp reted as prefolding remanences. Five dike swarms of Middle Jurassic to Early Cretaceous age yield similar directions to that obtained from t he La Negra Formation. Four of the five swarms have mixed polarity, su ggesting that they too carry a primary or very early remanence. The st ructural setting of the dikes suggests that they have not suffered any substantial tilting about nonvertical axes since acquisition of the r emanence. The clockwise rotation of the area is believed to have been the consequence of transpressional deformation of mid-late Cretaceous age, post-100 Ma, associated with abandonment of the Jurassic-Early Cr etaceous magmatic are in this region and its eastward migration to for m a new mid-late Cretaceous magmatic arc in the former back are region . This younger are is located east of the Coastal Cordillera and lies in the Central Valley region. The clockwise sense of rotation is consi stent with other paleomagnetic data from northern Chile and southern B olivia, south of the Arica Deflection in the Andean margin, although i t is the largest yet reported. To the north of the Arica Deflection, p aleomagnetic studies report counterclockwise rotations, and several la rge-scale models have been proposed to explain the overall pattern of rotations. Models include oroclinal bending of an originally straight margin, differential shortening across the margin at a preexisting ben d which is subsequently tightened by the passive relation of the limbs of the bend, and distributed shear throughout the margin as a consequ ence of oblique convergence at a preexisting bend. In contrast to thes e models, several workers have argued that rotation is better explaine d in terms of localized in situ rotations. We review these models in l ight of our results and present a domino-type model with blocks bounde d by left-lateral faults and rotating clockwise in response to mid-Lat e Cretaceous transpression within a crustal scale shear zone. This is consistent with the observed strike-slip fault systems identified in t he Coastal Cordillera.