PALEOMAGNETIC AND STRUCTURAL CONSTRAINTS ON ROTATIONS IN THE NORTH CHILEAN COAST RANGES

Paleomagnetic analysis of seventy-six samples from six sites in the Ju rassic to Cretaceous plutons (radiometric ages from 157 to 128 Ma) of the Coastal Cordillera of northern Chile yielded stable remanent magne tization directions. The plutons record both a normal and a reversed c haracteristic direction. The mean normal direction has a declination o f 3.4 degrees, an inclination of -38.5 degrees, and an alpha 95 of 10. 5 degrees. The mean reversed direction has a declination of 207.8 degr ees, an inclination of 33.8 degrees, and an alpha 95 of 10.8 degrees. Paleopoles calculated from these mean directions have the positions of latitude=85.8 degrees N, longitude=338.2 degrees E and latitude=63.5 degrees E, longitude=191.9 degrees E respectively. In comparison to an APW path, the reversed pole indicates a clockwise rotation of approxi mately 28 degrees, similar to other regional results, whereas the norm al pole is concordant with the present day magnetic field. These resul ts, in and of themselves, could support oroclinal bending or local blo ck rotations. However arrays of secondary faults to the Atacama fault system in the sampling area show characteristic spacings, orientations , and displacements. Restoration of displacements results in a 31 degr ees clockwise rotation of fault blocks; essentially removing the obser ved paleomagnetic discordance. Thus, structural data confirms the loca l block rotation model. The existence of clockwise discordant paleomag netic data within the sinistral (counterclockwise) Atacama Fault Zone (AFZ) underscores the ambiguity of using the sense of rotation to infe r the sense of motion on the larger shear zone. Age constraints for mo tions along the AFZ, suggest that these rotations were completed in th e Early Cretaceous; essentially coeval with magmatic activity within t he Coast Ranges, and underscores the magmatic are, rather than forearc , setting for this tectonic event.