Paleomagnetic and structural constraints on rotations in the North Chilean coast ranges

Abstract

Paleomagnetic analysis of seventy-six samples from six sites in the Jurassic to Cretaceous plutons (radiometric ages from 157 to 128 Ma) of the Coastal Cordillera of northern Chile yielded stable remanent magnetization directions. The plutons record both a normal and a reversed characteristics direction. The mean normal direction has a declination of 3.4° an inclination of −38.5°, and an alpha 95 of 10.5°. The mean reversed direction has a declination of 207.8°, an inclination of 33.8°, and an alpha 95 of 10.8°. Paleopoles calculated from these mean and latitude=63.5° S, longitude=191.9° E respectively. In comparison to an APW path, the reversed pole indicates a clockwise rotation of approximately 28°, similar to other regional results, whereas the normal pole is concordant with the present day magnetic field. These results, in and of themselves, could support oroclinal bending or local block 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° clockwise rotation of fault blocks; essentially removing the observed paleomagnetic discordance. Thus, structural data confirms the local block rotation model. The existence of clockwise discordant paleomagnetic data within the sinistral (counterclockwise) Atacama Fault zone (AFZ) underscores the ambiguity of using the sense of rotation to infer the sense of motion on the larger shear zone. Age constraints for motions along the AFZ suggest that these rotations were completed in the Early Cretaceous; essentially coeval with magmatic activity within the Coast Ranges, and underscores the magmatic arc, rather than forearc, setting for this tectonic event.