Paleomagnetic evidence for a very large counterclockwise rotation of the Madre de Dios Archipelago, Southern Chile

A paleomagnetic study of Late Paleozoic exotic units exposed in the Madre de Dios Archipelago and Diego de Almagro island, southern Chile, is reported. These units correspond to the Tarlton Limestone and the metabasaltic pillow lavas of the Denaro Complex. Nineteen sites were drilled (14 in limestone and five in lavas). Standard demagnetization procedures show that the primary remanence is not present in the investigated rocks, preventing the determination of the original latitudinal position of these rocks. However, a post-tectonic secondary magnetic component was isolated in eight sites at the Madre de Dios Archipelago, in both limestone and basaltic pillow lavas. Rock magnetic experiments and microscopic observations indicated that the carrier of this remanence is secondary titanomagnetite in the limestone and titanomagnetite-titanomaghemite in the lavas. The most likely event to produce this secondary magnetization is the nearby intrusion of the south Patagonian batholith in the Early Cretaceous. Mean direction from eight sites shows a very large (117.7 ± 29.9°) counterclockwise rotation of the study area with negligible paleolatitude anomaly. This rotation is only constrained as post-Early Cretaceous. Restoration of the structures to their pre-rotation position indicates that accretion of the exotic block took place from the northwest and not from the south as previously proposed. This is more consistent with a low latitude origin for the limestone and the general paleogeographic framework. Different tectonic mechanisms and ages for this large rotation are possible.