LARGE PALEOMAGNETIC DECLINATION ANOMALIES IN THE CERRO NEVADO (SNOW HILL) ISLAND, ANTARCTIC PENINSULA: EVIDENCE OF HIDDEN TECTONIC ROTATIONS?

We report anomalous paleomagnetic directions obtained in magnetostratigraphic studies carried out on Cretaceous marine sedimentary rocks of the Marambio Group, southeast James Ross Basin, at the Sanctuary Cliffs Nunatak and the Spath Peninsula, located in the Cerro Nevado (Snow Hill) Island. Data quality and field tests suggest a primary origin of the magnetization, ruling out the possibility that directions have been affected by alteration or remagnetization processes related to the Miocene magmatism of the James Ross Island Volcanic Group. A counterclockwise tectonic rotation of 47.9°±20.0° of the whole Snow Hill Island can account for the paleomagnetic declination anomaly. Bedding of Cretaceous units at and near the sampling localities show similar directional variations that those observed in paleomagnetic declinations, consistent with such interpretation. However, geological continuity between the exposed units at Marambio Island and the Spath Peninsula, the variable bedding strikes at different localities in the Snow Hill Island, plus geometric considerations strongly constrain any rigid body counterclockwise rotation of the whole island to a maximum of about 10°, around a vertical axis located close to the Picnic Passage (~64.4° S, 56.9° W). This indicates that most, if not all, of the hypothetical rotations suggested by the paleomagnetic data must occurred during the Eocene, before the tectonic tilting of the Cretaceous units. This need to be tested with new detailed paleomagnetic studies at different localities along the Cerro Nevado (Snow Hill) Island as well as with geophysical surveys along the island and its marine surroundings.