Application of AMS for reconstruction of the geological evolution of recent volcanic systems: Case of Deception Island (South Shetland Islands, Antarctica)

- AMS analysis of recent vocanic deposits.
- Tilting processes deciphering by kinematic analysis.
- Volcanic edifice evolution by means AMS.
- Caldera collapse and tectonic faults.

The analyses of recent volcanic systems can encounter limitations due to recent eruptions hiding part of the geological history, where the geomorphological expression can present non-univocal interpretations or where modern relief and abrupt cliffs can decrease the accessibility to places that are suitable for detailed analysis. In the case of Deception Island (Bransfield Trough), the recent tectonic and volcanic history and the interactions with glacial, periglacial and marine activities make traditional analysis difficult. Anisotropy of magnetic susceptibility (AMS), a petrofabric approach, has allowed for the identification of a clear correlation between the anisotropy of the magnetic ellipsoid, the axial distribution characteristics at the site-scale, the type of defined magnetic fabrics and the imbrication angle between the magnetic foliation and the emplacement surface. However, these aspects are independent of the dip of the emplacement surface. In some of the analysed sites, the shear plane (the plane that contains Kmax and Kmin axes and the flow direction) shows non-suitable kinematic patterns to respect the dip of the emplacement surface. A tilting process is needed to recover the expected original distribution, in some cases, affecting the analysed units through a concentric fracture system with respect to the main caldera or following the regional tectonic structures. A detailed analysis of AMS data permits us to constrain the recent evolution of a volcanic system through three stages: 1) a radial distribution of volcanic deposits, 2) a coaxial tilting of the analysed zones respect the caldera collapse and 3) a tectonic-related fracturing system responsible for the tilting of the volcanic system.