The reliability of anisotropy of magnetic susceptibility (AMS) data as flow direction indicators in friable base surge and ignimbrite deposits: Italian examples

Abstract

The base surge deposits in Linosa and Procida islands (Italy) show minimum axes ( K 3) of the anisotropy of magnetic susceptibility (AMS) that are mostly parallel to the flow directions inferred from field evidence, but sometimes perpendicular to them. Samples from different heights above the base of an ignimbrite bed (Vulsini area, Italy), showing a coarse tail reverse grading and a fine-grained basal layer, have different magnetic fabrics: at 10 and 40 cm from the base the declinations of maximum axes ( K 1) of the AMS are approximately 90 ° from the theoretical flow direction, whereas at 60 and 75 cm from the base the K 1 axes are approximately parallel to the flow direction. This bed also shows a decrease upward of the magnetic foliation, lineation and degree of anisotropy, suggesting that the AMS data could be used to obtain information about flow mechanisms. The AIRM data in the case of the ignimbrite bed show that the long shape axes of the ferromagnetic grains are probably aligned with the maximum axes of the AMS. For this reason the change in the magnetic fabric at different distances from the base is probably due to differences in the flow behaviour rather than the domain state of magnetite grains. This study shows that the AMS lineations can indicate the flow direction in base surge and ignimbrite deposits but other geological and magnetic information is necessary to assess if these lineations are flow perpendicular or flow parallel.