Models for interprétation of the magnetic anomaly of the Ivrea body

The extensive regional magnetic anomaly observed along the Western AIps from Locarno to Turin can be related to the Ivrea body. The models so far put forward to explain the anomaly are compatible with the seismic and gravimetrie data, and consist of a slab-like body dipping to the SE. One major difference, however, lies in the fact that the thickness of this slab is only 2 - 5 km in the magnetic models, whereas it is of the order of 12 - 20 km in the seismic models. This would imply a division of the Ivrea body into two parts, with magnetically differentiated rocks in the tectonically upper part only. The physical and geometrical findings in the magnetic models proposed are in agree ment. By contrast, no single interpretation is available with regard to the geological nature of the magnetically different materials. Two suggestions hâve been made : Ivrea-Verbano Zone metabasites; Lanzo Massif type serpentinized peridotites. More detailed investigation and a more extensive examination of the magnetic properties of these rocks are needed before a final choice can be made between these two candidates. There is also a possibility that the magnetic anomaly is not due to the Ivrea body at ail, but attributable to the top of the south-Alpine crystalline basement. The theories advanced with respect to the nature of the magnetically anomalous unit imply different relations between the Ivrea body and the south- Alpine and west-Alpine blocks.