Magnetic fabric and implications for pyroclastic flow and lahar emplacement, Albano maar, Italy

[1] The Albano polygenetic maar is the youngest eruptive center of the quiescent Colli Albani volcano, located near the city of Rome. The most recent activity of the Albano maar extends from similar to23 ka into the Holocene and produced the small volume, basic, phreatomagmatic Peperino Albano (PA) ignimbrite, and, more recently, phreatomagmatic surge and lahar deposits related to the overspill of the Albano maar lake. We have performed an anisotropy of magnetic susceptibility (AMS) study, in order to define the relationships between the magnetic fabric and the flow mechanisms of the PA and of the phreatomagmatic deposits and lahars. AMS results indicate different transport and/or depositional systems for the veneer and valley pond facies in the PA ignimbrite and for the lahar deposits. AMS also demonstrates that flow directions are mainly controlled by the paleotopography. The paleotopographic control has been interpreted in terms of talweg sedimentation even at proximal locations where deposition occurs from dilute pyroclastic flows. Furthermore, AMS results clearly evidence a southward provenance for Holocene post-PA ignimbrite units, cropping out in the Ciampino plain, and confirm their origin from Albano maar lake overspill. We demonstrate that AMS is a reliable marker to determine paleoflow directions also in small volume phreatomagmatic ignimbrites and in syneruptive lahar deposits and can be successfully used to define their depositional systems.