Syn-depositional substrate deformation produced by the shear force of a pyroclastic density current: An example from the Pleistocene ignimbrite at Monte Cimino, northern Lazio, Italy

Substrate deformation by pyroclastic density currents is very sparsely described in the literature. The rare occurrence of syn-depositional substrate deformation suggests that special circumstances are required to transmit shear from the base of a pyroclastic density current into the deposited ignimbrite and the substrate. One example of a substrate deformed by a pyroclastic density current is found at the base of the Pleistocene ignimbrite at Monte Cimino, central Italy. A series of reverse faults that offsets the basal contact were produced by the shear force of the pyroclastic current during deposition of the ignimbrite. The faults formed on the vent-facing side of a palaeo-slope that strikes sub-parallel to the flow direction of the pyroclastic current. Fault offsets suggest motion was parallel to the flow direction of the pyroclastic current, rather than down-slope. We propose that these faults resulted from fluctuations in the shear force of the pyroclastic density current as it was channelled down a palaeovalley. The lower flow boundary, which separated the deposited ignimbrite and the substrate from the moving pyroclastic density current, momentarily stepped down into the substrate, so that the upper 0.5 m of the substrate and about 1.5 m of the deposited ignimbrite became incorporated into the current. This momentary coupling of the current and the substrate induced reverse faulting in the substrate and the deposited portion of the ignimbrite. Deposition appears to have been ongoing during the formation of these faults, as well as afterward. Following the formation of the faults, the lower flow boundary seems to have been quickly re-established above the faults (approximately 1.5 m above the base of the ignimbrite), allowing deposition to continue without further deformation of the substrate.