Paleomagnetic study of the Capo di Bove lava flow, Rome, Italy

The Capo di Bove (CDB) lava flow was emplaced at ∼277 ka during the Faete eruptive Phase of Colli Albani volcanic district near the city of Rome. The CDB lava has a historical significance as it provided the slabs used in the paving of the ancient Appian Way, built in the 4th century BCE. Puzzlingly beyond the seventh milestone, the ancient Appian Way deviates briefly from an otherwise straight SE-NW direction, abandoning the top of the lava flow and resuming its elevation and the SE-NW trend within less than 1 km. This peculiarity raised a question as to whether the deviation could have been the result of a tectonic deformation caused by a (buried) fault. To test this hypothesis, we sampled the CDB lava flow at four locations over a ∼ 10 km transect near the ancient Appian Way around the bend and performed a detailed rock magnetic, paleomagnetic, and petrographic study. Rock magnetic data indicate that pseudo-single-domain magnetite and low-Ti titanomagnetite particles are the main magnetic carriers for three sampling locations, located in freshly cut quarries, which reliably recorded the paleomagnetic field at the time of emplacement. Conversely, the samples collected in the upper part of the lava flow, within the bent segment of the ancient Appian Way, show multi-domain low- and moderate-Ti titanomagnetites as main magnetic carriers which fail to record a paleomagnetic direction. Anisotropy of magnetic susceptibility data are consistent with an overall CDB lava flow direction from SE to NW and the paleomagnetic directional data from the three reliable sampling sites are statistically indistinguishable. Hence, data from this study show no evidence of post-emplacement tectonic rotations. We suggest that the origin of the bend could be identified in the pre-existing morphology (for the lava flow path) and in historical reasons (for the ancient Appian Way).