The origin and growth of a recently-active fissure ridge travertine over a seismic fault, Tivoli, Italy

An enigmatic, c. 2 km-long and 15 m-high travertine ridge, the Colle Fiorito ridge, occurs in the northwestern sector of the Tivoli travertine plateau, central Italy. The main questions addressed in this paper concern the origin and mode of growth of this prominent ridge. The presence of active structures beneath the studied ridge is inferred by recent and past earthquakes located at shallow depths immediately beneath Colle Fiorito. To understand the surficial structure of the Colle Fiorito ridge and the travertine depositional environment, we constructed a 10 m-resolution DEM, analyzed recent and past aerial photographs, and conducted field surveys and meso- to micro-scale sedimentological analyses. To understand the ridge subsurface structure, we studied a set of 32 stratigraphic well logs available from previous works and from the local decorative stone industry, and realized a 2D electrical resistivity tomography (ERT) across the ridge. Results show a gentle antiformal structure affected by subvertical zones of strata discontinuity. The Colle Fiorito structure is interpreted as a previously-unknown fissure ridge travertine grown at the edge of the Tivoli travertine plateau, perhaps when the volumetric deposition rate reached its climax in the plateau for the abundance of fluid discharge and the rise of the water table. Such a fluid pressure may have activated the faults and fractures beneath Colle Fiorito, thus opening new pathways for the ascension of geothermal fluids toward the surface.