Relationship between topography, rates of extension and mantle dynamics in the actively-extending Italian Apennines

To investigate the mechanism driving active extension in the central and southern Italian Apennines and the geography of seismic hazard, we compare spatial variations in upper crustal strain-rate measured across exposed fault scarps since 15 ± 3 ka with data on cumulative upper-crustal strain and topographic elevation, and free-air gravity, P-wave tomography and SKS splitting delay times that are a proxy for strain in the mantle. High extensional strain-rates across the Apennines since 15 ± 3 ka (0.4–3.1 mm/yr along 90 km transects) occur in two areas (Lazio-Abruzzo; SE Campania and Basilicata) where values for finite extensional strains that have developed since 2–3 Ma are highest (2–7 km cumulative throw), and where mean elevation in 5 × 90 km NE–SW boxes is > 600 m; the intervening area (NW Campania and Molise) with < 600 m mean elevation in 5 × 90 km boxes has extension-rates < 0.4 mm/yr and lower values for finite extensional strains (< 2 km cumulative throw). These two areas with high upper-crustal strain-rates overlie mantle that has relatively-long spatially-interpolated SKS delay times (1.2–1.8 s) indicating relatively-high mantle strains and free-air gravity values (140–160 mGals); the intervening area of lower extension-rate has shorter spatially-interpolated SKS delay times (0.8–1.2 s) and lower free-air gravity values (120 mGals). The two areas with high upper crustal strain-rates and strain, mean elevation, and mantle strain, coincide with the northern and southern edges of a slab window in the Tyrrhenian–Apennines subducting plate that has been inferred from published P-wave tomography. Together these correlations suggest that dynamic support of the topography by mantle flow through the slab window may control the present day upper crustal strain-rate field in the Apennines and the geography of seismic hazard in the region.