Deep geometry and rheology of an orogenic wedge developing above a continental subduction zone: Seismological evidence from the northern-central Apennines (Italy)

Abstract

Data from high-density seismic networks deployed between 2000 and 2007 in the north-central Apennines (Italy) yield unprecedented images of an active orogenic wedge. Earthquake foci from the northern Apennines define a Benioff zone deepening westward from the Adriatic foreland down to ~60 km depth below the chain. The seismicity shows that only the lowermost ~10 km of the Adriatic foreland crust is subducted, whereas the uppermost ~20 km is incorporated into the orogenic wedge. Farther west, an aseismic mantle with markedly negative P-wave-velocity (V p ) anomalies is interpreted as asthenosphere flowing toward an Adriatic slab in retrograde motion. Three crustal layers with different V p and seismicity characteristics are imaged below the northern Apennines: an uppermost 10-km-thick fast layer affected by extensional faulting, a slow layer with diffuse seismicity down to ~15 km depth, and a lowermost fast and aseismic layer resting directly above the asthenosphere. We interpret the latter layer as having formed by anhydrous crust undergoing granulitization, whereas trapped CO 2 (either from the underlying granulites or from the subducting Adriatic crust) is inferred to have been responsible for both low V p and diffuse seismicity in the middle crust. Trapped CO 2 is released along the easternmost normal fault systems breaking the Apennine upper crust, consistent with geochemical and seismotectonic evidence. Compressive earthquakes at 20-25 km depth along the external front suggest offscraping of the subducting foreland crust and show that asthenospheric flow represents the primary source of ongoing shortening along the belt front. © 2009 Geological Society of America.