Crustal structure in the area of the 2002 Molise earthquake: Clues for the evolution of the southern Apennines

摘要

The Apennines is a mountain belt with paired extension and compression that are explained by the subduction and delamination of the Ionian/Adria lithosphere. During the belt formation, the Adria crust is flexed downward and then incorporated into the wedge after peeling off along some weak level whose origin is still discussed. In southern Apennines, this process seems to have stopped at about 0.7 Ma by the development of a tear in the subducting lithosphere and consequent extension that spread along the mountain range. Seismological data acquired after the 2002 Mw 6.0 Molise strike-slip earthquake yield modeling the structure of the Adria crust as congealed close to the leading edge of the belt. Tomography images show inverted normal faults within the sedimentary cover (Apulian units). Fault inversion, incomplete close to the compressional front, testifies for a final squeezing of the entire, previously flexed, Apulian continental margin. Pronounced high Vp and low Vp/Vs anomalies are observed at depth greater than 12 km and undoubtfully attributed to heterogeneities of the basement, solving the long-lasting contention on the nature of similar high-speed bodies observed beneath the Apennines. Beneath these bodies, we observe a strong reflectivity, suggesting the presence of a fluid-filled layer in the middle lower crust that may act as a decoupling level in the Apennines tectonics. The deep fluids are also favoring the rupture of strike-slip earthquakes, like those that occurred in 2002, and the intermediate-depth seismicity diffused in the Apulian region.