Deep structural heterogeneities and the tectonic evolution of the Abruzzi region (Central Apennines, Italy) revealed by microseismicity, seismic tomography, and teleseismic receiver functions

The crustal structure of central Apennines (Italy) is still poorly defined, leaving uncertainties on the tectonic style (thin or thick-skinned) responsible for the development of the thrust-and-fold belt. The today active extension, which replaced compression since early Quaternary, is presumably influenced by the pre-existing structure that yields location and segmentation of the fault system. To focus on such issues, we computed P- and S-wave velocity models of the crust by using the independent methodologies of local earthquakes tomography and teleseismic receiver function. We document strong lateral and vertical heterogeneities that define shallow, imbricate sheets of the Mesozoic cover that overlay exceptionally high Vp and high Vs bodies. These bodies can be interpreted as either dolomitic or, partially hydrated, mafic rocks. The two alternative interpretations respectively imply an ultra-thick deposition of dolomitic rocks in the hanging wall of Triassic normal fault or a deep exhumation of the Pre-Mesozoic basement during the early Mesozoic sin-rift tectonic. In both cases, these bodies influenced the evolution of the thrust-and-fold belt. Very remarkably, active normal faults, like those ruptured during the still ongoing 2009 L'Aquila sequence, concentrate at the border of these bodies, suggesting that they have an active role in the segmentation of the normal fault system. The rheological behavior of such high Vp high Vs bodies, weak or strong, is still uncertain, but of utmost importance to understand the risk of future normal faulting earthquakes.