Present-day stress field and active tectonics in

Abstract

A systematic collection of present-day stress directions has allowed us to gain new insights into the recent tectonic evolution of the Italian Peninsula. Horizontal stress directions from breakouts in 40 deep wells drilled for oil exploration have been compared to compressional and tensional axes of 17 earthquake fault-plane solutions (4.2<M<7) and 17 stress axes obtained from structural analysis on Quaternary rocks. Notwithstanding the different scales and depths of the data considered in this work—near-surface for structural data, 0–6 km depth for breakouts, ≈ 3–20 km for earthquake focal mechanisms—the three distributions are surprisingly similar, all showing a predominant northeast-directed minimum horizontal stress in a broad region from the Tyrrhenian backarc through the Apenninic belt and up to the Apulian foreland. The belt, which corresponds to a region of high elevation, contemporary uplift and strong seismic release, with earthquakes as large as M = 7, is characterized by a normal faulting stress regime. The breakout data demonstrate that the NE-oriented extension associated with the seismogenic normal faults of the southern Apennines is determined by a regional stress with 3 = NE, in agreement with the seismological evidence that these faults are young (post-Middle Pleistocene). The breakout data show that even the foredeep and the most internal foreland region, where the (Apulian) foreland crops out, are dominated by northeast-oriented extension, even if we cannot establish the stress regime of this region due to the lack of seismicity. This suggests that a model in which the underthrusting Adriatic plate is in compression and in flexure, generating extension in the upper (Apenninic) plate, is not applicable, unless what we observe (NE-directed Shmin) is limited to the upper part of the Adriatic plate, while its deeper portion might be in compression. Compared to the recent compressional tectonics that affected parts of the study region in the Upper Pliocene and Lower-Middle Pleistocene, it seems likely that an important change in the stress field occurred in this area during the Quaternary, most probably around 0.7 Ma. Considering the main tectonic processes acting in southern Italy—NNW convergence between Africa and Eurasia and subduction of Ionian and Adriatic lithosphere with associated slab pull and backarc rifting we speculate that the extension observed in southern peninsular Italy can be explained either by a slab window or by a buoyant subducted continental lithosphere in the upper mantle between the two active arcs, the northern Apennines and the Calabrian arc.