Recent tectonic evolution and present stress in the Northern Apennines (Italy)

Abstract

The present-day stress field and its recent tectonic evolution in the Northern Apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data. We have considered 86 wells for breakout analysis, with depths down to 6–7 km, 125 fault plane solutions of crustal earthquakes with M<5 that occurred between 1988 and 1995 in the Northern and Central Apennines, and data of stronger earthquakes (M≤6) reported in other studies. The Tyrrhenian coastal region and the Apenninic belt are characterized by Shmin direction mainly trending NE-SW, with predominantly normal fault plane solutions. Along the outer front of the belt and the Adriatic offshore, Shmin is oriented NW-SE, and focal solutions are thrust or strike-slip, with maximum compression around NE-SW. Conversely, south of 43°N, breakouts evidence an orthogonal direction of horizontal compression (NW-SE), following the Southern Apennine trend, where a widespread NE-SW extension was recognized by previous investigations. Comparing these results to the recent tectonic evolution inferred from structural geology, we argue that the extension-compression pair, characteristic of the post-Tortonian evolution of the mountain belt, has been migrating in time from late Miocene to Present only in the northern sector of the arc, whereas the southern sector underwent a generalized extension, at least since middle Pleistocene. The striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension-compression pair has been controlled by progressive retreat of the slab.