Active deformation at the southwestern Alps–Ligurian basin junction (France–Italy boundary): Evidence for recent change from compression to extension in the Argentera massif

Abstract

Several hundred earthquakes of small and moderate magnitude occur each year at the southwestern Alps–Ligurian basin junction. In addition, the historical seismicity data attests for earthquakes of magnitude close to 6.5. Despite the seismic hazard and the vulnerability of this region, the present-day driving mechanism of deformation and the major structures accommodating these deformations remain unclear. This littoral area presents a strong topographic gradient from 3000 masl on the Argentera alpine massif to 2500 m.u.s.l. in the Ligurian basin and a strong structural inheritage resulting from 50 My geological evolution combining different processes such as continental collision and ocean basin opening. We present new results about the kinematics of the Corsica–Sardinia continental block with respect to the western alpine belt. From 7 years of continuous GPS measurements we show that a small part of the Nubia–Eurasia convergence is certainly transferred northward of the Maghrebides deformation belts to the Corsica–Sardinia block. In the southwesternmost Alps from Torino to the Mediterranean coast, the baselines reveal differential motions of lengthening between the Po plain and the Argentera and shortening between the Argentera and the Ligurian domain. In a region where few focal solutions are available we determine seven new focal mechanisms located in the Argentera massif. These new data underline a quaternary change from compression to extension in that part of the western alpine belt. The focal mechanisms calculated attest for extensional faulting at shallow level between 1 and 11 km depth. We add 7 focal mechanisms from the literature to calculate the stress tensor: a homogeneous solution indicates a direction of extension broadly perpendicular to the topographic axis culmination of the Argentera. In contrast, the earthquakes recorded below 15 km at the northwestern margin of the Po plain testify reverse faulting and a compressional state of stress in the deep crust. Thus, we point out that the stress field is vertically heterogeneous in the crust of the Argentera area. The Ligurian domain is mainly characterized by reverse and strike-slip faulting. The major earthquakes are located in the northern margin and at the ocean–continent transition. These earthquakes are assumed to result from the reactivation, in a compressional state of stress, of the structures formed during the Oligo-Miocene opening of the Ligurian basin. Finally, we propose three hypotheses to explain the current deformation at the Alps–Ligurian basin junction, taking into account the far-field kinematics between the Nubia and the Eurasia plates, the topographic gradient between the Argentera massif and the Ligurian oceanic floor and the distribution of the deformations.