The active fault system of SW Alps

Abstract

International audienceHistorical and active seismicity in the south-western Alps (France and Italy) shows the recurrence of relatively high-magnitude earthquakes (M ≥ 5.8), like the one that recently affected the Italian Apennine range (M = 6.3 on the 30th March 2009). However, up-to-date detailed mapping of the active fault network has been poorly established. The evaluation of seismological hazard in particular in the highly populated French and Italian coastal region cannot be done without this. Here, we present a detailed study of the main active fault system, based on geological observations along the south-western flank of the Alpine arc. This N140° right-lateral strike-slip active fault system runs along the edge of the Argentera-Mercantour range and can be followed down to the Mediterranean Sea. It is evidenced by (1) Holocene offsets of glacial geomorphology witnessing ongoing fault activity since 10 ka, (2) widespread recent (10–20 Ma) pseudotachylytes featuring long term activity of the faults, (3) active landslides along the main fault zone, (4) geothermal anomalies (hot springs) emerging in the active faults, (5) ongoing low-magnitude seismic activity and (6) localization of the main historical events. In the light of our investigations, we propose a new tectonic pattern for the active fault system in the south-western Alps

Geological setting of the southern termination of Western Alps

“…2, 3), interconnected with minor NE-SW and E-W fault segments (Perello et al 2001;Tricart 2004;Baietto et al 2008;Sanchez et al 2009). All these structures are here considered as a whole and named the Argentera boundary fault system (ABF).…”

Deciphering orogenic evolution

“…Several extensional features occur across the Alpine arc (Fig. 1), but they have generally been ascribed to transtensional domains (e.g., Simplon and High Durance; [Sanchez et al, 2010b] and [Sanchez et al, 2011b] ). Similarly, transpressional domains (e.g., Mont Blanc; Rolland et al, 2008) also connect with strike-slip faults.…”

“…Similarly, transpressional domains (e.g., Mont Blanc; Rolland et al, 2008) also connect with strike-slip faults. Continuous ductile to brittle deformation, seismicity and morphological offsets evidence the permanence of such motions, occurring during the Quaternary ( [Sanchez et al, 2010a] and [Sanchez et al, 2010b] ). The overall dextral motion that is observed across the Alpine arc contradicts any model of westward indentation of the European margin by the Apulian microplate.…”

A morphological study of Durance River terraces from Tallard to Avignon, South-East France

“…The reach between Mirabeau and Sisteron has the complete range of terraces revealing increased elevations upstream. This reach attracted earlier terrace studies [43,49,50] and it has been affected by recent tectonics [16,17,24,38,[51][52][53][67][68][69][70][71][72].…”

“…8) filling parts of large embayments of the lower valley. These were probably incised in the Messinian, which affected the lower Rhône, Crau and Durance into the southern parts of the Valensole Plateau [14,15,[67][68][69][70][71][72][73][74].…”