GEOLOGICAL CONTEXT OF THE BECCA FRANCE HISTORICAL LANDSLIDE (AOSTA VALLEY, NW ITALY)

Abstract

: The wide historical Becca France landslide occurred at dawn on July 6th, 1564. It involved the Becca France ridge (2313 m a.s.l.) a few kilometers NW of the town of Aosta (Aosta Valley, NW Italy). This event buried the Clusellaz Valley floor (a Dora Baltea left tributary) destroying the populous ancient village of Thora and resulting in an unknown number of victims (120-545). The occurrence of this landslide is documented in the local historical literature. Despite this, the geological and morphological features of the accumulation have never been investigated. The aim of this work is to map the geological context of the area affected by the gravitational event, to reconstruct the phenomenon and to make a first evaluation of the causes of the landslide. The examined area is located along the contact between the Middle Penninic and the Piedmont Zone. The geological survey has allowed to distinguish, above the tectonically deeper continental unit (Gran San Bernardo Nappe system), two oceanic units (Lower and Upper TMU of the Aouilletta Unit). This area was largely interested by the Pleistocene glaciers and by a wide deep-seated gravitational slope deformation (P. Leysser DSGSD). The detachment niche extends in the right side of the Clusellaz Valley and is shaped in the E slope of the Becca France ridge. It corresponds to a very steep cirque-like scarp, very remarkable for its width (about 850 m), height (more than 500 m) and the lack of vegetation cover. It consists of two calcschist units (the lower one with prasinite) separated by a tectonic contact underlain by cargneule and gypsum. The niche cuts the doubled Becca France ridges and other gravitational evidence of the P. Leysser DSGSD. The landslide accumulation, some tens of meters thick, also exhibits a wide extent (about 1.26 km2 ). It climbed for about 80 m on the opposite side of the valley. The landslide body partially developed immediately below the detachment niche (proximal sector) is probably connected to a rockfall. Most of the accumulation is lengthened for approximately 3 km into the Clusellaz Valley floor (middle-distal sector), showing several longitudinal ridges, up to ten meters high and some hundreds meters long. This sector, having travelled down the slope a long distance, is an example of a landslide body with longitudinal ridges, linked to a rock avalanche. The slope failure was triggered by the concurrence of various predisposing causes: i) the sufficiently strong relief energy, with difference of level of 700 m between the mountain crest and the glacially-deepened valley floor; ii) the fractured and slackened bedrock connected to the DSGSD; iii) the poor geomechanical properties of the outcropping calcschist; iv) the chemical dissolution of cargneule and gypsum along the tectonic contact between the two calcschist units. An interval of extremely heavy rainfall, that produced an increase in interstitial pressure, was probably the triggering cause.