I Lavini di Marco: il gruppo di frane oloceniche nel contesto morfoneotettonico dell'Alta Val Lagarina (Trentino)

The Lavini di Marco: a complex of Holocene landslides in the morphoneotectonic context of the upper Val Lagarina (Val d'Adige, Southern Alps, Italy)

Abstract

This report examines a group of seven large landslides on the western slope of Mount Coni Zugna, between Rovereto and Serravalle all'Adige (Trentino, Southern Alps, Italy), the largest of which is the complex landslide known as Lavini di Marco. The morphological features of the failure and accumulation zones are described, and the chronological order of events is tentatively established by means of dating buried soils and analysis of the different degree of development of the karstic corrosion microforms. The evolution of this large slope in the regional morphotectonic context is also considered. The landslides examined here, which involved the large dip slope of a monoclinal relief of hogback type, may be classified as translational slides, some of which evolved into debris avalanches. The differences in altitude range between 300 and 1200 m and the distances travelled between a few dozen metres and about 5 km. The thicknesses involved are generally small and in any case less than approximately 100 m; the volumes mobilised range between 0.5 and 200 million m³. In some landslides or parts of them, sliding soon stopped, without reaching complete dismembering and crushing of the rocky masses, while in other cases the mobilised masses were completely crushed and transformed into debris avalanches, capable of flowing for considerable horizontal distances, even along the valley bottom (more than 1.5 km in the case of the main debris deposit). The modes of flow of these debris avalanches may be interpreted by recalling some internal structures observed in artificial sections of the quarry faces in the Lavini di Marco: concentration and "floating" of the coarser boulders on the surface; fine matrix, clearly dominant at depth; flow sub-units, which maintain their identity to a certain extent both lithologically and morphologically, forming concentric imbricated structures whose thrust planes dip towards the interior of the accumulation zone. The superficial morphology of these structures is provided either by a succession of small crescent-shaped banks, or by belts of different grain size, as shown by vegetation. From the viewpoint of typologies, spatial interrelations and relative ages, obtained from analysis of the degree of development of corrosion microforms, most of the landslides may be coeval, except for the slide at 772 m, which is older than the main slide. This is also confirmed by a C14 dating of one of its buried soils (5630 ± 80 B.P.). The large Lavini slide is of complex type: parts of the niche and of the slide surface are still active or at least subject to slow descent and deformation. The accumulation zone is composed of both the "Lavini" and the "Gran Ruina" near the houses of Pinera (marked Lavini di Marco on the topographic map). Its emplacement may be coeval with that of the Lavini slide, to which an age of 1300 B.P., calculated on a buried soil, is attributed (this age, if calibrated, may be considered compatible or quasi-compatible with the High Medieval ages of 369 and 883 A.D., found in some historical records). The degree of development of karst microforms on the rocky surfaces indicates that, in this 6 km-long sector of the left slope of the Val d'Adige, at least two separate landslide events occurred, involving several accumulations, not always clearly separate from each other. The most spectacular of these two events was the more recent, which all data and findings so far available indicate as historical. This means that the phenomenon of large-scale landslides was repeated during the Holocene, and many morphological signs indicate that the great slope is still involved in deformational processes which may produce future landslides. The problem therefore arises of careful evaluation of morphotectonic signs, which may support the correlation between earthquakes and landslides. The literature contains many indications and evidence of active tectonics in this part of the Southern Alps. Many morphological aspects of the Coni Zugna ridge point to recent tectonic movements. In particular, large-scale ridge splitting with the formation of longitudinal furrows and opposed scarps may be observed for more than 3 km along the top of the ridge. These forms, caused by slope tectonics, may be the consequence not only of gravity, but also tilting of the entire ridge, with a tendency towards uplifting in the eastern edge. The ridge is cut obliquely by NW-SE faults causing small fault scarps and flexures, expressions of recent or active tectonic deformation. In particular, in the lower part of the large dip slope on the northern margin of the main landslide there is a fault scarp facing NE, and a SW-facing flexure scarp in the intermediate part of the slope. The flexure defining one part of the edge of the landslide niche may have formed during the landslide event. In this case the large slide may have been triggered by a seismo-tectonic event involving differential movements along the fault and the flexure. The collected data therefore tend to support the hypothesis of a seismo-tectonic trigger mechanism, which would also plausibly explain the contemporaneity of several contiguous landslides.