Geomorfologia e neotettonica della Val Gallenca ed aree limitrofe nell'Alto Canavese

Abstract

The research now brought to a conclusion with the publication of the Geomorphological and Neotectonic Map of the Val Gallenca has produced the following main results: - The region's dominant alteration processes, here conventionally referred to Middle Pleistocene, involve both the crystalline bedrock and the unconsolidated deposits. - The relicts of the pedogenesis products, here conventionally referred as Lower Pleistocene, are relatively scanty. Except in small areas, they are covered by and in continuity with Middle Pleistocene deposits. Substantial local relief stability can, therefore, be assumed down to the end of the Middle Pleistocene. - With the exception of the parts immediately underlying the ridges of the left side of the Val Gallenca, the whole area was subjected to repeated, intensive terracing from the commencement of Upper Pleistocene. This was not due to changes in the base level of the Valle dell'Orco, into which the Gallenca flows, since the last giaciers never reached high levels in the Cuorgne area, did not penetrated the Lower Val Gallenca and did not close it with occlusional deposits. Glacial action, what is more, cannot be postulated for the terraces on the SE side of the Belmonte Ridge because this faces the plain and was not reached by the glaciers, except for a tiny portion in its northern part. The terraces must thus be entirely attributed to epeirogenetic movements. - Epeirogenesis took place in the form of gradual, repeated upliftings completed in a differential way, through a thick network of subvertical neotectonic faults that divide the area into numerous prisms with independent mechanical behaviour. - The picture is mainly one of upliftings af various extent locally punctuated by pauses or inversions. This is demonstrated by the small zones to the NW of Valperga and around Rivara, where the Lower Pleistocene deposits, instead of being below the Middle Pleistocene as usual, have been terraced by them. - As to the Belmonte Ridge itself, the readily recognisable composition of its granite clasts shows that it was an outcrop in the Lower Pleistocene, buried under the Middle Pleistocene glaciofluvial deposits and only returned to be subjected to erosion in the upper Pleistocene. - It can be demonstrated that the substantially positive epeirogenesis of the sector reached its maximum development during the last Glacials. There is plenty of evidence to show that it has continued in an attenuated form down to the Present. The interest of this evidence, indeed, is such as to entitle it to be set out in detail below: 1) All the watercourses, even their terminal sections, are in active erosion and still producing terraces, often differently accentuated along their beds. 2) There are two sets of rapids in the bed of the Oreo, corresponding to neotectonic faults, by which even the Present-day alluvia are deformed. 3) The two small elliptical areas to the W of Rivara, indicated with a specific sign near the southern edge of the map (one of which also extends beyond it), are traversed by two meandering, ultra-excavated streams, clearly due to ongoing uplifting of their beds, resulting in the doming that forms the initial stage of an entrenched meanders morphology. 4) There is no lack of pivoting of the Present alluvia. Of greater significance, however, is the phenomenon, clearly visible in the Viano Valley, for which I have felt it necessary to coin the term "geomorphological flexure". Due to this, the slope of the fans, on which it is located, displays veritable steps for which neotectonic faults correspond on the valley sides. - In the Upper Val Gallenca there were local glaciers in the Lower Pleistocene and more extensive ones in the Middle Pleistocene. These glaciers were fed from the slopes SE of Mt Soglio whereas the only sure sign of Wurmian glaciers is found at the Pian della Mussa, a little to the East. In addition to the scattered morainal deposits, there are both Middle and Upper Pleistocene morainic ridges. Caution is, naturally, needed in interpreting these glacial features, since apart from the usual climatic factors account must be taken, perhaps even more so, of changes in height on topography caused by uplifting. To Neotectonics, for example, one can ascribe the lack of modellings and glacial deposits that, on the same left-hand side, are stilt observable further to the E on the slopes of Rocche San Martin o. - No less problematical is the reconstruction of the Viana Valley glaciers. Here there are Lower Pleistocene morainal deposits accompanied by a morainic ridge, whereas Middle and Upper Pleistocene fronts ere lacking. Moreover, at the head of the valley, now proportionally restricted with respect to the deposits that produced it, there are none of the forms reminescent of a glacial cirque. I am of the opinion that the morphology of the upper part of the valley retains very few of the characteristics it had in the Pleistocene. - A singular feature of the middie Val Gallenca is the presence of two imposing masses of boulders scattered over wide areas that cannot readily ascribed to either moraines or landslides. The first lies along the course of the Gallenca to the S of Mezzavilla. It appears to be of Middle Pleistocene age, but could also be Lower Pleistocenic. I have used the term "marocca" to describe it as its shape is similar to that of the typical "maroccas" of the Trentino, though its origin, which is certainly due to glacial transport, is probably partly different. For both, the first and the second deposit which is more recent and consists of a more scattered accumulation of boulders on the Upper Pleistocene alluvia of the Lower Valle Buasca, it is much more likely, in fact, that the collapse was linked to seismic events contemporaneous with the strong uplifting of the uphill slopes. - The energy of the relief, but above all the division of all the minor stream beds into segments, some in equilibrium, the majority in ultra-excavation (with rapids and waterfalis), definitely show that epeirogenesis occurred through numerous faults and starting from numerous terraced surfaces that repeatedly served as temporary base levels. - Since in view of the heights of the scarps, though with due allowance for subsequent deformations, the terracing may be regarded as of the order of 200 metres, its mean uplifting rate is 2.5 mm/a, if it is thought to spread over the Upper Pleistocene and the Holocene, or 3 mm/a if concentrated in the Upper Pleistocene; the first value, or one between the two,would appear the most acceptable.