Elementi per una ricostruzione dell'evoluzione morfotettonica delle Prealpi Venete

Morphotectonic evolution of the Venetian Prealps: some elements

Abstract

From the geological-structural and morphotectonic viewpoints, the Venetian Prealps show many similarities of forms, partly due to the prevalence of very thick carbonate formations. However, they also show specific peculiarities, due to their different tectonic histories and to the features and directions of the thrusts which caused their evolution during the most recent phases of Alpine orogenesis. The aim of this work is to re-examine and re-order the facts and problems characterising the Venetian Prealps region, partly on the basis of newly acquired knowledge. However, it should be noted that only by means of specific research, which this report will hopefully stimulate, will it be possible to provide exhaustive answers to some of the problems noted here. 
After a brief presentation of the few elements of chronological significance regarding the Neogene and Lower and Middle Pleistocene, the geologic-structural features of the prealpine belt are illustrated. This belt is involved in at least three large-scale tectonic systems, all with different directions. In relation to the features of these systems, the following morphotectonic styles are found: a) style with monoclinal blocks, prevailing in the western sector; b) tabular style, prevailing in the central sector; c) complex style with faulted folds (north-east sector). 

One can find large tectonic scarps both limiting the monoclinal blocks and at the southern border connected with flexures or fault-folds. More particularly, smaller fault scarps are present at some edges and interior parts of the mainly tabular structures (e.g. Lessini Mountains). Some of these are the expression of recent movements, of Pleistocene and in some cases Holocene age. 

There is remarkable congruence between large forms and structures, both in the tabular areas (plateaus) and in those with monoclinal blocks and fold areas, in the sense that both orographic trend and altitude reflect tectonics directly. The Venetian Prealps show very good examples of specific tectonic forms and derived structural forms modelled by erosional processes, with results which are congruent with tectonics. Naturally, there are also exceptions, e.g., large-scale inversions of relief mainly in the areas of maximum uplift, and minor but exemplary cases in single localities. Large erosional forms which are significant in reconstructing the history of the Prealpine relief are grouped in the following way: 

a) Linear still active, erosional forms (large valleysmainly ofcanyon type): these express strong downcutting of the drainage network and were caused by the most recent large uplifts; 

b) Linear relict erosional forms: these are represented by valley trunks isolated with respect to the current drainage network,as a consequence ofdeviations and sometimes captures. These trunks are generally clearly hanging, and have less steep slopes than the active valleys; 

c) Levelled forms: these extensive surfaces are slightly undulate and are also mainly relict forms; they also preserve systems of relict valleys,indicatingpast evolutionary forms which developed near the local or general base level; their current altitude is the result of later uplift phenomena. 

Three types of relict surfaces are distinguished: a) old planation surfaces with strong structural control; b) old planation surfaces slightly controlled by structure; c) hilly areas deriving from the dissection of hypothesised planation surfaces, of which some parts, preserved near the summits of the hills, approximately at the same altitudes, may still be recognised. 

These three types of surface involve both the plateaus and the hilly areas. Some plateaus, although being erosional surfaces with strong structural control, and in practice belonging to the category of substructural surfaces, reveal the ancient erosional phases, as well as later modifications due to karstic or periglacial processes. Other, deeper trunks of relict valleys indicate significant variations in the drainage network occurring during later evolutionary phases, during which the mountainous massifs may have been further dismembered and variously tilted, as an effect of tectonic movements. These movements may also be demonstrated in some cases by anomalies in the trends of the older valleys.