Considerazioni morfoneotettoniche sul versante padano dell'Appennino fra il F. Secchia e il F. Sant'Ermo in base all'analisi delle “superfici pianeggianti”

Remarks on the morphotectonic evolution of the Northern Apennines between the Secchia and Sant'Ermo rivers based on the analysis of the «nearly flat surfaces»

Abstract

In the Po Valley side of the Northern Apennines, between the Secchia and Santerno rivers, horizontal or slightly sloping areas with a flat or gently undulating morphology and of various sizes are present. All of these "nearly flat surfaces" have been mapped from aerial photographs and studied in order to assess their origin. In particular, the surfaces that are discordant with geological structure or occur regardless of lithological differences ("unconforming surfaces") have been highlighted, as they may represent remnants of old planation surfaces. The relative relief in the area has been evaluated and represented quantitatively by a relief amplitude map. The overall geomorphological features have also been considered and displayed by means of block diagrams. The results obtained allow us to recognise several sectors in the investigated area, both parallel and perpendicular to the chain axis, which have undergone different morphotectonic evolution. Specifically, the sector between the middle valleys of the Secchia and Panaro rivers shows a high number of unconforming surfaces and a low relief amplitude. Such a situation indicates that this area underwent a planation process and was subsequently affected by only moderate erosion. Previous studies on the Apennine margin close to this area report the presence of an erosional glacis formed at the end of the Middle Pleistocene. Since the paleosols present on the unconforming surfaces seem to be similar to those on the erosional glacis, a single planation process, Middle Pleistocene in age, can be postulated, which affected the Apennine margin and the median part of this chain. Taking into consideration the characteristics of the drainage pattern, this planation surface could be extended southward to include the upper course of the Rossenna river. The current morphological setting allows us to postulate that this sector has experienced post-Middle Pleistocene tectonic activity, represented by both faulting and tilting, as already partially reported in the existing neotectonic literature. Eastward, the sector between the Reno and Samoggia rivers shows only a limited number of unconforming surfaces. Nevertheless, they are extensive enough to be interpreted as resulting from a planation process, which most probably is synchronous with the previous one. In the sector between the Panaro and Samoggia rivers, the unconforming surfaces are virtually absent and the relief amplitude is low, thus indicating limited erosional activity. Therefore, it is inferred that the absence of unconforming surfaces is due to a continuous, albeit weak, uplift which prevented any planation process. The hypothesis of an independent morphological evolution of this sector is supported by its peculiar structural setting, characterised by a wide, gently undulating syncline isolated from the surrounding areas. In the sector east of the Reno river, unconforming surfaces are totally absent and this can be ascribed to vertical movements associated with the thrusting of this sector towards the Po plain. The thrusting of this sector, easily detectable in one of the block diagrams presented here, is supported by surface and subsurface evidence as widely documented in the literature. The lack of unconforming surfaces in the high Apennine belt is in agreement with the generally accepted assumption of significant uplift. This uplift could have been continuous enough to prevent any planation process, or severe enough in recent times to completely erode the former planation surface. However, the relief amplitude and the drainage pattern allow for some differentiation even in this area. In fact, a greater uplift seems to have affected the central part of the belt, in comparison with the eastern sector.