La tettonica quaternaria della Conca di Fossa (L'Aquila)

Abstract

The Fossa basin, which is located in the Aterno valley on the SE of L'Aquila (Abruzzi, central Italy), is one of the most interesting intermontane basins of the Latium-Abruzzi sector of the Apennines. The Fossa basin has been the object of a thorough study, which has improved in some details the results obtained from a previous investigation by the present authors. The Fossa basin substratum is formed by a marine series deposited in shelf and reef environments during meso-cainozoic times. This series has been markedly deformed during strong compressive tectonic phases in upper Miocene-middle Pliocene times. The continental series unconformably lying over the substratum, is formed by deposits of various sedimentary cycles; these deposits form the surface of a series of variously preserved terraces which are the product of strong erosional phases within a general uplift of the area. Main elements of these terraces are their stratigraphie (allostratigraphic) units and relic surfaces. Stratigraphie units are represented by ten different formations: five of them are the product of the oldest sedimentary cycle (called "Poggio Picenze Cycle"); four formations correspond to so many younger cycles (ValPOrsa; Valle dell'lnferno; San Giovanni and San Mauro cycles) and the tenth formation is a deposit that may probably be attributed to a cycle between the Vall'Orsa and Valle dell'lnferno cycles. The youngest cycle (San Mauro) is characterized by the presence of abundant tufites of uncertain origin. Relic surfaces, which may be either erosional or depositional, represent seven different units, which may be considered - with all due terminological reservation - actual "morphostratigraphic" units. On the basis of regional correlations, the entire terraced series can be dated to the lower Pleistocene-middle Pleistocene interval. From the neotectonic point of view, the most striking aspect of Fossa is the strict correlation between (mostly normal) displacements and morphological features. This is true both for main orographic features and for minor morphological elements. This characteristic and the well-developed morpho-lithostratigraphic sequence have favoured the identification of numerous normal faults; the displacement of many of these faults has been quantitatively evaluated on the basis of particular stratigraphie horizons ("alloformations") and/or morphologic forms (relic surfaces). The general structural pattern after quaternary dislocations has been defined from the structural features of ten relatively homogeneous areas. Roughly, these areas form two markedly different zones. The western zone is characterized by an asymmetric en-échelon structure with total displacement of more than 500 m with reference to units of the oldest cycle of sedimentation. The eastern zone is an almost undeformed symmetric triangular depression. The geological reconstruction as obtained on the basis of the strati graphical, morphological and structural data that have been collected during the study, has very well shown that the zone have had a persistent character of tectonic depression during the whole Pleistocene, and that single evolutionary phases were strictly controlled by tectonics. With regards to this, data collected suggest that at least three distinct phases of tectonic activity occurred.