Long term morphotectonic evolution of the Southern Apennines

ABSTRACT The main goal of this work is the study of the morphological and morphometrical features of the Southern Apennines thrust belt †" foredeep system, carrying out both a large scale and a small scale analyses of areas of high relevance, with the aim to determine new morphological and morphometrical constraints to the reconstruction of the main morphotectonic events that have interested the chain. This type of research has been based on the idea to compare such data with new thermochronological data that have been produced in many areas of the chain (Aldega et al., 2003; Mazzoli et al., 2006; Mazzoli et al., 2008), which highlighted the role of the exhumation processes in the evolution of the thrust belt. The thermochronological data indicate that the exhumation processes started about 10Ma and they have been active since recent time (in the last million years), resulting contemporary to the main morphogenetic events responsible of the actual morphostructural setting of the chain. The Southern Apennines chain formed has a consequence of the Neogene collision of the African (and in particular, the Adria microplate) and Euroasiatic plates, with the subduction of the Adria microplate beneath the Euroasiatic plate. The morphostructural setting of the Southern Apennines has been determined by its complex tectonic history (with the occurrence of both thrust faults and normal faults) and by the erosional processes that have sculptured the topography, also considering that the landforms are strongly influenced by the rock type too. In the last years the development of new techniques of analysis has provided new constraints useful to the reconstruction of the morphotectonic history of the Southern Apennines. This techniques are based on thermochronological analysis, and in particular on the Apatite Fission Tracks, the Ur-Th-He series, the Vitrinite Reflectance, the Clay Mineralogy and the Fluid Inclusions. These data, that have been extensively produced in the whole chain, have pointed out the attention on tectonic exhumation processes (we mean by rock exhumation a variation of the position of a rock in relation to the air-topograhy interface), which have determined rock uplift of thousands of meters in the last 2-3Ma (Aldega et al., 2003; Mazzoli et al., 2006; Mazzoli et al., 2008). The individuation of so enhanced vertical and horizontal tectonic motions in recent times has expected to have interacted with the processes responsible of the exogenic modelling of the topography, and they have probably played an important role in the morphotectonic evolution of the chain, leaving their signature in the topography. The research has been based on large scale geomorphological and morphometrical techniques of analysis, that have been used with the aim to describe the main morphological and morphometrical features of the chain, to compare the features of the Tyrrhenian, Adriatic and Ionian slopes of the Southern Apennines, and to relate these features with the proposed morphotectonic events. The large scale analysis has been accompanied by the small scale analysis of a selected transect namely the Noce-Sirino-Alpi-Sant'Arcangelo transect. This transect has been chosen because of the particular features that make this area one of the most relevant portion of the chain in order to reconstruct the morphotectonic evolution of the Southern Apennines, and in particular, to investigate the role played by the rock exhumation processes in the evolution of the relief. This transect assumes a high relevance because: - it includes tectonic units which have been exhumed in recent times; - it preserves a stratigraphical and morpho-stratigraphical record which is almost continuous both temporally (from the Middle Pliocene to the whole Quaternary) and spatially (from the Tyrrhenian to the Adriatic coasts); - the topography, in this portion of the Southern Apennines, has been only slightly dismembered by the post-orogenic extensional tectonic, and may be considered resulting by the major geodynamic processes (shortening, thrusting, extension and exhumation). One of the main parameters that can influence the geomorphological and morphometrical features of a determined region is the lithology, or more correctly the bedrock resistance to erosion. In fact, parameters such as the elevation, local relief, steepness, presence of knickpoints are strongly controlled by bedrock erodibility. As a result, the first step in the analysis of the landscape is represented by a clear depiction of the space distribution of the rock types with different erodibility. For the above mentioned reason, a “Map of the Morphostructural Units of the Southern Apennines” has been created. This map is a simplification of the “Geological Map of the Southern Apennines” in scale 1:250000, in fact the 81 formations distinguished in the “Geological Map of the Southern Apennines” have been reduced into the 20 morphostructural units which have been grouped based on the estimation of erodibility of each rock type relative to other rock types. The erodibility degree was basically assigned by the observations of the features (e.g. steepness, degree of development of the upper convexity/basal concavity of hillslopes, average elevation, etc.) associated with the various bedrocks. As regards the Quaternary deposits, these were grouped based on different criteria. Taking into account the main goal of this study, which consists in the reconstruction of the Plio-Quaternary relative/absolute vertical motions of the Southern Apennines, the grouping of the different Quaternary stratigraphical units was based on the depositional environment (marine vs continental), degree of correlation of the different units with the original depositional environment (i.e. whether and to what degree they are displaced/dissected), and tectonic context (e.g. peri-tyrrhenian grabens, foredeep and intramontane basins deposits). The large scale geomorphological analysis of the Southern Apennines has been based on the determination of the following parameters: elevation map and the derived maximum, medium and minimum elevation maps, swath profiles and the derived relief curves, analysis of the river long profiles and the derived parameters (drainage area vs distance, Stream Gradient Index, steepness (ks) and concavity (Q) indexes, slope of the first order channels). This type of analysis enhance a series of particular feature of the Southern Apennines that can be summarized as follows: - the minimum elevation map can be separated in two different sector, respectively located north and south of the hereinafter named “Sele-Ofanto line”: the north sector is characterized by the coincidence of the highest values with the apenninic divide, while the southern sector is characterized by the presence of a wide area with high values in the minimum elevations, which moves from the apenninic divide to the east, involving the foredeep (and the Lavello high) and the Murge-Salento area ; - this data regarding the minimum elevation map is very interesting in particular when compared with the “Map of the Morphostructural Units of the Southern Apennines”: this comparison show that the valleys on the Adriatic flank are higher than the valleys on the Tyrrhenian flank despite the Adriatic flank is characterized by the outcropping of very weak lithologies (external flyschs and Quaternary filling of the foredeep); - the minimum elevation map could be so considered a good representation of the differential uplift at the orogen scale; this fact let the maximum elevation map to play a less relevant role when we want to interpret it in terms of uplift, and it can be more correctly considered as a good representation of the distribution of the tectonic Quaternary lows; - the medium elevation map clearly enhance the presence of the hard carbonatic highs on the Tyrrhenian slope respect to more eroded surrounding areas where weaker lithologies crop out. This means that the Tyrrhenian slope has experienced a more intense erosion, or even that it is experiencing erosion since older times than the Adriatic flank (where the same weak lithologies crop out), and that the amount of eroded rock volumes is higher on the Tyrrhenian flank than on the Adriatic flank.