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. If this two sectors were experiencing erosion since the same time, than we cannot explain why the external flank of the chain is higher than its inner flank despite this two sectors are characterized by the same rock-type; - the analysis of the maximum, medium and minimum elevation maps suggests that the Adriatic flank of the Southern Apennines has experienced more enhanced uplift in recent times than the Tyrrhenian flank; - the Tyrrhenian and the Adriatic flanks of the chain have also other different morphological and morphometrical features, in particular the Tyrrhenian flank becomes steeper than the Adriatic flank as we move to the south, giving the typical asymmetrical feature to the Southern Apennine; - this asymmetrical feature of the chain is clearly showed by the envelop of the minimum elevation line of the five swath profiles, which enhance the presence of a Tyrrhenian steep slope and of an Adriatic gentle slope; - there is an important difference regarding the elevation of the valleys on Tyrrhenian and the Adriatic flanks, with a mean gradient that bring the valleys to reach elevations a.s.l. higher on both the flanks as we move to the south, but in general the valleys on the Tyrrhenian flank are always lower than the valleys on the Adriatica flank: such a difference suggest a more recent uplift on the Adriatic flank than on the Tyrrhenian one; The analysis of the river system shows how there is a spatial variations of the morphological and morphometrical features of the Southern Apennine rivers. If we consider the shape of the river long profiles we notice that the Tyrrhenian rivers have a clear concave-up shape with no important knickpoints, while the Adriatic rivers show a more rectilinear shape and the Ionian rivers show a less evident concave-up shape, in same cases close to the rectilinear, with evident knickpoints along the profiles. The Q (concavity index) values show a difference among the three sectors, with the Tyrrhenian rivers showing the highest value (Q=0.52), the Adriatic rivers showing a lower value (Q=0.45) and with the Ionian rivers showing the lowest value (Q=0.43). This data confirm what we noticed by the analysis of the river long profiles, in particular the Tyrrhenian rivers have a more evident concave-up shape and the Ionian rivers the less evident concave-up shape. The clear concave up shape of the Tyrrhenian rivers can be related to a more enhanced uplift on the Adriatic and Ionian slopes than on the Tyrrhenian slope. If we consider the Ks (steepness index) values, we suggest that in a geological setting such as the Southern Apennines, that is characterized by important lithological variations also in very close areas, the Ks index seems to reflect such variations more than recent rock uplift The geomorphological, morphometrical and sedimentological analysis of the Noce-Sirino-Alpi-Sant’Arcangelo transect allowed the individuation of two low relief landforms which are located on the western sector of the Sant’Arcangelo basin (700-900m a.s.l.) and in the area between the north side of Mt. Sirino, Mt. Raparo and Mt. Alpi (1200-1400m a.s.l.). The lowest surface (700-900m a.s.l.) corresponds to the eroded depositional surface of the Serracorneta Conglomerate, so it is temporally constrained at about 0.6Ma. The morphological relationships among this lower paleosurface and the highest one are not clear, we can anyway affirm that it is recognized in the area north of the Mt. Sirino and it involves both carbonates units than Lagonegro Units, so its modelling took place after the exhumation of the Mt. Sirino ended and so, considering the data we are going to talk about soon, it could temporally constrained in the Middle-Late Lower Pleistocene, and in particular between 1.5-0.6Ma. The field analysis let us to recognize the oldest units of the Qquaternary filling of the Sant’Arcangelo basin that contains clasts of the Lagonegro Units coming from the Mt. Sirino area: this unit is the subsynthem A2a (Benvenuti et al., 2006) which should be not older than 1.5Ma, so this means that at this time the Mt. Sirino was already a morphostructural highs that was experiencing erosion. This data agrees with the thermochronological analysis, which suggested that the rock exhumation of the Mt. Sirino started since 2.5Ma, and it has allowed us to give a lower temporal limit to the formation of the highest paleosurface. The analysis of the “map of the slope of the 1st order channels”, carried out within the Sant’Arcangelo basin, suggests that the area comprised between the Serrapotamo and the Sarmento river shows the highest values: these high slope values could be related to a more enhanced uplift that this area has experienced respect to the rest of the Sant’Arcangelo basin. If we combine this data with the uplift data obtained by the analysis of the marine terraces on the Ionian coast (Amato, 2000), we have that the southwestern portion of the Sant’Arcangelo basin seems to be aligned with the southernmost Ionian coast, that is the portion of the Ionian coast which has experienced a more enhanced uplift: this data could suggest a connection between these two sectors, highlighting the presence of this NW-SE oriented portion of the Southern Apennines that has been strongly uplifted. The analysis of the river terraces inside the Sinni valley has allowed the individuation of 7 orders of river terraces. The highest order, the 7th, doesn’t correspond to a real river terrace but it corresponds with the eroded depositional surface of the Serracorneta Conglomerates, whose age is of about 0.6Ma (Benvenuti et al., 2006). To date the lowest terraces we can try to correlate them with the dated marine terraces on the Ionian coast (Amato, 2000), this analysis let us to propose a late Upper Pleistocene for the 1st order terraces of the Sinni valley Considering the age of the highest river terraces and the actual elevation of the Sinni valley it is also possible to establish an incision rate of about 1mm/yr: the incision rate is always greater or equal to the uplift rate, so we can say that the uplift rate of the Sinni valley since 0.6Ma doesn’t exceed 1mm/yr. This uplift rate agrees with the uplift rate that have been proposed by Amato (2000) for the marine terraces on the Ionian coast, where the author proposed an uplift rate comprised between 0.3-1.6mm/yr. The analysis of the Noce valley river terraces has allowed the grouping of the several mapped fluvial terraces into three main orders: pre-lake, syn-lake and post-lake terraces. There are no absolute date available to date the lake time, so the age of the syn-lake terraces has been obtained using methods of relative chronology, by trying to correlate such terraces with dated marine terraces on the Tyrrhenian coast at the mouth of the Noce river: an Emilian-Sicilian age is proposed for the highest marine terraces at 170m and 140m a.s.l., while a Middle Pleistocene age is proposed for the 80m a.s.l. marine terrace. The oldest marine terraces are extended inside the Noce valley, so it means that at that time the Noce valley was already individuated. In addiction to this we have to consider that, on the basis of morphometrical considerations, such marine terraces are correlable with the river terraces at about 200m a.s.l. individuated in locality Feliceta, inside the Noce valley, and that are referred to the post-lake river terraces. Another important issue is given by the presence of Lagonegro clasts into this marine deposits and, considering that the only area from which these clasts could come from is Mt. Sirino, this data suggests that at the time of the formation of the oldest marine terraces (about 1Ma), Mt. Sirino was already experiencing erosion, so it was very close to the actual morphostructural setting. The combination of the sedimentological data of the SAnt’Arcagenlo basin and the analysis of the Noce river terraces allowed us to affirm that about 1.5-1Ma Mt. Sirino was already a morphological high which was experiencing erosion, so it means that the rock exhumation processes was finished: the comparison of these data with the thermochronological data suggest that in the period between 2.5Ma and 1.5-1Ma Mt. Sirino has experienced an enhanced rock exhumation that has brought it from an initial situation where it was covered by about 4km of rocks (2.5Ma) to a final situation where it outcrops on the Earth surface and it was subject to the exogenic processes (1.5-1Ma). This study has highlighted the importance of the morphotectonic approach in the reconstruction of the tectonic events occurred either at a regional scale or at a local scale. In particular, the numerical analysis of digital topographic data has been very useful to the large scale characterization of the Southern Apennines chain landscape. Furthermore, the integration of data provided by the digital analysis technique (e.g. swath profiles, river long profiles and the derived metrics), with the data obtained through the “classical” geomorphological approach, based on morphostructural and morphostratigraphical analyses, has provided new constraints to the reconstruction of the vertical motions which affected the entire chain during the Quaternary. The main results of this study can be summarized as follows: - long profiles, elevation of the valley bottoms and the minimum elevation map show that the outer portion of the Southern Apennines (Adriatic and Ionian slopes) has been uplifted more recently and with higher rates than its inner side (Tyrrhenian slope). These data agree with data provided by the analysis of the shorelines, marine terraces and coastal deposits observed on the Ionian belt (Amato, 2000) and the Tyrrhenian margin (Romano, 1992; Caiazzo et al., 2006), which indicate that the Ionian flank has experienced larger uplift, since the Middle Pleistocene, than the Tyrrhenian flank; - the Ionian rivers show a very steep long profile when they flow into the Sant’Arcangelo basin. This suggests that the post-orogenic uplift recognized by the marine terraces in the foredeep affected also the outer portion of the chain, involving at least the Sant’Arcangelo area; - by the comparison of the Agri and the Sinni long profile, by the map of the gradient of the first order channel, and by the Ks values (which are higher on the southern portion of the Sant’Arcangelo basin and that decrease moving towards its northern portion) it appears that the uplift in the Sant’Arcangelo area follows a N-S trend. However, further studies are necessary to discern about the reason of such different uplift; - as regards to the Middle Pleistocene to Present uplift trend, the above observations indicate that the uplift increases towards the west, probably reaching the chain axis. Coeval uplift in the Tyrrhenian margin (as estimated by elevation of Middle to Late Pleistocene marine terraces shorelines; Romano, 1992; Caiazzo et al., 2006; Filocamo, 2006) was much lower, not exceeding about 100 m. These evidences suggest that the uplift trend of the outer flank of the chain is not recognizable in the whole orogen. The western boundary of the more rapidly uplifting belt can be tentatively located in correspondence to the deep-seated normal faults that have formed the several Quaternary intramontane basins; - the stronger post-orogenic uplift occurred on the outer side of the chain since the Middle Pleistocene has determined a minor ability of the Adriatic and Ionian rivers (which experienced a continuous downcutting) to compete with the Tyrrhenian rivers. This fact is enhanced by the comparison of the valley bottoms, which are higher for the rivers flowing on the outer flank than for the rivers flowing on the inner flank of the chain. This has probably contributed, together with regressive river erosion due to the extensional tectonics on the Tyrrhenian margin (see sec. 3.5), in the decoupling between the maximum elevation line and the main divide, which is one of the peculiar features of the Southern Apennines chain; - the combination of the field analysis together with the morphological and morphometrical analysis of the Noce-Sirino-Alpi-Sant’Arcangelo transect allowed us to affirm that during the late Lower Pleistocene Mt. Sirino was already a morphological high which was experiencing erosion, so it means that the rock exhumation processes was finished; - this data is confirmed by the analysis of the river terraces of the Noce valley: the lacustrine conditions have been dated (by methods of relative chronology on the basis of the morphological relationships among the fluvial terraces of the Noce valley and dated marine terraces on the Tyrrhenian coast close to the Noce mouth) to the middle Lower Pleistocene, and the recognition of Lagonegro clasts inside the oldest marine deposits (Lower Pleistocene) suggests that at that time there was an active drainage from Mt. Sirino to the south, and so Mt. Sirino was already a morphological high subject to the erosional processes and able to produce debris, and the lake didn’t exist anymore; - in addiction to this, we have to consider that since its exhumation, Mt. Sirino corresponds to the location of the Apennine divide, representing one of the few portion of the Southern Apennines where there is a coincidence between the maximum elevation line and the divide location. This situation is partly recognized also in Monti Picentini area, where the two lines (divide and maximum elevation lines)