The geomorphological surveys allow for the mapping of structural landforms, slope landforms, karst landforms and fluvial and water erosion landforms.
The processes that have controlled the evolution of the escarpment are highlighted by the characteristics and degree of physical weathering, retreat of fault scarps and fault related slopes, and in particular by the analysis of transversal profiles (mostly rectilinear with more or less evident rock scarps, Fig 9) when compared to the distribution of slope depositional forms (rock landslides and talus slopes) (Fig. 7).
The variable degradation of the fault scarps and the morphology of the fault slopes (according to Brancaccio et al., 1978; Wallace, 1978; Blumetti et al., 1993; Bosi et al., 1993; Stewart and Hancock, 1994; Ascione and Cinque, 1997; Peulvast and Vanney, 2001), suggest a variable balance between the relative tectonic uplift, rejuvenating the fault scarps, and the slope denudation processes. Variability of rock resistance seems to have a control on the development of the geomorphic processes influencing the physical weathering because of the different type of stratification, degree of fracturing and local presence of cataclasite.
Moreover, it is worth noting that the upslope profile of several fault scarps is poliphasic (Fig. 9). This suggests again the cyclic alternation of relief building phases linked to tectonic activity and slope denudation events.
In the northern sector, the slope related to the Schiena d’Asino fault shows a profile made up of a clear fault scarp separating slope segments with different dip angles (Fig. 9b, c). Upslope there are many minor rock cliffs and secondary scarps, while downslope there is a talus slope. On the basis of the models proposed by the literature, particularly for the Apennine area (Demangeot, 1965; Brancaccio et al., 1978; Bosi et al., 1993; Ascione and Cinque, 1997), the slope is thought to be affected by a period of repeated tectonic activity with slope development by replacement with moderate sediment accumulation on the downfaulted block. A possible renewal of the tectonic activity would have formed the present basal fault scarp, which is only partly weathered. On the slope related to the Basal border fault, only triangular shaped fault related slopes, retreated and developed, are preserved (Fig. 9b, c; Brancaccio et al., 1978; Wallace, 1978). This clearly shows the role of drainage downcutting in the geomorphology of the lower part of the northern sector.
In the southern sector, the geomorphological characteristics of the escarpment indicate that the relative uplift has taken place mostly on the Basal border fault (Fig. 9f,g). The basal fault scarp has in many cases clearly retreated and the fault line is covered by scree (Demangeot, 1965; Ascione and Cinque, 1997). Furthermore, on the fault related slope, there are wide rock landslide bodies and remnants of relict alluvial fans, referable to Early?-Mid Pleistocene age. This suggests an early stage of strong activity on the Basal border fault, leading to slope development by wide and sudden mass movements together with early slope replacement processes on the resistant, but highly jointed rocks. This created a steep slope, mostly planar, and supplied slope deposits along the slope down to the base, which are now preserved in remnants. The continuation of the fault activity, possibly at a reduced rate, has brought about a gradual slope development, shaping the basal fault scarps with a high sediment supply that has partly covered the fault lines, the relative scarplets and the landslide bodies placed on them (Fig. 7, 9a,f,g).
Several slope landforms are mapped in the study area, even though non-homogeneously distributed: landslide scarps, rock slide bodies, talus slopes and debris cones, and also evidence of deep seated gravitational slope deformations (Fig. 7, 9). The most significant landforms are large rock landslides mapped on the escarpment. Based on the geomorphological analysis, these landforms are thought to have started as deep seated gravitational slide deformation (D.S.G.S.D.), then evolved as large landslides (Dramis and Sorriso-Valvo, 1994; Dramis et al., 1995).
The distribution of such landforms is linked to the distribution of slope and local relief. In the southern sector of the ridge, the slope and local relief is concentrated in the basal part of the slope, corresponding to the Basal border fault related slope, where the main landslide bodies are located. Poor evidence of D.S.G.S.D. is mostly located in the summit area of the ridge. In the northern sector, however, the distribution of the slope and local relief in two parallel belts seems to have partly prevented the evolution of D.S.G.S.D. into landslides. Evidence for the former is in fact distributed along the lower part of the slope, while landslides are found only on the upper part of the slope, where the gradient becomes steep again.
On the basis of morpho-lithostratigraphic correlations with the relict alluvial fan deposits, these landslides can be dated to the Early?-Mid Pleistocene. The preparatory morphostructural conditions, such as high steep slope on carbonate jointed rocks, and the trigger causes, possibly related to strong seismicity necessary for the occurrence of this type of landslide, could be linked to an important morphotectonic phase during this period. This would have had a great effect on the morphogenesis of the slope. This is confirmed by the intense tectonic activity that took place between the Early Pleistocene and the Mid-Pleistocene, highlighted by various authors in the chain and periadriatic piedmont (Dramis, 1993; Bigi et al., 1996; Centamore and Nisio, 2003). So, possibly a large part of the relief of the slope should have already been formed in the early stages of the slope evolution (Early?-Mid Pleistocene) and would have further growth in later times, as confirmed by the geometry of the foot of the slip surfaces now suspended hundreds of metres above the base of the slope (Fig. 9 b, f, g).
Karst landforms and complex origin landforms on Mt. Morrone are found in the summit areas, as well as on several ridges of the eastern-central Apennines (Montagna Grande, Mt. Godi, Mt. Sirente, Monti Peligni, Maiella). These features have been attributed by many authors to remnants of a summit paleo-landscape and to different periods of shaping from the Late Miocene (Demangeot, 1965) to Late Pliocene-Early Pleistocene (Dramis, 1993; Coltorti and Farabollini, 1995; Centamore and Nisio, 2003). When considering the surface of the mid-slope in the northern sector, it is possible to identify a displacement of the undulated surface brought about by the Schiena d’Asino fault.
In our case, the landform characteristics and the geomorphological correlations with slope forms seem to suggest that the shaping of undulated surfaces and karst depressions may have started before the activity of the landslides between C.le delle Nocelle and Pacentro. This would allow the dating of the first genesis of these forms to a period before the Early?-Mid Pleistocene.
Geomorphological analysis of the alluvial fans has provided a significant contribution to the understanding of the morphostructural evolution of the escarpment and of its base junction with the Sulmona basin. The fans have been useful in defining the morphostratigraphic relationships between the deposits on the slope and in the basin, and also because of the volcanoclastic levels and paleosoil inside them, which have allowed the deposits to be dated (Miccadei et al., 1999). The morphometric analysis of the main fan/catchment systems is summarized in Tab. 3 and Fig. 8. The law which governs the fan area/catchment area relationship (according to Oguchi and Ohmori; 1994; Oguchi, 1997; Allen and Hovius, 1998; Allen and Densmore, 2000) is: Af = 0,59 Ab0,63. Note that the constant k (0,63 in this case) has a direct relationship with the erodibility of the materials, as already indicated in Bull (1964), and an inverse relationship with the rate of the movement of the faults at the apex of the fans (Oguchi and Ohmori, 1994).
The analysis of the results that were obtained on the Montagna del Morrone SW escarpment has very clearly demonstrated how the values, and especially the value of the constant k, are among the lowest known in the relevant literature and similar to values measured on fault related slopes with a fault slip rate documented at some mm/yr (Fig. 8e,f; Allen and Hovius, 1998; Allen and Densmore, 2000). This can be only partly due to higher resistance of the bedrock and must therefore also be accounted for by the high slip rate of the slope’s basal fault. The relationships between the other morphometric parameters are also governed by a power law, as the graphs of Fig. 8 b’, c’ show. The data are more scattered, but they confirm the morphostructural considerations.
Another important aspect relates to the values for the fan area/catchment relationship, which are markedly far from the gathered data in Fig 8a, as similarly occurs for the other parameters (Fig. 8b, c, d). These values are of fan/catchment systems which have undergone noteworthy perturbation in their geometry (Basin C - Mancini fan; Basin N - Marane fan). In the first case there are several generations of fans that are built up one upon the other. The positioning of the alluvial terraces and the correlation with the terraces of the Sulmona basin demonstrate how the development of the fan itself was affected by external elements, such as the process of regressive erosion from the Gole di Popoli in the Sulmona basin (Ciccacci et al., 1999). This has extended its action headward, leading to a re-cutting of the fan and limiting its growth. The overall catchment geometry and the surface sediment distribution suggest that internal factors such as the existence of sediment storage points in the catchment, which tend to prevent the sediment supply to the fan, have also led to the fan being undersize. In the second case (Basin N) the geometry of the network, of the basin and its hypsometry (Fig. 4,5) show how a large part of the summit area of the basin itself may have been ‘captured’ during one of the recent phases of the slope’s development. This is confirmed by comparing the value of the relationships calculated and illustrated in the graphs of Fig. 8. If the area that is considered the object of capture is excluded from the calculation, the data (triangular dot) clearly approximates to the regression line (cfr. Fig. 8 a, b, c, d, e Fig. 8 a’, b’, c’, d’). Moreover, the anomalous value in the relationships studied shows that the phenomenon must have come about recently, as the re-equilibrium of the fan-basin system has not yet been achieved. Since Allen and Densmore (2000) point to re-equilibrium periods that are in fact rapid (to the order of tens of thousands of years), even considering the presence of resistant lithologies, it seems possible to date the capture to the Late-Pleistocene.
Therefore, it can be stated that the morphometric analysis of fan-basin systems can be exploited in morphostructural contexts such as the central Apennines, whether it be in morphotectonic analysis of fault related slopes or in the assessment of the conditions of equilibrium for single fan-catchment systems, which contributes to the study of local morphostructural evolution.
Finally, the geomorphological evolution of the alluvial fans in the central and southern sectors can be summarized. In the southern sector they are relatively small, with high dip angles, in clear aggradation, and are controlled by structural factors such as the resistant rocks of the catchment bedrock and the high slip rate on the Basal border fault. Considering the relationship between landslide scarps, catchments and alluvial fans, according to Blair (1999), it is possible to argue that the initiation of the catchments was due to the emplacement of the large landslide body.
In the northern sector, the alluvial fans are controlled more by interaction with the geomorphological evolution of the Popoli gorge, the northern outlet of the Sulmona basin, than by these same structural factors (Ciccacci et al., 1999). The regressive erosion due to the incision in the Popoli gorge deeply affected the alluvial fans of this sector, but only just touched those of the central sector, without reaching the southern sector.