Fredi, Paola

Extensional tectonics in the inner portion of the central Apennines began during the Late Pliocene-Early Pleistocene. It resulted in the formation of chain-parallel normal fault systems, whose activity through the Quaternary led to the formation of intermontane tectonic basins; these represented traps for continental sedimentary sequences. In particular, during the Early Pleistocene most of the central Apennine depressions hosted lakes, testifying to endorheic hydrographic networks. Afterwards, lacustrine environment was replaced by fluvial regimes, aged at the Middle Pleistocene, as the hydrographic systems of the basins were captured by headward regressive erosion coming from the outermost sectors of the chain. This is testified by a strong erosional phase that cut into the lake sequences, due to deepening of streams and river incisions, and the subsequent deposition of embedded fluvial deposits. This environmental change is commonly attributed to a regional relief enhancement, as a consequence of the increase of regional uplift of the central Apennines (and geologically seen in many parts of the Apennine chain), generically aged between the upper part of the Early Pleistocene and the lower part of the Middle Pleistocene [e.g. D’Agostino et al. 2001]. The Subequana Valley and Middle Aterno Valley are part of a cluster of Quaternary tectonic depressions distributed along the current course of the Aterno River - here termed the Aterno basin system - which also includes the L’Aquila and Paganica-Castelnuovo-San Demetrio basins to the north, and the Sulmona basin to the south. They are located in innermost sector of the central Apennines, in correspondence of the chain divide. These basins are hydrographically connected by the Aterno river, one of the moste important fluvial basins of the “Adriatic domain” which runs south-easterly along the eastern side of the Subequana basin and Middle Aterno Valley, flows to the Sulmona basin through the San Venanzio gorges, where it joins to the Pescara river. The depressions are bounded towards the NE by an active normal fault system that led the formation and the tectonic evolution of the basins [Falcucci et al. 2011]. The analysis of the early Quaternary geological evolution of this depression can represent a significant case study to refine the knowledge of the Early-Middle Pleistocene tectonic/environmental transition, especially in terms of timing, taking into account that uplift rate is defined as having been larger along the chain divide. We integrated geological, geomorphological, paleomagnetic and radiometric dating with the 40Ar/39Ar method to reconstruct the morpho-stratigraphic setting of the Subequana Valley-Middle Aterno river system, defining the paleo-environmental features and chronology of the depositional and erosive events that have characterised the Quaternary geological and structural evolution of these basins. In detail, a synchronous lacustrine depositional phase was recognised in the Subequana basin and the Middle Aterno Valley. Paleomagnetic analysis performed along some sections of these deposits exposed in the Subequana valley attested a reverse magnetisation, reasonably related to the Matuyama Chron. The lacustrine sequence of the Subequana valley passes upwards to sand and gravel, testifying for the infilling of the lake and the onset of a fluvial regime that displays a direction of the drainage towards the north, i.e. opposite to the present Aterno river flow. At the topmost portion of the lake deposits, two subsequent tephra layers were identified and dated by means of 40Ar/39Ar method, at ~890ka, for the lower tephra, and ~805ka for the upper one. It is worth noting that a “short” direct magnetisation event occurred just above the lower tephra, whose significance is still under investigation. This data constraints the infilling of the lake in the Subequana valley very close to the Early-Middle Pleistocene transition. Subsequent to the infilling of the Subequana basin, a fluvial regime, characterised by a northward drainage direction – i.e. opposite to the current one –, was established. Then, after a strong erosional phase, the presence of a new coeval fluvial depositional phase within the Subequana Valley and the Middle Aterno Valley, with flow direction towards the south-east, indicates the formation of a paleo-Aterno. We identified a further fluvial sequence, embedded within the lacustrine sequence through an evident erosional surface. These deposits are found at the northern part of the Subequana valley, where they laterally pass to fluvial deposits that crop out at the southern part of the Middle Aterno river valley; this sequence shows a flow direction consistent with the current direction of the Aterno river. This morpho-stratigraphic setting, schematized in Fig. 1, indicates that after an intense erosional phase, which dissected the lake sequence, the Subequana-Middle Aterno river valley system has been hydrographically connected by the course of a paleo-Aterno river; this river flowed southerly, towards the San Venanzio gorges.Such morpho-stratigraphic interpretation is corroborated by geological observations performed in the Sulmona basin. At the outlet of the Aterno river, we found slope derived breccias, commonly attributed to the Early Pleistocene, that lay over the bedrock Their depositional geometry suggests that the breccias deposited when the Aterno river thalweg was not present yet, that is when the Subequana Valley was hosting a lake and no drainage was hydrographically connecting the valley to the Sulmona basin. Then, an alluvial fan body unconformably overlays the breccias; the fan, suspended over the Aterno river thalweg, was fed by a stream incision coinciding with the paleo-San Venanzio gorges. Lastly, a fluvial deposit is found embedded within the breccias and the alluvial fan, sourcing from the San Venanzio gorges as well. A tephra layer was found interbedded to the sedimentary body. The volcanic deposit was related to the “Pozzolane Rosse” eruption of the Colli Albani district, dated at 456±4 ka BP [Galli et al. 2010]. This fluvial deposit indicates the presence a paleo-Aterno river flowing from the Subequana valley. Therefore, the described morpho-stratigraphic framework, and the obtained chronological elements constrain the capture of the endorheic hydrographic network of the Subequana valley-Middle Aterno Valley during a time span comprised between ~800ka and ~450ka. In this perspective, it is worth noting that endorheic hydrographic networks of other basins (e.g. the Leonessa basins) located along the innermost portion of the central Apennine chain were captured during the same time span by headward erosion of streams and rivers related to the “thyrrenian hydrographic system” [e.g. Fubelli et al 2009]. This provides new elements for unravelling coupling between river incision potential and capability, and the Apennine chain uplift.

La conca Subequana costituisce una delle depressioni intermontane più orientali dell’Appennino abruzzese, situata a sud delle conche di L’Aquila e di Fossa-San Demetrio. L’evoluzione e l’assetto strutturale della conca Subequana sono stati condizionati dall’attività di un sistema di faglie che interessano il margine orientale della depressione e che sono individuabili lungo il versante occidentale del Monte Urano-Le Serre. Tale sistema di faglie, con direzione NW-SE, si segue con una certa continuità per circa 8-10 km. Questa struttura tettonica, caratterizzata da una evoluzione strutturale complessa, è stata interessata da movimenti prevalentemente distensivi nel corso del Quaternario, testimoniati dalla dislocazione di depositi continentali pleistocenici (Miccadei et al., 1997). Sono stati effettuati rilevamenti geologici e geomorfologici all’interno del bacino, con particolare riguardo alla zona del versante del monte Urano, finalizzati all’individuazione di elementi utili alla definizione dell’attività tardo-pleistocenica-olocenica di tale elemento tettonico. A tal fine sono state effettuate delle trincee geognostiche che hanno permesso di individuare la dislocazione di depositi riferibili ad un contesto cronologico relativo all’Olocene (con un evento di fagliazione in superficie posteriore a 2615±19BP), definendo così l’attività della struttura tettonica (Fig.1). Inoltre, le osservazioni effettuate hanno permesso di individuare elementi strutturali orientati NE-SW, dunque trasversali al sistema di faglie analizzato, localizzati nel settore compreso fra la conca Subequana e la terminazione meridionale della Valle dell’Aterno, nello specifico nell’area compresa fra i paesi di Castelvecchio Subequeo e Molina Aterno. Questi elementi strutturali sono responsabili della dislocazione di depositi fluviali, individuabili in destra idrografica del fiume Aterno, a poche centinaia di metri a sud di Molina Aterno. Anche se tali depositi non sono stati ancora cronologicamente vincolati, la loro dislocazione lungo queste faglie trasversali indica l’attività quaternaria (probabilmente tardo-quaternaria) di questi elementi tettonici. Tali osservazioni suggeriscono che queste faglie trasversali rappresentino delle trasfert faults (e.g. Peacock, 2002) che collegano il sistema di faglie del bacino subequano con quello della media Valle dell’Aterno, quest’ultimo già riconosciuto da diversi autori (e.g. Galadini e Galli, 2000). Inoltre, nell’ottica di ottenere maggiori informazioni sull’evoluzione quaternaria della conca Subequana, è stata condotta una campagna di misure di noise sismico ambientale nel bacino, volta ad indagare l’andamento profondo dell’interfaccia substrato carbonatico-riempimento continentale. I risultati ottenuti attraverso la tecnica dei rapporti spettrali HVSR di Nakamura, mostrano chiari picchi spettrali grazie al forte contrasto di impedenza tra i depositi quaternari ed il substrato calcareo. In particolare, lo spostamento della frequenza fondamentale di vibrazione dei sedimenti indica un approfondimento del bacino della zona tra Castel di Ieri e Castelvecchio Subequo. Tale assetto sarebbe paragonabile a quello osservato nelle depressioni tettoniche adiacenti del Fucino (Galadini e Messina, 2001) e della conca di Sulmona (Miccadei et al., 1998). Questo permetterebbe di ipotizzare un’evoluzione tettonica simile di questi bacini.

This work aims at defining the contribution, in terms of earthquake probability assessment, of the integration of Coulomb stress diffusion analysis related to an earthquake with geological studies on fault activities, investigating the case of the April 6, 2009, L’Aquila (central Italy) earthquake (Mw 6.3). The analysis of the Coulomb stress diffusion induced by this earthquake has revealed a stress increase along two poorly-investigated active normal faults in the Apennines: the Subequana fault and the Middle Aterno Valley fault. No strong seismic events have been attributed to these tectonic structures over the past 800-1000 years, and they have therefore been considered as probable seismic gaps. Geological and paleoseismological investigations have since indicated that these tectonic structures belong to the same 25-30-km-long fault system that ruptured twice during the late Holocene. The last activation occurred between the 4th-1st century B.C. and the past millennium (probably during the 2nd-1st century B.C), with the penultimate between 6381±30 BP and 3511±37 BP. The data obtained indicate that this fault system might rupture during Magnitude up to 6.8 earthquakes and that the 2009 seismic event have brought these tectonic structures about 200 years closer to failure.

Ischia is an active volcanic island in the Gulf of Naples whose history has been dominated by a calderaforming eruption (ca. 55 ka) and resurgence phenomena that have affected the caldera floor and generated a net uplift of about 900 m since 33 ka. The results of new geomorphological, stratigraphical and textural investigations of the products of gravitational movements triggered by volcano-tectonic events have been combined with the information arising from a reinterpretation of historical chronicles on natural phenomena such as earthquakes, ground deformation, gravitational movements and volcanic eruptions. The combined interpretation of all these data shows that gravitational movements, coeval to volcanic activity and uplift events related to the long-lasting resurgence, have affected the highly fractured marginal portions of the most uplifted Mt. Epomeo blocks. Such movements, mostly occurring since 3 ka, include debris avalanches; large debris flows (lahars); smaller mass movements (rock falls, slumps, debris and rock slides, and small debris flows); and deep-seated gravitational slope deformation. The occurrence of submarine deposits linked with subaerial deposits of the most voluminous mass movements clearly shows that the debris avalanches impacted on the sea. The obtained results corroborate the hypothesis that the behaviour of the Ischia volcano is based on an intimate interplay among magmatism, resurgence dynamics, fault generation, seismicity, slope oversteepening and instability, and eruptions. They also highlight that volcano-tectonically triggered mass movements are a potentially hazardous phenomena that have to be taken into account in any attempt to assess volcanic and related hazards at Ischia. Furthermore, the largest mass movements could also flow into the sea, generating tsunami waves that could impact on the island’s coast as well as on the neighbouring and densely inhabited coast of the Neapolitan area.