Apenninic Arc Research Articles

The Italian story of HEMS: the HEMS Association

Mountain HEMS Association was born, with a notarial deed of incorporation, in April 2008 following the initiative of 7 autonomous Region /Provinces of the Alps and Apennines: in 2005, Dr. Enrico Visetti, Anesthetist Doctor of the Aosta hospital, organized a helicopter rescue training course aimed at new doctors to be included in the local helicopter rescue. In December 2006, access to the course was offered to all the other operational bases in the Alps. The invitation was extended to the various figures operating within the Service, therefore doctors, nurses, and subsequently the CNSAS Helicopter Rescue Technicians: with the participation of new external experiences, the course objective was also modified. In 2007, 90% of the participants in the training course were Doctors and Nurses who already had experience and actively operated in the helicopter rescue in the bases of origin: the participants themselves had to bring their own ideas and experiences and simultaneously cover the role of speakers. Until now, the main objective, concentrated in the training addressed to new doctors, became, thanks to the support of the various experiences, an exchange of opinions and a training ground for discussion between the various actors. The technical mountaineering part was managed thanks to the support of the mountain rescue, always present in the external activities alongside the staff. In the theoretical part, various lines of common behavior were discussed to be adopted in the interventions of Helicopter Rescue, while in the context of the scientific discussion, the total lack of a database of helicopter rescue services in which to obtain useful information was recognized. During the Courmayer course, in the spring of 2007, the idea of creating a website was born with the aim of developing a data acquisition system from the various operational bases of the Alpine and Apennine arc: from this initiative, the establishment of the Association.

Subduction Zones Interaction Around the Adria Microplate and the Origin of the Apenninic Arc

AbstractThe study of slab‐slab interactions has come to the front of geodynamics researches to explain geological and geophysical observations from tectonically complex areas. Here we aim to better understand the geodynamics of the Central Mediterranean, where the Adria plate subducts on its two opposite sides. Additionally, the slab below the Central South Apennines has been progressively breaking off during the last 3 Myr. The role of a slab window in a single slab or in an outward dipping double‐sided subduction system is addressed by analog models at the scale of the upper mantle, realized using glucose syrup and silicone putty, to model the interaction between the Earth's mantle and the lithosphere. Our results show that the presence of a slab window modifies the pattern of mantle circulation, as well as the trench geometry and kinematics. In particular, the opening of the slab window induces the formation of two arcs flanking the window, while the mantle flows through it and turns toward the arcs, creating a small‐scale toroidal flow. The effect of a slab window is more pronounced on double subduction systems, as the outflow through the window is amplified, while internal deformation is induced in the plate by the opposite slab pull force. These experimental results suggest that the origin of the Apenninic and the Calabrian arcs is the result of the formation of a slab window, providing a new interpretation of the surface deformation and the SKS shear wave splitting pattern of the Adria microplate.

Magmatism, mantle evolution and geodynamics at the converging plate margins of Italy

The Plio-Quaternary magmatism in the Tyrrhenian Sea area exhibits wide compositional variations, which cover almost entirely those observed for volcanic rocks worldwide. Some volcanoes (Etna, Iblei, Sardinia, etc.) range from tholeiitic to Na-alkaline, and display elemental and isotope signatures typical of FOZO and EM-1 ocean-island basalts (OIB). Other volcanoes (Aeolian Arc, Italian peninsula) range from calc-alkaline–shoshonitic to K-alkaline, exhibit typical ‘subduction-related’ trace element signatures (low Ta–Nb, high Rb–Cs–LREE), and show a large range of radiogenic isotope ratios, from mantle-like in the Aeolian Arc to crustal-like in central Italy. Geochemical data suggest that OIB-type magmatism originated in lithosphere–asthenosphere sources that were unaffected by recent subduction. In contrast, subduction-related magmas come from mantle sources that underwent Eocene to present mixing with various amounts and types of subducted crustal components. Fluxing of the mantle wedge by water-rich fluids from a mid-ocean ridge basalt-type slab occurred in the southern Tyrrhenian Sea, whereas interaction between peridotite and various types of sediments occurred in central Italy. These contrasting styles of mantle contaminations relate to the nature (oceanic or continental) of the foreland, slab geometry and pre-metasomatic mantle compositions, which vary greatly along the Apennine arc and are the reason for the formation of the wide variety of orogenic magmas in Italy.

Four-dimensional tectono-stratigraphic evolution of the Southeastern peri-Tyrrhenian Basins (Margin of Calabria, Italy)

We use seismic reflection profiles of diverse resolutions integrated with high resolution bathymetry and calibrated by deep wells, piston cores and field data to derive a structural and stratigraphic interpretation of the Southeastern peri-Tyrrhenian Basins. The study area is located in the Southeastern Tyrrhenian Sea offshore Calabria. We document a tectono-stratigraphic evolution of the sedimentary basins from the Serravallian to the Present. The basins formation started during the Serravallian and the area was successively affected by E–W normal faulting and approximately N–S transfer faults in the Messinian–Lower Pleistocene time range. During the Lower Pleistocene the tectonic depressions, completely filled with sediments, were affected by folding associated with transcurrent tectonics along a NW-trending left lateral fault. Finally, during the Middle Pleistocene, a change in the slip direction produced minor pull-apart basins that overprinted the folds. The basin analysis revealed the stratigraphic signature of the tectonic events and, the uplifting and subsiding areas and the direction from which sediments arrive into the tectonic depression. The four-dimensional geologic reconstruction of the Margin of Calabria furnished new constrains for the kinematic evolution of the Southeastern peri-Tyrrhenian Basins. We suggest that the superposition of different structural styles reflects a variation of the relative velocity between the Southern Apennine Arc and the Calabria Arc during the formation of the Apennine Thrust Belt.

Early–Middle Pleistocene eastward migration of the Abruzzi Apennine (central Italy) extensional domain

The evolution of the Apennine arc is related to the flexural retreat of the Adriatic lithosphere plate, the NE migration of the compressive front and the contemporaneous rifting of the Tyrrhenian basin. Evidence of the NE migration of the orogen can also be inferred from the analysis of the intra-Apennine Quaternary extensional tectonics. Stratigraphic and structural data collected along NE–SW transects in the Abruzzi Apennines (central Italy), indeed, indicate that the westernmost normal faults (activated during the Pliocene) have to be considered inactive since the Early Pleistocene (Turano valley fault) or the beginning of the Middle Pleistocene (Salto valley and Liri valley faults). In contrast, the easternmost faults (Campo Imperatore and Mt. Morrone faults) were activated during the Early Pleistocene and are still active, as demonstrated by the displacement of Late Pleistocene–Holocene deposits and landforms. Within the innermost portion of the extensional domain, the Fucino and upper Aterno valley fault systems show persistent activity since the Pliocene. Besides the evidence of a progressively NE-shifted intra-Apennine extension, available data suggest that a period of co-existing activity of the presently inactive and newly activated faults (e.g. Liri valley and Mt. Morrone faults, respectively) occurred during the Early Pleistocene. This indicates that for a few hundred thousand years active faults in the central Apennines were more numerous than in the period of the present tectonic regime. This may have resulted in a reduced activity per fault (lower slip rates) or an increased intra-Apennine extension rate. Quaternary faults in the Abruzzi Apennines generally coincide with pre-existing discontinuities such as old thrust ramps or syn-orogenic normal faults. Based on a simple geometric criteria, i.e. direction, dip, length of the pre-existing discontinuities located east of the investigated area, and assuming a still active NE-migration of the extensional domain, it is possible to hypothesize that the old and presently inactive normal faults of the Maiella and Montagna dei Fiori anticlines may accomodate active extension of the future. The NE-ward shifting of the extensional domain would be, instead, locked if the process of lithospheric flexural retreat which drove the Neogene-Quaternary p.p. structural evolution of the Apennine chain has ended during the Early Pleistocene.

Geodetic control on recent tectonic movements in the central Mediterranean area

During the Neogene and Quaternary, the western Mediterranean geodynamics was apparently dominated by the nearly eastward migration of the Apenninic arc and the associated opening (spreading) of the back-arc basin (Tyrrhenian Sea). However, during the last 5 My, the collision of the arc with the Apulian platform led to a dramatic change in the tectonic setting of the area. As geological processes require a long period of time to register the displacements of the different blocks, it is indispensable to take into account the present-day motion given by space geodesy data analysis in order to better constrain the geological models. Geodetic motions were derived from Global Positioning System (GPS), Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) observations collected from different networks. All the geodetic solutions have been computed and combined at the Centre of Space Geodesy (CGS), at Matera, Italy. The geodetic results show a NNE motion of the Adriatic plate with a small component of counter-clockwise rotation, in good agreement with the geological and geophysical observations. In the southern Tyrrhenian area, the lengthening of the Matera–Cagliari baseline should imply that convergence cannot be considered as the driving mechanism for the Apenninic subduction process. The estimated motion of Noto is in quite good agreement with the estimated motion of the African plate.

Belt bending driven by lateral bending of subducting lithospheric slab: geophysical evidences from the northern Apennines (Italy)

Many of the mountain belts displaying a curved shape are ‘oroclines’, i.e. are produced after progressive bending of an originally straight fold and thrust belt. The bending process was previously explained as a consequence of several possible events taking place in the crustal orogenic wedge, such as occurrence of obstacles, non-coaxial deformation, and movements on wrench faults. Recent paleomagnetic results from the northern Apenninic Arc document that this belt is properly an orocline and results from Late Messinian–Early Pliocene bending of a Messinian straight belt–foredeep system. Tomographic images in turn show the presence of a high-velocity body, interpreted as subducted slab, in the upper mantle beneath the northern Apennines, between 35 and 670km depth. Down to 100km, this body displays an arcuate shape which closely mirrors the geological outlines, while it appears to be straight (and parallel to the Messinian pre-rotated belt) at depth. We explore here the possibility that the arcuate shape of the northern Apennines is a consequence, closely following in time, of much deeper processes than previously suggested, i.e. the lateral bending of the subducting Adriatic plate.

Seismic study of the ‘41st Parallel’ Fault System offshore the Campanian–Latial continental margin, Italy

A set of seismic reflection lines, collected in the Southern Tyrrhenian Sea offshore the Campanian–Latial continental margin has been selected and interpreted. The aim is to characterize: (1) the structural features of the 41st Parallel Line (41PL), an E–W elongated magnetic anomaly zone separating the Northern Tyrrhenian domain from the Southern Tyrrhenian domain; and (2) to study the relationships between the 41PL and the Ortona–Roccamonfina Line (ORL), a tectonic structure transversal to the Apennines along which the Northern Apennine Arc merges the Southern Arc. The interpretation of the seismic lines is discussed in light of the available geological (stratigraphic and structural) and geophysical (gravimetric and magnetic) information. Results show that the Campanian–Latial continental margin is characterized by a series of structural lows and highs that match with the main structures on the mainland (Mt. Massico horst, the Volturno and Garigliano depressions). The study area is characterized by ESE–WNW to E–W and NE–SW striking faults. The activity of these faults developed during Pliocene–Early Pleistocene times. ESE–WNW to E–W faults display structures consistent with strike–slip movements. These faults, which are located on the maximum gradient of the E–W elongated magnetic alignment of 41PL, are responsible for the SSE translation of the offshore sector of the Mt. Massico horst. The horizontal dislocation of Mt. Massico suggests left-lateral movements for the ESE–WNW to E–W faults. The NE–SW faults that affect the Mt. Massico horst, which represents the southern tip of ORL, show seismic features consistent with normal movements, as also revealed by inland data. Since: (1) background seismicity is virtually absent along the study area; and (2) the uppermost seismic reflectors seem unaffected by faults, it is very likely that both the ORL and 41PL fault zones are now inactive, at least in the Campanian–Latial area. Seismic data indicate that ORL is older than 41PL and support the interpretation that the 41PL represents a deep-seated transfer fault system formed in response to the different rates of opening of the Tyrrhenian Sea. The strike–slip movements along the 41PL faults and the normal movements along the ORL faults are consistent with a NW–SE extension, which is responsible for the longitudinal extension in the Southern Apennines belt.

Paleomagnetic constraints on the Plio–Pleistocene geodynamic evolution of the external central–northern Apennines (Italy)

We report on new paleomagnetic results obtained from 27 sites sampled in the Plio–Pleistocene sequences at the external front of the central–northern Apennines. Previous analyses of Miocene (Messinian) sediments indicated that the present shape of the northern Apenninic arc is due to the oroclinal bending of an originally straight belt oriented around N320° and that vertical axis rotations accompanied the migration of the thrust fronts toward the Adriatic foreland [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153–3166]. We tried to provide new paleomagnetic constraints for the timing and rates of the oroclinal bending process during the Pliocene and the Pleistocene. The results suggest that CCW rotations observed in the northern part of the studied area are possibly younger than 3 Ma. No regional rotation is recorded in the Pliocene and Pleistocene sediments from the southern part of the study area, analogously to the Messinian sediments of the ‘Acquasanta’ domain of Speranza et al. [F. Speranza et al., J. Geophys. Res. 102 (1997) 3153–3166]. A local significant CCW rotation (23°±10°) is identified in the Early Pleistocene sediments that crop out along the Adriatic coast between Ascoli and Pescara, indicating differential motion of the thrust sheets. This rotation must be younger than 1.43 Ma.

Active stress along the ne external margin of the Apennines: the Ferrara arc, northern Italy

We have analysed borehole breakout data from 12 deep wells in order to constrain the direction of the minimum and maximum horizontal stress in a part of the Po Plain, northern Italy, characterised by a ∼N–S prevailing compressional stress regime, and in order to shed light on the regional state of stress and on the correlation between the active stress field and the orientation of tectonic structures. The results have been compared with seismological data relating to 1988–1995 crustal seismicity (2.5< M d<4.8) and to the 1983 Parma ( M s=5.0) and the 1996 Reggio Emilia ( M s=5.1) events. Plio-Pleistocene mesostructural data are also described in order to better define the present-day stress field and to understand the active tectonic processes in particular stress provinces. The borehole breakout analysis, in accordance with the seismicity and mesostructural data, shows the presence of a predominant compression area, characterised by approximately N–S maximum horizontal stress, along the outer thrust of the Ferrara arc. Particularly, the breakout analysis indicates a minimum horizontal stress, N81W±22° relative to a total of eleven analysed wells, with 3746 m cumulative total length of breakout zones. Among these, nine wells are located in the same tectonic structure, consisting of an arc of asymmetric folds overthrust towards the NE. The breakout results for these wells are quite similar in terms of minimum horizontal stress direction (∼E–W oriented). The other two wells are located in the outside sector of the arc and one of them shows a different minimum horizontal stress direction, probably distinctive of another tectonic unit. On the basis of these new reliable stress indicators, the active compressive front in this area is located along the termination of the external northern Apenninic arc.

On the post-25 Ma geodynamic evolution of the western Mediterranean

During the Neogene and Quaternary western Mediterranean geodynamics were dominated by the `eastward' migration of the Apenninic arc and associated back-arc basins. The migration was controlled by retreat of the Apenninic slab and was associated with `boudinage' of the lithosphere in the back-arc area. Palaeo-reconstruction of the kinematics of the arc suggests about 775 km of migration from the Late Oligocene to present along a transect from the Gulf of Lions to Calabria. A maximum of 135 km of N–S converge occurred between Africa and Europe during the same time span. The western Mediterranean was thus mainly shaped by the migration of the slab related to west-directed subduction. It is hypothesized that minor N–S convergence deformed the arc but was not the cause of its formation.

Spatial distribution of horizontal seismic strain in the Apennines from historical earthquakes

Horizontal principal seismic strain rate axes have been calculated within a regular mesh of triangles covering the Italian peninsula in a time interval of 700 years. I have used both the method of Kostrov (1974), that requires knowledge of the seismic moment tensor of earthquakes, and the modified version provided by England and Molnar (1997) that makes use of length and kinematics of the activated faults. Seismic moment tensor of historical earthquakes can be inferred from recent literature, while length of faults has been obtained from the observation that strain drop is almost constant for large Apenninic earthquakes. Spatial strain distribution from historical earthquakes shows that the Apennines can be divided into three homogeneous structural arcs (Northern Apenninic, Southern Apenninic and Calabrian arcs) within which strain is roughly constant. Although NE-SW extension is the main deformation process along the two Apenninic arcs it involves a velocity more than five times greater in the Southern Apennines. Along the Calabrian arc, I tested the effect on the strain field of the contemporaneous ~WNW-ESE and ~NNE-SSW extension due to the longitudinal dilatation of the arc during its still ESE migration.

A kinematic model for the Plio-Quaternary evolution of the Tyrrhenian–Apenninic system: implications for rifting processes and volcanism

During the frontal accretion due to the Late Miocene–Quaternary thrusting, the interior of the Apenninic chain underwent large-scale extension which produced the opening of the Tyrrhenian Oceanic Basin, a back-arc basin in respect to the late Cenozoic Apenninic chain, and the onset of the Quaternary volcanic activity in the Campanian Plain and more generally in the Tyrrhenian area. To outline the space/time distribution and the geotectonic setting of the Tyrrhenian volcanics we approached the problem from a kinematic point of view. A synthesis of the available geological and geophysical data leads us to suggest that the progressive migration of the Apenninic Arcs is responsible for the extension phenomena which took place during the last 5 Ma. At first, the extension resulted from the kinematic interaction between the Northern Apenninic Arc and the Southern Apenninic Arc during the Late Pliocene. Then, from the Early Pleistocene the extension was controlled by the SE migration of the Southern arc only, and therefore it can be regarded as part of the general Southern Tyrrhenian extension phenomenon. Due to the intense thinning, the isotherms migrated upward very rapidly within the Toscana, Latium and Campania lithosphere where the melting point was reached, giving rise to the onset of volcanic activity at the end of the Early Pleistocene.

Occurrence of Boron-Rich Potassic Melts in the Vulsini Volcanic District, Italy: Evidence From Melt Inclusions

Boron was analyzed by nuclear microanalysis technique, using the 11B(p, α) 8Be reaction, in potassic to highly potassic melts (Mg# [Mg 2+/(Mg 2++ Fe 2+)] > 0.70) trapped as melt inclusions in olivine (Fo 91.7–90.5) phenocrysts from the Vulsini Volcanic District (northwestern Roman Province, Italy). These data evidence unusual B concentrations from 60 to 100 ppm, up to five times higher than any documented values in primitive subduction-related magmas. The melt inclusions display variable concentrations in B, K, Sr, and F, with constant ratios between elements, for a single volcanic event, that strongly suggest variations of the extents of batch partial melting of a mantle source enriched in B, K, Sr, and F. Conversely B/K and B/Cl ratios in potassic magmas (Mg# > 0.70), as exemplified by melt inclusions in olivine (Fo > 90), are variable on the scale of the whole Roman Province. These variations cannot be explained by either fractional crystallization or differing extents of the mantle source melting alone. This is also supported by the range in Th/Ta ratios (52–10) of the whole rock samples. Variability of the B/K or B/Cl ratios cannot be related to a single-stage metasomatism event. The data indicate the possible occurrence of B-rich contaminant and/or high inputs of subducted-derived components in the mantle wedge beneath the Vulsini area, within the peculiar geodynamic setting of the northern part of the Apenninic arc. In addition, by determining boron concentrations using nuclear microanalysis involving the 11B (p, α) 8Be nuclear reaction, we have illustrated the sensitivity of the nuclear reactions in the analysis of the light elements.

PROVENANCE FROM OPHIOLITES AND OCEANIC ALLOCHTONS: MODERN BEACH AND RIVER SANDS FROM LIGURIA AND THE NORTHERN APENNINES (ITALY)

The Northern Apennine arc consists of allochtonous oceanic units (Ligurids) and synorogenic terrigenous wedges, and includes both alpine (Voltri Massif) and apenninic (Bracco Unit) ophiolitic complexes and isolated slabs of oceanic lithosphere (Casanova Complex). Detritus transported by streams in this vast region, comprised between Bologna and Alessandria, and sand of Liguria beaches from Savona to La Spezia can be distinguished - through analysis of framework composition and transparent heavy mineral suites - in four provinces, further subdivided in sub-provinces and variants. The Apennine Province and the Antola Province are carbonaticlastic and characterized by recycling of Ligurid flysches, whereas the Voltri Province and the Bracco Province are dominated by ophioliticlastic detritus. Along the boundaries of major ophiolitic complexes various sub-provinces with mixed provenances characerized by significant ophioliticlastic detritus are recognized (Bormida di Spigno and Polcevera river sands, and Celle-Varazze beach sands for the Voltri Massif; Vara, Lavagna and Ghiaiaro river sands for the Bracco Unit). Ophiolite-derived material dominates the heavy mineral fraction also in the sand transported by major streams draining the Emilia Apennines (Trebbia, Nure, Taro). Framework composition and heavy mineral suites clearly differentiate the serpentineschist- and amphibole-rich (glaucophane, actinolite) detritus derived from alpine ophiolites involved in thick-skinned deformation and high-pressure metamorphism (Voltri Massif), from the serpentinite- and pyroxene-rich (diallage) detritus derived from oceanic lithosphere involved in thin-skinned deformation within the Apennine belt (Bracco Unit; Casanova Complex). In the study of ancient perisutural-basin clastic wedges, these criteria can help reconstructing subduction style and paleogeodynamic evolution of associated suture belts.

Lateral shifts of Apenninic foredeep depocentres reflecting detachment of subducted lithosphere

The hypothesis of lateral migration of slab detachment was formulated on the basis of seismic tomography results on the 3-D structure of subduction zones in the Mediterranean region [M.J.R. Wortel and W. Spakman, Structure and dynamics of subducted lithosphere in the Mediterranenan region, Proc. K. Ned. Akad. Wet. 95 (1992) 325–347]. The redistribution of slab pull forces associated with the tearing process is taken to affect foredeep development by adding a lateral component to internal–external depocentre migrations. In the case of the Apenninic Arc the direction of detachment and associated depocentre migration is expected to be southeast. This prediction has been confronted with the tectonostratigraphy of Oligocene to Recent Apenninic foredeeps. Results show there are two episodes marked by important lateral shifts of foredeep depocentres. During the early Late Miocene (Tortonian) the depocentre of the Northern Apennines foredeep shifted towards the southeast. This is evidenced by the relative positions of the depocenters of the Burdigalian–Serravallian Marnoso Arenacea and the Tortonian Umbrian–Marchean and Lazian “Minor Basins”. A further, stepwise southeastward migration can be inferred from the Plio-Pleistocene records of the Central and Southern Apennines. In a more general sense, our results indicate that slab detachment may play a significant role in the evolution of foredeeps along convergent plate boundaries.

Crustal stress regime in Italy

In order to obtain a reliable map of the present-day stress field in Italy, needed to better understand the active tectonic processes and to contribute to the assessment of seismic hazard, in 1992 we started to collect and analyze new data from borehole breakouts in deep oil and geothermal wells and focal mechanisms of earthquakes (2.5 &lt; M &lt;5) occurred in Italy between 1988 and 1995. From about 200 deep wells and 300 focal mechanisms analyzed to date, we infer that: the internal (SW) sector of the Northern Apenninic arc is extending with minimum compressional stress (Shmin) oriented ? ENE, while the external front is thrusting over the Adriatic foreland (Shmin ? NW-SE). The entire Southern Apennine is extending in NE direction (from the Tyrrhenian margin to the Apulian foreland) and compression (in the foredeep) is no longer active at the outer (NE) thrust front. Between these two arcs, an abrupt change in the tectonic regime is detected with directions of horizontal stress changing by as much as 90º in the external front, around latitude 430N. Along the Ionian side of the Calabrian arc the stress directions inferred from breakouts and focal mechanisms are scattered with a hint of rotation from N-S Shmin close to the Southern Apennines, to ~ E-W directions in the Messina Strait. In Sicily, a NW-SE direction of SHmax is evident in the Hyblean foreland, parallel to the direction of plate motion between Africa and Europe. A more complex pattern of stress directions is observed in the thrust belt zone, with rotations from the regional trend (NW í directed SHmax) to NE oriented SHmax. A predominant NW direction of SHmax is also detected in mainland Sicily from earthquake focal mechanisms, but no well data are available in this region. In the northern part of Sicily (Aeolian Islands) a ~N-S direction of SHmax is observed.

Contemporaneous extension and compression in the Northern Apennines from earthquake fault-plane solutions

SUMMARY The Northern Apenninic arc belongs to the deformation zones surrounding the Adriatic plate, which behaves as a rigid block. It is a NE-verging fold-and-thrust belt, which developed since the Miocene with the migration of an extensional-compressional pair. Previous seismological data are roughly in agreement with this deformation picture, although the outer compressional front was not clearly defined by the few available solutions. In this work we confirm the existence of two adjacent zones of contemporaneous extension and compression in the Northern Apennines, defining the extent of these two zones and their interim1 deformation better than was previously done. This is accomplished through the analysis of 125 new focal mechanisms of earthquakes (2.6 < Md < 4.8) recorded by the national network of the lstituto Nazionale di Geofisica in the period 1988-1995. The two deformation zones are clearly separated and lie very close to each other with a partial overlapping in the Emilian-Tuscan Apennines. Strikeslip events are scattered in most of the study area. Thrust-faulting earthquakes are located in the internal part of the most recently active thrust front (middle-upper Pleistocene). There is a general trend of the compressional P-axes of thrust-fault and strike-slip solutions to be perpendicular to the Apenninic direction in the external zone, while the tensional axes of the normal-fault and strike-slip solutions in the belt do not show a uniform orientation. However, clearly evident is the -ENE direction of extension in the peri-Tyrrhenian region, consistent with the direction of Shmln in the Quaternary volcanoes of this region inferred from borehole breakouts and microearthquake fault-plane solutions. We determined the orientation of the stress tensor in the two main deformation zones by inversion of the 125 fault-plane solutions determined in this study. We found that the external belt is characterized by NE-SW compression (ol horizontal, oriented N45E), while a normal-faulting regime with o3 - E-W oriented is characteristic of the Umbria-Tuscany (back-arc) region.

Rheology and seismotectonic regime in the northern central Mediterranean

The connection between thermal field and mechanical properties is analysed in the northern central Mediterranean region, extending from the Ligurian-Provençal basin to the Adriatic foredeep. As the thermal regime is still far from equilibrium in most of the tectonic units, transient thermal models are used. The temperature-depth distribution is estimated in four areas affected by the volcanic activity, which from the Neogene to the Present shifted from Corsica to the Apenninic arc. In the Adriatic foredeep, the thermal effects of the recent thrust-faulting phase in the buried sectors of the northern Apennines are taken into account. The general context consists of convergence involving westward subduction of the Adriatic plate. This process caused anti-clockwise rotation of Corsica and Sardinia, which led to formation of the Ligurian marginal basin, and also resulted in crustal doubling and overthrusting in the northern Apennines and rifting in the northern Tyrrhenian. Seismic activity is focused in the internal and external zones of the Apenninic arc, where low surface heat flux is observed, and in the western margin of the Ligurian-Provençal basin. This is a consequence not only of lateral variations in the thermal field but also of the different tectonic settings. Regional extensional patterns in the shallow crust, with minimum principal stress axes trending N60°E and E-W, are observed in the northern and in the southern sectors of the Apenninic arc, respectively. A compressional regime at depths greater than 30 km is observed below the northern sector of the arc, while to the south a change in the structure of the lithosphere is marked by a decrease in deeper seismic activity. Thrust faults and strike-slip faults with a thrust component support a compressional regime along the western margin of the Ligurian basin with maximum principal stress axis oriented N120°E. Two lithospheric cross-sections across the study region are constructed, based on structural, thermal, gravity, rheological and seismic data. There is clear evidence of the presence of the subducting slab of the Adriatic plate, corresponding to a thickening of the uppermost brittle layer. The crustal seismicity cut-off corresponds to temperatures of 320–390°C. A brittle layer of considerable thickness is present in the uppermost mantle beneath Variscan Corsica and the Adriatic foredeep, with estimated seismic cut-off temperature of about 550 ± 50°C.

Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography

In this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.