Apenninic Belt Research Articles

Hypocenter location accuracy and seismicity distribution in the Central Apennines (Italy)

In this paper, we analyse the seismicity distribution in the Central Apennines (Italy) using the recordings of the Rete Sismometrica Marchigiana (RSM). In particular, the selected events are relocated using a 1-D model calculated by means of an inversion procedure. The robustness of the 1-D model and the location accuracy are tested. The capability of the RSM to well constrain crustal and subcrustal events in the studied area is discussed. We find that in the inner side of the chain the seismicity lies in the upper crustal layers, following the structural trend of the Apenninic belt. A W-deepening of the events is observed in the Apenninic foredeep, where the seismicity is mainly confined in the lower crust. This evidence implies the deepening of the brittle to ductile transition. Some well-located subcrustal events are found. Their locations seem to confirm the W-dipping subduction of the Adriatic lithosphere beneath the Apennines.

Geodynamic implications of “subduction related” magmatism: insights from the Tyrrhenian–Apennines region

The location and age of potassic and ultra-potassic magmatism, mostly recognized as derived from mantle sources hybridized by subducted crustal rocks, in the Tyrrhenian–Apennines system do not show any plausible causal relationship with the evolutionary history of subduction processes, reconstructed from the time pattern of accretionary activity in the Apenninic belt. On the other hand, since magmatism in the study area was always associated in space and time with major phases of crustal stretching, one could think that the uprise of magmas through the uppermost lithosphere is strictly conditioned by the occurrence of extensional tectonics and, in particular, by the formation of significant fractures in the upper brittle crust. This would imply that using the distribution of “subduction related” magmatism for recognizing the timing and location of paleosubduction processes could be misleading. The geochemical features of these magmas can provide information on the kind of tectonophysical processes which took place in the mantle during the previous evolution but, as far as we know, the delay between mantle hybridization and magmatic activity cannot easily be assessed.

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.

Contribution of numeric dynamic modelling to the understanding of the seismotectonic regime of the northern Apennines

In this paper we investigate the forces possibly active in the area of the northern Apennines by means of two-dimensional finite element modelling assuming a viscoelastic rheology. The forces included in the models are related to continental convergence between Africa and Eurasia, to negative buoyancy of the subducted Adriatic lithosphere and to positive buoyancy of anomalously hot mantle material underneath Tuscany. Model-predicted stress distribution is compared with seismotectonic data available for the area of the northern Apennines and with earthquakes distribution. Our results indicate that continental convergence cannot be the only mechanism acting in the study area since it causes insignificant subcrustal stresses, in disagreement with intermediate seismicity observed under the Apennines. Asthenospheric upwelling under Tuscany (back-arc region) is shown to play a crucial role in the present-day dynamics of the Tyrrhenian–Apennines. Positive buoyancy causes an upwards and eastwards flow that generates extensional bending stresses in Tuscany and at the top of the slab under the Apennines. Horizontal pushing of the slab is proposed as an alternative mechanism to slab pull to generate subcrustal stresses. Activation of slab pull does not result in a significant change in orientation of the principal stress axes at shallow depths with respect to the model in which only asthenospheric diapirism is active. Therefore, the existence of a continuous gravitationally sinking slab cannot be ruled out on the basis of comparison with seismotectonic observations. Both models successfully reproduce extension in Tuscany and compression in the outer margin of the Apenninic belt, but fail to reproduce extension along the inner portion of the chain. Our modelling highlights the importance of better constraining the lateral extent of the asthenospheric diapir in order to find out whether the extension and tectonic uplift in the Apennines is caused by asthenospheric upwelling as in the area of Tuscany.

Active stress map of Italy

We present a new map of the present‐day stress field in Italy obtained from all the available data. The map reports 200 horizontal stress directions inferred from 109 borehole breakout data, 44 centroid moment tensor solutions, 34 other focal mechanisms, most of which are from polarity distributions, seven stress inversions of microearthquake data, two averages of T and P axes of earthquake focal mechanisms in zones of diffuse seismic activity, and four fault slip data. The integration of breakout data, which yield horizontal stress directions, with fault plane solutions, which reflect the stress regime, allows us to obtain an improved map of the present‐day stress in Italy. This stress field map can be used for a better comprehension of active tectonic processes, for seismic hazard assessment, and to foresee the behavior of faults recognized with other methods. Stress directions obtained from different data, although relative to different depth intervals (e.g., 0–7 km for breakouts and 0–20 km for most of the earthquakes) and to different tectonic units, are consistent. Since many regions in Italy are characterized by an extensional stress regime, we report the minimum horizontal stress (Shmin) orientations. The map shows that an extensional regime affects most of the Apenninic belt. Conversely, a compressional (or transpressional) regime characterizes the eastern Alps, the eastern side of the northern Apennines, and the southern Tyrrhenian to northern Sicily zone. An abrupt change in stress directions marks the transition between northern and southern Apennines, suggesting that the two arcs are characterized by a different tectonic setting and recent evolution. In this paper we report all the data analyzed to date, with their geographic coordinates and average stress directions, and we describe the main stress provinces in Italy in the framework of the tectonic evolution of the region.

Foredeep geometries at the front of the Apennines in the Ionian Sea (central Mediterranean)

A new regional seismic section in the Ionian Sea across the Apennines belt and related foreland shows how the present foredeep geometry may be an example for interpreting discontinuous, tilted and deformed earlier basins now incorporated in the internal parts of the accretionary wedge. Onlap stratal terminations of the foredeep sediments on the foreland monocline may simulate downlap geometries once involved and tilted by back-thrusting. The geometry of the Ionian foredeep is controlled by the dip of the regional monocline, and internally by the variable dip and length of the limb of the external fold, which may be either foreland-verging or hinterland-verging. The generation of a new fold within the foredeep splits the basin into a new foredeep toward the foreland and a thrust-top basin toward the hinterland. The thrust-top basin dimension is primarily controlled by the distance between the two folds and related thrusts at its margins. The foredeep, in its overall history, is composed by a series of concave heterogeneous lenses, progressively displaced and piled up toward the foreland to the east. The formation of each sedimentary lens is controlled by the development of a new fold and the contemporaneous retreat of the regional monocline which creates new accommodation space. The complex 3D geometry of the Apennines foredeep mainly results from lateral variations of the latter parameters, as well as variations in sediment supply and eustasy.

Depth and modes of Pliocene–Pleistocene crustal extension of the Apennines (Italy)

Since early Pliocene times the Apenninic chain has been dissected by normal faults propagating towards the Adriatic foreland. In the Tyrrhenian Sea extension involved deep crustal sections, whereas in the ‘Central Apennines Downfaulted Area’ it affected the shallow crust. The Tyrrhenian back‐arc domain is connected to the overall flexural retreat of the Adriatic–Apulia plate in front of the Apenninic collisional wedge. In the outer Apenninic belt thin‐skinned delamination and gravitational collapse occurred in the hanging wall of a thickened wedge, overthrusting the uplifted, buoyant crust of the Apulia foreland. Differential sinking velocity of the foreland plate results from the inherited competence contrast between the rigid Gargano–Apulia carbonate platform to the south, and the low‐competence lithology of the pelagic sequence of the Adriatic basin to the north. During late Messinian–early Pliocene times this palaeogeographical boundary acted as a lithospheric tear, separating segments of the Apulia plate subjected to different subduction modes.

Syn- versus post-orogenic extension: the case study of Giglio Island (Northern Tyrrhenian Sea, Italy)

Combined structural, petrographic and fluid inclusion data from Giglio Island (Northern Tyrrhenian Sea, Italy) are used here to constrain kinematics, timing and thermal regimes associated with the post-thickening tectonic evolution of the innermost sector of the Northern Apennine belt. Giglio Island is composed of Pliocene intrusives in tectonic contact with a poorly retrograded HP/LT, Fe-carpholite bearing nappe pile. Peak pressure estimates for the carpholite bearing Verrucano metasediments range between 10 and 14 kbar for temperatures lower than 350°C. Data from this study indicate a nearly isothermal exhumation of the Giglio HP metamorphic complex, which occurred during the Early–Middle Miocene as a consequence of a top-to-the-east syn-orogenic extensional shear. This resulted in the transposition of the HP fabric while the lithostatic pressure was decreasing, with a final re-equilibration in the greenschist facies along a cool geothermal-gradient regime. This relatively cold regime is referred to the continuous underthrusting of the cool continental Adria plate during exhumation. Afterwards, during the Late Miocene–Early Pliocene, the onset of the post-orogenic crustal thinning also showed an eastward asymmetry in an HT/LP back-arc environment, leading to partial melting of previously exhumed deep crustal levels and genesis and emplacement of the Giglio intrusives. A comparison with other peri-Tyrrhenian regions allows us to propose that the post-thickening Miocene extension in the Northern Apennines was driven by the collapse of the previously thickened Northern Apennine orogenic wedge, combined with the progressive eastward retreat of the Adria subduction boundary.

Recent tectonic evolution and present stress in the Northern Apennines (Italy)

The present‐day stress field and its recent tectonic evolution in the Northern Apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data. We have considered 86 wells for breakout analysis, with depths down to 6–7 km, 125 fault plane solutions of crustal earthquakes with M<5 that occurred between 1988 and 1995 in the Northern and Central Apennines, and data of stronger earthquakes (M≤6) reported in other studies. The Tyrrhenian coastal region and the Apenninic belt are characterized by Shmin direction mainly trending NE–SW, with predominantly normal fault plane solutions. Along the outer front of the belt and the Adriatic offshore, Shmin is oriented NW–SE, and focal solutions are thrust or strike‐slip, with maximum compression around NE–SW. Conversely, south of 43°N, breakouts evidence an orthogonal direction of horizontal compression (NW–SE), following the Southern Apennine trend, where a widespread NE–SW extension was recognized by previous investigations. Comparing these results to the recent tectonic evolution inferred from structural geology, we argue that the extension‐compression pair, characteristic of the post‐Tortonian evolution of the mountain belt, has been migrating in time from late Miocene to Present only in the northern sector of the arc, whereas the southern sector underwent a generalized extension, at least since middle Pleistocene. The striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension‐compression pair has been controlled by progressive retreat of the slab.

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.

Spatial distribution of scalar seismic moment release in Italy (1983-1996): seismotectonic implications for the Apennines

We analyzed the distribution of seismic moment in Italy, computed from instrumental seismicity recorded by the Italian National Seismic Network in the past 14 years, to map the areas where seismic deformation processes have been active in this time interval. Seismic moment is the most suitable parameter to quantify earthquake size. It is related to the geometric characteristics of faults, to seismic energy and it is a quantity that can be summed and represented in its cumulative value. The maps of seismic moment distribution display more information than epicentral maps, better showing actively deforming regions. They provide further and original evidence for the existence, within the Apenninic belt, of two regions (north and south) characterized by different seismic energy release. The seismic moment is almost continuously and homogeneously distributed all along the belt in the Northern Apennines, whereas in the Southern Apennines it is concentrated in the zones recently activated by mainshock-aftershock seismic sequences. Seismic deformation takes place in a 30 km narrow belt along the Apennines, while in the transition zone between the Northern and Southern Apennines this belt is about 100 km wide. Comparing instrumental seismic moment release with the areas struck by the largest historical earthquakes which have occurred in the past six centuries, we qualitatively extended back to the past the information on where the seismic deformation occurs. In the Southern Apennines background energy release from instrumental seismicity is very low in the several areas hit by large historical earthquakes, suggesting that these seismogenic zones are currently quiescent.

Minimum 1D velocity models in Central and Southern Italy: a contribution to better constrain hypocentral determinations

We computed one-dimensional ( I D) velocity models and station corrections for Centrai and Southern Italy, in- verting re-picked P-wave alTival times recorded by the Istituto Nazionale di Geofisica seismic network. The re-picked data yield resolved P-wave velocity results and proved to be more suited than bulletin data for de- tailed tomographic studies. Using the improved velocity models, we relocated the most significant earthquakes which occurt.ed in the Apennines in the past 7 years, achieving constrained hypocentral determinations for events within most of the Apenninic belt. The interpretation of the obtained lD velocity models allows us to infer interesting features on the deep structure of the Apennines. Smooth velocity gradients with depth and low P-wave velocities are ob,'ierved beneath the Apennines. We believe that our results are effective to constrain hypocentral locations in Italy and may represent a first step towards more detailed seismotectonic analyses.

Present-day stress field and active tectonics in southern peninsular Italy

SUMMARY A systematic collection of present-day stress directions has allowed us to gain new insights into the recent tectonic evolution of the Italian Peninsula. Horizontal stress directions from breakouts in 40 deep wells drilled for oil exploration have been compared to compressional and tensional axes of 17 earthquake fault-plane solutions (4.2 < M < 7) and 17 stress axes obtained from structural analysis on Quaternary rocks. Notwithstanding the different scales and depths of the data considered in this worknear-surface for structural data, 0-6 km depth for breakouts, GZ 3-20 km for earthquake focal mechanisms-the three distributions are surprisingly similar, all showing a predominant northeast-directed minimum horizontal stress in a broad region from the Tyrrhenian backarc through the Apenninic belt and up to the Apulian foreland. The belt, which corresponds to a region of high elevation, contemporary uplift and strong seismic release, with earthquakes as large as M=7, is characterized by a normalfaulting stress regime. The breakout data demonstrate that the NE-oriented extension associated with the seismogenic normal faults of the southern Apennines is determined by a regional stress with o,=NE, in agreement with the seismological evidence that these faults are young ( post-Middle Pleistocene). The breakout data show that even the foredeep and the most internal foreland region, where the ( Apulian) foreland crops out, are dominated by northeast-oriented extension, even if we cannot establish the stress regime of this region due to the lack of seismicity. This suggests that a model in which the underthrusting Adriatic plate is in compression and in flexure, generating extension in the upper (Apenninic) plate, is not applicable, unless what we observe (NE-directed Shmin) is limited to the upper part of the Adriatic plate, while its deeper portion might be in compression. Compared to the recent compressional tectonics that affected parts of the study region in the Upper Pliocene and Lower-Middle Pleistocene, it seems likely that an important change in the stress field occurred in this area during the Quaternary, most probably around 0.7 Ma. Considering the main tectonic processes acting in southern Italy-NNW convergence between Africa and Eurasia and subduction of Ionian and Adriatic lithosphere with associated slab pull and back-arc riftingwe speculate that the extension observed in southern peninsular Italy can be explained either by a slab window or by a buoyant subducted continental lithosphere in the upper mantle between the two active arcs, the northern Apennines and the Calabrian arc.

Upper-crustal structure of the Benevento area (southern Italy): fault heterogeneities and potential for large earthquakes

SUMMARY The Benevento region is part of the southern Apennines seismogenic belt, which experienced large destructive seismic events both in historical and in recent times. The study area lies at the northern end of the Irpinia fault, which ruptured in 1980 with a Ms= 6.9 normal faulting event, which caused about 3000 casualties. The aims of this paper are to image lateral heterogeneities in the upper crust of the Benevento region, and to try to identify the fault segments that are expected to generate such large earthquakes. This work is motivated by the recognition that lithological heterogeneities along major fault zones, inferred from velocity anomalies, reflect the presence of fault patches that behave differently during large rupture episodes. In this paper, we define the crustal structure of the Benevento region by using the background seismicity recorded during 1991 and 1992 by a local seismic array. These data offer a unique opportunity to investigate the presence of structural discontinuities of a major seismogenic zone before the occurrence of the next large earthquake. The main result that we obtained is the delineation of two NW-trending high-velocity zones (HVZs) in the upper crust beneath the Matese limestone massif. These high velocities are interpreted as high-strength regions that extend for 30-40 km down to at least 12 km depth. The correspondence of these HVZs with the maximum intensity regions of historical earthquakes (1688 AD, 1805 AD) suggests that these anomalies delineate the extent of two fault segments of the southern Apenninic belt capable of generating M= 6.5−7 earthquakes. The lateral offset observed between the two segments from tomographic results and isoseismal areas is possibly related to transverse right-lateral faults.

Interfering paths of deformation and development of arcs in the fold‐and‐thrust belt of the central Apennines (Italy)

In the foreland fold‐and‐thrust belt of the central Apennines, the major thrust sheets design systems of arcs, interfering with noncoaxial trends. Localization of shortening by thrusting and shearing appears to be controlled by the squeezing of low‐competence, basinal units (Marche, Gran Sasso‐Monte Picca and Monte Genzana) in between the rigid blocks of the Lazio‐Abruzzi and Adriatic carbonate platforms. The geometrical interlocking has been resolved by differential rotations of the superposed thrust sheets, as also substantiated by paleomagnetic data. The structural analysis and the age of the syntectonic siliciclastic deposits suggest an irregular sequencing of superposed contractional and extensional events during late Miocene‐middle Pliocene times. Interfering kinematic paths characterize the arcuate belts, where slip partitioning has adjusted the coeval deformation of the noncoaxially arranged tectonic units. In the arcuate Gran Sasso‐Monte Picca thrust belt, out‐of‐sequence thrusting, displacement transfer on high‐angle, strike‐slip faults and growth of high‐angle normal faults behind the most rotated blocks have resulted in the back and forth superposition of contractional and extensional structures in adjacent domains. Our kinematic reconstruction of the central Apennines indicates that it is impossible to perform the retrodeformation of tectonic events by assuming a regular outward progression of deformation pulses. In such a strongly arcuate belt, the irregular trajectories of the displacement field and the complex overprinting relations between different sets of structures reflect the kinematic accommodation of blocks with contrasting rigidity and are not fitted easily within a regionally smoothed stress field.

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.

Seismic anisotropy beneath the Northern Apennines (Italy) and its tectonic implications

We examined shear wave splitting in SKS and S phases from 22 teleseisms at 10 temporary stations on a transect across the Northern Apenninic arc. The array, near 43°N, spans from Corsica Island across the Tyrrhenian region and the Apenninic belt to the Adriatic coast. We applied particle motion, covariance matrix decomposition, and cross correlation methods to estimate the polarization direction of the fast split‐shear wave (ϕ) and the delay time between split phases (δt). Most of the analyzed shear waves show clear evidence of splitting. The ϕ in the Adriatic foreland and in the Apennines are approximately parallel to the strike of the mountain belt (NW‐SE). The largest δt correspond to the highest elevations, suggesting that anisotropy is related to the compressional tectonics which built the Apennines, and that this tectonic compression involved at least the entire lithosphere. In the Tyrrhenian area we observe ϕ oriented about E‐W, suggesting a reorientation of the mantle fabric due to astenospheric flow, responsible for the E‐W post‐orogenic extension observed at the surface.

Pre-Quaternary orogen-parallel extension in the Southern Apennine belt, Italy

The Southern Apennine fold and thrust belt differs from other parts of the peri-Tyrrhenian orogen. In most of the peri-Tyrrhenian belt, hinterland extension is oriented at a high-angle to the orogen axis and appears to be related to rifting and formation of oceanic crust within the Tyrrhenian basin. The Southern Apennines share the late-stage development of normal faults related to the opening of the Tyrrhenian Sea, but also experienced an episode of extension parallel to the strike of the tectonic belt. The orogen-parallel extension was apparently formed in response to the increase in length of the deformed belt during arcuation. Arcuation ostensibly was related to asymmetrical rifting in the hinterland, which was greater in the Southern Tyrrhenian Sea than in areas to the north, and proportionately greater shortening in the frontal parts of the southern belt as compared to regions in the north. During arcuation, extension was spatially concentrated within structural domains and was accomplished by displacement on low-angle detachment faults cutting through a previously imbricated thrust stack. During the Miocene-Pliocene, NNW-SSE extension in the interior of the Southern Appennine belt formed coveally with ENE-WSW shortening in the foreland. Longitudinal extension ceased in the Pleistocene, when younger high-angle normal faults formed in response to the easterly migration of Tyrrhenian Sea rifting and NE-SW extension associated with lithospheric stretching.

Synchorology of the Northern Apennine summit vegetation. An outline

Abstract A numerical chorological analysis of the 28 vegetation types still described in the boreal vegetation belt of the Northern Apennines was performed. Chorotypes were grouped into 7 main types: arctic-alpine, circumboreal, cosmopolitan, endemics, European orophytes, Eurasian, Mediterranean. Phytosociological vegetation types were classified into 9 groups on the basis of their chorological composition. Such groups include vegetation types with high ecological similarities. The reciprocal ordering of chorotypes and vegetation groups show that most chorotypes are distributed into two or more vegetation groups. The chorotypes which locally have the less number of taxa (Mediterranean and endemic elements) are linked to single vegetation types. The synchorological analysis supplies useful information for a preliminary ecological description of communities as well as suggestions for the biological conservation.

Evolution of the tyrrhenian basin and surrounding regions as a result of the Africa-Eurasia convergence

It is widely accepted that the northern, central and southernmost Tyrrhenian basins opened up during three extensional phases, clearly differentiated in time (upper Tortonian to Messinian, late Messinian to upper Pliocene, late Pliocene to Present) and characterized by rather different deformation patterns in the surrounding Appenninic belt and Adriatic-Ionian foreland. Here, it is argued that the peculiar evolution mentioned above and several other major post-Tortonian deformation events in the central Mediterranean region can be coherently explained as direct consequences or side effects of the shortening processes, which accommodated the Africa-Eurasia convergence. These processes mainly consisted in the eastward and SEward lateral escape of buoyant crustal wedges of the Apenninic belt, at the expense of the adjacent Adriatic-Ionian foreland, which sunk into the underlying mantle, after decoupling from its buoyant cover. The extensional episodes which formed the Tyrrhenian basins were connected with local block divergences in the framework of an overall compressional régime. The principal changes of deformation styles which occurred around the upper Tortonian, the Messinian and the late Pliocene are attributed to the occurrence of major tectonic events, which modified the distribution of resistive forces in the zone considered. The final stage of the proposed evolutionary pattern can provide plausible explanations for the main shallow and deep structural tectonic features evidenced by geophysical observations.