Enzo Mantovani

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

Abstract 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 regime. 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.

Deformation pattern in the central Mediterranean and behavior of the African/Adriatic promontory

Abstract The Adriatic platform has been described in the literature both as an Africa promontory, moving in close connection with the main continent and as an independent microplate. This evident incongruity is mainly due to the wide spectrum of kinematic hypotheses proposed on the basis of paleomagnetic data and to the fact that the Adriatic-Africa transition is not marked by any clear decoupling fracture or by any interruption of lithological facies along the marginal belts (Apennines-Maghrebides and Dinarides-Hellenides). In this work it is argued that the counterclockwise rotation of the Adriatic plate, driven by Africa pushing beneath the Calabrian Arc and southern Tyrrhenian, may coherently account for all major Plio-Quatemary events in the central Mediterranean.

AFRICA-EURASIA KINEMATICS: MAIN CONSTRAINTS AND UNCERTAINTIES

Abstract It is widely believed that the Africa-Eurasian relative motion in the Mediterranean region is oriented SE-NW to S-N. This result has been deduced from the analysis of North Atlantic kinematic data by assuming that Eurasia is a unique coherent plate from the North Atlantic ridges to the Pacific trenches. However, this assumption cannot easily account for the not negligible tectonic activity inside and around Western Europe (Rhine Graben system, Pyrenees and off-shore Portugal). This paper shows that, if one allows Western Europe to move independently from main Eurasia, the kinematic indicators in the North Atlantic do not exclude other kinematic solutions which are significantly different from those currently accepted. In particular, it is demonstrated that a SSW-NNE to SW-NE trending Africa-Eurasia convergence, beside allowing plausible explanations of major post-Tortonian deformation events in the Central Mediterranean region, can fit North Atlantic data within experimental errors.

Role of kinematically induced horizontal forces in Mediterranean tectonics: insights from numerical modeling

Abstract Finite element modeling of the central–eastern Mediterranean region has been carried out to show that the recent/present deformation pattern of this zone, inferred from neotectonic observations and seismic strain rates, may be satisfactorily reproduced as effect of the relative motion of Africa and eastern Anatolia with respect to Eurasia. Numerical modeling involved 2D elastic elements in a plane-stress approximation. The model is constituted by a mosaic of poorly deformable blocks separated by much more deformable decoupling zones, representing consuming boundaries, extensional zones and transcurrent discontinuities, whose location and geometry have been deduced by neotectonic, morphological and seismological information. The calculated displacement field obtained with the modeling parametrization which allows to match the observed strain regimes is compatible with geodetic observations in the study area, but for the Hellenic Arc, where geodetic velocities are higher than those predicted by modeling. This discrepancy could be considerably reduced by adopting a higher deformability of the model in the Hellenic trench, but this condition would contrast with the Plio-Quaternary deformation pattern of the southern Aegean zone, which suggest a considerable slowdown of western Crete since the late Pliocene. Furthermore, geodetic velocities are considerably higher than the motion rates derived by moment tensor analysis in the Hellenic trench and in the internal Aegean area and cannot easily account for the low Quaternary deformation observed in the southern Aegean zone. The above discrepancy could be due to a difference between the “instantaneous” kinematic behavior of the Aegean zone, indicated by geodetic measurements, and the average behavior over longer time intervals, inferred from geological and seismological strain indicators.

Numerical simulation of the observed strain field in the central-eastern Mediterranean region

Abstract The highly heterogeneous strain field indicated by neotectonic and seismological data in the central-eastern Mediterranean region has been reproduced, at a first approximation, by finite element modelling, of a 2D elastic thin plate. The zone considered is modelled as a mosaic of poorly deformable zones decoupled by highly deformable belts, simulating the major tectonic structures indicated by geological and geophysical evidence. The deformation of the model is obtained by imposing kinematic boundary conditions, representative of the motion of Africa and eastern Anatolia relative to Eurasia. Experiments carried out with different boundary conditions and model parameterisations have provided information on the sensitivity of the model and some insights into the geodynamic behavior of the study area. The deformation pattern of the central Mediterranean area is strongly conditioned by the mechanical properties assumed in the border zones between the Aegean and Adriatic systems. The match of the complex strain pattern observed in the western Anatolian–Aegean–Balkan zones is significantly favoured if high rigidity is assigned to the inner part of this structural system. A motion of Africa with respect to Eurasia compatible with an Eulerian pole located offshore Portugal best accounts for the observed strains in the central Mediterranean region. The match of the strongly heterogeneous strain field observed in the study area can hardly be achieved by simplified models not including major tectonic features and lateral heterogeneity of mechanical properties. The kinematic field resulting from the model configuration which best simulates the observed strain field presents some differences with respect to geodetic measurements in the Aegean–Western Anatolian area, where the computed velocities are systematically lower than the geodetic ones. It is suggested that the most plausible explanation of such differences is related to the fact that the present deformation pattern, inferred from geodetic data, may be different from the middle–long term one, inferred from seismological and geological data.

Short and long term deformation patterns in the Aegean-Anatolian Systems: Insights from space geodetic data (GPS)

Geodetic measurements (GPS) in the eastern Mediterranean suggest higher rates of motion, of about 10 mm/yr, in the Aegean - Western Anatolian zone with respect to those in the central-eastern Anatolia. In this work we explore the plausibility of the hypothesis that the observed kinematics may be significantly influenced by post - seismic relaxation processes induced by the seismic activation of the North Anatolian Fault since 1939. The major implications of this hypothesis are tentatively quantified by a simplified model, constituted by an elastic lithosphere (100 km thick) coupled with a viscous asthenosphere (250 km thick with a viscosity of 10 19 Pas). The predicted perturbation of the displacement and stress fields is consistent with geodetic velocities and could also account for the major features of seismic activity in the period considered.

Seismic activity in North Aegean region as middle-term precursor of Calabrian earthquakes

Abstract A comparative analysis of seismicity patterns in the Italian and Balkan regions (both related to the dynamics of the Adriatic Plate) has revealed a noticeable correlation between the time patterns of seismic energy release in the Calabrian Arc (southern Italy) and in the northernmost Aegean region. The systematic occurrence of damaging earthquakes in the Calabrian Arc within a few years after the most intense seismic periods in the North Aegean zone may be useful for middle-term earthquake prediction.

A distribution-free analysis of magnitude-intensity relationships: an application to the Mediterranean region

Abstract A statistical analysis of the relations between macroseismic intensity and magnitude is presented. The examined data set contains earthquakes characterized by epicentral or maximum intensity ≥ VI which occurred in the Mediterranean region. As a first step, an empirical magnitude-intensity relationship has been determined by using the whole data set. Then, differences between experimental magnitude values and the ones expected on the basis of the empirical relationship have been correlated with some features related both to physical and data sources characteristics. On this basis, a distribution-free statistical approach has been developed to attempt a regionalization of the examined area, able to locally optimize the performances of magnitude-intensity relations. However, the results showed that data relative to larger events (intensity ≥ VII) are not sufficient to perform any reliable zonation of the area. Thus, well-constrained relationships determined for the whole Mediterranean region should be preferred to ill-defined local ones. Concerning smaller earthquakes (intensity VI), the analysis suggests that an efficient zonation could only be obtained if medium-scale variations (lower than 200 Km) are taken into account.

Post-Tortonian Deformation Pattern in the Central Mediterranean: A Result of Extrusion Tectonics Driven by the Africa-Eurasia Convergence.

The tectonic activity which has occurred in the Central Mediterranean since the late Tortonian is explained as a result of the Africa-Eurasia convergence roughly along a SSW-NNE direction. This convergence has been first accommodated by a considerable reduction of the Adriatic foreland, through the consumption of its eastern and western margins, and then by the lateral escapes of crustal wedges, accompanied by crustal thickening, in the zone comprised between the Adriatic and African forelands. The lateral escapes of the Calabria and Sicily blocks, towards SE and NW respectively, have been allowed by the presence, at the sides of the most strongly compressed zone, of poorly constrained boundaries, corresponding to the thinned Ionian foreland and, to the zone of crustal stretching in the Tyrrhenian basin. This interpretative scheme allows physically plausible explanations of a considerable amount of geological, geophysical and volcanological evidence in the framework of relatively simple and coherent tectonic mechanisms.

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

Abstract 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.