Daniele Babbucci

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.

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.

Regularities in time and space distribution of seismicity in the Periadriatic regions: tectonic implications

Abstract The pattern of energy release in the very active seismic belt along the Hellenides and southern Dinarides, which is interpreted as the main collision boundary between the Adriatic plate and the Balkan regions, has been characterized in the last three centuries by a fairly regular succession of active and quiescent phases. Statistical tests have indicated a very low probability of observing this kind of pattern associated with a random generation of earthquakes. The discontinuous release of seismic energy might be connected with a periodic activation of the underthrusting of the Adriatic Mesozoic platform beneath the Dinarides-Hellenides belts. This effect could condition the kinematic behavior of the whole Adriatic plate. This hypothesis might explain the fact that the Italian regions, lying on the other side of the Adriatic platform, also show a seismic energy release pattern characterized by alternating active and quiescent periods, which, furthermore, proves to be well correlated in time with those occurring at the Adriatic-Balkan border.

Recognition of periAdriatic Seismic Zones Most Prone to Next Major Earthquakes: Insights from a Deterministic Approach

The mitigation of seismic risk in Italy could be considerably helped by the recognition of the seismic zones most prone to next strong earthquake. An attempt at achieving such information has been made by considering the present knowledge about the tectonic setting in the study area and its possible connection with the spatio-temporal distribution of major historical earthquakes. The results of such investigation suggest that at present the probability of major socks is highest in the Northern Apennines.