Geodynamics of the South Balkan and Northern Aegean Regions Driven by the Westward Escape of Anatolia

The Plio-Quaternary deformation pattern of the northern Aegean and south Balkan regions is interpreted as an effect of the interaction between the Anatolian-Aegean-Pelagonian system (Tethyan belt), undergoing westward extrusion and strong deformation, and the surrounding plates (Nubia, Europe and Adriatic). Since the middle-late Miocene, the collision of the Tethyan belt with the continental Adriatic domain has caused strong E-W shortening in the outer Hellenides and Albanides, also involving the southward extrusion of the Peloponnesus wedge, at the expense of the Ionian oceanic domain. The roughly E-W extension recognized in the western South Balkan zones (Macedonia and eastern Albania) is related to the divergence between the Pelagonian belt (Albanides and Hellenides) and the Rhodope-Moesia domain. Stressed by the westward displacement of the central Anatolian plateau and by the southward bowing of the Cycladic Arc, the northern Aegean zone has contemporaneously undergone E-W compression and N-S extension, which has generated a series of dextral shear faults, delimiting a number of slats. The westward displacement and deformation of such slats can explain the morphological features of the northern Aegean zone. During this phase, the push of the central Anatolian plateau also caused the separation of the Rhodope massif from the Moesian European domain, with the consequent formation of the upper Thrace basin. This hypothesis can explain the Plio-Quaternary compressional deformations recognized in a sector of the North Anatolian fault system, the Ganos-Gelibolu zone. The proposed geodynamic/tectonic interpretation may help to explain some features of the time-space distribution of major earthquakes in the study area.

Possible Multiple Sources of the Strong 1117 Po Plain Earthquake, Inferred from the Plio-Quaternary Evolution of the Northern Adriatic Area

The strongest documented seismic disaster ever occurred in the Po Plain area (January 3, 1117, M = 6.5) involved significant damage over a large zone. The genetic mechanism of such an event, most probably caused by more than one earthquake, is still an object of debate. Above all, the sources so far proposed cannot account for significant features of the observed macroseismic field. In this work, we suggest that the damage in the Verona zone was caused by the activation of a fault in the Lessini tectonic district, while damage in the central Po Plain may be related to a thrust fault located beneath the Giudicarie belt. The effects felt in northern Tuscany might derive from the seismic activation of the presumed SW-ward buried prolongation of the Giudicarie fault. The presence of such transpressional lithospheric discontinuity in the Adriatic domain since the upper Miocene and its reactivation (Pliocene-Pleistocene) as a thrust zone is mainly suggested by an accurate analysis of the observed deformation pattern in the central Mediterranean region. The proposed Giudicarie source may also help to explain the damage observed in the central Po Plain on December 25, 1222, which is not compatible with the seismic sources so far proposed.

Seismotectonics of the Padanian Region and Surrounding Belts: Which Driving Mechanism?

It is argued that the complex tectonic pattern observed in the study area can plausibly be explained as an effect of the kinematics of the Iberia and Adria blocks, induced by the NNE ward motion of Africa and the roughly westward motion of the Anatolian-Aegean system with respect to Eurasia. These boundary conditions cause the constrictional regime which is responsible for the observed shortening processes in the Padanian region and Western Alps. The proposed dynamic context can plausibly account for the peculiar distribution of major seismic sources, located in the northern Apennines, the Giudicarie fault system, the offshore of the western Ligurian coast and the Swiss Alps. The observed tectonic pattern in Western Europe and the study area can hardly be reconciled with the implications of the roughly NWward convergence between Africa and Eurasia proposed by global kinematic models, whereas it is compatible with the alternative Africa-Eurasia kinematics and plate mosaic proposed by [1].

Short-Term Kinematics of the Adria Plate and Space-Time Distribution of Major Peri-Adriatic Earthquakes

Seismic activity is quite strong in the peri-Adriatic zones, whereas the internal part of the Adria plate is almost aseismic. This pattern suggests that Adria is a solid block that interacts with the surrounding belts, trying to move roughly northward. Each major earthquake in a peri-Adriatic zone triggers the acceleration of the decoupled Adria sector, which induces a perturbation of the stress/strain fields in the still blocked boundaries of the plate. Step by step, the displacement of Adria involves more and more northern zones to finally reach the northern front of the plate (eastern Southern Alps). This interpretation seems to be compatible with the time patterns of seismic activity in the main peri-Adriatic zones since 1600 A.D., which may suggest repeated northward migrations of seismic crises. Each supposed migrating sequence involves major earthquakes in most zones. The main features of the first 4 seismic sequences (1600-1930) are used to get insights into possible regularities in the progressive activations of the peri-Adriatic zones. This information and the main features of the ongoing migrating sequence (since 1931) are then used to tentatively recognize the peri-Adriatic zones where the occurrence of next major earthquakes may be most likely.

Generation of Back-Arc Basins as Side Effect of Shortening Processes: Examples from the Central Mediterranean

The evolution of the Mediterranean area since the Oligocene-Lower Miocene has been driven by the convergence of the surrounding plates. This implies that the observed deformation pattern in that region must be the most convenient shortening pattern, i.e. the one controlled by the minimum action principle. To understand why the fulfilment of such condition has required a complex spatio-temporal distribution of major tectonic events, such as uplift, lateral displacement and bending of orogenic belts, consumption of large lithospheric domains and formation of back arc basins, it may be very useful to take into account a basic tectonic concept, which helps to identify the process that can minimize the resistance of tectonic forces. Such concept starts from the fact that the most convenient consumption process is the one that involves low buoyancy oceanic lithosphere (Tethyan domains). However, such process is highly favoured where the oceanic lithosphere is stressed by vertical forces, a situation that develops when orogenic wedges are forced to over thrust and load the oceanic domain to be consumed. This interpretation can provide plausible and coherent explanations for the complex pattern of the observed deformations. In this view, the generation of back arc basins is taken as a side effect of an extrusion process, as suggested by numerical and mechanical experiments.

Present Velocity Field in the Italian Region by GPS Data: Geodynamic/Tectonic Implications

The analysis of geodetic observations carried out by 478 continuous GPS stations in the Italian region since 2001 has allowed a fairly good definition of the ongoing horizontal velocity field with respect to Eurasia. It is argued that such evidence can provide important insights into the geodynamic context in the central Mediterranean area. Numerous velocity vectors in the Apulia zone coherently indicate that the southern Adriatic domain is moving roughly NE ward. Since no significant decoupling zone between this domain and Nubia has so far been recognized, one could expect that the kinematics of these two plates is compatible. However, this condition is not fulfilled if the Nubia-Eurasia relative motion is taken from the global kinematic models, either deduced by long-term evidence?[1]?or short-term geodetic data?[2]?[3]. This problem is considerably reduced if the alternative Nubia-Eurasia rotation pole suggested by?[4]?is taken into account. This choice is also suggested by other major long-term evidence in the Mediterranean region. The numerous geodetic vectors available in two Adriatic sectors, the Apulia zone and the Venetian plain, would imply an Adria-Eurasia rotation pole incompatible with all Nubia-Eurasia Eulerian poles so far proposed. Since a significant relative motion between these plates is not compatible with the absence of a tectonic decoupling zone, we suppose that the short-term kinematics of Adria might be influenced by a transient non-rigid behaviour of that plate. This hypothesis is compatible with the expected effects (post seismic relaxation) of the major decoupling earthquakes that have occurred along Periadriatic zones in the past tens of years. The compatibility of the GPS kinematic pattern in the Apennine belt, Calabria Arc and Sicily with the implications of the geodynamic/tectonic interpretations so far proposed for the central Mediterranean area is then discussed.

Present Tectonic Setting and Spatio-Temporal Distribution of Seismicity in the Apennine Belt

In previous papers, we have argued that a close connection may exist between the discontinuous northward displacement of the Adria plate and the spatio-temporal distribution of major earthquakes in the?periAdriatic?regions?[1]-[3]. In particular, five seismic sequences are tentatively recognized in the post 1400 A.D. seismic history, each characterized by a progressive migration of major shocks along the eastern (Hellenides, Dinarides), western (Apennines) and northern (Eastern Southern Alps) boundaries of Adria. In this work, we describe an attempt at gaining insights into the short-term evolution of the strain field that underlies the migration of seismicity in the Apennine belt. The results of this study suggest that seismicity in the study area is mainly conditioned by the fact that the outer (Adriatic) sector of the Apennine belt, driven by the Adria plate, is moving faster than the inner (Tyrrhenian) belt. This kinematics is consistent with the observed Pleistocene deformation pattern and the velocity field inferred by GPS data. The spatio-temporal distribution of major shocks during the last still ongoing seismic sequence (post 1930) suggests that at present the probability of next major shocks is highest in the Northern Apennines. Within this area, we suggest that seismic hazard is higher in the zones located around the outer sector of the Romagna-Marche-Umbria units (RMU), since that wedge is undergoing an accelerated relative motion with respect to the inner Apennine belt. This hypothesis may also account for the pattern of background seismicity in the Northern Apennines. This last activity might indicate that the Upper Tiber Valley fault system is the most resisted boundary sector of the RMU mobile wedge, implying an higher probability of major earthquakes.

Belt-Parallel Shortening in the Northern Apennines and Seismotectonic Implications

Major seismic activity in the Northern Apennines concentrates in few zones, distributed in a peculiar way. It is argued that such context may be plausibly explained as an effect of belt-parallel?shortening, which has caused oroclinal bending of the longitudinal ridges formed during the Late Miocene to Lower Pliocene evolutionary phase. The main effects of this process, developed since the upper Pliocene, have mainly affected the outer sectors of the belt. The major seismic sources have generated in the zones where different oroclinal bendings of adjacent ridges have produced extensional/transtensional deformation. In the inner side of the Northern Apennines, belt parallel shortening has occurred at a lower rate. The main effects have resulted from the shortening of the?Albano-Chianti-Rapolano-Cetona ridge. In particular, the proposed tectonic setting may account?for the moderate seismic activity that occurs in the Firenze, Elsa, Pesa, Siena and Radicofani basins.

Possible Location of the Next Major Earthquakes in the Northern Apennines: Present Key Role of the Romagna-Marche-Umbria Wedge

It is argued that in some zones of the Northern Apennines, in particular the Rimini-Ancona thrust system, the Romagna Apennines and the Alta Valtiberina trough, the probability of major earthquakes is now higher than in other Apennine zones. This hypothesis is suggested by the comparison of the present short-term kinematics of the Romagna-Marche-Umbria wedge in the Northern Apennines, deduced by the distribution of major shocks in the last tens of years, with the previous repeated behavior of the same wedge, evidenced by the distribution of major earthquakes in the last seven centuries. The seismotectonics of the Apennine region here considered is closely connected with the larger context that involves the progressive migration (from south to north) of seismicity along the peri-Adriatic zones. The information provided by this study can be used to better manage the resources for prevention in Italy.

Generation and Disruption of Subducted Lithosphere in the Central-Western Mediterranean Region and Time-Space Distribution of Magmatic Activity Since the Late Miocene

The long migration of the Balearic Arc (Alpine-Apennine and Alpine-Maghrebian belts) in the Early-Middle Miocene caused the formation of a subducted lithospheric edifice in the western and central Mediterranean regions. Then, since the Late Miocene, this slab was almost completely disrupted, only maintaining a narrow and deformed remnant beneath the southernmost Tyrrhenian basin. This work describes a tentative reconstruction of the tectonic processes that caused the formation of major tears and breakoffs in the original slabs and the consequent disruption of the subducted lithosphere. In particular, it is suggested that this relatively fast process was produced by the collision between the Anatolian-Aegean system and the continental Adriatic domain, which triggered a number of extrusion processes. Possible connections between the proposed tectonic evolution and the spatio-temporal distribution and geochemical signatures of magmatic activity are then discussed. It is supposed that such activity has been mainly conditioned by the occurrence of transtensional tectonics in the wake of escaping orogenic wedges.