The Mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt. Lithospheric dynamics are affected by slab rollback and collision of two large, slowly moving plates, forcing fragments of continental and oceanic lithosphere to interact. This paper reviews the rich and growing set of constraints from geological reconstructions, geodetic data, and crustal and upper mantle heterogeneity imaged by structural seismology. We proceed to discuss a conceptual and quantitative framework for the causes of surface deformation. Exploring existing and newly developed tectonic and numerical geodynamic models, we illustrate the role of mantle convection on surface geology. A coherent picture emerges which can be outlined by two, almost symmetric, upper mantle convection cells. The downwellings are found in the center of the Mediterranean and are associated with the descent of the Tyrrhenian and the Hellenic slabs. During plate convergence, these slabs migrated backward with respect to the Eurasian upper plate, inducing a return flow of the asthenosphere from the back-arc regions toward the subduction zones. This flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath Anatolia and eastern Iberia. This convection system provides an explanation for the general pattern of seismic anisotropy in the Mediterranean, first-order Anatolia, and Adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as Anatolia and eastern Iberia. More generally, the Mediterranean is an illustration of how upper mantle, small-scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the Tethyan collision.

[1] A laboratory analogue of a three-layer linear viscous slab-upper mantle-lower mantle system is established in a silicone putty, honey and crystallized honey tank experiment. The same setup as in the numerical investigation (part 1) is used. We focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions. In our numerical experiments the lateral volume flux was set to zero. In interpreting the results relative to the real Earth, the base of the box is taken as the bottom of the mantle convection system, while the lateral boundaries may be associated with the presence of other nearby slabs. Dynamic force equilibrium, assessed on the basis of an analytical review of forces, is described for four different phases: (1) the subduction initiation instability, (2) the accelerating dynamic free fall phase of the slab, (3) the dynamic interaction with the 660-km discontinuity, and (4) a final phase of steady state trench retreat. Phase 3 is an important feature not observed in the numerical experiments. This highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux. On the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux: one in which the lateral boundary can be considered open and the other class where it is ''closed.'' Closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab. Assuming a one-to-one relation between trench retreat and back arc spreading, enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations.

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[1] Based on global earthquake catalogs, the hypocenters, nodal planes, and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007. Assuming that the seismogenic zone coincides with the distribution of 5.5 ≤ M < 7 earthquakes, the subduction interface seismogenic zones were mapped for 80% of the trench systems and characterized with geometrical and mechanical parameters. Using this database, correlations were isolated between significant parameters to identify cause-effect relationships. Empirical laws obtained in previous studies were revisited in light of this more complete, accurate, and uniform description of the subduction interface seismogenic zone. The seismogenic zone was usually found to end in a fore-arc mantle, rather than at a Moho depth. The subduction velocity was the first-order controlling parameter for variations in the physical characteristics of plate interfaces, determining both the geometry and mechanical behavior. As such, the fast subduction zones and cold slabs were associated with large and steep plate interfaces, which, in turn, had large seismic rates. The subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches. Events with M w ≥ 8.5 preferentially occurred in the vicinity of slab edges, where the upper plate was continental and the back-arc strain was neutral. This observation was interpreted in terms of compressive normal stresses along the plate interface. Large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally.Components: 16,300 words, 15 figures, 1 table.

[1] Three-dimensional dynamically consistent laboratory models are carried out to model the large-scale mantle circulation induced by subduction of a laterally migrating slab. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. The circulation pattern is continuously monitored and quantitatively estimated using a feature tracking image analysis technique. The effects of plate width and mantle viscosity/density on mantle circulation are systematically considered. The experiments show that rollback subduction generates a complex three-dimensional time-dependent mantle circulation pattern characterized by the presence of two distinct components: the poloidal and the toroidal circulation. The poloidal component is the answer to the viscous coupling between the slab motion and the mantle, while the toroidal one is produced by lateral slab migration. Spatial and temporal features of mantle circulation are carefully analyzed. These models show that (1) poloidal and toroidal mantle circulation are both active since the beginning of the subduction process, (2) mantle circulation is intermittent, (3) plate width affects the velocity and the dimension of subduction induced mantle circulation area, and (4) mantle flow in subduction zones cannot be correctly described by models assuming a two-dimensional steady state process. We show that the intermittent toroidal component of mantle circulation, missed in those models, plays a crucial role in modifying the geometry and the efficiency of the poloidal component.

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We present lithospheric-scale analog models,\ud investigating how the absolute plates’ motion and\ud subduction of buoyant oceanic plateaus can affect both\ud the kinematics and the geometry of subduction,\ud possibly resulting in the appearance of flat slab\ud segments, and how it changes the overriding plate\ud tectonic regime. Experiments suggest that flat\ud subductions only occur if a large amount of a\ud buoyant slab segment is forced into subduction by\ud kinematic boundary conditions, part of the buoyant\ud plateau being incorporated in the steep part of the slab\ud to balance the negative buoyancy of the dense oceanic\ud slab. Slab flattening is a long-term process (10 Ma),\ud which requires the subduction of hundreds of\ud kilometers of buoyant plateau. The overriding plate\ud shortening rate increases if the oceanic plateau is large\ud enough to decrease the slab pull effect. Slab flattening\ud increases the interplate friction force and results in\ud migration of the shortening zone within the interior of\ud the overriding plate. The increase of the overriding\ud plate topography close to the trench results from (1) the\ud buoyancy of the plate subducting at trench and (2) the\ud overriding plate shortening. Experiments are\ud compared to the South American active margin,\ud where two major horizontal slab segments had\ud formed since the Pliocene. Along the South\ud American subduction zone, flat slab segments below\ud Peru and central Chile/NW Argentina appeared at\ud 7 Ma following the beginning of buoyant slab\ud segments’ subduction. In northern Ecuador and\ud northern Chile, the process of slab flattening\ud resulting from the Carnegie and Iquique ridges’\ud subductions, respectively, seems to be active but not\ud completed. The formation of flat slab segments below\ud South America from the Pliocene may explain the\ud deceleration of the Nazca plate trenchward velocity