The role of decoupling in the low-velocity zone is crucial for understanding plate tectonics and mantle convection. Mantle convection models fail to integrate plate kinematics and thermodynamics of the mantle. In a first gross estimate, we computed at >300 km 3 /yr the volume of the plates lost along subduction zones. Mass balance predicts that slabs are compensated by broad passive upwellings beneath oceans and continents, passively emerging at oceanic ridges and backarc basins. These may correspond to the broad low-wavespeed regions found in the upper mantle by tomography. However, west-directed slabs enter the mantle more than three times faster (~232 km 3 /yr) than in the opposite east- or northeast-directed subduction zones (~74 km 3 /yr). This difference is consistent with the westward drift of the outer shell relative to the underlying mantle, which accounts for the steep dip of west-directed slabs, the asymmetry between flanks of oceanic ridges, and the directions of ridge migration. The larger recycling volumes along west-directed subduction zones imply asymmetric cooling of the underlying mantle and that there is an “easterly” directed component of the upwelling replacement mantle. In this model, mantle convection is tuned by polarized decoupling of the advecting and shearing upper boundary layer. Return mantle flow can result from passive volume balance rather than only by thermal buoyancy-driven upwelling.

Plume tracks at the Earth's surface probably have various origins, such as wet spots, simple rifts, and shear heating. Because plate boundaries move relative to one another and relative to the mantle, plumes located on or close to them cannot be considered as reliable for establishing a reference frame. Using only relatively fixed intraplate Pacific hotspots, plate motions with respect to the mantle in two different reference frames, one fed from below the asthenosphere, and one fed by the asthenosphere itself, provide different kinematic results, stimulating opposite dynamic speculations. Plates move faster relative to the mantle if the source of hotspots is taken to be the middle-upper asthenosphere, because hotspot tracks would then not record the entire decoupling occurring in the low-velocity zone. A shallow intra-asthenospheric origin for hotspots would raise the Pacific deep-fed velocity from a value of 10 cm/year to a faster hypothetical velocity of ∼20 cm/year. In this setting, the net rotation of the lithosphere relative to the mesosphere would increase from a value of 0.4359°/m.y. (deep-fed hotspots) to 1.4901°/m.y. (shallow-fed hotspots). In this framework, all plates move westward along an undulated sinusoidal stream, and plate rotation poles are largely located in a restricted area at a mean latitude of 58°S. This reference frame seems more consistent with the persistent geological asymmetry that suggests a global tuning of plate motions related to Earth's rotation. Another significant result is that along east- or northeast-directed subduction zones, slabs move relative to the mantle in the direction opposed to the subduction, casting doubts on slab pull as the first-order driving mechanism of plate dynamics.

During the Late Miocene to Pleistocene, Western Anatolia and the Aegean area were affected by scattered alkali basaltic activity that was temporally distinct from the older orogenic magmatism related to the subduction of Africa beneath the Anatolian and Aegean plates. On the basis of geochemical and isotopic data, two groups of alkali basalts have been distinguished. The first group (Foça, Urla, Selendi, Samos, Chios, Patmos, and Psathoura) exhibits a wide variability in isotopic composition ( 87 Sr/ 86 Sr 0.7043–0.7079; 143 Nd/ 144 Nd 0.51278–0.51243) and trace-element distribution (Th/Ta 2.4–12.3; Ba/Nb 14–49) probably acquired from a subduction-related component. The second group (Kula, Biga, Kalogeri, and Thrace), on the other hand, retains typical intraplate features with no subduction-related imprinting ( 87 Sr/ 86 Sr 0.7031–0.7035; 143 Nd/ 144 Nd 0.5130–0.51275; Th/Ta 1.2–1.7; Ba/Nb 5–10). The first group of basalts marks the transition between subduction-related and intraplate activity, characterized by the interaction of a mantle source with residual slab fluids, whereas the second group is an expression of a mantle with any subduction signature. Within the second group, the geochemical and isotope variations highlight the involvement of both mid-ocean-ridge basalt (MORB)–like and ocean-island basalt (OIB)–like mantle domains. Overall, the intraplate character of this alkaline association indicates that the mantle wedge, previously metasomatized by slab-derived material, was replaced by the upwelling of subslab mantle. This process is considered to be the consequence of the extension of the hanging-wall Aegean-Anatolian lithosphere, coupled with the subducted African slab, which was stretched and torn. In this interpretation, the track of the alkali basalts would be a useful marker tool of ruptures in the slab.

The northwestern side of the Sicily Channel in the central Mediterranean has been shaped by the occurrence of two independent tectonic processes that overlap each other, the Maghrebides-Apennines accretionary prism and the Sicily Channel rift. Since at least the Pliocene, these two processes have acted simultaneously, being respectively related to the Apennines subduction and to the African rift. Thrust sheets of the accretionary prism crosscut the almost orthogonal rift-related normal faults and vice versa. Analog modeling supports the kinematics inferred from regional structural data. Alkaline magmatism associated with the rift is more pronounced in the foreland of the prism, where the extension is more concentrated. This peculiar setting confirms how independently geodynamic processes can interact in the same area at the same time, suggesting that plate boundaries are passive features responding to far-field velocity fields of the lithosphere.

Intraplate migrating hotspots, which are unrelated to rifts or plate margins in general, regardless of their origin in the mantle column, indicate relative motion between the lithosphere and the underlying mantle in which the hotspot source is located. Pacific plate hotspots are sufficiently fixed relative to one another to represent an independent reference frame to compute plate motions. However, the interpretation of the middle asthenosphere rather than the deep lower mantle as the source for intraplate Pacific hotspots has several implications. First, decoupling between the lithosphere and subasthenospheric mantle is greater than recorded by hotspot volcanic tracks (>100 mm/yr) due to undetectable shear in the lower asthenosphere below the magmatic source. The shallower the source, the larger the décollement. Second, computation of the westward drift is linked to the Pacific plate and assumes that the deep lower mantle, below the decoupling zone, sources the hotspots above. The Pacific plate is the fastest plate in the hotspot reference frame and dominates the net rotation of the lithosphere. Therefore, if decoupling with the subasthenospheric mantle is larger, the global westward drift of the lithosphere must be faster than present estimates, and may possibly vary between 50 and 90 mm/yr. In this case, all plates, albeit moving at different velocities, move westward relative to the subasthenospheric mantle. Finally, faster decoupling can generate more shear heating in the asthenosphere (even >100°C). This amount of heating, in an undepleted mantle, could trigger scattered intraplate Pacific volcanism itself if the viscosity of the asthenosphere is locally higher than normal. The Emperor-Hawaiian bend can be reproduced when bent viscosity anisotropy in the asthenosphere is included. Variations in depth and geometry in the asthenosphere of these regions of higher viscosity could account for the irregular migration and velocities of surface volcanic tracks. This type of volcanic chain has different kinematic and magmatic origins from the Atlantic hotspots or wetspots, which migrate with or close to the oceanic spreading center and are therefore platemargin related.