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Highlights: [•] Shear-wave splitting in Transylvanian Basin shows highly variable upper mantle fabrics in the vicinity of the Vrancea body. [•] Apparent isotropy near Ciomadul volcano is likely due to either vertical flow or lack of upper mantle fabric development. [•] The regional upper mantle flow field from NW to SE is diverted along the northwestern facing edge of the Vrancea structure. [Copyright &y& Elsevier]

The plasticity of the mantle is still not well constrained, and satisfactory mineral-physics-based rheological laws are still missing. Despite olivine being the major component of the upper mantle, it is still debated which deformation mechanism (dislocation creep, diffusion creep, grain boundary sliding) dominates deformation. High-pressure research developments (state-of-the-art presses, synchrotron experiments, and so on) as well as competitive analysis utilities (software analysis, microscopy, and so on) allow considering intra- and intergranular mechanisms (grain boundary sliding accommodated by diffusion/dislocation creep) simultaneously. To study the contribution of individual deformation mechanism to the overall deformation in the upper mantle, we deformed polycrystalline forsterite at 3.5-5.0 GPa, 1000-1200 °C, 2 x 10[sup -5]s[sup -1] at different strains in a 6-axis Mavo press. Split-cylinder experiments allowed to characterize an "internal" surface of the sample before and after the deformation experiments. Intra- and intergranular deformation was tracked using a focus ion beam milled reference grid on this surface. Grain internal misorientation where obtained from electron backscatter diffraction (EBSD) data. Both techniques suggest the dominance of intragranular deformation, in agreement with the fact that the samples have been deformed in the dislocation creep regime, as usually defined. Moreover, strain markers and out-of-plane displacements of grains provide the first microstructural evidence for a contribution of grain boundary sliding to plastic deformation at upper mantle pressure. Whether these displacements are grain boundary sliding or involve grain boundary migration cannot be clarified, given the resolution of the strain markers. Our EBSD data suggest that grain boundary processes become increasingly relevant at temperatures above 1100 °C and ensure homogenous plastic strain distribution in the aggregate. [ABSTRACT FROM AUTHOR]

South China Cenozoic basalts ( SCCB) are regionally distributed at the south-east coastal area and can be grouped into three zones: a western zone (>38 Ma), a central zone (17-8 Ma) and an eastern zone (<8 Ma), leaving a temporal gap at 38-17 Ma between the western and central zones. An eastward migration of the SCCB could therefore be identified by the systematic decrease in the eruption age from the western inland to the eastern coast. We propose that most SCCB were associated with the lateral asthenospheric flows moving along the lithosphere/asthenosphere boundary ( LAB) beneath South China and the subsequent decompressional partial melting. The lateral asthenospheric flows had been pulled by the eastward retreat of the pan-Pacific plate subduction before 38 Ma and have been pushed by the northward indentation of the Indian plate into the Eurasian plate since 17 Ma. [ABSTRACT FROM AUTHOR]

Geological and geodynamic models of the mantle often rely on joint interpretations of published seismic tomography images and petrological/geochemical data. This approach tends to neglect the fundamental limitations of, and uncertainties in, seismic tomography results. These limitations and uncertainties involve theory, correcting for the crust, the lack of rays throughout much of the mantle, the difficulty in obtaining the true strength of anomalies, choice of what background model to subtract to reveal anomalies, and what cross-sections to select for publication. The aim of this review is to provide a relatively non-technical summary of the most important of these problems, collected together in a single paper, and presented in a form accessible to non-seismologists. Appreciation of these issues is essential if final geodynamic models are to be robust, and required by the scientific observations. [ABSTRACT FROM AUTHOR]

Western Mexico, where the young and small Rivera Plate and the adjacent large Cocos Plate are subducting beneath the North American Plate, is a unique region on Earth where tearing of subducting oceanic plates, as well as fragmentation of the overriding continental plate, is occurring today. Characterizing the mantle flow field that accompanies the subduction of the Rivera and adjacent Cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary. Here we report observations of seismic anisotropy, as manifested by shear wave splitting derived from local S and teleseismic SKS data collected by the Mapping Rivera Subduction zone array that was deployed from 2006 January to 2007 June, in southwestern Mexico, and from data collected by two of Mexico's Servicio Sismológico Nacional stations. SKS and local S-wave splitting parameters indicate that the fast directions of the split SKS waves for stations that lie on the central and southern Jalisco Block are approximately trench-normal, following the convergence direction between the Rivera Plate and Jalisco Block. S-wave splitting from slab events show a small averaged delay time of ∼0.2 s for the upper 60 km of the crust and mantle. Therefore, the main source of anisotropy must reside in the entrained mantle below the young and thin Rivera Plate. Trench-oblique fast SKS split directions are observed in the western edge of the Rivera Plate and the western parts of the Cocos slab. The curved pattern of fast SKS split directions in the western Jalisco block and beneath the Rivera–Cocos slab gap indicates 3-D toroidal mantle flow, around the northwestern edge of the Rivera slab and the Rivera–Cocos gap, which profoundly affect the finite strain field in the northwestern edge of the Rivera slab and the mantle wedge. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a downdip direction but also have recently rolled back towards the trench and that the Colima rift is intimately related to the tearing between the Rivera and Cocos plates. [ABSTRACT FROM AUTHOR]

Physical models of subduction investigate the impact of regional mantle flow on the structure of the subducted slab and deformation of the downgoing and overriding plates. The initial mantle flow direction beneath the overriding plate can be horizontal or vertical, depending on its location with respect to the asthenospheric flow field. Imposed mantle flow produces either over or underpressure on the lower surface of the slab depending on the initial mantle flow pattern (horizontal or vertical, respectively). Overpressure promotes shallow dip subduction while underpressure tends to steepen the slab. Horizontal mantle flow with rates of 1–10 cm yr−1 provides sufficient overpressure on a dense subducting lithosphere to obtain a subduction angle of , while the same lithospheric slab sinks vertically when no flow is imposed. Vertical drag force (due to downward mantle flow) exerted on a slab can result in steep subduction if the slab is neutrally buoyant but fails to produce steep subduction of buoyant oceanic lithosphere. The strain regime in the overriding plate due to the asthenospheric drag force depends largely on slab geometry. When the slab dip is steeper than the interplate zone, the drag force produces negative additional normal stress on the interplate zone and tensile horizontal stress in the overriding plate. When the slab dip is shallower than the interplate zone, an additional positive normal stress is produced on the interplate zone and the overriding plate experiences additional horizontal compressive stress. However, the impact of the mantle drag force on interplate pressure is small compared to the influence of the slab pull force since these stress variations can only be observed when the slab is dense and interplate pressure is low. [ABSTRACT FROM AUTHOR]

We present 3-D laboratory experiments constructed to investigate the pattern of mantle flow around a subducting slab under different boundary conditions. In particular we present a set of experiments, characterized by different conditions imposed at the trailing edge of the subducting plate (that is, plate fixed in the far field, plate detached in the far field, imposed plate motion). Experiments have been performed using a silicone slab floating inside a honey tank to simulate a thin viscous lithosphere subducting in a viscous mantle. For each set, we show differences between models that do or do not include the possibility of out-of-plane lateral flow in the mantle by varying the lateral boundary conditions. Our results illustrate how a subducting slab vertically confined over a 660-km equivalent depth can be influenced in its geometry and in its kinematics by the presence or absence of possible lateral pathways. On the basis of these results we show implications for natural subduction systems and we highlight the importance of suitable simulations of lateral viscosity variations to obtain a realistic simulation of the history of subduction. [ABSTRACT FROM AUTHOR]

Recent evidence indicates that beneath the Caribbean a tongue of sublithosphere mantle is flowing from the Pacific to the Atlantic, dragging the overlying lithosphere eastward: (i) Shear-wave splitting results from beneath the Andean subduction zone and Venezuela suggest mantle flow eastward through the Caribbean. (ii) Volcanic chemistry in Central America indicates a slab source beneath Nicaragua, but a different source in Costa Rica, above the proposed Pacific outflow. (iii) An extinct volcanic arc accreted to the margins of the Caribbean swept eastward through the Caribbean gap between North & South America. The 1982 ‘continental undertow’ model requires shallow-mantle flow through the Caribbean gap from the Pacific to the Atlantic, if continents have deep roots and if shallow-mantle flow beneath oceans is decoupled from convection at deeper levels. The new evidence from the Caribbean is thus compatible with the continental undertow model, and perhaps with other models involving decoupled shallow flow. [ABSTRACT FROM AUTHOR]