Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. It is debated whether fluids and earthquakes are directly related. By combining seismic observations and geodynamic models from western Greece, and comparing across other subduction zones, we find that earthquakes effectively track the flow of fluids from their slab source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface, facilitating intraslab earthquakes. In some locations, the seal breaks and fluids escape through vents into the mantle wedge, thereby reducing the fluid supply and seismicity updip in the slab. The vents themselves may represent nucleation sites for larger damaging earthquakes.

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Subduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intraoceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of 25 km. Equilibration temperatures are high for these depths, averaging 13008C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic reequilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma reequilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localized below the arc, as kinematic thinning above the wedge corner would lead to a hot fore arc, incompatible with fore-arc surface heat flow and seismic properties. Localized subarc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution.

A preliminary assessment of the results obtained from a comprehensive field instrumentation and monitoring scheme of a deep basement in London clay, initiated in 1981 and expected to continue until at least 1987 and probably beyond, is presented. The aspect of particular interest is the performance of the 18 m deep perimeter diaphragm wall and the associated temporary works during construction of the basement. Details of the recorded wall movements, strut forces and strains developed within the wall are given together with observations of water pressures in the ground behind the wall. In addition the results of line and level surveys of the wall and the surrounding area are presented. Soils information extracted from the original site investigation report, together with results obtained from samples and in situ tests conducted during the installation of a deep borehole extensometer, are given. A comparison is made with the original design predictions of wall and ground movements, bending moments and strut loads. Tentative conclusions are drawn which suggest that the forces developed within the struts are within the design values; similarly strains in the wall derived from the design bending moments have not been exceeded. The dependence of the wall deformations on the speed and sequence of excavation is illustrated. The measurements to date suggest that the zone of influence with respect to movements of the surrounding ground is confined to the immediate vicinity of the excavation. L'article présente une évaluation préliminaire des résultats d'un projet d'ensemble contrôlé à instrumentation sur place qui a été entrepris dans un sous sol profond dans l'argile de Londres en 1981 et qui va continuer probablement jusqu'à 1987 voire plus longtemps. On s'intéresse en particulier au comportement d'une paroi périphérique d'une profondeur de 18 m et aux travaux associés temporaires au cours de la construction du sous sols. Des détails sont présentés concernant les mouvements enregistrés de la paroi, les forces dans les étais et les déformations crées dans la paroi, aussi bien que les pressions d'eau dans le sol derrière la paroi. L'article donne en outre les résultats des études d'alignement et de niveau de la paroi et du terrain environnant et présente des renseignements au sujet des sols contenus dans le rapport original concernant la reconnaissance du sol initiale aussi bien que les résultats obtenus à partir d'échantillons et d'essais effectués in situ au cours de l'installation d'un extensomètre dans un sondage profond. Une comparaison est faite avec les prévisions originales des mouvements de la paroi et du sol, des moments de flexion et des charges dans les étais. Les conclusions tirées suggestent que les forces développées dans les étais n'excèdent pas les valeurs du projet, et de façon analogue les déformations dans la paroi qui découlent des moments de flexion du projet n'ont pas été dépassées. On démontre la dépendance des déformations de la paroi avec la vitesse et le cours de l'excavation. Les mesures faites jusqu'à présent donnent l'impression que la zone d'influence des mouvements du sol environnant est confinée au voisinage immédiat de l'excavation.

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