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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The origin of 3‐D seismic heterogeneity in Precambrian lithosphere has been enigmatic, because temperature variations in old stable shields are expected to be small and seismic sensitivity to major‐element compositional variations is limited. Previous studies indicate that metasomatic alteration may significantly affect average 1‐D structure below shields. Here, we perform a grid search for 3‐D thermochemical structure, including variations in alteration, to model published Rayleigh wave phase velocities between 20 and 160 s for the eastern part of the Archean Superior and Canadian Proterozoic Grenville Provinces. We find that, consistent with constraints from surface heat flow and xenoliths, the lithosphere is coolest (Moho heat flow 12–17 mW/m2) and the thermal boundary layer thickest (>250 km) in the northeastern Superior and warmest in the southeastern Grenville (Moho heat flow 20–25 mW/m2, thermal boundary thickness 160–200 km). Compositionally, the phase velocities for most of the Superior within our study region require little alteration, but in a few regions, fast velocities need to overlie slower velocities. These can be modeled with an eclogite layer in the midlithosphere, consistent with active seismic and xenolith evidence for remnants of subducted Archean crust. The phase velocities from the Grenville Province require significant metasomatic modification to explain the relatively low velocities of the shallow lithosphere, and the required intensity of alteration is highest in parts of the Grenville associated with arc accretion. Thus, the composition of the northeastern Canadian Shield appears to reflect different stages and styles of craton assembly.

The thermal and compositional structure of lithospheric keels underlying cratons, which are stable continental cores formed during the Precambrian, is still an enigma. Mapping lithospheric temperatures and compositional heterogeneities is essential to better understand geodynamic processes that control craton formation and evolution. Here we investigate the northeastern part of North America which comprises the Superior Craton, the largest Archean craton in the world, and surrounding Proterozoic belts. We model Rayleigh-wave dispersion curves from a previous study, which were regionalized based on cluster analysis. Next, we perform a grid search for sub-crustal thermal and compositional structures that are consistent with the average dispersion curve for each cluster. We apply constraints on crustal structure and use thermodynamic methods to map thermo-compositional structures into seismic velocity. In agreement with previous studies, most regions require concentrations of metasomatic minerals over certain depth intervals to fit the seismic profiles. Our results further require vertical as well as lateral variations in compositional and thermal structures, which appear to reflect different stages of formation and modification of the lithosphere below the region, with distinct structures found under Archean cores, Archean/Paleoproterozoic collision belts, mid-late Proterozoic collision belts, and zones affected by rifting.

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