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The origin of mid-ocean ridge segmentation-the systematic along-axis variation in tectonic and magmatic processesremains controversial. It is commonly assumed that mantle flow is a passive response to plate divergence and that between transform faults magma supply controls segmentation. Using seismic tomography, we constrain the geometry of mantle flow and the distribution of mantle melt beneath the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge. Our results, in combination with prior studies, establish a systematic skew between the mantle-divergence and plate-spreading directions. In all three cases studied, mantle divergence is advanced with respect to recent changes in the plate-spreading direction and the extent to which the flow field is advanced increases with decreasing spreading rate. Furthermore, seismic images show that large-o set, non-transform discontinuities are regions of enhanced mantle melt retention. We propose that oblique mantle flow beneath mid-ocean ridges is a driving force for the reorientation of spreading segments and the formation of ridge-axis discontinuities. The resulting tectonic discontinuities decrease the e ciency of upward melt transport, thus defining segment-scale variations in magmatic processes. We predict that across spreading rates mid-ocean ridge segmentation is controlled by evolving patterns in asthenospheric flow and the dynamics of lithospheric rifting.S ince the discovery that Earth's mid-ocean ridge system is divided into segments 1-3 a wealth of observations have shown that there are systematic, along-axis variations in tectonic and magmatic processes 2,4-6 . Between transform faults, the boundaries of ridge segments are defined by long-lived, non-transform tectonic offsets or second-order ridge crest discontinuities 2,3 ( Fig. 1) that often occur at axial depth maxima and that migrate along the plate boundary. These second-order discontinuities include overlapping spreading centres (OSCs) at fast-and intermediate-spreading rates, and oblique shear zones at slow-spreading rates. The origin of non-transform tectonic offsets and their relations to segment-scale magmatic processes remains actively debated 1,2,5-9 .The prevailing hypothesis for segmentation of spreading centres attributes second-order offsets to variations in magma supply from the upwelling mantle. In this view, segment centres overlie sites of increased melt supply and magma is redistributed along axis at crustal or mantle depths toward magma-starved segment ends 1,2,8,10 . Alternatively, competing hypotheses suggest that changes in the plate-spreading direction are related to the formation of tectonic offsets 7 and to a misalignment between sub-ridge mantle and crustal processes 9 . Here, we seismically image the geometry of mantle flow and the distribution of shallow mantle melt beneath the intermediate-spreading Endeavour segment of the Juan de Fuca Ridge (JdFR). We synthesize our results with observations from other spreading environments to identify the mechanisms responsible ...

The anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo‐relative plate motion (RPM). However, we find evidence that this assumption is invalid beneath the Juan de Fuca (JdF) plate. Using travel times of seismic energy propagating through the topmost mantle, we find that the fast direction of P wave propagation is rotated 18° ± 3° counterclockwise to the paleo‐spreading direction and strikes between Pacific‐JdF relative and JdF absolute plate motion (APM). The mean mantle velocity is 7.85 ± 0.02 km/s with 4.6% ± 0.4% anisotropy. Synthesis of the plate‐averaged Pn anisotropy signal with measurements of Pn anisotropy beneath the JdF Ridge and SKS splits across the JdF plate suggests that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with APM. We infer that the oceanic mantle lithosphere may record the influence of both paleo‐RPM and paleo‐APM.

Hydrothermal circulation at mid‐ocean ridges is responsible for ~25% of Earth's heat flux and controls the thermal and chemical evolution of young oceanic crust. The heat flux of black smoker hydrothermal systems is thought to be primarily controlled by localized magma supply and crustal permeability. Nevertheless, magma chamber characteristics and the nature of crustal permeability beneath such systems remain unclear. Here we apply three‐dimensional full‐waveform inversion to seismic data from the hydrothermally active Endeavour segment of the Juan de Fuca Ridge to image the upper crust in high resolution. We resolve velocity variations directly above the axial magma chamber that correlate with variations in seismicity, black smoker heat flux, and the depth of the axial magmatic system. We conclude that localized magma recharge to the axial magma lens, along with induced seismogenic cracking and increased permeability, influences black smoker heat flux.

The occurrence of methane hydrate in marine reservoirs often correlates with the physical properties of the host sediments. High hydrate saturations (>60% of the pore volume) found in association with coarser‐grained strata have been attributed to both enhanced advective transport through more permeable sediment layers and to perturbations in phase equilibrium related to pore space geometry that results in increased diffusive transport. To assess the relative importance of these mechanism in controlling hydrate occurrence, we develop a 1‐D model for hydrate growth along dipping, coarse‐grained layers embedded in a fine‐grained sediment package. We explicitly account for pore size effects on methane solubility and permeability‐driven variations in fluid flux. We show how the vertical distribution of hydrate varies in response to changes in grain size and rates of fluid advection, sedimentation, and in situ methane production. We then use our model to simulate centimeter‐scale variations in hydrate saturation observed at Walker Ridge Block 313, Hole H in the Gulf of Mexico. We find that the largest concentrations of hydrate are controlled by diffusion, while increased advective methane supply favors more distributed growth throughout high‐permeability regions. Our results hold promise for using well log‐derived estimates of hydrate saturation to infer sediment properties and the sources and rates of methane supply during reservoir emplacement.