Alaska has been a site of subduction and terrane accretion since the mid‐Jurassic. The area features abundant seismicity, active volcanism, rapid uplift, and broad intraplate deformation, all associated with subduction of the Pacific plate beneath North America. The juxtaposition of a slab edge with subducted, overthickened crust of the Yakutat terrane beneath central Alaska is associated with many enigmatic volcanic features. The causes of the Denali Volcanic Gap, a 400‐km‐long zone of volcanic quiescence west of the slab edge, are debated. Furthermore, the Wrangell Volcanic Field, southeast of the volcanic gap, also has an unexplained relationship with subduction. To address these issues, we present a joint ambient noise, earthquake‐based surface wave, and P‐S receiver function tomography model of Alaska, along with a teleseismic S wave velocity model. We compare the crust and mantle structure between the volcanic and nonvolcanic regions, across the eastern edge of the slab and between models. Low crustal velocities correspond to sedimentary basins, and several terrane boundaries are marked by changes in Moho depth. The continental lithosphere directly beneath the Denali Volcanic Gap is thicker than in the adjacent volcanic region. We suggest that shallow subduction here has cooled the mantle wedge, allowing the formation of thick lithosphere by the prevention of hot asthenosphere from reaching depths where it can interact with fluids released from the slab and promote volcanism. There is no evidence for subducted material east of the edge of the Yakutat terrane, implying the Wrangell Volcanic Field formed directly above a slab edge.

The southern Alaskan margin captures a transition between compression and strike‐slip‐dominated deformation, accretion of the overthickened Yakutat terrane, termination of Aleutian arc magmatism, and the enigmatic Wrangell Volcanic Field. The extent of subduction and mantle structure below this region is uncertain, with important implications for volcanism. We present compressional and shear wave mantle velocity models below south central Alaska that leverage a new seismometer deployment to produce the most complete image of the subducting Pacific‐Yakutat plate to date. We image a steeply dipping slab extending below central Alaska to >400 km depth, which abruptly terminates east of ~145°W. There is no significant slab anomaly beneath the nearby Wrangell volcanoes. A paucity of volcanism is observed above the subducting Yakutat terrane, but the slab structure below 150 km depth and Wadati‐Benioff zone here are similar to those along the Aleutian‐Alaska arc. Features of the mantle wedge or overlying lithosphere are thus responsible for the volcanic gap.

Large-scale extraction of power from tidal streams within the Pentland Firth is expected to be underway in the near future. The Inner Sound of Stroma in particular has attracted significant commercial interest. To understand potential environmental impacts of the installation of a tidal turbine array a case study based upon the Inner Sound is considered. A numerical computational fluid dynamics model, Fluidity, is used to conduct a series of depth-averaged simulations to investigate velocity and bed shear stress changes due to the presence of idealised tidal turbine arrays. The number of turbines is increased from zero to 400. It is found that arrays in excess of 85 turbines have the potential to affect bed shear stress distributions in such a way that the most favourable sites for sediment accumulation migrate from the edges of the Inner Sound towards its centre. Deposits of fine gravel and coarse sand are indicated to occur within arrays of greater than 240 turbines with removal of existing deposits in the shallower channel margins also possible. The effects of the turbine array may be seen several kilometres from the site which has implications not only on sediment accumulation, but also on the benthic fauna

Tectonic plates are underlain by a low viscosity mantle layer, the asthenosphere. Asthenospheric flow may be induced by the overriding plate or by deeper mantle convection 1. Shear strain due to this flow can be inferred using the directional dependence of seismic wave speeds-seismic anisotropy. However, isolation of asthenospheric signals is challenging; most seismometers are located on continents, whose complex structure influences the seismic waves en-route to the surface. The Cascadia Initiative, an offshore seismometer deployment in the US Pacific Northwest, offers the opportunity to analyze seismic data recorded on simpler oceanic lithosphere 2. Here we use measurements of seismic anisotropy across the Juan-de-Fuca and Gorda plates to reconstruct patterns of asthenospheric mantle shear flow from the Juan-de-Fuca mid-ocean ridge to the Cascadia subduction zone trench. We find that the direction of fastest seismic wave motion rotates with increasing distance from the mid-ocean ridge to become aligned with the direction of motion of the Juan-de-Fuca Plate, implying that this plate influences mantle flow. In contrast, asthenospheric mantle flow beneath the Gorda Plate does not align with Gorda Plate motion and instead aligns with the neighbouring Pacific Plate motion. These results show that asthenospheric flow beneath the small, slow-moving Gorda Plate is controlled largely by advection due to the much larger, faster-moving Pacific Plate. The Juan-de-Fuca plate system is the northernmost section of the Farallon slab, which is approaching complete subduction beneath the North American continent 3. The system is subdivided into the Explorer, Juan-de-Fuca and Gorda segments,