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The Queen Charlotte plate boundary (QCPB), a transform separating the Pacific and North American plates, accommodates ~55 millimeters per year of motion, is a source of large earthquakes in the northeast Pacific, and may be a modern site of subduction initiation. The southern QCPB experiences oblique convergence, showcased by the 1949 magnitude (M) 8.1 strike-slip earthquake and the 2012M7.8 tsunamigenic thrust earthquake, both offshore Haida Gwaii, British Columbia. We present seismic reflection images of the southern QCPB, which constrain the crustal structure in unprecedented detail. The Queen Charlotte Terrace is underthrust by oceanic crust topped by a throughgoing, low-angle plate-boundary thrust, which ruptured in the 2012 earthquake. The Queen Charlotte Terrace is analogous to strain-partitioned, thin-skinned forearc slivers seen at oblique subduction zones, captured between a localized plate-boundary thrust and a mature strike-slip fault. Our imaging suggests that the system rapidly evolved from distributed to partitioned strain and is currently an incipient subduction zone. 

Abstract Plate boundaries in the oceans are often poorly monitored. Though typically less remote than the deep sea, shallow marine environments with seafloor depths <0.5 km can be especially challenging for seismic experiments due to natural and anthropogenic hazards and noise sources that can affect instrument survival and data quality. The Queen Charlotte fault (QCF) is part of a transform plate boundary that follows the continental shelf of the Alaska Panhandle and central British Columbia. This fault system accommodates dextral slip between the Pacific and North American plates and has hosted several historic Mw > 7 earthquakes. In August 2021, we deployed 28 broadband ocean-bottom seismometers (OBSs) along the central QCF for the “Transform Obliquity along the Queen Charlotte Fault and Earthquake Study” (TOQUES) to investigate fault architecture and local seismicity. Deployment depths varied between 0.2 and 2.5 km below sea level, with half of the instruments deployed in shallow water (<0.5 km depth). We describe the scientific motivations for the TOQUES broadband OBS array, present data metrics, and discuss factors that influence data quality and instrument survival. We show that many opportunities exist for scientific study of shallow marine environments and the solid earth. Despite concerns that shallow water was responsible for the risk of data or instrument loss, direct relationships between instrument success and water depth are inconclusive. Rather, instrument success may be more related to the ability of different instrument designs to withstand shallow-water conditions. 

Abstract Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the incoming lithosphere. Debate remains about the relative importance of pre‐existing faults, plate curvature and other factors controlling the extent and style of bending‐related faulting. The subduction zone off the Alaska Peninsula is an ideal place to investigate controls on bending faulting as the orientation of the abyssal‐hill fabric with respect to the trench and plate curvature vary along the margin. Here, we characterize faulting between longitudes 161°W and 155°W using newly collected multibeam bathymetry data. We also use a compilation of seismic reflection data to constrain patterns of sediment thickness on the incoming plate. Although sediment thickness increases over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus should have limited impact on the expression of bend‐related fault strikes and throws in bathymetry data. Where magnetic anomalies trend subparallel to the trench (<30°) west of ∼156°W, bending faults parallel magnetic anomalies, implying that bending faults reactivate pre‐existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156°W, no bending faults are observed. Summed fault throws increase to the west, including where pre‐existing structure orientations are constant (between 157 and 161°W), suggesting that another factor such as the increase in slab curvature must influence bending faulting. However, the westward increase in summed fault throws is more abrupt than expected for gradual changes in slab bending alone, suggesting potential feedbacks between pre‐existing structures, slab dip, and faulting. 

Abstract Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events. 

Abstract P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive theVp/Vsstructure of Cascadia Basin sediments. We usedP‐to‐Swaves converted at the basement to derive an empirical function describing the averageVp/Vsof Cascadia sediments as a function of sediment thickness. We derived one‐dimensional intervalVp/Vsfunctions from semblance velocity analysis ofS‐converted intrasediment and basement reflections, which we used to define an empiricalVp/Vsversus burial depth compaction trend. We find that seaward from the Cascadia deformation front,Vp/Vsstructure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevatedVp/Vs. These zones with elevatedVp/Vsare likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneousVp/Vsfunctions of Cascadia Basin sediments.