Continental rifting is a fundamental plate tectonic process that leads to the formation of new passive margins and ocean basins. Large igneous provinces have the potential to accelerate the breakup of continents, since they can produce voluminous mantle melts that weaken the extending lithosphere. Here, we present seismic velocity models from a controlled-source seismic dataset across the passive margin of the eastern United States, where the Late Triassic Central Atlantic Magmatic Province (CAMP) impinged on a broad incipient continental rift zone along the eastern flank of the Appalachian orogen. In the Early Jurassic, continued rifting at the present-day outer continental shelf led to complete crustal separation -50 km east of where CAMP magmatism intruded the mid-Triassic rift basins. Our results show that the first pulse of breakup magmatism occurred along the present-day East Coast Magnetic Anomaly when the continental crust offshore thinned to -15 km, -15 m.y. after CAMP. We infer that slow extension rates and a strong mantle lithosphere prevented a rapid continental rupture beneath the present-day coastal plain, despite the previous emplacement of CAMP volcanics in this region.

At subduction zones, the occurrence of slow slip events and tremor is often attributed primarily to fluids released from the subducting plate. Yet, sufficiently high-resolution constraints on along-strike variations in the extent and nature of incoming plate hydration required to assess the role of fluids are lacking. Using a high-resolution P-wave velocity model, we estimate the water contents of the young incoming Cocos plate before it subducts beneath the Guerrero Gap and adjacent segments, which exhibit variations in slip behavior. Seaward of the Guerrero Gap, large seamounts contain 1.5-3 times more water, suggesting a possible link between seamount subduction and the aseismic character of the gap. However, widespread upper-crustal hydration and subducting seamounts outside the gap where large earthquakes regularly occur suggest incoming plate hydration alone cannot account for megathrust slip variability offshore Mexico. Subduction of large hydrated seamounts alone is unlikely to be the primary driver of along-strike variations in megathrust slip behavior offshore Mexico, according to a high-resolution P-wave velocity model of the Guerrero Gap

Continental rifting initiates the transition to breakup when the crust is necked and deformation localizes at the rift axis. However, the slow crustal-stretching rates and >20-km deep mantle beneath many active rifts worldwide suggest that present-day breakup may not be imminent. High-resolution seismic data from the Turkana Rift Zone of the East African Rift System (EARS) reveal the rift's subsurface structure. Here, we show that Turkana's crystalline crust has thinned to ~13 km along the rift axis, revealing an active rift undergoing crustal necking. Onset of necking is constrained to ~4 Ma and facilitated the accumulation of Turkana's world-famous fossil record of human evolution. Identification of necking in the EARS indicates that eastern Africa is primed for continental breakup.

The structure and properties of mature upper oceanic plates may evolve through mechanisms such as magmatism, hydrothermal circulation, and faulting. However, high-resolution constraints, especially those involving both P- and S-waves, remain scarce, limiting our ability to detect these processes and assess their impacts on crustal properties. We present high-resolution P- and S-wave velocity models from traveltime tomography of downward-continued long-offset streamer data acquired along a margin-parallel profile in the outer rise of the Sumatra subduction zone. The data reveal high-quality, doubly converted S-wave arrivals from the upper crust. Layer 2A (uppermost crust) exhibits high and laterally uniform Poisson's ratios (0.3-0.35), whereas the underlying Layer 2B is more heterogeneous with lower Poisson's ratios (0.26-0.33). We interpret Layer 2B heterogeneity as reflecting widespread deformation within the Wharton Basin. In contrast, the more uniform and elevated Poisson's ratio in Layer 2A likely indicates the opening of cracks by plate bending in the outer rise.

The Shumagin Gap, a creeping segment of the Alaska subduction zone characterized by tsunamigenic structures, experienced a deep rupture during the July 2020 M7.8 earthquake. However, shallow slip behavior and the upper boundary of the rupture remain poorly understood. Here we utilize controlled-source electromagnetic data to image subsurface electrical resistivity, investigating the role of fluids in modulating megathrust locking state within the Shumagin Gap. Results reveal pronounced trench-normal heterogeneity in electrical resistivity both along the shallow plate interface and within the overriding plate, showing fluid presence but low overall porosity at the interface. An observed conductive channel extending into the overriding plate may facilitate upward fluid drainage. Our findings suggest that the volumes of fluids and inferred pore pressures are not sufficient to explain megathrust creep at the Shumagin Gap. Rather, the intricate interplay between heterogeneous structure and fluid distribution contributes to the region's seismogenic behavior and tsunami hazards, particularly in the shallow portion of the megathrust.

Gas hydrates and shallow gas are common hazards encountered during drilling. They also are important components of the global carbon cycle and a potential energy resource. Predicting the occurrence of free gas and hydrates, particularly highly concentrated accumulations, is therefore important for scientific and safety purposes. Hydrates accumulate following principles that are analogous to conventional hydrocarbons: the hydrate system requires at least one gas source, migration pathways, a suitable reservoir, and a seal. Here, we apply the hydrate system framework to several high-concentration accumulations to demonstrate the geologic controls on the different aspects of the system and the overall propensity of certain locations to host abundant hydrate and free gas. We find that a deep source of gas is typically required in these settings, even if the gas has microbial origins. This gas may be highly pressured and can generate fractures that reach from the base of the hydrate stability zone to the seafloor. At the Cape Fear Slide offshore North Carolina, newly acquired high-resolution 2D multichannel seismic data show details of the association among gas, hydrates, gas venting, and slope failure. Overall our analysis demonstrates the importance of considering the site-specific details of the local hydrate system in any shallow hazard analysis.

Based on measurements of near-trench deformations of the oceanic and overriding plates, in this investigation, we elucidate the tectonic and mechanical processes leading to the Mw7.0 (moment magnitude of 7.0) Acapulco, Mexico, earthquake in 2021. We exploit unprecedented ocean-bottom observations using ultralong-period "tilt mechanical amplifiers," along with hydrostatic pressure, global navigation satellite system, and satellite interferometric synthetic aperture radar data. The joint inversion of these geodetic data, template-matching seismicity, and repeating earthquakes, revealed the first two shallow slow slip events (SSEs) observed in Mexico. The first one migrated from the trench to the earthquake hypocenter before rupture, and the second one occurred following an Mw7.3 long-term SSE induced by the earthquake. Episodic near-trench oceanic-crust deformations (i.e., tilt transients) associated with shallow and deep synchronous decoupling of the plate interface reveal the occurrence of "slab-pull surges" before three regional earthquakes of magnitude 7 or greater, including the Acapulco event, suggesting that they may serve as rupture precursors observable in subduction zones.

Subduction may terminate when a mid-ocean ridge approaches a trench, introducing buoyant lithosphere that resists subduction, leading to slab detachment and plate boundary reconfiguration. Yet, the spatial and temporal dynamics of slab tearing remain enigmatic due to a lack of modern examples. Here, we integrate new seismic images with regional seismicity to investigate an actively fragmenting subduction system at northern Cascadia's ridge-trench-fault triple junction where subduction termination is imminent. Our analyses reveal a broad shear-zone initiated at ~4 Ma by exploiting ridge-parallel fabrics of nascent oceanic lithosphere and progressively localized into a mature trench-perpendicular transform boundary. This process severed an oceanic microplate and enabled its diminished subduction relative to adjacent subducting lithosphere. Downdip, we image trench-parallel slab tears offset by the transform, suggesting lateral tear propagation was intersected by the transform boundary, facilitating efficient decoupling of the microplate while allowing adjacent subduction to continue. We propose a 4D model where transform boundaries drive laterally diachronous slab fragmentation and subduction termination.

The amount of water entering subduction zones and how it is stored within the slab is debated. This limits our understanding of where subducted fluids are released and therefore how pore pressure influences slip behavior along megathrusts. Here we present 3-D compressional and shear-wave velocity models, and their ratio Vp/Vs, of the Alaska Peninsula subduction zone using local earthquake tomography. We investigate the hydration of, and fluid release from, the downgoing plate, and how this impacts recent megathrust ruptures. First, we identify a wide-spread oceanic crust and upper mantle water reservoir, resulting from fluid-filled porosity. Second, slab dehydration is inferred from velocity anomalies along the plate interface interpreted as high pore fluid pressure regions. These regions bound the 2020 Mw7.8 Simeonof, 2021 Mw8.2 Chignik, and 2023 Mw7.2 earthquake rupture zones, demonstrating how locations of fluid release and elevated pore pressure impact megathrust frictional properties and act as rupture barriers.

The Unimak and Shumagin segments of the Alaska Aleutian Subduction Zone show extensional deformation of the forearc since the Miocene. Using legacy seismic profiles and modern multichannel seismic data, we update the structural map of the area, focusing on the intersection between trench-parallel, landward-dipping normal faults rooting in the plate interface and trench-oblique to trench-perpendicular normal faults, all showing signs of recent activity. We investigate for the first time the origin of the trench-parallel extension and explain the horsetail geometry of the Central Sanak Basin as the termination of a slip-partitioning right-lateral strike slip fault. Re-analysis of subduction zone thrust earthquakes slip vectors indicates a possible onset of slip partitioning in the vicinity of the Central Sanak Basin. In the hypothesis of a continuum of deformation, finite deformation from normal fault offsets show a slow sliver motion of less than 1 mm/yr, which is below the resolution of GNSS measurements. Both trench oblique and trench parallel faults have been cited as reactivated terrane sutures, the presence of which may act as upper-plate weaknesses needed to allow slip partitioning in a context of low convergence obliquity. Weak landward-dipping normal faults rooting in the plate interface have also been linked to the tsunamigenic rupture of the shallow plate interface. We infer that the trench parallel Unimak Ridge, associated with the 1946 Mw 8.6 tsunami earthquake, is the last expression of terrane sutures reactivation before their westward vanishing in the more recent Aleutian arc.

We present seismic reflection images of the plate interface up to depth of ~65 km across a ~130,000 km 2 area offshore the Alaska Peninsula. These images capture systematic along-strike and downdip variations in the thickness of the plate interface reflection package, which we interpret in terms of megathrust properties and slip nature. A relatively thin reflection package (<2 km) characterizes the megathrust at depths <24-34 km, marking an area of localized brittle deformation that is likeliest to host the largest seismic asperities. The plate boundary reflection band thickness transitions from 2-5 km over depths of ~24-41 km, suggesting a thickening zone of cumulative deformation and greater heterogeneity corresponding to conditionally stable megathrust behavior. At depths >35-41 km, the reflection package maintains a thickness of 5-6 km and marks the predominantly aseismically slipping megathrust areas of which only the shallower part can host earthquake slip. Historic and recent megathrust earthquakes in the SW Kodiak Asperity and Semidi Segment generally occur within the thin and/or transitional reflection bands. In contrast, the 2020 Simeonof and earlier M7.x earthquakes in the Shumagin Gap are estimated to also rupture the shallowest section of megathrust characterized by the thick reflection band (~35-41 km depth). Although the shallow part of the plate boundary ( in the Semidi and Shumagin segments did not rupture in recent 2020 M7.8 Simeonof and 2021 M8.2 Chignik earthquakes, the seismic properties of the megathrust suggest it may be capable of earthquake slip in the future, as may have occurred in 1788.

Oceanic plates experience extensive normal faulting as they bend and subduct, enabling fracturing of the crust and upper mantle. Debate remains about the relative importance of pre-existing faults, plate curvature and other factors in 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-related faulting as the orientation of abyssal-hill fabric with respect to the trench and plate curvature vary along the margin. Here we characterize bending faulting between longitudes 161°W and 155ºW 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 by over 1 km from 156°W to 160°W, most sediments were deposited prior to the onset of bending faulting and thus 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 ~156ºW, bending faulting parallels magnetic anomalies, implying bending faulting reactivates pre-existing structures. Where magnetic anomalies are highly oblique (>30°) to the trench east of 156ºW, no bending faulting is observed. Summed fault throws increase to the west, including where pre-existing structure orientations do not vary between 157-161ºW, suggesting that the increase in slab curvature directly influences fault throws. However, the westward increase in summed fault throws is more abrupt than expected for changes in slab bending alone, suggesting potential feedbacks between pre-existing structures, slab dip, and faulting.

The role of fluids in earthquake rupture is key to understanding seismic hazards, particularly at subduction zones. The Shumagin Gap, Alaska, is notable due to a paucity of large earthquake nucleation and weak coupling between the overriding and subducting plates. Fluids have been hypothesized to explain these observations, but the source of the fluids remains unclear. Here we present an image of the subsurface electrical resistivity derived from marine magnetotelluric data collected in the Shumagin segment. The model reveals an approximately 50-km-wide conductive (that is, fluid-rich) zone near the plate interface with fluids sourced from the dehydration of slab mantle (15–25 km beneath the crust–mantle boundary). We find that the July 2020 megathrust earthquake—which nucleated near the Semidi segment and propagated westwards into the Shumagin segment—only ruptured the conductive portion of the plate interface. This suggests that slab mantle fluids can influence the seismogenic zone by, for example, creating patches that are prone to dynamic rupture. In contrast, updip of the slip patch is simultaneously resistive and weakly coupled, suggesting that fluids alone are not responsible for weak coupling and that plate roughness plays a role. More broadly, these results suggest that slab mantle fluids could be an underappreciated fluid source in the water budgets of forearc subduction zones.

In the past decade, marine geophysical observations have led to the discovery of thin channels at the base of oceanic plates with anomalous physical properties that indicate the presence of low-degree partial melts. However, mantle melts are buoyant and should migrate toward the surface. We show abundant observations of widespread intraplate magmatism on the Cocos Plate where a thin partial melt channel was imaged at the lithosphere-asthenosphere boundary. We combine existing geophysical, geochemical, and seafloor drilling results with seismic reflection data and radiometric dating of drill cores to constrain the origin, distribution, and timing of this magmatism. Our synthesis indicates that the sublithospheric channel is a regionally extensive (>100,000 km 2 ) and long-lived feature that originated from the Galápagos Plume more than 20 Ma ago, supplying melt for multiple magmatic events and persisting today. Plume-fed melt channels may be widespread and long-lived sources for intraplate magmatism and mantle metasomatism.

The hydration state of subducting oceanic crust has been proposed to influence subduction zone processes like seismic coupling at the megathrust interface and arc magmatism downdip. Plate bending in the outer rise region is thought to help rehydrate the incoming oceanic lithosphere before subduction. Although numerous seismic refraction studies provide constraints on the amount of water stored in the lower crust and uppermost mantle, little information exists about how much free water is present in the upper 1–2 km of oceanic crust. Here, we present results from the application of advanced techniques to long‐offset multi‐channel seismic data acquired outboard the Alaskan and South Sumatran subduction zones. Our results show that the incoming upper crustal seismic layer, layer 2A, is significantly hydrated in both areas. Favorable conditions offshore the Alaska Peninsula promote the creation of a dense system of bending‐related faults, facilitating the infiltration of fluids that increases average water estimates to ∼3.9 wt.% H 2 O in the outer rise. As the crust subducts and temperature increases, some free water may react with the host rocks to form mineral‐bound water that is carried to greater depths, in agreement with elevated water contents found in arc lavas of Shumagin Gap volcanoes. Offshore Sumatra, we propose that similar layer 2A water estimates (∼3.2 wt.% H 2 O average) and heterogeneous hydration within 2B are associated with the ongoing, slow, complex deformation occurring in the Wharton Basin and thus potentially contribute to the presence of a long‐lived slow slip event recently inferred there at seismogenic depths.

Subduction zone architecture and properties are thought to control megathrust slip behavior, but few constraints on crustal structure and megathrust properties are available at sufficient resolution and depth, hindering understanding of linkages between structure and behavior. Here we present a P-wave seismic velocity model based on wide-angle seismic data integrated with collocated reflection imaging in the weakly coupled Shumagin Gap in the Alaska subduction zone, where a M7.8 occurred in July 2020. We show that this earthquake occurred near and below the Moho of the overriding plate where the megathrust is characterized by a 3- to 5-km-thick reflection band interpreted to represent tectonic mixing. The rheological heterogeneity of the plate boundary near and below the Moho could account for abundant interplate seismicity, repeated M7.x events, and patchiness of the 2020 rupture. Velocity variations in the overriding continental crust imply changes in rigidity that could further influence megathrust slip.

We interpret a region of undulatory sediments adjacent to a major headwall of the Cape Fear submarine landslide system offshore of North Carolina, USA, as sediment waves rather than creep or fault-related deformation. The wave package extends 19 km upslope from the S4 landslide headwall and thickens upslope from approximately 250 to 450 m. The field of undulating sediments displays the continuity of seismic horizons, upslope-migrating crests, downslope thinning, and wave heights and lengths of approximately 26 m and approximately 1 km, respectively, which are consistent with sediment wavefields. The Western Boundary Undercurrent formed contourites on the nearby Blake Ridge and it is possible that these sediment undulations were deposited via similar mechanisms. Ocean Drilling Program (ODP) cores near the sediment undulation field suggest that the turbidity currents also may have played a role in wave formation. Although most of the 19 km long field comprises unaltered sediment waves, we observe an approximately 5 km long zone adjacent to the landslide scarp that expresses evidence of faults that offset and deform the sediment wave strata. We interpret this deformation as the result of reduction in stress following the removal of the landslide mass. Given that the Cape Fear system has generated several episodes of potentially tsunamigenic slope failure, the future stability of the system is pertinent. Redefining these undulatory sediments as sediment waves eliminates a major slope instability mechanism of the system and is important for understanding the future slope stability hazards of Cape Fear. Our analysis highlights the importance of understanding sediment waves in hybrid submarine landslide-sediment wave systems. Geological feature: Cape Fear submarine landslide sediment waves Seismic appearance: Continuous undulating horizons Alternative interpretations: Downslope creep or faults Features with similar appearance: Extensional slope failure Formation: Alongslope and downslope currents Age: Quaternary Location: Cape Fear submarine landslide complex, offshore North Carolina Seismic data: High resolution multichannel seismic data Analysis tools: Multichannel seismic data, multibeam bathymetry, sub-bottom Chirp

A decade-long research collaboration has revealed that the split between Africa and North America roughly 200 million years ago was more drawn out than previously thought.