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Heat flow measurements indicate hydrothermal activity in oceanic crust continues at least for 65 m.y. after formation. Hydrothermal activity progressively fills cracks and pores with alteration products, which is expected to lead to a trend of increasing seismic velocities with age. Compilations of seismic-P-wave velocity models inverted from ocean bottom seismometer (OBS) data have failed to detect such an aging trend beyond crustal ages of ca. 10 Ma. However, in these models, the velocities of the uppermost crust, where fluid flow would be most concentrated, are poorly resolved. This is because as the oceanic crust matures, the first crustal arrivals on OBS records (which best resolve upper crustal velocities using tomographic inversion), become hidden in the coda of the water wave. This may lead to the masking of any aging trend in the seismic velocities. For the first time, we show how including downward continuation (DC) in the analysis of OBS data collected across 65 Ma seafloor significantly improves measurements of the P-wave velocities of the upper crust. Our new analysis reveals a highly heterogeneous upper crust, with ridge-parallel P-wave velocity variations of 25%, implying local porosity values that are up to double that of global averages. Our new results, combined with other most recent advanced seismic analyses, reveal that seismic velocities indeed evolve with age up to at least 70 Ma, confirming that hydrothermal activity continues in mature oceanic crust. [ABSTRACT FROM AUTHOR]

The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for vp and vp=vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw D 8:7) and to the south in 2007 (Bengkulu earthquake, Mw D 8:5). The 2-D and 3-D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ~180 km distance to the trench. vp velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s-1 at 10 km depth and the vp=vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp=vs values of 1.85 at ~150 km distance to the trench in the region of the Mentawai Fault. vp=vs ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust. [ABSTRACT FROM AUTHOR]

The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel time data of local earthquakes for vp and vp/vs velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, Mw=8.7) and to the south in 2007 (Bengkulu earthquake, Mw=8.5). 2D and 3D vp velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3D velocity model shows prevailing trench parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at ~ 180 km distance to the trench. Vp velocities beneath the magmatic arc and the Sumatran fault zone SFZ are around 5 km/s at 10 km depth and the vp/vs ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated vp/vs values of 1.85 at ~ 150 km distance to the trench in the region of the Mentawai fault. Vp/vs ratios suggest absence of large scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced vp velocities beneath the forearc basin covering the region between Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust. [ABSTRACT FROM AUTHOR]

Determining reliable cable ampacities for marine High Voltage Cables is currently the subject of significant industry and academic reassessment in order to optimize (maximizing load while maintaining safe operating temperatures) design and reduce costs. Ampacity models can be elaborate, and inaccuracies are increasingly predicated on the uncertainty in environmental inputs. A stark example is the role of ambient temperature at cable depth, which, due to the scale of cables and the inaccessibility of the seafloor, is commonly estimated at 15 °C. Oceanographic models incorporating ocean bottom temperature are increasingly available, and they achieve coverage and spatiotemporal resolutions for cable applications without the requirement for project specific measurements. Here, a rudimental validation of the AMM15 and AMM7 mean monthly ocean bottom temperature models for the NW European Shelf indicates encouraging accuracies (MBE ≤ 1.48 °C; RMSE ≤ 2.2 °C). A series of cable case studies are used to demonstrate that cable ratings can change between −4.1% and +7.8% relative to ratings based on a common static (15 °C) ambient temperature value. Consideration of such variations can result in both significant ratings (and hence capital expenditure and operating costs) gains and/or the avoidance of cable overheating. Consequently, validated modelled ocean bottom temperatures are deemed sufficiently accurate, providing incomparable coverage and spatiotemporal resolutions of the whole annual temperature signal, thereby facilitating much more robust ambient temperatures and drastically improving ampacity estimates. [ABSTRACT FROM AUTHOR]

Seismic activity in the Lesser Antilles (LA) is characterized by strong regional variability along the arc reflecting the complex subduction setting and history. Although routine seismicity monitoring can rely on an increasing number of island stations, the island-arc setting means that high-resolution monitoring and detailed studies of fault structures require a network of ocean bottom seismometers (OBS). As part of the 2016–2017 Vo lat i le recycling at the L esser A ntilles arc (VoiLA) project, we deployed 34 OBS stations in the forearc and backarc. During the deployment time, 381 events were recorded within the subduction zone. In this paper, we perform full-waveform regional moment tensor (RMT) inversions, to gain insight into the stress distribution along the arc and at depth. We developed a novel inversion approach, AmΦB—'Amphibious Bayesian', taking into account uncertainties associated with OBS deployments. Particularly, the orientation of horizontal components (alignment uncertainty) and the high noise level on them due to ocean microseisms are accounted for using AmΦB. The inversion is conducted using a direct, uniform importance sampling of the fault parameters within a multidimensional tree structure: the uniXtree-sampling algorithm. We show that the alignment of the horizontal OBS components, particularly in high noise level marine environments, influences the obtained source mechanism when using standard least-squares (L2) RMT inversion schemes, resulting in systematic errors in the recovered focal mechanisms including high artificial compensated linear vector dipole (CLVD) contributions. Our Bayesian formulation in AmΦB reduces these CLVD components by nearly 60 per cent and the aberration of the focal geometry as measured by the Kagan angle by around 40 per cent relative to a standard L2 inversion. Subsequently, we use AmΦB-RMT to obtain 45 (M w > 3.8) regional MT solutions, out of which 39 are new to any existing database. Combining our new results with existing solutions, we subsequently analyse a total of 151 solutions in a focal mechanism classification (FMC) diagram and map them to the regional tectonic setting. We also use our newly compiled RMT database to perform stress tensor inversions along the LA subduction zone. On the plate interface, we observe the typical compressional stress regime of a subduction zone and find evidence for upper-plate strike slip and normal fault behaviour in the north that becomes a near arc-perpendicular extensional stress regime towards the south. A dominant slab perpendicular extensional stress regime is found in the slab at 100–200 km beneath the central part of the arc. We interpret this stress condition to be a result of slab pull varying along the arc due to partial slab detachment along previously hypothesized lateral slab tear near Grenada, at the southern end of the LA arc, leading to reactivation of pre-existing structures around the subducted Proto-Caribbean ridge. [ABSTRACT FROM AUTHOR]

In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and provides a window into its temporal evolution. V-shaped ridges of thick oceanic crust observed south of Iceland are thought to record pulses of upwelling within the plume. Specifically, excess crust is thought to form during the quasi-periodic generation of hot solitary waves triggered by thermal instabilities in the mantle. Here we use seismic reflection data to show that V-shaped ridges have formed over the past 55 million years-providing the longest record of plume periodicity of its kind. We find evidence for minor, but systematic, asymmetric formation of crust, due to migration of the mid-ocean ridge with respect to the underlying plume. We also find changes in periodicity: from 55 to 35 million years ago, the V-shaped ridges form every 3 million years or so and reflect small fluctuations in plume temperature of about 5-10 °C. From 35 million years ago, the periodicity changes to about 8 million years and reflects changes in mantle temperature of 25-30 °C. We suggest that this change in periodicity is probably caused by perturbations in the thermal state at the plume source, either at the mantle-transition zone or core-mantle boundary. [ABSTRACT FROM AUTHOR]

Estimating the location of geologic and tectonic features on a subducting plate is important for interpreting their spatial relationships with other observables including seismicity, seismic velocity and attenuation anomalies, and the location of ore deposits and arc volcanism in the over-riding plate. Here we present two methods for estimating the location of predictable features such as seamounts, ridges and fracture zones on the slab. One uses kinematic reconstructions of plate motions, and the other uses multidimensional scaling to flatten the slab onto the surface of the Earth. We demonstrate the methods using synthetic examples and also using the test case of fracture zones entering the Lesser Antilles subduction zone. The two methods produce results that are in good agreement with each other in both the synthetic and real examples. In the Lesser Antilles, the subducted fracture zones trend northwards of the surface projections. The two methods begin to diverge in regions where the multidimensional scaling method has its greatest likely error. Wider application of these methods may help to establish spatial correlations globally. [ABSTRACT FROM AUTHOR]

Kevin Burke played a major role in the development of the "Pacific origin model" of Caribbean tectonics. The emplacement of the Caribbean plateau between the Americas, behind a series of north and east facing subduction zones known as the Great Arc of the Caribbean (GAC) throughout the Late Cretaceous and Paleogene is now widely accepted as the fundamental mechanism for the long-term evolution of the region. However, within this framework there are still many areas whose structure and tectonic history are poorly constrained. We present a compilation of new (collected in 2017) and historical marine magnetic anomaly and seismic data. We combine this with constraints from published geochronological and geochemical data in order to propose a new tectonic model for the Eastern Caribbean. We interpret phases of both arc retreat and advance along the eastern plate boundary with significant implications for the crustal affinity of the modern day Lesser Antilles Arc, Grenada and Tobago Basins, with clear evidence for east-west orientated back-arc spreading either side of the Aves Ridge (a now extinct Late Cretaceous segment of the GAC). It was previously recognised that in the north the arc migrated from the Aves Ridge to the islands of the Limestone Caribbees during the Eocene. We now suggest that this easternmost magmatic front extended the full length of the current arc, below the modern day Barbados accretionary prism. This outer arc went extinct as a result of westward arc advance and formation of the modern-day Lesser Antilles, subdividing the previous backarc basin into the southern Grenada and Tobago Basins during the Oligocene. Globally, whilst slab retreat and backarc spreading in oceanic subduction systems are commonplace, arc advance is rare. To our knowledge this is the first known example of wholesale arc advance in an intra-oceanic setting and may help explain along arc variations in Lesser Antilles geochemistry. The back and forth motions of the arc are probably expressions of the constraints the two Americas impose on this isolated subduction system rather than of variations in subducting plate buoyancy. [ABSTRACT FROM AUTHOR]

Much of our current knowledge on the seismic structure of mature oceanic crust comes fromstudies older than 25 years, which suffer from limited lateral resolution, because receiverspacings at the time were typically on the order of 20 – 25 km. These studies show thatfracture zones and non-transform offsets are expressed as varying degrees of thinning of theoceanic crust, while oceanic crustal structure varies little. However, recent high-resolutionstudies along the Mid-Atlantic Ridge have emphasised the extent of along axis variation incrustal accretion, providing new insights into processes such as the tectonic accommodationof extension at segment ends, through the exhumation of mantle peridotites alonglarge detachment faults. This motivates the re-examination of mature fracture zonesand oceanic crust formed at slow-spreading ridges at a comparable along-strikeresolution. We present results from an active seismic experiment, with 54 ocean-bottomseismometers spaced every 4 km, conducted over ∼65 Ma central Atlantic crust in 2017. The225 km long profile crosses three discontinuities, one of which can be seen on satellitegravity data to follow Atlantic spreading flowlines (Marathon FZ) and two which do not. Wetherefore have an opportunity to compare the structure generated at transform andnon-transform offsets. We have developed a 2D compressional velocity model by forwardand inverse modelling of wide-angle seismic data. In the resulting model we observe transform and non-transform offsets as asignificant thinning of the oceanic crust from 6.9 ± 1.5 km thick in the segment centres,to 4.0 – 5.0 km, over lateral distances of 9.0 – 20.0 km. Crustal thickness withinthe oceanic segments is asymmetric, with a rapid shoaling of the Moho at insidecorners (∼260 m/km) and a gradual shoaling at outside corners (∼75 m/km), withthe difference being attributed to enhanced mechanical deformation at the insidecorners. Within the segment centres two clear basement layers are observed; uppermost crustalvelocities of 4.6 ± 0.3 kms 1 increase to 6.4 ± 0.4 kms1 in layer 2 (gradient of∼0.9 s−1), and then increase to 7.5 ± 0.3 kms1 at the base of layer 3 (gradient of∼0.2 s1). In contrast, within the offsets the seismic structure is characterised by asingle velocity gradient, increasing from uppermost crustal velocities to mantlevelocities (∼7.8 kms−1) at depths of 4.0 – 5.0 km below top basement. Nowherewithin these domains is there a clear velocity structure indicative of oceanic layer3. These results are consistent with an interpretation that these structures formedthrough the pervasive hydration and serpentinization during active deformationprocesses. However, there are structural and seismic differences between the Marathon FZ and thetwo non-transform offsets. Nontransform offsets exhibit no clear Moho reflections,velocity gradients of ∼0.6 0.7 s1, and one is flanked by significant basementtopography. Conversely, the Marathon FZ shows muted basement topography,exhibits Moho reflections and a velocity gradient of ∼0.4 s1. This difference invelocity gradient indicates either a differing degree of serpentinization or structuralcomposition, while Moho reflections suggest the presence of a reflective alteration front. [ABSTRACT FROM AUTHOR]