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Improvements in structure-from-motion techniques are enabling many scientific fields to benefit from the routine creation of detailed 3D models. However, for a large number of ...applications, only a single camera is available for the image acquisition, due to cost or space constraints in the survey platforms. Monocular structure-from-motion raises the issue of properly estimating the scale of the 3D models, in order to later use those models for metrology. The scale can be determined from the presence of visible objects of known dimensions, or from information on the magnitude of the camera motion provided by other sensors, such as GPS.
This paper addresses the problem of accurately scaling 3D models created from monocular cameras in GPS-denied environments, such as in underwater applications. Motivated by the common availability of underwater laser scalers, we present two novel approaches which are suitable for different laser scaler configurations. A fully unconstrained method enables the use of arbitrary laser setups, while a partially constrained method reduces the need for calibration by only assuming parallelism on the laser beams and equidistance with the camera. The proposed methods have several advantages with respect to existing methods. By using the known geometry of the scene represented by the 3D model, along with some parameters of the laser scaler geometry, the need for laser alignment with the optical axis of the camera is eliminated. Furthermore, the extremely error-prone manual identification of image points on the 3D model, currently required in image-scaling methods, is dispensed with.
The performance of the methods and their applicability was evaluated both on data generated from a realistic 3D model and on data collected during an oceanographic cruise in 2017. Three separate laser configurations have been tested, encompassing nearly all possible laser setups, to evaluate the effects of terrain roughness, noise, camera perspective angle and camera-scene distance on the final estimates of scale. In the real scenario, the computation of 6 independent model scale estimates using our fully unconstrained approach, produced values with a standard deviation of 0.3%. By comparing the values to the only other possible method currently usable for this dataset, we showed that the consistency of scales obtained for individual lasers is much higher for our approach (0.6% compared to 4%).
Seafloor spreading at slow and ultraslow rates is often taken up by extension on large-offset faults called detachments, which exhume lower crustal and mantle rocks, and in some cases make up domed ...oceanic core complexes. The exposed footwall may reveal a characteristic pattern of spreading-parallel corrugations, the largest of which are clearly visible in multibeam bathymetric data, and whose nature and origin have been the subject of controversy. In order to tackle this debate, we use available near-bottom bathymetric surveys recently acquired with autonomous deep-sea vehicles over five corrugated detachments along the Mid-Atlantic Ridge. With a spatial resolution of 2 m, these data allow us to compare the geometry of corrugations on oceanic detachments that are characterized by differing fault zone lithologies, and accommodate varying amounts of slip. The fault surfaces host corrugations with wavelengths of 10–250 m, while individual corrugations are finite in length, typically 100–500 m. Power spectra of profiles calculated across the corrugated fault surfaces reveal a common level of roughness, and indicate that the fault surfaces are not fractal. Since systematic variation in roughness with fault offset is not evident, we propose that portions of the exposed footwalls analyzed here record constant brittle strain. We assess three competing hypotheses for corrugation formation and find that the continuous casting and varying depth to brittle–ductile transition models cannot explain the observed corrugation geometry nor available geological observations. We suggest a model involving brittle strain localization on a network of linked fractures within a zone of finite thickness is a better explanation for the observations. This model explains corrugations on oceanic detachment faults exposed at the seafloor and on normal faults in the continents, and is consistent with recently imaged corrugations on a subduction zone megathrust. Hence fracture linkage and coalescence may give rise to corrugated fault zones, regardless of earlier deformation history and tectonic setting.
•Five oceanic detachment faults mapped at 2 m resolution using underwater vehicles.•Corrugation geometry does not depend upon fault lithology or deformation history.•Fracture coalescence can lead to corrugated faults in the oceans and continents.
Oxide gabbros are a minor but diffuse component of the lower oceanic crust. Their presence poses questions on lower crust exhumation processes and magma differentiation at mid ocean ridges because ...they are systematically associated with shear zones and are hardly explained by classical fractionation and melt migration models. Here, we report on a study of lower-crust gabbros recovered from the Vema Lithospheric Section at 11°N along the Mid Atlantic Ridge, where oxide gabbros are abnormally abundant relative to ridge centred magmatic intrusives and where we found a peculiar lithological occurrence represented by deformed diorites extremely enriched in Fe-Ti-oxides and apatites. Their complex genetic history reveals a hybrid nature consistent with derivation from high pressure injections of Fe-Ti-P saturated nelsonitic melts in a primitive gabbroic groundmass that induced fracturing, de-compaction, mineral resorption and chemical re-equilibration. Melt injections may occur after intense ductile shearing at the edges of the axial magma chamber following lateral differentiation of primitive melts injected at the centre of the ridge axis segment. We propose a regime of lateral, instead of vertical, melt differentiation along the ridge axis and a possible role for melt immiscibility in the formation of Fe-Ti-P melt pockets in oceanic domains.
•Anomalously abundant apatite-rich ferrodiorites and evolved gabbros are sampled at the Vema Lithospheric Section (MAR).•Ferrodiorites are hybrid rocks formed by injection of nelsonitic melts at the ductile/brittle transition.•Strong melt differentiation results from lateral melt percolation possibly fostering fluid immiscibility.•Melt injection is controlled by the ridge parallel normal faults rooting close to the axial magma chamber.•Major fault slip events generate melt squeezing, overpressurized injection, cataclasis and resorption during relaxation.
The Lucky Strike vent field, located on the Mid‐Atlantic Ridge (MAR), is hosted on enriched mid‐ocean ridge basalt associated with the nearby Azores hotspot. In this study, we present bulk rock ...geochemistry coupled with in situ sulfur isotope analysis of hydrothermal samples from Lucky Strike. We assess the geological controls on the differences in the major and trace element content and sulfur isotopic composition of the hydrothermal deposits within the vent field. The hydrothermal deposits contain elevated concentrations of elements typically enriched in mid‐ocean basalt (E‐MORB), such as Mo, Ba, and Sr, compared to typical values for other hydrothermal deposits hosted on the MAR. The range in sulfur isotope compositions of hydrothermal marcasite and chalcopyrite (−2.5 to 8.7‰) is similar to the range recorded at other sediment‐free basalt‐hosted seafloor hydrothermal sites. However, at Lucky Strike, the Capelinhos vent, situated 1.4 km east of the main field, is enriched in 34S (by ∼3.5‰ for both marcasite and chalcopyrite), relative to the main field. This difference reflects contrasting subseafloor fluid/rock interactions at these two sites, including subseafloor sulfide precipitation at the main field that results in <20% of reduced sulfur within the upwelling hydrothermal fluid reaching the seafloor. We also compare the geochemistry of the hydrothermal deposits at Lucky Strike to other hydrothermal sites along the MAR and show that the average hydrothermal deposit Ba/Co is useful to discriminate between E‐MORB and other mafic/ultramafic hosted deposits.
Plain Language Summary
We investigate the variations in composition of metal‐ and sulfur‐rich hydrothermal deposits that form on the seafloor at a cluster of high‐temperature hot springs called the Lucky Strike hydrothermal vent field, on the Mid‐Atlantic Ridge. We find that the mineralogy and geochemistry of the deposits do not vary spatially within this vent field. However, variations in the relative abundances of different sulfur isotopes within these deposits differ between the central cluster of vents and a newly discovered site called Capelinhos that is located 1.4 km east of the main vent field. Isotopic variations are usually interpreted to indicate differences in sulfur sources, with seawater and sulfur from the mantle as the two primary sources. However, our results instead show that significant mineral precipitation below the seafloor at the main vent cluster is the likely source of these isotopic variations. In addition, we show that the relative abundances of various trace elements within the hydrothermal deposits can be used to fingerprint the composition of the volcanic rocks that host these deposits. In particular, the ratio of Ba to Co can be used to fingerprint specific tectonic settings for different hydrothermal vent sites on mid‐ocean ridges.
Key Points
Spatial variations in in situ sulfur isotope compositions at Lucky Strike indicate differences in fluid/rock interactions in the sub‐surface
In situ sulfur isotope data suggests that >80% of the available H2S in the ascending hydrothermal fluid has precipitated in the subseafloor
The Ba/Co ratio of hydrothermal deposits discriminates those associated with enriched mid‐ocean basalts from other mafic/ultramafic hosted deposits
Oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid-ocean ridges, and is associated with dramatic changes in seafloor morphology. Detachments form ...expansive dome structures with corrugated surfaces known as oceanic core complexes (OCCs), and often transition to multiple regularly-spaced normal faults that form abyssal hills parallel to the spreading axis. Previous studies have attributed these changes to along-axis gradients in lithospheric strength or magma supply. However, despite the recognition that magma supply can influence fault style and seafloor morphology, the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along-axis variations in magma supply remain poorly understood. This study investigates this issue using two complementary modeling approaches. The first consists of semi-analytical, two-dimensional (2-D) cross-axis models designed to address the fundamental mechanical controls on the longevity of normal faults. These 2-D model sections are juxtaposed in the along-axis direction to examine the response of the plan-view pattern of faults to along-axis variations in magmatic accretion in the absence of along-axis mechanical coupling. The second approach uses three-dimensional (3-D), time-dependent numerical models that simulate faulting and magma intrusion in a visco-elasto-plastic continuum. The primary variable studied through both approaches is the along-axis gradient in the fraction M of seafloor spreading that is accommodated by magmatism. The 2-D and 3-D results predict different abyssal hill spacing and orientation, however the plan-view geometry of self-emerging detachment faults predicted by the 3-D numerical models are well explained by the juxtaposed 2-D models. This indicates a first-order control by cross-axis effects of changing values of M. These models are also shown to explain the along-axis extent and plan-view curvature of the well-developed 13°20′N and Mt. Dent OCCs (Mid-Atlantic Ridge and Cayman Rise) in terms of quantifiable along-axis gradients in magma emplacement rates.
•Map-view geometry of Oceanic Core Complexes relates to magma supply variations.•Magma supply variations can induce transition from OCC to linear abyssal hills.•2-D physics predict some 3-D fault behaviors, and may be used to infer magma supply.•Future seismic studies could test between continuous and consecutive fault models.
Oceanic core complexes are massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor. They form at mid-ocean ridges through slip on detachment faults rooted below the ...spreading axis. To date, most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes. Here we present a survey of the Mid-Atlantic Ridge near 13 degrees N containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A field of extinct core complexes extends away from the axis for at least 100 km. Our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity. Within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity. Preliminary examination of seismicity and seafloor morphology farther north along the Mid-Atlantic Ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected.
The region of the Mid‐Atlantic Ridge (MAR) between the Fifteen‐Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults ...show large amounts of rotation, dome‐shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off‐axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward‐facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long‐lived, a dome‐shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.
We investigate the potential of satellite imagery to map and monitor the activity of shallow-water hydrothermal systems, which are often found around volcanic islands. For this study, we used ...publicly available data and proprietary WorldView-2 satellites images, with spectral bands that can penetrate up to water depths of 30 m. Shallow water hydrothermal sites are visible on satellite imagery, primarily with publicly available data, demonstrating the potential of satellite imagery to study and monitor shallow water hydrothermal activity. We focus our work on volcanic islands, showing intense near-shore, shallow-water hydrothermal activity, and distinct styles of hydrothermal venting. Satellite imagery constrains regional outflow geometry and the temporal variability or stability of these systems. Milos Island shows hydrothermal outflow associated with reflective mineral precipitates and/or bacterial mats, which are stable over time (2010–2014). These outflows locally define polygonal patterns likely associated with hydrothermal convection in porous media. In Kueishantao Island individual hydrothermal plumes charged with particles are visible at the sea surface, and display great variability in intensity and distribution of plume sources (2002–2019). Worldwide we have identified ~15 shallow water hydrothermal sites with satellite imagery, that are similar to either the Milos system (e.g., Vulcano and Panarea, Italy), or the Kueishantao system (numerous sites in Pacific volcanic islands). This study demonstrates that satellite imagery can be used to map and monitor different types of shallow-water hydrothermal systems, at regional scale, and monitor their evolution. Satellite data provide not only regional and temporal information on these systems, unavailable to date, but also the regional context for follow-up in situ field data and observations (e.g., instrumental monitoring, sampling, observations and mapping with divers or AUVs) to understand both the nature and dynamics of these systems, and ultimately the associated fluxes.
•On satellite images we mapped hydrothermal outflow at the seafloor and at sea surface.•Plumes at sea surface and seafloor outflow were observed over several years.•Hydrothermal outflow framework at the seafloor is mapped at a regional scale.•Polygons geometry is identified, reminiscent of possible free convection.
The arc of the Lesser Antilles is associated with a significant tectonic activity due to the subduction of the Atlantic oceanic plate under the Caribbean plate. Earthquakes in this context have the ...potential to trigger landslides and tsunamis due to the important vertical seafloor displacement. The historical tsunamigenic earthquakes in this region are rare, but the damages they may have generated before along the coasts show that they pose a considerable threat to the closest inhabited islands. The most recent tsunamigenic earthquake occurred in 2004 in the area of Les Saintes, along a normal fault system located in the back-arc of the subduction. This
M
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6.3
earthquake generated small waves with 2 m of run-up in several bays of Les Saintes, a group of islands in the South of Guadeloupe. A recent survey conducted in the source area using deep-sea vehicles revealed for the first time an important coseismic slip on the Roseau fault plane, attributed to the 2004 event, which had not been predicted in the seismic inversion models. This event and the dataset on the Roseau fault gives the opportunity to model precisely the earthquake, to compare the simulation results with the observations and to evaluate the impact of the rupture heterogeneity and rupture shallowness on the height of the tsunami waves. Extending our earlier work (Cordrie et al. in OCEANS 2019—Marseille, Jun 2019, Marseille, France, pp 1–9, 2019.
https://doi.org/10.1109/OCEANSE.2019.8867447
) and in order to avoid a loss of quality from the dataset in the modeling of the initial sea-surface deformation especially in shallow depth and near field context, a transfer function of the deformation from the seafloor to the sea surface and different numerical schemes were used. Results on the tsunami height distribution indicate some local tsunami amplification phenomena linked to the bathymetry or the coastline geometry and highlight the most endangered areas of the islands. The simulations give additional constraints on the source, show the impact of the slip heterogeneities on the tsunami and finally provide a complementary estimation of the 2004 coseismic slip intensity.
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