Subducting slabs carry water into the mantle and are a major gateway in the global geochemical water cycle. Fluid transport and release can be constrained with seismological data. Here we use joint ...active-source/local-earthquake seismic tomography to derive unprecedented constraints on multi-stage fluid release from subducting slow-spread oceanic lithosphere. We image the low P-wave velocity crustal layer on the slab top and show that it disappears beneath 60-100 km depth, marking the depth of dehydration metamorphism and eclogitization. Clustering of seismicity at 120-160 km depth suggests that the slab's mantle dehydrates beneath the volcanic arc, and may be the main source of fluids triggering arc magma generation. Lateral variations in seismic properties on the slab surface suggest that serpentinized peridotite exhumed in tectonized slow-spread crust near fracture zones may increase water transport to sub-arc depths. This results in heterogeneous water release and directly impacts earthquakes generation and mantle wedge dynamics.
IODP Expedition 357 used two seabed drills to core 17 shallow holes at 9 sites across Atlantis Massif ocean core complex (Mid-Atlantic Ridge 30°N). The goals of this expedition were to investigate ...serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. More than 57 m of core were recovered, with borehole penetration ranging from 1.3 to 16.4 meters below seafloor, and core recovery as high as 75% of total penetration in one borehole. The cores show highly heterogeneous rock types and alteration associated with changes in bulk rock chemistry that reflect multiple phases of magmatism, fluid-rock interaction and mass transfer within the detachment fault zone. Recovered ultramafic rocks are dominated by pervasively serpentinized harzburgite with intervals of serpentinized dunite and minor pyroxenite veins; gabbroic rocks occur as melt impregnations and veins. Dolerite intrusions and basaltic rocks represent the latest magmatic activity. The proportion of mafic rocks is volumetrically less than the amount of mafic rocks recovered previously by drilling the central dome of Atlantis Massif at IODP Site U1309. This suggests a different mode of melt accumulation in the mantle peridotites at the ridge-transform intersection and/or a tectonic transposition of rock types within a complex detachment fault zone. The cores revealed a high degree of serpentinization and metasomatic alteration dominated by talc-amphibole-chlorite overprinting. Metasomatism is most prevalent at contacts between ultramafic and mafic domains (gabbroic and/or doleritic intrusions) and points to channeled fluid flow and silica mobility during exhumation along the detachment fault. The presence of the mafic lenses within the serpentinites and their alteration to mechanically weak talc, serpentine and chlorite may also be critical in the development of the detachment fault zone and may aid in continued unroofing of the upper mantle peridotite/gabbro sequences.
New technologies were also developed for the seabed drills to enable biogeochemical and microbiological characterization of the environment. An in situ sensor package and water sampling system recorded real-time variations in dissolved methane, oxygen, pH, oxidation reduction potential (Eh), and temperature and during drilling and sampled bottom water after drilling. Systematic excursions in these parameters together with elevated hydrogen and methane concentrations in post-drilling fluids provide evidence for active serpentinization at all sites. In addition, chemical tracers were delivered into the drilling fluids for contamination testing, and a borehole plug system was successfully deployed at some sites for future fluid sampling. A major achievement of IODP Expedition 357 was to obtain microbiological samples along a west–east profile, which will provide a better understanding of how microbial communities evolve as ultramafic and mafic rocks are altered and emplaced on the seafloor. Strict sampling handling protocols allowed for very low limits of microbial cell detection, and our results show that the Atlantis Massif subsurface contains a relatively low density of microbial life.
•Seabed rock drills and real-time fluid monitoring for first time in ocean drilling•First time recovery of continuous sequences along oceanic detachment fault zone•Highly heterogeneous rock type and alteration in shallow detachment fault zone•High methane and hydrogen concentrations in Atlantis Massif shallow basement•Oceanic serpentinites potentially provide important niches for microbial life
Receiver ghosts attenuate marine seismic reflection data at harmonic frequencies that depend on the propagation angle and the streamer depth below the sea surface. The resulting loss of bandwidth is ...one of the major factors hampering seismic resolution. In near-surface and legacy multi-channel data, receivers depth is often unknown and may vary significantly both along the streamer length and during the survey, making frequency–slowness deghosting techniques unsuitable. In this work, we present a method for the attenuation of receiver ghost reflections in data with an arbitrary streamer depth profile varying during the survey. For each trace, a different deghosting operator is estimated and applied at different two-way-time windows, in order to account for depth-dependent changes in reflection angles. The ghost null frequencies are picked on the time-varying power spectrum via an automatic algorithm, guided by a user-dependent a priori function, and optimised to respect the harmonics’ periodicity. The power of the inverse filter is adjusted by adaptively damping abnormal amplitudes in the deghosted spectra. The algorithm is applied to high-resolution (GI-gun, 20–400 Hz) and ultra-high-resolution (Sparker, 0.2–3.0 kHz) multi-channel datasets, yielding an excellent bandwidth recovery and gain in resolution in the final stacks. Limited computing time and straightforward application make the method widely applicable and cost-effective.
•An unprecedented detailed tectono-thermal history of a magma-poor margin is revealed.•Deformation mechanisms laterally vary across active faults during extreme extension.•Mantle hydration occurs ...through brittle deformation in the footwalls of active faults.•Detachments form through ductile shearing in the hangingwalls of active faults.•Detachment formation is a byproduct but not a root cause of margin asymmetry.
A long-standing problem in solid Earth science is to understand how low-angle normal faults form, their role in the development of tectonic asymmetry of conjugate margins, and how they relate to mantle hydration during continental breakup. The latter requires water to reach the mantle through active brittle faults, but low angle slip on faults is mechanically difficult. Here, we incorporate observations from high-resolution multichannel seismic data along the West Iberia-Newfoundland margins into a 2D forward thermo-mechanical model to understand the relationship between evolving rift asymmetry, detachment tectonics, and mantle hydration. We show that, during extreme extension, slip on active faults bifurcates at depth into brittle and ductile deformation branches, as a result of the cooling of the faults' footwall and heating of their hangingwall. The brittle deformation penetrates the Moho and leads to mantle hydration, while ductile deformation occurs in localized shear zones and leads to the formation of detachment-like structures in the distal margin sections. Such structures, as for example ‘S’ in the West Iberia-Newfoundland margins, are thus composed of several shear zones, active at low-angles, ∼25°-20°, and merging with the Moho at depth. The final sub-horizontal geometry of these structures is the result of subsequent back-rotation of these shear zones by new oceanward faults. Our results reproduce remarkably well the final sedimentary, fault, crustal architecture, and serpentinisation pattern observed at the West Iberia-Newfoundland margins. However, they challenge widely accepted ideas that such detachment-like structures formed by brittle processes, separate crust from mantle and caused conjugate margin asymmetry. Our model provides a quantitative framework to study hydrothermal systems related to serpentinization during extreme extension, their associated hydrogen, methane production, and the chemosynthetic life they sustain.
SUMMARY
Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically ...to densely sampled seismic data. In this study, we applied FWI to 3-D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our data set samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2-D, 2.5-D and 3-D-FWI performances using a real sparsely spaced data set. We performed 3-D FWI in the time domain and compared the results with 2-D and 2.5-D FWI results from a profile through the 3-D model. When overlaid on multichannel seismic images, the 3-D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2-D result. Furthermore, we estimate variable serpentinization of the upper mantle below the S-reflector along the profile using 3-D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2-D, 2.5-D and 3-D inversions suggest that 2-D inversion can be prone to overfitting when using a sparse data set. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic data sets. Based on our comparison of the velocity models, we conclude that the use of 3-D data can partially mitigate the problem of receiver sparsity in FWI.
The offshore Bangladesh includes the northern Bengal fan, where sediment supply from the Ganges and Brahmaputra rivers has resulted in the accumulation of up to 20 km of shallow-marine, ...fluvio-deltaic and slope sediments that have accumulated during rapid tectonic subsidence since the late Miocene. The high sedimentation rates, along with high organic matter content, make this area favorable for the formation of natural gas from both microbial and thermogenic sources. Here we use multichannel seismic reflection profiles and modelling of the gas hydrate stability zone (GHSZ) to present the first evidence for the occurrence of natural gas hydrate in the offshore Bangladesh. First, we analyze the sediments of the shelf and slope areas, which are characterized by downslope sediment transport features and by the presence, in places, of faults/fractures as well as widely distributed amplitude anomalies and seismic facies that we relate to the presence of gas. A high-amplitude reversed polarity reflection of variable continuity that mimics the seafloor and cross-cut stratigraphy is interpreted as a Bottom Simulating Reflector (BSR). The BSR is observed in several areas that are predominantly located in the E-SE of the study area, in water depths of 1300–1900 m and at depths below seafloor of 250–440 m. Sediments above BSR locations generally show higher seismic interval velocities reaching values of ∼1920–1940 m/s, which are consistent with the presence of gas hydrate in shallow marine sediments. Furthermore, the BSR lies at approximately the same depth as the theoretical base of the gas (methane) hydrate stability zone (BGHSZ), calculated assuming a 3.5 % wt pore water salinity and using existing geothermal gradient and seafloor temperature data from the study region. However, in places, the BSR lies deeper or shallower than the base of the modelled BGHSZ. These discrepancies include areas where faults/fractures and seismic evidence linked to fluid flow from deeper reservoirs reach the GHSZ disrupting its stratigraphic continuity. At these locations, we suggest that faults/fractures act as fluid migration pathways causing localized heat-flow perturbations and/or changes in the hydrate-forming gas composition both likely affecting the depth of the GHSZ. Our results provide the first evidence of the gas hydrate potential in the offshore Bangladesh and should drive future research and data acquisition aiming to understand the composition, saturation and thickness of the gas hydrate-bearing sediments in this region.
•2D seismic reflection data reveal the presence of BSRs offshore Bangladesh.•Modelling of the GHSZ supports the hydrate-related nature of the BSRs.•Fluid flows along faults/fractures in places shift the BSRs from the calculated base of GHSZ.
SUMMARY
Subsurface fluid escape structures are geological features which are commonly observed in sedimentary basins worldwide. Their identification and description have implications for various ...subsurface fluid flow applications, such as assuring integrity of overburden rocks to geological CO2 storage sites. In this study, we applied 3-D first-arrival traveltime tomography to a densely sampled wide-azimuth and wide-angle ocean bottom seismometer (OBS) data set collected over the Scanner Pockmark complex, a site of active gas venting in the North Sea. Seismic reflection data show a chimney structure underlying the Scanner Pockmark. The objective of this study was to characterize this chimney as a representative fluid escape structure in the North Sea. An area of 6$\times $6 km2 down to a depth of 2 km below sea level was investigated using a regularized tomography algorithm. In total, 182 069 manually picked traveltimes from 24 OBS were used. Our final velocity model contains compressional wave velocity perturbations ranging from −125 to +110 ms−1 relative to its average 1-D model and compares favourably with a coincident seismic reflection data set. The tomographic velocity model reveals that the chimney as observed in seismic reflection data is part of a larger complex fluid escape structure, and discriminates the genuine chimney from seismic artefacts. We find that part of the seeping gas migrates from a deep source, accumulates beneath the Crenulate Reflector unconformity at ∼250 m below seafloor (mbsf) before reaching the porous sediments of the Ling Bank and Coal Pit formation at <100 mbsf. In addition, the model shows that the venting gas at Scanner Pockmark is also being fed laterally through a narrow NW–SE shallow channel. Quantitative velocity analysis suggests a patchy gas saturation within the gas-charged sediments of the Ling Bank and the Coal Pit formations. Confined to the well-resolved regions, we estimate a base case average gas saturation of ∼9 per cent and in-situ gas volume of ∼1.64 $\times {10^6}\ {{\rm{m}}^3}$ across the Ling Bank and Coal Pit Fm. that can sustain the observed methane flux rate at the Scanner Pockmark for about 10 to 17 yr.
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