SUMMARY The lithosphere–asthenosphere boundary (LAB) separates the rigid lithospheric plate above with the ductile and convective asthenosphere below and plays a fundamental role in plate tectonic processes. The LAB has been imaged using passive geophysical methods, but these methods only provide low-resolution images. Recently, seismic reflection imaging method has provided high-resolution images of the LAB, but imaging of the LAB at younger ages has been difficult. Here, we present the image of the LAB using wide-angle seismic reflection data covering 11–21 Ma old lithosphere in the equatorial Atlantic Ocean. Using ocean bottom seismometers (OBSs), we have observed wide-angle reflections between 150 and 400 km offsets along with crustal and mantle refraction arrivals. We first performed traveltime tomography to obtain the velocity in the crust and upper mantle. The Pn arrivals provide the information about P-wave velocity down to 4 km below the Moho. The disappearance of Pn arrivals beyond 130 km offset suggests that vertical P-wave velocity gradient is negligible or negative below this depth. We extended these velocities down to 90 km depth and then applied two imaging techniques to wide-angle reflection data, namely traveltime mapping of picked reflection arrivals and pre-stack depth migration of full wavefield data. We find that these reflections originate between 34 and 67 km depth, possibly from the LAB system. We have carried out extensive modelling to show that these reflections are real and not artefacts of imaging. Comparison of our results with coincident passive seismological and magnetotelluric results suggests that wide-angle imaging technique can be successfully used to study the lithosphere and the LAB system. We find that the LAB gradually deepens with age, but becomes very deep at 17–19 Ma, which we interpret to be due to the anomalous geology along this part of the profile.
Oceanic crust forms at mid-ocean spreading centres through a combination of magmatic and tectonic processes, with the magmatic processes creating two distinct layers: the upper and the lower crust. While the upper crust is known to form from lava flows and basaltic dikes based on geophysical and drilling results, the formation of the gabbroic lower crust is still debated. Here we perform a full waveform inversion of wide-angle seismic data from relatively young (7-12-million-year-old) crust formed at the slow spreading Mid-Atlantic Ridge. The seismic velocity model reveals alternating, 400-500 m thick, high and low velocity layers with ±200 m/s velocity variations, below ~2 km from the oceanic basement. The uppermost low-velocity layer is consistent with hydrothermal alteration, defining the base of extensive hydrothermal circulation near the ridge axis. The underlying layering supports that the lower crust is formed through the intrusion of melt as sills at different depths, that cool and crystallise in situ. The layering extends up to 5-15 km distance along the seismic profile, covering 300,000-800,000 years, suggesting that this form of lower crustal accretion is a stable process.
Abstract Oceanic crust is formed at mid-ocean spreading centres by a combination of magmatic, tectonic and hydrothermal processes. The crust formed by magmatic process consists of an upper crust generally composed of basaltic dikes and lava flows and a lower crust presumed to mainly contain homogeneous gabbro whereas that by tectonic process can be very heterogeneous and may even contain mantle rocks. Although the formation and evolution of the upper crust are well known from geophysical and drilling results, those for the lower crust remain a matter of debate. Using a full waveform inversion method applied to wide-angle seismic data, here we report the presence of layering in the lower oceanic crust formed at the slow spreading Mid-Atlantic Ridge, ~7-12 Ma in age, revealing that the lower crust is formed mainly by in situ cooling and crystallisation of melt sills at different depths by the injection of magma from the mantle. These layers are 400-600 m thick with alternate high and low velocities, with ± 100-200 m/s velocity variation, and cover over a million-year old crust, suggesting that the crustal accretion by melt sill intrusions beneath the ridge axis is a stable process. We also find that the upper crust is ~400 m thinner than that from conventional travel-time analysis. Taken together, these discoveries suggest that the magmatism plays more important roles in the crustal accretion process at slow spreading ridges than previously realised, and that in-situ lower crustal accretion is the main process for the formation of lower oceanic crust.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing an older version [v2]Go to new versionSeismic structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°WAuthorsKevinGroweiDIngoGrevemeyeriDSatish ChandraSinghMilenaMarjanovicEmma PMGregoryCordPapenbergiDVenkata AbhishikthVaddineniLauraGómez de la PeñaiDZhikaiWangiDSee all authors Kevin GroweiDCorresponding Author• Submitting AuthorGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-0822-3562view email addressThe email was not providedcopy email addressIngo GrevemeyeriDGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-6807-604Xview email addressThe email was not providedcopy email addressSatish Chandra SinghInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressMilena MarjanovicInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressEmma PM GregoryInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressCord PapenbergiDGeomar, Kiel, GermanyiDhttps://orcid.org/0000-0001-8790-558Xview email addressThe email was not providedcopy email addressVenkata Abhishikth VaddineniInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressLaura Gómez de la PeñaiDGEOMAR Helmholtz Centre of Ocean ResearchiDhttps://orcid.org/0000-0001-7443-0503view email addressThe email was not providedcopy email addressZhikai WangiDInstitut De Physique Du Globe De ParisiDhttps://orcid.org/0000-0003-0852-2658view email addressThe email was not providedcopy email address
SUMMARY Seismic full waveform inversion (FWI) is a powerful method for estimating quantitative subsurface physical parameters from seismic data. As the FWI is a nonlinear problem, the linearized approach updates model iteratively from an initial model, which can get trapped in local minima. In the presence of a high-velocity contrast, such as at Moho, the reflection coefficient and recorded waveforms from wide-aperture seismic acquisition are extremely nonlinear around critical angles. The problem at the Moho is further complicated by the interference of lower crustal (Pg) and upper mantle (Pn) turning ray arrivals with the critically reflected Moho arrivals (PmP). In order to determine velocity structure near Moho, a nonlinear method should be used. We propose to solve this strong nonlinear FWI problem at Moho using a trans-dimensional Markov chain Monte Carlo (MCMC) method, where the earth model between lower crust and upper mantle is ideally parametrized with a 1-D assumption using a variable number of velocity interfaces. Different from common MCMC methods that require determining the number of unknown as a fixed prior before inversion, trans-dimensional MCMC allows the flexibility for an automatic estimation of both the model complexity (e.g. the number of velocity interfaces) and the velocity–depth structure from the data. We first test the algorithm on synthetic data using four representative Moho models and then apply to an ocean bottom seismometer (OBS) data from the Mid-Atlantic Ocean. A 2-D finite-difference solution of an acoustic wave equation is used for data simulation at each iteration of MCMC search, for taking into account the lateral heterogeneities in the upper crust, which is constrained from traveltime tomography and is kept unchanged during inversion; the 1-D model parametrization near Moho enables an efficient search of the trans-dimensional model space. Inversion results indicate that, with very little prior and the wide-aperture seismograms, the trans-dimensional FWI method is able to infer the posterior distribution of both the number of velocity interfaces and the velocity–depth model for a strong nonlinear problem, making the inversion a complete data-driven process. The distribution of interface matches the velocity discontinuities. We find that the Moho in the study area is a transition zone of 0.7 km, or a sharp boundary with velocities from around 7 km s−1 in the lower crust to 8 km s−1 of the upper mantle; both provide nearly identical waveform match for the field data. The ambiguity comes from the resolution limit of the band-limited seismic data and limited offset range for PmP arrivals.
Earth and Space Science Open Archive This preprint has been submitted to and is under consideration at Journal of Geophysical Research - Solid Earth. ESSOAr is a venue for early communication or feedback before peer review. Data may be preliminary.Learn more about preprints preprintOpen AccessYou are viewing an older version [v1]Go to new versionSeismic structure of the St. Paul Fracture Zone and Late Cretaceous to Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°WAuthorsKevinGroweiDIngoGrevemeyeriDSatish ChandraSinghMilenaMarjanovicEmma PMGregoryCordPapenbergiDVenkata AbhishikthVaddineniLauraGómez de la PeñaiDZhikaiWangiDSee all authors Kevin GroweiDCorresponding Author• Submitting AuthorGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-0822-3562view email addressThe email was not providedcopy email addressIngo GrevemeyeriDGEOMAR Helmholtz Centre for Ocean Research KieliDhttps://orcid.org/0000-0002-6807-604Xview email addressThe email was not providedcopy email addressSatish Chandra SinghInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressMilena MarjanovicInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressEmma PM GregoryInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressCord PapenbergiDGeomar, Kiel, GermanyiDhttps://orcid.org/0000-0001-8790-558Xview email addressThe email was not providedcopy email addressVenkata Abhishikth VaddineniInstitut De Physique Du Globe De Parisview email addressThe email was not providedcopy email addressLaura Gómez de la PeñaiDGEOMAR Helmholtz Centre of Ocean ResearchiDhttps://orcid.org/0000-0001-7443-0503view email addressThe email was not providedcopy email addressZhikai WangiDInstitut De Physique Du Globe De ParisiDhttps://orcid.org/0000-0003-0852-2658view email addressThe email was not providedcopy email address
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