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AJMV LUCKY STRIKE FULL
Lucky Strike has several characteristics typical of a slow spreading ridge segment, including a full spreading rate of ∼2.1 cm yr −1 (Demets et al.
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Lucky Strike volcano lies on the Mid-Atlantic Ridge (MAR) south of the Azores triple junction at 37.3° N. ( 2002a, b) performed 1-D waveform inversion of surface multichannel streamer data, obtaining better resolution but analysis was restricted to areas of smooth seafloor where the 1-D assumption could be justified.
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Collier & Singh ( 1998) and Hussenoeder et al. ( 1997) modelled the shear and compressional-wave structure of Layer 2A at the East Pacific Rise, but resolution remained a major issue. By using multiple components of the seismic wavefield Christeson et al. ( 1994) provided the first well-constrained velocity structure of the uppermost several hundred metres of the crust. By deploying both the sources and the receivers close to the seafloor during a NOBEL experiment, Christeson et al. ( 1993) stacked the turning rays present in seismic reflection data to determine the thickness of Layer 2A. Since there is a strong velocity gradient at the base of Layer 2A, Harding et al. 2010a, b), but the nature of Layer 2A is poorly constrained by OBS arrays because the spacing of instruments is generally too large for detailed studies. Layers 2B and 3 are well studied using ocean bottom seismometers (OBS Cudrak & Clowes 1993 Van Avendonk et al. 1993) or as a porosity limit within the extrusive section associated with an hydrothermal alteration front or a fracture front (McClain et al. Over the last few decades, the origin of the velocity transition at the base of Layer 2A has been interpreted as the lithological boundary between extrusives and sheeted dyke complex (Toomey et al. The lowest layer, Layer 3, has a small velocity gradient with velocity reaching up to 6.8 km s −1 at its base. Layer 2B has a P-wave velocity of 4.5–6 km s −1, and roughly encompasses the sheeted dyke complex. The uppermost (150–1000 m) Layer 2A has a low-velocity (∼2.5 km s −1) and high-velocity gradient at its base that separates it from Layer 2B. 1988), the igneous oceanic crust was divided into three layers: Layer 2A, Layer 2B and Layer 3. Based on early seismic studies (Spudich & Orcutt 1980 Lewis & Garmany 1982 White & Purdy 1983) and observations from ophiolites (Nicolas et al. Oceanic crust is formed by a combination of magmatic and tectonic processes.
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Tomography, Composition of the oceanic crust, Controlled source seismology, Mid-ocean ridge processes, Submarine tectonics and volcanism 1 INTRODUCTION Finally, the rapid thinning of the entire Layer 2A in the vicinity of the main normal faults suggests the tectonic thinning of a geologically defined layer, further confirming the lithological origin of the high-velocity gradient zone at the base of seismic Layer 2A. Hydrothermal deposition sealing of small-scale porosity is shown to be a secondary process, which likely explains the upper crustal velocity increase with age, but is not responsible for the high-velocity gradient Layer 2A. Thick (>400 m) units of anomalously low-velocity material (<2.5 km s −1) beneath different summital edifices on the central volcano indicate that a thick pile of high-porosity extrusive rocks can be supported without collapsing, suggesting that while in general there is pore closure with depth this is not the cause of high velocities we observe. The base of Layer 2A is defined as a lithological boundary that can be offset by faulting. Our results clearly demarcate two layers within seismic Layer 2A a low-velocity, highly heterogeneous layer likely reflecting the complexity of accretion that is underlain by a more homogeneous high-velocity gradient layer. Since both sources and receivers are downward continued to the seafloor, the computational cost of FWI is reduced, as one does not need to model the thick water layer. The downward continuation procedure enhances the refracted arrivals and wide-angle reflections, and reduces the scattering noise due to rough seafloor. We have used a two-step process combining downward continuation with a time-domain, elastic FWI. Here, we present results of elastic full waveform inversion (FWI) along three multichannel seismic lines at the Lucky Strike volcano on the Mid-Atlantic ridge that provides a velocity image of the upper oceanic crust with unprecedented resolution (50–100 m). Seismic full waveform is an emerging technique for determining the fine-scale velocity structure of the subsurface.