Fine-scale crustal velocity structures along and across the Lucky Strike segment of Mid-Atlantic Ridge from full waveform inversion of wide-angle seismic data

The 70-km long Lucky Strike segment at 37oN on the Mid-Atlantic Ridge is characterized by a well-defined median valley bounded by ridge-ward dipping faults and a volcano located at the segment centre. The central volcano is fed by an axial magma chamber at ~3 km depth below seafloor. Away from the axial valley, the seafloor morphology is dominated by fault-controlled abyssal hills. Several active hydrothermal vents have been observed on the summit of the Lucky Strike volcano, and the axial magma chamber has been suggested to be the source supplying heat. Seismic velocities of the subsurface provide constraints on porosity and thus permeability, as well as the distribution of hot or molten rock. Therefore, determining fine-scale velocity structure of crust formed on the Lucky Strike segment is critical for understanding the interactions between magmatic, tectonic and hydrothermal processes during crustal accretion. We performed two-dimensional elastic full waveform inversion (FWI) to wide-angle ocean bottom seismometer (OBS) data to constrain the velocity of the crust beneath a profile along the entire Lucky Strike segment and another across the axis extending to ~50 km distance on both flanks. The seismic data were acquired during the SEISMOMAR survey in 2005. The OBS intervals vary between 4.5 and 14.0 km, with denser OBS deployed around the central volcano. Both profiles were shot multiple times, with shot spacings of 425 m and 150 m. Starting models for FWI were obtained from travel time tomography of the OBS dataset. The FWI results show a low velocity anomaly (LVA) beneath the Lucky Strike volcano at ~3 km depth below seafloor, just below the axial magma chamber reflector imaged in seismic reflection data. The LVA extends ~10 km along axis and ~4 km across axis and is ~0.7-1 km thick in depth. Taking the depth of 6.5 km/s velocity contour as the base of upper crust, the upper crust thickens along the axis from 2.2 km at segment centre to 3.5 km at the segment ends. In contrast, the crustal thickness reduces from ~8.3 km at the segment centre to 4.0-4.5 km at distal ends, assuming the 7.1 km/s velocity contour corresponds to the crustal base. This contrast is due to the significant reduction in lower crustal thickness towards the segment ends, where the upper-to-lower crustal thickness ratio increases from ~0.4 to >3.5 from segment centre to ends. These observations suggest the presence of focused magma supply to the segment centre along the Lucky Strike segment and that the igneous crust at the segment ends is formed primarily by magma eruption and/or diking. The upper crustal thickness has smaller variations in the across-axis direction to ~40 km distance, suggesting the current magmatic accretion mode could have been going for ~3.5 Myr. Beneath the central volcano, the crustal velocity is higher in the along axis direction above the LVA, suggesting seismic heterogeneity in the upper crust.