We propose that fault-steering of lava flows along-axis, limiting off-axis transport as observed at fast-spreading systems, may be common at both slow-and ultra-slow spreading ridges with significant along-axis changes in magma supply linked to topographic gradients. This results in local lava emplacement due to a melt supply that is lower than at the segment center. This mode of lava emplacement transitions away from the segment center to pillow-dominated seafloor near the segment ends, associated often with hummocks and axial volcanic ridges. Surface lineations of sheet lava flows tend to be subparallel to fault scarps and the overall segment orientation, and show whorls, well-developed lava channels with associated levees, and surface fold structures at flow fronts. In these settings, sheet flows may efficiently transport melt away from magmatically robust segment centers in the along-axis direction, steered by normal faults, and exploiting along-axis topographic gradients, with limited across-axis flow. Based on optical and high-resolution bathymetry data from the Lucky Strike segment (Mid-Atlantic Ridge), we document a mode of volcanic emplacement that likely operates at some magmatically robust slow-and ultra-slow spreading ridge segments, in the presence of strong gradients in magma supply. Its construction is dominated by pillow lavas, reflecting lower effusion rates than those at fast spreading ridges, where sheet and lobate flows are common and flow off-axis while thickening the extrusive volcanic layer. The structure of the oceanic upper volcanic crust is less understood at slow-spreading ridges than at faster ones. Shallow dipping reflectors imaged in the upper-crust beneath the volcano flanks are interpreted as buried lava flow surfaces. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across-axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. The magma chamber roof is imaged at 3.4☐.4 km depth beneath the volcano and major faults are imaged with dips ranging between 33° and 50°. We also generate a high-resolution bathymetric map using the seismic reflection data.
We use a 3D tomographic velocity model derived from a co-incident ocean bottom seismometer experiment to depth convert the post-stack time-migrated seismic volume and provide 3D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We present results from three-dimensional (3D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid-Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid-Atlantic Ridge (SISMOMAR) survey.
Within the axial valley, the upper crust is nearly uniform over the central two thirds of the segment, but the layer 2A reflector shallows, and the 4.5 km s−1 isovelocity contour deepens at the segment ends, suggesting that the spreading regime changes from predominately magmatic at the segment center to predominantly tectonic at the segment ends. Upper crustal velocities are ~0.5 km s−1 slower and the layer 2A event 0.2 s later (two-way travel time) inside the axial valley than outside. Faults penetrating from the west flank of the volcano arrive within <1 km of the top of the AMC reflector. The axial valley’s Eastern Boundary Fault penetrates to within ~1 km of the eastern edge of the AMC reflector, and the lower crustal LVZ stops abruptly beneath the bottom of this fault. The underlying LVZ spans the lower crust and has a maximum anomaly of at least 0.5 km s−1. The AMC reflector lies 3.0–3.8 km beneath the seafloor and is up to 2 km wide by at least 5 km long.
The experiment reveals an axial magma chamber (AMC) reflector beneath the hydrothermal field, an underlying low-velocity zone (LVZ), and faults that may facilitate fluid flow and heat transfer between the AMC and the seafloor. The Seismic Studies for the Monitoring of the Mid-Atlantic Ridge (MOMAR) program (SISMOMAR) experiment constrained crustal structure over a range of scales from the central volcano to the entire segment. The Lucky Strike hydrothermal field sits at the summit of the Lucky Strike segment central volcano, at 37☁7′N on the Mid-Atlantic Ridge.
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