Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field

Abstract

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

Recent volcanic events and the distribution of hydrothermal venting at the Lucky Strike hydrothermal field, Mid‐Atlantic Ridge

“…This inferred melt lens lies at a depth of 3 to 4 km beneath seafloor (bsf). It extends 1 to 3 km across axis and 7 km along-axis [Singh et al, 2006]. It is underlain by a low-velocity lower crustal domain, interpreted as hot gabbroic rocks with a small proportion of melt .…”

Composition of the Oceanic Crust

“…Steady-state magma chambers appear to be much rarer on slow-spreading ridges, as only a couple of examples have been found, thus far (e.g., Singh et al, 2006), although this may partly reflect the difficulty of identifying them in rough seafloor terrain. Based on present evidence, magma chambers on slow-spreading ridges appear to be ephemeral rather than permanent.…”

The response of two-phase hydrothermal systems to changing magmatic heat input at mid-ocean ridges

“…Seismic data show that the sub-axial magma lens (AMC) typically exists between approximately 1 and 3 kilometers beneath the seafloor [Detrick et al, 1987;Canales et al, 2006;Singh et al, 1999Singh et al, , 2006Van Ark et al, 2007]. The width across-axis is typically 0.5 -4 km [Collier and Sinha, 1990;Kent et al, 1990;Singh et al, 2006] and the vent field along-axis 21 spaces 1-3 km [Gente et al, 1986;Kelley et al, 2002].…”

“…The width across-axis is typically 0.5 -4 km [Collier and Sinha, 1990;Kent et al, 1990;Singh et al, 2006] and the vent field along-axis 21 spaces 1-3 km [Gente et al, 1986;Kelley et al, 2002]. Measurements of hydrothermal heat output show that it ranges between approximately 50 and 500 MW [Lowell and Germanovich, 2004;Ramondenc et al, 2006;Baker, 2007].…”