In 1995, a multidisciplinary geophysical experiment targetted the intermediate spreading Valu Fa Ridge (full rate 60 mm yr−1 ), which is centred on 22°20′S, 176°40′W in the Lau Basin. As part of this experiment, wide‐angle and normal‐incidence seismic profiles were collected both along‐ and across‐axis to determine the crustal structure of the Central Valu Fa Ridge (CVFR) and its overlap with the Northern Valu Fa Ridge (NVFR). Controlled‐source electromagnetic profiles and underway gravity, magnetic and bathymetry data were also collected.  In this paper we describe the results of forward modelling of the along‐ and across‐axis wide‐angle and normal‐incidence seismic data. An axial low‐velocity block and its underlying slightly broader zone of depressed seismic velocities (low‐velocity zone) have been identified, and these features are interpreted as corresponding to a melt lens and underlying magma chamber. The low‐velocity block is 1–2 km wide and has a first‐order upper boundary, from which large‐amplitude reflections are observed; amplitude analysis of these indicates an interconnected melt fraction. The nature of the lower boundary is more poorly constrained, as no reflection event corresponding to the base of the low‐velocity block is observed. Modelling indicates that velocities similar to those observed at the base of layer 2 within the axial region (~5.5 km s−1) are achieved by 250 m below the upper boundary, possibly suggesting a gradational lower boundary with high velocity gradient. The low‐velocity zone (LVZ) is interpreted as an ~4 km wide magma chamber delineated by a seismic velocity anomaly of −0.2 km s−1, extending down through layer 3 to within 1.5–2 km of the Moho. The velocity anomaly and dimensions of the LVZ are generally smaller than those observed at the East Pacific Rise (EPR) and Mid‐Atlantic Ridge (MAR).  The observed along‐axis continuity of the low‐velocity block is remarkable, extending from the southern tip of the CVFR to the overlapping spreading centre (OSC) with the NVFR. A low‐velocity block is modelled beneath the inside flanks (i.e. the slopes that dip into the overlap basin) of both ridges at the OSC, although the existence of a single low‐velocity block beneath the overlap basin itself cannot be ruled out. The identification of a single LVZ centred on the overlap region, rather than two merged LVZs beneath each segment, implies that the material in each low‐velocity block originates from the same crustal magma source. A reflection event from the Moho is observed from directly beneath the axis on both across‐axis profiles, which indicates that a distinct crust–mantle boundary may be formed within the axial region. Many of the observations at the Valu Fa Ridge are consistent with those at the EPR and the Reykjanes Ridge (MAR), which implies that, regardless of spreading rate, crustal accretionary processes at mid‐ocean ridges with similar magmatic budgets are also broadly similar.

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In extensional geologic systems such as mid-ocean ridges, deformation is typically accommodated by slip on normal faults, where material is pulled apart under tension and stress is released by rupture during earthquakes and magmatic accretion. However, at slowly spreading mid-ocean ridges where the tectonic plates move apart at rates <80 km m.y.-1 , these normal faults may roll over to form long-lived, low-angled detachments that exhume mantle rocks and form corrugated domes on the seabed. Here we present the results of a local micro-earthquake study over an active detachment at 13°20′N on the Mid-Atlantic Ridge to show that these features can give rise to reverse-faulting earthquakes in response to plate bending. During a 6 month survey period, we observed a remarkably high rate of seismic activity, with >244,000 events detected along 25 km of the ridge axis, to depths of ~10 km below seafloor. Surprisingly, the majority of these were reverse-faulting events. Restricted to depths of 3-7 km below seafloor, these reverse events delineate a band of intense compressional seismicity located adjacent to a zone of deeper extensional events. This deformation pattern is consistent with flexural models of plate bending during lithospheric accretion. Our results indicate that the lower portion of the detachment footwall experiences compressive stresses and deforms internally as the fault rolls over to low angles before emerging at the seafloor. These compressive stresses trigger reverse faulting even though the detachment itself is an extensional system.

The RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics) targeted an apparently magmatically active axial volcanic ridge (AVR), centred on 57°45′N at the Reykjanes Ridge, with the aim of investigating the processes of crustal accretion at a slow spreading mid‐ocean ridge. As part of this multicomponent experiment, airgun and explosive wide‐angle seismic data were recorded by 10 digital ocean‐bottom seismometers (OBSs) along profiles oriented both across‐ and along‐axis. Coincident normal‐incidence seismic, bathymetry and underway gravity and magnetic data were also collected.  Forward modelling of the seismic and gravity data has revealed layer thicknesses, velocities and densities similar to those observed elsewhere within the oceanic crust near mid‐ocean ridges. At 57°45′N, the Reykjanes Ridge has a crustal thickness of approximately 7.5 km on‐axis. However, the crust is modelled to decrease in thickness slightly off‐axis (i.e. with age), which implies that full crustal thickness is achieved on‐axis and that it is subsequently thinned, most likely, by off‐axis extension. Modelling also indicates that the AVR is underlain by a thin (∼100 m), narrow (∼4 km) melt lens some 2.5 km beneath the seafloor, which overlies a broader zone of partial melt approximately 8 km in width. Thus the results of this study provide the first clear evidence for a crustal magma chamber beneath any slow spreading ridge. The size and depth of this magma chamber (the melt lens and underlying zone of partial melt) are similar to those observed beneath fast and intermediate spreading ridges, which implies that the processes of crustal accretion are similar at all spreading rates. Hence the lack of previous observations of magma chambers beneath slow spreading ridges is probably temporally related to the periods of magmatic activity being considerably shorter and more widely spaced in time than at fast and intermediate spreading ridges.

This paper is the first in a series of three (this issue) which present the results of the RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis from Electromagnetics and Seismics). RAMESSES was an integrated geophysical study which was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57°45′N. It consisted of three major components: wide‐angle seismic profiles along and across the AVR, using ocean‐bottom seismometers, together with coincident seismic reflection profiles; controlled‐source electromagnetic sounding (CSEM); and magnetotelluric sounding (MT). Supplementary data sets included swath bathymetry, gravity and magnetics.  Analyses of the major components of the experiment show clearly that the sub‐axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids. The AVR is underlain by a significant crustal magma body, at a depth of 2.5 km below the sea surface. The magma body is characterized by low seismic velocities constrained by the wide‐angle seismic data; a seismic reflection from its upper surface; and a region of anomalously low electrical resistivity constrained by the CSEM data. It includes a thin, ribbon‐like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone, which flanks and underlies the melt lens. RAMESSES is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge. The result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion, but are short‐lived rather than steady‐state features. The magma body can exist for only a small proportion of a tectono‐magmatic cycle, which controls crustal accretion, and has a period of at least 20 000 years. These findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle, and of its delivery into the crust, beneath slow‐spreading mid‐ocean ridges.