The RAMESSES experiment - II. Evidence for accumulated melt beneath slow spreading ridge from wide-angle refraction and multichannel reflection seismic profiles

The RAMESSES study (Reykjanes Axial Melt Experiment: Structural Synthesis f rom Electromagnetics and Seismics) targeted an apparently magmatically acti ve axial volcanic ridge (AVR), centred on 57 degrees 45N at the Reykjanes R idge, 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 o cean-bottom seismometers (OBSs) along profiles oriented both across- and al ong-axis. Coincident normal-incidence seismic, bathymetry and underway grav ity and magnetic data were also collected. Forward modelling of the seismic and gravity data has revealed layer thickn esses, velocities and densities similar to those observed elsewhere within the oceanic crust near mid-ocean ridges. At 57 degrees 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 ag e), which implies that full crustal thickness is achieved on-axis and that it is subsequently thinned, most likely, by off-axis extension. Modelling a lso indicates that the AVR is underlain by a thin (similar to 100 m), narro w (similar to 4 km) melt lens some 2.5 km beneath the seafloor, which overl ies a broader zone of partial melt approximately 8 km in width. Thus the re sults of this study provide the first clear evidence for a crustal magma ch amber beneath any slow spreading ridge. The size and depth of this magma ch amber (the melt lens and underlying zone of partial melt) are similar to th ose 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 spr eading ridges is probably temporally related to the periods of magmatic act ivity being considerably shorter and more widely spaced in time than at fas t and intermediate spreading ridges.