The influence of ridge migration on the magmatic segmentation of mid-ocean ridges

Synchronous motion of the Easter mantle plume and the East Pacific Rise

The Easter mantle plume has produced one of the longest hotspot tracks in the Pacific Ocean. While previous studies have focused on the eastern side extending across the Nazca Plate, we use 40Ar/39Ar isotopic and geochemical data to investigate the less explored western side around the Easter Microplate. We propose a dynamic model in which a deeper (600 km-depth), less buoyant mantle exerts a westward force on the East Pacific Rise (EPR), while a more buoyant plume region drives Easter hotspot volcanism and a localised acceleration in seafloor spreading. Our findings suggest that the Easter hotspot is the more focused surface expression of the most buoyant region of a vast, deep-seated mantle plume extending from the Pacific Large Low Shear Velocity Province (LLSVP). This challenges the traditional view of hotspots as isolated phenomena and suggests they are part of broader LLSVP-related mantle structures. Our results imply a more intricate, large-scale relationship between hotspots, mantle plumes, spreading ridges, and mantle dynamics.

Discovery of flat seismic reflections in the mantle beneath the young Juan de Fuca Plate

Crustal properties of young oceanic lithosphere have been examined extensively, but the nature of the mantle lithosphere underneath remains elusive. Using a novel wide-angle seismic imaging technique, here we show the presence of two sub-horizontal reflections at ∼11 and ∼14.5 km below the seafloor over the 0.51-2.67 Ma old Juan de Fuca Plate. We find that the observed reflectors originate from 300-600-m-thick layers, with an ∼7-8% drop in P-wave velocity. They could be explained either by the presence of partially molten sills or frozen gabbroic sills. If partially molten, the shallower sill would define the base of a thin lithosphere with the constant thickness (11 km), requiring the presence of a mantle thermal anomaly extending up to 2.67 Ma. In contrast, if these reflections were frozen melt sills, they would imply the presence of thick young oceanic lithosphere (20-25 km), and extremely heterogeneous upper mantle.

Seismic reflection images of the Moho underlying melt sills at the East Pacific Rise

The determination of melt distribution in the crust and the nature of the crust-mantle boundary (the 'Moho') is fundamental to the understanding of crustal accretion processes at oceanic spreading centres. Upper-crustal magma chambers have been imaged beneath fast- and intermediate-spreading centres but it has been difficult to image structures beneath these magma sills. Using three-dimensional seismic reflection images, here we report the presence of Moho reflections beneath a crustal magma chamber at the 9 degrees 03' N overlapping spreading centre, East Pacific Rise. Our observations highlight the formation of the Moho at zero-aged crust. Over a distance of less than 7 km along the ridge crest, a rapid increase in two-way travel time of seismic waves between the magma chamber and Moho reflections is observed, which we suggest is due to a melt anomaly in the lower crust. The amplitude versus offset variation of reflections from the magma chamber shows a coincident region of higher melt fraction overlying this anomalous region, supporting the conclusion of additional melt at depth.