S-wave processing and interpretation of wide-angle seismic refraction data, Malpelo Ridge, Eastern Panama Basin
On a seismic wide-angle profile across Malpelo Ridge, located in the Eastern Panama Basin, converted shear waves of very high quality were recorded. The evaluation of a model of Poisson's ratio versus depth enabled an interpretation in terms of the lithological composition of the underground. Values of compressional and shear wave velocities and Poisson's ratios were mainly compared to laboratory measurements obtained from ophiolite sequences, and match the expected layering for the oceanic crust and upper mantle very well. A sedimentary layer consisting of calcareous ooze covers a layer of extrusives, that is underlain by a sheeted dike complex and subsequently by pyroxene- and olivine gabbros. The upper mantle consists of ultramafites. The profile is divided into two distinct areas: the thickened crust beneath Malpelo Ridge and normal oceanic crust northwest off the ridge. For the thickened crust a higher degree of metamorphism is suggested. For the crust and upper mantle underneath Malpelo Ridge and northwest off the ridge a melt with a different chemical composition is indicated. The oceanic crust between Regina Ridge, situated to the northwest of Malpelo Ridge, and Malpelo Ridge is proposed to have been formed at an abandoned spreading center in this area. Spreading at this abandoned rift is suggested to have occurred after Malpelo Ridge has been formed. A long-term influence of the Galapagos Hotspot is made responsible for a high magnesium content in the area northwest off Malpelo Ridge, which decreases with increasing distance to the hotspot due to ongoing spreading.