Trummer, Irmgard (2002) S-wave processing and interpretation of wide-angel seismic refraction data, Malpelo Ridge, Eastern Panama Basin. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 106 pp.

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Abstract

The Panama Basin is defined as the area bounded by the continental margins of Panama and Colombia to the north and east, and by the aseismic Cocos and Carnegie Ridges to the west and south. The Cocos and Carnegie Ridges are considered as hotspot tracks of the Galapagos Hotspot, which is located beneath the Galapagos Archipelago today. The Galapagos Hotspot is believed to have influenced the tectonic history of the Panama Basin and its main tectonic features to a great extent. So is the origin of the Cocos and Carnegie Ridges attributed to the interaction of the hotspot and the nearby Cocos-Nazca Spreading Center. Malpelo Ridge is also believed to originate from such an interaction, but its existence as a former continuation of Cocos Ridge, shifted south along the Panama Fracture Zone, or as a former part of Carnegie Ridge, moved north through continuous rifting at the now inactive Malpelo Rift, is still disputed. On a seismic wide-angle profile across Malpelo Ridge converted shear waves of very high quality were recorded, which enabled the evaluation of a model of Poisson’s ratio versus depth, that was interpreted 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 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.

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