Deformation and melt in natural mantle rocks: The Hilti Massif (Oman) and the Othris Massif (Greece)

Abstract

For a full understanding of plate tectonics, one of the central paradigms in Earth Sciences, it is critical to know the mechanical properties of the material of which the earth's upper mantle consists, i.e., peridotite. The cold outer shell of the Earth, the lithosphere, is broken up into strong and almost rigid plates, which can move with relatively little resistance over a weak substratum in the upper mantle, the asthenosphere. The weakness of the asthenosphere is primarily caused by the high, near-solidus ambient temperature in this zone. However, questions remain as to the effects of the presence of small fractions of molten material ('melt') on the deformation properties and strength of these asthenospheric peridotites. Recent laboratory experiments have shown that small amounts of melt (4- ro%) can have a drastic weakening effect on peridotitelike materials. If this weakening effect of melt plays a role in the earth's upper mantle, then the asthenosphere could be significantly weaker than commonly believed. The current research was therefore initiated, aiming to investigate the factors which govern melt-weakening of peridotites and to determine whether evidence could be found for melt-weakening in natural peridotites deformed under asthenospheric conditions. However, samples from the asthenosphere are generally unavailable, mainly because of the great depth of this zone (generally >roo km). Only at mid-ocean ridges, the asthenosphere reaches relatively shallow levels of only a few kilometres. Ophiolites, fossil fragments of mid-ocean ridges, occasionally emplaced onto continents as a result of the closure and disappearance of oceans during the earth's geological history, are therefore the most suitable source of samples of upper mantle rocks deformed under asthenospheric conditions. Peridotites in ophiolites can thus provide a window into the asthenosphere, provided that the effects of deformation related to emplacement of the ophiolite superimposed on asthenospheric deformation can be distinguished and that relics of the early asthenospheric deformation have been preserved. This thesis comprises a detailed geological study of peridotites found in two ophiolites, the Oman Ophiolite (Oman) and the Othris Ophiolite (Greece). Both ophiolites represent remnants of an ancient ocean, the Neotethys, which existed in Mesozoic times. Structural geological and lithological maps and sections of selected areas were made, and rock samples were subjected to detailed microstructural and petrographic analysis using light microscopy, electron microscopy, and chemical (electron microprobe) analysis.