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
... read more
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.
show less
