Abstract

M. F. Osmaston writes: It has been fascinating to follow, through their published papers over a number of years, the remarkable degree to which Dr Smith and his co-workers have built up our knowledge of Othris geology and structure. I find wholly convincing the weight of evidence which leads the authors to conclude that in Kimmeridgian time a major sedimentary basin extended southwestward from the Pelagonian Massif.

On the other hand, I believe the means which they propose, whereby the sedimentary slices became superposed by ophiolite and stacked against the Massif, to be completely untenable. The work of Hubbert & Rubey (1959), extended by Forristall (1972), showed that for most known allochthonous rock slices their thickness-to-width ratio is too small for them to have been emplaced by being pushed without suffering major disruption, the only viable mechanism known being that they slide down a slope upon an incompletely compacted substratum, whose pore fluid thereby becomes overpressured and provides low-friction support for the slice(s). Whereas some variations upon the subduction theme may well be capable of uplifting and tilting the former sedimentary basin floor enough to cause décollement and sliding of sedimentary rock slices, the sufficient elevation of oceanic crust to permit down-slope décollement of an ophiolitic slice, complete with sub-Moho material (as seen in Othris and in ophiolites generally) is a completely different matter.

A Triassic ocean floor crust would have been at least 8 km thick by the late Jurassic. Furthermore it is characteristic of ophiolite slices that they

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