Professor R. Shackleton enquired abouthe remarkably thin inverted metamorphic sequence. Did the authors think that this thinness was the result of intense deformation—presumably mainly ductile, since the sequence, though thin, appeared to be complete—or did they think that the thinness was an original feature, in which case since the temperature at the contact was above 800°C, a very thin hot ophiolite slab, a steep thermal gradient and rapid loss of heat seems to be implied.
In reply, the Authors said that most metamorphic soles are less than 500 m thick, but so few have been studied that it is premature to consider the Ballantrae sole relatively thin or otherwise. However, there may well have been tectonic thinning of the sole during its formation. If the sole is pressure inverted as well as temperature inverted, as the evidence suggests, it cannot have formed statically like a conventional aureole, but must have formed over a range of P-T conditions at different depths in the lithosphere. While the sole now exists as an apparently coherent sequence it is likely that it originally comprised rocks at different positions and metamorphic grades within the fault zone which were subsequently tectonically juxtaposed by the fault movements. As we see it now its apparent coherence may be due to successive ‘welding’ of the rock layers together, and/or failure in the field to recognize discrete tectonic contacts through the sequence (mylonites, etc.). If the sole is composite then neither steep and extensive thermal gradient nor a rapid