Dr E. L. McLellan writes: In their very valuable paper, Baker & Droop (1983) have argued that contrasts in metamorphic cooling ages and grades across the andalusite-kyanite isograd are attributable to emplacement of the Banff Nappe. Specifically, 'it is suggested that the eastern high temperature zones were thrust over the western kyanite zone and that the rocks to the west of the kyanite isograd were metamorphosed by thermal relaxation beneath this thrust sheet.' My comments concern:
(1) the proposed direction of overthrusting; and
(2) the differing thermal settings of the eastern and western areas.
Baker & Droop worked in an area marginal to the Banff Nappe and were thus unable to provide direct evidence to support their proposed westerly-directed nappe emplacement. My work (unpublished) on the Deeside gneisses forming the southern base of the Banff Nappe, together with that of Treagus & Roberts (1981) on the northern coast, suggests that nappe emplacement involved SE-directed motion. Thus the Banff Nappe was emplaced from the interior of the orogen, as in the tectonic models of Henderson & Robertson (1982), not from the exterior as required by Baker & Droop.
I agree with Baker & Droop that the high grade assemblages of Glen Muick formed under the same high geothermal gradient as the overlying Buchan assemblages, and that this high geotherm is best ascribed to an island-arc environment, as proposed for Connemara by Yardley & Senior (1982). However, Baker & Droop's model requires that these high geotherms be established prior to and during