Discussion of ‘The Highland Boundary Fault and the Highland Border Complex’ by B. J. Bluck, Scottish Journal of Geology , 46 , 113–124

Abstract

P.W. Geoff Tanner writes: In this paper, Bluck has failed to provide a balanced and accurate assessment of the progress that has been made over many years to find a robust, modern geological interpretation of the Highland Border. This is no trivial matter, for the rocks exposed in this narrow strip of ground retain many of the clues to understanding the causes, and subsequent development, of the Caledonian c . 470 Ma Grampian Event, and the role played by the Highland Boundary Fault in continental-scale plate-tectonic reconstructions (Strachan & Dewey 2003; Tanner 2008). The Highland Boundary Fault is considered by Bluck to define the contact between the Southern Highland Group (Dalradian) and the ‘Highland Border Complex’. However, at outcrop this boundary is variously seen as a fault (North Esk), a lithological transition (Keltie Water), or the sole of an ophiolite (Innellan), and there is no evidence of a major, through-going fault between Arran and Stonehaven, as conventionally depicted (Tanner 2008). Conflicting interpretations of Highland Border geology have generated two diametrically opposed models, essentially summarized below, variations notwithstanding, as A & B. In model A (Curry et al . 1984; Bluck 1985), the rocks between uncontested Southern Highland Group (Dalradian) to the NW and the unconformable Old Red Sandstone to the south, were assigned to the ‘Highland Border Complex’ (HBC). The latter was once ‘considerably separated’ from the Dalradian block, and acquired a northerly dip and younging direction before the two were amalgamated, following the Grampian Event, by movement on the Highland Boundary Fault. In model B (Tanner & Sutherland 2007), the HBC was divided into two parts: the Trossachs Group (now the youngest part of the Dalradian Supergroup), and an overlying, allochthonous unit, the Highland Border Ophiolite (HBO). The main differences between the two models are: all of the …