Reply to Dewey and Ryan Comment on Searle, MP (2021) ‘Tectonic evolution of the Caledonian orogeny in Scotland: a review based on the timing of magmatism, metamorphism and deformation’

Abstract

I welcome the comment from Dewey and Ryan on my paper (Searle, 2021), and the chance to clarify some conflicting Caledonian tectonic models and new ideas. As outlined in the paper, my review was based on the geology of Scotland, not Ireland, Newfoundland or the rest of the Caledonian orogeny beyond, mainly because of length constraints, and the fact that my personal field experience lies mainly in Scotland. The Great Glen fault and the Moine Supergroup are not exposed at all in Ireland, so critical parts, and half of the Caledonian orogeny are missing in Ireland. I argue that the Caledonian geology of Scotland can be explained by a Laurentia – Midland valley arc-microcontinent collision, progressing from NW-directed ophiolite obduction in the Late Cambrian to crustal thickening and regional metamorphism during the Ordovician and Silurian, across the Dalradian and Moine metamorphic rocks. Caledonian structures do continue north into Greenland, as summarized by Leslie et al. (2008), but the geological evolution of the Scottish Caledonides does not require any collision with Baltica. The Dewey and Ryan comment raises major tectonic questions, which this reply addresses.

Structure of the Assynt window, Moine Thrust Zone and relationship of thrusts to alkaline igneous complexes, Caledonian orogeny, NW Scotland

“…Post-collisional regional Barrovian metamorphism occurred during either a two-phase orogenic event comprising a 480-470 Ma Grampian event, and a 445-425 Ma Scandian event (Dewey, 1969(Dewey, , 1971(Dewey, , 2005Dewey & Shackleton, 1984;Johnson et al 1985), or a single, protracted orogenic-metamorphic event . Readers are referred to the recent discussion between Searle (2022) and Dewey and Ryan (2022) for arguments for and against the two short-lived orogenies (Ordovician Grampian and Silurian Scandian), or a single longer-lasting event in the Northern Highland terrane .…”

“…1985), or a single, protracted orogenic-metamorphic event (Searle, 2021). Readers are referred to the recent discussion between Searle (2022) and Dewey and Ryan (2022) for arguments for and against the two short-lived orogenies (Ordovician Grampian and Silurian Scandian), or a single longer-lasting event in the Northern Highland terrane (Searle, 2021).…”

“…Despite more than 150 years of active research along the Moine Thrust Zone, the Caledonian thrust front in NW Scotland (Nicol, 1861; Lapworth, 1883, 1885a, b; Peach et al 1907; Elliott & Johnson, 1980; Butler, 1982, 1987; 2010, 2023; Coward, 1983, 1985; Trewin, 2002; Law et al 2010; Mendum et al 2009; Searle et al 2010, 2019; Searle, 2022), there remain several controversial aspects concerning the geometry of thrust sheets, the timing of motion along major thrusts and the relationship between thrusts and a suite of alkaline igneous rocks exposed in NW Scotland (Fig. 1).…”