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Discussion of Searle, ‘Tectonic evolution of the Caledonian orogeny in Scotland: a review based on the timing of magmatism, metamorphism and deformation’

Published online by Cambridge University Press:  05 September 2022

John F. Dewey Open the ORCID record for John F. Dewey [Opens in a new window]  and
Paul D. Ryan Open the ORCID record for Paul D. Ryan [Opens in a new window]
John F. Dewey*
Affiliation:
University College, High Street, OxfordOX1 4BH, UK
Paul D. Ryan
Affiliation:
School of Natural Sciences, Earth and Ocean Sciences, National University of Ireland Galway, University Road, Galway, Ireland
*
Author for correspondence: John F. Dewey, Email: jfdeweyrocks@gmail.com
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Abstract

Searle (2022) argues, based on data from Scotland alone, that the deformation and metamorphism of the Scottish Highlands is continuously diachronous from the early Ordovician (Grampian) in the Dalradian of the Grampian Highlands to the mid-Silurian (Scandian) in the Moine of the Northwest Highlands. Necessarily, he disputes substantial offset along the Great Glen Fault (GGF). He compares the Scottish Caledonides with the Himalayas. We take the different view, which also considers evidence from along-strike in Ireland and Newfoundland, that the Grampian and Scandian orogenies were discrete events. The Grampian Orogeny involved major deformation and metamorphism across the Grampian and Northwest Highlands to the Moine Thrust Zone, whereas the Scandian Orogeny affected only the Northwest Highlands. The early Ordovician Grampian Orogeny was generated by the collision and obduction of an oceanic arc-ophiolite with and onto the Laurentian continent. The mid-Silurian Scandian Orogeny was caused by continental collision between Laurentia and Baltica. Late Silurian – early Devonian sinistral slip along the GGF emplaced the the Moine of the Northwest Highlands from a distant location to the north in the Scandian collision zone, against the Dalradian, which had not been involved in the Scandian collision. We outline several (eight) arguments that we believe contradict evidence for progressive northwestward propagation of a Himalaya-style thrust wedge from the early Ordovician to the mid-Silurian.

Keywords

GrampianScandianOrogeny
Type
Discussion
Information
Geological Magazine , Volume 159 , Issue 10 , October 2022 , pp. 1833 - 1836
Copyright
© The Author(s), 2022. Published by Cambridge University Press

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