245 Rock Deformation and the Dynamics of Mountain Building: A Session Honoring the Scientific Contributions of John P. Platt, Part I

We welcome contributions inspired by the diverse research of John Platt in structural geology and tectonics showing how the rock record can be used to illuminate the fundamental processes of crustal deformation and mountain building.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217D

New P-T-t results for the southern Northern Highlands terrane, Scotland: Constraints for Caledonian deformation along the Sgurr Beag Thrust

Paper number: 245-3
Program: Technical Sessions
Time: 8:40 AM – 8:55 AM

Authors

Abstract

The Paleozoic collision of the eastern margin of Laurentia with an outboard magmatic arc at ~490-470 Ma (Grampian/Taconic Event) followed by amalgamation with Baltica and Avalonia at ~430 Ma (Scandian Event) initiated thrusting in the northwest Scottish Highlands and built the Caledonian orogenic wedge in Scotland. In the Northern Highlands terrane (NHT), the orogenic retrowedge is composed of an imbricate stack of ductile thrust sheets that progressively increase in metamorphic grade from west to east, reaching upper amphibolite facies (sillimanite grade) in the core of the orogenic retrowedge. In the southern NHT, Caledonian shortening was mostly accommodated along two dominant thrusts: the lower-grade Moine thrust to the west and the higher-grade Sgurr Beag thrust (SBT) to the east. However, modern geochronologic and quantitative metamorphic studies have been notably limited in this part of the NHT, hindering our ability to unravel the tectonic and metamorphic evolution in this area. Here, we couple laser ablation split-stream dating of monazite and xenotime with isochemical pseudosection modeling to provide new constraints on the tectonometamorphic evolution of the southern NHT.

 

In the SBT hanging wall, monazite dates are 478 to 432 Ma. Temperatures within this unit were consistently elevated (696-561°C) during this time, based on monazite-xenotime thermometry. Peak metamorphic temperatures were reached at ~455 Ma, coincident with garnet and kyanite growth. Surface directed thrusting and erosion cooled the eastern hanging wall, while further west, temperatures display an increasing trend from 559°C at 471±18 Ma to 653°C at 453±14 Ma. We interpret that emplacement of high temperature rocks from the east drove heating and prograde metamorphism down into the SBT footwall, leading to a second phase of garnet growth. Internal deformation in the hanging wall of the SBT largely occurred during the Grampian event, and the younging trend from east to west within the hanging wall is consistent with published observations made along the northern coast of Scotland. We interpret that exhumation and cooling of the SBT sheet was driven by activation of the structurally lower Moine thrust during the Scandian orogenic event. However, no dateable phases were observed in the structurally underlying Moine thrust sheet below the SBT shear zone, which can likely be attributed to the lack of appropriate lithologies or P-T conditions conducive for monazite formation.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414107

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