32-12 Linking Microstructures to Ages- An Investigation of the Multi-Phase Orogenic History of the Southern extent of the Northern Highlands Terrane in NW Scotland

Session: Latest Research Advances in Structural Geology and Tectonics (Posters)

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The Northern Highlands Terrane (NHT) of NW Scotland formed over a series of orogenic events including the Knoydartian orogeny (~870-720 Ma) and the Caledonian orogenies divided into the Grampian I (~475-460 Ma), Grampian II (~450-440 Ma), and the Scandian (~440-415 Ma) events. While there is evidence of each orogeny scattered throughout the NHT through folding relationships and isotopic dates, each event successively overprints the previous event, leading to uncertainty in the timing of deformation structures and P-T conditions of these mountain building events. The southern NHT is an opportune location to investigate this complex orogenic history as detailed mapping reveals regional polyphase folding and isotopic dating studies, while limited, demonstrate that metamorphism occurred through the Knoydartian and Caledonian orogenies.

The southern NHT is divided into two structural regions by the Loch Quoich Line (LQL): the western Steep Belt and the eastern Flat Belt. Both regions are composed of metasedimentary rocks which have been deformed under amphibolite facies conditions and transported/sheared along the SE dipping Moine and Sgurr Beag thrusts. Recent investigations of deformation microstructures along the Sgurr Beag thrust and within the Flat belt show that a period of high-temperature penetrative deformation caused widespread development of grain boundary migration recrystallization in quartz and feldspar in both Steep Belt and Flat Belt samples, which were followed by a later stage of shearing, possibly during cooling, that appears to have affected the two structural regions differently.

Most Steep Belt samples show top-down-to-the-SE motion with temperatures between ~490-655 °C, while the Flat Belt samples show top-up-to-the-NW shearing with temperatures between ~552-655 °C. U-Pb ages collected from titanite, monazite, and xenotime show a range of metamorphic growth periods from the Knoydartian (763 ±16 Ma) to the end of the Caledonian (412 ±26 Ma) in both the Steep Belt and Flat Belt. Zr-in-Titanite thermometry indicates that metamorphic temperatures during the late Caledonian events in the Steep Belt are significantly higher than subsequent deformation temperatures, but this may correspond to earlier high-temperature recrystallization of quartz and feldspar. In contrast, late Caledonian metamorphic temperatures of the Flat Belt overlap the calculated deformation temperature, which may indicate that late penetrative shearing in the Flat Belt occurred during ~412 Ma metamorphism.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8862

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