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

Spatially and temporally changing tectonic regimes in the southern part of the Northern Highland Terrane, Caledonian mountain belt, Scotland

Paper number: 245-2
Program: Technical Sessions
Time: 8:20 AM – 8:40 AM

Authors

Abstract

In this GSA Special Session honoring the career of John Platt we contribute an overview of currently on-going structural, metamorphic petrology and isotopic dating based studies in the southern part of the Northern Highland Terrane. The southern part of the NHT is divided into two tectonic units: the Flat Belt located between the Great Glen and the Loch Quoich Line (LQL) and to the west the Steep Belt located between the LQL and the Moine thrust zone. Foliated and lineated Neoproterozoic Moine metasedimentary rocks deformed under amphibolite facies conditions in the core of the Caledonian mountain belt make up both tectonic units. In our contribution we concentrate on Grampian – Scandian (Ordovician-Silurian) age structural processes operating in these tectonic units.

Sub-horizontal mineral lineations on gently dipping foliation in the Flat Belt trend NNW-SSE traced southwards for 70 km from Cannich to Loch Eil. Quartz c-axis fabrics demonstrate that lineation is parallel to the maximum principal stretch direction (X) and fabric asymmetry and opening angles indicate orogen-parallel top to the NNW shearing at deformation temperatures of 575-620 °C. In contrast the Steep Belt is characterized by intense upright folding with steeply plunging (often sheath-like) fold hinges. In the Steep Belt a series of transects have been made across the Sgurr Beag thrust (SBT); foliation dips steeply to the ESE/SE and lineation generally plunges steeply (60-80°) to the SE/S. Quartz fabrics indicate a NW (footwall) side up relative shear sense with deformation temperatures increasing from hanging wall to footwall of the SBT - e.g. at Kinloch Hourn, 530-500 °C in hanging wall and 585-530 °C in footwall. This regionally pervasive shearing, with higher temperatures towards the core of the Steep Belt, post-dates motion on the SBT.

Isotopic ages from the Flat Belt that may be associated with sub-horizontal orogen-parallel top to N shearing range from ~475-465 Ma while formation of the Steep Belt has been bracketed at 448 - 432 Ma. These ages are significantly older than the ~437-430 Ma ages obtained for motion on the Moine thrust at the foreland edge of the Caledonian orogen, indicating a complex history of sub-horizontal orogen parallel shearing, followed by orogen perpendicular shortening and vertical stretching, that pre-dates final foreland-directed shearing on the Moine thrust zone.

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

doi: 10.1130/abs/2025AM-412375

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