SHEAR SENSES, TECTONIC TRANSPORT DIRECTIONS AND DEFORMATION TEMPERATURES IN THE SOUTHERN PART OF THE NORTHERN HIGHLAND TERRANE, SCOTLAND: MICROSTRUCTURAL AND QUARTZ C-AXIS FABRIC DATA FROM THE FLAT BELT AND TRANSECTS ACROSS THE SGURR BEAG THRUST IN THE STEEP BELT

The southern part of the Northern Highland Terrane 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. Foliated and lineated Moine metasedimentary rocks deformed under amphibolite facies conditions make up both tectonic units. Sub-horizontal mineral lineations on gently dipping foliation in the Flat Belt from Cannich southwards to Glen Garry trend NNW-SSE. Quartz c-axis fabrics demonstrate that lineation is parallel to the maximum principal stretch direction (X) and fabric asymmetry and opening angles indicate top to the NNW shearing at deformation temperatures of 575-620 °C.

In the Steep Belt transects have been made across the Sgurr Beag thrust (SBT) at (from north to south) Loch Monar, Glen Shiel, Kinloch Hourn and Lochailort. In all four transects foliation dips steeply to the ESE/SE. At Loch Monar lineation (X) plunges moderately (35-60°) to the SW, while at Glen Shiel, Kinloch Hourn and Lochailort lineation (X) generally plunges steeply (60-80°) to the SE/S. Quartz fabrics indicate a SE (hanging wall) side down shear sense with deformation temperatures decreasing from footwall to hanging wall of the SBT - e.g. at Kinloch Hourn, 585-530 °C in footwall and 530-500 °C in hanging wall. Grain size of dynamically recrystallized quartz on these transects remains fairly constant from footwall to hanging wall, rather than decreasing towards the mapped position of the SBT, suggesting that this shearing postdates formation of the SBT. In present day coordinates these observed shear senses (and distribution of deformation temperatures) suggest a normal/extensional sense of shearing. However, it remains to be determined if the observed structures have been rotated from an original more gently inclined orientation on later formation of the Steep Belt, or if they developed during folding and shearing associated with formation of the Steep Belt.

Formation of the Steep Belt has been bracketed between 448 and 432 Ma. Dating the age of foliation/lineation development on these transects across the SBT (and also in the Flat Belt) is therefore a potentially promising method for determining if the observed shearing and associated transport directions developed before or during formation of the Steep Belt.