Difference between revisions of "Geology of the Aberfoyle district: Structural history"

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Latest revision as of 13:57, 29 July 2021

This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
File:AberfoyleSD fig9.jpg
Figure 9    The distribution of faults and major folds in the Aberfoyle district. The loci of the traces of fold axial planes are inferred from structural observations and are generalised. The monoformal Highland Border Downbend (HBDB) separates the ‘Flat Belt’ to the north from the ‘Steep Belt’ to the south. Note the obliquity of the trend of the major fold traces to the Highland Boundary Fault Zone (HBFZ). Note also the way in which the Loch Tay Fault Zone (LTFZ) and related faults to the west, such as the Dukes Pass Fault (DPF), swing in to the HBFZ, with sinistral strike-slip displacement being transferred on to the western structures. Strike-slip displacement on the Loch Tay Fault is progessively transferred to the south-west onto the Loch Ard Fault (LAF).

The structural history of the Aberfoyle district involves episodic and protracted deformation. This history began with the Caledonian Orogeny, represented In the Aberfoyle district by the mid Ordovician Grampian event. Explain here how the Scandian and Acadian orogenies fit with the Caledonian. In the following account, successive episodes of deformation are labelled D1, D2, etc. Folds and cleavages resulting from each of these deformation episodes are labelled F1, etc and S1, etc. Bedding is labelled S0 by convention.

The Aberfoyle district contains important structures that have been key in elucidating the Palaeozoic evolution of Grampian Highlands and the Midland Valley. The structural history of the Aberfoyle district is a complex of episodic and protracted deformation. Rocks of the Highland Border Complex and the Dalradian Supergroup were pervasively deformed during Caledonian orogenic events, mainly during the Ordovician. During the late Caledonian, Scandian and Acadian (late Silurian to Early Devonian) deformation significantly affected the disposition of rocks within the Dalradian terrain and deformed Lower Devonian strata in the Midland Valley. Variscan deformation during the late Carboniferous to Permian played a key role in the deposition and subsequent erosion of Devonian and Carboniferous rocks north and south of the Highland Boundary Fault Zone (HBFZ). The fault zone is the key structural feature of the Aberfoyle district, dividing the Caledonian highlands from the Midland Valley graben. Other important structural elements in the district include the Loch Tay and related faults, that principally affect Dalradian rocks; the major Dalradian folds, including the Aberfoyle Anticline, that define the nose of the Tay Nappe; the monoformal Highland Border Downbend, and the oversteepened north-western limb of the Strathmore Syncline in which lie Lower Devonian strata immediately adjacent to the HBFZ. The distribution of the major structural features within the Aberfoyle district are shown in Figure 9.

Geology of the Aberfoyle district - contents[edit]