The nature and origin of D1 and D2 structures in the upper parts of the Dalradian succession in the Southern Highlands of Scotland
John MENDUM
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA
Immediately NW of the Highland Boundary Fault (HBF), Dalradian Southern Highland Group rocks are exposed in a ‘Steep Belt’, formed during the later stages of the Ordovician Grampian orogenic event. They are mainly turbiditic arenites, wackes, pelitic and volcaniclastic rocks, metamorphosed under greenschist facies conditions in which four deformation phases are recognised. Bedding, S1, S2 and S3 cleavages have all been rotated c. 90°, such that they now dip steeply NW. Facing on S1, S2 and S3 is generally downwards. The ‘Steep Belt’ provides a cross section through some 5 to 10 km of structural level, bounded to the NW by the monoformal ‘Downbend’ axis. Only D1 structures are generally present adjacent to the Highland Boundary but D2 structures are progressively developed to the NW.
A structural cross section between Callander and Loch Earn exposes the highest structural and stratigraphical levels in the Southern Highlands. Here, D1 structures dominate for some 5 km NW from the HBF and small- to large-scale, close to tight folds (F1) and related slaty/spaced/crenulation cleavages (S1) can be distinguished. Cleavage orientations lie close to bedding and dislocations are common on all scales. F1 axes generally plunge moderately westwards. D2 deformation is first manifest as discrete structures some 5 km NW of the HBF. It results in attenuation, rotation and modification of the D1 structures, formation of centimetre to rare kilometre-scale, NNW-vergent F2 folds with gently ENE plunging axes, and development of a penetrative S2 spaced/crenulation cleavage. D3 effects are largely absent here, and D4 deformation is of variable intensity.
Based on the structural geometry and petrography, a model for the generation of the D1 and D2 structures has been determined. In thin section S1 and S2 spaced cleavages show similar mineralogies with muscovite and chlorite defining the cleavage laminae and quartz, feldspar (albite) ± calcite dominant in the lithons. D1 structures formed under high non-coaxial strain (top to S) with shear acting near parallel to the regional bedding. Large- and small-scale dislocations formed in the folded sections, probably associated with expulsion of pore fluid. D2 deformation is interpreted as a product of pervasive non-coaxial strain (sub-simple shear), again with the principal shear direction acting sub-parallel to the regional bedding, but now top to the NW. Differential rotation of S1 relative to bedding is common, particularly on the upper right-way-up limbs of F1 folds where the S0-S1 geometry is favourable. On the lower inverted limbs of F1 folds, a more common circumstance, the abundant minor folding of the spaced S1 is attributed to vorticity variations. Jiang (1994) quantified the theoretical variations in shear-induced vorticity in layered rocks for various flow regimes and different layer competencies, and his results readily explain the observed D2 structural geometries. Build up of vorticity, particularly in the incompetent mica-rich layers, is relieved by spin (top to NW) of the competent layers. Quartz and calcite have migrated into the lithons coeval with maximum spin and formation of S2. Cathode luminescence studies show that quartz in the S2 lithons is pervasively recrystallised in D2.The model predicts that stress-induced mass transfer has occurred over distances of at least15 cm on geologically rapid timescales and has consequences for the mechanisms of pressure solution and crenulation cleavage formation.
Jiang, D. 1994. Flow variation in layered rocks subjected to bulk flow of various kinematic vorticities: theory and geological implications. Journal of Structural Geology, 16, 1159-1172.