Structural Geology of the Lewisian Complex North of Loch Maree, NW Scotland

Fernandes, Luis Alberto D'Avila (1987) Structural Geology of the Lewisian Complex North of Loch Maree, NW Scotland. PhD thesis, University of Glasgow.

Abstract

Field and laboratory studies of the Lewisian Complex of the Loch Maree district in NW Scotland have confirmed the presence of tectonic interleaving of late Archaean basement gneisses and early Proterozoic volcano-sedimentary cover (Loch Maree Group). Structural analysis of these rocks has demonstrated the presence of four generations of approximately coaxial folds and a relative chronology of events has been established. During D1 a strong NW-SE stretching fabric was developed. At a late stage of this phase the basement was tectonicaIIy emplaced over the cover with the widespread formation of myIonites particuIarIy at major Iitho IoqicaI contacts. D2 affected both basement and cover giving rise to a large SE-plunging HE-verging structure. This fold which shows perfectly coaxial relations with the D1 fabrics, controls the distribution of the lithoiogical units with gneisses and marbles representing basement and detachment zone, respectiveIy, in the core of the structure. During D5 D4 recumbent and upright folds were formed. These display clockwise relations with the L1-L2 stretching lineation and were formed under conditions of continuous deformation at higher structural levels indicating post-D2 uplift. Climaic metamorphic conditions were operative during early stages of D1 deformation: middle to upper amphibolite conditions (Barrovian type metamorphism) have been deduced from garnet-biotite and garnet-hornblende geothermometry and coexisting mineral assemblages. The temperature was lower (greenschist facies conditions) during myIonitization. M2 temperatures, also determined by geothermometry (garnet-biotite and garnet-hornblende) corresponded to upper greenschist to lower amphibolite facies conditions. The changes in P-T conditions are interpreted as being indicative of ' thermaI relaxation' associated with thickening of the crust produced by (D1) thrusting. Structures indicating transient brittle deformation along the main thrust horizons are ascribed to pore fluid effects. Evidence for abundant fluid activity is given by chemical transformations producing mineralogical convergence in uItramyIonites. Interplay between metamorphism and deformation was complex and the application of the classical microtectonic approach was not possible. Similarly for the determination of the brittle or ductile character of the deformation in 'fault-related' rocks the recently proposed methods do not seem to be applicable: careful analysis of deformation mechanisms on the grain scale is considered to be essential. Uiscosity contrast variations between early D1 and late D4 are ascribed to changes in the strain rate, geometric softening and even variations of the temperature: there is no evidence that mineralogical changes exercised a control. A model based on simple transpression with a middle crustal flat-lying shear zone formed in the step-over of a Ieft-lateral right- stepping wrench system is capable of explaining the deformation of these rocks in a continuous fashion under conditions of progressive uplift of the crust. If such a model is considered realistic, no reliable correlation of structures based on style is possible over distances of few tens of kilometers where physical continuity of structures in not available. Instead features indicating the predominant kinematic pattern are more reliable.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Geology
• Date of Award: 1987
• Depositing User: Enlighten Team
• Unique ID: glathesis:1987-77569
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77569