Mylonites: lessons from Eriboll

Abstract

Eriboll has been an important area in understanding the geology and structure of the NW Highlands of Scotland. It came to prominence during the Highland Controversy between Nicol and Murchison in the 1850s. Nicol recognized a major regional, or grand, dislocation (the Moine Thrust Zone) at Eriboll whereas Murchison denied its existence. An important element in the resolution of the controversy was the repeated observation of a distinctive schistose rock that often marked Nicol's dislocation. Lapworth named it a ‘mylonite’ and related its development to mechanical metamorphism resulting from tectonic (milling) movements along the dislocation. Peach, Horne and co-workers, whose contributions to early mylonite studies are often overlooked, described the effects of increasing strain on mylonite development, recorded the prominent lineation within mylonites and identified its kinematic significance via thrust geometries and a first use of shear sense indicators. In this contribution those elements of the Highland Controversy that led to the identification of mylonites will be reviewed, concentrating on the contributions of Lapworth and of Peach, Horne and co-workers. It will include an examination and discussion of the mylonites from Lapworth's locality and of the mylonitized Moine schists at Eriboll. It will conclude with comments on mylonite classification and nomenclature.

Moine Thrust zone mylonites at the Stack of Glencoul: I – microstructures, strain and influence of recrystallization on quartz crystal fabric development

“…It was in this paper that Lapworth coined the name 'mylonite' for rocks produced by such grain-scale processes of crushing and fracture (see reviews by Teall 1918, pp. 1-3;White 1998White , 2010Law & Johnson 2010).…”

Structure and internal deformation of the Arnaboll Thrust Sheet, NW Scotland: implications for strain localization in thrust belts

“…1). Their conclusions followed Peach et al (1907) in that the main thrusts in the Eriboll district were interpreted with the structurally lowest (Sole Thrust) being the oldest and the structurally highest (the Moine Thrust) being Lapworth's (1885) type locality for mylonites (see White 2010). Note that the hanging wall Lewisian gneisses only acquire intense mylonitic fabrics close to the thrust surface.…”

“…It was at Arnaboll that Lapworth (1883Lapworth ( , 1885 established not only the importance of horizontal displacements in building the geological structure of the NW Highlands, but also developed understanding of fault rock evolution. Thus the Arnaboll Thrust is the type locality for mylonites (Lapworth 1885;White et al 1982;White 1998White , 2010. It is also where Geikie (1884) coined the term 'thrust' and was the focus of the early mapping by the Geological Survey (Peach & Horne 1884;Peach et al 1888).…”

The Geological Structure of the North-West Highlands of Scotland – revisited: Peach et al. 100 years on

“…Aspects are reviewed in later chapters (Law & Johnson 2010;White 2010) and discussed by Cheeney (2002). Lapworth's insights on fault rocks were quickly adopted by the Survey geologists, especially through the efforts of Teall, as he recounted in his later review (Teall 1918).…”

“…A companion paper explores in greater detail the evolution of thrust tectonic models in NW Scotland, especially following the 1907 memoir. Reviews of the late-19th century Survey work in the NW Highlands, emphasizing microstructural studies and contemporary connections with metallurgy/materials science, are also given by Law & Johnson (2010) and White (2010) respectively.…”