A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe, SW Highlands, Scotland

Accepted Manuscript A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe, SW Highlands, Scotland P.W. Geoff Tanner PII:

S0191-8141(15)30052-3

DOI:

10.1016/j.jsg.2015.11.007

Reference:

SG 3284

To appear in:

Journal of Structural Geology

Received Date: 1 July 2015 Revised Date:

9 November 2015

Accepted Date: 16 November 2015

Please cite this article as: Geoff Tanner, P.W., A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe, SW Highlands, Scotland, Journal of Structural Geology (2015), doi: 10.1016/j.jsg.2015.11.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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D2 microlithon

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Stretching lineation

L2

D2 cleavage seam

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Crenulation fold axis

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Orthogonal tipping point D1 microlithon Back-rotation

Bulk non-coaxial strain

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A new model for the formation of a spaced crenulation (shear band)

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cleavage in the Dalradian rocks of the Tay Nappe, SW Highlands, Scotland

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P. W. Geoff Tanner

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School of Geographical and Earth Sciences, University of Glasgow, Glasgow

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G12 8QQ, Scotland

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Keywords:

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microlithon

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vergence

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non-coaxial

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flexural-slip

back-rotation

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E-mail address: [email protected]

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ABSTRACT

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The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D1 spaced cleavage was responsible for the formation of the D2

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spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-

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Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D2

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microlithons are asymmetric; have a geometrically distinctive nose and tail; and

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show a thickened central portion resulting from back-rotation of the constituent

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D1 microlithons. The current terminology used to describe crenulation

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cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is

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shown how embryonic D2 flexural-slip folds developed into a spaced cleavage

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comprising fold-pair domains wrapped by anastomosing cleavage seams. The

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bulk strain was partitioned into low-strain domains separated by zones of high

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non-coaxial strain. This new model provides a template for determining the

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sense of shear in both low-strain situations and in ductile, higher strain zones

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where other indicators, such as shear folds, give ambiguous results. Analogous

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structures include tectonic lozenges in shear zones, and flexural-slip duplexes.

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Disputes over the sense and direction of shear during emplacement of the Tay

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Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appears to be resolved.

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1. Introduction

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This work is based on a small area of exceptionally well-exposed shear bands on the Cowal Peninsula near Glecknabae, Isle of Bute, in the SW

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Highlands of Scotland (Fig. 1). These exposures are unique in preserving, in

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3D, early stages in the development of the D2 spaced crenulation cleavage.

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This fabric occurs widely in the greenschist facies rocks of the Dalradian

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Supergroup. In Spring 2014, hurricane-force storms deposited a layer of gravel

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several metres thick, (with boulders > 50 cm across) over almost the entire site,

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and it is important that a fully illustrated account of these structures is made

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available.

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--------------------------FIGURE 1 ABOUT HERE ----------------------------

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The terminology used to describe shear bands and crenulation cleavages, and the mechanisms proposed for their formation, are reviewed briefly. The

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main types of crenulation cleavage are illustrated in Figure 2, in which

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examples of fabrics from the Cowal Peninsula are supplemented by three

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examples from a Tentaculites-bearing grey slate from the Rosenun Formation (Middle Devonian) (Tanner, 1985) (Fig. 2a, c-e) near Liskeard, South Devon; and one example from the Precambrian rocks of Anglesey (Fig. 2f). There

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follows a detailed examination of the 3D external and internal geometry of the

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D2 spaced crenulation cleavage, developed in the Dalradian rocks of the SW

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Highlands, culminating in a model for the formation of this cleavage.

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--------------------------FIGURE 2 ABOUT HERE ----------------------------

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1.1. Nomenclature and general aspects of cleavage formation

79 Cleavages in low-grade metasediments may be divided into: primary

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cleavages, which form in rocks where there was no pre-existing planar tectonic

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fabric, and secondary cleavages, which require the presence of an earlier

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cleavage or fabric for their formation. All of these cleavages are to some extent

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domainal (Williams, 1972; Weber, 1981; Bell, 1981), and it is the scale of

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observation that dictates whether a cleavage is described as penetrative or

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spaced. As emphasized by Bell (1981), and supported here, most types of

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cleavage have an intrinsic anastomosing habit.

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Slaty cleavage is a primary continuous or penetrative cleavage (Fig. 2a) that appears all pervading to the naked eye, whereas a primary spaced cleavage

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(Fig. 2b) is clearly discontinuous and differentiated into narrow, dark,

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anastomosing cleavage seams (Bell, 1981; Gray, 1979), that alternate with

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thicker, pale-coloured microlithons (De Sitter, 1959). This process of

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mineralogical and compositional differentiation on a m–mm-scale, results in a

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thinly banded rock that may be mistaken for a laminated metasediment, or vice versa (see the discussion by Treagus et al. (2013)). In these cases, a careful search of the exposure usually reveals that the spaced cleavage lies at a high

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angle to bedding (see Fig. 2b), or preserves evidence of an earlier spaced

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cleavage (Fig. 2h).

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The cleavage seams are phyllosilicate-rich, whereas the microlithons

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contain more quartz and/or calcite, depending upon the whole rock

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composition. An important difference between them is that the microlithons

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commonly present evidence of an earlier structural history, whereas this record

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is no longer preserved in the cleavage seams.

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Early penetrative or spaced cleavages provide the essential, thin, parallelsided folia that are necessary for the development of a crenulation cleavage

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(Knill, 1960). This cleavage type is associated with the later stages of

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deformation in polyphase-deformed regional metamorphic terrains, and in its

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simplest form consists of aligned trains of m–mm-scale buckle folds, which

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lack discernable structural breaks, and formed as a result of bulk homogeneous

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coaxial strain affecting rocks with a pre-existing anisotropy (see section 1.2).

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The origin of other forms of crenulation cleavage, in which dark seams occur on the limbs of the micro crenulations, roughly parallel to their axial

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surfaces, is more uncertain. The dark seams appear to mark structural

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discontinuities, and Turner and Weiss (1963) queried how they could have

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developed parallel to the axial surfaces of minor crenulation folds, when these

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generally coincide with the XY principal plane. The probable answer is that

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they are composed almost entirely of 'debris' accumulated on seams orthogonal

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to the maximum principal stress where solution transfer has been active.

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This problem was discussed by Bell and Hobbs (2010) who noted the

dichotomy between those workers who accept that some shearing along the preexisting foliation was necessary for the development of the crenulation folds,

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and those who did not. Bell and Hobbs (2010) emphasized that all crenulation

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cleavages required a component of shear for their development. They reiterated

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the main tenet of Bell's (1981) model that during orogenesis the bulk regional

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strain is partitioned into zones dominated by either coaxial or non-coaxial

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strain. The cleavage seams arising from the non-coaxial component form an

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anastomosing pattern around elliptical domains affected by coaxial strain.

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In finely banded or laminated sediments, the crenulation cleavage is marked by a crumpling or crenulation of the primary D1 slaty cleavage in

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mudrocks (Fig. 2c), accompanied by the development of buckle folds in the

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thin, sandy, more competent beds or layers. In this so-called zonal form of

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crenulation cleavage, the parallel trains of micro folds remain intact and, like

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the bedding traces, show little or no apparent displacement across the dark

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seams that occur on their limbs (Fig. 2c).

Alternatively, the onset of the D2 deformation is heralded by the

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appearance of a D2 discrete crenulation cleavage (Fig. 2d). Thin dark seams

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marking D2 structural discontinuities divide the rock into slices, which were

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enriched in calcite or quartz, or both, and within which bedding-early cleavage

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relationships are unaffected by D2 (Fig. 2d). Subsequent shear parallel to the S1

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surfaces resulted in the formation of sigmoidal folds (Fig. 2e).

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Individual hinge zones of the crenulation folds, or eventually even the

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entire microlithon (Fig. 2g), may become preferentially enriched in quartz

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and/or calcite (Fig. 2f) (Williams, 1972; Cosgrove, 1976). This results in a

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rock with a noticeably striped appearance. In some cases, the microlithons become broader, and more prominent, with evidence of an earlier set of solution stripes (S1) having been virtually obliterated. At this stage, the fabric

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is called a disjunctive cleavage (Fig. 2h). In Figure 2g, a series of parallel D2

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microlithons preserve S1 traces, which when connected reveal the profile of a

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Type 2 fold (D2).

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Having regard to the problems posed by the identification and kinematic interpretation of C-S bands, extensional crenulation cleavages (Platt and

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Vissers,1980) and related structures (Passchier and Trouw, 2005), the

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recommendation that the term shear band be used in all of these cases (Fossen,

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2010; Carreras et al., 2013), is adopted here.

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Solution transfer (formerly 'pressure-solution') is responsible for the marked contrast in mineralogy, and in some cases, colour, between the

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microlithons and the adjoining cleavage lithons. The cleavage seams have been

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depleted in quartz, or calcite (depending on rock type) and in some cases

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contain newly-grown chlorite and white mica, plus detrital heavy mineral

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grains, whereas the microlithons have been considerably enriched in quartz.

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The mechanisms responsible for these changes have been studied in depth

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(Gray, 1979, and references therein) and are not discussed further, except to

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note that loss of calcite and quartz from the local system by solution transfer is

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the most effective way in which low grade rocks of suitable composition can

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undergo a significant loss in volume. Solution transfer plays a major role in the

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formation of most types of crenulation cleavage and in some cases has a greater

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effect on the bulk shape change than does the ductile deformation.

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1.2. Previous work on mechanisms for the formation of crenulation cleavage

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Since the earliest research into the origin of crenulation cleavage there has

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been a tacit agreement that it formed by bulk, homogenous coaxial strain: a

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viewpoint reiterated in recent publications and enshrined in textbooks (De

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Sitter, 1959, p. 94-98; Turner and Weiss, 1963 p. 463-465; Ramsay and Huber,

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436, 442; Passchier and Trouw, 2005 p. 84, fig. 4.18 ; Fossen, 2010, p. 254).

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Turner and Weiss (1963) concluded that slip on S2 was followed by coaxial

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strain, in contrast to Ghosh (1983) who pointed out that there was no evidence

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that crenulation cleavage could develop parallel to a shear surface. Problems

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with the application of a pure shear model led to the formation of two schools

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of thought. Gray and co-workers (Gray, 1977a, b, 1979; Gray and Durney,

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1979) stressed the over-riding importance of solution transfer in forming both

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primary and secondary crenulation cleavages in low-grade metasediments.

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Conversely, Bell and co-workers (Bell, 1981, 1986; Bell and Cuff, 1989; Bell

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and Johnson, 1989; Bell and Rubenach, 1980, 1983) proposed a more radical

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solution based on the premise that when bulk, progressive, inhomogeneous

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coaxial strain affects rocks of suitable composition, it results in the

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development of a network of anastomosing shear planes enclosing well-defined

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areas of low strain.

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1.3. Vergence and minor fold asymmetry

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As there is some confusion in the literature over the meaning of these

terms, they are defined below, together with recommendations for their use. Classical vergence was defined by Roberts (1974) as ' the horizontal

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direction within the plane of the fold profile, towards which the upper

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component of (viz: apparent) rotation is directed', a definition more easily

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understood when considered in terms of the congruous or parasitic minor folds

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found on the limbs and in the hinge zones of contemporaneous major folds.

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indicates in which direction the next antiformal fold hinge is positioned relative

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to the observer, expressed in terms of right-handed (RH) or left-handed (LH)

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vergence, or, with reference to geographical coordinates, as 'E-directed

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vergence' etc. (Fig. 3).

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Minor fold asymmetry is a general term best reserved to describe the

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down plunge pattern of a minor fold present in a mylonite or shear zone or, as

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in the present case, a microlithon domain. Such asymmetry is not linked to any

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structure, nor does it indicate the local sense of shear.

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Minor fold-pairs in microlithons should be described as having right- or

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left-handed asymmetry or, E-directed asymmetry etc. (Fig. 3), when viewed

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down-plunge. The minor folds in microlithons generally form by back-rotation

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and have a different kinematic significance to both the congruous folds on

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major structures (vergence) and to the sense of shear in zones of non-coaxial

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deformation (Fig. 3). Minor fold asymmetry in microlithons is most obvious

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where the internal folds are sigmoidal but an early stage in the development of

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sigmoids may be seen in many fault and shear lozenges. So-called shear folds

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are unreliable indicators of shear sense compared with -porphyroclasts (Fig.

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3).

------------------------------FIGURE 3 ABOUT HERE---------------------

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2. Regional geological setting

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The minor structures described here occur on the lower limb of the Tay Nappe, a regional-scale fold nappe that provided the foundation for the ~470

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Ma Grampian Fold Belt (Grampian Terrane). This structure can be traced NE-

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SW for several hundred km across the breadth of Scotland, being excised in

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part by the Highland Boundary Fault (Fig. 1). The Grampian Terrane consists

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of rocks belonging to the Dalradian Supergroup, which are of Neoproterozoic-

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Lower Ordovician age (Tanner and Sutherland, 2007). The rocks studied here

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belong to the Southern Highland Group, and crop out <10 km NW of the

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Highland Boundary Fault. They are of turbidite facies and comprise semi-

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pelite, pebbly and gritty psammite, and micro conglomerate, with rare

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carbonate lenses.

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The Grampian Orogeny is divided into four main structural episodes (D1D4) (for reviews, see Stephenson et al., 2013; Tanner et al., 2013, and

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references therein), with the main ductile deformation occurring during D1-D2,

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as the Tay Nappe was being emplaced to the SE. Regional metamorphism does

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not exceed greenschist facies in the three localities studied in this paper. The

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D1 folds are accompanied by two symmetrical sets of cleavages in the

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metasandstones: a grain-alignment cleavage and a spaced cleavage (Harris et

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al., 1976). The D1 spaced cleavage consists of quartz-rich microlithons separated by darker cleavage seams. The D2 deformation is represented by two types of cleavage: a penetrative, commonly finely banded, zonal crenulation

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cleavage in slate and a spaced crenulation cleavage in metasandstone. The

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microlithon component of the shear bands provides a distinctive structural

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marker throughout a large part of the Grampian Terrane. The D2 folds in the

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microlithons show an anomalous micro fold asymmetry that is opposed to the

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observed bulk sense of shear, an (?) unusual situation analyzed by Krabbendam

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and Leslie (1996). L2 maintains a constant N-S trend and plunges at < 35º S.

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D3 folds are spatially restricted to the SW Highlands and show, like D2, a

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consistent westerly vergence. The D4 deformation is represented by the Highland Border Downbend

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Antiform (Fig. 4), which is accompanied by a widely spaced crenulation

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cleavage. All of the sites studied here lie on the southern limb of this structure,

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and according to most workers, this limb was 'flat lying' prior to D4

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deformation. A measure of the degree of tilting during D4 is shown by the

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mean dip of S2, which, on the southern limb of the Antiform, varies from 25–

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45º SE (Fig. 4a). The mean stretching lineation plunges at 25–40º ESE (Fig.

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4a).

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Most aspects of the geology of the Grampian Terrane are summarized by Stephenson et al. (2013), and a more detailed summary of the structural

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evolution of the SW Highlands is provided by Tanner et al. (2013).

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3. Previous work on the formation of D2 microlithons in the Tay Nappe

Charles Clough (in Gunn et al., 1897) was the first geologist to describe

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the D2 microlithons from the Cowal Peninsula (Fig. 4). In a seminal study of

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the structure of the Dunkeld district, Harris et al. (1976) subsequently

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addressed the twin problems, of the origin and kinematic significance of the D2

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shear bands. From the morphology and orientation of the D1 microlithons in

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emplacement of the Tay Nappe was controlled by top-to-the-E shear.

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However, Roberts' (1977) suggestion that the NW-SE stretching lineation

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provided a more reliable measure of the movement direction of the Tay Nappe

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than did the D2 microlithons (and not 'eastward' as clearly stated by Harris et al.

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(1976)) was generally accepted.

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Over the past twenty years, several alternative models have been suggested for the evolution of the Tay Nappe. Krabbendam et al. (1997) proposed that

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major upright D1 folds in the Dalradian rocks, collectively referred to as the

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Tay Nappe (Mendum and Fettes, 1985), had been affected by sub-horizontal,

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heterogeneous non-coaxial strain with a constant sense of shear. They reasoned

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that, given specific geometrical constraints, 'passive' shear folds could form,

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that had the same 'anomalous' asymmetry as that shown by the minor D2 folds

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in microlithons, in the field. The most serious drawback to this model is that it

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requires repeated and systematic variations in shear strain for these folds to

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develop. In addition, the known field relationships do not agree with the

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geometrical constraints required by this process, which are very specific.

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The shear bands at Dunkeld are similar in all respects to those found at a

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comparable structural level in Cowal, and throughout the Highland Border region (Fig. 4). In Cowal, Dunkeld and Glen Shee, L2, the line of intersection of S1 with S2 trends N-S (Stringer, 1957; Harris et al., 1976; Krabbendam et al.,

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1997). L2 should not be confused with the stretching lineation, as they

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represent different kinematic events, perhaps separated by a significant time

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interval. The kinematic relationship between them is problematical, and is not

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referred to further in this paper.

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An alternative hypothesis, that the apparent shear sense given by the D2 microlithons throughout the lower limb of the Tay Nappe was top-to-the-NW,

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was proposed by Mendum and Fettes (1985), Mendum and Thomas (1997), and

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Thomas (2013). However, ample evidence is presented here which shows that

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D2 non-coaxial shear was top-to-the-E and, in the SW Highlands, at least, the

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above model cannot be applied.

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Rose and Harris (2000) made a very detailed SE-NW structural traverse

along the Sma' Glen 12 km SW of Dunkeld (Fig. 1). It runs from the Highland

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Boundary Fault NW across rocks dominated by D1 structures, through the D1/

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D2 transition zone, and passes into the upper part of the D2-dominated zone.

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This transect equates to traverse 3 (Fig. 4a), described here from the Cowal

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Peninsula. Rose and Harris (2000) concluded that:

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1) In the D2-dominated zone, the early spaced cleavage was initiated by layer-

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parallel shortening during D1.

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2) There was a large anticlockwise rotation (as viewed towards the SW) of the XY plane that brought upright D1 major folds to the horizontal.

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3) D2 shear took place on the D1 spaced-cleavage, not bedding as previously

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thought (Harris et al., 1976)

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4) D1 and D2 fabrics were essential parallel prior to their rotation by D4 to form the Highland Border Downbend Antiform.

5) The D2 folds were 'passive shear augen folds generated during bedding-

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parallel shear' (Rose and Harris, 2000, p. 388).

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--------------------------FIGURE 5 ABOUT HERE ----------------------------

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4. Morphology and general features of spaced cleavages from critical

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localities in the SW Highlands

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In order to understand the mode of formation of the D2 microlithons described here, it was necessary, firstly, to determine the nature of the S1 spaced

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cleavage where it was unaffected by the D2 deformation, as at Camsail Bay,

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Rosneath Peninsula; and secondly, to find D2 microlithons that had preserved

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early stages in the development of such structures. The prime targets for this

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study were the Rosneath Peninsula (Fig. 4); where D2 structures are first seen to

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overprint those of D1 in the D1/ D2 transition zone at Cove; and fully developed

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D2 structures from a deeper level within the Tay Nappe, at Glecknabae, Isle of

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Bute (Fig. 4, locs 1–3) (Roberts, 1974; Tanner, 1992b, 2013). All of the rocks

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described here belong to the Beinn Bheula Schist Formation (Southern

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Highland Group), and show a similar range of lithologies, namely,

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metasandstones, metapelites and micro conglomerates. The D1 and D2

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microlithons are composed mainly of quartz and feldspar.

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4.1. D1 structures 'unaffected' by D2: Camsail Bay, Rosneath Peninsula

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The structural geometry of the Dalradian rocks on the small headland on

the north side of Camsail Bay (Fig. 4, loc. 1) [NS 262 822] is considered to

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proxy for that of the (nowhere exposed) hinge zone of the Tay Nappe, and was

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chosen to provide samples for characterizing the D1 cleavage where unaffected

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by D2. This locality lies on the upper limb of the Tay Nappe (Fig. 4b) and

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comprises undulating, near-horizontal bedding which is inverted (as shown by

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cross-lamination and graded bedding), and cut by a steep to vertical, slaty to

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spaced, cleavage (S1) (Fig. 2b) (Tanner, 1992b).

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Dark, slightly calcareous, metasandstones and slates develop a prominent spaced cleavage, which is markedly anastomosing, both in situ (Fig. 5a, b), and

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under the microscope (Fig. 5c). The dark cleavage seams contain a high

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proportion of chlorite, white mica, and opaque minerals, and alternate with

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much thicker, paler and more quartz-rich microlithons, that are laterally

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impersistent. Cleavage seams bifurcate and recombine as they pass around the

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lens-shaped microlithons (Fig. 5c).

Early quartz veins folded during D1 occur in rocks dominated by the S1

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slaty cleavage. The all-pervading nature of the solution transfer that occurred

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during cleavage formation is indicated by the irregular, corroded margins to the

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folded quartz veins, and by both the black, quartz-poor shadows seen in their

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inner arcs (Fig. 5d); and the paler solution-transfer 'tails' on their outer arcs

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(Fig. 5d, A, B). The pecked line at B indicates where the margin of the quartz

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vein was probably located prior to the onset of the solution-transfer process.

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The resulting tail (B) now occupies this position.

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--------------------------FIGURE 6 ABOUT HERE ----------------------------

4.2. Morphology of the D2 spaced cleavage from the D1/D2 transition zone:

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Cove, Rosneath Peninsula

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Moving west from Camsail Bay, D2 minor structures are first seen on the

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coast at locality 2 [NS 223 809] (Fig. 4) (Tanner, 1992b), and within a few 100

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m of the D2 spaced cleavage becoming the main fabric. Thin sections cut normal to the D2 cleavage seams show that they are

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thicker than the earlier seams, having subsumed a number of them, in addition

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to hosting new mineral growth. The seams form anastomosing patterns (Fig.

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6b-e) superficially similar to those of the D1 cleavage seams. They divide the

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rock into doubly-tapered microlithons of approximately equal thickness whose

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internal fabric ranges from slightly curved D1 seams, to truncated pairs of D2

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folds (Fig. 5a-e). The overall impression is that the D2 cleavage seams have a

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more pronounced symmetry and regularity than those of D1. One of the D1

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microlithons ((A) on Fig. 6e) has been highlighted to emphasize the amount of

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strain that they have undergone.

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In thin section, some of the D2 cleavage seams are seen to contain minute

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quartz-rich slivers ((B) on Fig. 6d), whose shape may be indicative of high

387

strain or of corrosion by solution transfer. Close to their tip points, some of the

388

D2 seams shown in Figure 6 develop a series of micro Riedel shears, that make

389

apparent angles of 17–22º to the parent dislocation and show alternating dextral

391 392

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390

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386

and sinistral shear displacements at their tips (Fig. 6). Figure 6f shows a set of extremely attenuated D2 microlithons, which from the variable trend of the included traces of S1, is a dismembered 'D2 fold' (see Fig. 2g). This example is

393

similar to that figured in Krabbendam et al. (1997), who considered that it

394

typified microlithons in the Glen Shee area. It is generally difficult to detect

395

megascopic D2 folds in the field, but one such structure has been described in

396

detail from this locality by Tanner (1992b), who also provided further structural

16

17

ACCEPTED MANUSCRIPT 397

details of the site. The best locality for examining downward-facing minor D2

398

folds with an axial planar crenulation cleavage and hinges plunging to the SW,

399

is on the coast by the Knockderry Hotel [NS 215 834] (Tanner, 1992b, p. 173).

400 --------------------------FIGURE 7 ABOUT HERE ----------------------------

402

4.3. Geometry and main features of the D2 spaced cleavage: Glecknabae,

403

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401

Isle of Bute

The principal study area lies on the west coast of the Isle of Bute, 0.5 km

SC

404

north of Glecknabae, near the termination of the minor coastal road (Fig. 4,

406

loc. 3). The rocks consist of thinly banded metasandstones and metasiltstones

407

interstratified with sequences of massive microconglomerate in beds several

408

metres thick. Thick units of gritty metasandstone form prominent features on

409

the hillside NE of the site, and have an apparent dip of 25º as viewed from the

410

west.

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411

Part of the main study area is pictured in Figure 7. The two sandstone horizons labelled X and Y contain between them a package of D2 spaced

413

cleavages, which deform an equally strong D1 spaced cleavage. Both

414

cleavages die out within a short distance of reaching the contact with the

416 417

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412

structurally overlying sandstone bed, and with the top of the underlying bed. This situation is identical to that figured by Dennis and Secor (1990, fig. 6); who classified the fabric as a 'normal-slip crenulation', and similar sets of shear

418

bands showing early signs of back-rotation were figured by Alsop (1993, fig.

419

6e).

420

Bedding dips gently SE (dip 28º, strike 064º (this is a calculated mean, as

421

are all of the readings given below)). This is defined by occasional

17

18

ACCEPTED MANUSCRIPT metasandstone beds, which weather to a rusty brown colour, appear to be

423

calcareous (?dolomitic), and do not develop D1 or D2 cleavages. It is unusual to

424

see bedding so well preserved at this structural level within the Tay Nappe, as it

425

is normally completely reworked by the combined effects of the D1 and D2

426

spaced cleavages. Way-up indicators are absent.

427

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The early spaced cleavage (S1) is strongly developed and varies in

orientation from near horizontal to near vertical, depending on the intensity of

429

the local D2 deformation. Where least affected by D2 it has a dip of 27º SE,

430

strike 070º. The D2 cleavage seams have, in places, been preferentially eroded

431

to give flat-lying S2 surfaces (dip 30º SE, strike 070º) (Fig. 7a). A prominent L2

432

intersection lineation, seen as narrow, parallel stripes on S2 plunges at 26º to

433

185º. Where successive S2 surfaces are viewed orthogonally, L2 is occasionally

434

seen to have a slightly curved trace on a single surface, with different curved

435

trajectories from one S2 surface to the next. This feature is reminiscent of the

436

variations in slip vector seen on successive slip surfaces on the limbs of a

437

flexural-slip fold (Tanner, 1989).

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The stretching lineation lies 30–50º anticlockwise of L2 on S2, but has not

EP

438

440 441 442

been seen with L2 on the same surface. The explanation for this is two-fold:

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firstly, the two lineations developed in different lithologies, and secondly, a gently plunging, finely spaced, D4 crenulation lineation, affects most of the S2 surfaces, and has most likely obscured any pre-existing stretching lineation.

443 444

----------------------FIGURE 8 ABOUT HERE ----------------------------

445

18

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ACCEPTED MANUSCRIPT 446

5. Field observations at Glecknabae leading to a new model for the

447

formation of the D2 crenulation cleavage

448 449

5.1. D2 shear sense and direction

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When the above exposure is viewed in an up-plunge direction (Fig. 7a),

452

what appears to be a chaotic structure becomes more readily interpretable. The

453

two small areas ringed in white give profile views of the W-directed

454

asymmetric microlithon folds. Figure 7b shows six cleavage seams (solid

455

white), each of which throws down the base of sandstone X, demonstrating a

456

top-to-the-E displacement on the D2 cleavage seams. Exceptionally, a complete

457

D2 microlithon is exposed, as at A-B (Fig. 7b).

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At Glecknabae, two localities a few tens of metres apart provide critical

459

evidence of the role played by D2 cleavage seams in the emplacement of the

460

Tay Nappe. At the first locality (Fig. 8a), L2 plunges due S, away from the

461

observer, and into the inclined rock face. Quasi-longitudinal sections through

462

the D2 microlithon fold-pair domains are shown in grey on Figure 8b, and

463

labelled 1-4. The 'longitudinal' section of domain 4 shows a very good example

465 466

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of back-rotation of the D1 microlithons within the D2 domain, about the orthogonal tipping point. Shears responsible for the consistent sinistral displacement of the quartz veins (in black) are marked in red on Figure 8.

467

Some of these shears also define the margins of microlithon domains 1-3,

468

suggesting that the quartz veins were displaced by shear movement during the

469

growth of these domains. The net effect of these movements along the D2

19

20

ACCEPTED MANUSCRIPT 470

cleavage seams is a top-to-the-E displacement from which field measurements

471

yield  = 1.2.

472

At the second locality (Fig. 8c), a set of strongly developed 'ribs' seen trending top-right to bottom-left on this Figure, is accompanied by spindle-

474

shaped bodies (two of which are ringed in white (1)), and are crossed at a high

475

angle by the L2 lineation. This puzzling situation is again made clearer by

476

down-plunge viewing parallel to L2. This exposure is more difficult to interpret

477

than the previous one, the reason being that the D1 microlithons contained

478

within the D2 microlithons are mainly planar so that the top and side views have

479

the same appearance. The spindle-shaped body (1) encircled in white presents

480

a profile view of a microlithon domain, which has a periclinal geometry and,

481

like many of the other bodies, has a tongue-like shape in 3D.

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The main feature in Figure 8c is the quartz vein (white arrows) that is

483

displaced by some, but not all, of the cleavage seams. It is locally parallel to S1

484

and oblique to L2. In the ringed area (2), it has been drawn out into a series of

485

minute boudins, by non-coaxial strain acting within the D2 cleavage seam. This

486

example is further evidence that the displacement across the D2 cleavage seams

487

has a top-to-the-E shear sense. This displacement is difficult to quantify, but in

488

the three examples discussed above,  = 1-2.

490

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The interpretation of the above relationships is not entirely straightforward

as the seams are sites of solution transfer where quartz has been removed, and

491

much of the apparent displacement could have resulted from this, as is in the

492

examples given by Ramsay and Lisle (2000, p. 873). However, for at least part

493

of their course the quartz veins are approximately orthogonal to the cleavage

494

seams, and it is unlikely that much, or all, of this apparent displacement was by

20

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ACCEPTED MANUSCRIPT 495

solution transfer. The estimates of shear strain quoted above are maximum

496

values.

497 ------------------------FIGURE 9 ABOUT HERE ----------------------------

499 500

5.2. Identification of D2 microlithon fold-pair domains

501

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Figure 8c shows the presence of spindle-shaped forms (circled in white),

503

which are of similar shape and size from one exposure to the next. Individual

504

D1 microlithons may be traced continuously through them, and both the D1

505

microlithons and cleavage seams are at maximum thickness in the D2

506

microlithons, and reduced to paper-thin sheets in the enclosing D2 cleavage

507

seams.

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A perfect example of a fully evolved set of four microlithon fold-pair

509

domains is seen on a cross-sectional view oblique to the profile plane (Fig. 9a;

510

an arrow marks the D2 fold hinge). They are still partially linked to one another

511

across the intervening cleavage seams. Domain A is a good example of a

512

complete asymmetric microlithon domain with a well-developed nose (B) and

514 515

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tail (C).

The D2 folds in the fold-pair domains in Figure 9a appear to be modified

buckle folds with characteristic triangle zones of brown weathering calcareous

516

pelite in their hinge zones (D). The more competent bands have the appearance

517

of flattened parallel folds which, when combined with the shapes of the

518

intervening pelite, give a classical Class 2 profile (Ramsay, 1967). The folds

519

form isolated packages of folded D1 spaced cleavages, here named microlithon

21

22

ACCEPTED MANUSCRIPT 520

fold-pair domains, which in some cases bifurcate to form new single-fold

521

domains. Rose and Harris (2000, fig. 8) figured an excellent example from the

522

Sma' Glen of a group of D2 domains, which are identical to those in Figure 9a.

523

A unique exposure (Fig. 9b) near Glecknabae reveals how the present day D2 microlithon domain architecture was achieved. The flat-lying D1 fabric was

525

buckled by a D2 antiform-synform pair, in which both folds die out together

526

away from the domainal structure. The upper surface of the slab-like feature in

527

the centre of the figure is an S2 surface, and carries the L2 intersection lineation.

528

A side view (longitudinal) of a second fold pair that occupies the bottom half of

529

the figure, shows an arching-up of the central portion, and an apparent rotation

530

of the internal D1 microlithons.

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531

A diagnostic feature of these structural domains, seen in longitudinal section in the lower part of Figure 9b, is the back-rotation of the D1

533

microlithons about the orthogonal tipping point, from the head to the middle

534

of the microlithon domain. This rotation of D1 microlithons then reverses and

535

they return to near horizontal as the tail is approached. To extend the

536

interpretation further requires exceptional 3-D exposures of the internal

537

structure of the D2 microlithons such as are shown in Figure 10. The top

539 540

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illustration (a) shows a complete microlithon (A), which has a spade-like nose, back-rotated middle portion, and a clearly defined tail. The crenulation fold axes vary from N- to S- plunging, giving rise to local 'eye' structures.

541 542

----------FIGURE 10 ABOUT HERE ----------------------------

543

22

23

ACCEPTED MANUSCRIPT Figure 10b and c are complementary views of an early stage in the

545

development of 3 pairs of D2 microlithon folds from a stack of flat-lying D1

546

platy microlithons. From W-E, the D1 cleavage is progressively deformed by

547

these folds, which back-rotate the D1 microlithon sheets to a steep attitude, a

548

process that led to attenuation of the D1 microlithons, and of the D2 cleavage

549

seams by a combination of solution transfer and shearing. This process was

550

almost certainly accompanied by removal of quartz from the two shear zones

551

by solution transfer. The microlithons generally appear in stepped rows as in

552

Figure 10 but careful examination reveals the presence of the diagnostic

553

transverse cross-section.

555

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To conclude the morphological description, two complete microlithon domains are shown in Figure 11, viewed as near as possible down-plunge of

557

their respective D2 fold hinges. Each D1 microlithon has been coloured red to

558

emphasize how the pair of D2 folds nucleated on the flat-lying cleavage, and

559

reached their maximum development, before dying out in unison. Both fold

560

pairs show how back rotation was important in developing their SE-directed

561

asymmetry, and how this was in the opposite direction to the regional shear.

563 564

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----------FIGURE 11 ABOUT HERE ----------------------------

565

6. Summary of the main features of the D2 spaced crenulation cleavage in

566

Cowal

567 568

6.1. Morphology and geometry

23

24

ACCEPTED MANUSCRIPT 569 570

The D2 spaced cleavage consists of two components – pale-coloured,

571

quartz-rich microlithons that are the main subject of this account, and thinner,

572

darker cleavage seams that separate them. The D2 microlithons vary widely in external shape from spindles to

RI PT

573

parallel-sided slab-like bodies that are generally up to 1 – 2 cm thick (Fig. 7a).

575

They each consist of regularly spaced D1 microlithons that range in shape from

576

slightly curved, through sigmoidal, to tightly folded. Each microlithon is

577

characterized by a particular internal pattern and separated from its neighbours

578

by D2 cleavage seams.

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579

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574

The block diagram (Fig. 12) shows a typical D2 microlithon with, on the section transverse to L2, a sigmoidal pattern of deformed D1 microlithons, and

581

on the longitudinal section, a very much simpler pattern of D1 microlithons,

582

oblique to section; both sections show back-rotation, with the early microlithon

583

being arranged about an orthogonal tipping point. The D2 microlithons display

584

a variety of shapes in cross-section, ranging from ovoid, to parallel-sided, and

585

are wrapped by D2 cleavage seams. Their geometry is summarized as follows:

586

1) S1 and S2 are statistically parallel in the D1-D2 transition zone that lies astride

588 589

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587

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580

'the line of onset of D2' on Figure 4.

2) The D1 and D2 minor structures belong to a symmetry-constant regime (Voll, 1960; Roberts, 1974).

590

3) The D2 minor folds within the microlithons are bound by D2 cleavage seams

591

and have a consistent west-directed asymmetry.

592

4) L2 maintains a constant N-S orientation across the width of the D2 zone and

593

for 200 km between Bute and Glen Shee.

24

25

ACCEPTED MANUSCRIPT 594 595

6.1. Shear sense and direction

596 The limbs of D2 buckle folds contained within D2 microlithons become drastically

598

thinned at the margins of the fold-pair domains, and their asymptotic relationship to

599

the external D2 cleavage indicates the sense of shear.

600

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597

Examples of bedding or early quartz veins displaced by movement on the D2 cleavage seams have not previously been reported from the Dalradian rocks of the

602

Grampian terrane. Displacement of a bedding plane (Fig. 7b), and of two cross-

603

cutting quartz veins (Fig. 8) on D2 cleavage seams, yield a maximum value of  = 1

604

– 2.

605

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601

The shear sense is given by thinning of D1 microlithons at the margins of D2 microlithon fold-pair domains (Fig. 9a); and by the shape of the asymmetrical D2

607

domains in longitudinal section (Fig. 8a, b). Estimates of shear strains of = 2 – 10

608

for such structures by Krabbendam et al. (1997) do not take into account the

609

possible effect of solution transfer and are therefore maximum values.

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606

EP

610

612 613 614

----------FIGURE 12 ABOUT HERE ----------------------------

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611

6.2. Minor fold asymmetry

615 616

The asymmetry shown by fold pairs in the D2 microlithons is, with few

617

exceptions, W-directed. The opposite, E-directed asymmetry is seen locally,

618

particularly where D2 folds of bedding are developed. An attempt to quantify

25

26

ACCEPTED MANUSCRIPT 619

such changes over a km-long strike-normal section on Loch Lomondside was

620

abandoned due to a lack of D2 microlithon folds showing an E-directed

621

asymmetry.

622 6.3. Pre-D2 tectonic setting

RI PT

623 624

There are no means of deciding whether the D1 major folds were initially

626

upright or recumbent, but it would seem logical they began as upright, open to

627

tight structures such as are found in slate belts. It is generally agreed that the

628

D1 structures and cleavages were gently dipping to horizontal following D2

629

(Tanner, 2013, and references therein), but there are several alternative ways by

630

which this could have occurred. For example, either the folds were rotated

631

during by non-coaxial strain to become flat lying before they were affected by

632

the D2 deformation, or the D1 folds formed, and were later rotated to the

633

horizontal by the D2 deformation. The interpretation followed here is that both

634

the formation and rotation of the early folds occurred during a single (D1-D2)

635

progressive, diachronous event (Fig. 13) (Tanner, 2013).

EP

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637 638 639

The formation and rotation of a single set of folds in a non-coaxial shear

AC C

636

regime has been demonstrated in the Culm Basin in North Devon (Sanderson, 1979) and in the Cumberland Bay Formation on South Georgia, South Atlantic (Tanner and Macdonald, 1982), and applied to the Dalradian basin by Tanner

640

(2013). In both cases, initially upright, open folds were rotated by over 80° to

641

become tight folds accompanied by a well-developed slaty cleavage. The shear

642

strain involved in all three cases was about  = 5 – 6.

643

26

27

ACCEPTED MANUSCRIPT 644

----------FIGURE 13 ABOUT HERE ----------------------------

645 7. The new microlithon fold-pair model It is inferred that the initial structural configuration for this model was of

648

major recumbent D1 folds with a near horizontal axial-planar spaced cleavage

649

(S1). Formation of the D2 spaced crenulation cleavage commenced only when

650

the bulk regional XY plane had been rotated into near-parallelism with S1 (Fig.

651

13) (Tanner, 2013). It is inferred that layer-parallel slip then took place,

652

possibly preceded by a small amount of layer-parallel shortening, to facilitate

653

nucleation of the D2 folds. The D1 spaced cleavage was not perfectly planar,

654

but consisted of alternating layers of mm-cm-thick D1 microlithons separated

655

by cleavage seams rich in chlorite and white mica, probably ornamented by

656

minor asperities upon which folds could have nucleated. In addition to having

657

a fanned arrangement, the early cleavage had an overall anastomosing character

658

(Fig. 5 a-c). This feature, together with numerous irregularities, such as

659

bifurcations and cut-offs, provided 'sticking points' for the nucleation of the D2

660

microlithon folds and possibly obviated the need for an initial layer-parallel

661

shortening event.

662 663 664

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647

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646

----------FIGURE 14 ABOUT HERE ----------------------------

665

The process of D2 microlithon formation began by the flexing and layer-

666

parallel slippage of several pairs of D1 microlithons and their associated

667

cleavage seams, causing them to be rotated by a small amount in a direction

668

opposite to that of the bulk shear. As an increasing number of D1 microlithons

27

28

ACCEPTED MANUSCRIPT became involved in this structure, they formed a pair of back folds whose

670

asymmetry was opposite to that of the bulk regional shear. In order to

671

accommodate the increasing space problem caused by amplification of the

672

minor folds in response to more layer-parallel shear, thickening and uplift of

673

the central portion of individual microlithons took place. Each microlithon

674

behaved independently, was affected by a shear strain that varied from

675

microlithon to microlithon, and as a result, each microlithon developed a

676

characteristic internal pattern. The term orthogonal tipping point is

677

introduced here to mark the point at which a line drawn orthogonal to the

678

external cleavage is tangential to the middle limb of the microlithon fold-pair

679

i.e. a back-rotation of 90º.

680

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669

Partition of the bulk strain into normal and shear components was probably operative throughout, resulting in a polarization of the structure into discrete

682

domains affected only by co-axial strain, separated by well defined zones of

683

high shear strain. The noses of the growing microlithon developed chisel-

684

shaped profiles that may be distinguished from their more bulbous tails (Fig.

685

12), and in some cases, the whole structure adopted a spindle-like shape.

686

Subsequently, each domain became isolated from its neighbours and the

688 689

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connections between domains were severed.

7.1. The origins of spaced crenulation cleavages

690 691

Two main mechanisms operate in deformed metasediments at low

692

metamorphic grade: layer-parallel shortening resulting in flexural flow and

693

buckle folds, and layer-parallel slip forming chevron-style folds. The same

28

29

ACCEPTED MANUSCRIPT 694

dichotomy is evident in the mechanics of spaced cleavage formation, as

695

represented by models A and B (Fig. 14).

696

In Model A (Fig. 14a-c) a rock consisting of well defined parallel-sided layers (such as in a spaced cleavage in a metasandstone resulting largely from

698

solution transfer), will respond by layer-parallel slip in which layers slip freely

699

over one another, accompanied by negligible internal deformation.

RI PT

697

700

In model B, a finely cleaved rock such as a slate, when deformed by layerparallel shortening forms a zonal crenulation cleavage, either symmetrical or

702

asymmetrical depending on a number of factors, including its position on the

703

D2 major fold profile. In this model (Fig. 14d-f) layer-parallel shortening forms

704

buckle folds, which are subsequently rendered asymmetric by shear strain

705

generated by rotation on the limb of a growing major fold (Ham and Bell,

706

2004). This is the classical model for forming tight asymmetric crenulations, as

707

outlined by Bell and Johnson (1989, fig. 1). The most significant feature of

708

these crenulations is that, having been formed by layer-parallel shortening,

709

modified by non-coaxial strain, and rotated during the D2 folding (Ham and

710

Bell, 2004) (Fig. 14d-f), their vergence (s.s.) remains in the same direction as

711

the regional bulk shear, and maintains this relationship to moderate strains. The

713 714

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important difference between these two mechanisms is that, in Model A the folds develop because of non-coaxial strain, whereas in Model B the folds formed first, and were then modified by this strain.

715

Different PT conditions and tectonic settings will result in a spectrum of

716

different types of crenulation cleavages. These may be assigned to one of the

717

two main categories outlined above, but in reality, they are members of a

29

30

ACCEPTED MANUSCRIPT 718

continuum and there is no unique mechanism by which spaced crenulation

719

cleavages may form.

720 721

------------------------FIGURE 15 ABOUT HERE ----------------------------

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722 723

8. Comparison of the Cowal microlithons with other types of shear bands

724

The field evidence that the fold-pair domains on Cowal formed in response

SC

725

to simple shear strain is supported by their close similarity to structures known

727

to have formed because of non-coaxial strain (Fig. 15a, c, f). These include

728

lozenge-shaped bodies within mylonite and fault zones (Fig. 15b), and duplexes

729

associated with the formation and amplification of flexural-slip folds (Fig. 15d).

730

The internal pattern in the D2 microlithons is asymmetric with a

M AN U

726

recognizably different structure in the nose and toe, and with back-rotation

732

evident in the central part of the body. It is noticeable that the upper contact of

733

the microlithon with the cleavage seam (the 'roof', Fig. 15f) diverges from the

734

trend of the external cleavage, opening up an angle  in the direction of shear.

735

For comparison, Figure 15d shows the streamlined, asymmetrical profile of a

737 738

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736

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731

typical flexural-slip duplex.

8.1. Fault and shear lozenges

739 740

A recent review by Ponce et al. (2013), of the morphology and kinematic

741

interpretation of tectonic lozenges in sheared rocks within fault and shear

742

zones, showed that most types of fault lozenge are reliable indicators of shear

30

31

ACCEPTED MANUSCRIPT

sense. Ponce et al. considered that contractional overstep structures (Fig. 15b),

744

which have features almost identical to those of the microlithon domains

745

described from the Cowal Peninsula (Fig. 15c, f), are dependable shear-sense

746

indicators. The similarities between the lozenges and duplexes, were also

747

pointed out by these authors who concluded that lozenges developed in foliated

748

rocks are the least well understood of this family of minor structures.

RI PT

743

749

In an early study, Passchier (1990) figured a weakly deformed lens within a shear zone that had the geometric properties of the type of shear band

751

described in this paper and shows minor fold asymmetry having the opposite

752

sense to bulk shear strain.

754

8.2. Flexural-slip duplexes

755

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750

These structures resemble microlithons or shear bands, both in their

757

internal organization and their external morphology. Flexural-slip duplexes

758

were first recognized in the rocks of the Culm Basin at Hartland Quay, North

759

Devon (Tanner, 1989, 1992a), and were identified as such by the fact that (as

760

predicted by the flexural slip model), their shear sense reversed from one fold

762 763

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756

limb to the next. They formed in response to bedding-parallel slip, and nucleated on bedding irregularities, such as sedimentary slump beds. Most of the features listed below and shown on Figure 15 were also recognized by

764

Horne and Culshaw (2001) from their work on chevron folded turbidities in

765

Nova Scotia.

31

32

ACCEPTED MANUSCRIPT 766

(1) Flexural-slip displacements only take place on movement horizons, which

767

divide the rock into packets within which individual beds or layers remain

768

bonded.

769

(2) Duplexes result from layer-parallel slip. Thickening of the central part of the duplexes occurs due to the back-rotation of horses consequent upon

771

accretion of new layers to the duplex (Fig. 15d).

772

(3) The noses and tails of flexural-slip duplexes have different shapes that give the duplexes a distinctive streamlined profile.

(4) The roof thrust makes an acute angle, with the trace of the D2 cleavage in

774

the overlying cleavage seam.

776

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775

(5) Minor variations in plunge and trend of L2 occur (and hence of the slip

777

vector) within a single S2 surface and between successive surfaces.

779

9. Discussion

780 781

9.1. Problems associated with shear-related folds

EP

782

The origin and interpretation of folds in shear zones has been reviewed by

AC C

783

786

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778

785

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773

784

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770

Carreras et al. (2005) who divided them into three main types: 1) pre-existing folds modified by shearing; 2) folds formed during the shearing event; and 3) shear-related late folds. Neither the first nor the third categories are relevant

787

here and are not discussed further. Type 2 folds develop where a discrete shear

788

or shear zone reworks an earlier foliation, and it is increasingly clear that back-

789

rotation has an important part to play in the formation of folds resulting from

790

layer-parallel shear. Bell and Hobbs (2010, fig. 10a) figure trains of

32

33

ACCEPTED MANUSCRIPT asymmetric minor folds with which give the correct sense of shear. This is

792

possibly because the small folds to the right of the figure are arranged in

793

microlithon-like packets, and are back-folds, not drag folds. A possible

794

complication is that minor fold asymmetry in shear zones is reversed at high

795

shear strain, such as at the base of the Morcles Nappe in the European Alps,

796

where a complete reversal occurred at high strain (> 8) (Ramsay et al. 1983).

797

This has clearly not happened in the case of the Cowal microlithons, for the

798

anomalous vergence was already established at  = < 2.

SC

9.2. Back-folding

801 802

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799 800

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Early work by Soula and Bessiere (1980) figured a pair of folds in a shear band, based on an illustration by Ramberg (1975), in which they noted that

804

back-rotation had affected the middle limb of the structure. Hanmer and

805

Passchier (1991), following work by Hanmer (1986), recognized that antithetic

806

rotation of small domains can take place in shear zones, but concluded that this

807

was limited to a counter-clockwise rotation of < 10-15º.

EP

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809 810 811

Harris (2003) summarized the main characteristics of back-folds and

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808

recognized the following types: (1) Drag fold and shear folds have been traditionally considered reliable indicators of shear sense in shear and mylonite zones (Williams et al.,

812

1994). However, several authors have cautioned against the use of such

813

fold asymmetry as shear sense indicator, e.g. Ramsay and Huber (1983),

814

and it is now clear that the caution was merited because of the prevalence

815

of back-folding in such settings. For example, the pair of folds figured in

33

34

ACCEPTED MANUSCRIPT 816

the classic paper by Lister and Williams (1983, fig. 7b) and interpreted as

817

'drag-folds', display most of the features of the microlithon domains

818

described here (Fig. 12), suggesting that the shear sense given by this

819

structure is the opposite to that proposed by the authors. (2) Folds formed between pairs of ductile shear zones. This style of

821

deformation (Fig. 2c) has not been recognized in the Cowal area.

822

(3) Shearing of an inclined layer that dips in the same direction as the bulk

823

shear. This configuration is a special case and requires a considerable

824

shear displacement on sets of regularly spaced shear zones.

SC

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820

(4) Non-coaxial strain ( < 2) acting parallel to the pre-existing cleavage

826

produces folds in which the micro fold asymmetry is opposed to the

827

regional shear: the case which is analyzed in this paper.

828

Mandal et al. (2004) carried out a series of physical experiments to

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825

determine the conditions under which folds are able to nucleate and grow from

830

a surface that is parallel to the XY plane (i.e. Ramsay, 1980), and concluded

831

that some form of mechanical heterogeneity (ideally in the form of a weak

832

material layer) was necessary for the nucleation of folds to take place. Johnson

833

(1999) investigated the geometry and origin of back-folds associated with the

835 836

EP

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834

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829

development of the crenulation cleavage and concluded that the maximum angle of back-rotation is 90º, but several of the examples presented here show that this value is commonly exceeded. When the back-rotation exceeds 90º, the

837

limbs of crenulation folds start to rotate towards one another.

838

Rajlich (1993) envisaged a complex scenario in which Riedel shears

839

govern the development of the crenulation cleavage but it has been shown here

840

that these shears are a product of the process, and not the cause. For example,

34

35

ACCEPTED MANUSCRIPT 841

the formation of micro en-echelon sets of Riedel shears at the tips of

842

propagating D2 cleavage seams (Fig. 6a–e) is taken as evidence that layer-

843

parallel shearing had occurred.

844 9.3. A published mathematical model for shear band formation.

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845 846 847

Few mathematical modelling experiments have been designed specifically to study the initiation and growth of microlithons or shear bands. However,

849

those by Ord (1990) to examine the growth of shear bands resulting from non-

850

coaxial deformation acting in the plane of a finite difference grid, are an

851

exception, and are highly relevant to this study. Ord used different values for

852

volume change and mean normal stress in this model and compared the results

853

of one of these experiments with a field example of a spaced crenulation

854

cleavage from low-grade schists. She noted that the artificially-formed

855

'microlithons' corresponded to the more widely-spaced gridlines (A on Fig.

856

15e), and to the quartz-rich microlithons in the field example. The highest

857

values of normal stress occurred within the shear zones, and this would have

858

facilitated the movement of quartz-rich fluid out of the local system and into

860 861

M AN U

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859

SC

848

the more 'porous' adjoining microlithons where some, or all, of it could be deposited. The same model applies equally well to the D1 spaced cleavage in the Cowal metasasandstones as to the D2 spaced cleavage that reworks it. The

862

linear zones of closely spaced grid lines (B) equate with the dark cleavage

863

seams in natural examples. The surprising result was that at = 1 the deformed

864

grid replicated most of the features displayed by the naturally occurring spaced

865

cleavage in Cowal. These features include their asymmetry; distinctive nose

35

36

ACCEPTED MANUSCRIPT

and tail; inclined 'roof' to the microlithon (that makes a small forward-directed

867

angle () (Fig. 15f) with the shear zone boundary); and, of most importance,

868

back-rotation of planes in the microlithons. Ord's results also explain how the

869

D1 spaced cleavage could have formed from a non-foliated rock, with the D1

870

microlithons proxying subsequently for one of the sets of lines in the

871

experimental grid.

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866

The most significant result was that, at  =1, the bulk shear sense was in the

872

opposite direction to the micro fold asymmetry (Fig. 15a), so mimicking the

874

naturally-occurring shear bands whose interpretation has caused controversy

875

(Krabbendam and Leslie, 1996). This results demonstrates conclusively that it

876

is possible for such shear bands to form during a single continuous deformation

877

at low shear strain.

878

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SC

873

The theoretical controls on the spacing of the shear bands are not known (Ord, 1990), and further experimentation is required to determine the controls

880

on size, spacing and regularity of the shear bands.

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881

------------------FIGURE 16 ABOUT HERE ----------------------------

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884 885 886

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9.5. Strain partitioning and the formation of microlithon domains

Carreras et al. (2013) reviewed the role of strain partitioning in the

887

formation of shear bands and commented that the development of networks of

888

shear zones is a common occurrence, with low-strain domains co-existing with

889

domains of highly strained rock. They referred to the result of a specific

890

numerical experiment by Jessell et al. (2009), in which a shear lozenge that had

36

37

ACCEPTED MANUSCRIPT 891

formed by layer-parallel slip at  = 2, had a very similar profile and dichotomy

892

of vergence as the Cowal microlithons. Carreras et al. (2013) commented that

893

the structure was consistent with its initiation as a flexural flow structure, with

894

strain partitioning occurring antithetically to the bulk shear sense. Carreras et al. (2013, fig. 9a) figured an example of a microlithon fold-pair

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895

domain, developed in a mylonite zone, that reproduced the essential 2D

897

features of the model presented here. The fold pair lies within the shear zone

898

close to the centre of the line drawing in their figure 14. These authors

899

acknowledged the importance of back-rotation in the formation of such shear-

900

related folds.

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901

SC

896

The illustration, based on field observations, showing D2 microlithon domains wrapped by cleavage seams (Fig. 16a), is modified after the widely

903

quoted model of Bell (1981). Figure 16b shows one of the many variations of

904

this model in which the regional bulk strain has been partitioned into small,

905

regularly distributed domains, affected by low strain (or remaining unstrained

906

(Bell and Johnson, 1989)), wrapped by zones of intense non-coaxial strain.

907

Other examples of this phenomenon include a field sketch by Fusseis et al.

908

(2006) showing a metre-scale pattern of interconnected shear zones that

910 911

EP

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909

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902

resembles the domains from Cowal (Fig. 9a), as does a similar example from the Sma' Glen (Rose and Harris, 2000, fig. 8d). The Dalradian domains contain fold pairs, whereas that of Fusseis et al. follows the model of Bell (1981) in

912

containing a planar fabric.

913 914

10. Conclusions

915

37

38

ACCEPTED MANUSCRIPT 916

The spaced cleavages described in this paper occur in turbidite facies rocks belonging to the Southern Highland Group (Dalradian Supergroup) in SW

918

Scotland. These rocks are of late Neoproterozoic to early Ordovician age

919

(Tanner and Sutherland, 2007), and during the ~ 470 Ma Grampian Orogeny

920

were affected by four main phases of deformation (D1–D4) and by greenschist

921

facies regional metamorphism.

922

RI PT

917

The Tay Nappe is a regional-scale structure, which originated as a plexus of major upright D1 folds that were rotated to become near horizontal during the

924

progressive D1/D2 deformation. The resulting D2 spaced crenulation cleavage

925

(S2) is restricted to the lower, inverted limb of the nappe, and formed because

926

of regional shear acting approximately parallel to the plane of the flat-lying D1

927

spaced cleavage. The model presented here for the development of S2 is based

928

largely on field observations, supplemented by published analogues and

929

mathematical modelling.

TE D

M AN U

SC

923

930

The D2 microlithons have a number of features that make them easy to identify, namely: a distinctive asymmetry; spindle–to–slab-shaped form; well-

932

defined nose and tail; and internally, planar to sigmoidally folded D1 spaced

933

cleavage. Solution transfer played a major role in the formation of both sets of

935 936

AC C

934

EP

931

spaced cleavages (S1 and S2). The metamorphic differentiation that occurred during D1 resulted in couplets that consist of narrow, sharply-defined, quartzrich microlithons, with thinner, dark, cleavage seams. This combination of

937

quartzose ribs and mechanically weaker, more easily eroded cleavage seams,

938

have proven invaluable in the analysis of the D2 fold-pair microlithon domains.

939

During this process, D1 microlithons were sequentially back-rotated about a

940

newly-identified 'orthogonal tipping point'. A similar mechanism is thought to

38

39

ACCEPTED MANUSCRIPT

have operated in fault and shear zone lozenges from elsewhere, which show an

942

internal morphology similar to that found in the D2 spaced microlithonss but the

943

closest analogy is with flexural-slip duplexes. The involvement of layer-

944

parallel slip in the formation of a crenulation cleavage represents a considerable

945

departure from the accepted model whereby the crenulation cleavage results

946

from shortening parallel to the plane of the pre-existing anisotropy.

RI PT

941

At the heart of this model is the recognition that the D2 spaced cleavage

948

comprises linked arrays of microlithon domains. These domains, which have

949

only been interpreted previously from their 2D transverse profiles, consist of

950

pairs of cm–dm-scale D2 minor folds wrapped by an anastomosing composite

951

D1/D2 cleavage, and are the basic building blocks for the D2 spaced cleavage.

M AN U

SC

947

952

An apparent conflict between opposing directions of microlithon asymmetry and bulk shear (Fig. 3) is shown to be a natural consequence arising from the

954

formation of the D2 microlithons. Although counter-intuitive, this mechanism

955

is considered to be the 'normal' relationship in shear zones, rather than the

956

exception.

This study demonstrates conclusively that the sense of shear during the D2

EP

957

960 961

deformation in the SW Highlands was top-to-the-E, as determined by Harris et

AC C

958 959

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953

al. (1976), who were not misled by the W-directed asymmetry of the minor folds in the microlithons, and correctly relied upon general shear sense criteria to reach that conclusion. The next question to be decided is whether the Tay

962

Nappe was emplaced parallel to the NW-SE D1 stretching lineation or E-W as

963

given by the D2 microlithons. At the current level of exposure the latter is

964

generally seen as a weak deformation ( <2) but could develop into a major

965

shear zone at depth. This problem is discussed further elsewhere. Having

39

40

ACCEPTED MANUSCRIPT resolved the apparent conflict between the W-directed asymmetry shown by

967

minor folds contained within the D2 microlithons, and the E-directed shear

968

sense shown by the D2 cleavage seams. It also solves the puzzle of the origin

969

of the dark seams running parallel to the limbs of the micro folds, as identified

970

by Turner and Weiss (1963) (see section 1.1) and explained in Figure 16c.

971

The next step is to examine in detail the relationship between these

972

structures and the stretching lineation. The results presented here should

973

remove any doubts as to the direction and sense of the D2 component of the

974

emplacement of the Tay Nappe, and possibly provide a template that may be

975

used as a tool in the kinematic modelling of other orogenic belts during the

976

main ductile deformation.

977 978

Acknowlegements

I thank the Journal referees Graham Leslie and Jordi Carreras for detailed scrutiny

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979

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966

of the paper, and suggestions for improving the presentation of the data; Bill

981

Henderson for helpful discussion; Mike Shand for help and advice with the

982

cartography; and Judith Tanner for her generous and unwavering support.

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984 985 986

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Tanner, P.W.G., Thomas, C.W., Harris, A.L., Gould, D., Harte, B., Treagus, J.E.,

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Stephenson, D., 2013. The Dalradian rocks of the Highland Border region of

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Scotland. Proceedings of the Geologists' Association 124, 215–262.

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Tanner, P.W.G., 2013. 3. Cove to Kilcreggan. In: Tanner, P.W.G., Thomas, C.W.,

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Harris, A.L., Gould, D., Harte, B., Treagus, J.E., Stephenson, D., 2013. The

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Dalradian rocks of the Highland Border region of Scotland. Proceedings of the Geologists' Association 124, 215–262.

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Thomas, C.W., 2013. 5. Bealach nam Bo and 6. Duke's Pass. In: Tanner, P.W.G.,

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Thomas, C.W., Harris, A.L., Gould, D., Harte, B., Treagus, J.E., Stephenson, D.,

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2013. The Dalradian rocks of the Highland Border region of Scotland. Proceedings

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of the Geologists' Association 124, 233-240.

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Treagus, J.E., Treagus, S.H., Woodcock, N.H., 2013. Discussion on: 'The structural

1210

interpretation of domainal trace lineation: an example from the Mona Complex,

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Anglesey'. Journal of the Geological Society, London 170, 627–630.

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Turner, F.J., Weiss, L.E., 1963. Structural analysis of metamorphic tectonics.

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McGraw-Hill, USA, 545pp.

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Voll, G., 1960. New work on petrofabrics. Liverpool and Manchester Geological

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Journal 2, 503-567.

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Weber, K., 1981. Kinematic and metamorphic aspects of cleavage formation in very

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low-grade metamorphic slates. Tectonophysics 78, 291-306.

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Williams, P.F., 1972. Development of metamorphic layering and cleavage in low

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grade metamorphic rocks at Bermagui, Australia. American Journal of Science 272,

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147-271.

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Williams, P.F., Goodwin, L.B., Ralser, S., 1994. Ductile deformation processes. In: Hancock, P.L. (Ed.), Continental deformation. Pergamon Press, 1-27.

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Figure Captions

1234 1235

Fig.1. Location of the study area (boxed) in the Grampian

1236

Terrane of Scotland. D, Dunkeld/Birnam; G, Glen Shee; S, Sma' Glen.

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1237

Fig. 2. This Figure shows the main types of crenulation cleavage that form from

1239

an early penetrative cleavage (D1: green double arrow) being partially reworked by

1240

a spaced cleavage (D2: red double arrow). D1 and D2 each refer to local structural

1241

episodes. (a), (c)-(e) are from the Liskeard area, East Cornwall; (f) is from the

1242

Rhoscolyn Anticline, Holy Island, Wales; (b), (g) and (h) are from the Cowal

1243

Peninsula (this paper). Note: the scale bars represent lengths from 200 m to 10

1244

cm.

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(a) Thin section showing a D1 penetrative cleavage. The large white areas

1246

(calcite) consist of tentaculites pseudomorphed by calcite that has been

1247

subsequently corroded by solution transfer, resulting in rectilinear margins to the

1248

clasts. (b) D1 spaced cleavage (upper half of picture) orthogonal to bedding. The

1249

scale is 7 cm long. (c) Zonal spaced crenulation cleavage reworking a penetrative

1250

D1 cleavage and bedding. (d) Discrete D2 cleavage (irregular black seams) cutting

1252 1253

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1251

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1245

across a strongly developed D1 cleavage.

(e) D2 spaced cleavage with

microlithons comprising the folded early fabric, associated with dark D2 seams, and with some migration of calcite into the microlithons. (f) D2 zonal spaced

1254

crenulation cleavage, in which quartz has migrated into the hinge areas of

1255

individual crenulation folds.

1256

development of a D2 spaced fabric comprising quartz-rich microlithons separated

1257

by cleavage seams. (g) D2 spaced crenulation cleavage consisting of prominent

51

This fabric represents an early stage in the

52

ACCEPTED MANUSCRIPT 1258

pale microlithons preserving traces of the early cleavage, and dark cleavage seams

1259

containing chlorite, white mica and opaque minerals. Note the inferred 'similar'

1260

style of the folding. The two green lines indicate how the individual traces may be

1261

related.

1262

preserving traces of the D1 fabric.

1263

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(h) D2 spaced cleavage showing pale-coloured lensoid microlithons

Fig. 3. A cartoon that highlights the differences between the ASYMMETRY of

1265

minor folds in shear zones, mylonite, and microlithons in a spaced cleavage, and

1266

VERGENCE in folded metasediments. In both cases the minor folds are viewed

1267

down-plunge.

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1268

Fig. 4. Outline structural map (a) and true scale N-S cross-section (b) of the study

1270

area (see Fig. 1) in the SW Highlands of Scotland. (a) 1–3, localities mentioned in

1271

the text. The structural symbols contained in boxes 2 and 3 are in their correct

1272

orientations and show the computed means for S1, S2, L2, and the stretching

1273

lineation. Dunkeld is located 56 km NNE along strike from Callander. (b)

1274

Computed apparent dips for S1 (red) and S2 (blue), are shown by thin coloured

1275

lines in the cross section. Labels 13 refer to the approximate locations of sites

1277 1278

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1269

shown in (a).

Fig. 5. Anastomosing D1 spaced cleavage seams from the Cowal Peninsula as

1279

seen in the field ((a) and (b)), and in thin section ((c) and (d)). (d) shows a folded

1280

quartz vein that has been corroded and reconstituted by solution transfer (see text).

1281

It occurs in metamudstone with a planar slaty cleavage. Camsail Bay, Rosneath

1282

[NS 262 822].

52

53

ACCEPTED MANUSCRIPT 1283

Fig. 6. Morphology of the D2 spaced cleavage in the Cowal area. Example (a)

1285

shows a finely spaced D1 cleavage, affected by a flat-lying D2 spaced cleavage. In

1286

(b) – (e), widely spaced D2 cleavage seams cut and displace the earlier spaced

1287

cleavage. The annotated photomicrographs (c) and (e) are copies of parts of (b)

1288

and (d) that show en-echelon Riedel shears developed adjacent to the tips of

1289

propagating D2 shears. (f). An example of the contrast between D2 microlithons

1290

with thick internal D1 microlithons that make a high angle with their margins, and

1291

adjacent areas in which the D1 microlithons have been strongly rotated and

1292

sheared (see text for discussion).

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Fig. 7. Morphological features of the D2 spaced crenulation cleavage in a packet

1295

bounded by metasandstone beds X and Y (see text). (a) D1 microlithons (A)

1296

paired with narrower cleavage seams, deformed during the formation of the D2

1297

spaced crenulation cleavage.

1298

dipping at <25º S that carry L2, a ribbed intersection lineation (C). The D2

1299

microlithons consist of sigmoidally folded D1 microlithons, (i.e. ringed in white),

1300

with Z-shaped patterns indicating W-directed asymmetry. (b) An enlarged view

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S2 has weathered out as flat-lying surfaces (B)

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of the boxed area in (a). It shows spindle-shaped D2 microlithons (i.e. DE), bounded by S2 cleavage seams (thick white lines), on which the structural base of metasandstone bed Y (thin white line) is displaced repeatedly by top-to-the-E

1304

shear. In order to show the effect of D2, a single S1 trace is highlighted in red

1305

across the exposure.

1306

53

54

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Fig. 8. Evidence for top-to-the-E displacement on D2 cleavage seams as discussed

1308

in text. (a) shows the relationship between the D2 spaced crenulation cleavage and

1309

the laminated D1 spaced cleavage. (b) which covers most of (a) (see box), shows

1310

a thin quartz vein (Q-Q) displaced by top-to-the-E shear (see text) and cut by a late

1311

quartz vein (V). L2 is at a high angle to the rock face (which dips at a moderate

1312

angle towards the observer) in (a) and (b), and several oblique sections through

1313

microlithon domains are shown in grey (in (b)), and labelled 1–4 (see text). (c)

1314

shows D2 microlithon domains, some of which are spindle-shaped. One example

1315

of the latter is ringed and labelled in white (1). The exposure carries a strong

1316

ribbed L2 lineation. A quartz vein (arrowed) that runs diagonally across the figure

1317

has been repeatedly displaced top-to-the-E on D2 cleavage seams. One segment

1318

(ringed in white, 2) has been drawn out into minute boudins (see text).

1319

Small mm-scale white spots in this and other illustrations are juvenile limpets (the

1320

adults are up to 2cm across) and are not of geological origin.

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1321

Fig. 9. (a) A lateral section through a nested group of four microlithon fold-pair

1323

domains, mantled by D2 cleavage seams. A–D, see text. (b) Two D2 microlithon

1324

fold pair domains in which the D1 microlithons form narrow protruding ribs on the

1326 1327

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surface, separated by hollowed-out cleavage seams. In the upper part of this figure, a single D1 microlithon was selected (in red) to outline the structure of this domain. The sausage-shaped structure in the lower third of the picture is a

1328

longitudinal section through a domain within which there are a few widely spaced

1329

internal D1 microlithons (in red).

1330

orthogonal tipping point, and their wide spacing is due to the 'cut-effect'. Marine

54

They have been back-rotated about the

55

ACCEPTED MANUSCRIPT 1331

erosion has stripped away the carapace of highly sheared rock that enclosed the

1332

domain, to reveal the internal geometry.

1333 Fig. 10. 3D views of the interiors of both isolated and stacked D2 microlithons. (a)

1335

Included here is a complete microlithon fold-pair (A). (b) A platy flat-lying D1

1336

microlithon fabric affected by two pairs of partially developed D2 microlithon

1337

folds.

1338

microlithon (B) showing how the D1 microlithons have been rotated about the

1339

orthogonal tipping point (open circle). The scale in each case is 5 cm long.

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(c) Side view of

1340

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The arrow indicates the direction of view for (c).

Fig. 11. Two examples of complete D2 microlithon domains, exposed on an

1342

irregular flat-lying surface, viewed as near as possible normal to their profile

1343

planes. The structures are outlined by complete sets of D1 microlithon traces (in

1344

red), and fault traces (in green).

1345

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Fig. 12. A block diagram showing the main features of the D2 microlithon fold-

1347

pair domains.

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1346

1349 1350 1351

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Fig. 13. A sketch cross-section showing the possible means by which the major D1 folds were rotated to become nearly horizontal. Based on Tanner (2013, fig. 16b).

1352 1353

Fig. 14. Simplified stages 1-3 in the development of the two contrasting models

1354

for the formation of a crenulation cleavage. Model A is that presented in this

1355

paper and Model B is based on Ham and Bell (2004, fig. 1).

55

56

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Fig. 15. Minor structures similar in morphology and origin to the microlithon

1358

fold-pair domains from the Scottish Dalradian. In all cases, the bulk shear sense

1359

is sinistral. (a) A microlithon domain from Dunkeld redrawn (and flipped

1360

horizontally) from Harris et al., (1976, fig. 3a). (b) A contractional understep-type

1361

shear zone lozenge redrawn from Ponce et al., (2013, fig. 8b; flipped

1362

horizontally). (c) An early stage in the development of a D2 microlithon domain,

1363

showing back-folding of the prominent D1 microlithons. Cowal Peninsula. (d) A

1364

line drawing showing the main features of a flexural-slip duplex from the Culm

1365

Basin, N Devon (redrawn from Tanner, 1989, fig. 3). (e) The result obtained from

1366

mathematical modelling of non-coaxial strain acting parallel to the undeformed

1367

planar surface at  = 1. Redrawn from Ord (1990, part of fig. 2d, (rotated 90º anti-

1368

clockwise)). (f) An idealized sketch of a typical D2 microlithon domain from the

1369

Cowal Peninsula (this paper).

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1370

Fig. 16. a) A schematic drawing showing the microlithon fold-pair domains

1372

described in this paper wrapped by D2 cleavage seams. Folded D1 microlithons,

1373

one of which is shown in red, have an asymmetry opposite to that shown by the

1375 1376

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1371

cleavage seams. b)

Bell and Johnson's (1989) modification of Bell's (1981)

model, in which the bulk strain is partitioned into low-strain domains (represented by elliptical shapes aligned in the XY plane), enclosed by high strain zones. c)

1377

Formation of dark seams (in red) by late-stage shearing and solution transfer in the

1378

limbs of D2 micro folds (after Ham and Bell, 2004, fig. 1). Note that the sense of

1379

shear in the high strain zone (1) is opposite in direction to the asymmetry of

1380

related small D2 folds (2).

56

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M GH oine LA T ND hrus t TE RR AN E

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Post-Cambrian rocks not shown separately

Fa Gl en

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FIG. 1

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STUDY AREA

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SHEAR FOLD(?)

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δ-PORPHYROCLAST

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LEFT-HANDED ASYMMETRY

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minor fold

ASYMMETRY

RIGHT-HANDED VERGENCE

S h e a r Z o n e

microlithon domain RIGHT-HANDED ASYMMETRY

S2

VERGENCE M e t a s e d i m e n t s

LEFT-HANDED (LH) or NW-DIRECTED VERGENCE

3 Glecknabae

Katrine

39 35

44

Stretching lineation

lltt Be d lan at l h F g Hi

L2 Fault Highland Boundary Fault

l wa w o o CC

rd Bo

2

p ee t l BeSCowal r Cowal

Isle of Bute

25 27

25 18

EP

A

10 km

Upper

GLASGOW

Ophiolite

Dalradian Supergroup Southern Highland Group & Trossachs Group

Upper limb of Tay Nappe

o 2n rds fD a o ngthw mni or o Inf cD 2 go

pemin p a co 2

Ta

Fault Old Red Sandstone & younger rocks

2 Cove

Unconformity

Limb

1

Azimuth of mean D1 stretching direction

Highland Border Ophiolite

AC C

HBF

Est uar y

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Bute, West Coast

A

Camsail Bay

30

Toward

S

Loch Lomond

Rosneath Peninsula HBF Innellan

3

200

Aberfoyle

Balmaha

1 Helensburgh

Dunoon

HBF

A

pe p Na

Clyd e

d an Glecknabae l h

Isle of Arran

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d en b wn o D

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pe eed t o SB

B

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S1 cleavage S2 cleavage

Keltie Water Callander

or

if nt

48

Leny Quarry

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a

HBF

Loch Tay Fault

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200

yN

b

In

Lower Dunoon Phyllite Formation ( not shown on the map above )

w rth

SE limit of D2 reworking on lower limb of Tay Nappe

ard

s

N B 3

Limb 2 km

Bedding trace S1 trace S2 trace Fault Younging direction Lithological boundary

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SE Tay Nappe

Single progre ssive d efor ma D tio 1 n

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D 2 Superimposed

2

A

B

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(this paper)

(modified from Ham and Bell (2004)

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BACK-ROTATION

1

2

1 BUCKLING-zonal

crenulation cleavage

2 MAJOR FOLD-

LIMB ROTATION asymmetric crenulation cleavage

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LAYERPARALLEL SLIP

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BACK-ROTATION ABOUT THE ORTHOGONAL TIPPING POINT ( )

3

STRAIN PARTITION AND FORMATION OF A FOLDPAIR DOMAIN

BULK SHEAR COMPONENT

3 SHORTENING accentuated asymmetry

D2 cleavage seam

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b a

D1 cleavage seam D1 microlithon

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2 cm

nose

High D2 strain

d

Back-rotation

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1-50 cm

Ba

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-ro

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in

ta

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Be

Horse

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&

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us

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α Roof

B

f

D2 cleavage seam

Tail

c

Model B

Model A

a

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a

Strain Strain Partition modelsMICROLITHON

δ-PORPHYROCLAST 1

2

VERGENCE

Dark cleavage seam

on n ith ai ol om r ic d

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c

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b

m

S h e a r Z o n e

SHEAR FOLD(?)

S2

M e t a s e d i m e n t s

VERGENCE

BULK SHEAR SENSE

NORMAL VERGENCE

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Ms. Ref. No.: SG-D-15-00110 Bullet points for Tanner

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1) Unique 3-D analysis of shear band morphology in greenschist facies Dalradian rocks [82] 2) Strain partition results in microlithon fold-pair domains enveloped by D2 cleavage seams [88]

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3) Layer-parallel slip and back-rotation of D1 spaced cleavage generate D2 microlithons [85] 4) Analogous structures include fault and shear zone lozenges and flexural-slip duplexes [85]

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5) Discussion of kinematic interpretation of minor fold asymmetry and vergence [76]