Jack E. Treagus

Studies of strain and rheology of conglomerates

Abstract Conglomerates can show significant strain variations among their component rock types. Previous studies have exploited this feature as a method of quantifying the effective viscosity ratios among different rock types, using inclusion–matrix models that we review here. We further investigate the rheology of conglomerates via models of multiphase mixtures of spherical clasts with varying phase fractions. Using idealised models with two to four phases, we can quantify the influence of phase viscosities and volume fractions on the bulk viscosity, and reveal how this controls the strain partitioning among the different phases. We consider practical methods of analysing conglomerates, and choose R f - φ analysis for measuring clast strain, and Fry analysis for whole-rock strain. The methods are illustrated through case studies of two very different conglomerates, the Port Askaig Tillite, Scotland, and Cesson Conglomerate, Brittany. We define the important rock phases in each, and assess their strain variations. Although providing strain measurements of regional interest, our main purpose is to quantify the viscosity ratios among the component rocks. We find that among the most common rock types (semipelite, quartzite, psammite, volcanics, granite), the viscosity ratios span about one order of magnitude, agreeing with previous studies. These small numerical values for viscosity ratios among many rock types, in many conglomerates, lead us to conclude that these rocks each deformed at approximately constant viscosity: i.e. as linearly viscous, rather than according to a power law.

A structural cross-section of the Moine and Dalradian rocks of the Kinlochleven area, Scotland

The structure of the Dalradian rocks at the eastern end of Loch Leven has been re-examined. The geometry of the major folds as determined by Bailey (1934) is largely confirmed but a reappraisal of their relative ages, based on the evidence of minor structures, leads to a new interpretation of the overall structure. The new interpretation is extended into the Moine in the Blackwater area. The dominant structural element is the Di Kinlochleven Anticline that faces down to the NW in the west of the area. To the east the lower, inverted, limb of this fold has been deformed by three D3 folds which restore it to an upward-facing attitude at the Moine-Dalradian boundary at Blackwater. A major deflection in strike of the earlier structures is attributed to the D4 Loch Leven Antiform. The relation of these structures to the regional structure is briefly considered.

Transected folds and transpression: how are they associated?

Abstract Folds transected by cleavage are a distinctive structural feature in the Caledonian-Appalachian orogenic belt. With the advent of a tectonic model of transpression for this belt, there has been an assumption that transected folds are a symptom of transpressive deformation: in particular that clockwise transection signifies sinistral transpression. We examine this tenet, and conclude that there is no unique explanation for folds transected by a synchronous cleavage. Explanations can be found for either clockwise or anticlockwise transection, for sinistral (or dextral) transpression, each requiring assumptions about the inclination of bedding to the transpression vector, or requiring that cleavage does not track the finite XY plane. A further explanation, which may fit the regional data best, requires a non-constant transpressive deformation, with initial pure-shear-dominated deformation giving way to increasing transcurrent movement in time. None of these models can be assumed to be right, without supporting data on the incremental and finite strain histories of such regions.

Effects of object ellipticity on strain, and implications for clast-matrix rocks

We examine the influence of object shape on strain, taking the example of an elliptical object in a matrix of different viscosity. The Eshelby–Bilby equation for the cross-sectional strain of circular cylindrical objects in a matrix of different viscosity is differentiated, to express the relationship of developing sectional ellipticity on incremental strain. This leads to an expression for the strain of initial elliptical objects whose axes are parallel or perpendicular to pure shearing. Graphs show that competent elliptical objects with axial ratios of Ri=3 or more, will strain significantly more than circular objects of the same viscosity; less if the objects are incompetent. While the effect is likely to be insignificant for competent objects with initial ellipticity of <2, which is indicated from statistics for undeformed pebbles in conglomerates, any clast with greater ellipticity, such as Ri=5 to 10, could deform significantly more than an equant clast, and thus appear to be materially less competent. These principles have implications for geological strain studies and for competence contrasts in rocks.

Movement and mineralization in the Tyndrum Fault Zone, Scotland and its regional significance

Major and minor fracture analysis of the Tyndrum Fault Zone, Scotland, reveals a late Silurian history of transtensional deformation with opening across the zone as well as left-lateral strike-slip movements. The extensional phases are characterized by hydrothermal quartz veins and breccias associated with the early stages of precious-metal mineralization. The strike-slip movements are characterized by cataclastic textures and are associated with the later stages of the precious-metal mineralization. Further transtensional deformation in the Carboniferous is indicated by right-lateral strike-slip movements, associated with fractures throughout the zone containing both cataclastic and extensional hydrothermal quartz veins: the quartz veins are associated with base-metal mineralization. The pre-Devonian, WSW–ENE-directed, transtensional deformation of the zone is extrapolated to the whole Dalradian Terrane, the driving force being the gravitational collapse of the orogenic welt parallel to the tectonic trend. The necessary area increase is signified by the intrusion of the end-Caledonian granitic magmas and quartz-veins. The Carboniferous right-lateral movements resulting from N-S extension are related to similar movements, also transtensional, on the Great Glen Fault Zone and in the Midland Valley; the associated mineralization is related to a broader Dinantian base-metal mineralizing event.

A structural reinterpretation of the Tummel Belt and a transpressional model for evolution of the Tay Nappe in the Central Highlands of Scotland

Structural mapping from the Flat Belt into the Tummel Steep Belt of the Dalradian Supergroup of the Central Highlands provides evidence for a new interpretation of the relationship between the two belts and their evolution. The open upright D 3 major folds of the Flat Belt intensify into the Steep Belt and are responsible for the steeper dips. These D 3 folds refold four newly recognized D 2 major folds. It is the recognition of the interference of these two sets of folds that leads to the radically revised structural interpretation presented here. A reconstruction of the pre-D 3 structural history shows that the stack of major D 2 folds, on the lower limb of the Tay Nappe, originally verged to the northwest, as did three D 1 fold-pairs. A model is proposed for the D 2 deformation in a transpressional setting, simplified as partitioned into two sub-horizontal zones. The lower zone is represented by the pure-shear-dominated Tummel Belt with extension parallel to the regional orogenic trend; the upper zone is represented by the simple-shear-dominated Flat Belt where extension is perpendicular to that trend. The curvature of the minor D 2 fold hinges supports a more refined model of smoothly continuous partitioning of transpressive deformation between the basement and high-level, southeast-propagating nappes above the Flat Belt.

Superposed deformations and their hybrid effects: the Rhoscolyn Anticline unravelled

This study of the controversial structures of the Rhoscolyn Anticline suggests a different result of two-phase coaxial deformation from Ramsays Type 3 interference fold patterns. From detailed field observations of the sequence of bedded quartzites, psammites, pelites and oblique quartz veins, with their strong competence contrasts, we conclude that the Rhoscolyn Anticline was an original tight, upright F1 anticline that has undergone modification and distortion in a second deformation (D2). This second deformation is an oblique, but near-vertical, pure shear, with a quantifiable strain ratio (R=3) that altered the Rhoscolyn Anticline and its minor structures into a more open, SE-overturned antiform, with c. 260 m hinge migration. Refolded folds are rare, but hybrid F1 + F2 minor folds and their fabrics, especially in the region between old and new hinges, provide clues to the two-stage history. Oblique distortion of originally NW-verging F1 minor folds has resulted in their apparent neutral vergence in the present-day hinge of the Rhoscolyn Anticline. We regard the structures and fabrics in quartzites and psammites as more reliable indicators of the regions deformation history than those in pelites or quartz veins, and this may prove true for other regions of polyphase deformation.

The significance of the boundary between the Rhoscolyn and New Harbour formations on Holy Island, North Wales, to the deformation history of Anglesey

The boundary between the Rhoscolyn and New Harbour formations on Holy Island, Anglesey, has been described as a high strain zone or as a thrust. The boundary is here described at four localities, with reference to the contrasting sedimentary and deformational character of the two formations. At one of these localities, Borth Wen, sandstones and conglomerates at the top of the Rhoscolyn Formation are followed, without any break, by tuffs and then mudstones of the New Harbour Formation. It is concluded that there is clear evidence of depositional continuity across the boundary here, and that both formations subsequently shared a common two-phase deformation. The first (D1) was manifestly different in intensity and scale in the two formations, whereas the second (D2) produced very similar structures in both. The other three localities provide continuity of sedimentary and tectonic features at this boundary in a traverse along the length of Holy Island, leading us to identify two previously unrecognized major D1 folds in addition to the Rhoscolyn Anticline. At one of these localities (Holyhead), we confirm the presence of Skolithos just below the boundary, supporting radiometric evidence for a lower Cambrian or later age for the Rhoscolyn Formation. A turbidite interpretation for both the Rhoscolyn and New Harbour formations best fits the available evidence. A deep-water depositional environment is still compatible with the sporadic presence of Skolithos burrows, but less so with reported observations of hummocky and swaley cross-stratification lower down the South Stack Group.

Major folds affecting the Lower Old Red Sandstone Group at Lligwy, Anglesey, North Wales, and their regional significance

The Old Red Sandstone on Anglesey, North Wales, presumed Lower Devonian in age, is folded and locally cleaved, but the intensity of this deformation has previously been understated. We describe two S-verging anticline–syncline pairs, one with a strongly overturned middle limb, their associated minor folds and an axial-planar cleavage. The intensity of the deformation calls into question a proposed link to Variscan fault displacements, and the angular unconformity below the Old Red Sandstone precludes the deformation being part of a continuous ‘late Caledonian’ phase. We consider this deformation of the Old Red Sandstone to be mid-Devonian, correlating with the Acadian phase in mainland Wales. It is predated by a Silurian shortening deformation on Anglesey that is possibly related to the closure of the Iapetus Ocean, absent in mainland Wales and the Lake District, but perhaps preserved also on the Isle of Man.

Discussion on ‘The structural interpretation of domainal trace lineation: an example from the Mona Complex, Anglesey’: Journal, Vol. 170, 2013, pp. 627–630

In a recent short note, Lisle (2013) uses an example from the New Harbour Group of Anglesey, UK, to illustrate a type of structural lineation he terms ‘domainal trace lineation’. This is a form of intersection lineation manifested in fine banding, considered to be the intersection of one domainal foliation (e.g. crenulation cleavage) on another domainal foliation. If Lisle’s interpretation is correct, it implies that the New Harbour Group has at least four phases of deformation, compared with the two that are recognized in the underlying South Stack Group at Rhoscolyn (e.g. Shackleton 1969; Treagus et al . 2003). Furthermore, if the pervasive lineation in the New Harbour Group is considered to be an intersection lineation, this has significant consequences for the regional tectonics. As noted by Lisle, the much-described lineation in the New Harbour Group has been described as a ‘stretching lineation’ by Shackleton (1969), Powell (1979) and Roper (1992), and a ‘quartz stretching lineation’ by Phillips (1991). In a paper overlapping Lisle’s, we describe it as a ‘quartz elongation lineation’ (L1) that is folded with the S1 foliation (Treagus et al . 2013). Lisle notes that only Hudson & Stowell (1997) considered the prominent lineation in the New Harbour Group to be an ‘intersection lineation’. Nevertheless, on the basis of one locality, …