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Strain Study of the Caledonides in the Islay

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Strain Study of the Caledonides in the Islay Jf geol. Soc. Land. Vol. 136, 1979 pp. 77-88, 9 figs., 1 table. Printed in Northern Ireland. Strainstudy of theCaledonides in the Islay region, SW Scotland: implications for strainhistories and deformation mechanisms in greenschists G. J. Borradaile SUMMARY: Metasediments of the NW foreland have a deformation phase older than the first phase in metasediments of the internal part of the Caledonides to the SE. They also suffered constrictive deformation with NE-SW extensionof more than 100°/~.In contrast, rocks in the adjacent internal zone were extended 70* 10% upwards in a NW direction and also thrust in the same direction; they were extended less than 10% along strike. An important concealed boundary separates the foreland from the internal zone. Total strains determined for foreland metasediments (Colonsay Group) and for metasedimentsof the internal zone (Dalra- dian Supergroup) allow a partial pretectonic reconstruction. Total cross-strike shortening for both zones averages55 *S%. It is not possible completelyto unfold the profile through Islay by removing the measured strains. This is partly because those strain analyses based on grain shapesfail to detectthe important component of intergranularslip and partly because mechanisms not involving penetrative rock deformation helped produce the folds. Strain histories for the Dalradian rocks are generally coaxial the within limits of a test applied so that the primary cleavageof the Dalradian rocks and its extension lineation may be mapped as total strain trajectories. The fan of primary cleavage and examples of extreme non-coaxial strain near the hinge of the Islay Anticline may be associated with hinge migration during its growth. Severallines of evidencefavour grain boundary sliding as an important deformation mechanism in these low grade metasediments. It may have been enhanced by elevated pore fluid pressures and three classes of -erain boundary sliding are proposed, dependent on the prevailing fluid pressure. This isa study of a cross-section throughthe NW bols X, Y and Z are also used to refer to the orienta- margin of the Caledonianmobile belt in the SW tions of those principal strains. Scottish Highlands. The sectioncomprises deformed The shape of the strainellipsoid is expressed numer- greenschist-facies metasediments of the Dalradian ically by k (= (a- l)/(b - 1)) and referred to as flat- Supergroup and of the foreland, where remnants of tened (k < 1) or constricted (k> 1). The shape of the basement rocks crop out. Attention will be drawn to fabric ellipsoid-pebble or grain shapes-is described features of general structural relevance which are par- loosely with the L-S nomenclature of Flinn (1965a). ticularly well illustrated in this area. These include the The deformation phases in the region are continu- nature of natural strain histories; relationship of cleav- ous (Ramsay 1969), i.e., they do not involve much age to the principal strain directions; the inadequacy faulting, and justify the extrapolation of total strains of strain analyses based on grain shapes;and the determined for outcrops to bulk strains on the scale of importance of grain boundary sliding in rock deforma- many outcrops. Homogeneity of total strain is recog- tion. nised whereboth bedding and cleavage are planar. This is generally the case on the scale of many out- crops. Definitions and terminology Facing, or more specifically structural facing, is used The strain of rocks or grains determined from field strictly in the sense intended by Shackleton (1958): it data is termed total strain. This is preferred to ‘finite’ is the component of the younging direction projected strain since increments of strain, or components attri- on to a (axial planej cleavage. It is not a synonym for buted to different deformation episodes are in them- the local younging or way up of strata. selves finite (Means 1976). All strain analyses yielded In the interestsof precision, the author has followed the principal strain ratios a ( = X/y) and b ( = Y/Z), Harland (1969) in specifying deformation events both where X > Y 3 2 are thelengths of the principal radii temporally and spatially. Thusthe D1 eventin the of the strain ellipsoid. Where helpful, the author has Dalradian rocks is suffixed with ‘d’ to denote therock calculated X, Y and Z (or the percentage extensions sequence to which it refers-D,,. Appropriately, first they represent) from a and b assuming constant vol- cleavage and first folds are designated S,, and F,+ The ume strain (Flinn 1962). Standard errors of the mean suffix‘c’ is used in connection with the Colonsay strain values are quoted orindicated graphically. Sym- Group rocks. 0016-7649/79/0100-0077$02.0~~@ 1979 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/1/77/4886137/gsjgs.136.1.0077.pdf by guest on 01 October 2021 78 G. J. Bor radaile Regional setting and Islay Anticline (Fig. 1). The coeval cleavage is usually penetrative,approximately axial planarto The Caledonian metamorphic mobile belt is rather folds of bedding andhas been traced regionally as narrow in the SW Highlands. Its NW margin on Islay shown.It has afanning pattern; vertical on the W brings metasediments of the internal part in juxtaposi- flank of the LochAwe Syncline but progressively tion with foreland metasediments and their underlying more gently dipping across strike to the NW and SE. tectonisedbasement. Metasediments of theinternal The stretching or extension lineation on S,, also shows zone comprise various low-grade rocks belonging to a gradual spatial variation, plunging NW in the north the Bowmore Group and the late Precambrian partof of the area and plunging SE in the south. the Dalradian Supergroup (Harris & Pitcher 1975). The Bowmore Group metasediments lie to the NW The Dalradian stratigraphy is disposed about three and beneath the Dalradianrocks, separated from them major F,,, folds: the Tay Nappe, Loch Awe Syncline by the Loch Skerrols Thrust. Nevertheless they have D4LRADlAN wlthSld cleavage trace. Arrows lndlcate mlneral or shape Lineation azimuth. GROUP ... LEWlSlAN GNEISS 15 km FIG. 1. Map of the primary fabric elements of the Dalradian and Colonsay Group rocks of the SW Scottish Highlands. Corrections have been made for the displacements on the two major, late tear faults. The S,, cleavage and its extension lineation approximately correspond to the XY plane and X-direction of the total strain ellipsoid in most localities. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/1/77/4886137/gsjgs.136.1.0077.pdf by guest on 01 October 2021 Strain study of the Islay Caledonides 79 TABLE1 C olonsay Group Dalradian SupergroupDalradianGroup Colonsay (h Bowmore Group) D3, rnrnor close folds, gentle NEplunge minor close folds, gentle NE plunge subhorrzontal axtal plones subhonzontol axial planes oxtol plane crenulation cleovage axial plane crenulotron cleavage DZcmojor tight folds lFrg.21, D,, mopr trght folds Fig.501 gentle NE plunge, NW vergence gentle NE plunge, NW vergence axtal plane cleovoge (seetext)penetrative axtol plane cleovoge extensron ltneotion on cleavage, steep-dtpptng D,, ?!nor folds. gentle NE plunge tnconsrstent vergence LzS fabric often. "cleavage" ortgtnally dtpped SE extension lineofion, gentle NE plunge the structural characteristics of the internalzone The Colonsay Group rocks (Table I). They are a low-grade metamorphosed se- quence of monotonous grey-brown sandstones of un- The group is an overall right-way-up sequence, con- known affinity but possibly Torridonian or Moine in tinuously deformed in three main episodes. This study age. refers particularly to theoutcrops in the central part of Still further NW there occurs a different sequenceof W Islay wherea useful profile may beconstructed Moine-r Tomdonian-like-rocks, the Colonsay (Fig. 2). Group (Stewart 1975). These have a longer structural The first episode produced an L> S or L-fabric with history than the Bowmore and Dalradian rocks of the strike-parallel fabric lineation (Fig. 1). Sometimes the internal zone and lie unconformably on retrogressed planar component is difficult to detect but the group and cataclastically deformed caledonised Lewisian facesupwards to the NW on it. Cleavage is well Gneiss. They comprise low-grade arkoses, sandstones developed higher in the Group where pelites are more and slates with a pronounced sub-horizontal NE-SW common. Only minor folds developed and their ptyg- L-fabric. In Fig. 1 the position of the foreland rocks matic form gives rise to inconsistent vergence direc- has been corrected for the lateral displacement along tions (NR 189 595, NR 212 606, NR 251 579). the Loch Gruinart Fault(Westbrook & Borradaile The secondphase produced the only large folds 1978). This fault also throws 4 km down to the SE. (Fig. 2), refolding the earlier fabric (NR 235 713). The All three rock groups are deformed more than once axial plane cleavage is often penetrative and not the but they do not share the same sequence of events. spaced,crenulation texture common in secondary The Bowmore Group (Amos 1960) and the Dalradian cleavages. This is because the preceding fabric usually Supergroup on Islay havea similar history, but the lacked a good planar component and its linear compo- Colonsay Group's deformation sequence corresponds nent was nearly parallel to thesecond cleavage so that only from the second episode onwards (Table I). The it could not be crenulated. Second cleavage thus varies second phase in the Colonsay Group produces major from a fine 'slaty' texture to an undisputed crenulation folds with the same plunge, vergence and style as the cleavage. It may oftenbe traced intothe Lewisian first phase in the Dalradian. A similar correspondence gneiss although the more prominent first fabric may exists between the distinctive third phase in the Colon- not. Lack of suitable shape fabricindicators in the say Group and the second phase in the Islay Dalra- gneiss may explain this. dian. Roberts (1974, p. 113) and the author suggest thirdA phase locally superimposedregular that the Colonsay Group experienced an older defor- wavelength minor folds onthe flanks of themajor mation phase preceding the first Dalradian deforma- second folds (NR 219 618).
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