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. Theyalso 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 donot 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

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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 thearea and plunging SE inthe 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.

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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). tion phase. This is also implied by Stewart & Hackman (1973, p. 199). Note that Table I refers to events in Shape fabrics and total strains in the Islay: in particular, the secondphase in the Islay Colonsay Group Dalradian rocks is the sixth phase recognised in the Dalradian over thewhole of the SW Highlands accord- Coarse-grainedsandstones of the EileanLiath and ing to Roberts (1974, fig. 4). Rubha Gaidealachformations (Stewart & Hackman

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/1/77/4886137/gsjgs.136.1.0077.pdf by guest on 01 October 2021 80 G. J. Borruduile 1973) provided suitable material for study at 45 loca- For the following reasons the author believes the tions through the profile (Fig. 2). Clastic grains were grain elongation is parallel to X. Firstly, the rocks are severely plastically deformed and aligned by the first L-or L>S tectonites in all positions aroundthe two deformation phases, although cataclastic deforma- profiles of major folds and any shape lineation is thus tion also occurred. much more likely to be parallel to X. Secondly, the elongated clasts of quartzand feldspar are often matrix-supported.They wouldhave beenfree to Significance of the orientation of the shape rotate during deformation withoutimpinging upon one fabric another.Alignment parallel to X would thushave The ubiquitous strike-parallelclastic shape lineation occurred long before the grainscould passively change presents a problem. Is it a maximum extension linea- shapeto produce L-tectonites parallelto Y in the tion or did it arise from a fortuitous combination of manner described by Ramsay (op. cif.). Thirdly, in original grain shape orientations with the orientation some pelites a mineral lineationis parallel to the' shape of the total strain ellipsoid? The latter can produce a elongation of the coarsersediments. This is more shape lineation parallel to Y rather than X (Ramsay likely to be due to rotation towards, or recrystallisa- 1967, p. 220). tion parallel to X than to any other direction.

GRAPH SYMBOLS

P \ FELDSPAR

WNW (mean traverse d!rectIonJ - TRAVERSE A WITH GRAPH ORlGlN AT GRID REFERENCE INR 787 593)

Colonsay Group ( =samples) aLewisian gneiss 1 2 3 4 km L NE -TRAVERSE E WITH GRAPH ORIGIN AT GRID REFERENCE INR 260 598)

FIG. 2. Variationin strain ellipsoidshape, denoted by k, for deformation of quartz or feldsparclasts in two traversesaround the ColonsayGroup's major folds(see inset). Tielines joindata from adjacent localities. For location see Fig. 5a.

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Strain estimates from grain shape fabrics In a Two thin sections were made fromeach sample. One was parallel to L and perpendicular to S; the other was perpendicular to L. Sufficient continuously deformed grain shapeswere measured to allow a simple &/c#J analysis (Dunnet & Siddans 1971, Ram- say 1967, p. 210). This used the most extreme grain shapes and assumed the maximum extension to be parallel to the grain elongation observed in the thin section. (This method is only marginally more accurate $' than the more efficient methodproposed by Lisle / (1977).) Data from each pair of sections were com- 0 / / bined to define (a, b) for the strain ellipsoid responsi- 0 / ble for deformation of the quartz or feldspar grains 0 / (Fig. 3). / The strain responsible for the continuous deforma- tion is the sum of D1, and D2=. Buthow far has each of thesecontributed to the total strain reflected in the grain shapes? Fortunately, D2=produced big folds and variation in total strain around them helps answer this question (Fig. 2). Theshape of the strain ellipsoid varies with position around the folds although X re- mains subparallel with the hinges, evenwhere the strain is of a flattening type (k< 1). Almost perfect flattening occurs near hinges (Fig. 2, anticlines 1, 2 & 3) and, where the data suffice, a positive correlation exists between k and the distance of the sample from the hinge as far as the inflexion point on the flank concerned.Flattened ellipsoids occur on or near Results of Rfl$ analyses of mefasandslone hinges, while more constricted strain ellipsoids are handspectmcns. found nearer the inflexion points. On the west flanks 0 for deformed quarlz clost shopes of anticlines 1 and 2 the correlation coefficients be- for deformedfelspar clastshapes tween k and distance from the hinge are respectively 1 datafrom some sample ore connected 0.86 and 0.96. by he- lino Since S2c is only well developed near hinges, the RcsuIIs of Rf/+ analyses for deformed author attributes the flattening component to D2=.D,, pebbles of gnerss at produced a more constrictive deformation as witnes- ci] D& Mder, Islay I NR 225 608) sed by the prevalence of L-tectonites on the flanks of Mach,,Bay, Islay INR 2% 6181 the major second folds where second cleavage is less W N. Colonsay INM 428 0&1

well developed. Result Iw,th error borsl for Slrom OnalYSrS

There is no evident relationship between frequency of dcforrncd sand dykelets in pclrtc of cataclastic deformation and distance fromthe Colonsay -+- INR 354 940 I Lewisian contact or position within the profile. Quartz grains experiencedgreater strains than FIG. 3. Logarithmic Flinndiagram of Colonsay feldspar (plots lie further from the origin on a Flinn Group strain data. diagram, Fig. 3). Quartz was more constrictively de- formed too. These generalisations are apparent from total strain ellipsoid shape for the deformation: the mean total strain ellipsoid axes for the deforma- tion: of rock 2.51 0.660.61 This will be discussed further (p. 86). of quartz 1.47 1.04 0.66 of feldspar 1.29 1.11 0.69 The Dalradian rocks The author emphasises that these data only refer to intragranular deformation. The few data available for Bailey's (1917) major outline of the structural geol- multigranularstrain markers in thesame lithologies ogy remainunaltered. The Islay Anticline is the indicate much highertotal rock strains with a mean largest Ftd structure with approximately axial planar

6

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/1/77/4886137/gsjgs.136.1.0077.pdf by guest on 01 October 2021 82 G. J. Borradaile (but fanning) cleavage on which the succession faces N upwards and generally NW, throughout Islay. Local- ised minor secondary folds and crenulation cleavage (Table I) contribute little to the regional strain in Islay: The D,, structural pattern may also be traced beyond Islay (Fig. l), because the multitude of local secondary deformation episodes in the SW Highlands contribute hardly anything to the major structure (Roberts 1974, fig. 4). In part, however, the regional fan of S,, on the mainland may bedue to late large scale flexuring (Borradaile 1970, 1973, Roberts & Treagus 1977a).

Signifwance of S,, and its accompanying shape or fabric lineation As an illustration, consider first the special case of the oolitic limestones which have been used for strain determination in the Dalradian rocks. In these, S,, is defined by the preferredorientation of the oolites themselves. Since these are almost perfect strain mar- kers, S,, is by definition parallel to the XY plane of the strain ellipsoid at each stage in the deformation. N S,, is in these circumstances a trajectory of the XY planes of the total strain ellipsoid. In other lithologies the strain markers were origi- nally non-spherical and multigranular in scale. There is no reason to expect that S,, is parallel to the XY planes associated with the deformation of such strain markers.This is becausenatural strain histories are generally non-coaxial (successive strain increments are superimposedobliquely onthe existing strain ellip- soid) and whatever defines the cleavage may have a history quite independent of that of the strain mar- kers. Where sedimentary sand dykes are the strain mar- kers a geometrical test may be applied, the ‘sand dyke test’, to ascertain whether or not there exists even an approximate parallelism between cleavage and XY. The method involves defining an area ona stereogram in which the Z-direction of the total strain ellipsoid mustlie (Borradaile 1977, fig. 5). If the pole to S,, also lies in this field, and if the field is fairly small, we can saythat the strain history hasbeen sufficiently coaxial to use S,, as a surface approximately parallel pole to sand dyke(let) to XY. 0 mean pole to bedding Many localities provided sufficient metre-scale sand dykes to make this test very sensitive (e.g. Fig. 4a, mean pole to cleavage (Sld) locality 40). However, where the strainmarkers are Al’extension direction on Sld centimetre-scaledykelets it is extremely difficult to field containing Z for measure their orientations with sufficient accuracy to .;: a coaxial strainhistory

auulv.. ~ thesand dvketest. Nevertheless. in the few cases where this was possible, S,, may still be recog- nised as an approximate XY trajectory (e.g. Fig. 4b, FIG. 4. Lower hemisphere, equal area projections illustrating the use of the ‘sand dyke test’ to estab- locality 22B). lishwhether approximate parallelism exists be- In the majority of cases S,, is approximately parallel tween cleavage and the XY plane. (a)Sand dykes to XY. Moreover the lineation on S,, may be iden- from the Port Ellen Phyllites atNR 527 657, local- tified asa maximum extension in that surface.For ity 40. (b) Sand dykelets from the Dolomitic For- example, from the symmetrical shapes of secretionary mation at NR 364 698, locality 22B.

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strain shadows about the ends of rigid inclusions. This rather similar and the thinning of formations near the lineation fixes the X-direction in XY. Thus to a first thrust is attributed to attenuation by discontinuous approximation, the regionalmap of SI, and its deformation (sliding) along discrete surfaces at a low mineral/fabric lineation is a map of the trajectories of angleto bedding. No furthersystematic variation in X and XY for the total strain ellipsoid (Fig. 1). strain around the anticline has been detected. In anumber of instances thesand dyke test has The degree of strain is lithology dependent to some proven that s1d is markedly oblique to XY.There the degree (Fig. 7). The ‘Phyllites’, a well-bedded pelite strainhistory has been so non-coaxial that cleavage andsandstone formation, show the highest strains: and itslineation no longer reliably predict principal data points plot furthest from the origin of the Flinn strainorientations. Four very convincing examples diagram,‘Quartzites’ of variouswell-bedded forma- were found at NR 464 530, NR 431 698, NR 682 006 tions are the next most deformed while the carbonate and NM 672 997. Furthermore,examination of lithologies are least deformed. Spencer’s (1971) data on cross-beds fron the Garvel- lachs (see Fig. 1) also supports such a discrepancy between XY and S,, although his sand dyke data do notreveal this (Borradaile, in prep.). His cross-bed A partial restoration of theprofile data show that SE-dipping foresetshave been reduced through Islay to its pre-tectonic form in dipand NW-dipping foresets oversteepened.A gradualvariation in foreset angles occurs with Knowing the total strain at discrete points in a princi- azimuths between these two extremes. This variation pal planesection through a continuouslydeformed could only be accounted for by an XY plane bisecting body, it is possible to restore the section to itsoriginal the acuteangle between NW- andSE-dipping configuration by removing or reversing the observed foresets. This hadto make an angle of >40” with strainsat each point. Ramsay (1969) first suggested bedding whereas S,, makes only 27” with bedding. this and Hossack (1978) has applied it. The method is Wherever Sld is not an XY trajectory it dips more only easily appliedto a plane containingprincipal gently than XY. Andwhere it is only just an XY- directions of the strain ellipsoids. Otherwise the plane trajectory according to the sand dyke test, it has the section being restored becomes distorted to a surface most gentle dip within the range of possible orienta- of complex configuration (Schwerdtner 1977). For this tions. All these examples occur on the SE flank of the reason a map of strain estimatelocalities is not used as Islay Anticline and near its hinge. The regional and a basis forthe restoration. Instead, a down-plunge general implications are discussed on p. 85. view or plunge profile (Fig. 5a) is used because this is approximately perpendicular to Y in each of the Dal- radian localities and perpendicular to X in the Colon- Total strain estimates for Dalradian rocks say Group localities (Fig. 1). This fortunateset of circumstances allows the author to use what are very Estimates of total strain were obtained for various nearly principal plane strain values at most localitiesin locations in Islay, Jura and for the Garvellachs. The order to restore theplunge profile of Islay’s geology to groups of localities are designated in Fig. 5a and the its pretectonic form. The author emphasises that the data are presented in Figs. 6 and 7. Sedimentary sand plunge profile plane whichis restored is not a real dykes anddykelets were the mostcommon strain section through the geology but merely a projectionof marker (localities 1, 6, 16-27, 34, 35, 38). The anal- all the map data on to a single plane perpendicular to yses used the orientations in three-dimensions, or the the mean fold plunge. Fortunately, thehomoaxiality of orientations in two-dimensionson joint surfaces, or themajor folds in Islay andthe small variation in the angulardistributions on bedding surfaces (Bor- strain perpendicular to the profile plane (1.25 +0.05 radaile 1976, 1977, Borradaile& Johnson 1973). 001- for the Dalradian) make this a realistic proposition. iteswere thestrain markers usedin the Islay and Ramsay’s method was concerned with the restora- Ballygrantlimestones (localities 2-4, 9-15) and tion of bedlengths. Herethe present author has elsewhere vesicles (36)and concretions (37) were extended its use to fix the original separation of strain used. estimate localities, original bed thicknesses, and origi- Although all theDalradian rocks are good S- nal angular orientations of bedding with respect to the tectonites, strains are sometimes constrictive.Seven of principal strain directions at each locality. The recon- theten groups of outcrops with constrictivetotal struction (Fig. Sb) was achieved by arbitrarily choosing strains are situated on or near the hinge of the Islay the lowermost right data point in each of the profiles Anticline (Fig. 5a). Greater total strainshave not been for the Dalradian and Colonsay rocks and rebuilding recorded from thevicinity of the Loch Skerrols Thrust the profiles stepwise using the recalculated locality (adjacent to localities 12-15) nor from near the only separations, bed thicknesses and orientations. Oertel other possible primarydislocation (locality 6).The restoreda minor fold in a similar fashion (Oertel degree of strain on the two flanks of the anticline is 1974, fig. S).

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FIG. 5. (a) Down-plunge view of Dalradian and Colonsay Group major folds on Islay. Major dislocations: thicker lines. Numbers: groupsof strain estimates localities (data are presented in Figs.6 and 7). Localities with constrictive deformationindicated byfilled circles. Colonsay Graupstrain data are given in Fig. 2.(b) Partialpretectonic restoration of the plane of the plunge profile in (a)at same scale. In o I In a -18A- I

l v 0 I’ /‘ (In2 ,g// A ,/’ .A 12 / 9 0 0

B __ U30 -&- (h2) i (h4) (lG6) i lnb-> (In21 f (lA4) ClAS 2 hb-> 0 Thln-bedded phylhtes B quartzltes (Port Ellen Phyllltes) FIG. 6. Logarithmic Flinn diagram of mean strain Ooltticllrnestone (Islay hestone) states for Dalradian rocks of Islay. Numbers referto 0 Quartzltes(several forrnatlons) A Well-bedded dolomitlc sllt- 8 sandstones (Dolomltlc Beds) localities in Fig. 5a. T Tllllte rnotrlx (Port Askalp Boulder Bed) V VesiclesIn metodolertte dyke FIG. 7. LogarithmicFlinn diagram of Dalradian strain data distinguished by lithology. Data from Fig. 6 and also extra data from Jura (29-40) and the Garvellachs (46).

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 Caledonidesthe Islay 85 The recqnstructed profiles forthe Dalradian and Regional structural development Colonsay rocks are internally consistent. Errors in the overall dimensions are those of individual strain esti- The internal zone: Dalradian Bowmore mates: 5-10%. However, the relative positions and and Group rocks orientations of the two restored parts of the profile remain unknown since the strain data yield no infor- The Dalradian sequence was shortened by an aver- mation concerning the translation or rotations of the age ‘55f 5% across strike, at a high angle to the steep structural units. The relative positions of the Colonsay, primary cleavage. It was also squeezed upwards to the Bowmore and Dalradian structural units are indicated NW, parallel to the mean extension lineation, by 70 * arbitrarily and the Bowmore rocks, in which no suit- 16%.(These data areweighted averages for the differ- able strain markers were found,are indicated schemati- ent lithologies). Thisdeformation could accountfor cally. The Colonsay Group is positioned somewhat the presence of the Islay Anticline by more or less lower than the other rocks in the reconstruction since homogeneous shortening of an embryonic buckle fold. the author has corrected for the throw on the Loch Up to 25% extensionoccurred parallel tostrike in Gruinart Fault. At the onset of deformation the rela- some localities but the average extension parallel to tive levels may have been different from those shown strike was 8%. This modest NE-SW extension might since the amount of upthrusting on the Loch Skerrols account for the gently curving hinge lines of the major Thrust is unknown andthe nature of the original folds in the SW Highlands. For example, the measured contact between the Colonsay Group and therocks to length along the hinge of the Loch Awe Syncline at a the SE is completely unknown. given stratigraphic level is about 11% longer than the axial trace onthe ground. Such longitudinal wavelength components may partly owe their origin to the constraint of strike-parallel extensions. The restoration (Fig. 5b) The Islay Anticline has a marked cleavage fan and in several localities on the SE flank the author found The Islay Anticline reduces to an open fold when that cleavage dips more gently than XY of the rocks’ the observed total rock strains are removed. (The large totalstrain ellipsoid. Bothfeatures are recognisable original thickness variation of Jura Quartzite is not because the strains in this area are lower than in the unique to this area: Harris & Pitcher (1975)). The more internal parts of the belt to the SE. Their origin Anticline is not completely unrolled when theob- may lie in hinge migration duringthe anticline’s served strains are removed. This indicates that some growth. S,, cleavage planes initiated at an early stage folding-involving an amplitudesomething like that would fan as the hinge migrated along the layers remaining in Fig. 5b-preceded or overlapped with duringthe rolling up of the anticline as it moved penetrative D,, strain. Perhaps this was by a flexural- along the Loch SkerrolsThrust. Initially steep em- slip process involving no penetrative deformation. bryonic cleavage planes would adoptmore gentle The ‘restored’ Colonsay Goup part of the profile is attitudes as they were carriedaround to the overturned littledifferent from the presentday onefor two flank (Fig. 8). This could occur during a single, con- reasons. Firstly, the principal directions are oriented tinuing episode of deformation. Overprinting of cleav- with X perpendicular to the profile and often Y = Z. age(s) need not be expected. This process could also Consequently removing the effects of the YZ strain account for the rotation of early-initiated S,, planes ellipse removes littledistortion in the profile plane. away from theconsistently steep orientation of the XY Secondly, the strains measured are those responsible planes of the strain ellipsoid at eachstage in the for the continuous deformation of clastic grains. They deformation. In this way the marked obliquity of are not total strains for the whole rock. The weakest cleavage to the XY plane of the total strain ellipsoid parts of a rock are grain boundaries and interstices, recognised at several localities can be explained. not the grains themselves. This is confirmed where Regionally, the Dalradian rocks show an interesting strain markers on a multigranular scale indicate more spatial pattern of primary fabric (Fig. 1). Because SI, severestrains (Fig. 3). Theattempt to restorethe and its extension lineation broadly correspond to XY Colonsay Group to anything faintly resembling a uni- and X, the fabric pattern indicates the spreading paths formly dipping set of strata fails for these reasons. Fig. of the Dalradian rocks during the primary deforma- 5b onlypresents a correction forthe grain-shaping tion. At the NE end of Loch Awe the rocks squeezed component mainly due toD1,. The heterogeneous (fold) up to the SW and outwards along the cleavage fan to components due to DZc are not removed at all. The the S and W respectively. A mirror image is observed magnitude of these fold remnants in the reconstruction on Islay, atthe SE end of the Loch Awe Syncline indicates the extent to which intergranular deforma- where the rocks squeezed up to the NE and out to the tion (grain boundary sliding), cataclasis and flexural- N on the NW margin of the internal zone where the slip folding have contributed to the Colonsay Group’s cleavage dips more gently. Apparently from two sites total heterogeneous strain. of more intense deformation,one NE of Loch Awe and

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/1/77/4886137/gsjgs.136.1.0077.pdf by guest on 01 October 2021 86 G. J. Borradaile XY theforeland rocksseveral kilometres relative to l the other rocks to the present level of exposure. Theattempt to undeform the Colonsay Group's profile failed becausethe strain markers are grains. Nevertheless, correcting only for the intragranular de- formation indicates that the profile was 20% broader than now (Fig. 5). Similarly, the group was, at most, between 59 and 77% of its present length along strike (NE-SW). Simply unrolling the F2cfolds remaining in the partly restored profile of Fig. 5b indicates that it was originally 120% broader,i.e., ithas suffered a contraction of 55%. (This procedure is not strictly valid but gives some idea of total shortening). The tectonic significance of the unconformable con- tact between the Colonsay Group and the caledonised Lewisian is not known, further than that it pre-dates the first deformation of the Dalradian. Stewart & Hack- man (1973)termed it the Bruichladdich Slide but there may have been no more movement along this contact than that required by the differential strain of the very contrastinglithologies whichit juxtaposes. Displacement on the contact due to differential strain effects would probably have been NE-SW, parallel to the maximum extension in the foreland. This is per- pendicularto thesubsequent overthrusting and stretching of the Dalradian rocks.

FIG. 8. Possible hinge-migration growth model for Rock deformation, strain analysis the Islay Anticliie. (a) Initial, and (b) later stages in D,, episode. Discrepancies (E) between XY and and grain boundary sliding S,, detected from field data. The strains experienced by coherent, whole grains in the Colonsay Groupare much less than those of one just S of Islay, the rocks squeezed up and out- multigranular strain markers in the same lithologies. wardsrelative tothe intervening LochAwe plunge Therefore,the removal of strainsdetermined from depression. To some extent this is evident from the grain shapes fails to completely unfold the profile of wayin which the major folds' axial traces pinch to- the group.This is becausedeformation has largely gether in these areas (Roberts & Treagus 1977a, b). beenachieved by intergranular deformation-grain boundary sliding (gbs). In the case of the Colonsay Group this effect has The foreland rocks: Colonsay Group and been exaggerated by cataclasis which generated even Lewisin gneiss more grain boundaries and further reduced the resis- Although it is now obscured by the effects of the tance of the rocks to gbs. The maximum extension of late Loch Gruinart Fault, a significant boundary sepa- quartz grains in the Colonsay Group is 47% while the ratesthe Bowmore and Dalradian rocks from the whole rock may be extended as much as 150% in the foreland. The Colonsay Group of the foreland has an same direction. Furthermore, the shapes of the strain olderdeformation phase and is characterised by ellipsoids determined for the deformationof quartz, of severestrike-parallel extension. The magnitude and feldspar and of the wholerock each have different orientation of its total strains contrasts sharply with shapes. In general, and also in specific cases where those of the adjacent internal zone, to the SE. The strains for quartz and feldspar have been determined original boundary between the Colonsay Group and from the same specimen, quartzis more constrictively the rocks to the SE is much more significant than the deformed (Fig. 3). Since quartz is more deformed it Loch Skerrols thrust. probably started deformation earlier than feldspar and The Colonsay Group is presumably a Torridonian- it is thus inferred that the strain history usually began or Moine-likesequence, older than the Bowmore with high k-values which reduced in value as deforma- Group, and separated from the rocksto the SE by tion progressed. The strainellipsoid forthe whole rock either a now concealed unconformity, a major tectonic is evenmare constrictive. Since rock deformation boundary,or both. The Loch GruinartFault raised began beforethe grains startedto deform this may

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 87 also confirm that thestrain path progressed from higher to lower k-values. In the Bowmore and Dalradian rocks cataclasis is very rare. Psammites show only weak preferred shape orientations and strained extinction of quartz. Even the most cursory examination of texturesand grain shapesindicates that grains have suffered negligible 'Pf= strain (e.g. Bayly et al. 1977, pl. Eu 3 & 4). Neverthe- 0 less, largestrains areapparent from multigranular strainmarkers (datafor the rocks illustrated in the work just cited are from localities 25 & 34, Fig. 6). Intragranular deformation contributes little to the de- formation of these rocks. Grain boundary sliding : Other evidence indicates the separateness of grain and rock deformation histories in the Dalradian rocks. Naturalstrain histories are generally non-coaxial: strain increments are superimposed obliquely on pre- controlled existing strain ellipsoids. Discreteplanar structures developing parallel to XY in only a part of the strain history cannot be parallel tothe XY plane of the ultimate,total strain ellipsoid. They may of course FIG. 9. Tentative deformation mechanism map for approach parallelism if strains are severe enough. In monomineralic rock. Deformation mechanisms for the Dalradian rocks the widespread, penetrative first a given mineral may be indicated on a differential cleavage is manifest as a poor grain alignment and stress (a)us temperature (T) graph (rear wall of weak grain-shaping in non-pelitic lithologies. In some the diagram). To account for the behaviour of an cases it is grossly oblique to the XY plane of the total aggregate of such grains a pore fluid pressure axis strain ellipsoid for therock (p. 83). Those data indicate is added (Pf). that cleavage formed during only a part of the defor- mation history of the rocks. rock or rock with normal Pf. This effective stress The reason for the differences in amount and syn- principle must considerably increase the importance of chronisation of grain vs whole rock deformation is the role of gbs as a rock deformation mechanism. that intergranulardeformation (gbs) accomplishes In Fig. 9 the two-dimensional deformation mechan- much or most of the total strain of the rocks. The very ism map used to illustrate the behaviour of polycryst- nature of gbs makes it elusive to normal petrographic als is extended to the case of a monomineralic rock. study and it is probably for this reason that its impor- This is done by adding a P, axis which accounts for the tance has been overlooked and underestimated, except modification inbehaviour expected from fluid-filled by Hinn (1965b). Means' (1977) experiments with an pores. The usual deformation mechanism map for, say, artificial rock nicely illustrate just how much the bulk quartz or calcite (after Rutter 19761, is indicated on strain of an aggregate can owe to the relative displace- the P, = 0 wall of the diagram. Even atP, = 0, some gbs ments of grains without any grain deformation at all. will generally be necessary for the mapped mechanism However, under geological conditions it is not clear to to operate, otherwise voids would open up between what extent gbs will be controlled by the small adjust- grains, dilating the aggregate. Elliot (1973, p. 2659) ments of grain shape by intragranular processes which quoted an example in which the greater part of the are required to remove locking asperities on contigu- strain of an aggregate is achieved in this way. This gbs ous grain surfaces (Flinn 1965b). at P, = 0 is, however, dependenton, and a conse- While the locking of unfavourably shaped grains quence of,grain deformation. It is proposed to call this undoubtedly hinders gbs, the author believes that an dependent gbs. equally important effect promotes it. This supporting With increasing Pf, gbs becomes increasingly easier process arises from the effects of pore fluid pressure because of the effective stress principle. Under these (P,). Compaction, tectonism andthe thermalexpan- conditions gbs may occur independently of grain de- sion of water (Norris & Henley 1976) generate and formation although the rate at which it proceeds may help sustain abnormally high P,. This may be supp- be controlled by the speed with which grain deforma- lemented by water exuded by metamorphic dehydra- ion processes remove obstacles which preventun- tionreactions of the rocksconcerned and more so favourablyshaped grains from slipping past one fromthe underlying rocks (Fyfe 1974). Pf, acting another. This could be called controlled gbs. hydrostatically, reduces the normal stresses acting At very high P,, especially under non-metamorphic, across grain contacts of all orientations and allows gbs quicksand-type conditions, internal grain deformation to occur more readily than would be possible in dry processes may be entirely supplanted by independent

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/1/77/4886137/gsjgs.136.1.0077.pdf by guest on 01 October 2021 88 G. J. Bonadaile gbs (see front wall of diagram, Fig. 9). This may also ACKNOWLEDGMENTS.The field work was financed by the apply in low-grademetamorphic rocks where dis- Royal Society following a pilot study P~Yfinanced by the aggregation of the rock mayoccur, sometimes in discrete Staf€ Research Fund of the university of Durham. zones(Bayly et al. 1977, PI. Eu 27, 28 & 29), perhaps FliM, Win Means and PatersonPro- voked and clarified my thoughts about rock deformation, and as a result of liquefaction during tectonic dewatering discussion with Fried Schwerdtner helped my revision of the alongselectedchannels (Powell 1972). manuscript.final

References

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Received 23 January 1978; revised typescript received 15 May 1978. GmJOHN BORRADAILE, Geology Department, Lakehead University, Thun- der Bay, Ontario, Canada.

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