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Progressive simple shear deformation on the Laxford Shear Zone, Sutherland
Progressive simple shear deformation on the Laxford Shear Zone, Sutherland F. BRY AN DAVIES
DAVIES, F. B. 1978. Progressive simple shear deformation on the Laxford Shear Zone, Sutherland. Proc. Geol. Ass., 89(3), pp 177-196. The Laxford front is a candidate for a partial section across a major sub-vertical ductile shear belt which was active for more than 1000 Ma from at least 2800 Ma. Determinations of heterogeneous shear strain from major structural patterns indicate a minimum finite dextral displacement of 35 km, Three minimum strain estimates have been determined; finite strain from the end of the granulite facies metamorphism, strain from the late-Scourian and the Laxfordian strain. Most of the determined shear strain occurred in the Scourian; dextral displacements with movement directions plunging about 40° SE played a major part. The highest shear strains for each stage occurred over an early Scourian synform containing supracrustal rocks and the width of the shear belt increased throughout progressive deformation. The shear belt may be wedge-shape, narrowing westwards and downwards as the result of modification by superposed strain in the north and west of the structure. A model is proposed for both the spatial and temporal distribution of strain throughout this major shear belt which may be of use in the study of generalised shear belt evolution. It may be profitable to review the interpretations of other NW trending belts in the Precambrian basement of Britain in the light of this evidence. c/o Watts, Griffis & McQuat, Consulting Geologists, P.O. Box 5219, Jeddah, Saudi Arabia.
CONTENTS page
1. 2. 3. 4. 5. 6.
INTRODUCTION ... PREVIOUS ESTIMATES OF SHEAR DISPLACEMENTS FORM OF THE MAJOR SHEAR BELT ESTIMATES OF SHEAR STRAIN DEVIATIONS FROM THE SIMPLE SHEAR MODEL CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
177 178 179 181 187 193 195 195
1. INTRODUCTION
Since the realisation by the geological survey (Peach, Home, Gunn, Clough, Hinxman & Teall, 1907) that the transition between the granites and migmatites to the north of Loch Laxford and the pyroxene-bearing gneisses around Scourie occurred at an anomalous NW trending zone characterised by an abundance of relatively minor belts across which relative displacements had occurred and in which new foliations were progressively developed, knowledge of the structural evolution of the zone has been regarded as essential in determining the original relationships between the parents of the Scourian and Laxfordian complexes. Whilst the original view of Sutton & Watson (1951) was that the zone developed after the emplacement of the Scourie dykes and was therefore a Laxfordian structure, recent workers have demonstrated that zones of 177
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F. DRYAN DAVIES
shearing also existed during the late-Scourian (Beach, Coward & Graham, 1974). The present author previously presented evidence which demonstrates that these structures were sited on an even earlier major NW trending structure which was in existence by the end of the granulite facies metamorphism and which controlled to a large degree the position and type of the later structures (Davies, 1976). Further, recent work has shown that many of the contrasts between the Scourian and Laxfordian complexes were established before the end of the granulite facies metamorphism (Davies & Watson, 1977) and so it is now of particular importance to determine the relative sense and magnitude of tectonic displacements which brought together the complexes during the long evolution of the Laxford front from at least the end of the granulite facies metamorphism. 2. PREVIOUS ESTIMATES OF SHEAR DISPLACEMENTS Several workers have interpreted the structural evolution of the Laxford front in terms of major transcurrent components of finite displacements since the late Scourian; dextral displacements by Watson (1973) and Davies (1976) based on large-scale deflections of major earlier markers and sinistral displacements by Beach & others, (1974) and Beach (1974) based on the sense of displacement of minor shear zones which cut the metadolerites derived from Scourie dykes. This paper aims to present new quantitative data on shear strains, displacements and related parameters during several stages of the evolution of this major structure in the high-grade crust. The only previous quantitative estimate has been made on Laxford shear zones which cut metamorphosed Scourie dykes (Beach, 1974) and therefore only record the closing stages of the deformation. These estimates made use of the metadykes as marker horizons external to.the shear zones; thickness variations of the metadykes and deflections coupled with fabric intensity variations across discrete shear zones were interpreted as due to strain in Laxfordian shear zones. Although primary deflections and thickness variations are common in Scourie dykes particularly in the late Scourian and Laxfordian belts (Park & Cresswell, 1973) the assessment of the Laxfordian strain in the metadykes by means of fabric intensity variation is clearly valuable, and shows that sinistral displacements occurred on many of the minor shear zones outside the shear belt from Kylesku to Scourie, particularly those with easterly trends. This led to the postulation that the sum of the displacements on individual shear zones across the Laxford front equalled a total finite Laxfordian sinistral displacement of the order of 25 km. However, minor ductile shear zones in initially homogeneous dyke material indicate both sinistral and dextral displacements. Large-scale Scourian isoclinal folds, which are distinctive structures with NE trends in the Scourian complex, are rotated with a clockwise sense into NW trends near Loch Laxford. This rotation is only compatible with a dominant dextral sense of displacement along the Laxford front (Watson, 1973; Davies, 1976). This contradiction between overall large-scale dextral displacement across the shear belt associated with minor shear zones with both dextral and sinistral sense displacements is puzzling and at first sight contradictory. Here an analysis is made of the shear strain pattern across the major shear belt of the Laxford front since the end of the granulite facies metamorphism by using large-scale changes in orientations of structures and so understand something of the progress of the long deformation on this zone. The method makes use of pre- Badcallian layered basic-ultrabasic sheets as markers also present in regions outside the belts, and of a structural analysis across the region from Kylesku to south of Loch Inchard (Fig. 1); it is based on relationships determined by Ramsay & Graham (1970) and a technique used by Coward, Graham, James & Wakefield (1973) to
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAR ZONE
179
determine displacements across the shear belt along the northern margin of the Limpopo mobile belt, South Africa. 3. FORM OF THE MAJOR SHEAR BELT There is a broad symmetry across the region; granulites are exposed to the south of Scourie and are inferred from high positive magnetic anomalies (Bott, Holland, Storry & Watts, 1972) to lie below the Laxfordian complex, north of Loch Inchard. The southern region, at least, is the site of Scourian structures and discordant Scourie dykes. The region between these granulite blocks is occupied by amphibolite facies gneisses containing examples of pre-Badcallian rocks which can by matched in both the Scourian and Laxfordian complexes, where early Scourian structures are modified by the late-Scourian and Laxfordian shear belts. The southern margin of the shear belt is defined by the deflection of early Scourian isoclines with NE trends south of the belt into NW trends. This change which is abrupt in the northwest (north of Scourie) but takes place over about 1 km in the southeast corresponds to the southern limit of late-Scourian folds (see Beach & others, 1974) with near vertical axial surfaces along the southern margin. The approximate centre lies along Loch Laxford in the belt of concordant metamorphosed Scourie dykes and granite sheets. Several distinct sinistral shears extend along strike for up to 10 km across the southern transition. In the granulites of the south, the zones are usually only a few metres wide and are sometimes made up of fine-grained crushed matrix containing rock and mineral fragments suggesting a brittle or semi-brittle deformation. At the margin of the main belt, these shears widen to a few tens of metres and are associated with minor isoclinal folds with a new penetrative tectonite S-fabric defined by amphibolite facies minerals. Within the NW trending major shear belt they are characterised by complex ductile shear zones up to 50 m wide where the strain is heterogeneously distributed in minor ductile shear zones flowing around lozenges of low finite strain. This change in character northwards along the strike of the shears indicates an increase in ductility into the main belt. Late-Scourian folds are confined to the shear belt and its margins. On the southern margin the folds are open, with near-vertical axial surfaces which strike E-W. Significantly, lateScourian folds become progressively tighter together with an increase in development of planar fabrics toward the centre of the shear belt which suggests that the folds grew in progressively changing orientations during shearing rather than being passively rotated. Continued fold growth in progressively changing orientations during the Late-Scourian and Laxfordian explains the extreme difficulty in distinguishing between late-Scourian and Laxfordian folds on style and orientation. The northern margin is more difficult to define owing to the heterogeneity of Laxfordian strain and the abundance of discordant granite sheets. However, an important change occurs where remnants of Scourian basic bodies become less deformed north-east of the Skerricha antiform, which is a late-Scourian arcuate structure defined by Chowdhary & Bowes (1972). This regional change, both of orientation of structures and finite strain, probably coincides with the northern margin of the shear belt. The entire zone has suffered heterogeneous simple shear (Beach & others, 1974; Coward, 1974; Davies 1974) although the southern part (Fig. 1) is the site of major late-Scourian folds with near vertical dipping axial surfaces, whereas in the northern part the axial surfaces of late-Scourian and Laxfordian major folds have moderate south-westerly dips. It is suggested that the southerly part, from the sub-vertical southern margin which runs NW to SE along the transition between granulites and retrogressed granulites
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F. BRYAN DAVIES
o
o
Lf)
tf'l
.....
N
N
500 1 I
Z
- +
-- ---
- _
~O-O!Q
..,.-
_
65~10 _
_
I
--- -.,..
90-110
Fig. 1. Outline structural geological map of the shear belt. Solid lines, early Scourian axial surfaces; dots-dashes, late-Scourian axial surfaces; dashes, Laxfordian sinistral shear zones; hatched zone, region of granite sheets.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAR ZONE
181
south of Scourie to the granite sheets along Loch Laxford is a candidate for a partial section across a major sub-vertical ductile shear zone, and much of the following analysis refers to this zone. Fig. 1 is a structural map of this region which records the orientation of three important sets of structures; the axial surfaces of isoclinal early Scourian major folds which have a NE trend (with moderate NW dips) south of the shear belts, but have suffered progressive clockwise rotation into the Laxford front, sub-vertical axial surfaces oflate-Scourian folds, and discrete sub-vertical major Laxfordian shear zones. All of these indicate progressive dextral rotation from easterly or NE trends in the granulites ofthe Scourian complex to NW trends in the retrogressed granulites between Scourie and Loch Laxford. The early Scourian folds were described in detail by Davies (1976) and were shown to have developed axial surfaces with NW trends within the shear belt by the time the late-Scourian structures developed. Particularly significant to this work was the determination that within the shear belt thinner normal limbs of early Scourian synforms were consistently more deformed than thicker inverted limbs. The thinner limbs must have formed prior to late-Scourian folds since they are folded by them. L-S shape fabrics defined by clots of plagioclase crystal aggregates in a plagioclase-hornblende matrix occur in both folded Scourian basic sheets and metadolerites derived from Scourie dykes. At some localities metadolerites without penetrative tectonite fabrics cut across the L-S shape fabrics in the Scourian basics and at localities where both rocks carry fabrics significant differences in orientation and intensity of fabrics occur. These relationships demonstrate that an early fabric in the Scourian basics formed before dyke emplacement; its symmetry and orientation reflects the resultant of any original orientation and superimposed Scourian strain or a combination of Scourian and Laxfordian strains. Other lineations, produced by the intersection of the S-fabric and earlier surfaces which occur near to the margins of Scourian basic sheets and particularly along the margins of the metadolerites, deviate considerably from the orientation of the L-S shape fabrics. Whilst in the regions of low-moderate finite strains these two lineations can be distinguished, in the region of high finite strain a few kilometres south of Loch Laxford the distinctions become.very much more difficult to make as at maximum intensity the orientation of both types of fabrics coincides. The L-eomponent of an L-S shape fabric plunges more steeply in the less deformed than in highly deformed limbs of early folds, with plunges of 30-S0° SE in the thinned limbs; the lower value, commoner in limbs further modified by late-Scourian and Laxfordian deformation, corresponds to the orientation of the lineation within the metadykes which represents the Laxfordian movement direction (Beach, 1974). Lineations with this orientation are broadly co-axial with the axes of major late-Scourian folds with plunges of about 40° at 140° (see Davies, 1976, fig. Sc, S49). It is therefore deduced that from the time of thinning of the early Scourian fold limbs a L-component of the shape fabric plunged between 30-S0° SE and probably closely reflected the changing movement direction of the major shear zone. 4. ESTIMATES OF SHEAR STRAIN The shear strain profiles (Fig. 3) are determined from the orientation of the above markers relative to the shear zone margins, in the X-Z plane containing the movement direction (40° SE/1400) and perpendicular to the southern margin of the shear zone (80° SE/31 SO), i.e. the true profile plane (42° SE/0400). At the shear zone margin, the external angle between the zone and the axial surfaces of early Scourian folds in the X-Z plane is close to 90°, and that of both late-Scourian folds and Laxfordian shear zones varies between 40-4So. Calculation of shear strain values is a simple geometrical technique. In this analysis the axial surfaces of early
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F. BRYAN DAVIES
Fig. 2. Contoured map of plunge of linear fabric element. Within shear belt, solid shading 50°; solid line 40° contour; dashed line 30° contour.
Scourian folds are used as passive markers which also occur outside the shear zones and which can be used to estimate the finite shear strain across the southern margin of the shear belt by means of equation (i) (derived by Ramsay & Graham, 1970). (i) cot a'
= cot a -
y
where y is the shear strain required to deform an initial angle a between a marker line and the shear zone margin, to the deformed angle a'. The axial surfaces of both major and minor late-Scourian folds are parallel to the contemporaneous schistosity and are therfore considered to represent the orientation of the X-Y plane of the finite strain ellipsoid since the late-Scourian. This conforms to the pattern predicted for X-Y planes developed contemporaneously during simple shear in a homogeneous material (Ramsay & Graham, 1970), and so the shear strain since the late-Scourian was determined using equation (ii) (ii) tan
2f)
= 2/y
where y is the shear strain required to deform axial surfaces initially formed at 45° to the shear zone margin to the deformed angle 0'. However, the use of finite amplitude folds may lead to underestimates of displacement since it is not known whether significant shear strain would have occurred before finite amplitude folds developed. To minimise any possible discrepancy between actual and calculated displacements the orientation of axial surfaces of minor folds and schistosity were compared with those of local major folds and deviations never exceeded more than ±5°. This problem is further discussed in a later section. Two independent estimates of Laxfordian strain have been attempted. Firmly rooted in the literature is the observation that Scourie dykes are progressively rotated with increasing Lax-
PROGRESSNE SIMPLE SHEAR DEFORMATION ON TIlE LAXFORD SHEAR ZONE
Shear
st rai n
183
Y
(J'I
N
Zone of granites en
x: 3
Fig. 3. Dextral shear strain profiles from southern margin across section Y- Y' of shear belt. Solid line, shear strain from early Scourian; dot-dashes, shear strain from Iate-Scourian; dashed line, Laxfordian shears; shaded zone, Laxfordian shear strain determined from rotation of Scourie dykes.
fordian strain from steep NE dips in the south to SW dipping metadykes concordant with the foliation in the main shear belt (Sutton & Watson, 1962). Whilst the dykes make excellent markers for the qualitative estimates of Laxfordian strain outside the main shear belt the angle between them and the margin of the belt is smaller and more variable than the limits of accuracy of determination of the orientation of the margin and so quantitative estimates are difficult because small discrepancies due to errors or local variations in the initial angle mean possible large variations in the calculated shear strain. Little confidence is held in this result; it is included merely for comparison. The other method of estimating Laxfordian shear strain is from the overall rotation of Laxfordian shears trending 2700 outside the belt, which are progressively rotated into the shear belt. This is an estimate of the overall dextral Laxfordian strain although Laxfordian sinistral displacements clearly occurred along the minor shear zones (Beach, 1974). This apparent contradiction will be discussed later. In this analysis Laxfordian dextral shear strain is determined by treating both dykes and discrete shears as rotated markers by using equation (i), and their results can be compared. Since it is uncertain from the finite strain state whether Laxfordian shear zones making angles of 45° to the shear zone margin acted as passively rotated markers or reflected the XY-plane of the finite strain ellipsoid, shear strains calculated for markers making 45° to the margins produced by both processes are compared in Fig. 14. Within the zone of granites south of Loch Laxford all the above markers have suffered a finite rotational strain greater than that which can be accounted for by the simple shear model investigated here, and so this analysis refers to the region between the southern margin of the shear belt and the zone of granites where the analysis breaks down. The above uncertainties are general problems in understanding the strain histories of large-scale shear belts.
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F. BRYAN DAVIES
(8) Stages of Progressive Simple Shear Strain
Each set of structures is compatible with overall dextral displacements on the Laxford front in both Scourian and Laxfordian times. The results are recorded in Fig. 3 which shows that the shear strain (y) determined for each marker progressively increases from margin to centre reaching a maximum (y =10) in the region of supracrustal rocks which contains near concordant metadykes, south of Loch Laxford. Determinations of minimum displacements from the shear strain variations for each marker (summarised in Table I) indicate a minimum of 35 km of dextral displacement along the northern margin of the Scourian complex since the development of the early Scourian folds, approximately 18-20 km of which occurred during the late-Scourian and Laxfordian stages, and 10 km during the Laxfordian. Important results are that about 50 per cent of the determined displacement occurred before the development of late-Scourian folds and that maximum shear strain and minimum displacements are lower for the younger markers. Finite strains for each of the three determined stages of deformation (Fig. 4) indicate that the southern zone increased in width by about 2 km from the early Scourian to the Laxfordian (about 30 per cent increase in width) and that the maximum shear strain decreased for later increments; i.e. the gradients of the shear strain profiles for each increment decrease with progressive deformation. b. Resolution of the Method using Finite Amplitude Folds The relatively large proportion (45-50 per cent) of the finite dextral displacement estimated by this method to have occurred prior to late-Scourian deformation requires further consideration, since such a major shear strain component earlier than the late-Scourian deformation has not been previously recognised. The identification of this earliest component depends on the
)( 6
, (b)
, (a) 2 kms
(c)
kms Fig. 4. Stages in build up offinite strain for section Y- Y'. (a) Early Scourian shear strain. (b) Late-Scourian shearstrain. (c) Laxfordian shear strain.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAll. ZONE
185
x-x!
Fig. 5. Finite shear strain profiles from southern margin across shear belt to southern margin of granite sheets (S.M.G.).
-
•
Km 30
I
c
CIJ
E 20 CIJ u .!2 0.. III 10 0
)
/
•
•
/
..- ..2
/
4
6Km
Width Fig. 6. Displacement plotted against width from southern margin of shear belt to southern margin of gran ites.
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F. BRYAN DAVIES
resolution of the methods used to determine the shear strain displacement. It is particularly important to understand if the method of using the orientation of axial surfaces of late-Scourian folds significantly underestimates the finite displacement. To be true this would mean an underestimate of 50 per cent of the finite displacement. The development of finite amplitude folds is a major deviation of most major shear belts from the idealised parallel walled shear zone in a homogeneous material considered by Ramsay & Graham (1970). One method of testing the validity of the above method is by comparing the actual and calculated displacements from the axial surfaces of folds produced in a model shear zone. The model adopted was based on an idea of R. Graham (described at the Tectonic Studies Group meeting, 1974) where wet tissue paper, representing a thin competent layer was laid over two blocks free to slide along their mutual boundaries and so produce Reidel-like shears. In this experiment the blocks were merely the method of carrying the tissue layer and imposing a simple shear strain and only structures developed in the tissue layer were considered. Whilst it must be recognised that there are 5 orders of magnitude difference between the wrinkles in wet paper and the natural structure, the model seems a reasonable approximation for shear strains in a competent folded layer. The method adopted was to impart small displacements to markers on the tissue sheet (by shear movement of the supporting blocks) and to record the orientation of the axial traces of folds developed in the tissue. The folds developed in a parallel sided zone parallel to the slip boundary in the underlying blocks and the width of this zone increased during deformation. The results of a number of trials are recorded in Fig. 7. Over the shear strain range )'=0 to 1 (the most critical range of any possible discrepancy between actual displacement and that recorded by finite amplitude folds) it can be seen that after an initial stage during the development of folds, when use of their orientation underestimates the actual displacement across the shear zone in the tissue layer, the calculated displacement underestimates the actual value by less than 10 per cent, which is within the error of measurement of this experiment and well within the 50 per cent of the finite displacement estimated to have occurred before the development of late-Scourian structures. It seems reasonable to conclude that the use of axial surfaces of minor folds yields a result well within an error of 10 per cent of the actual displacement.
•
••
o
••
•
••
2 3 4 5 6 Actual displacement
Fig. 7. Calculated versus actual displacement for Reidel-type model.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON
ras
LAXFORD SHEAR ZONE
187
s. DEVIATIONS FROM SIMPLE SHEAR MODEL a. Wedge-Shaped Belt Although this work has concentrated on the southern margin of the shear belt, which has undergone ductile simple shear since at least the end of the granulite facies metamorphism, the northern zone of the belt also suffered shear strain at least in the earlier part of its evolution which does not simply fit the proposed model of a sub-vertical dipping shear zone. Axial surfaces and schistosity of late-Scourian and Laxfordian deformation which are sub-vertical in the southern zone, have moderate or sub-horizontal dips in the northern zone. Whilst the bulk ofthe Laxfordian complex has suffered high Laxfordian strains, eastern regions of the complex (east of the Laxford Bridge to Rhiconich road; Davies & Watson, 1977) have suffered extremely heterogeneous strain and relatively large areas of low finite strain containing early migmatite complexes and intrusive metadykes are bounded by zones of high Laxfordian finite strain dipping about 25°-50° SW. This change in orientation of XY-planes means that along the exposed strike length the shear belt narrows westwards with the northern margin dipping about 25°-50° SW: its intersection with the sub-vertical southern margin plunges 30°-40° SE which lies close to the movement direction of the shear belt. It is significant that the zone of Laxfordian granite sheets, suggested by Beach & others, (1974) to be a strain discontinuity between the Scourian and Laxfordian complexes, broadly marks the northern limit of the systematic rotation of structures which can be accounted for by the simple shear strain model outlined in this paper. In other words, north of this line the finite strain trajectories are the result of further Laxfordian rotational strain superimposed on the strain developed in the shear belt. Evidence presented in this paper of the regional deviation of the finite strains from the simple shear strain model which holds well over the southern part of the belt, allows a more specific delineation of the boundary of the region of superimposed strain. The southern part of the shear belt varies in width from the southern margin to the zone of granite sheets where the anaiysis breaks down. Estimates of finite shear strain determined from rotation of early Scourian isoclines along three sections which included the narrowest (western) and widest (eastern) section indicate that the finite dextral displacement across this part of the shear belt is constant at about 35 km, the shear strain across the belt changing with varying widths to accommodate constant offset. Contoured shear strain patterns are not however parallel, as predicted for simple shear deformation alone, but shear strain contours converge in the north-west where the strain is greatest. Thus, as well as the additional rotational component already identified there must have been a component of flattening in the west or extension in the east of the belt. These alternatives can be tested by using large-scale shape fabric symmetries which have been defined by using the orientation of the long axes of deformed mineral aggregates and poles to flattening surfaces described above. Several hundred elements from each of four sub-zones near the boundary of superimposed deformation define fields of elongation and fields of flattening (Fig. 10). Whilst it is impossible to split a belt of heterogeneous simple shear into zones of entirely homogeneous strain, it is suggested that the shape fabric symmetries might reflect the symmetry of the bulk finite strain ellipsoid in each domain. In these large-scale shape fabric patterns the surfaces separating the fields of elongation from the fields of contraction can be reasonably defined. In the south eastern domain these surfaces are planes intersecting in the Y-direction of the bulk strain ellipsoid whereas in the northern and western domains these surfaces define cones about the X-axis defined by the highest concentration of lineation orientations. These surfaces might then define the surfaces of NO FINITE LONGITUDINAL
c.
L--_ _---'"~_ __'
Fig. 8. Card-deck redefonnation of shear belt structures in erosion plane. Slip planes parallel to shear belt margin. (c) Present erosion surface pattern. (b) Pattern after redefonnation using sinistral Laxfordian shear profile corrected for horizontal section. (c) pattern after redefonnation using sinistral shear strain profile since late-Seourian corrected for horizontal section restoring type 2 (mushroom) interference patterns.
STRAIN (N.F.L.S.) and in the SE domain, where these surfaces are planes intersecting the Y-axis, the finite strain ellipsoid would have a K=l value which corroborates the simple shear deformation interpretation or a value of K~l. The fact that these surfaces which best define the boundaries are planes intersecting in the V-direction of the finite strain ellipsoid, and that the orientation ofthe relevant surface of N.F .L.S. closely coincides with the margin of the shear belt, suggest that this interpretation is correct for the SE domain. Further, the angle between the surfaces of N.F.L.S. have been compared to those calculated from the relevant strain ellipsoid of simple shear strain determined in this work. They compare well in the east of the shear belt but deviate markedly in the northern and western domains which record highest finite shear strains and indicate prolate strains. The plunges of mean X-directions vary from 30°-40°/140° in the domain compatible with simple shear alone but increase in the western and northern domains to plunges of 50°/150°. This means that the northern and western domains where the shear strain contours converge have suffered a superimposed component of flattening and rotation. Gentle southerly dipping regional late-Laxfordian shear movement planes suggested by Beach & others, (1974) to occur north of the granite sheets can account for this superimposed strain although it's initiation cannot be confidently dated.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON TIlE LAXFORD SHEAR ZONE
189
>10.
--'--,. O---~_1 O.~ 0.5
--------.
Fig. 9. Contoured finite dextral shear strain across southern part of shear belt. Note reduction in dextral shear strain in vicinity of sinistral shears.
190
NE
F. BRYAN DAVIES
NW
sw
Fig. 10. Large-scale shape fabric symmetries using long-axes of deformed mineral aggregates for four sub-zones covering shear belt zones B and C of Fig. 13. Dotted lines, boundaries between fields of finite elongation and finite contraction X, Y. &: Z principal axes of finite strain ellipsoid.
PROGRESSIVE SIMPLE SHEAR. DEFORMATION ON TIlE LAXFORD SHEAR ZONE
191
b. Strain Intensity Variation Although bulk finite strain has been shown to vary systematically across the shear belt, strain intensity is often heterogeneous from large-scale to crystal aggregate scale characterised by undeformed 'pods' flanked or enclosed by shear zones where the strain becomes extreme. As with shear strain distribution, strain intensity has varied both with time and position in the shear belt. Some elements of finite strain intensity variation in a spatial context have already been described (section 2) and are summed up in Fig. 1. An important feature is the increase in heterogeneity away from the centre of the shear belt displayed by both late-Scourian and Laxfordian structures (Fig. 13). Coupled with this is the transition within distinct Laxfordian shears from a ductile mode of deformation within the shear belt to a semi-brittle (mylonite products) and locally brittle deformation (cataclasite products) in the granulites south of the main shear belt. A puzzling observation which leads to an important conclusion concerning strain intensity variation with time is that whilst 50 per cent of the determined finite displacement took place perhaps during granulite facies metamorphism but certainly before the development of amphibolite facies assemblages, distinct minor shear zones flanking low strain pods are either of late-Scourian or Laxfordian age. This large early component of the finite displacement took place within the central part of the shear belt where finite strain intensity variation is least and can only reasonably be interpreted as due to an early stage of homogeneous deformation. This variation in the products of deformation with time must clearly have been related to the changing depth at which deformation took place (cf, Grocott, 1977) but from the above evidence must also have been related to inherited characters of the shear belt which made the central zone more ductile relative to the outer zones. Structures developed in all three stages of evolution of the shear belt converge within this central tract of homogeneous deformation, and it is therefore important to examine the products of the interaction of these structures. Within this tract, on a small scale, shape fabric symmetry changes have been investigated across shear zones within early basic rocks which contain equidimensional clots of crystal aggregates of mafic and felsic material in the undeformed state. Strain ellipsoid symmetries determined by the Rf/0 method (Dunnet, 1969) vary from K=O to K=oo from the margin to
B
6
X 4
2 2
4
~
6
8
Fig. 11. Strain symmetry variation across minor dextral shears. Arrows indicate direction of symmetry change into centres of shears.
192
F. BRYAN DAVIES
-~
.. _- . -._----_.- --._ ... - - - --- _.
B _••• _.~_._.,.
_
6
¥ 4
2
o
3
6
9
12
15Cms
Distan ce
Fig. 12. Relationship between finite shear strain and qualitative characteristics of minor shear zones. (a) Shear strain range of grain size reduction. (b) Loss of earlier fabric elements, extreme grain size reduction.
centres of most shear zones greater than 1 m wide (Fig. 11). Few minor zones lie in the field where K=1. In zones of extreme strain, early layers and original mineral aggregates which define the tectonite fabrics become indistinguishable and shape changes cease to be observable in the amphibolites studied at about y=5~ in minor shears. L-S fabrics then appear extreme, axial ratios of mineral aggregates become indeterminable and low strain pods are lost, which agrees with the larger scale variation. This state is accompanied by considerable grain size reduction. Within the central tract the zones are annealed and it is difficult to identify strain variation on the crystal scale. Grain size reduction is obvious between y =5-7'5, whereas above y=7'5 earlier fabric elements are completely lost and extreme grain size reduction is dominant (Fig. 12). On a larger scale the interaction of sinistral shears with the ductile shear strain in the main belt can be examined. Although a few late-Scourian sinistral shears have been found, most are Laxfordian and so they cannot belong to a contemporaneous conjugate shear set. Most minor sinistral shears that extend into the major dextral shear belt occur in the narrower (western) part of the main belt where the shear strain contours are closer together. In the southern and western part of the belt a reduction in the overall dextral shear strain accompanies the minor sinistral shears. These relationships suggest that the minor sinistral shears are boundary effects where the main shear belt narrows and are a mechanism for accommodating some of the displacement on the narrower part of the shear belt. These curving sinistral shears break up the rock material outside the major shear belt into blocks which have moved against each other with opposite relative movement to the main shear belt (Figs 9 &13). They are probably analagous to the smaller scale shears separating lozenge shaped blocks described by Coward (1976). The interaction of the sinistral shears with the main belt has resulted in complex strain patterns where deformation has taken place by a combination of sinistral block sliding and simple shear. Corroboration of this interpretation may be obtained by comparing the actual displacement in the narrowest (western) end, with the hypothetical displacement obtained from a generalisation of the converging pattern of shear strain contours (Fig. 9). The general converged pattern in the narrow part would have produced a displacement of about 50-55 km compared with the actual displacement of 32-35 km. This difference compares well with the 20-25 km sinistral displacement computed by Beach (1974) to have taken place on the sinistral shears.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAR ZONE
193
(0)
Ie)
Fig. 13. Sub-division of shear belts in terms of deformation intensity. ES, early Scourian dextral shear strain. LS, late-Scourian strain. Lax, Laxfordian dextral shear strain. A, Zone of discrete Laxfordian sinistral shears exterior to main ductile dextral shear belt, change from semi-brittle to ductile deformation at margin of main belt. Antipathetic rotation of axial surfaces outside dextral shear belt. B, Zone of interaction between discrete Laxfordian ductile sinistral shears and ductile dextral shearing. Note low finite Laxfordian strain lozenge-shaped blocks bounded by discrete ductile shears. C, Zone of reduction of discrete Laxfordian sinistral shears, low finite strain lenses present on megascopic and macroscopic scale but reduction in size and frequency towards zone D. D, Zone of late superposed strain.
6. CONCLUSIONS The Laxford front is a major shear belt which has undergone a progressive dextral simple shear deformation since the early Scourian. Most of the determined strain occurred in the Scourian and about 50 per cent of the finite displacement occurred before the Inverian (late-Scourian) episode. The early movement directions plunged about 40° SE and probably only 30° SE during the Laxfordian stage, and whilst vertical displacement components were important, transcurrent components played a major part in the evolution of the structure. At the very least, this might mean that earlier Archaean zonations might be obscured. The very nature of such a deformation system dominated by progressive heterogeneous simple shear means that structures outside the belt would have a different orientation to contemporaneous structures within the belt. The long strain history means that early structures were so strongly modified as to be often impossible to recognise as early components. Some Scourie dykes are located within late-Scourian minor dextral shear belts parallel to the margin of the major zone and often acted as loci of Laxfordian shears and therefore may have accommodated some Laxfordian deformation. 2
194
F. BR.YAN DAVIES
~ a'
Fig. 14. Graph of shear strain determined from markers treated as reflecting the XY-plane of the strain ellipsoid (yO') initiated at 45° to the margin of the shear belt versus shear strain determined from same markers treated as passively rotated markers (ra') that have suffered further rotation from 45° outside the shear belt. (For relevance see Fig. 13A.)
From the grade of the rocks involved, this shear belt must be primarily a structure of the middle-crust, and there is therefore the question of its expressions near the surface and at depth in the crust. Whilst widening of the belt upwards into a zone of ductile deformation would be compatible with the idea of a heterogeneous high-grade crust where deep seated granulites that deformed with a low ductility were overlain by amphibolite facies gneisses that responded to deformation with a high ductility (Watson, 1973), it is not taken as evidence of a continued wedge-shape throughout the crust. Knowledge of the history and progress of deformation on the Laxford front is important since a number of similar NW trending belts occur throughout the mainland Lewisian. On the type of evidence used above, the shear belt at Loch Inver has long been recognised to have existed in the late-Scourian (Inverian) and the maps of Sheraton, Tamey, Wheatley & Wright, (1973) indicate a component of finite dextral displacement, and vertical displacements have also been recognised (Evans & Lambert, 1974). The pattern of NW trending shear belts continues further south in the Gairloch-Torridon region, where these have previously been interpreted as due mainly to vertical displacements (Sutton & Watson, 1969; Park, 1970). Very few regions of Archaean rocks in the mainland Lewisian record low finite strains, so it may be more valuable-to interpret this high grade crust as part of a large shear belt that has suffered heterogeneous deformation. Another question of regional importance is the relationship between displacements on the Laxford shear belt and the grade of metamorphism of the complexes juxtaposed by these movements. The relative south side (Scourian complex) up displacement on the Laxford front is compatible with the movement needed to juxtapose the Scourian and Laxfordian complexes required by the petrological model of Sheraton, Skinner & Tamey, (1973) which suggested that the Scourian granulites were coeval with the Laxfordian complex but originated at deeper crustal levels and therefore suffered the depletion associated with the formation of deep-seated granulites. The evidence summarised by Dickenson & Watson (1976) that deep-seated granu-
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON TIlE LAXFORD SHEAR ZONE
195
lites were transformed into successively lower pressure assemblages and that the vertical displacement which took place from early Scourian to late-Laxfordian times was of the order of 15-20 km agrees well with the vertical displacement component determined from this work, which is strong evidence that the actual displacement was the rising of the Scourian granulite complex; the radiometric age of 2800 Ma probably indicates the beginning of its rise upwards in the crust. Such vertical displacements, approaching the order of thickness of the normal crust, present space problems in a crust of normal thickness unless new material is added at its base. It is significant that the early stage of this movement was probably broadly contemporaneous with the period of crustal thickening postulated to account for the occurrence of deep-seated granulites at the surface in what is now a crust of normal thickness. Whereas factors such as grain size reduction (Watterson, 1975) and perhaps hydraulic fracturing influence the rate of deformation (Beach, 1976) and probably the susceptibility of such shear zones in the high-grade crust to further deformation, the origin of such a major structure requires further explanation. Several workers (Bak, Korstgard & Sorensen, 1975; Grocott, 1977) have suggested that similar major shear belts in west Greenland are deep-seated expressions of what would be major fault zones near the surface. With displacements of the order of thickness of the crust or, in the case of the Greenland shear belts, the lithosphere, the only likely modem analogues of these Precambrian belts are the major transform fault systems. Such transform systems are often characterised by high heat flow and this might agree with the evidence of this work that a large proprtion of the displacement on the Laxford shear zone took place during granulite facies metamorphism. If these are Archaean-Proterozoic analogues of transforms then they may be structures for maintaining strain compatibility between early ridgeor trench-features. ACKNOWLEDGMENTS The author thanks Professor J. Watson, B. Windley, M. P. Coward, J. Hossack, R. J. Lisle, J. Watterson and A. Beach for discussions of earlier drafts of this paper. REFERENCES BAK,J.,J. KORSTGARD& K.SORENSEN.1975. A major shear zone within the Nagssugtoqidian of west Greenland. Tectonophysics, 27, 191-209. BEACH, A. 1974. The measurement and significance of displacements on Laxfordian shear zones, N.W. Scotland. Proc. Geol. Ass., 85(1), 13-21. BEACH, A. 1976. The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in the Lewisian complex. Phil. Trans R. Soc. Lond., A.280, 569--604. BEACH, A., M. P. COWARD & R. H. GRAHAM. 1974. An interpretation of the structural evolution of the Laxford front. Scott. I. Geol., 9(4), 294-308. BOTT, M. H. P., J. G. HOLLAND, P. G. STORRY & A. B. WATTS. 1972. Geophysical evidence concerning the structure of the Lewisian of Sutherland, N.W. Scotland. II geol. Soc. Lond., 128,599--612. BRIDGWATER, D., A. ESCHER & J. WATTERSON. 1973. Tectonic displacements and thermal activity in two contrasting Proterozoic mobile belts from Greenland. Phil. Trans. R. Soc. Lond., A.273, 513-33.
CHOWDHARY, P. K. & D. R. BOWES. 1972. Structure of Lewisian rocks between Loch Inchard and Loch Laxford, Sutherland. Krystallinikum, 9,25-51. COWARD, M. P. 1974. Flat-lying structures within the Lewisian basement gneiss complex of N.W. Scotland. Proc. Geol. Ass., 85(4),459-72. COWARD,M. P., R. H. GRAHAM, P. R.JAMES&J. WAKEFIELD. 1973. A structural interpretation of the northern margin of the Limpopo orogenic belt, southern Africa. Phil. Trans. R. Soc. Lond., A.273, 487-91. COWARD, M. P. 1976. Strain within ductile shear zones. Tectonophysics, 34, 181-97. DA VIES, F. B. 1974. The evolution oftwo assemblages ofmetamorphosed supracrustal and intrusive rocks in the Lewisian complex. Unpublished PhD thesis, University of London. DAVIES. F. B. 1976. Early Scourian structures in the Scourie-Laxford region and their bearing on the evolution of the Laxford Front. II geol. Soc. Lond., 132(5), 543-54. DAVIES, F. B. & J. V. WATSON. 1977. The early
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history of the type Laxfordian complex, N.W. Sutherland. JI geol. Soc. Lond., 133(2), 123-33. DICKINSON, B. B. & J. V. WATSON. 1976. Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of N.W. Scotland . Precambrian Research, 3,363-74. DUNNET, D. 1969. A technique of finite strain analysis using elliptical particles . Tectonophysics, 7, 117-36. EVANS, C. R. & R. St. J. LAMBERT. 1974. The Lewisian of Loch Inver, Sutherland; the type area for the Inverian metamorphism.JI geol. Soc. Lond., 130, 125-50. GROCOTT, J. 1977. The relationship between Precambrian shear belts and modem fault systems. JI geol. Soc. Lond., 133, 257-62. PARK, R. G. 1970. The structural evolution of the Tollie antiform-a geometrically complex fold in the Lewisian, north east of Gairloch, Ross-shire. Q. JI Geol. Soc. Lond., 125,319-49. PARK, R. G. & D. CRESSWELL. 1973. The dykes in the Laxfordian belts. In R.G.Park,&J.Tamey,(eds.). Early Precambrian of Scotland and related rocks of Greenland. Keele. 119-30. PEACH, B. N., J. HORNE, W. GUNN, C. T. CLOUGH, L. W. HINXMAN & J. J. H. TEALL. 1907. The geological structure ofthe N.W. Highlands ofScodand. Geol. Surv. Gt . Brit ., Mem. Geol , Surv., Seot., 1907: 191-252. RAMSAY, J. G. & R. G. GRAHAM. 1970. Strain variations in shear belts. Can. JI Earth Sci., 7(3), 786-813.
SHERATON,J. W., A. C. SKINNER&J. TARNEY. 1973 . The geochemistry of the Scourian gneisses of the Assynt district. In Park, R. G. & J. Tamey, (eds.) . The early Precambrian history ofScotland and related rocks of Greenland. Univ. Keele. SHERATON, J. W., J. TARNEY, T. J. WHEATLEY & A. E. WRIGHT. 1973 . The structural history of the Assynt district . In Park, R. G . & J. Tamey, (eds.) . The early Precambrian of Scotland and related rocks of Greenland. Univ. Keele . SUTTON, J. & J. V. WATSON. 1951. The preTorridonian metamorphic history of the Loch Torridon and Scourie areas in the N.W. Highlands and its bearing on the chronological classification of the Lewisian. Q. II geol. Soc. Lond., 106,241-308. SUTTON, J. & J. V. WATSON. 1962 . Further observations of the margin of the Laxfordian complex near Loch Laxford, Sutherland. Trans. R. Soc. Edin ., 65, 90-106. SUTTON, J. & J. V. WATSON. 1969. ScourianLaxfordian relationships in the Lewisian of N.W. Scotland. Geol. Assoc., Can., Spec. Paper,.5, 119-28. WATSON, J. V. 1973. Effects of reworking on highgrade gneiss complexes. Phil. Trans. R. Soc. Lond., A.273, 443-55. WATTERSON, J. 1975. Mechanism for the persistence of tectonic lineaments. Nature, 253, 5492 , 520-1. Received 15 October 1976 Revised version received 10 July 1977
DAVIES, F. B. 1978. Progressive simple shear deformation on the Laxford Shear Zone, Sutherland. Proc. Geol. Ass., 89(3), pp 177-196. The Laxford front is a candidate for a partial section across a major sub-vertical ductile shear belt which was active for more than 1000 Ma from at least 2800 Ma. Determinations of heterogeneous shear strain from major structural patterns indicate a minimum finite dextral displacement of 35 km, Three minimum strain estimates have been determined; finite strain from the end of the granulite facies metamorphism, strain from the late-Scourian and the Laxfordian strain. Most of the determined shear strain occurred in the Scourian; dextral displacements with movement directions plunging about 40° SE played a major part. The highest shear strains for each stage occurred over an early Scourian synform containing supracrustal rocks and the width of the shear belt increased throughout progressive deformation. The shear belt may be wedge-shape, narrowing westwards and downwards as the result of modification by superposed strain in the north and west of the structure. A model is proposed for both the spatial and temporal distribution of strain throughout this major shear belt which may be of use in the study of generalised shear belt evolution. It may be profitable to review the interpretations of other NW trending belts in the Precambrian basement of Britain in the light of this evidence. c/o Watts, Griffis & McQuat, Consulting Geologists, P.O. Box 5219, Jeddah, Saudi Arabia.
CONTENTS page
1. 2. 3. 4. 5. 6.
INTRODUCTION ... PREVIOUS ESTIMATES OF SHEAR DISPLACEMENTS FORM OF THE MAJOR SHEAR BELT ESTIMATES OF SHEAR STRAIN DEVIATIONS FROM THE SIMPLE SHEAR MODEL CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
177 178 179 181 187 193 195 195
1. INTRODUCTION
Since the realisation by the geological survey (Peach, Home, Gunn, Clough, Hinxman & Teall, 1907) that the transition between the granites and migmatites to the north of Loch Laxford and the pyroxene-bearing gneisses around Scourie occurred at an anomalous NW trending zone characterised by an abundance of relatively minor belts across which relative displacements had occurred and in which new foliations were progressively developed, knowledge of the structural evolution of the zone has been regarded as essential in determining the original relationships between the parents of the Scourian and Laxfordian complexes. Whilst the original view of Sutton & Watson (1951) was that the zone developed after the emplacement of the Scourie dykes and was therefore a Laxfordian structure, recent workers have demonstrated that zones of 177
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F. DRYAN DAVIES
shearing also existed during the late-Scourian (Beach, Coward & Graham, 1974). The present author previously presented evidence which demonstrates that these structures were sited on an even earlier major NW trending structure which was in existence by the end of the granulite facies metamorphism and which controlled to a large degree the position and type of the later structures (Davies, 1976). Further, recent work has shown that many of the contrasts between the Scourian and Laxfordian complexes were established before the end of the granulite facies metamorphism (Davies & Watson, 1977) and so it is now of particular importance to determine the relative sense and magnitude of tectonic displacements which brought together the complexes during the long evolution of the Laxford front from at least the end of the granulite facies metamorphism. 2. PREVIOUS ESTIMATES OF SHEAR DISPLACEMENTS Several workers have interpreted the structural evolution of the Laxford front in terms of major transcurrent components of finite displacements since the late Scourian; dextral displacements by Watson (1973) and Davies (1976) based on large-scale deflections of major earlier markers and sinistral displacements by Beach & others, (1974) and Beach (1974) based on the sense of displacement of minor shear zones which cut the metadolerites derived from Scourie dykes. This paper aims to present new quantitative data on shear strains, displacements and related parameters during several stages of the evolution of this major structure in the high-grade crust. The only previous quantitative estimate has been made on Laxford shear zones which cut metamorphosed Scourie dykes (Beach, 1974) and therefore only record the closing stages of the deformation. These estimates made use of the metadykes as marker horizons external to.the shear zones; thickness variations of the metadykes and deflections coupled with fabric intensity variations across discrete shear zones were interpreted as due to strain in Laxfordian shear zones. Although primary deflections and thickness variations are common in Scourie dykes particularly in the late Scourian and Laxfordian belts (Park & Cresswell, 1973) the assessment of the Laxfordian strain in the metadykes by means of fabric intensity variation is clearly valuable, and shows that sinistral displacements occurred on many of the minor shear zones outside the shear belt from Kylesku to Scourie, particularly those with easterly trends. This led to the postulation that the sum of the displacements on individual shear zones across the Laxford front equalled a total finite Laxfordian sinistral displacement of the order of 25 km. However, minor ductile shear zones in initially homogeneous dyke material indicate both sinistral and dextral displacements. Large-scale Scourian isoclinal folds, which are distinctive structures with NE trends in the Scourian complex, are rotated with a clockwise sense into NW trends near Loch Laxford. This rotation is only compatible with a dominant dextral sense of displacement along the Laxford front (Watson, 1973; Davies, 1976). This contradiction between overall large-scale dextral displacement across the shear belt associated with minor shear zones with both dextral and sinistral sense displacements is puzzling and at first sight contradictory. Here an analysis is made of the shear strain pattern across the major shear belt of the Laxford front since the end of the granulite facies metamorphism by using large-scale changes in orientations of structures and so understand something of the progress of the long deformation on this zone. The method makes use of pre- Badcallian layered basic-ultrabasic sheets as markers also present in regions outside the belts, and of a structural analysis across the region from Kylesku to south of Loch Inchard (Fig. 1); it is based on relationships determined by Ramsay & Graham (1970) and a technique used by Coward, Graham, James & Wakefield (1973) to
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAR ZONE
179
determine displacements across the shear belt along the northern margin of the Limpopo mobile belt, South Africa. 3. FORM OF THE MAJOR SHEAR BELT There is a broad symmetry across the region; granulites are exposed to the south of Scourie and are inferred from high positive magnetic anomalies (Bott, Holland, Storry & Watts, 1972) to lie below the Laxfordian complex, north of Loch Inchard. The southern region, at least, is the site of Scourian structures and discordant Scourie dykes. The region between these granulite blocks is occupied by amphibolite facies gneisses containing examples of pre-Badcallian rocks which can by matched in both the Scourian and Laxfordian complexes, where early Scourian structures are modified by the late-Scourian and Laxfordian shear belts. The southern margin of the shear belt is defined by the deflection of early Scourian isoclines with NE trends south of the belt into NW trends. This change which is abrupt in the northwest (north of Scourie) but takes place over about 1 km in the southeast corresponds to the southern limit of late-Scourian folds (see Beach & others, 1974) with near vertical axial surfaces along the southern margin. The approximate centre lies along Loch Laxford in the belt of concordant metamorphosed Scourie dykes and granite sheets. Several distinct sinistral shears extend along strike for up to 10 km across the southern transition. In the granulites of the south, the zones are usually only a few metres wide and are sometimes made up of fine-grained crushed matrix containing rock and mineral fragments suggesting a brittle or semi-brittle deformation. At the margin of the main belt, these shears widen to a few tens of metres and are associated with minor isoclinal folds with a new penetrative tectonite S-fabric defined by amphibolite facies minerals. Within the NW trending major shear belt they are characterised by complex ductile shear zones up to 50 m wide where the strain is heterogeneously distributed in minor ductile shear zones flowing around lozenges of low finite strain. This change in character northwards along the strike of the shears indicates an increase in ductility into the main belt. Late-Scourian folds are confined to the shear belt and its margins. On the southern margin the folds are open, with near-vertical axial surfaces which strike E-W. Significantly, lateScourian folds become progressively tighter together with an increase in development of planar fabrics toward the centre of the shear belt which suggests that the folds grew in progressively changing orientations during shearing rather than being passively rotated. Continued fold growth in progressively changing orientations during the Late-Scourian and Laxfordian explains the extreme difficulty in distinguishing between late-Scourian and Laxfordian folds on style and orientation. The northern margin is more difficult to define owing to the heterogeneity of Laxfordian strain and the abundance of discordant granite sheets. However, an important change occurs where remnants of Scourian basic bodies become less deformed north-east of the Skerricha antiform, which is a late-Scourian arcuate structure defined by Chowdhary & Bowes (1972). This regional change, both of orientation of structures and finite strain, probably coincides with the northern margin of the shear belt. The entire zone has suffered heterogeneous simple shear (Beach & others, 1974; Coward, 1974; Davies 1974) although the southern part (Fig. 1) is the site of major late-Scourian folds with near vertical dipping axial surfaces, whereas in the northern part the axial surfaces of late-Scourian and Laxfordian major folds have moderate south-westerly dips. It is suggested that the southerly part, from the sub-vertical southern margin which runs NW to SE along the transition between granulites and retrogressed granulites
180
F. BRYAN DAVIES
o
o
Lf)
tf'l
.....
N
N
500 1 I
Z
- +
-- ---
- _
~O-O!Q
..,.-
_
65~10 _
_
I
--- -.,..
90-110
Fig. 1. Outline structural geological map of the shear belt. Solid lines, early Scourian axial surfaces; dots-dashes, late-Scourian axial surfaces; dashes, Laxfordian sinistral shear zones; hatched zone, region of granite sheets.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAR ZONE
181
south of Scourie to the granite sheets along Loch Laxford is a candidate for a partial section across a major sub-vertical ductile shear zone, and much of the following analysis refers to this zone. Fig. 1 is a structural map of this region which records the orientation of three important sets of structures; the axial surfaces of isoclinal early Scourian major folds which have a NE trend (with moderate NW dips) south of the shear belts, but have suffered progressive clockwise rotation into the Laxford front, sub-vertical axial surfaces oflate-Scourian folds, and discrete sub-vertical major Laxfordian shear zones. All of these indicate progressive dextral rotation from easterly or NE trends in the granulites ofthe Scourian complex to NW trends in the retrogressed granulites between Scourie and Loch Laxford. The early Scourian folds were described in detail by Davies (1976) and were shown to have developed axial surfaces with NW trends within the shear belt by the time the late-Scourian structures developed. Particularly significant to this work was the determination that within the shear belt thinner normal limbs of early Scourian synforms were consistently more deformed than thicker inverted limbs. The thinner limbs must have formed prior to late-Scourian folds since they are folded by them. L-S shape fabrics defined by clots of plagioclase crystal aggregates in a plagioclase-hornblende matrix occur in both folded Scourian basic sheets and metadolerites derived from Scourie dykes. At some localities metadolerites without penetrative tectonite fabrics cut across the L-S shape fabrics in the Scourian basics and at localities where both rocks carry fabrics significant differences in orientation and intensity of fabrics occur. These relationships demonstrate that an early fabric in the Scourian basics formed before dyke emplacement; its symmetry and orientation reflects the resultant of any original orientation and superimposed Scourian strain or a combination of Scourian and Laxfordian strains. Other lineations, produced by the intersection of the S-fabric and earlier surfaces which occur near to the margins of Scourian basic sheets and particularly along the margins of the metadolerites, deviate considerably from the orientation of the L-S shape fabrics. Whilst in the regions of low-moderate finite strains these two lineations can be distinguished, in the region of high finite strain a few kilometres south of Loch Laxford the distinctions become.very much more difficult to make as at maximum intensity the orientation of both types of fabrics coincides. The L-eomponent of an L-S shape fabric plunges more steeply in the less deformed than in highly deformed limbs of early folds, with plunges of 30-S0° SE in the thinned limbs; the lower value, commoner in limbs further modified by late-Scourian and Laxfordian deformation, corresponds to the orientation of the lineation within the metadykes which represents the Laxfordian movement direction (Beach, 1974). Lineations with this orientation are broadly co-axial with the axes of major late-Scourian folds with plunges of about 40° at 140° (see Davies, 1976, fig. Sc, S49). It is therefore deduced that from the time of thinning of the early Scourian fold limbs a L-component of the shape fabric plunged between 30-S0° SE and probably closely reflected the changing movement direction of the major shear zone. 4. ESTIMATES OF SHEAR STRAIN The shear strain profiles (Fig. 3) are determined from the orientation of the above markers relative to the shear zone margins, in the X-Z plane containing the movement direction (40° SE/1400) and perpendicular to the southern margin of the shear zone (80° SE/31 SO), i.e. the true profile plane (42° SE/0400). At the shear zone margin, the external angle between the zone and the axial surfaces of early Scourian folds in the X-Z plane is close to 90°, and that of both late-Scourian folds and Laxfordian shear zones varies between 40-4So. Calculation of shear strain values is a simple geometrical technique. In this analysis the axial surfaces of early
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F. BRYAN DAVIES
Fig. 2. Contoured map of plunge of linear fabric element. Within shear belt, solid shading 50°; solid line 40° contour; dashed line 30° contour.
Scourian folds are used as passive markers which also occur outside the shear zones and which can be used to estimate the finite shear strain across the southern margin of the shear belt by means of equation (i) (derived by Ramsay & Graham, 1970). (i) cot a'
= cot a -
y
where y is the shear strain required to deform an initial angle a between a marker line and the shear zone margin, to the deformed angle a'. The axial surfaces of both major and minor late-Scourian folds are parallel to the contemporaneous schistosity and are therfore considered to represent the orientation of the X-Y plane of the finite strain ellipsoid since the late-Scourian. This conforms to the pattern predicted for X-Y planes developed contemporaneously during simple shear in a homogeneous material (Ramsay & Graham, 1970), and so the shear strain since the late-Scourian was determined using equation (ii) (ii) tan
2f)
= 2/y
where y is the shear strain required to deform axial surfaces initially formed at 45° to the shear zone margin to the deformed angle 0'. However, the use of finite amplitude folds may lead to underestimates of displacement since it is not known whether significant shear strain would have occurred before finite amplitude folds developed. To minimise any possible discrepancy between actual and calculated displacements the orientation of axial surfaces of minor folds and schistosity were compared with those of local major folds and deviations never exceeded more than ±5°. This problem is further discussed in a later section. Two independent estimates of Laxfordian strain have been attempted. Firmly rooted in the literature is the observation that Scourie dykes are progressively rotated with increasing Lax-
PROGRESSNE SIMPLE SHEAR DEFORMATION ON TIlE LAXFORD SHEAR ZONE
Shear
st rai n
183
Y
(J'I
N
Zone of granites en
x: 3
Fig. 3. Dextral shear strain profiles from southern margin across section Y- Y' of shear belt. Solid line, shear strain from early Scourian; dot-dashes, shear strain from Iate-Scourian; dashed line, Laxfordian shears; shaded zone, Laxfordian shear strain determined from rotation of Scourie dykes.
fordian strain from steep NE dips in the south to SW dipping metadykes concordant with the foliation in the main shear belt (Sutton & Watson, 1962). Whilst the dykes make excellent markers for the qualitative estimates of Laxfordian strain outside the main shear belt the angle between them and the margin of the belt is smaller and more variable than the limits of accuracy of determination of the orientation of the margin and so quantitative estimates are difficult because small discrepancies due to errors or local variations in the initial angle mean possible large variations in the calculated shear strain. Little confidence is held in this result; it is included merely for comparison. The other method of estimating Laxfordian shear strain is from the overall rotation of Laxfordian shears trending 2700 outside the belt, which are progressively rotated into the shear belt. This is an estimate of the overall dextral Laxfordian strain although Laxfordian sinistral displacements clearly occurred along the minor shear zones (Beach, 1974). This apparent contradiction will be discussed later. In this analysis Laxfordian dextral shear strain is determined by treating both dykes and discrete shears as rotated markers by using equation (i), and their results can be compared. Since it is uncertain from the finite strain state whether Laxfordian shear zones making angles of 45° to the shear zone margin acted as passively rotated markers or reflected the XY-plane of the finite strain ellipsoid, shear strains calculated for markers making 45° to the margins produced by both processes are compared in Fig. 14. Within the zone of granites south of Loch Laxford all the above markers have suffered a finite rotational strain greater than that which can be accounted for by the simple shear model investigated here, and so this analysis refers to the region between the southern margin of the shear belt and the zone of granites where the analysis breaks down. The above uncertainties are general problems in understanding the strain histories of large-scale shear belts.
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F. BRYAN DAVIES
(8) Stages of Progressive Simple Shear Strain
Each set of structures is compatible with overall dextral displacements on the Laxford front in both Scourian and Laxfordian times. The results are recorded in Fig. 3 which shows that the shear strain (y) determined for each marker progressively increases from margin to centre reaching a maximum (y =10) in the region of supracrustal rocks which contains near concordant metadykes, south of Loch Laxford. Determinations of minimum displacements from the shear strain variations for each marker (summarised in Table I) indicate a minimum of 35 km of dextral displacement along the northern margin of the Scourian complex since the development of the early Scourian folds, approximately 18-20 km of which occurred during the late-Scourian and Laxfordian stages, and 10 km during the Laxfordian. Important results are that about 50 per cent of the determined displacement occurred before the development of late-Scourian folds and that maximum shear strain and minimum displacements are lower for the younger markers. Finite strains for each of the three determined stages of deformation (Fig. 4) indicate that the southern zone increased in width by about 2 km from the early Scourian to the Laxfordian (about 30 per cent increase in width) and that the maximum shear strain decreased for later increments; i.e. the gradients of the shear strain profiles for each increment decrease with progressive deformation. b. Resolution of the Method using Finite Amplitude Folds The relatively large proportion (45-50 per cent) of the finite dextral displacement estimated by this method to have occurred prior to late-Scourian deformation requires further consideration, since such a major shear strain component earlier than the late-Scourian deformation has not been previously recognised. The identification of this earliest component depends on the
)( 6
, (b)
, (a) 2 kms
(c)
kms Fig. 4. Stages in build up offinite strain for section Y- Y'. (a) Early Scourian shear strain. (b) Late-Scourian shearstrain. (c) Laxfordian shear strain.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAll. ZONE
185
x-x!
Fig. 5. Finite shear strain profiles from southern margin across shear belt to southern margin of granite sheets (S.M.G.).
-
•
Km 30
I
c
CIJ
E 20 CIJ u .!2 0.. III 10 0
)
/
•
•
/
..- ..2
/
4
6Km
Width Fig. 6. Displacement plotted against width from southern margin of shear belt to southern margin of gran ites.
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F. BRYAN DAVIES
resolution of the methods used to determine the shear strain displacement. It is particularly important to understand if the method of using the orientation of axial surfaces of late-Scourian folds significantly underestimates the finite displacement. To be true this would mean an underestimate of 50 per cent of the finite displacement. The development of finite amplitude folds is a major deviation of most major shear belts from the idealised parallel walled shear zone in a homogeneous material considered by Ramsay & Graham (1970). One method of testing the validity of the above method is by comparing the actual and calculated displacements from the axial surfaces of folds produced in a model shear zone. The model adopted was based on an idea of R. Graham (described at the Tectonic Studies Group meeting, 1974) where wet tissue paper, representing a thin competent layer was laid over two blocks free to slide along their mutual boundaries and so produce Reidel-like shears. In this experiment the blocks were merely the method of carrying the tissue layer and imposing a simple shear strain and only structures developed in the tissue layer were considered. Whilst it must be recognised that there are 5 orders of magnitude difference between the wrinkles in wet paper and the natural structure, the model seems a reasonable approximation for shear strains in a competent folded layer. The method adopted was to impart small displacements to markers on the tissue sheet (by shear movement of the supporting blocks) and to record the orientation of the axial traces of folds developed in the tissue. The folds developed in a parallel sided zone parallel to the slip boundary in the underlying blocks and the width of this zone increased during deformation. The results of a number of trials are recorded in Fig. 7. Over the shear strain range )'=0 to 1 (the most critical range of any possible discrepancy between actual displacement and that recorded by finite amplitude folds) it can be seen that after an initial stage during the development of folds, when use of their orientation underestimates the actual displacement across the shear zone in the tissue layer, the calculated displacement underestimates the actual value by less than 10 per cent, which is within the error of measurement of this experiment and well within the 50 per cent of the finite displacement estimated to have occurred before the development of late-Scourian structures. It seems reasonable to conclude that the use of axial surfaces of minor folds yields a result well within an error of 10 per cent of the actual displacement.
•
••
o
••
•
••
2 3 4 5 6 Actual displacement
Fig. 7. Calculated versus actual displacement for Reidel-type model.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON
ras
LAXFORD SHEAR ZONE
187
s. DEVIATIONS FROM SIMPLE SHEAR MODEL a. Wedge-Shaped Belt Although this work has concentrated on the southern margin of the shear belt, which has undergone ductile simple shear since at least the end of the granulite facies metamorphism, the northern zone of the belt also suffered shear strain at least in the earlier part of its evolution which does not simply fit the proposed model of a sub-vertical dipping shear zone. Axial surfaces and schistosity of late-Scourian and Laxfordian deformation which are sub-vertical in the southern zone, have moderate or sub-horizontal dips in the northern zone. Whilst the bulk ofthe Laxfordian complex has suffered high Laxfordian strains, eastern regions of the complex (east of the Laxford Bridge to Rhiconich road; Davies & Watson, 1977) have suffered extremely heterogeneous strain and relatively large areas of low finite strain containing early migmatite complexes and intrusive metadykes are bounded by zones of high Laxfordian finite strain dipping about 25°-50° SW. This change in orientation of XY-planes means that along the exposed strike length the shear belt narrows westwards with the northern margin dipping about 25°-50° SW: its intersection with the sub-vertical southern margin plunges 30°-40° SE which lies close to the movement direction of the shear belt. It is significant that the zone of Laxfordian granite sheets, suggested by Beach & others, (1974) to be a strain discontinuity between the Scourian and Laxfordian complexes, broadly marks the northern limit of the systematic rotation of structures which can be accounted for by the simple shear strain model outlined in this paper. In other words, north of this line the finite strain trajectories are the result of further Laxfordian rotational strain superimposed on the strain developed in the shear belt. Evidence presented in this paper of the regional deviation of the finite strains from the simple shear strain model which holds well over the southern part of the belt, allows a more specific delineation of the boundary of the region of superimposed strain. The southern part of the shear belt varies in width from the southern margin to the zone of granite sheets where the anaiysis breaks down. Estimates of finite shear strain determined from rotation of early Scourian isoclines along three sections which included the narrowest (western) and widest (eastern) section indicate that the finite dextral displacement across this part of the shear belt is constant at about 35 km, the shear strain across the belt changing with varying widths to accommodate constant offset. Contoured shear strain patterns are not however parallel, as predicted for simple shear deformation alone, but shear strain contours converge in the north-west where the strain is greatest. Thus, as well as the additional rotational component already identified there must have been a component of flattening in the west or extension in the east of the belt. These alternatives can be tested by using large-scale shape fabric symmetries which have been defined by using the orientation of the long axes of deformed mineral aggregates and poles to flattening surfaces described above. Several hundred elements from each of four sub-zones near the boundary of superimposed deformation define fields of elongation and fields of flattening (Fig. 10). Whilst it is impossible to split a belt of heterogeneous simple shear into zones of entirely homogeneous strain, it is suggested that the shape fabric symmetries might reflect the symmetry of the bulk finite strain ellipsoid in each domain. In these large-scale shape fabric patterns the surfaces separating the fields of elongation from the fields of contraction can be reasonably defined. In the south eastern domain these surfaces are planes intersecting in the Y-direction of the bulk strain ellipsoid whereas in the northern and western domains these surfaces define cones about the X-axis defined by the highest concentration of lineation orientations. These surfaces might then define the surfaces of NO FINITE LONGITUDINAL
c.
L--_ _---'"~_ __'
Fig. 8. Card-deck redefonnation of shear belt structures in erosion plane. Slip planes parallel to shear belt margin. (c) Present erosion surface pattern. (b) Pattern after redefonnation using sinistral Laxfordian shear profile corrected for horizontal section. (c) pattern after redefonnation using sinistral shear strain profile since late-Seourian corrected for horizontal section restoring type 2 (mushroom) interference patterns.
STRAIN (N.F.L.S.) and in the SE domain, where these surfaces are planes intersecting the Y-axis, the finite strain ellipsoid would have a K=l value which corroborates the simple shear deformation interpretation or a value of K~l. The fact that these surfaces which best define the boundaries are planes intersecting in the V-direction of the finite strain ellipsoid, and that the orientation ofthe relevant surface of N.F .L.S. closely coincides with the margin of the shear belt, suggest that this interpretation is correct for the SE domain. Further, the angle between the surfaces of N.F.L.S. have been compared to those calculated from the relevant strain ellipsoid of simple shear strain determined in this work. They compare well in the east of the shear belt but deviate markedly in the northern and western domains which record highest finite shear strains and indicate prolate strains. The plunges of mean X-directions vary from 30°-40°/140° in the domain compatible with simple shear alone but increase in the western and northern domains to plunges of 50°/150°. This means that the northern and western domains where the shear strain contours converge have suffered a superimposed component of flattening and rotation. Gentle southerly dipping regional late-Laxfordian shear movement planes suggested by Beach & others, (1974) to occur north of the granite sheets can account for this superimposed strain although it's initiation cannot be confidently dated.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON TIlE LAXFORD SHEAR ZONE
189
>10.
--'--,. O---~_1 O.~ 0.5
--------.
Fig. 9. Contoured finite dextral shear strain across southern part of shear belt. Note reduction in dextral shear strain in vicinity of sinistral shears.
190
NE
F. BRYAN DAVIES
NW
sw
Fig. 10. Large-scale shape fabric symmetries using long-axes of deformed mineral aggregates for four sub-zones covering shear belt zones B and C of Fig. 13. Dotted lines, boundaries between fields of finite elongation and finite contraction X, Y. &: Z principal axes of finite strain ellipsoid.
PROGRESSIVE SIMPLE SHEAR. DEFORMATION ON TIlE LAXFORD SHEAR ZONE
191
b. Strain Intensity Variation Although bulk finite strain has been shown to vary systematically across the shear belt, strain intensity is often heterogeneous from large-scale to crystal aggregate scale characterised by undeformed 'pods' flanked or enclosed by shear zones where the strain becomes extreme. As with shear strain distribution, strain intensity has varied both with time and position in the shear belt. Some elements of finite strain intensity variation in a spatial context have already been described (section 2) and are summed up in Fig. 1. An important feature is the increase in heterogeneity away from the centre of the shear belt displayed by both late-Scourian and Laxfordian structures (Fig. 13). Coupled with this is the transition within distinct Laxfordian shears from a ductile mode of deformation within the shear belt to a semi-brittle (mylonite products) and locally brittle deformation (cataclasite products) in the granulites south of the main shear belt. A puzzling observation which leads to an important conclusion concerning strain intensity variation with time is that whilst 50 per cent of the determined finite displacement took place perhaps during granulite facies metamorphism but certainly before the development of amphibolite facies assemblages, distinct minor shear zones flanking low strain pods are either of late-Scourian or Laxfordian age. This large early component of the finite displacement took place within the central part of the shear belt where finite strain intensity variation is least and can only reasonably be interpreted as due to an early stage of homogeneous deformation. This variation in the products of deformation with time must clearly have been related to the changing depth at which deformation took place (cf, Grocott, 1977) but from the above evidence must also have been related to inherited characters of the shear belt which made the central zone more ductile relative to the outer zones. Structures developed in all three stages of evolution of the shear belt converge within this central tract of homogeneous deformation, and it is therefore important to examine the products of the interaction of these structures. Within this tract, on a small scale, shape fabric symmetry changes have been investigated across shear zones within early basic rocks which contain equidimensional clots of crystal aggregates of mafic and felsic material in the undeformed state. Strain ellipsoid symmetries determined by the Rf/0 method (Dunnet, 1969) vary from K=O to K=oo from the margin to
B
6
X 4
2 2
4
~
6
8
Fig. 11. Strain symmetry variation across minor dextral shears. Arrows indicate direction of symmetry change into centres of shears.
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F. BRYAN DAVIES
-~
.. _- . -._----_.- --._ ... - - - --- _.
B _••• _.~_._.,.
_
6
¥ 4
2
o
3
6
9
12
15Cms
Distan ce
Fig. 12. Relationship between finite shear strain and qualitative characteristics of minor shear zones. (a) Shear strain range of grain size reduction. (b) Loss of earlier fabric elements, extreme grain size reduction.
centres of most shear zones greater than 1 m wide (Fig. 11). Few minor zones lie in the field where K=1. In zones of extreme strain, early layers and original mineral aggregates which define the tectonite fabrics become indistinguishable and shape changes cease to be observable in the amphibolites studied at about y=5~ in minor shears. L-S fabrics then appear extreme, axial ratios of mineral aggregates become indeterminable and low strain pods are lost, which agrees with the larger scale variation. This state is accompanied by considerable grain size reduction. Within the central tract the zones are annealed and it is difficult to identify strain variation on the crystal scale. Grain size reduction is obvious between y =5-7'5, whereas above y=7'5 earlier fabric elements are completely lost and extreme grain size reduction is dominant (Fig. 12). On a larger scale the interaction of sinistral shears with the ductile shear strain in the main belt can be examined. Although a few late-Scourian sinistral shears have been found, most are Laxfordian and so they cannot belong to a contemporaneous conjugate shear set. Most minor sinistral shears that extend into the major dextral shear belt occur in the narrower (western) part of the main belt where the shear strain contours are closer together. In the southern and western part of the belt a reduction in the overall dextral shear strain accompanies the minor sinistral shears. These relationships suggest that the minor sinistral shears are boundary effects where the main shear belt narrows and are a mechanism for accommodating some of the displacement on the narrower part of the shear belt. These curving sinistral shears break up the rock material outside the major shear belt into blocks which have moved against each other with opposite relative movement to the main shear belt (Figs 9 &13). They are probably analagous to the smaller scale shears separating lozenge shaped blocks described by Coward (1976). The interaction of the sinistral shears with the main belt has resulted in complex strain patterns where deformation has taken place by a combination of sinistral block sliding and simple shear. Corroboration of this interpretation may be obtained by comparing the actual displacement in the narrowest (western) end, with the hypothetical displacement obtained from a generalisation of the converging pattern of shear strain contours (Fig. 9). The general converged pattern in the narrow part would have produced a displacement of about 50-55 km compared with the actual displacement of 32-35 km. This difference compares well with the 20-25 km sinistral displacement computed by Beach (1974) to have taken place on the sinistral shears.
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON THE LAXFORD SHEAR ZONE
193
(0)
Ie)
Fig. 13. Sub-division of shear belts in terms of deformation intensity. ES, early Scourian dextral shear strain. LS, late-Scourian strain. Lax, Laxfordian dextral shear strain. A, Zone of discrete Laxfordian sinistral shears exterior to main ductile dextral shear belt, change from semi-brittle to ductile deformation at margin of main belt. Antipathetic rotation of axial surfaces outside dextral shear belt. B, Zone of interaction between discrete Laxfordian ductile sinistral shears and ductile dextral shearing. Note low finite Laxfordian strain lozenge-shaped blocks bounded by discrete ductile shears. C, Zone of reduction of discrete Laxfordian sinistral shears, low finite strain lenses present on megascopic and macroscopic scale but reduction in size and frequency towards zone D. D, Zone of late superposed strain.
6. CONCLUSIONS The Laxford front is a major shear belt which has undergone a progressive dextral simple shear deformation since the early Scourian. Most of the determined strain occurred in the Scourian and about 50 per cent of the finite displacement occurred before the Inverian (late-Scourian) episode. The early movement directions plunged about 40° SE and probably only 30° SE during the Laxfordian stage, and whilst vertical displacement components were important, transcurrent components played a major part in the evolution of the structure. At the very least, this might mean that earlier Archaean zonations might be obscured. The very nature of such a deformation system dominated by progressive heterogeneous simple shear means that structures outside the belt would have a different orientation to contemporaneous structures within the belt. The long strain history means that early structures were so strongly modified as to be often impossible to recognise as early components. Some Scourie dykes are located within late-Scourian minor dextral shear belts parallel to the margin of the major zone and often acted as loci of Laxfordian shears and therefore may have accommodated some Laxfordian deformation. 2
194
F. BR.YAN DAVIES
~ a'
Fig. 14. Graph of shear strain determined from markers treated as reflecting the XY-plane of the strain ellipsoid (yO') initiated at 45° to the margin of the shear belt versus shear strain determined from same markers treated as passively rotated markers (ra') that have suffered further rotation from 45° outside the shear belt. (For relevance see Fig. 13A.)
From the grade of the rocks involved, this shear belt must be primarily a structure of the middle-crust, and there is therefore the question of its expressions near the surface and at depth in the crust. Whilst widening of the belt upwards into a zone of ductile deformation would be compatible with the idea of a heterogeneous high-grade crust where deep seated granulites that deformed with a low ductility were overlain by amphibolite facies gneisses that responded to deformation with a high ductility (Watson, 1973), it is not taken as evidence of a continued wedge-shape throughout the crust. Knowledge of the history and progress of deformation on the Laxford front is important since a number of similar NW trending belts occur throughout the mainland Lewisian. On the type of evidence used above, the shear belt at Loch Inver has long been recognised to have existed in the late-Scourian (Inverian) and the maps of Sheraton, Tamey, Wheatley & Wright, (1973) indicate a component of finite dextral displacement, and vertical displacements have also been recognised (Evans & Lambert, 1974). The pattern of NW trending shear belts continues further south in the Gairloch-Torridon region, where these have previously been interpreted as due mainly to vertical displacements (Sutton & Watson, 1969; Park, 1970). Very few regions of Archaean rocks in the mainland Lewisian record low finite strains, so it may be more valuable-to interpret this high grade crust as part of a large shear belt that has suffered heterogeneous deformation. Another question of regional importance is the relationship between displacements on the Laxford shear belt and the grade of metamorphism of the complexes juxtaposed by these movements. The relative south side (Scourian complex) up displacement on the Laxford front is compatible with the movement needed to juxtapose the Scourian and Laxfordian complexes required by the petrological model of Sheraton, Skinner & Tamey, (1973) which suggested that the Scourian granulites were coeval with the Laxfordian complex but originated at deeper crustal levels and therefore suffered the depletion associated with the formation of deep-seated granulites. The evidence summarised by Dickenson & Watson (1976) that deep-seated granu-
PROGRESSIVE SIMPLE SHEAR DEFORMATION ON TIlE LAXFORD SHEAR ZONE
195
lites were transformed into successively lower pressure assemblages and that the vertical displacement which took place from early Scourian to late-Laxfordian times was of the order of 15-20 km agrees well with the vertical displacement component determined from this work, which is strong evidence that the actual displacement was the rising of the Scourian granulite complex; the radiometric age of 2800 Ma probably indicates the beginning of its rise upwards in the crust. Such vertical displacements, approaching the order of thickness of the normal crust, present space problems in a crust of normal thickness unless new material is added at its base. It is significant that the early stage of this movement was probably broadly contemporaneous with the period of crustal thickening postulated to account for the occurrence of deep-seated granulites at the surface in what is now a crust of normal thickness. Whereas factors such as grain size reduction (Watterson, 1975) and perhaps hydraulic fracturing influence the rate of deformation (Beach, 1976) and probably the susceptibility of such shear zones in the high-grade crust to further deformation, the origin of such a major structure requires further explanation. Several workers (Bak, Korstgard & Sorensen, 1975; Grocott, 1977) have suggested that similar major shear belts in west Greenland are deep-seated expressions of what would be major fault zones near the surface. With displacements of the order of thickness of the crust or, in the case of the Greenland shear belts, the lithosphere, the only likely modem analogues of these Precambrian belts are the major transform fault systems. Such transform systems are often characterised by high heat flow and this might agree with the evidence of this work that a large proprtion of the displacement on the Laxford shear zone took place during granulite facies metamorphism. If these are Archaean-Proterozoic analogues of transforms then they may be structures for maintaining strain compatibility between early ridgeor trench-features. ACKNOWLEDGMENTS The author thanks Professor J. Watson, B. Windley, M. P. Coward, J. Hossack, R. J. Lisle, J. Watterson and A. Beach for discussions of earlier drafts of this paper. REFERENCES BAK,J.,J. KORSTGARD& K.SORENSEN.1975. A major shear zone within the Nagssugtoqidian of west Greenland. Tectonophysics, 27, 191-209. BEACH, A. 1974. The measurement and significance of displacements on Laxfordian shear zones, N.W. Scotland. Proc. Geol. Ass., 85(1), 13-21. BEACH, A. 1976. The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in the Lewisian complex. Phil. Trans R. Soc. Lond., A.280, 569--604. BEACH, A., M. P. COWARD & R. H. GRAHAM. 1974. An interpretation of the structural evolution of the Laxford front. Scott. I. Geol., 9(4), 294-308. BOTT, M. H. P., J. G. HOLLAND, P. G. STORRY & A. B. WATTS. 1972. Geophysical evidence concerning the structure of the Lewisian of Sutherland, N.W. Scotland. II geol. Soc. Lond., 128,599--612. BRIDGWATER, D., A. ESCHER & J. WATTERSON. 1973. Tectonic displacements and thermal activity in two contrasting Proterozoic mobile belts from Greenland. Phil. Trans. R. Soc. Lond., A.273, 513-33.
CHOWDHARY, P. K. & D. R. BOWES. 1972. Structure of Lewisian rocks between Loch Inchard and Loch Laxford, Sutherland. Krystallinikum, 9,25-51. COWARD, M. P. 1974. Flat-lying structures within the Lewisian basement gneiss complex of N.W. Scotland. Proc. Geol. Ass., 85(4),459-72. COWARD,M. P., R. H. GRAHAM, P. R.JAMES&J. WAKEFIELD. 1973. A structural interpretation of the northern margin of the Limpopo orogenic belt, southern Africa. Phil. Trans. R. Soc. Lond., A.273, 487-91. COWARD, M. P. 1976. Strain within ductile shear zones. Tectonophysics, 34, 181-97. DA VIES, F. B. 1974. The evolution oftwo assemblages ofmetamorphosed supracrustal and intrusive rocks in the Lewisian complex. Unpublished PhD thesis, University of London. DAVIES. F. B. 1976. Early Scourian structures in the Scourie-Laxford region and their bearing on the evolution of the Laxford Front. II geol. Soc. Lond., 132(5), 543-54. DAVIES, F. B. & J. V. WATSON. 1977. The early
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history of the type Laxfordian complex, N.W. Sutherland. JI geol. Soc. Lond., 133(2), 123-33. DICKINSON, B. B. & J. V. WATSON. 1976. Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of N.W. Scotland . Precambrian Research, 3,363-74. DUNNET, D. 1969. A technique of finite strain analysis using elliptical particles . Tectonophysics, 7, 117-36. EVANS, C. R. & R. St. J. LAMBERT. 1974. The Lewisian of Loch Inver, Sutherland; the type area for the Inverian metamorphism.JI geol. Soc. Lond., 130, 125-50. GROCOTT, J. 1977. The relationship between Precambrian shear belts and modem fault systems. JI geol. Soc. Lond., 133, 257-62. PARK, R. G. 1970. The structural evolution of the Tollie antiform-a geometrically complex fold in the Lewisian, north east of Gairloch, Ross-shire. Q. JI Geol. Soc. Lond., 125,319-49. PARK, R. G. & D. CRESSWELL. 1973. The dykes in the Laxfordian belts. In R.G.Park,&J.Tamey,(eds.). Early Precambrian of Scotland and related rocks of Greenland. Keele. 119-30. PEACH, B. N., J. HORNE, W. GUNN, C. T. CLOUGH, L. W. HINXMAN & J. J. H. TEALL. 1907. The geological structure ofthe N.W. Highlands ofScodand. Geol. Surv. Gt . Brit ., Mem. Geol , Surv., Seot., 1907: 191-252. RAMSAY, J. G. & R. G. GRAHAM. 1970. Strain variations in shear belts. Can. JI Earth Sci., 7(3), 786-813.
SHERATON,J. W., A. C. SKINNER&J. TARNEY. 1973 . The geochemistry of the Scourian gneisses of the Assynt district. In Park, R. G. & J. Tamey, (eds.) . The early Precambrian history ofScotland and related rocks of Greenland. Univ. Keele. SHERATON, J. W., J. TARNEY, T. J. WHEATLEY & A. E. WRIGHT. 1973 . The structural history of the Assynt district . In Park, R. G . & J. Tamey, (eds.) . The early Precambrian of Scotland and related rocks of Greenland. Univ. Keele . SUTTON, J. & J. V. WATSON. 1951. The preTorridonian metamorphic history of the Loch Torridon and Scourie areas in the N.W. Highlands and its bearing on the chronological classification of the Lewisian. Q. II geol. Soc. Lond., 106,241-308. SUTTON, J. & J. V. WATSON. 1962 . Further observations of the margin of the Laxfordian complex near Loch Laxford, Sutherland. Trans. R. Soc. Edin ., 65, 90-106. SUTTON, J. & J. V. WATSON. 1969. ScourianLaxfordian relationships in the Lewisian of N.W. Scotland. Geol. Assoc., Can., Spec. Paper,.5, 119-28. WATSON, J. V. 1973. Effects of reworking on highgrade gneiss complexes. Phil. Trans. R. Soc. Lond., A.273, 443-55. WATTERSON, J. 1975. Mechanism for the persistence of tectonic lineaments. Nature, 253, 5492 , 520-1. Received 15 October 1976 Revised version received 10 July 1977