Tectonophys~cs, 88 (1982) 347-360 347

Elsevier Scientific ~blishing Company, Amsterd~-Printed in The Netherlands

AbCUATE LINEATION TRENDS IN A DEEP LEVEL, DUCTILE THRUST

BELT, SYROS, GREECE

JOHN RKDLEY

Granr Institute of Geology, University of Edinburgh, Edinburgh (Great Britain)

(Submitted June 12, 1981; revised version received December 15, 1981)

ABSTRACT

Ridley, J., 1982. Arcuate lineation trends in a deep level, ductile thrust belt, Syros, Greece. In: G.D.

Williams (Editor), Strain within Thrust Belts. Tecfonophysics, 88: 347-360.

The metamorphic rocks of blueschist facies on the Aegean island of Syros show structures which

indicate deformation in a ductile thrust zone at 40-50 km depth. The associated dominant linear fabric

shows systematic variations in orientation across the island, tracing out a series of arcuate patterns on

various scales. Differing ~~rost~ctural development along these lineation arcs suggests that they are the

result of localized and variable superim~~d wrench shears in the overall thrust regime. The angular

relations of these microstructures show a ciose correspondence with those predicted theoretically from the

superimposition of thrust and wrench shears and a possible irrotational component.

INTRODUCTION

Much of the Aegean island of Syros (Fig. 1) consists of a structural succession of metasedimentary and meta-igneous rocks, 7.5 km thick metamorphosed to blueschist facies assemblages. This metamo~~sm has been dated as early Tertiary, probably Eocene by the K-Ar technique (Bonneau et al., 1980). The assemblages jadeite + quartz and zoisite, paragonite f quartz, seen in metabasites, indicate PT conditions at the climax of metamorphism of approximately 500°C at greater than 14 kbar pressure (Dixon, 1976).

The whole succession is seen to be intensely deformed. The dominant fabric is a flat lying, layer parallel schistosity almost completely penetrative (Dixon, 1969). Scattered intrafolial tight to isoclinal folds folding this schistosity show a constant sense of asymmetry and a constant vergence to the south or southeast. These features both indicate deformation with horizontal, thrust sense, shear. The struct- ural ~nte~retation of the rocks now exposed on Syros is as part of an exhumed, deep level (40-50 km depth), ductile thrust belt.

0040- 195 1/82/oooO-0000/$02.75 0 1982 Blsevier Scientific Publishing Company

348

2

Thiro a

0

Fig. 1. Location map for Syros and the Cyclades, Greece.

There is no penetrative deformation later than this ‘thrusting’; deformation

ceased with the cessation of prograde blueschist facies metamorphism. The textures

record the later stages of the tectonothermal event when the ‘peak’ mineralogies were

stable yet being deformed and annealed in the folding observed. Static overprinting

by down-pressure greenschist facies mineralogies is sporadic so enough textural

detail is preserved to allow some analysis of the tectonic processes that took place

during this event.

This paper describes the variation in orientation and aspect of asymmetric

microfolds across the island, a distance of a few kilometers, and suggests that they

record wrench displacements superimposed on an overall thrust regime. Similar

inte~retations have been suggested for certain, possibly analogous features of near

surface thrust belts, e.g. anomalous oblique fold trends and intra-thrust sheet tear

faults (Dahkstrom, 1970; Coward and Kim, 198 1; Rattey and Sanderson, 1982, this

volume).

DESCRIPTION OF FABRICS

Foliation

The rocks of interest in this paper are the micaceous schists which generally show

the best development of small-scale structures and form about half the total

349

Marbles

Fig. 2. Simplified geological map of Syros showing the major lithoiogicai units and late faults.

structural section seen on the island. They are interleaved with marbles and some

metabasites (Fig. 2) and are essentially quartz-mica schists with variable amounts of

glaucophane, garnet, epidote, chlorite, calcite, sphene, opaque minerals and locally

lawsonite.

The dominant schistosity is formed from the alignment of mica plates, glaucophane

needles and lenticular quartz aggregates. Fine scale, 1 - 10 mm, but variable repeated

micaceous and quartzose layering seen locally show that the foliation is in part an

350

351

inherited and transposed sedimentary fabric. Elsewhere there are indications that the mica schistosity cross-cuts a previous layering though the two surfaces are now

almost parallel. Small-scale intrafolial folding always folds an existing fabric without new axial

planar grain growth; mica grains and glaucophane welts are simply bent and fractured or annealed around fold hinges. A progression of textures can be seen, folds tightening until a point is reached after which further strain is taken up solely on the limbs which show a dearth of quartz and concentration of opaque minerals and mica and the effective formation of a second schistosity (Fig. 3a). Rarely, fold interference structures are seen, always of type-3 geometry (Ramsay, 1967) and have exactly parallel fold hinge directions suggesting formation within a single deforma- tion event.

The mica schistosity clearly formed prior to the nucleation of these folds and hence the geometries seen do not reflect the total strain suffered by the rocks. The minimum strain indicated is however large, several hundred percent. The fold

forms, notably amplitude variations along axial planes (Berthe and Brun, 1980) are most consistent with a strain regime dominated by simple shear. The nucleation of individual folds through the pile need not have been synchronous and hence it is not possible to make quantitative comparisons of strain magnitude across the pile using fold profiles.

Lineations

The schists almost always show a lineation on foliation surfaces. In hand specimen this appears as open crenulations of the mica fabric on the scale of the grain size of the micas (OS-2 mm). The lineation is composite, resulting essentially from the tight asymmetric folding of the mica fabric described above, but also frequently from aligned glaucophane needles, elongate quartz aggregates, fold hinges in quartz veins and thin continuous quart&c bands and sometimes quartz strain shadows about garnets. It generally keeps a constant orientation over a single outcrop and can perhaps best be visualized as an intersection lineation between the original schistosity and an often cryptic second axial pianar foliation. fn the rest of this paper it will be termed the dominant Iineation.

-- Fig. 3. Photomicrographs of quartz-mica schists from Syros. The plates are all orientated as if the gross

lithological layering was horizontal. For localities see Fig. 4. a. Typical microfolding as seen over most of

the island with flat lying asymmetric folds and an incipient second schistosity along fold limbs. b. Schist

with isolated gtaucophanes showing the bimodaiity of the linear fabric; glaucophane prisms are orientated

parallel to the fold hinges only immediately at them and are elsewhere appro~mately pe~endicular. c.

Inclined and weakly asymmetric folds with the preservation of a lithological layering from the belt of

NNW trending lineation. d. Tighter, almost symmetrical folds with axial planes close to perpendicular to

the gross layering. Fields of view: (a) 15 mm, (b) 4 mm, (c) 5 mm, (d) 8 mm.

352

---+ Trend of the

dominant lineation

==+Glaucophone

lineation

0 1 2 3km

Fig. 4. Trend of the lineation in the main structural unit of Syros. The ‘thrust’ shoun in the north is an

almost continuous serpentinite horizon which breaks the succession into two mechanically distinct units.

Locally, in lithologies with isolated large glaucophane needles in a quartz and

mica matrix. there is a second lineation formed from the alignment of these

gfaucophanes variably inclined to the dominant lineation but tending towards

orthugonality. Figure 3b shows the two lineations and the relationship between

them. The fold hinge seen is parallel to the do~nant ~ineation; gIaucophane prisms

353

are orientated parallel to the hinge only at the hinge and elsewhere are approxi-

mately perpendicular to it (cf. Schwerdtner, 1970).

Figure 4 shows the trend of the dominant lineation across the island and reveals a

series of large scale arc-like patterns with various degrees of tightness. The lineation

plunge is never steep, The thrust shown is an almost continuous horizon of

serpentinite subparallel to the schistosity which appears to have acted as a mechani-

cal discontinuity during the bulk of the deformation. The lineation arcs above and

below this horizon do not match. Also shown is the trend of the separate glaucophane

lineation where developed.

Late, approximately NE-trending open folds affect all the penetrative structures.

The variation in the glaucophane lineation can be effectively removed by unfolding

these folds (assuming a similar folding model and some layer parallel shortening), to

give a constant trend at 140 * IO*. Unfolding only has a minor effect on the arcuate

patterns of the dominant lineation as it trends nearly parallel to the axial traces of

these folds. The local orientation of these folds may well have been influenced by the

presence of a pervasive linear fabric in the rocks.

South of and beneath the main serpentinite horizon the arcuate patterns are open

and broad. The microfolding varies little in aspect when viewed along the lineation

over the whole area. The angle between the fold axial planes and the previous

foliation ranges between about 10’ and 25”. Possible asynchronous fold nucleation

and visible changes in fold tightness over distances of l-2 m perpendicular to the

foliation mask any systematic changes in fold geometry around the lineation arcs.

Note that the glaucophane lineation is only developed where the angle between it

and the dominant lineation is greater than about 40”.

North of (i.e. above) the serpentinite horizon the patterns show no relation to

those below. Here there is a much tighter arcuate pattern in which there is a clear

contrast in microfabric development between the areas of ENE and NNW trending

lineations.

Where the lineation trends ENE the microstructures seen are not markedly

different from those in the rest of the island, though perhaps here indicating a higher

strain: fold hinges are more frequently seen as isolated lenses surrounded by a

re-formed continuous mica schistosity. The angle between this schistosity and the

lithological layering remains small and often unmeasurable.

Where the dominant lineation trends NNW a clearly different microstructural

development is seen. In hand specimen the lineation appears much stronger,

repeated tight microfolding giving rock fracture resembling pencil cleavage and

overwhelming the foliation. In detail, fine quartz-rich and micaceous layering is

often preserved.

Figures 3c and d show two examples of quartz-mica schists from this belt of

NNW trending lineation. Figure 3c shows the good preservation of a lithological

layering, the micaceous layers show close to tight asymmetric folding largely

localized within one single layer. This folding differs from the predominant textures

354

described above in showing a high angle between the microfold axial planes and the

original schistosity, here about 45”. Figure 3d shows a more extreme example with

tight, repeated, disharmonic and almost symmetrical folds with axial planes at

approximately 70” to the original schistosity. Note garnets show extension cracks

perpendicular to the fold axial planes and no strain shadows in this section

orientation.

DISCUSSION AND MODELLING

Arcuate lineation patterns could be formed by a variety of processes, the most

likely would seem to be the following:

(1) A later heterogeneous strain or folding superimposed on a previous lineation

giving local reorientation (Ramsay, 1967).

(2) Reorientation of the lineation and fold hinges as passive markers during a

single deformation (Sanderson, 1973 and Escher and Waterson, 1974).

(3) Heterogeneous strain during the event that produced the lineation.

On Syros there are no structures that could be assigned to an appropriate later

deformation event. The variation in the lineation orientation seen in the sequence

above the serpentinite would require local reorientation of up to 80” and corre-

sponding strains of a few hundred percent.

Observed folds in outcrop are cylindrical, no sheath forms have been found. This

suggests that they were active rather than passive elements during deformation

(Cobbold and Quinquis, 1980), as would be expected in a layered micaceous and

quartzose rock with marked competence contrasts. The variation in microstructural

style along the arcs precludes passive reorientation being a mechanism and suggests

that differing strain fields through the rock pile were the fundamental cause of the

arcuate patterns.

Analysis of this possi-ble variation in strain requires a knowledge or estimation of

the tectonic displacement direction or u-direction of shearing. The constant vergence

and facing of minor asymmetric folds suggests a thrust sense of displacement

somewhere towards south or southeast. Schwerdtner (1970) and Ramsay and Stuart

(1973) suggest that in foliated rocks amphiboles and other linear elements will tend

to be aligned parallel to the major axis of the strain ellipse in the plane of the

foliation (the sectional ellipse). If the strain approximates to simple shear then the

discordant glaucophane lineation described above is a close estimate of the orienta-

tion of the displacement vector. This surmise is supported by the presence of

fractured and pulled apart glaucophane and epidote crystals aligned parallel to the

dominant lineation in the far south of the island. Here the dominant lineation is only

20”-30” from the suggested displacement direction of 140”. In such a situation a

strong component of elongation parallel to the dominant lineation would be ex-

pected.

The implication is that on Syros there are localized belts where thrust related

355

folds formed oblique to or had their axes rotated towards par~leljsm with the displacement direction. Similar localized oblique structures in southern Cornwall have been shown by Rattey and Sanderson (1982, this volume) to be related to differential forward movement of thrust segments giving localized wrench tectonics.

t $5 konstant)

*’ L,,u,,,

4 9

60 I b&h

Fig. 5. Theoretical and actual plots of the angle between the fold axial planes and lithological layering (S, ,,AP) against the orientation of the dominant lineation with respect to the Y-axis of thrust shear (L ,, Y,). The variation is in each case with increasing wrench shear (y,): {a) keeping thrust shear constant (yh) and varying the shear parallel elongation (A), taken up vertically, (b) keeping the shear parallel elon~tion constant with various thrust shears ( y,, ), (c) keeping thrust shear constant with shear parallel elongation taken up along thrust strike, (d) data from Syros pfotting the tightest folds from any one area.

356

The total strain fields produced by superimposing wrench and thrust shears and

their variation with the addition of layer parallel shortening or extension have been

analysed by Coward ( 1980), Coward and Kim ( 198 1) and Sanderson ( 1982, this

volume). On Syros the only measurable variables reflecting reliably the change in the

strain field are the lineation orientation and the angle between the fold axial planes

and the original schistosity, These are both related to the Xl’-plane of the finite

strain ellipsoid, One can use fhe same mathematical treatment as Sanderson (this

volume), (Flinn, 1979. gives the theoretical basis), rearranged to produce theoretical

plots of lineation orientation against fold axial plane inclination and their coupled

co-variation with changing strain parameters.

Typical plots are shown in Fig. 5; all are orientated with wrench shear (y, )

increasing to the right. as measured by the rotatiol~ of the donlinant lineation away

from the expected direction parallel to the h-axis of thrust shear. The vertical axis in

each case is the expected angle between the original schistosity and the subsequent

fold axial planes, i.e. the inclination of the XY-plane of the finite strain ellipsoid.

The essential feature to notice is that for all cases with a sufficiently large thrust

shear ( y,, > 4) the inclination of the XY-plane of the finite strain ellipsoid does not

increase s~gl~~fi~ant~y until wrench shears exceed about 2. A corollary of this is that

the lineation may deviate by up to 60’ from the expected orientation perpendicular

to the displacement before a significant change in the geometry of the minor

structures is seen.

The constructed lines assume a constant thrust shear along the lineation arcs. It is

equally possible to have a coupled decrease in thrust shear as wrench shear increases.

This is possible given the constraint of constant area on the bounding surfaces of the

shear zone (Ramsay and Graham, 1970) which may or may not apply. Approximate

plots can be drawn up using the data in Fig, S and showing that the effect on the

microstructures would be slight given the large strains inferred for Syros.

Moderate strains paiallel to the thrust strike have little effect. Fig. 5c shows that

the only significant effect is to change the magnitude of wrench shear required to

produce a certain geometry.

Data from Syros is plotted on the same axes (Fig. 5d), distinguishing between the

separate sequences above and below the serpentinite horizon. The fit is good but not

for a unique set of strain parameters. It is possible to increase the thrust shear ( Yh)

and decrease the shear parallel longitudinal strain (X ). or vice versa. without

seriously affecting the fit.

There is a clear qualitative correspondence between the geometries of the minor

structures seen on Syros and those predicted from superimposed and synchronous

thrust and wrench shears. In the lower part of the pile with broad open arcuate

patterns, the microfolding on the west coast, where the lineation trends at approxi-

mately 100’. is indistinguishable from that on the east coast, where it trends at

40”-50*. This would correspond to an increase in wrench shear from y, = 0 to about

y, = 1.6- 1.8, from west to east giving a theoretical increase in fold axial plane

357

inclination of 5”-8” or about 40% which would not be noticeable.

The two samples described above with contrasting upright folding (Figs. 3c and

d) both come from an area where the dominant lineation is at only about 10” to the

displacement direction. The analysis suggests that in these cases the wrench shear is

comparable to or greater than the thrust shear. The two samples show differing

textures; that of Fig. 3c is from a locality where the lineation trends 12’ further from

the displacement direction and shows markedly more asymmetric fold forms. The

modelling suggests that at least a doubling of the wrench shear is required to give the

extra rotation; the separation in the field is about 200 m. Little theoretical or

experimental work has been attempted on the shearing of a layered sequence with

the b-axis of shear perpendicular to the layering (the work of Graham, 1978 assumes

different mechanics). Intuitively a geometric strain softening might be expected as

the micaceous layers rotate (an artifice of folding), towards the &plane of shearing.

We may be seeing here, in this rapid increase in recorded wrench shear, an analogue

of shear zone localization in isotropic rocks.

It is not possible to quantify either the form or the magnitude of finite strain from

these structures alone. The components (X, yh, y,) can be added together in a

number of ways to produce the same result.

The nature of the shear parallel strain, if any, may be a tectonic indicator but it

can not be deduced from the above. An extra control would be provided by reliable

three dimensional strain markers. As pointed out by Coward and Kim (1981) and

Sanderson (1982, this volume) the longitudinal strain affects the shape of the finite

strain ellipsoid. Shear parallel shortening produces an oblate fabric, elongation a

prolate one.

Metaconglomerates are found in the sequence on Syros. They are comprised of

competent quartz-jadeite clasts in a schistose micaceous matrix and as a whole

clearly deform inhomogeneously. The shape fabrics are however invariably oblate.

Vialon (1979) suggests that a component of dilatancy perpendicular to the

foliation is required before asymmetric folding can nucleate in a shear zone. Such

folding is pervasive on Syros and early layer parallel quartz veining is locally

common. The dilation is, however, only required for the nucleation stage of folding.

CONCLUSIONS

There is a close fit between the observed changes in microfold geometry across

the island of Syros and those predicted theoretically if localized wrench shear is

synchronous and superimposed on an overall thrust regime. Moderate irrotational

strain can be added to the total strain field without qualitatively affecting the

geometry.

The exact ‘in thrust’ geometry is not clear from the limited area1 extent of Syros.

The suggestion is of relatively forward-moving lobes along the thrust length bounded

either by diffuse wrench zones (as in the lower sequence on Syros) or fairly sharp

zones (as in the upper).

358

Figure 6 is a schematic sketch of the overall structure envisaged. The continuation

or termination of the lobes along strike is open to speculation. There is a suggestion

from a few anomalous NW-trending fold axes seen on the north coast of Syros

above the serpentinite horizon, that neighbouring ‘lobes’ may interfere or overprint

(Coward, 1981). Alternatively they may join at backward pointing cusps or be

bounded by simple wrench zones with a classical sigmoidal shear zone geometry.

Whatever the lateral geometry is, it is clear that within this deep level thrust zone

there are vertical and lateral changes in the form and magnitude of strain on an

‘Aegean island scale’ of a few kilometers. These heterogeneities must be mapped out

before a thrust or displacement direction is inferred from an analysis of the minor

structures. The overthrust direction of 140 i IO0 consistent with the structures

described above is in agreement with that obtained from the neighbouring island of

Andros (Papanikolaou, 1979) and is parallel to rather than perpendicular to the

Aegean Arc. The regional significance of this will be discussed elsewhere but the

northwesterly directed movement is more consistent with the Aegean blueschists

being related to the Izmir-Ankara belt in Anatolia (Sengiir and Yilmaz, 1981, and

references therein) rather than the Vardar ‘ocean’ of mainland Greece.

Fig. 6. Schematic sketch of the relationships of the minor structures to the varying strain above and below

the serpentinite horizon.

359

ACKNOWLEDGEMENT

This work was carried out during the tenure of a N.E.R.C. research studentship

which was gratefully received. I thank Drs. J.E. Dixon, M.R.W. Johnson and R.F.

Cheeney for critically reading through the manuscript and suggesting improvements.

I also thank Dr. J.E. Dixon for introducing me to Syros and to the staff at I.G.M.E.

for help and support while in Greece.

REFERENCES

Berthe, D. and Brun, J.P., 1980. Evolution of folds during progressive shear in the South Armorican shear

zone, France. In: J. Cameras and S. White (Editors), Shear Zones in Rocks. .I. Strttct. Geol., 2:

127-133.

Bonneau, M., Geyssant, J., Kienast, J.-R., Lepvrier, C. and Malinski, H., 1980. Tectonique et metamor-

phisme Haute Pression d’Lge Co&me dans les Hell&ides; exemple de I’ile de Syros (Cyclades, Grece).

C.R. Acad. Sci. Ser. D, 291: 171-174.

Cobbold, P.R. and Quinquis, H., 1980. Development of sheath folds in shear regimes. In: J. Carreras and

S. White (Editors), Shear Zones in Rocks. J. Struct. Geol., 2: 119-126.

Coward, M.P., 1980. The analysis of flow profiles in a basaltic dyke using strained vesicles. J. Geol. Sot.

London, 137: 605-615.

Coward, M.P., 1981. Fold nappes: examples from the Maine and Himalayan thrust zones. Meeting on

Strain within Thrust Belts, Cardiff, 1981, presented paper.

Coward, M.P. and Kim, J.H., 1981. Strain within thrust sheets. In: K.R. McClay and N.J. Price (Editors),

Thrust and Nappe Tectonics. Geol. Sot. London, Spec. Publ., 9: 275-292.

Dahlstrom, C.D.A., 1970. Structural geology in the eastern margin of the Canadian Rocky Mountains.

Bull. Can. Pet. Geol., 18: 332-406:

Dixon, J.E., 1969. The Metamorphic Rocks of Syros, Greece. PhD Thesis, Cambridge University

(unpublished).

Dixon, J.E., 1976. Glaucophane schists of Syros, Greece. Bull. Sot. G&o]. Fr., (7), 18: 280 (Abstr.)

Escher, A. and Waterson, J., 1974. Stretching fabrics, folds and crustal shortening. Tectonophysics, 22:

223-231.

Flinn, D., 1979. The deformation matrix and the deformation ellipsoid. J. Struct. Geol., 1: 299-308.

Graham, RX, 1978. Wrench faults, arcuate fold patterns and deformation in the southern French Alps.

Proc. Geol. Assoc., 89: 12% 142.

Papanikolaou, D., 1979. Contribution to the Geology of the Aegean sea: The island of Andros. Ann.

Gtol. Pays Hell., 29(2): 477-533.

Ramsay, D.M. and Stuart, B.A., 1973. An analysis of noncylindrical and incongruous fold patterns from

the Eo-Cambrian rocks of Soroy, northern Norway. II. The significance of synfold stretching lineation

in the evolution of noncylindrical folds. Tectonophysics, 18: 109- 122.

Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York, N.Y., 568 pp.

Ramsay, J.G. and Graham, R.H., 1970. Strain variation in shear belts. Can. J. Earth Sci., 7: 786-813.

Rattey, R.P. and Sanderson, D.J., 1982. Patterns of folding within nappes and thrust sheats: examples

from the Variscan of southwest England. In: G.D. Williams (Editor), Strain within Thrust Belts.

Tectonophysics, 88: 247-267.

Sanderson, D.J., 1973. The development of fold axes oblique to the regional trend. Tectonophysics, 16:

55-70.

Sanderson, D.J., 1982. Models of strain variation in nappes and thrust sheets. In: G.D. Williams (Editor),

Strain within Thrust Belts. Tectonophysics, 88: 201-233.

360

Schwerdtner, W.M., 1970. Hornblende lineations in Trout Lake area, Lx la Rouge map sheet,

Saskatchewan. Can. J. Earth Sci.. 7: 884-899.

Sengor, A.M.C. and Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach.

Tectonophysics, 75: 18 I-242.

Vialon, P., 1979. Les deformations continues-discontinues des roches anisotrope. Eclog. Geol. Helv.. 72:

531-550.