Palaeomagnetic evidence for clockwise rotations related to dextral shear along the Southern Troodos Transform Fault, Cyprus

250

Earth and Planetary Science Letters, 99 (1990) 250-262 Elsevier Science Publishers B.V., Amsterdam

[CH]

Palaeomagnetic evidence for clockwise rotations related to dextral shear along the Southern Troodos Transform Fault, Cyprus A. Morris, K.M. Creer and A.H.F.

Robertson

Department of Geology and Geophysics, University of Edinburgh, West Mains Roa~ Edinburgh, EH9 3JZ (U.K.) Received November 2, 1989; revised version accepted April 6, 1990 ABSTRACT The Southern Troodos (Arakapas) fault represents a Late Cretaceous oceanic fracture zone which was locally disrupted during Late Cretaceous-Eocene palacorotation of the Troodos microplate. Opinions are divided as to the sense of displacement along the transform fault and the exact timing of initiation of palaeorotation. Palaeomagnetie studies of Turonian zeolite fades lavas and sediments exposed within the transform domain have revealed considerable variations in the declination of remanent magnetisation between sites along the Arakapas fault belt, the western margin of the Limassol Forest Complex and the eastern flank of the Troodos ophiolite. At seven sites clockwise rotation of fault blocks has occurred about steeply inclined axes. One fault block at the western end of the Arakapas fault belt has experienced a net anticloekwise rotation, while at six other sites only simple tilting about sub-horizontal axes is indicated. The overall clockwise sense of block rotation and initial dyke strikes calculated at three sites are consistent with right-lateral slip along the transform. Cross-cutting relationships revealed by the analysis of one site demonstrate that these rotations took place during crustal genesis and are not a product of post-spreading disruption of the fracture zone. Sites located further south, in the Eastern Limassol Forest Complex, show no relative rotation with respect to the main Troodos ophiolite to the north and have experienced only simple tilting. However, if our results are considered in conjunction with existing palaeomagnetie data from the area, it appears that the entire area lay within a complicated zone of localised and predominantly clockwise block rotations produced by dextral slip along the transform. Some areas were rotated by over 100 o about steeply inclined axes, whereas others experienced only simple tilting. The whole area was later subjected to a bulk 90 o anticlockwise rotation along with the Troodos microplate. Data obtained from the umbers and radiolarites (Perapedhi Fro.) exposed on the eastern flank of the Limassol Forest block indicate that at least 30 °, and possibly up to 45 °, of this 90 ° rotation took place over a maximum of 15 Ma.

1. Introduction

2. G e o l o g i c a l s e t t i n g

R e c e n t p l a t e t e c t o n i c studies h a v e d e m o n strated the difficulty of measuring deformation a n d s t r a i n t a k i n g p l a c e in o c e a n basins. W h i l e t h e development of side-scan sonar and other remote sensing techniques have made direct observation of the ocean floor possible, much important inform a t i o n a b o u t p r o c e s s e s at p r e s e n t - d a y c o n s t r u c tive m a r g i n s a n d t r a n s f o r m f a u l t s c a n still b e g a i n e d b y s t u d y i n g a n c i e n t o p h i o l i t i c terrains. However, field structural studies frequently only tell us a b o u t h o r i z o n t a l c r u s t a l m o v e m e n t s (i.e. t h r u s t i n g ) , since r o t a t i o n s a b o u t s t e e p l y i n c l i n e d a x e s a r e o f t e n n o t a p p a r e n t in t h e field. A u n i q u e p r o p e r t y o f t h e p a l a e o m a g n e t i c t e c h n i q u e is t h a t it c a n b e u s e d to d o c u m e n t t h e s e r o t a t i o n s . H e r e w e p r e s e n t t h e results o f s u c h a s t u d y in t h e T r o o d o s ophiolite of Cyprus.

It is n o w well e s t a b l i s h e d t h a t t h e T r o o d o s massif represents an uplifted fragment of Late Cretaceous Neotethyan oceanic crust which f o r m e d in a s u p r a - s u b d u c t i o n z o n e s e t t i n g [1]. Subsequently, the Troodos ophiolite underwent a 90 ° a n t i c l o c k w i s e r o t a t i o n as a d i s t i n c t m i c r o p l a t e d u r i n g the L a t e C r e t a c e o u s ( C a m p a n i a n ) t o E a r l y E o c e n e [2-3]. A c o m p l e t e o p h i o l i t e s t r a t i g r a p h y is p r e s e r v e d w i t h i n t h e massif, w i t h t e c t o n i s e d h a r z burgites, cumulate gabbros, sheeted dykes, mafic e x t r u s i v e s a n d a n in situ p e l a g i c s e d i m e n t a r y c o v e r . R e c e n t r a p i d u p l i f t h a s r e s u l t e d in a c o n c e n t r i c o u t c r o p p a t t e r n w i t h t h e d e e p e s t s t r u c t u r a l levels e x p o s e d in t h e c e n t r e (Fig. 1). T h e m a i n o p h i o l i t i c m a s s i f is b o u n d e d to the s o u t h b y t h e A r a k a p a s f a u l t belt. T h i s m a j o r e a s t - w e s t l i n e a m e n t h a s a s t r i k e t h a t is p e r p e n d i c -

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PALAEOMAGNETIC EVIDENCE FOR CLOCKWISE ROTATIONS

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Fig. 1. Simplified geological m a p of the Limassol Forest Complex and the southern margin of the main Troodos ophiolite (adapted from Simonian and Gass [5]), showing the locations referred to in the text. The genetic term "Southern Troodos T r a n s f o r m Fault" has been proposed [7] to describe the combination of the Arakapas fault belt and that part of the Limassol Forest Complex formed within the transform zone. Note the progressive change in the trend of dykes into near parallelism with the Arakapas fault belt over a distance of 10-15 km.

ular to the general dyke trend observed in the sheeted dyke complex of the main massif, and represents the remnants of a Late Cretaceous oceanic transform fault [4]. To the south of the fault belt lies the ophiolitic crust and mantle of the Limassol Forest Complex. This consists of strongly deformed, predominantly mantle sequences generated within a "leaky" transform zone [5,6], together with a small fragment of crust that was possibly formed at an "Anti-Troodos" spreading axis [7]. The term "Southern Troodos Transform Fault" has been proposed [7] to describe the combination of the Arakapas fault belt and that part of the Limassol Forest Complex to the south which was formed within the transform zone. Recent debate has centred on the sense of offset of the spreading axes along the fossil transform. This contribution aims to address this problem and at the same time to demonstrate that rotational strain plays an important part in deformation occurring within transform zones. At present, within the sheeted dyke complex of the Troodos massif there is a general north-south

orientation of dykes. However, as the Arakapas fault belt is approached a progressive change in dyke trend into eventual alignment with the transform lineament is seen (see Fig. 1). This observation has led to the suggestion [5] that the dykes were injected into a sigmoidal stress field that operated across the transform zone (see Fig. 2a). This would imply that the transform operated as a sinistral fault system between dextrally displaced spreading axes. This interpretation was supported by a suggestion that abundant N E - S W orientated mafic and picritic dykes within the Western Limassol Forest Complex were injected along en echelon fractures oblique to the principal eastwest direction of shear, in response to sinistral transform movement [6]. In addition, large phacoidal blocks with N E long axis orientations were found entrained within serpentinite shear zones. The obliquity of these blocks to the predominantly east-west trending shear zones was believed to be due to rotation associated with sinistral shearing [6]. An alternative explanation for the swing in

252

A. M O R R I S E T AL.

a) Dykeinjectionintoa sigmoidalstress~/K~fij/~ eld ~J~ t

'11' b) Rotationof faultblocks bydextralshear

,11,

Fig. 2. Possible alternative settings in which deviations in dyke trend could occur close to the Southern Troodos Transform Fault (adapted from Clube and Robertson [3]): (a) shows dyke injection into a sigmoidal stress field operating across a sinistrally slipping transform between dextrally offset spreading axes; (b) shows dyke deviation attributed to frictional rotation of fault blocks related to dextral slip along the active transform domain. In this model, clockwiserotation of fault blocks would be expected to occur between the sinistrally offset spreading axes.

dyke trend is that right-lateral movement along the transform has frictionally rotated originally n o r t h - s o u t h striking dykes around vertical axes into alignment with the fault lineament (see Fig. 2b). This would therefore require sinistral offset of ridge segments along the transform. This spreading configuration is more consistent with the geology of Cyprus and was previously incorporated into a tectonic model for the palaeorotation of the Troodos microplate within the regional plate tectonic framework of the Eastern Mediterranean [1,31. The geological arguments are therefore conflicting but palaeomagnetic studies provide a direct method of determining whether either of the models apply. The first palaeomagnetic data obtained from the Troodos ophiolite [8] demonstrated that a stable westerly directed magnetisation vector was retained by the extrusive series, indicating that a 90 o anticlockwise rotation had taken place. Subsequent studies [2] have confirmed this result

and have further shown that there was no rotation of the ophiolite during its formation in the Late Cretaceous [3]. The structurally corrected remanence direction of declination 276 °, inclination 32 ° [8] may therefore be assumed to represent the geomagnetic field direction at the time of magnetisation of the ophiolite. This direction is hereafter referred to as the Troodos magnetisation vector (TMV) [9]. If the deviations in dyke trend observed to the north of the Arakapas fault belt are a primary feature of the oceanic crust, as required by the sigmoidal stress field model, then magnetisation vectors obtained from units within the deviation zone and along the transform should cluster around the TMV. In contrast, a deviation of magnetisation vectors away from the T M V would indicate that significant tectonic rotations have taken place. In this paper we present new palaeomagnetic results obtained from the extrusive and sedimentary sequences of the Arakapas fault belt and the Limassol Forest Complex to the south. These data confirm that tectonic rotations of fault-bounded blocks have occurred within the transform zone and yield important information concerning the sense of offset along the Troodos spreading axis. In addition, we report data that further constrain the timing of the initial stages of palaeorotation of the Troodos microplate as a whole.

3. Choice of sampling sites We have obtained orientated cores mainly from zeolite facies pillow lavas, massive lava flows, dykes, sills and interlava sediments exposed around the periphery of the Limassol Forest block and at a single site on the eastern flank of the Troodos ophiolite (see Fig. 3). The sampled units form part of the Lower Pillow Lava (LPL) and U p p e r Pillow Lava (UPL) units and are assumed to be of Turonian age [10]. In addition, small orientated rock chips were taken from the overlying metalliferous umbers and radiolarites of the Perapedhi Formation, of assumed Turonian to C a m p a n i a n age [11]. A total of 24 sites were collected, 19 of which yielded reliable palaeomagnetic results. Sites were located away from areas of intense shearing and brecciation. Visibly weathered and altered exposures were avoided. Samples were only

PALAEOMAGNETIC EVIDENCEFOR CLOCKWISEROTATIONS

collected when an accurate structural tilt correction could be defined. Sites with only minor structural tilt were preferred (normally less than 40 o), to minimise any declination errors due to tectonic rotation about inclined axes. Tilt corrections were based on the attitude of massive lava flow units, together with primary lamination in interlava sediments and in the overlying umbers and radiolarites. The orientation of the margins of any dykes and sills present were also recorded. Between five and twenty independent samples were collected at each site. Cores were orientated using both magnetic and sun compasses, whereas rock chips were orientated using a magnetic compass only. 4. Results

4.1. Rock magnetic characteristics Magnetic measurements were carried out using a Molspin fluxgate spinner magnetometer for the pillow lava, massive flow, sill and dyke samples, and a 2-axis C C L superconducting magnetometer for the more weakly magnetised interlava sediments, umbers and radiolarites.

90

NC

0

i

~

253

The frequency distributions of the natural remanent magnetisation ( N R M ) intensities of the igneous samples and the sediments show clear log-normal distributions. The mean intensity of the lava samples is 1.3 A m -1 which is substantially lower than the values of 13.0 A m -1 found for the extrusives of the main Troodos ophiolite by the Cyprus Drilling Project [12] and 6.0 A m -1 found for the upper levels of oceanic layer 2 sampled by D S D P Leg 83 [13]. However, this value is comparable to the mean intensity of 1.16 A m - 1 found for the U p p e r Pillow Lava unit by Vine et al. [14]. The mean intensity of the sediments is two orders of magnitude lower than that of the lavas, but is much greater than the noise level of the cryogenic magnetometer (5.0 # A m - 1 ) . The rate of acquisition of isothermal remanence (IRM) in fields up to 1.0-1.5 T has been studied for at least two samples per site. Typical results, shown in Fig. 4, indicate that magnetite is the main magnetic mineral present in all the sampled lithologies except for the interlava sediments, where a continuous increase in the I R M for fields higher than 0.2 T indicates the presence of a higher coercivity fraction. The dominant presence

10 km

Fig. 3. Geological map showing the location of the 19 palaeomagnetic sites. The "clock" diagrams show the 95~g confidence limits associated with the tilt corrected mean declination and inclination at each site (see Fig. I for the location of site MP at Mosphilati on the eastern flank of the Troodos ophiolite).

254

A. M O R R I S E T A L .

of magnetite is confirmed by thermomagnetic analyses carried out on magnetic extracts prepared from lava samples showing Curie points of 580°C. These Curie points may indicate that the lavas have been altered by sea-floor weathering, since unaltered lavas tend to include titanomagnetites with significantly lower Curie temperatures [15]. Magnetite Curie points have also previously been obtained for the umbers of the Perapedhi Formation [16]. Stepwise alternating field (AF) demagnetisation of a minimum of two samples per site was carried out up to peak fields of 100 m T to define cleaning fields for the remaining samples. AF treatment was found to be effective in removing all the magnetisation, even in the interlava sediment sampies where high coercivity minerals were identified by the IRM analyses. This indicates that any high coercivity minerals present do not contribute significantly to the N R M of these samples. Typical Zijderveld demagnetisation diagrams are shown in _Fig. 5 for sites situated along the Arakapas fault belt, in the Eastern Limassol Forest Complex and at Mosphilati on the eastern flank of the Troodos ophiolite. Apart from a minor northdipping secondary component, attributed to viscous magnetisation in the present field direction, single stable components of magnetisation were isolated in fields of less than 20 mT for all samples studied. Cleaning fields of 10, 15 or 20 mT were subsequently applied to the remaining samples at each site. After demagnetisation, within-site scatter is small, as shown by the stereographic projections

RU 01 A

DH 01 B

SIRM= 26.0 Am -1

of Fig. 6. A reduction in a95 and increase in the precision parameter K after tilt correction at sites where variations in palaeohorizontal orientation were recorded indicates that magnetisation predates folding. No reversed sample polarities were identified, as may be expected for units formed during the long Cretaceous normal polarity period [4].

4.2. Palaeomagnetic results The results obtained from our 19 reliable sites before and after application of a standard tilt correction are reported in Table 1. Results have been grouped into four sub-areas and the site means for each area are shown in the "clock" diagrams of Fig. 3. These diagrams show the 95% confidence limits associated with the mean declination and inclination at each site [17]. Considerable variations in the declination of cleaned remanence directions occur between sites along the Arakapas fault belt, the western margin of the Limassol Forest Complex and the eastern flank of the Troodos ophiolite at Mosphilati (Fig. 3). Seven sites show more northerly declinations than the TMV, indicating that these sites have experienced a net clockwise rotation with respect to the Troodos ophiohte. Two adjacent sites at the western end of the Arakapas fault belt have declinations in the SW quadrant, indicating a net anticlockwise rotation of the sampled block. Declinations at the remaining six sites are indistinguishable from that of the TMV. By contrast, sites located further south in the Eastern Limassol Forest Complex, near Asgata (in

SIRM=1.68 A m

1.0-

1.o-

a:

fl LL

40.0 ~ ,

;Z

_¢ (.D

VL 01 B

-1

i

10o

Applied field (mT)

10'00

1.0-

A m -1

2.0

41

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LL

IJ. 10

SIRM=5.78

0

10

100

Applied field (mT)

lOO0

10

1(30

1000

Applied field (mT)

Fig. 4. Examples of normalised IRM acquisition curves and coercivity spectra for (a) pillow lava, (b) umber and (c) interlava sediment specimens. Magnetite is the main magnetic mineral present in all the sampled lithologics except for the interlava sediments, where a higher coercivity fraction also occurs but which does not contribute to the NRM.

PALAEOMAGNETICEVIDENCEFORCLOCKWISEROTATIONS

255

extrusives and interlava sediments, whereas the inclinations of the umbers and radiolarites are generally low. These discrepancies cannot be attributed to latitudinal drift during crustal genesis since the variations are unsystematic. Alternative possibilities are that: (1) Physical rotation of remanence carriers has occurred during compaction of the sediments studied. This could explain the shallow inclina-

the "Anti-Troodos" plate [18]), exhibit consistent westerly directed declinations. N o relative rotation has therefore occurred between these sites and the Troodos microplate to the north. Three previously reported sites at Asgata, Kalavassos and Parekklisha [16] gave similar results. The mean inclination of all sites (36 o) is comparable to the inclination of the TMV (32°). However, significant variations are seen within the

N

W m=

-

UP

RK01 A

NIUP E

Interlava mudstone

z o o6 A

Picritic pillow basalt MD 04 A

\

Low

NrUP

Sheeted dyke screen in pillow lavas

N

EP01 A W . . . . . . . .

i

'E

/sIow

UP

Dyke in pillow lavas

W W'

~

'E

'E

MD I O A

MP01 A

Interlava sediment

Umber N

UP

AS 08 B Pillow lava

MP 1 3 B

DH01 B

Pillow lava

Umber

Fig. 5. Typical Zijderveld plots of A F demagnetisation data for a range of lithologies. Single stable components of magnetisation were isolated for all samples. A minor north-dipping secondary component seen in some cases was removed in fields of less than 20 mT.

256

A. M O R R I S E T AL.

N

N

N

N

N

N

Margi on the northeastern margin of the Troodos o p h i o l i t e [16]. It h a s b e e n s h o w n t h a t i n t h i s a r e a the u m b e r s u n d e r w e n t c o m p a c t i o n i n t o h o l l o w s in t h e s u r f a c e o f t h e T r o o d o s lavas, r e d u c i n g t h i c k n e s s e s b y u p t o 50% [19]. (2) T h e a t t i t u d e o f t h e p a l a e o h o r i z o n t a l r e l a tive t o t h e p a l a e o f i e l d w a s i n c o r r e c t l y i d e n t i f i e d a t

Fig 6. Stereographic projections of cleaned sample directions at six typical sites. After demagnetisation, within-site scatter is small. Significant deviations of magnetisation directions away from the Troodos magnetisation vector (TMV) are attributed to predominantly clockwise block rotations occurring within the transform domain.

s e v e r a l sites. T i l t c o r r e c t i o n s a r e d i f f i c u l t t o d e f i n e in s u b m a r i n e extrusive sequences. Se a f l o o r lava slopes can b e i n c l i n e d at a c o n s i d e r a b l e angle to the h o r i z o n t a l so that r e c o r d e d p a l a e o h o r i z o n t a l s f r e q u e n t l y r e p r e s e n t p a l a e o s l o p e s . I t is k n o w n t h a t lavas along the Arakapas fault belt were erupted down significant slopes into hollows in the breccia t e d b a s e m e n t [5]. T h e a p p l i c a t i o n o f a s i m p l e tilt

tions o b s e r v e d in the P e r a p e d h i F o r m a t i o n u m bers and radiolarites. Similar inclinations have

c o r r e c t i o n m a y t h e r e f o r e b e u n j u s t i f i e d in such

b e e n f o u n d in u m b e r s a n d radiolarites e x p o s e d at

cases.

TABLE 1 Palaeomagnetic results Site

Lithology

N

In situ

Tilt corrected

MBC

Dec

Inc

a95

K

Dec

Inc

232 224 317 313 267 338 167 270 266 268 307 358 284

14 18 49 45 33 59 75 15 - 3 11 - 5 33 21

4.4 2.4 9.6 3.4 4.8 7.4 9.5 9.0 6.1 8.0 3.7 5.1 10.9

121 773 34 112 198 33 24 46 35 92 95 101 31

231 240 317 311 269 314 345 267 270 266 304 358 274

28 30 36 29 22 45 30 51 23 30 28 34 29

4.4 2.4 5.5 2.6 4.8 7.4 9.5 8.7 5.9 8.0 3.2 5.1 9.8

121 773 101 193 198 33 24 49 38 92 127 101 39

330/14 250/37 223/13 212/16 200/12 182/23 255/75 006/37 314/32 019/20 056/34 112/01 078/22

Western margin of Limassol Forest Complex KA1 Flows/Sills 7 343 KA2 TAS Pillows 4 26 RU UPL Pillows 8 292

57 29 46

6.2 12.3 5.9

95 57 88

277 39 304

22 3 40

6.2 12.3 5.9

95 57 88

151/72 003/59 275/14

Arakapas TR SA MD ZO RK EP LY VL VR PU1 PU2

Fault Belt UPL Pillows UPL Pillows UPL Dykes UPL Sediments UPL Pillows UPL Seds/dyke UPL Pillows/dyke LPL Sediments LPL Haem. Shales Perapedhi Umbers Perapedhi Rads Perapedhi Umbers Perapedhi Umbers

Eastern flank of Troodos massif MP UPL Pillows Perapedhi Umbers

10 6 8 17 6 13 11 7 17 5 17 9 7

0t95

K

9 10

294 284

4 - 3

6.3 7.1

68 47

305 286

33 17

6.3 7.1

68 47

340/43 344/23

Eastern Limassol Forest Complex AS UPL Pillows/seds 14 AC Perapedhi Umbers 5 DH Perapedhi Umbers 9 AU Perapedhi Umbers 4 Perapedhi Rads 8

266 293 269 285 304

2 41 21 26 23

6.2 3.1 8.2 11.8 7.0

42 597 40 61 63

282 282 277 274 306

46 39 26 23 20

6.2 3.1 5.8 11.8 7.0

42 597 81 61 63

326/54 115/13 288/20 107/24 270/05

N = number of samples; a95 = semi-angle of 95% cone of confidence; K = Fisher precision parameter; MBC = strike and dip of mean bedding correction (strike direction anticlockwise from dip direction).

PALAEOMAGNETIC

EVIDENCE

FOR CLOCKWISE

257

ROTATIONS

A further criticism of the s t a n d a r d tilt correction is that it a r b i t r a r i l y divides the t o t a l m o t i o n e x p e r i e n c e d at a site i n t o a tilt plus a r o t a t i o n a b o u t a vertical pole. D e c l i n a t i o n a n o m a l i e s m a y arise where r o t a t i o n has t a k e n p l a c e a b o u t an i n c l i n e d axis [20]. V a r i a t i o n s in d e c l i n a t i o n a l o n g the t r a n s f o r m m a y therefore b e p a r t i a l l y d u e to errors arising f r o m the use of the simple tilt correction. T o o v e r c o m e these p r o b l e m s , a m e t h o d of resolving net tectonic r o t a t i o n p a r a m e t e r s [9] has b e e n a p p l i e d to the d a t a in the p r e s e n t study. 4. 3. Derivation o f net tectonic rotation parameters A l l e r t o n a n d Vine [9] d e s c r i b e an analysis which yields b o t h the initial o r i e n t a t i o n of a s a m p l e d unit a n d the p o l e a n d a m o u n t of tectonic r o t a t i o n which has affected the unit in o n e o p e r a t i o n . T h e m e t h o d was o r i g i n a l l y u s e d in the sheeted d y k e t e r r a i n of the Solea g r a b e n on the n o r t h e r n f l a n k of the T r o o d o s o p h i o l i t e [9], b u t is also a p p l i c a b l e to the p a l a e o h o r i z o n t a l case [21]. T h e r e are four a s s u m p t i o n s in the m e t h o d : (1) the o b s e r v e d stable m a g n e t i s a t i o n was a c q u i r e d b e f o r e d e f o r m a t i o n t o o k place; (2) a reference m a g n e t i s a t i o n v e c t o r c a n be f o u n d which represents the g e o m a g n e t i c field d i r e c t i o n at the time of m a g n e t i s a t i o n ; (3) d y k e s are i n t r u d e d vertically a n d original b e d d i n g / s i l l o r i e n t a t i o n s are as close to h o r i z o n t a l as possible; a n d (4) n o i n t e r n a l def o r m a t i o n o f the s a m p l e d unit has o c c u r r e d so the angle fl b e t w e e n the m a g n e t i s a t i o n v e c t o r a n d the p o l e to the d y k e / b e d is c o n s t a n t d u r i n g d e f o r m a tion [9]. T h e analysis involves f i n d i n g a single p o l e of r o t a t i o n which s i m u l t a n e o u s l y restores the s a m p l e m a g n e t i s a t i o n v e c t o r (i.e. the in situ r e m a n e n c e d i r e c t i o n ) b a c k to the T r o o d o s m a g n e t i s a t i o n vect o r and the p r e s e n t b e d d i n g (dyke) p o l e as close to the vertical (horizontal) as possible, while conserving the angle ft. T h e net tectonic r o t a t i o n is d e s c r i b e d b y the d e c l i n a t i o n a n d i n c l i n a t i o n of this p o l e of r o t a t i o n , a n d the angle of r o t a t i o n ; a positive angle r e p r e s e n t i n g an anticlockwise r o t a tion [9]. A n e x a m p l e of this analysis for the p a l a e o h o r i z o n t a l case is given in Fig. 7, using d a t a o b t a i n e d f r o m site A S ( l o c a t e d in the E a s t e r n L i m a s s o l Forest Complex).

N

Fig. 7. An example of the palaeohorizontal case of the technique of Allerton and Vine [9] using data from site AS. The cleaned in situ magnetisation vector for this site (SMV; dec = 266 o, inc = 2 ° ) makes an angle fl of 44 ° with the present pole to the bedding (PBP). If this angle is conserved during deformation then a circle of radius fl centred on the TMV gives the locus of all possible initial bedding poles. The pole chosen in the analysis (IBP) is that which is closest to the origin of the stereonet and corresponds to the most horizontal initial bedding dip possible. The position of the net tectonic rotation pole which restores the SMV to the TMV and the PBP to the IBP is then found by constructing the great circle bisectrices of both pairs of vectors; the intersection of the great circles gives the pole of rotation and the angle of rotation is readily measured. In this case, 45 o of clockwise rotation has occurred about a horizontal axis.

T h e net tectonic r o t a t i o n p a r a m e t e r s f o u n d b y a p p l y i n g this t e c h n i q u e to o u r d a t a are given in T a b l e 2, while the stereonet of Fig. 8 shows the d i s t r i b u t i o n of r o t a t i o n poles. Sites l o c a t e d a l o n g the A r a k a p a s fault belt w h i c h e x h i b i t e d significant tilt c o r r e c t e d declination anomalies, with respect to the T M V , have e x p e r i e n c e d large m a i n l y clockwise r o t a t i o n s a b o u t i n t e r m e d i a t e to steeply i n c l i n e d axes. I n s o m e cases r o t a t i o n angles exceed 100 o. R o t a t i o n poles for these sites cluster in the southwest q u a d r a n t , e x c e p t for site M P on the eastern flank o f the T r o o d o s o p h i o l i t e where m o r e n o r t h e r l y axes are found. T h o s e sites with tilt c o r r e c t e d d e c l i n a t i o n s close to that o f the T M V have poles o f net tectonic r o t a t i o n close to the p r i m i t i v e of the s t e r e o n e t of Fig. 8 a n d angles of r o t a t i o n c o m p a r a b l e to the structural d i p s at the sites. R o t a t i o n a l d e f o r m a t i o n here has b e e n l i m i t e d to s i m p l e tilt a b o u t subh o r i z o n t a l a n d m a i n l y s t r i k e - p a r a l l e l axes.

258

A.

TABLE 2 Net tectonic rotation parameters Site

Pole of rotation

TR SA MD dykes sediments ZO RK EP Stage 1 EP Stage 2 LY VL PU1 PU2 KA1 KA2 RU MP pillow lavas numbers AS AC DH AU

170/67 091/42 204/67 193/69 078/55 178/48 281/54 247/54 201/25 149/05 205/89 252/06 326/03 335/66 239/54 325/33 352/16 311/01 138/23 302/07 266/09

Angle of rotation 48 51 -44 - 37 8

-52 -118 -92 20 41 82 23 64 141 -33 -51 -38 - 45 -18 -22 24

A t three sites along the A r a k a p a s fault belt ( M D , R K , EP), it has b e e n p o s s i b l e to d e t e r m i n e the initial strike of single a n d sheeted d y k e s within the U p p e r Pillow Lavas. F o r each site the analysis y i e l d e d two solutions for the d y k e o r i e n t a t i o n a n d

N

MORRIS

ET

AL.

net tectonic rotation. T o d e c i d e which s o l u t i o n was correct, the p i l l o w lavas a n d s e d i m e n t s at the sites were r e s t o r e d to their original o r i e n t a t i o n s using b o t h solutions. I n each case o n e s o l u t i o n gave o v e r t u r n e d p a l a e o s l o p e s a n d was rejected. T h e c a l c u l a t e d initial d y k e strikes were 320 o, 315 o a n d 317 o for sites M D , R K a n d E P respectively. A s s u m i n g t h a t d y k e s were injected perp e n d i c u l a r l y to the m i n i m u m p r i n c i p a l stress axis, this indicates that the stress ellipsoid h a d a n orient a t i o n c o n s i s t e n t with dextral slip a l o n g the transform. A t site E P there is a significant difference between the c l e a n e d r e m a n e n c e d i r e c t i o n s o b t a i n e d from the s a m p l e d d y k e a n d the s u r r o u n d i n g pillow lavas. It seems therefore that tectonic r o t a t i o n at this site was s y n c h r o n o u s with crustal genesis. U s i n g the t e c h n i q u e of A l l e r t o n a n d Vine [9], the c o m p l e t e r o t a t i o n a l h i s t o r y for this site m a y b e d e t e r m i n e d : (1) pillow lavas were e r u p t e d a n d then r o t a t e d b y 118 ° clockwise a b o u t a n i n c l i n e d axis ( d e c l i n a t i o n 281 ° , i n c l i n a t i o n 5 4 ° ) ; (2) d y k e injection o c c u r r e d along a strike of 317 o; a n d (3) a further clockwise r o t a t i o n of 92 ° a b o u t a n inclined axis ( d e c l i n a t i o n 247 °, i n c l i n a t i o n 54 ° ) affected b o t h the pillows a n d the dyke. T h e p o l e s of r o t a t i o n for this site are s h o w n o n Fig. 8 as E P Stage 1 a n d E P Stage 2. T h e m o r e r o b u s t net tectonic r o t a t i o n analysis a p p l i e d here c o n f i r m s that s u b s t a n t i a l clockwise r o t a t i o n s of small-scale fault b l o c k s (100's of metres to several k m in size) a b o u t steeply inclined axes have o c c u r r e d w i t h i n the t r a n s f o r m zone. C r o s s - c u t t i n g r e l a t i o n s h i p s revealed b y the analysis at site E P d e m o n s t r a t e the s y n m a g m a t i c n a t u r e of these rotations. O n the o t h e r h a n d , r o t a t i o n a l d e f o r m a t i o n at o u r sites in the E a s t e r n L i m a s s o l F o r e s t C o m p l e x to the s o u t h of the fault belt has b e e n shown to b e r e s t r i c t e d to s i m p l e tilt a b o u t s u b - h o r i z o n t a l axes.

4.4. Constraints on the timing of palaeorotation

Fig. 8. Stereographic projection showing the poles of net tectonic rotation derived for each site using the method of Allerton and Vine [9] (circles = poles of clockwise rotation, triangles = poles of anticlockwise rotation). Poles close to the primitive indicate simple tilting about near-horizontal axes. Poles close to the origin of the stereonet indicate rotation about near-vertical axes.

Previous p a l a e o m a g n e t i c d a t a have e s t a b l i s h e d that the 90 o p a l a e o r o t a t i o n o f the T r o o d o s microp l a t e b e g a n s o o n after its o c e a n i c genesis in the T u r o n i a n , with at least 60 o of r o t a t i o n o c c u r r i n g b y the e n d of the L a t e Palaeocene. R o t a t i o n was essentially c o m p l e t e b y the e n d o f the E a r l y E o c e n e [3,16]. It a p p e a r s then that the t i m e p e r i o d represented b y the P e r a p e d h i F o r m a t i o n ( T u r o n i a n to

PALAEOMAGNET1C EVIDENCE FOR CLOCKWISE ROTATIONS

Campanian) is a critical interval during which much of the rotation m a y have taken place. At two sites (AU, 0.5 k m southwest of Asgata and VR, 0.5 km southwest of Vavla) we have sampled both the metalliferous brown umbers and the stratigraphically overlying radiolarites of the Perapedhi Formation. Cleaned remanence directions for both sites are listed in Table 1. The mean declination of the stable remanent magnetisation carried by the umber samples is indistinguishable from that of the T M V (i.e. 276°). This is in contrast to the more northerly directions recorded by the radiolarite samples. N o relative rotation has occurred between these sites and the main ophiolitic massif to the north. Therefore, s i n c e the umber-radiolarite sequences sampled are stratigraphically continuous, we may assume that the observed declination difference is due to a 30 o anticlockwise rotation of both the Troodos microplate and the sampled area of the Eastern Limassol Forest Complex. In addition, radiolarite sequences exposed near Margi on the northeastern margin of the Troodos ophiolite [16] yielded a mean declination of 296 o. Significantly, two samples from within the top three metres of the radiolarite interval at Margi showed stable magnetisations with even more northerly declinations of 321 ° [16]. These results therefore indicate that at least 30 o and possibly up to 45 o of the 90 o anticlockwise rotation of the Troodos microplate took place over a m a x i m u m of 15 Ma, between Turonian umber deposition and the end of Campanian radiolarite deposition.

5. Discussion of spreading axis configuration It is now widely accepted that the Southern Troodos Transform Zone represents a fossil fracture zone which linked originally east-west orientated segments of a Neotethyan spreading axis [5]. We have shown that significant variations in declination of primary magnetisation between sites located within the transform zone are due to real tectonic rotations and not apparent rotations caused by the application of inappropriate structural corrections. Our results demonstrate that predominantly clockwise rotation of small faultbounded blocks about steeply inclined axes has occurred within the fracture zone. The results are

259

consistent with dextral shear along the transform and hence a sinistral offset between the Troodos and "Anti-Troodos" ridge segments. Original dyke orientations recovered at three sites along the Arakapas fault belt indicate that dykes were injected along a N W strike direction (present coordinates). This orientation is consistent with dyke intrusion in a sigmoidal stress system at a dextrally slipping transform. Our results are consistent with a preliminary study of five sites located along the Arakapas fault belt [3,16] in which northwesterly remanence directions were recovered from U p p e r Pillow Lavas pillows and interlava sediments. In addition, significant variations in the direction of magnetisation vectors have been reported [22] at thirteen dyke sites located in the zone of dyke deviation to the north of the Arakapas lineament. Cleaned remanence directions were found to cluster in the northwest and northeast quadrants and became more tightly grouped with a westerly declination after reorientating the dyke strikes to a n o r t h south direction. The tilt-corrected inclination values found in this latter study were significantly higher than the inclination of the T M V (32°), with a mean value of 48 °. This m a y indicate that the structural corrections applied were invalid as the technique of AUerton and Vine [9] was not used to determine the net tectonic rotation affecting each site. Even so, an overall clockwise sense of dyke motion would still be required to account for the deviation of the magnetisation vectors with respect to the TMV. M o r e recent work b y Allerton [23] on greenschist and zeolite facies dykes exposed further to the east in the Lefkara region, to the north of the eastward extension of the Arakapas fault belt, has also demonstrated the presence of major clockwise block rotations about steeply inclined axes.

All four studies therefore show that substantial clockwise tectonic rotations have occurred in the vicinity of the Southern Troodos Transform Zone. Palaeomagnetic evidence therefore does not appear to support a dextral offset configuration for the Troodos axis system. A modern day analogue of the Southern Troodos Transform Zone is the Tj~Srnes Fracture Zone, Iceland. This transform is known to have an overall right-lateral sense of motion [24]. However, to

260

the south of the transform a progressive change in the strike of lava units is seen, similar to that observed to the north of the Arakapas fault belt. Left-lateral strike-slip along the fracture zone could be inferred if this curvature is compared with similar lineations seen in G L O R I A sonograph studies of the Q u e b r a d a - G o f a r fracture zones along the East Pacific Rise [25]. However, the k n o w n slip direction and preliminary palaeomagnetic data indicating significant tectonic rotations within the TjiSrnes Fracture Zone clearly show that the observed swing in the orientation of lava units is not a primary feature of the spreading process [24]. Following recent detailed remapping of the Eastern Limassol Forest Complex, MacLeod [18] has proposed that the Southern Troodos Transform Zone was locally disrupted during the early stages of palaeorotation of the Troodos microplate. The model of MacLeod [18] involves initial genesis of the Eastern Limassol Forest Complex crust at an "Anti-Troodos" spreading axis adjacent to a sinistrally slipping transform, accepting the conclusions of Murton [6]. This "Anti-Troodos" crust exhibits a similar stratigraphy and geochemistry to that of the Troodos ophiolite as a whole. Post-oceanic disruption of the transform domain is then believed to have occurred in two stages. Firstly, regional extension was initiated during deposition of the umbers and radiolarites, which show evidence of progressive tilting. Extension in the northern part of the Eastern Limassol Forest Complex was primarily accommodated by reactivation of transform structures. At this stage fault blocks were rotated about northwest axes above a sub-horizontal drcollement surface located in the lower part of the layered plutonic complex (the Akapnou Forest Drcollement). During the second stage, initial rotation of the Troodos microplate in the Campanian-Maastrichtian interval imposed a frictional drag along the southern margin of the microplate. The Arakapas fault belt lay close to the inferred position of this margin, and this drag resulted in dextral reactivation of previously developed extensional structures. MacLeod [18] believes that the clockwise block rotations observed within the transform zone are related to this second stage of deformation. MacLeod [7] reports palaeomagnetic data from seven sites within the Eastern Limassol Forest

A. MORRIS ET A L .

Complex which are used to support this model of post-oceanic rotational deformation. The application of the method of Allerton and Vine [9] to these data yielded inclined poles of clockwise rotation clustering in the southwest quadrant for five sites, while the remaining two sites showed subhorizontal rotation axes. Our data from site EP indicate that rotations were occurring while dykes were still being injected. This clearly shows that rotation of blocks along the Arakapas fault belt was concurrent with Turonian crustal genesis. Thus, block rotation there cannot reflect dextral reactivation of originally sinistral east-west transform lineaments by later anticlockwise rotation of the microplate. In addition, the Turonian lavas, interlava sediments and umbers sampled at our four sites in the Eastern Limassol Forest Complex, and at three previous sites analysed by Clube [16], exhibit only simple tilting about nearly strike-parallel subhorizontal axes. A further site at Monagroulli studied by Clube [16] showed a tilt corrected declination of 313 ° . If these data are considered in conjunction with those from the seven sites reported by MacLeod [7] then the style of rotational deformation in the Eastern Limassol Forest is not palaeomagnetically distinguishable from that found along the Arakapas fault belt. A suggestion of MacLeod [7] which cannot be excluded is that the rotations occurring about inclined poles in the Eastern Limassol Forest Complex are due to local reactivation of extensional structures during microplate rotation. However, we prefer a model in which both the mapped transform fault and adjacent oceanic crust both to the north [22] and to the south was the site of a complicated system of localised, predominantly clockwise block rotations; some areas were rotated by over 100 ° about steeply inclined axes, whereas others experienced only simple tilting. The whole area underwent later passive anticlockwise rotation by 90 o as part of the Troodos microplate. Finally, structural [26] and palaeomagnetic [9] studies along the northern margin of the Troodos ophiolite suggest that the Troodos spreading system may have experienced non-steady state spreading, possibly involving ridge jumping. If correct this could have given rise to a complex situation with areas of crust being incorporated within or stranded outside the region of active slip

PALAEOMAGNETIC

EVIDENCE

FOR CLOCKWISE

ROTATIONS

b e t w e e n the two plates. T h e possibility of reversals of m o t i o n a l o n g the S o u t h e r n T r o o d o s T r a n s f o r m therefore could n o t be excluded. However, the extensive p a l a e o m a g n e t i c evidence of widespread clockwise r o t a t i o n of lavas, dykes a n d sediments p o i n t s to a d o m i n a n c e of right-lateral slip w i t h i n the t r a n s f o r m d o m a i n . Such rotations could n o t have b e e n identified b y field structural studies alone a n d c o n f i r m the i m p o r t a n c e of rotations a b o u t steeply i n c l i n e d axes in deformed oceanic crust adjacent to t r a n s f o r m faults.

6. Conclusions S i g n i f i c a n t i n t r a c r u s t a l r o t a t i o n s of small f a u l t - b o u n d e d blocks have taken place within the S o u t h e r n T r o o d o s T r a n s f o r m Zone. These rotations are considered to represent p r i m a r y features of crustal genesis a n d c a n n o t be a t t r i b u t e d simply to post-oceanic d i s r u p t i o n of the fracture zone. T h e p r e d o m i n a n t l y clockwise sense of block rotation suggests that the T r o o d o s ridge system h a d a sinistral offset configuration. This is s u p p o r t e d b y original northwest dyke strikes f o u n d along the A r a k a p a s fault belt which are consistent with a stress field operating across a dextrally slipping transform. A d d i t i o n a l p a l a e o m a g n e t i c d a t a o b t a i n e d from the u m b e r s a n d radiolarites of the Perapedhi Form a t i o n c o n f i r m that the C a m p a n i a n period was a time of rapid r o t a t i o n of the T r o o d o s microplate, with u p to 45 o of r o t a t i o n occurring over 15 Ma.

Acknowledgements W e t h a n k S i m o n Allerton, F r e d Vine, Chris M a c L e o d , Bramley M u r t o n a n d T r i s t a n C l u b e for s t i m u l a t i n g discussions. W e particularly t h a n k Tristan C l u b e for assisting i n the field a n d d e m o n strating the geology of C y p r u s to one of us (Morris). M a n y t h a n k s also go to A l a n Pike for technical support. W e t h a n k the Geological Survey Dep a r t m e n t of C y p r u s for s u p p l y i n g necessary permits. This work was s u p p o r t e d in part b y a N a t u ral E n v i r o n m e n t Research C o u n c i l research stud e n t s h i p to Morris.

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2 T.M.M. Clube, K.M. Creer and A.H.F. Robertson, The palaeorotation of the Troodos microplate, Nature 63, 522525, 1985. 3 T.M.M. Clube and A.H.F. Robertson, The palaeorotation of the Troodos microplate, Cyprus, in the Late MesozoicEarly Cenozoic plate tectonic framework of the Eastern Mediterranean, Surv. Geophys. 8, 375-437, 1986. 4 E.M. Moores and F.J. Vine, The Troodos massif, Cyprus and other ophiolites as oceanic crust: evaluation and implications, Phil. Trans. R. Soc. London A. 268, 443-466, 1971. 5 K.A. Simonian and I.G. Gass, Arakapas Fault Belt Cyprus: A fossil transform belt, Geol. Soc. Am. Bull. 89, 1220-1230, 1978. 6 B.J. Murton, Anomalous oceanic lithosphere formed in a leaky transform fault: evidence from the Western Limassol Forest Complex, Cyprus, J. Geol. Soc. London 143, 845854, 1986. 7 C.J. MacLeod, The tectonic evolution of the Eastern Limassol Forest Complex, Cyprus, 231 pp., Unpubl. PhD Thesis, Open Univ., 1988. 8 F.J. Vine and E.M. Moores, Palaeomagnetic study of the Troodos igneous massif, EOS 50, p. 131, 1969. 9 S. Allerton and F.J. Vine, Spreading structure of the Troodos ophiolite, Cyprus: Some paleomagnetic constraints, Geology 15, 593-597, 1987. 10 S.B. Mukasa and J.N. Ludden, Uranium-lead ages of plagiogranites from the Troodos ophiolite, Cyprus, and their tectonic significance, Geology 15, 825-828, 1987. 11 C.D. Blome and W.P. Irwin, Equivalent radiolarian ages from ophiolite terrains of Cyprus and Oman, Geology 13, 401-404, 1985. 12 G.C. Smith and F.J. Vine, Physical property sections through the Troodos Ophiolite, Cyprus; a geophysical analogue of oceanic crust?, in: Ophiolites and Oceanic Lithosphere, Proc. Int. Conf., Nicosia, Cyprus, in press, 1988. 13 G.M. Smith, Source of marine magnetic anomalies: Some results from DSDP Leg 83, Geology 13, 162-165, 1985. 14 F.J. Vine, C.K. Poster and I.G. Gass, Aeromagnetic survey of the Troodos Igneous Massif, Cyprus, Nature (Phys. Sci.) 244, 34-38, 1973. 15 S. Beske-Diehl and S.K. Banerjee, Metamorphism in the Troodos ophiolite: Implications for marine magnetic anomalies, Nature 285, 563-564, 1980. 16 T.M.M. Chibe, The palaeorotation of the Troodos microplate, 275 pp., Unpubl. PhD. Thesis, Univ. Edinburgh, 1986. 17 H.H. Demarest Jr., Error analysis for the determination of tectonic rotation from paleomagnetic data, J. Geophys. Res. 88, 4321-4328, 1983. 18 C.J. MacLeod, Role of the Southern Troodos Transform Fault in the rotation of the Cyprus microplate: evidence from the Eastern Limassol Forest Complex, in: Ophiolites and Oceanic Lithosphere, Proc. Int. Conf., Nicosia, Cyprus, in press, 1988. 19 J.F. Boyle, The origin and geochemistry of the metalliferous sediments of the Troodos massif, Cyprus, 277 pp., Unpubl. PhD. Thesis, Univ. Edinburgh, 1984. 20 W.D. Macdonald, Net tectonic rotation, apparent tectonic rotation, and the structural tilt correction in palaeomagnetic studies, J. Geophys. Res. 85, 3659-3669, 1980.

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A. MORRISET AL. 24 K.D. Young, M. Jancin, B. Voight and N.I. Orkan, Transform deformation of Tertiary rocks along the Tj/Srnes Fracture Zone, North Central Iceland, J. Geophys. Res. 90, 9986-10010, 1985. 25 R.C. Searle, Multiple closely spaced transform faults in fast-slipping fracture zones, Geology 11, 607-610, 1983. 26 R.J. Varga and E.M. Moores, Spreading structure of the Troodos ophiolite, Cyprus, Geology 13, 846-850, 1985.