extreme tectonic rotations within an eastern mediterranean ophiolite (baër–bassit, syria)

Extreme tectonic rotations within an eastern Mediterraneanophiolite (Bae«r^Bassit, Syria)

Antony Morris a;�, Mark W. Anderson a, Alastair H.F. Robertson b,Khalil Al-Riyami b;1

a Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UKb Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK

Received 8 March 2002; received in revised form 10 June 2002; accepted 19 June 2002

Abstract

Palaeomagnetic results from 27 sites at five localities within the dismembered Bae«r^Bassit ophiolite of northernSyria are presented. The ophiolite forms part of a series of thrust sheets emplaced over Mesozoic carbonates of theArabian platform in the middle Maastrichtian. A positive inclination-only area-wide tilt test applied to four localitymean remanences and positive fold and reversal tests from palaeohorizontal units (pillow lavas, lava flows) within oneof these localities indicate that the ophiolite preserves pre-deformation magnetisations. Variable directions ofremanence between localities demonstrate that the ophiolite has experienced extreme relative anticlockwise rotationson a kilometric scale. Within the most extensively sampled ophiolite massif (Bassit sheet) there is a progressiveincrease in rotation from north to south. The southernmost units at the lowest structural level in the imbricate thruststack record the highest rotation (exceeding 200‡). Although tectonic rotation during imbricate thrusting has beenreported in a number of orogenic belts, the pattern of rotations in the Bassit sheet is difficult to explain by differentialthrust sheet rotation. Instead, regional comparisons with the Hatay ophiolite of southern Turkey and the Troodosophiolite of Cyprus suggest that a significant component of rotation may be ascribed to intraoceanic deformation of acoherent region of oceanic crust within the southern Neotethyan basin prior to ophiolite emplacement. The partiallyrotated Bae«r^Bassit ophiolite was then emplaced and structurally dismembered by thrust faulting. During the LateTertiary the ophiolitic units were further rotated during the initiation and development of a major sinistral strike-slipfault zone, linking the Cyprus subduction zone to the Dead Sea Transform system. The extreme rotations observed inthe study are therefore of composite origin, and reflect the complex development of structural fabrics within theophiolite. < 2002 Elsevier Science B.V. All rights reserved.

Keywords: paleomagnetism; rotation; tectonics; ophiolite; Syria

0012-821X / 02 / $ ^ see front matter < 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 7 8 2 - 3

* Corresponding author. Tel. : +44-1752-233120; Fax: +44-1752-233117.E-mail addresses: [email protected] (A. Morris), [email protected] (M.W. Anderson), [email protected]

(A.H.F. Robertson).

1 Present address: Petroleum Development Oman, Muscat, Sultanate of Oman.

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Earth and Planetary Science Letters 202 (2002) 247^261

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1. Introduction

The eastern Mediterranean represents a com-plex orogenic zone formed during progressiveMesozoic to Recent collision of the African andEurasian lithospheric plates. Late Cretaceousophiolitic rocks found throughout the region areremnants of small oceanic basins destroyed duringplate convergence [1] (Fig. 1). The most promi-nent ophiolitic chain extends from Cyprus (Tro-odos terrane), through Turkey and Syria (Hatayand Bae«r^Bassit terranes) to Oman (Semail ter-rane). Paleomagnetism in the Troodos terranehas provided fundamental insights into theregional tectonic evolution through the identi-¢cation of crustal rotations on a range of timeand spatial scales [2^7], the most signi¢cant being

the protracted 80^90‡ rotation of the Troodos‘microplate’ [8^10]. It has been suggested thatthis was initiated by impingement of the Arabianpassive continental margin with a subductionzone [9,11], an event which also emplaced theHatay and Bae«r^Bassit ophiolites onto theArabian passive margin as a series of thrustsheets [12]. Here we report the ¢rst palaeo-magnetic data from the structurally dismembered,emplaced Bae«r^Bassit ophiolite (Fig. 1). Thesedata indicate some of the largest rotationsyet reported in any tectonic setting, suggestingthat the polyphase structural evolution of theophiolite during its oceanic detachment, emplace-ment and subsequent neotectonic modi¢cationwas accompanied by extreme rotational deforma-tion.

Fig. 1. Outline tectonic map of the eastern Mediterranean, showing the distribution of Cretaceous ophiolites.

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A. Morris et al. / Earth and Planetary Science Letters 202 (2002) 247^261248

2. Geological setting

The dismembered Bae«r^Bassit ophiolite of NWSyria (Figs. 1 and 2) represents the westernmostpart of the ‘Ophiolitic Crescent’ of the northernmargin of Arabia [13]. It is underlain by the‘Bae«r^Bassit Me¤lange’ [14] (Figs. 2 and 3), whichconsists of deformed Mesozoic rocks of continen-tal margin and oceanic a⁄nities. The ophioliticoutcrop (Fig. 2) is dominated by two massifs :Bae«r in the NE (inland) and Bassit in the NW(near the coast). The Bae«r massif is relativelystructurally intact and consists principally of harz-burgites, overlain by cumulate ultrama¢cs, lay-

ered gabbros and dolerite dykes. The Bassit mas-sif comprises a lower sequence of peridotites andgabbros, which are overthrust by a slice of me-lange and then by thin (6 100 m thick) imbricatethrust sheets of gabbro, sheeted dykes and pillowlavas. K^Ar dates from dykes in the range of 73^99 Ma suggest that the ophiolite is Late Creta-ceous in age, whereas the metamorphic sole ofthe ophiolite yields a minimum age of 85^95 Ma[12]. This latter age is commonly adopted as thelatest age of formation of the ophiolite, assumingthat the metamorphic sole was formed during ini-tial detachment of the oceanic crust near thespreading axis [15^17]. The ophiolite is inferred

Fig. 2. Simpli¢ed geological map of the Bae«r^Bassit ophiolite, northern Syria, showing the locations of the ¢ve localities fromwhich 27 palaeomagnetic sites have been sampled. BAK/QT=Bassit to Ain Kebira road/Qara Tate locality. Map modi¢ed from[14].

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A. Morris et al. / Earth and Planetary Science Letters 202 (2002) 247^261 249

to have formed in a south-Tethyan oceanic basin(part of Neotethys), and geochemical characteris-tics indicate formation in a supra-subduction zonesetting (in common with other eastern Mediterra-nean and Middle Eastern ophiolites; [14]).The ophiolite was thrust onto the Arabian car-

bonate platform in the middle Maastrichtian (ca.70 Ma), along with the underlying Bae«r^BassitMelange (Fig. 3). In contrast to the poorly de-¢ned age of the ophiolite itself, the timing of em-placement is precisely bracketed by the ages of theyoungest carbonates in the underlying autochthon(early Maastrichtian) and the oldest post-emplace-ment sedimentary cover sequences (late Maas-trichtian). Emplacement has been suggested tobe mechanically linked to the coeval initiation ofthe palaeorotation of the Troodos microplate tothe west [9]. A well-developed metamorphic soleconsists of amphibolites and greenschists, bothderived from alkali basalt and pelagic sedimentprotoliths [18]. Lineations in the metamorphicsole, de¢ned by elongation of amphibole porphy-roblasts, together with fold facing and vergencedirections within the underlying Bae«r^Bassit Me¤-lange indicate that ophiolitic thrust sheets wereemplaced towards the SE [18]. The disrupted al-lochthon (Bae«r^Bassit ophiolite, metamorphicsole and Bae«r^Bassit Melange) was later uncon-formably overlain by a sedimentary sequence oflate Maastrichtian to Pliocene age (Fig. 3).The Neogene sedimentary rocks onshore are

cut by mainly ENE^WSW trending strike-slipfaults that extend o¡shore (Fig. 2). This fault sys-tem represents part of the extension of the plateboundary zone between the African plate and theTurkish microplate, which runs eastwards fromsouth of Cyprus as a zone of distributed deforma-tion and then comes onshore, passing through theBae«r^Bassit region to link with the Dead SeaTransform Fault system to the east [14] (Fig. 1).

3. Sampling and methods

We have sampled layered gabbro, sheeted dykeand extrusive sequences at 27 sites for palaeomag-netic analyses in order to quantify di¡erential ro-tations which have a¡ected the ophiolitic thrust

sheets. Six to eight samples per site were drilledin situ following standard palaeomagnetic proce-dures. Sampling was restricted to exposures show-ing either consistent palaeohorizontal indicators(laterally continuous, planar layering in gabbroicrocks; coherent sequences of pillowed lava £ows)or palaeovertical indicators (exposures of multi-ple, sub-parallel sheeted dykes). The orientationsof these indicators were measured in the ¢eld toan accuracy of U 5‡. Sites are further groupedinto ¢ve localities (Fig. 2), four of which occurin the Bassit sheet utilising fresh coastal exposuresand road sections.Remanences were measured in the laboratory

using a Molspin £uxgate spinner magnetometerand samples were subjected to either alternating

Fig. 3. Summary tectonostratigraphy of the Bae«r^Bassit area(modi¢ed from [14]; see text for explanation).

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¢eld or thermal stepwise demagnetisation. Char-acteristic components of magnetisation werefound using orthogonal vector plots and principalcomponent analysis [19] and site and localitymean remanence directions computed using Fish-

erian statistics [20]. Isothermal remanent magnet-isation, thermal demagnetisation data and hightemperature magnetic susceptibility experimentswere used to characterise the magnetic mineralogyof selected samples.

Table 1Palaeomagnetic and net tectonic rotation results from the Bae«r^Bassit ophiolite

Site Lithology N In situ Tilt corrected k K95 NTR solution

Dec Inc Dec Inc Axis Angle

Qastal Maaf localityBB37 Dolerite dykes 5 198.6 317.2 154.4 341.4 109.3 7.4 358/16 75BB38 Dolerite dykes 6 199.1 318.9 152.3 342.2 194.3 4.8 359/17 75BB39 Dolerite dykes 5 202.5 322.0 148.5 345.8 229.8 5.1 003/17 76

Mean : 3 200.0 319.4 663.4 4.83 151.8 343.2 645.0 4.9

North Coast localityBB01 Dolerite dykes 6 66.5 335.1 71.6 345.5 135.0 5.8 127/78 103BB02 Dolerite dykes 6 66.3 335.8 67.2 337.3 79.0 7.6 128/88 114BB03 Dolerite dykes 6 111.5 337.1 106.9 333.2 210.7 4.6 134/85 67BB04 Dolerite dykes 5 103.2 335.7 98.0 329.7 293.4 4.5 315/88 80BB05 Dolerite dykes 6 81.1 335.8 83.5 339.5 20.6 15.1 134/87 98BB06 Dolerite dykes 5 68.9 338.3 69.8 340.0 72.6 9.0 105/86 109

Mean : 6 82.7 337.7 26.1 13.46 83.5 338.5 32.5 11.9

Bassit Road localityBB07 Dolerite dykes 7 257.1 388.8 74.8 375.2 217.7 4.1 075/27 62BB08 Dolerite dykes 6 294.6 388.4 42.0 382.6 195.8 4.8 068/37 71BB09 Dolerite dykes 6 229.3 381.6 194.3 386.7 38.7 10.9 067/27 60BB10 Dolerite dykes 7 197.4 388.4 216.7 386.6 89.9 6.4 072/32 64BB12 Dolerite dykes 6 347.4 383.5 271.9 368.4 46.8 9.9 087/07 61BB34 Dolerite dykes 6 99.8 387.4 55.7 358.2 439.4 3.2 057/50 86BB35 Dolerite dykes 6 20.3 382.4 55.6 355.2 60.3 8.7 072/42 79BB36 Dolerite dykes 6 72.8 378.5 67.0 368.6 78.7 7.6 077/47 73

Mean : 8 28.2 388.6 153.0 4.58 54.2 379.2 18.5 13.2

Bassit to Ain Kebira road/Qara Tate localityBB48 Gabbro/dolerite 6 311.4 367.6 10.7 346.0 465.6 3.1 033/66 174BB49 Gabbro/dolerite 7 333.7 366.0 15.1 337.7 462.5 2.8 036/69 168BB50 Pillow lavas 4 198.6 14.1 200.5 50.1 155.7 7.4 183/79 162BB51 Pillow lavas 5 194.1 318.1 197.7 39.7 36.5 12.8 170/62 167BB52 Lava £ow 6 209.4 321.5 217.7 31.0 310.4 3.8 137/42 152BB53 Pillow lavas 5 206.2 329.9 207.3 31.3 272.4 4.6 142/43 160

Mean : 6 12.7 312.3 2.9 48.66 22.1 339.6 56.0 9.0

West Coast localityBB42 Dolerite sill 6 191.8 358.1 319.1 351.0 165.4 6.2 000/48 217BB43 Dolerite sill 6 206.1 361.9 313.1 344.7 196.1 4.8 003/52 220BB44 Dolerite sill 7 205.4 365.2 317.4 345.8 432.0 2.9 001/53 218BB45 Layered gabbro 5 219.7 363.4 308.6 339.5 320.2 4.3 004/55 226

Mean : 4 205.2 362.5 171.5 7.04 314.3 345.3 197.1 6.6

N=number of specimens; Dec= declination; Inc= Inclination; k=Fisher precision parameter; K95 = semi-angle of 95% cone ofcon¢dence; axis = azimuth/plunge of preferred axis of net tectonic rotation; angle =net tectonic rotation angle.

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4. Results and analysis

4.1. Magnetic mineralogy and palaeomagneticresults

Isothermal remanent magnetisation acquisitioncurves reach saturation by ¢elds of 300 mT (Fig.4a), indicating presence of only low coercivityminerals in these rocks, whereas median destruc-tive ¢elds during alternating ¢eld demagnetisationof 25^40 mT indicate a dominance of stable sin-gle/pseudo-single domain grain sizes. Thermal de-magnetisation of natural remanence indicatesmaximum unblocking temperatures of 560^580‡C, and high temperature susceptibility mea-surements indicate Curie temperatures of 580‡C(Fig. 4b). These data together indicate that mag-netite is a signi¢cant remanence carrier in theserocks. This is supported by SEM images of pol-ished thin sections which reveal exsolution inter-growths of magnetite and ilmenite. During hightemperature susceptibility experiments on some

samples, a ‘bump’ in the heating curve is observedbetween 150^400‡C (Fig. 4b). The rising limb ofthis bump is reversible up to 250‡C but curvesbecome irreversible upon heating to 350‡C. Thesedata, together with partial unblocking of naturalremanences at temperatures of between 300‡Cand 400‡C in some samples, suggest the presenceof a titanium-rich ferrimagnetic phase (titanomag-netite or titanomaghemite). The magnetic miner-alogy of these rocks is dominated, therefore, byferrimagnetic phases which can be readily relatedto assemblages observed within in situ oceaniccrust and other ophiolites (with formation bylow temperature oxidation of titanomaghemitefrom a titanomagnetite precursor (TM60) duringsea-£oor weathering, followed by subsequent lowtemperature inversion to intergrowths of magne-tite and ilmenite [21]).Stable components of magnetisation were iso-

lated at all sites, following removal of minorsecondary components during initial demagne-tisation. Typical examples of demagnetisation

Fig. 4. Typical examples of magnetic behaviour of rocks from the Bae«r^Bassit ophiolite: (a) Isothermal remanent magnetisationacquisition curve showing saturation by 300 mT, consistent with remanence carried by low coercivity minerals; (b) Variation oflow ¢eld magnetic susceptibility with temperature, showing a Curie Temperature of 580‡C consistent with the presence of magne-tite and a lower temperature ‘bump’ suggesting the presence of titanomaghemite; (c) Examples of orthogonal demagnetisation di-agrams, showing well-de¢ned stable end point remanence directions isolated by both alternating ¢eld and thermal treatment. Sol-id circles, horizontal plane; open symbols, vertical N^S plane. Note that the vertical projections are on the horizontal axes andthat both normal and reversed polarities of magnetisation are present.

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behaviour are shown in Fig. 4c. In situ and tilt-corrected site mean directions are shown in thestereographic equal area projections of Fig. 5and are given in Table 1, along with the corre-sponding locality mean data. Reversed polarityremanences are observed at 23 sites with four sitesrecording normal polarities (Fig. 5). Magnetisa-tion directions are unrelated to the present geo-centric axial dipolar ¢eld direction in NW Syria(Dec= 000‡; Inc = 55‡) and are widely di¡erentbetween localities. These large di¡erences canonly result from the e¡ects of relative tectonicrotations of the sampled units subsequent to rem-anence acquisition. Scatter of directions betweensites within a locality may re£ect the e¡ects ofsecular variation. Consideration of the dispersionof site mean virtual geomagnetic poles at each

locality [22] indicates that secular variation isaveraged out e¡ectively at the North Coast, BassitRoad and Bassit to Ain Kebira road/Qara Tate(BAK/QT) localities. A component of unaveragedsecular variation may remain at the West Coastand Qastal Maaf localities, where the numbers ofsites are lower.Tilt-corrected mean magnetisations at four lo-

calities show consistent inclinations of around41‡, whereas in situ vectors at these localitieshave a wide range of inclinations (Table 1; Fig.5), suggesting that pre-tilt magnetisations havebeen isolated from these rocks. At the BassitRoad locality (eight sites), however, a vertical up-wards remanence is observed both prior to andafter tilt correction in a series of sheeted dykes(Fig. 5). In all other respects, the magnetic prop-

Fig. 5. Stereographic projections showing the distribution of site mean directions of magnetisation from the ¢ve sampling local-ities in the Bae«r^Bassit ophiolite. Note the large di¡erences in declination between localities, indicating extreme relative rotationswithin the ophiolite. Also note the near-vertical remanence recorded at the Bassit Road locality. Solid/open symbols = lower/upperhemisphere projections; ellipses = projection of K95 cones of con¢dence around site mean remanences; black/white stars = normal/reversed polarity reference directions for a site at 35.75‡N, 35.9‡E, derived from the Late Cretaceous African mean palaeomag-netic pole [27].

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erties of samples from this section are indistin-guishable from those of rocks from other sections.The in£uence of these data on ¢eld tests of pa-laeomagnetic stability is discussed below, andtheir interpretation is given separately.

4.2. Timing of magnetisation acquisition

This is determined using ¢eld tests of palaeo-magnetic stability, the most common of which isthe palaeomagnetic fold test [23,24]. Di¡erentialvertical axis rotations, however, invalidate use ofarea-wide fold tests based on full remanence vec-tors (declination and inclination). An alternativeapproach which is independent of the structuralhistory is to determine the e¡ect of untilting onthe distribution of inclinations only. The anglebetween the inclination and the palaeohorizontalat a site may be assumed to remain constant dur-ing rigid body rotation, regardless of the axis ofrotation. Signi¢cant improvement in clustering ofinclinations upon tilt correction of mean direc-tions from sites with di¡erent structural orienta-tions therefore suggests that a pre-tilt magnetisa-tion has been identi¢ed. However, where a samplecollection consists of several groups of sites, eachfrom a separate coherent block, the declinationinformation within each block is usable andneed not be discarded. In these circumstancesthe ‘block-rotation Fisher’ analysis of Enkin andWatson [25] is applicable. This maximises use ofthe remanence data and yields improved estimatesof mean inclination for subsequent use in a para-metric re-sampling tilt test formulation [25].Data from four localities (excluding the result

from the Bassit Road section initially) yield thefollowing inclination statistics using the block-ro-tation Fisher method of Enkin and Watson [25] :

In situ : II ¼ 334:3� 10:9; UU ¼ 5:5Tilt corrected : II ¼ 341:1� 3:4; UU ¼ 56:8

where IŒ, UU are the maximum likelihood estimatesof the true mean inclination in degrees and theFisher precision parameter, respectively.The increase in UU following tilt correction

strongly suggest that pre-deformation magnetisa-tions are recorded at these localities. Stepwise un-

tilting gives a maximum UU value of 56.8 at 100%untilting (Fig. 6). A parametric re-sampling imple-mentation of the tilt test [25], using 1000 re-sam-pling trials indicates an optimum untilting of105.2 U 2.7% (N=4). The e¡ects of estimated un-certainties in true initial verticality of dykes andpalaeohorizontal of pillowed lava £ows can bemodelled by incorporating a circular standard de-viation of 10‡ on the determination of structuraldata into the tilt test. This indicates an optimumuntilting of 101.7þ9:337:5% (N=4).This distribution straddling 100% of unfolding

constitutes a positive inclination-only tilt test [25].This is supported by a positive reversal test (ClassRc, Qc = 10.8‡; [26]) and a standard fold test whichis positive at the 99% con¢dence level [24] at theBassit^Ain Kebira/Qara Tate locality (Table 1;Fig. 5). The data, therefore, unequivocally indi-cate acquisition of remanence prior to tectonicdisruption of the sampled units.Inclusion of data from the Bassit Road locality

in the block-rotation Fisher tilt test calculationsyields the following results (N=5):

In situ : II ¼ 352:3� 10:2; UU ¼ 4:4Tilt corrected : II ¼ 352:2� 6:1; UU ¼ 12:6

Stepwise untilting: maximum UU value = 12.6 at100% untilting (Fig. 6)Optimum untilting (no uncertainties in structur-

al data) = 102.2 U 2.5%%Optimum untilting (10‡ c.s.d. on structural da-

ta) = 95.7þ8:737:5%These data, therefore, also yield a parametric

distribution centred on 100% untilting. However,inclusion of the Bassit Road result clearly invalid-ates the tilt test by producing an unacceptabledecrease in the maximum likelihood estimate ofthe tilt-corrected Fisher precision parameter, UU .The tectonic signi¢cance of data from this localityis discussed later.

4.3. Determination of tectonic rotations usingtraditional methods

Tectonic rotations are traditionally speci¢ed bycomparing site or locality mean remanences witha reference vector derived from the apparent polar

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wander path of an appropriate undeformed re-gion or major plate. The pre-deformational rema-nences identi¢ed here at four localities are in-ferred to represent magnetisations acquiredduring or shortly after oceanic crustal genesis.Magnetisation vectors are therefore compared toa reference direction of Dec= 002‡, Inc = 36‡ (orits antipode) derived from the Late Cretaceousmean palaeomagnetic pole for Africa given byWestphal et al. [27]. Con¢dence limits on calcu-lated rotation angles may be found using themethodology of Butler [22].Localities within the western, Bassit massif of

the ophiolite show a progressive increase in anti-clockwise rotation, with magnitudes varying from98.5‡ U 13.6‡ (North Coast locality), through159.9‡ U 11.1‡ (Bassit^Ain Kebira/Qara Tate lo-cality), and reaching a maximum of 227.7‡ U 9.6‡

(West Coast locality). The single Qastal Maaf lo-cality within the eastern Bae«r massif has also beenrotated in an anticlockwise sense by 30.2‡ U 8.0‡.The possibility that the larger rotations occurredin a clockwise sense can not be excluded, but theresultant variation in the sense of the extremerotations is then di⁄cult to reconcile with theobserved structural framework of the ophiolite.

4.4. Determination of net tectonic rotations

Standard palaeomagnetic corrections for the ef-fect of tectonic tilting upon magnetisation direc-tions involve rotating palaeohorizontal surfacesback to horizontal around strike-parallel axes.The total deformation at a site is, therefore, arbi-trarily decomposed into components of tilting andvertical axis rotation. In complexly deformed ter-rains, where fold axes are seldom horizontal andwhere multiple phases of deformation may occur,this procedure can introduce serious declinationerrors [28,29]. It is more appropriate, therefore,to describe the deformation at a site in terms ofa single rotation about an inclined axis, whichrestores both the palaeohorizontal/vertical to itsinitial orientation and the site mean magnetisationvector to the appropriate palaeomagnetic refer-ence direction. This single rotation may then bedecomposed into any number of component rota-tions on the basis of additional structural data[29].The net tectonic rotation algorithm employed

here is that devised by Allerton and Vine [2] foruse within the sheeted dyke terrain of the Troodosophiolite of Cyprus, and which was later modi¢edby Morris et al. [7] to yield estimates of uncertain-ties in rotation axis orientations and angles. Thistechnique can be applied to both palaeovertical(dyke) and palaeohorizontal (£ow) cases, withthe key assumption being that no internal defor-mation of a sampled unit has occurred. Underthis circumstance, the angle L between the mag-netisation vector and the normal to the dyke/£owis constant during deformation [2].The analysis involves ¢nding an initial dyke

(£ow) normal which conserves the angle L andis as horizontal (vertical) as possible. The meansite magnetisation vector (SMV) is then restored

Fig. 6. Variation in the Fisher precision parameter with pro-gressive untilting of sampled localities, indicating a positiveblock-rotation Fisher inclination-only tilt test [25] (N=4curve). Inclusion of the mean direction of magnetisation atthe Bassit Road locality reduces the maximum k value to anunacceptably low value of 12.6 (N=5 curve).

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to the reference magnetisation vector (RMV) andthe present dyke (£ow) normal to its initial ori-entation. The rotation axis which allows this res-toration is located at the intersection of the greatcircle bisectrix of the SMV and RMV and that ofthe present and initial dyke/£ow normals (Fig.7a). The net tectonic rotation is described by theazimuth and plunge of the axis of rotation, andthe angle of rotation; a positive angle represent-ing an anticlockwise rotation [2]. Two solutionsare generated in those cases where dykes can berestored to the vertical, and additional criteria(e.g. compatibility with observed structures) maybe used to select a preferred solution.In the modi¢cation of this method [7], the Al-

lerton and Vine [2] algorithm is applied to allcombinations of each of ¢ve orientations for thethree vectors input into the analysis (i.e. RMV,SMV, normal to dyke/£ow; Fig. 7b). These ori-entations are distributed around the K95 circles foreach vector (an K95 of 5‡ is assigned to the struc-tural data). This yields 125 combinations of inputvectors, and an output consisting of a minimumof 125 and maximum of 250 estimates of the nettectonic rotation axis and angle at each site. The

envelope on a stereonet which completely enclosesthe set of estimated rotation axes (Fig. 7c) pro-vides a ¢rst-order approximation of the associated95% con¢dence region. This envelope is the onlypracticable means of describing the con¢dence re-gion, since the net tectonic rotation techniquedoes not yield rotation axis estimates which aresymmetric around the mean estimate. The associ-ated rotation angles can be plotted as histograms(Fig. 7c). This method indicates the range of ro-tation axes and angles which are possible at a sitegiven the uncertainties in orientation of the vari-ous input vectors.The net tectonic rotation parameters found by

single application of the Allerton and Vine [2]technique to our data are given in Table 1, where-as the stereonets of Fig. 8 show the site-level en-velopes of potential rotation axes within each lo-cality together with histograms of estimatedrotation angles compiled at the locality level.Again, treatment of data from the Bassit Roadlocality is deferred. Within the Bassit sheet local-ities, rotation angles in all cases exceed 90‡ andreach a maximum of s 200‡ within the WestCoast locality (Fig. 8). Solutions indicate rotation

Fig. 7. An example of the net tectonic rotation analysis. (a) The Allerton and Vine [2] algorithm. SMV=Site magnetisation vec-tor (in situ remanence); RMV=reference magnetisation vector; PDN=present dyke normal; IDN=calculated initial dyke nor-mal; R=axis of net tectonic rotation; dashed line indicates circle of radius L ( = angle between SMV and PDN) centred on theRMV; subscripts 1 and 2 refer to alternative rotation solutions; (b) multiple application of this method to all combinations of¢ve estimates of SMV, TMV and PDN, distributed around their respective K95 cones of con¢dence [7] ; (c) gives 125 estimates ofthe net tectonic rotation axis for each solution (only NE initial dyke strike solution is shown). These de¢ne an envelope whichrepresents a ¢rst-order approximation of the 95% region of con¢dence around the true rotation axis. Inset histogram illustratesthe associated distribution of net tectonic rotation angles.

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axes with varying azimuths and moderate to sub-vertical plunges. At these localities the rotationaldeformation is dominated by the vertical axiscomponent with only moderate tilt components.In contrast, rotation axes are shallowly plungingat the single Bae«r sheet locality (Qastal Maaf)with rotation angles of approximately 75‡. Thedi¡erence between these solutions and the30.2‡ U 8.0‡ vertical axis rotation found using thetraditional analysis given above re£ects the signif-icant present day tilt of the dykes at this locality(dip of dyke margins = 20‡). Hence, at QastalMaaf the net rotation is dominated by the com-ponent of tilting over the vertical axis component.

5. Discussion

5.1. Geological interpretation

The tilt-corrected mean inclination of341.1‡ U 3.4‡ indicates a palaeolatitude of 21^26‡N (assuming a reversed polarity) at the timeof magnetisation acquisition. This is in closeagreement with the palaeolatitude of 20‡N deter-mined for the Troodos ophiolite to the west ofBae«r^Bassit [8], and is consistent with a palaeo-position for the Neotethyan spreading axis be-tween the Arabian margin and the Late Creta-ceous position of the Pontides [30].Observed rotations are likely to be composite in

origin and may potentially be due to successivephases of intraoceanic thrusting, emplacementon to the Arabian continental margin and laterneotectonic deformation. Thus polyphase mecha-nisms involving combinations of coherent rota-tion of the ophiolite sheet, di¡erential rotationduring thrust sheet imbrication and late-stagestrike-slip faulting are possible. Here we considerboth the internal pattern of rotations within theBassit sheet and the wider, regional context of theophiolite as a whole to propose a two-phase ro-tation model.Within the Bassit sheet, rotation angles system-

atically increase from north to south. This maypotentially re£ect the in£uence of two changesin structural style in this direction: (1) the ophio-lite becomes structurally thinner and more in-tensely imbricated by thrust faulting towards thesouth (Fig. 2); and (2) to the south, the ophioliteis cut by an increasing number of neotectonic,sinistral strike-slip faults (Fig. 2) which representpart of the extension of the plate boundary zonebetween the African plate and the Turkish micro-plate, linking the Cyprus subduction zone to theDead Sea Transform in the east.Components of rotation about vertical axes

within thrust stacks have been documented in nu-merous orogenic belts (e.g. [29,31^35]). However,in the Bae«r^Bassit case, the largest rotations areassociated with the structurally lowest, mostsoutherly imbricate slices. This is di⁄cult to rec-oncile with accepted models for the evolution ofimbricate thrust stacks [36,37]. In foreland prop-

Fig. 8. Results of the net tectonic rotation analyses. Note thegeneral north^south increase in rotation angles within theBassit sheet localities.

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agating thrust systems, the structurally highestand earliest thrust slice should experience all com-ponents of rotation associated with the develop-ment of subsequent, structurally lower thrusts,and should therefore exhibit the largest rotation.In alternative hinterland propagating systems, thestructurally lowest and earliest thrust slice be-comes inactive following the initiation of the sub-sequent, structurally higher thrust fault. In thissituation there are no incremental increases in ro-tation during the evolution of the system. Alter-native mechanisms of thrust sheet rotation suchas di¡erential shortening along strike [35] are alsodi⁄cult to reconcile with observed structuraltrends in the Bae«r^Bassit case. The variations inrotation across the Bassit sheet are, therefore, dif-¢cult to ascribe to di¡erential thrust sheet rota-tion during imbrication.Rotations associated with strike-slip fault sys-

tems have been widely reported. For example,Sonder et al. [38] demonstrated a progressive in-crease in clockwise rotation with proximity to thedextrally slipping Las Vegas Valley Shear Zone,reaching 100‡ at the closest localities. Fault blockrotations associated with the sinistral fault systemtraversing the Bae«r^Bassit ophiolite (Fig. 2) arelikely to be anticlockwise in sense, and couldtherefore account for the observed progressive in-crease in rotation southwards.The Bae«r^Bassit ophiolite is interpreted as the

structurally thinned, southernmost ‘feather-edge’of a more extensive ophiolitic sheet which is dom-inated by the Hatay ophiolite exposed to thenorth in southern Turkey [18]. Preliminary pa-laeomagnetic data from the Hatay ophiolite alsoindicate large (c. 70‡) anticlockwise rotation ofthe ophiolite sheet (J. Inwood, personal commu-nication). In contrast to Bae«r^Bassit, the Hatayophiolite does not appear to have experienced ex-tensive neotectonic strike-slip deformation [39].Furthermore, an 80^90‡ anticlockwise rotationhas been previously documented in the Troodosophiolite to the west [8], approximately 60‡ ofwhich was completed by the Maastrichtian [9],i.e. by the time of emplacement of the Bae«r^Bassitand Hatay units. These observations suggest thata signi¢cant component of the Bae«r^Bassit rota-tions may be due to initial detachment of the

ophiolite and/or its emplacement prior to ¢nal im-brication. Interestingly, such large (c. 75‡) intra-oceanic vertical axis rotations have been reportedpreviously for the Oman ophiolite [40,41] wherediscrete phases of extrusive activity have allowedphases of rotation to be distinguished.We suggest that an early phase of regional

scale, intraoceanic rotation was driven by obliqueconvergence between a subduction zone to thesouth of the Troodos and Bae«r^Bassit/Hatay oce-anic crust and the Arabian margin, as previouslyproposed for the Troodos palaeorotation [9]. Bythe Maastrichtian, some 60‡ of anticlockwise ro-tation of the supra-subduction zone crust had oc-curred. At this stage, the Bae«r^Bassit and Hatayophiolites began to be emplaced onto the Arabiancontinental margin, leaving the Troodos ‘micro-plate’ to complete a further 30‡ of rotation inan intraoceanic setting. We cannot exclude thepossibility that the detached Bae«r^Bassit ophio-litic sheet underwent further bulk rotation duringemplacement over the Arabian margin, but priorto ¢nal imbrication. However, the pattern of ro-tations recorded in this study suggests that thepreviously rotated, emplaced ophiolite experi-enced a ¢nal phase of di¡erential rotations duringthe Late Tertiary, associated with the initiationand development of the present plate boundarycon¢guration. During this phase, neotectonic si-nistral strike-slip faulting resulted in ampli¢cationof rotation angles during kilometric-scale dissec-tion of the thin, structurally weakened leadingedge of the ophiolitic sheet. Importantly, this hy-pothesis can be tested by palaeomagnetic analysisof the neoautochthonous Tertiary sedimentarycover sequences of the Bae«r^Bassit ophiolite,which will only have experienced rotation duringthe neotectonic phase.

5.2. Interpretation of data from the Bassit Roadlocality

Magnetisation directions from the sheeteddykes of the Bassit Road section are clearlyanomalous with respect to those observed at otherlocalities and the expected reference direction.These dykes are presently in a sub-vertical orien-tation with a NNW^SSE trend. Application of a

EPSL 6311 30-8-02


tilt correction to restore dyke orientations to thevertical therefore results in very little change inmagnetisation direction, which remains sub-verti-cal in both in situ and tilt-corrected referenceframes (Table 1; Fig. 5). The magnetic propertiesof samples from this section are indistinguishablefrom those observed elsewhere, and there is nogeological reason to propose that these rockshave been selectively remagnetised. We assume,therefore, that remanences at the Bassit Road lo-cality also represent pre-deformational magnetisa-tions.The simplest explanation for the anomalous di-

rection of magnetisation at this locality is that thesampled, sub-vertical dykes have been a¡ected bya component of tilting around a dyke-normalaxis. For initially vertical dykes, this would resultin no visible change in orientation, but wouldrotate magnetic remanences around small circlescentred on the tilt axis. These data may be use-fully analysed using the net tectonic rotation al-gorithm described above [2,7]. This approachyields a preferred solution indicating a shallowlyplunging, dyke-normal axis about which approx-imately 80‡ of anticlockwise rotation has occurred(Fig. 8), consistent with a signi¢cant componentof otherwise un-resolvable tilt. Interestingly, thisrotation axis is normal to the SE-directed em-placement direction for the ophiolitic thrust sheets[14], and is therefore kinematically compatiblewith tilting during thrust faulting. The net rota-tion about a shallowly plunging axis may be de-composed into 54‡ of tilting around a purely hor-izontal, dyke-normal axis and 30‡ anticlockwiserotation around a purely vertical axis.These data provide a cautionary note concern-

ing the interpretation of palaeomagnetic datafrom dykes in general, since any component ofrotation of a vertical dyke about a dyke-normalaxis will result in a tilt which can not be resolvedin the ¢eld. In the present dataset, however,we are con¢dent that the sheeted dykes at theNorth Coast and Qastal Maaf localities havenot been a¡ected in this way, since their tilt-cor-rected magnetisations have inclinations that areconsistent with those obtained from palaeohori-zontal sites.

5.3. Implications for the age of the Bae«r^Bassitophiolite

There is unequivocal evidence that the ophioliterecords a pre-deformational remanence. Both nor-mal and reversed polarities of magnetisation arepresent, but reversed polarities dominate the site-level data. These data are di⁄cult to reconcilewith the radiometric ages available for the ophio-lite. In particular, the 85^95 Ma K^Ar ages ob-tained for the metamorphic sole [12] suggest for-mation of the ophiolite prior to 95 Ma, within thelong Cretaceous normal interval (Chron C34n).Two possibilities exist if these ages are consideredreliable: (1) the ophiolite formed prior to C34nduring the mixed polarity part of the Early Creta-ceous (s 120 Ma; [42]). This would imply a longtime period between formation of the ophioliteand its metamorphic sole, in contrast to other,better dated Tethyan ophiolites (e.g. Oman); or(2) the ophiolite formed during a poorly docu-mented reverse polarity event within C34n. Suchsubchrons have been identi¢ed in the Albian byanalyses of DSDP cores [43^45], but have proveddi⁄cult to correlate globally because of theirshort durations of 2^3 Myr [46]. Alternatively,the age estimate of the metamorphic sole maybe in error, with the ophiolite forming at theend of chron C34n (80^83 Ma; [42]). In this re-spect, it is interesting to note that the K^Ar agesof 73^99 Ma obtained from the ophiolite itself[12] span reversed polarity chron C33r in theLower Campanian. In either case, there is nowneed for urgent re-evaluation of the age of theBae«r^Bassit units through more reliable radiomet-ric methods (e.g. 40Ar^39Ar) and possibly biostra-tigraphic dating of rare outcrops of ferromanga-niferous sediments associated with the extrusiveseries.

6. Conclusions

The Bae«r^Bassit ophiolite of northern Syria re-tains a stable magnetisation which is unequivo-cally shown to be pre-deformational in origin bypositive tilt and reversal tests. The age of the

EPSL 6311 30-8-02


ophiolite is at present poorly constrained by ra-diometric dating, and requires re-evaluation in thelight of the dominance of a reversed polarity ofmagnetisation. Remanence vectors after tilt cor-rection indicate signi¢cant and, in most cases, ex-treme amounts of anticlockwise tectonic rotationof the ophiolitic units. Rotations are variable on akilometric scale and are of composite origin, re-£ecting the complex progressive structural devel-opment of the ophiolite. The pattern of rotationswithin the ophiolite and the regional context sug-gest that the most likely sequence of events in-volves: (1) a substantial (c. 60‡) rotation of theophiolite as a coherent sheet in an intraoceanicsetting (with a possible additional component dur-ing emplacement on to the Arabian margin) ;(2) disruption of the ophiolitic sheet by thrustimbrication (without relative thrust sheet rota-tion); and (3) a ¢nal phase of di¡erential rotationassociated with neotectonic strike-slip faulting. Amajor sinistral strike-slip system, representing theextension of the plate boundary zone between theAfrican plate and the Turkish microplate, trans-ects the thinned ophiolitic thrust stack along aNNE^SSW trend. Future research is aimed atproviding further constraints on the evolution of¢nite rotations in this setting via palaeomagneticanalyses of the neoautochthonous sedimentarycover of the ophiolite.

Acknowledgements

We are extremely grateful to the Director of theSyrian Geological Establishment, Dr Talal Balla-ni, for permission to conduct ¢eldwork, to DrAbdul Salam Turkmani for logistical supportand discussion, and to Shell Syria for logisticalsupport. The demagnetisation data were analysedand the inclination-only tilt tests performed usingprograms developed by Dr Randy Enkin. Prof.Henry Halls and Dr Randy Enkin are thankedfor their comments on the tilt test implementa-tion. The paper bene¢ted from helpful reviewsby Dr Nigel Woodcock, Prof. Michel Delaloyeand Dr Graham Borradaile.[RV]

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extreme tectonic rotations within an eastern mediterranean ophiolite (baër–bassit, syria)

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