Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes

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1 Tectonophysics 418 (2006) Magnetic studies of magma-supply and sea-floor metamorphism: Troodos ophiolite dikes G.J. Borradaile, D. Gauthier Geology and Physics Department, Lakehead University, Thunder Bay, Canada ON P7B 5E1 Received 29 August 2005; received in revised form 28 October 2005; accepted 5 December 2005 Available online 3 March 2006 Abstract Dikes of the eastern Troodos ophiolite of Cyprus intruded at slow ocean-spreading axes with dips ranging up to 15 from vertical and with bimodal strikes (now NE SW and N S due to post-88 Ma sinistral microplate rotation). Varied dike orientations may represent local stress fields during dike-crack propagation but do not influence the spatial-distributions or orientation-distributions of dikes' magnetic fabrics, nor of their palaeomagnetic signals. Anisotropy of magnetic susceptibility (AMS) integrates mineral orientation-distributions from each of 1289 specimens sampled from dikes at 356 sites over 400 km 2 in the eastern Troodos ophiolite of Cyprus. In 90% of dikes, AMS fabrics define a foliation (k MAX k INT ) parallel to dike walls and a lineation (k MAX ) that varies regionally and systematically. Magma-flow alignment of accessory magnetite controls the AMS with a subordinate contribution from the mafic silicate matrix that is reduced in anisotropy by sea-floor metamorphism. Titanomagnetite has less influence on anisotropy. Occasionally, intermediate and minimum susceptibility axes are switched so as to be incompatible with the kinematically reasonable flow plane but maximum susceptibility (k MAX ) still defines the magmatic flow axis. Such blended subfabrics of kinematically compatible mafic-silicate and misaligned multidomain magnetite subfabrics; are rare. Areas of steep magma flow (k MAX plunge 70 ) and of shallow magma-flow alternate in a systematic and gradual spatial pattern. Foci of steep flow were spaced 4 km parallel to the spreading axes and 6 km perpendicular to the spreading axes. Ridge-parallel separation of steep flow suggest the spacing of magma-feeders to the dikes whereas ridge-perpendicular spacing of 6 km at a spreading rate of 50 mm/a implies the magma sources may have been active for 240 Ka. The magma feeders feeding dikes may have been 2 km in diameter. Stable paleomagnetic vectors, in some cases verified by reversal tests, are retained by magnetite and titanomagnetite. In all specimens, the stable components were isolated by three cycles of low-temperature demagnetization (LTD) followed by 10 steps of incremental thermal demagnetization (TD). 47% of primary A-components [338.2/+57.2 n=207, α 95 =3.9; mean T UB =397±8 C] are overprinted by a B-component [341.4/+63.5, n=96, α 95 =8.7; mean T UB =182±11 C]. A- and B-components are ubiquitous and shared equally by the N S and NE SW striking dikes. A-component unblocking temperatures (T UB ) are zoned subparallel to the fossil spreading axis. Their spatial pattern is consistent with chemical remagnetization at some certain off-axis distance determined by sea-floor spreading. A-components indicate less microplate rotation and more northerly palaeolatitudes that are consistent with metamorphic remagnetization after some spreading from the ridge-axis. Thus, their magnetizations are younger than those of the overlying volcanic sequence for which ChRMs are commonly reported as 274/+33 (88 Ma) Elsevier B.V. All rights reserved. Keywords: Ophiolite; Sheeted dikes; Seafloor metamorphism; Magnetic fabrics; Spacing of magma feeders; Chemical remagnetization; Unblocking temperature Corresponding author. address: borradaile@lakeheadu.ca (G.J. Borradaile) /$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.tecto

2 76 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) Introduction The Cretaceous Troodos ophiolite of Cyprus was the first most completely documented example of a terrestrial exposure of oceanic crust (Gass, 1968, 1990). Unlike fragmented ophiolite slivers obducted in numerous orogenic belts, the Troodos ophiolite is devoid of significant penetrative secondary tectonic deformation (Nicolas, 1989). The gently domed sequence exposes fresh oceanic-mantle rocks at its centre near Mount Olympus (1951 m), ringed by successively higher igneous stratigraphy. Successively, these include the dike complex, a basal group of dikes with thin vertical screens of extrusive material, umbers, exhalative deposits and pillowed basalts (Fig. 1a) (Gass, 1968; Moores and Vine, 1971; Robertson, 1990). These are capped by an upwards shallowing limestone sequence almost continuously deposited from 54 Ma to the present day. The ophiolite comprises 1600 km 2 of which dike exposures account for approximately 60% of the area Fig. 1. (a) Characteristic tectonic regions of Cyprus. (b) Simplified geology after the Geological Survey Department of Cyprus with 356 sampling sites of ophiolite dikes used in this study. (c) Spatial averages of 1169 dike orientations, averaged as unit-vectors at 500 grid locations with overlapping counting cells. Cell-counts were weighted according to the inverse-square distance of the observation from the cell centre's location. (d) Spatial averages of 1289 maximum susceptibility axes (k MAX ), determined as mean tensor orientations from groups of sites. Tensor-mean values were cell-counted and averaged as were dike orientations in (c).

3 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) and for approximately 2 km of its stratigraphy. Graben, fossil transforms and domains of slightly different dikeorientations are now well-documented (Moores et al., 1990). Extreme dike and lava orientations are due to local stress trajectory heterogeneity or paleo-topography, respectively. Tectonic deformation is confined to moderately intense brittle cataclastic deformation within a 5 km wide zone along the Southern Troodos Transform Fault Zone (STTFZ, Fig. 1a, Simonian and Gass, 1978). The fracture-zone deformation postdates sea-floor metamorphism and postdates the paleomagnetization (Bonhommet et al., 1988). The crust is tilted by 10 away from the centre of the Troodos dome but most of the variation in dike orientation is original. At individual outcrops, dikes of different near-synchronous ages crosscut one another obliquely (Fig. 2), commonly with a bimodal distribution of strikes (NW and NS; Figs. 1b, 2) with either one or both margins chilled. The range of dike-dips at any single outcrop greatly exceeds regional dip-variation and indicating that paleomagnetic-tilt corrections from individual sites may be inappropriate, even if tilt axes may be precisely determined. With strata, it is impossible to obtain a unique solution for their original declination, merely assuming that they were horizontal (MacDonald, 1981). For dikes, one can neither obtain a unique solution for declination or inclination from the assumption that they were once vertical (Borradaile, 2001a). Furthermore, the variance in dike dip in individual outcrops indicates that not all dikes could have been vertical originally (Fig. 2). Regional mean paleomagnetic declinations (Moores and Vine, 1971; Clube et al., 1985; Clube and Robertson, 1986; Robertson, 1990) proved sinistral microplate rotation about a vertical axis, corroborated by subsequent more detailed and localized studies (Bonhommet et al., 1988; Allerton and Vine, 1990; Macleod et al., 1990; Morris et al., 1990, 1998). Determination of the apparent polar wander path (APWP) from recent and older work indicates that the early rotation axis was located near or within the microplate and subsequently lay to the west, over which interval the microplate was translated northward from an equatorial latitude to the present 35 N (Borradaile and Lucas, 2003). Dilek et al. (1998) compare the Troodos ophiolite to a modern slow-spreading axis, for which the typical ridge separation rate would be 50 mm/a. 2. Goals We have examined the regional variation in the acquisition of remanent magnetism relative to the exposed spreading axis and compare this to the location of potential magma-feeders inferred from magma flow axes. Flow axes were interpreted from the anisotropy of low field magnetic susceptibility (AMS) with due allowance for mineralogical sources of susceptibility, partitioning of bulk susceptibility between different minerals and subfabrics with different orientation distributions. AMS rarely yields kinematic information directly (Borradaile and Jackson, 2004), even in magmatic rocks. That required supplementary techniques to isolate different subfabrics such as determining anisotropy of anhysteretic remanent magnetism (AARM), comparing AMS fabrics for different bulk susceptibilities, comparing mean tensors for normalized and nonnormalized AMS, and traditional techniques to determine magnetic mineralogy (hysteresis, thermomagnetic curves, mineral separations) (Borradaile, 2001b; Borradaile and Lagroix, 2001; Borradaile and Gauthier, 2001, 2003; Borradaile and Lucas, 2003). Sampling extended 15 km east and west of the Mitsero spreading axis, covered 400 km 2, and recorded Fig. 2. Primary dip-variation of ophiolite dikes, viewed along strike near Sykopetra. (a) Vertical road-cut (b) mountain slope, with some topographic cut-affect.

4 78 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) the orientations and other details of 1169 dikes from which 440 oriented hand specimens were taken. There is a slight bias for specimens taken further from the centre of the Troodos dome to sample from higher levels within the dike complex (Fig. 6a). However, the substantial topographic relief in all parts of the study area tends to cancel such geographically-dictated sampling bias. The inclination of magma flow directions in the dike complex should indicate proximity to underlying magma supply. First, heterogeneity in their geographical distribution may be inferred from magnetic fabrics that proxy for magma-flow aligned minerals. These provide clues to the location of high level magmatic feeders underneath the dike complex. Second, the ridge-normal spacing of those feeders is proportional to their longevity that may be estimated from an approximate spreading rate (say, 50 mm/a). Third, the intensity of early chemical thermal re-magnetizations is zoned parallel to the spreading axis. This verifies its metamorphic origins and its patchy distribution indicates its loose correlation to the earlier magnetizations of the overlying pillow lavas. 3. Mineral alignments and magma flow from magnetic fabrics (AMS) Anisotropy of low field susceptibility (AMS) merges induced magnetization anisotropy contributions of all minerals in a rock. In the dikes, this principally constitutes high-susceptibility remanence-bearing minerals such as titanomagnetite and magnetite, and the paramagnetic mafic silicates pyroxene and olivine and their low-grade metamorphic by-products, chlorite, epidote, and amphibole. The metamorphic by-products also produce exsolutions of remanence-bearing iron oxides. In these rocks, the diamagnetic contribution from the minerals is negligible in comparison with the specimens' mean susceptibilities (Fig. 3a) (Jackson and Tauxe, 1991; Rochette et al., 1992). For single crystals of high susceptibility ( 0.2 SI), principal susceptibility axes are parallel to the dimensional axes. This is chiefly applicable to polydomain magnetite and Ti-poor titanomagnetite; they yield AMS fabrics with simple kinematic interpretations in which k MAX defines flow axis and the k MAX k INT foliation defines the flow plane. For rock-forming minerals, Fig. 3. (a) Bulk susceptibility (κ) frequency-distribution of dikes showing modes that correspond to specimens for which κ is dominated by either magnetite or titanomagnetite. (b) AMS anisotropy of dikes grouped according to bulk susceptibility (κ). Pj=eccentricity of ellipsoid; Tj=shape (Jelinek, 1981); higher susceptibility specimens for which κ is dominated by magnetite have higher anisotropy and generally more oblate than specimens dominated by titanomagnetite. The polar plot (Borradaile and Jackson, 2004) restricts isotropic (and low anisotropy fabrics) to one location (or near the origin), unlike Cartesian plots (e.g., Flinn diagram).

5 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) kinematic interpretations of AMS are only possible for minerals of high symmetry (preferably better than monoclinic). In the ophiolite dikes, the mafic silicates could provide a petrofabric contribution but, apart from being poorly aligned due to hydrothermal metamorphic overprinting, their maximum theoretical susceptibility is so low that the alignment of iron titanium oxide dominates the magnetic fabric (Borradaile and Jackson, 2004). Exceptionally in these rocks, single domain magnetite may reveal its characteristic inverse fabric due to the inverse mapping of maximum/minimum principal magnitudes on minimum/maximum dimensions (Stephenson et al., 1986). However, where present, this contribution is so small that its worst complication is to deflect a kinematically compatible AMS slightly from the flow directions. Such blended fabrics (Borradaile and Jackson, 2004) were first recognized by Rochette et al. (1992) as intermediate fabrics. Rare examples of blended fabrics were isolated by Borradaile and Gauthier (2001, 2003). However, for most minerals including all common matrix-forming minerals, AMS axial orientations are crystallographically controlled. For diabase dikes and their ocean-metamorphic equivalents, the high-susceptibility silicate matrix mostly comprises monoclinic minerals in which only one AMS axis may be parallel to a crystallographic axis. Furthermore, one cannot simply predict the correlation between principal susceptibility magnitudes and crystallographic axis or habit-dimension, even for the case of pure silicates. For impure crystals, oxide inclusions or exsolutions may further reduce the correlation of AMS and crystallographic symmetry further (Borradaile, 1994; Borradaile and Werner, 1994; Lagroix and Borradaile, 2000). Triclinic minerals such as clinopyroxene can have no axis parallel to a principal AMS axis. Nevertheless, previous work in the Troodos dike complex inferred that AMS was a rapid means of identifying flow axes determined by other methods (Staudigel et al., 1992; Varga et al., 1998). We have shown that this is valid simply because the bulk susceptibility is dominated by polydomainal Fe Ti oxides for which grain-shape correlates with AMS axial orientations and magnitudes. We also investigated the problems of within-dike fabric heterogeneity and the appropriate sampling density for regional-scale work. For example, the appropriate minimum valid sampling density was established with considerations of magnetic mineralogy, anisotropy of anhysteretic remanence (AARM) and AMS for 139 specimens from seven dikes at each of two sites (Borradaile and Gauthier, 2003). This established that flow-imbrication was rare and that most meaningful dike-flow was inferred from AMS of central portions of dikes N0.5 m thick. We validated AMS axes as flowpetrofabric proxies by three procedures. First, AMS summarizes the total fabric and was usefully compared with AARM that isolates the ferromagnetic subfabric. Second, comparing the AMS of specimens classified according to their bulk susceptibility produces a similar discrimination of relative subfabric contributions (370 specimens from 79 dikes; Borradaile and Gauthier, 2001). Third, Jelinek (1978) observed that the mean tensor for a suite of specimens may be calculated using either the measured principal susceptibilities or their normalized equivalents (normalized avoids confusion with the statistical procedure of standardization). Normalized principal susceptibility values are achieved by dividing each of the principal susceptibility values by the bulk susceptibility. Borradaile (2001a,b) shows that differences between normalized and non-normalized mean tensors (mean tensor orientation, confidence cones and fabric symmetry) give direct clues to the relative strengths and orientations of different subfabrics. Bulk susceptibilities (averaged from the seven measurements along different specimen axes in AMS determination), have a bimodal frequency distribution (Fig. 3a). With a logarithmic susceptibility scale, the modes are symmetrically humped, typical of frequency distributions related to low abundance constituents, e.g., accessory minerals. The modes correspond to specimens in which either titanomagnetite or magnetite dominates the susceptibility. Both minerals greatly exceed the theoretical maximum susceptibility ( 2000 μsi) for any of the matrix silicates. There is no geographical or structural control on the distribution of bulk susceptibilities. Higher susceptibility specimens (N 9000 μsi) in which magnetic fabrics are dominated by magnetite have more eccentric and more oblate AMS (Fig. 3b). We prefer the parameters Pj for eccentricity (1=sphere) and Tj (+1=oblate; 1=prolate) for symmetry of the susceptibility magnitude ellipsoid, following Jelinek (1981). These plot conveniently on the polar plot (Borradaile and Jackson, 2004) in which isotropic fabrics (Pj=1) have a unique location, in contrast to other projections (e.g., Flinn diagram). Despite all the possible complications recognized by rock-magnetic research (e.g., Jackson and Tauxe, 1991; Rochette et al., 1992), we have previously established that AMS foliation and lineation are valid proxies for magmatic flow foliation and flow lineation at the 95% confidence level for dikes m thick (Borradaile and Gauthier, 2001, 2003). Within-dike variance in fabric-

6 80 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) orientation is less than the variation between dikes, which leads to the conclusion that sampling fabrics of four specimens from each of a large number of dikes is an efficient strategy for understanding regional petrofabrics. Large within-site samples simply duplicate the variance detected by a small sample from each of several dikes, which separately may better detect inter-site variance; such sampling strategies are discussed by Borradaile (2003). Regional conclusions are drawn from a study of 1289 cores (10.5 cm 3 ) from 440 oriented hand-specimens at 356 sites in the eastern 400 km 2 of the dike complex (Fig. 1b). The orientations of 1169 dikes were recorded. AMS was measured using a Sapphire Instruments SI2B susceptibility meter in a field of 80 A/m ( 1 Oe) at 19,200 Hz using seven measurement axes per specimen, which enhances the directional assessment of anisotropy balanced with speed that in turn reduces measurement noise (Borradaile and Stupavsky, 1995). Specimen- AMS is represented by a symmetrical second-rank tensor, visualized as a magnetic fabric ellipsoid with mutually perpendicular axes of maximum (k MAX ), intermediate (k INT ), and minimum (k MIN ) susceptibility. Site-mean orientations for samples of AMS ellipsoids are tensor-averaged, using normalized susceptibility. Thus, all specimens are reduced to unit-susceptibility so that outliers (high susceptibility specimens with spurious orientations) do not bias the mean orientation distribution shown by the mean tensor (Fig. 4). The most common and simplest kinematic pattern has k MAX (magnetic lineation) parallel to the magmatic flow lineation, which lies in the magnetic foliation plane and is sub-parallel to the dike-plane. Mean k MAX orientations may be more than a few degrees from the mean dike plane, usually due to the presence of an anomalous oxide sub-fabric or rarely due to a singledomain magnetite inverse-fabric effect (Rochette et al., 1991; Borradaile and Gauthier, 2003), or intrinsic flow Fig. 4. For subareas A to E of Fig. 1. Subareas A D show steep k MAX within the dike-plane and inferred steep flow; sub-area E has shallow flow. Poles to dikes are contoured and mean standardized AMS tensor is shown with 95% confidence limits for the principal axes of the mean tensor, normalized by the bulk susceptibility. Some sub-areas blend subfabrics from mafic silicates and iron-oxides to yield superficially misleading kinematic interpretations. For example, k INT may be at a high angle to the mean dike plane, rather than parallel to it. However, caution must also be applied in any attempt to draw simple statistical comparisons between density contours of dike-poles and mean AMS tensors. Investigation of individual sites and subareas with respect to the difference between normalized and non-normalized mean tensor orientations, and the differences in AMS according to bulk susceptibility usually identify such problems even without AARM, although that was used in some pilot studies (Borradaile, 2001b; Borradaile and Gauthier, 2001, 2003).

7 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) heterogeneity (Tauxe et al., 1998). Of course, k MAX generally deviates from the dikes' strikes in map projection since it plunges obliquely down the magnetic foliation plane (flow-plane) (Fig. 1c, d). Only two sites show convincing non-coaxiality of Fe Ti oxide subfabrics and silicate orientation-distribution using the supplementary AARM technique (Jackson, 1991). The majority of dikes show AMS foliation subparallel to dike walls but their k MAX ( flow-axis) plunge varies regionally and systematically (Figs. 1c,d; 4). This cannot simply trace dike-crack propagation since individual dikes are far too irregular, discontinuous and short to explain systematic regional variations over thousands of dikes. Secondly, the regional variations comprise contributions from many asynchronous dikes. It is improbable that they would conspire to produce the systematic regional changes in flow orientation (Fig. 1d). 4. Flow-geometry from AMS Dilek et al. (1998) correlate magmatic style and hydrothermal alteration with spreading rate. They demonstrate that fast spreading axes, with bilateral rates 100 mm/a, are fed by relatively extensive, robust magma chambers that keep pace with oceanfloor spreading. On the other hand, slow-spreading ridges may be fed by smaller short-lived and discretely spaced magma chambers, permitting a more effective hydrothermal-metamorphism. A consequence of spreading geometry is that ridge-parallel separations of steep magma-flow record the geographical spacing of magma feeders. On the other hand, ridge-perpendicular spacing provides a clue to their longevity since it relates to ocean-floor spreading-rate. Smaller, magma foci may be expected to feed dikes in a radial upward fashion with vertical flow above a feeder and at a gentler plunge distally, as in this area (Fig. 1d). The variation in plunge of magma-flow axis ( k MAX ) localizes foci of steep flow and of magmasupply along the spreading ridge, in coeval crustalsegments as suggested by the slow-spreading model of Nicolas (1989). Flow-inclinations from three traverses to the east and to the west of the Mitsero graben-axis have been analyzed by Fast Fourier Transform (FFT) to yield the predominant separation-frequencies (Fig. 5a). Ninety per cent of the vertical flow-foci mean are spaced at intervals of 4±1.5 km, parallel to the ridge axis, as shown by the box-and-whiskers summary statistics (inset, Fig. 5). Therefore, their magma-supply sources were probably 2 km in diameter. Similarly, the characteristic spacing of vertical-flow foci may be inferred along traverses perpendicular to the Mitsero spreading axis; these relate to spreading rate. Fast Fourier Transform (FFT) analysis isolates characteristic frequencies of spacing of steep flow along each of five ridge-perpendicular traverses. This technique shows that 90% of first harmonics indicate a + mean spacing lying in the range km (Fig. 5b). Fig. 5. Plunge angle of the maximum susceptibility (k MAX ) represents mean mineral alignment and magma flow-axis. (a) k MAX plunge plotted against distance for six traverses parallel to the Mitsero graben. The mean spacing of steep-flow foci is 4 km with a range from 2 to 8 km. (b) k MAX plunge plotted against distance for six traverses perpendicular to the Mitsero spreading axis. This identifies spacing influenced by the spreading-rate; mean separation of steep-flow foci is 6 km along traverses perpendicular to the spreading axis. The inset box-and-whiskers plots show the range, 5th and 95th percentiles and mean for the predominant spacing-frequencies obtained by Fast Fourier Transform analysis.

8 82 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) Accepting the slow-spreading model plate-divergence of 50 mm/a, 90% of such magma foci would be + separated by a time interval in the range Ka. The presence of significant areas of shallow magma flow may exclude the fast-spreading model entirely, in contrast to findings in the Oman ophiolite (Rochette et al., 1991). The inference that magma supply to the base of the dike complex occurred at foci with intervals of 240 Ka is compatible with the model of Dilek et al. (1998) of less robust, less continuous magmatic activity. Our relatively dense sampling over 400 km 2 characterizes flow through all levels of a sheeted dike complex, above localized magma conduits. Our spatial and temporal distribution of AMS-inferred flow directions relate only to the feeders immediately beneath the dikes, not necessarily to the spacing and distributions of magma chambers sensu stricto (Calvert, 1995; Dilek et al., 1998; Sinha et al., 1998). 5. Spatial-distribution of paleomagnetic unblocking temperatures Depending on the spreading rate, magma supply and submarine topography (e.g., Kusznir, 1980; Schouten and Denham, 2000), the extrusive sequence may be 0.5 to 2 km thick. Thus, by the time it has spread 20 km offaxis, all pillowed basalts and most upper dikes reveal ocean-floor hydrothermal greenschist metamorphism or, more rarely, brownschist facies metamorphism. Confining pressure is insufficient to inhibit open cracks and fissure-circulation in the upper 3 km, so that the upper dikes are also hydrothermally metamorphosed, although possibly to amphibolite facies. The absence of open fractures below 3 km restricts hydrothermal alteration so that some lower dikes and most mantlesequence oceanic rocks preserve a primary magnetic mineralogy to some extent and in some cases some ridge-acquired thermoremanent magnetism (TRM) persists. Of course, terrestrial exposures of oceanic crust (ophiolites) have secondary tectonic histories that may complicate the paleomagnetic record at any stratigraphic level, e.g., the exhumation of the Troodos ophiolite to a mountain range. Ocean-floor metamorphism is attributable to ephemeral, ridge-parallel zones of hydrothermal circulation, especially effective in slow-spreading environments and active to depths of 2 km and at distances 20 km from the spreading axis (Raymond and LaBreque, 1987; Gillis and Robinson, 1988; Gillis and Roberts, 1999). Regional magnetic anomalies in modern oceans are ridge-parallel zones of intense remanent magnetization. They provide geometrical and temporal control on the spreading mechanism and their recognition initiated the notion of ocean-floor spreading and plate-tectonic motion (Vine and Matthews, 1963). The Troodos ophiolite was the first terrestrial exposure in which the spreading model was tested paleomagnetically (Vine, 1966; Varga and Moores, 1985; Allerton and Vine, 1987). However, chemical remanent magnetization (CRM), actually a remagnetization process in this case due to sea-floor metamorphism, overprints or completely replaces primary TRM in most of the dike complex (Varga et al., 1999). Furthermore, subsequent paleomagnetic work revealed the importance of incremental demagnetization to isolate characteristic paleomagnetic vectors from later overprints. CRMs are carried by titanomagnetite and its oxidized versions, with unblocking temperatures (T UB ) b300 C. However, vector components with higher T UB s (N400 C) are also common, carried by magnetite. Whereas the titanomagnetite be primary, its magnetization may not be. Secondary magnetite is produced in by serpentinization of deeper dikes and of the mantle sequence rocks but primary magnetite is preserved in some mantle sequence beneath the dike swarm. Most chemical remagnetization occurs when titanomagnetite grows above the superparamagnetic grain-size in a hydrothermal, oxidizing environment. Viscous remagnetization (VRM) and postmetamorphic cooling TRM may also contribute to the final multi-component remanent magnetization (Hall et al., 1995), although laboratory demagnetization cleans these out easily, either by LTD in this study, or early thermal demagnetization in other studies. Troodos volcanic rocks and, by implication, their underlying plutonic and hypabyssal rocks, formed in a short Cretaceous interval, mostly in the Turonian (90 88 Ma). Three spreading-axes have been mapped. Their present trend is N S, with an associated E W transform fault along the southern margin of the Troodos mountain range (Moores and Vine, 1971; Simonian and Gass, 1978; Varga and Moores, 1985; Gass et al., 1994). Ocean-floor divergence was moderately slow ( 50 mm/ a), from petrological and tectonic inferences. Moores et al. (2000) recognized that their geochemical signature might be explained easily if they were fed by recycled mantle sources. This explanation provides a simple alternative to the supra-subduction setting offered to explain the mantle-depleted geochemical affinities (Simonian and Gass, 1978; Robinson et al., 1983; Malpas et al., 1993). Paleomagnetic studies reveal that the ophiolite terrane rotated anticlockwise at a decelerating rate (Clube and Robertson, 1986), since the extrusion of the pillowed sequence at 88 Ma. Net anticlockwise rotation has been approximately 90, so

9 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) that the original spreading axes were aligned E W. For the initial 38 Ma, rotation was 2 /Ma about a vertical axis located within the Troodos microplate, and thereafter more rapid about an external vertical axis (Borradaile and Lucas, 2003). The later component of rotation moved the microplate first slightly south of the Equator and then north to its present latitude at 35 N. 6. The Troodos ophiolite and its dike-complex Within the study area, dike-orientations (n = 679) fall into N S and NE SW striking modes, distributed ubiquitously and homogeneously across the area (Fig. 6a c). Along the southern edge of the area, dikes were mechanically re-oriented towards an EW trend by Fig. 6. (a) AMS sample sites studied in the eastern dike complex of the Troodos ophiolite. (b c) Orientations of individual dikes, average widths 1.5 m, fall into two modes whose spatial-distribution is homogenous. (c d) Synopsis of A- and B-component paleomagnetic vectors, respectively (Lower hemisphere, equal-area projections.) These are equally distributed between NE SW and N S trending dikes.

10 84 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) dextral cataclastic shearing on the southern Troodos transform fault (Bonhommet et al., 1988). Regionally, dike-orientations may be broadly partitioned into geographical sub-domains (Versoub and Moores, 1981; Varga and Moores, 1985; Hurst et al., 1992; Dietrich and Spencer, 1993). Nevertheless, within this study area, it appears that two simultaneous or pervasively alternating fracture patterns permitted dikes to intrude in either mode at any location, with similar probabilities (Fig. 6b c). The presence and the intensity of primary A-components and their B-overprints are independent of dike orientation (Fig. 6c d). The A-magnetizations are clearly due to early sea-floor metamorphism since they predate shearing along the Southern Troodos Transform zone (Bonhommet et al., 1988; here see Fig. 1b). Apart from regional tilting, tectonism is negligible within this study area; sitespecific tilt-correction for dikes are unwise since there is no reliable paleohorizontal marker and the traditional procedures would introduce errors in both declination and inclination (MacDonald, 1981; Borradaile, 2001a). Instead, small regional tilt corrections were applied to our paleopoles using the mean formational dips of the closest overlying extrusive volcanic rocks. 7. Paleomagnetic sampling campaign Oriented hand-specimens were drilled in the laboratory to yield, in geographic coordinates, 1289 standard paleomagnetic cores (right cylinders, 25 mm diameter 22 mm high), plus 300 cores from earlier magnetic fabric work (Borradaile and Gauthier, 2001, 2003). Reliable paleomagnetic information requires that every specimen is incrementally demagnetized to ensure directional stability of the characteristic vectors and isolate multiple components of magnetization (e.g., Butler, 1992; van der Voo, 1993). For tectonic purposes, the orientation of a vector component is usually the only important information, most particularly the characteristic (ChRM) or primary component ( A ). In these rocks, conventional tests for its primary age are not feasible, with the exception of a few polarity reversaltests (three for the A-component and six for the B- component). Slow chemical remagnetization and metamorphic cooling suffice to average secular variation in the paleofield so that meaningful paleopole positions may be determined from the ChRM directions. The relative position of successive paleopoles defines an APWP which may be used to judge relative ages and rotations rates of terranes (see below). Here we focus on the supplementary information provided by our paleomagnetic demagnetization experiments. The temperatures at which different directional magnetic components are removed, the unblocking temperatures (T UB ), provide indirect information on the extent and timing of ocean-floor re-mineralization. The ultimate demagnetization temperature will always correspond to the highest Curie temperature (T C ) present in the mineral assemblage, diagnostic of the most refractory ferro -magnetic mineral. Lower T UB s usually represent only the removal of a younger magnetic component. Thus, CRM unblocking temperatures determined in the course of our paleomagnetic cleaning experiments do not relate directly to magnetic-mineralogy although any T UB N250 C and b580 C must reside in magnetite or an oxidized titanomagnetite (e.g., Fig. 7e, f). Thermomagnetic experiments using a horizontal translation Curie balance verified the minerals' characteristic Curie temperatures (e.g., Borradaile and Lagroix, 2001; Borradaile and Gauthier, 2003). 8. Demagnetization experiments Specimens were first treated with three cycles of LTD (Dunlop and Argyle, 1991; Muxworthy and McClelland, 2000). Each cycle requires lowering the specimens' temperatures below 114 K, for which liquid nitrogen is convenient (77 K). Temperatures are equilibrated inside a magnetically shielded space, in which the specimens are subsequently allowed to warm to room temperature. Warming through the Verwey and isotropic transitions involves symmetry and other crystallographic transitions that mechanically clean out less significant geological remanences associated with domain walls in multi-domain magnetite. The Verwey transition is suppressed or absent in titanomagnetite. Experience shows that LTD prior to thermal demagnetization more clearly isolates the orientations of different vector-components with angular turning points, (e.g., Fig. 7b). In practical terms, LTD sharpens the turning points between successive vector components because it selectively removes parts of contiguous vector-components for which T UB s overlap (Hoffman and Day, 1978; Dunlop, 1979). After three cycles of LTD, each specimen was demagnetized in a Magnetic Measurements MMTD80 Thermal Demagnetizer at temperatures of 150, 200, 250, 275, 300, 325, 350, 375, 400, 450, 500, 550 and 600 C, although some specimens were completely demagnetized at temperaturesb600 C. Remanences were measured before each step, in a Molspin spinner magnetometer using an interactive software package that permits rapid visual isolation of vector components in rotatable 3D vector plots as well as conventional

11 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) Fig. 7. All demagnetization experiments used three cycles of low temperature demagnetization (LTD) followed by ten steps of thermal demagnetization. (a) Vector plot for a specimen in which only an A component is present. (b) Specimen with A-component and B-overprint. (c) Frequency distribution of unblocking temperatures (T UB ) for A-components and for B-components. (d) Frequency distribution for A-components with and without a B-overprint. (e) Triangular plot of Fe Ti oxides with Curie temperatures (after O'Reilly, 1984; Dunlop and Özdemir, 1997). (f) Arrows simplify trends of progressive alteration and resulting T C. Zijderveld plots. Principal component analysis defined the orientations of the visually identified vectors (Kirschvink, 1980). Most specimens show a ChRM or A-component vector stable at higher temperatures and defined by four or more demagnetization steps. Approximately half of the specimens also carry a B-component partial overprint. Mean unblocking temperatures (T UB ) for A and B are 397 ±8 C and 182±11 C, respectively. Representative vector plots show a specimen with both components and a spurious viscous overprint and another specimen with just an A-component (Fig. 7a, b). Many specimens carry spurious viscous magnetization

12 86 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) components (VRMs) sub-parallel the axial dipole field. These were removed by heating to 150 C or just by the pre-thermal LTD treatments. We do not consider further some specimens that exclusively carried what we infer to be younger VRMs, identified by low stability (T UB b150 C) and occasional reversed polarity, as noted by Gee et al. (1993). Stable A-vectors were defined in 207 specimens, 85 of which also bore B- overprints (Fig. 7c, d). A further 17 specimens carried B-components alone, identifiable by their low T UB and direction. 9. Unblocking temperatures (T UB ) Unblocking temperatures (or coercivities in the case of AF demagnetization) serve to isolate characteristic remanences (ChRM) with stable orientations that persist through several steps of demagnetization. From these, paleofields are inferred and from which paleopoles are calculated. Geologically significant unblocking temperatures may persist to the Curie temperature (T C ), at which the all paleomagnetization is removed. T C is characteristic for many important remanence bearing minerals (Magnetite = 580 C) but for solid solutions like titanomagnetite and oxidized titanomagnetite there is a range of composition-dependent T C, and consequently of potential T UB s in which paleomagnetic information may be stored. The frequency-distribution and the spatial-distribution of T UB s may be significant historically in the cases of uplifted plutonic rocks or metamorphosed rocks. In our case, spreading ocean floor has been metamorphosed by ephemeral hydrothermal circulation-cells. Even with slow spreading, such alteration will not be perfectly parallel to the ridge nor extend to uniform depths. The resulting heterogeneous spatial-distribution of T UB s reflects the spatial variations and relative timing of sea-floor metamorphism through its effects on magnetic mineralogy and chemical remagnetization. There is no reason to suspect significant re-heating of the dikes, thus the T UB s are below the effective Curie/ Néel temperature of their mineral alteration products (Fig. 7e, f). Although magnetite (T C =580 C) is the primary oxide in ocean crust and oceanic mantle, the dikes contain substantial amounts of titanomagnetite. The composition of the initial ferro -magnetic remineralization is close to Fe 2.4 Ti 0.6 O 4 (TM60) with an effective T C 250 C; its presence is ubiquitous and dominant in ocean-floor extrusive rocks but its proportion decreases downwards through the stratigraphy (Hall et al., 1997). In mantle harzburgites lherzolite the abundance of primary pure magnetite is enhanced by later serpentinization. According to the intensity of hydrothermal metamorphism, titanomagnetite is progressively replaced by oxidized versions, titano-maghemite (sensu lato) with T C 450 C (Fig. 7e). The T UB 's frequency distribution reveals a unimodal broad hump for B-components that peaks just below 200 C (Fig. 7c). They are superimposed on higher T UB A-components that we infer to be of early metamorphic age (e.g., also Hall et al., 1995). They are similarly oriented to the present geomagnetic field (Fig. 6d). The broad unimodal hump would be consistent with VRMstability (=T UB ) controlled by grain-size frequency distribution. The frequency-distribution of A-component T UB sis bimodal (Fig. 7c). One mode, negatively skewed just above 500 C, is attributed to residual magnetite in the deeper dikes. The other mode is positively skewed, peaks in the range C, and is associated with extensively metamorphosed specimens for which the T C (and maximum T UB ) are consistent with TM60 or an oxidized version (Fig. 7e, f). Positively skewed distributions are typically associated with the low concentrations for which the abundances are approximately lognormally distributed. The frequency distribution of T UB for specimens that only bear A-components is different from those with both A and B-components (Fig. 7d). Specimens that only have A, show the bimodal distribution of T UB whereas the A-component's T UB is negatively skewed from high TRM-values ( 550 C) toward lower values more characteristic of seafloor metamorphism. This suggests that the spatial distribution of A-component T UB s may indicate the geographical heterogeneity of seafloor metamorphism. 10. Spatial distribution of A-component T UB s Magnetic fabrics have identified sub-areas where dikes had steep magma-flow, which, in turn, implied that they lay directly above magma feeders (with an average dike thickness b1 m, it is unlikely that the dikes were individually extensive enough, nor that they remained fluid long enough for crack propagation to have controlled the flow-fabric). Areas of inferred steep magma-flow are boxed in Fig. 8. Density contours are an objective assessment of T UB spatial-distribution; they have been averaged within cells of 4.5 km diameter, site values being weighted inversely according to their distance from the centre of the counting cell, using the software SigmaPlot. Trial-and-error adjustments to contouring parameters result in the optimum balance between generalization, detail, boundary-effects and sparser-sampled areas.

13 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) Fig. 8. Contoured spatial distribution of unblocking temperature (T UB ) for A-components; note alternating zones of high and low values, slightly oblique to the Mitsero spreading axis. The first, cursory observation is that higher T UB s ( C), carried by magnetite, occur in zones alternating with subareas of lower T UB carried by titanomagnetite (Fig. 8). The zones are slightly oblique to the Mitsero graben (Varga and Moores, 1985; van Everdingen and Cawood, 1995). Bands of more intense metamorphism represented by zones of T UB b400 C are slightly oblique to the graben, a reasonable consequence of diachronous metamorphism along the spreading axis. The T UB 400 C bands are about 5 km wide. From the inferred divergence-spreading rate of 25 mm/a (25 km/ Ma) the bands may have been associated with hydrothermal metamorphic pulses enduring 200 Ka. 11. Geographical control on A-component orientation-distributions The first inspection of A-vector orientation-distributions groups them according to areas of steep or shallow magma-flow determined from magnetic fabric (Fig. 1d). Fig. 9. Early or A-component vectors grouped by subareas that have steep or shallow magma flow, as inferred from the AMS fabric. Note some smearing of vectors between the geocentric axial dipole field (GAD) and the mean vector for the overlying Turonian age Troodos extrusive volcanic sequence (TMV 88 Ma).

14 88 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) This would test the hypothesis that some aspect of flow anisotropy is inherited to bias ChRMs; e.g., permeability anisotropy that influences hydrothermal metamorphism. Fig. 9a shows A-vectors (n = 101 specimens) grouped from five (N=5) areas of steep flow (k MAX axes plunge N 70 ). Adjacent, site-mean A-vectors from six subareas of shallow flow with 103 specimens differ significantly at the 95% level (N=6, n =103), Fig. 9b. The mean vector for all sites above steep-flow subareas is closer to the dipole field. For sites with shallow flow, further from the dikes' magma supply, the mean vectors are closer to the Turonian Formation-mean vector (TMV Fig. 10. A-component vectors contoured geographically according to their unblocking temperature (T UB ). (a, b) from areas with T UB 400 C and T UB N400 C, respectively. (c f) Grouped according to geographical zones separated by the T UB =400 C contour. These zones are slightly oblique to the spreading axis. GAD=geocentric axial dipole field, TMV=mean-vector for Turonian volcanic rocks.

15 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) Ma, Fig. 9) determined from the overlying volcanic sequence. The second geographical classification is to consider vectors grouped according to their distance from the spreading axis. More precisely, they are grouped in bands of similar T UB, which are sub-parallel with the ridge (Fig. 8). However, they are more likely to detect any synchronicity of chemical remagnetization. First, compare all subareas with T UB 400 C against those with T UB N400 C (Fig. 10a, b). The orientationdistribution is less regular for the higher T UB s despite adequate sample-sizes, small Fisher confidence cones and Fisher means that do not differ at the 95% confidence level. Evidently, the A-remanences' acquisition processes were heterogeneous. For example, intermittent early hydrothermal metamorphism may have inadequately or unevenly averaged secular variation and it may have failed to overprint primary magnetizations completely and ubiquitously. Subareas C, D and E represent alternating bands of higher and lower T UB but share a common Fisher-mean vector orientation ( 330/ 55) (Fig. 10c f). There is some smearing between the ancient Turonian vector ( 270/24) and the geocentric axial dipole field. This is consistent with the rotating microplate having trapped successive paleofields during its anticlockwise rotation. 12. Conclusions Magnetic petrofabrics determined by anisotropy of low field susceptibility (AMS) invariably identify magnetite or titanomagnetite grain-shape alignments with k MAX // to mineral flow-alignment which is parallel to the walls of dikesb1 m thick and with k MIN Fig. 11. (a) Apparent pole positions calculated from previously published work (see Borradaile and Lucas, 2003). Some of that data were tilt-corrected but deformed limestones were not corrected for finite strain. (b) Apparent pole positions from dikes, this paper, with positions corrected for regional tilt. No dikes are penetratively strained and cataclastically sheared dikes, for example from the Southern Troodos Transform Zone (Fig. 1), were not sampled.

16 90 G.J. Borradaile, D. Gauthier / Tectonophysics 418 (2006) perpendicular to the dike walls. Rarely, subfabrics due to the mafic silicates blend with the iron oxide shape fabric to produce blended fabrics in which k INT and k MIN switch orientations. Our previous work in these rocks validated the sources of susceptibility and susceptibility anisotropy using AMS, AARM and statistical analysis of densely sampled sites. This regional study recognizes gradual systematic variations in the plunge of k MAX (magma flow proxy) within dikes and we infer that areas with steep flow may overlie magma feeders. These are spaced 4 km along the length of the spreading axis and are probably 2 km in diameter. They recur with a spacing of 6 km perpendicular to the spreading axis, implying an activation interval of 240 Ka if the plates diverge at 50 mm/a. Whereas the overlying Turonian submarine volcanic sequence (88 Ma) has a reported ChRM of (274/+33) (e.g., Gee et al., 1993), the feeding dikes studied here have a Fisher-mean ChRM near 335/+ 55, whether they are grouped by T UB value [N400 C versus 400 C], or according to locations (Fig. 10c f). This dates their metamorphic re-magnetization to a post-turonian time when the Troodos terrane had moved northward from the equator to a palaeolatitude 30 N (Fig. 11b). The provisional APWP from paleopoles calculated by Borradaile and Lucas (2003, here Fig. 11a) has a large degree of internal consistency and corroborates the overall sinistral microplate rotation (Clube et al., 1985; Clube and Robertson, 1986; Allerton and Vine, 1987; Gee et al., 1993). We applied small regional tilt corrections to the paleopoles in some sub-areas (see Fig. 11b). The APWP appears to place the sea-floor metamorphic remagnetization of the dikes' A-magnetizations in the range Ma although the sparse data and form of the APWP could extend this range to Ma. Interpolated paleopole-ages on that part of the APWP are insensitive to the accuracy of tilt-corrections since appropriate tilt-axes all trend roughly N S. However, relevant factors from previous research suggest, in order of decreasing confidence: (1) the oldest dikes, furthest from the spreading axis, were closer in age to the overlying volcanic sequence; (2) the volcanic sequence is as old as 88 Ma; (3) the plate-separation rate was 50 mm/a. Therefore, we could conclude that metamorphic remagnetization close to the spreading axis should have occurred within a few Ma of the pillow basalt extrusion, say Ma. The age discrepancy could be explained, at least partly, if sea-floor remagnetization persisted much further off-axis (N 50 km) or if sea-floor spreading was slower than assumed ( 20 mm/a). Acknowledgements This research was supported financially by NSERC (Canada) operating and equipment grants to Graham Borradaile at the Lakehead University Rock Magnetism and Deformation Laboratory. Fieldwork was possible with the encouragement and assistance of the Geological Survey of Cyprus, its Director Dr. George Petrides and his Liaison Officer Dr. Ioannis Panayides. We thank Anne Hammond for the preparation of several thousand excellent cores (for this and pilot studies) from oriented blocks, not to mention many perfect mineral separations and thin sections. Panicos Emilianou provided superb logistical support and generous help in Cyprus, and the villagers of Askas and Paleokhori were very hospitable. Frantisek Hrouda and an anonymous reviewer provided valuable criticism of the manuscript. References Allerton, S., Vine, F.J., Spreading structure of the Troodos ophiolite, Cyprus: some paleomagnetic constraints. 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