magnetic fabric and palaeomagnetic analyses of the plio–quaternary calc–alkaline series of...

Magnetic fabric and palaeomagnetic analyses of thePlio^Quaternary calc^alkaline series of Aegina Island,

South Aegean volcanic arc, Greece

Antony Morris *Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

Received 10 August 1999; received in revised form 15 December 1999; accepted 21 December 1999

Abstract

The South Aegean volcanic arc represents the magmatic expression of active subduction of the African plate beneathEurasia in the region of Greece. Plio^Quaternary calc^alkaline (dacite and andesite) lava flows exposed on the island ofAegina have been sampled for palaeomagnetic analyses to determine whether the western arc area has experiencedvertical axis rotations. In addition, these units allow an assessment of the use of magnetic fabric (anisotropy of magneticsusceptibility; AMS) data as indicators of lava flow directions in silicic extrusive rocks, providing a complement to theexisting literature on AMS of basaltic flows. The magnetic mineralogy of both dacites and andesites is dominated byfine-grained magnetite/Ti-poor titanomagnetite. Sampled flows are characterised by oblate magnetic fabrics at site levelwhich are co-axial to flow surfaces, although most sites also contain specimens with prolate AMS ellipsoids. Wheremagnetic lineations are developed, they are systematically aligned either parallel or orthogonal to average (exposure-scale) lava flow directions. Magnetic lineations in these rocks, therefore, may be interpreted in terms of flow directionsonly when the latter are known a priori. Observed variability in the directions of remanent magnetisation recorded bythe Aegina lava flows may be explained wholly in terms of geomagnetic variations. Several sites record spot readings ofthe geomagnetic field during polarity transitions. Scatter of directions at the remaining sites is consistent with thatexpected from secular variation. Post-eruption tilting of magnetisation vectors is shown to be limited. The in situ meandirection of magnetisation of the non-transitional sites is indistinguishable from the geocentric dipolar field used as areference direction for these young (6 5 Ma) rocks. This indicates that the island has not experienced significant Plio^Quaternary tectonic rotations, in contrast to other parts of the Aegean volcanic arc. The magnetic data also furtherconstrain the timing of volcanic activity in the western part of the South Aegean arc, when used in conjunction withexisting radiometric and stratigraphic ages. The most recent volcanic products are shown to correlate with thePleistocene part of the Matuyama polarity epoch. The Aegina volcanic centre has, therefore, been inactive for at leastthe last 720 000 yr, in contrast to the historical activity further to the east on Santorini. ß 2000 Elsevier Science B.V.All rights reserved.

Keywords: magnetic minerals; palaeomagnetism; magnetic properties; island arcs; Greece

1. Introduction

Magnetic fabric analyses are increasingly com-

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 3 1 8 - 0

* Tel. : +44-1752-233120; Fax: +44-1752-233117;E-mail: [email protected]

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Earth and Planetary Science Letters 176 (2000) 91^105

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bined with palaeomagnetic remanence analyses toprovide constraints on regional deformation his-tories. For example, anisotropy of magnetic sus-ceptibility (AMS) data may be used as a proxy forstructural stretching lineations. They may then becorrected for the e¡ects of late-stage vertical axisrotations using concurrent palaeomagnetic analy-ses in order to reconstruct regional stress ¢elds [1].Elsewhere, magnetic fabric data have been e¡ec-tively used in volcanic terrains to provide insightsinto lava and pyroclastic £ow dynamics and mor-phology [2^7]. A primary aim of these studies hasbeen to allow AMS principal axes to be used asproxies for £ow azimuths, for example to enabledetermination of vent sources for non-historiclava and pyroclastic £ows. Again, local-scale,post-eruption rotations must be accounted for inorder to reconstruct primary £ow patterns.

This paper presents integrated magnetic fabricand palaeomagnetic analyses of Plio^Quaternarylava £ows from the island of Aegina (Greece),which forms part of the South Aegean volcanicarc (Fig. 1). The Aegean region represents themost tectonically active segment of the Tethyan(Alpine^Himalayan) orogenic belt [8], and hasbeen a focus of research aimed at determining

the crustal response to plate convergence andback-arc extension. Substantial advances inunderstanding of this system have come from pa-laeomagnetic studies of Neogene to Recent sedi-mentary and volcanic rocks exposed in fore-arcand back-arc locations [1,9^17]. An earlier modelin which the arc acquired its curvature in twodistinct phases by progressive, opposing, roll-back-induced tectonic rotations along its westernand eastern limbs [9] is now being re¢ned throughdetailed palaeomagnetic analyses with precisetemporal control provided by cyclo-, bio- andmagnetostratigraphic studies [18]. These more re-cent studies have highlighted the importance ofrapid and intermittent Plio^Quaternary rotationsthroughout the Aegean [19].

The aims of this investigation are three-fold: (i)to assess the e¡ectiveness of AMS data as £owdirection proxies in silicic and intermediate vol-canic rocks. The analyses herein therefore comple-ment recent studies of the magnetic fabrics of ba-saltic £ows [4^6] which have focussed on examplesfrom the North American and Paci¢c plates; (ii)to determine whether Plio^Quaternary £ow unitson Aegina have been a¡ected by vertical axis ro-tations of a magnitude su¤cient to be detected bypalaeomagnetic analysis. This is achieved using adata set which is su¤ciently extensive to accountfor the e¡ects of geomagnetic variations on ob-served magnetic remanence directions; and (iii) toprovide additional constraints on the timing ofvolcanism in the western segment of the SouthAegean volcanic arc through consideration ofthe magnetic polarity of Aegina lava £ows.

2. Geological setting and previous palaeomagneticresearch

The South Aegean volcanic arc extends fromCrommyonia in the west, through the islands ofAegina, Methana, Milos and Santorini, to Ni-syros and Kos in the east (Fig. 1). The arc isthe magmatic expression of active subduction ofthe African plate beneath Eurasia. The volcanicproducts form a typical calc^alkaline associationwhich displays a continuous evolution from ba-salts to rhyolites, although dacites and andesites

Fig. 1. Location of the South Aegean volcanic arc above theHellenic subduction zone. Black arrow = relative convergencevector of the African and Eurasian plates. Volcanic centresare: 1 = Crommyonia; 2 = Methana; 3 = Aegina; 4 = Milos;5 = Kimolos; 6 = Santorini (Thera); 7 = Kos; 8 = Nisyros.

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A. Morris / Earth and Planetary Science Letters 176 (2000) 91^10592

are the dominant, less evolved members [20]. Inthe western sector of the arc (Crommyonia, Aegi-na, Milos; Fig. 1), the volcanic centres are domi-nated by the presence of domes and lava £owswith subordinate pyroclastics. Volcanism was ini-tiated in the early Pliocene, diminished in the latePliocene and became re-established in the Pleisto-cene. The timing of volcanic activity correlateswith two phases of basin subsidence in the areaof the arc [21,22]. Hence, the timing and locus ofvolcanism may be partially controlled by regionalextension [23].

The most recent of several attempts [24^27] atestablishing the lithostratigraphy of the volcanicedi¢ces on Aegina is the systematic study of Die-trich et al. [28], who remapped the island at a1:25 000 scale. They recognised eight distinct pet-rographic types within the extrusive sequence (inaddition to pyroclastic and epiclastic £ows) andidenti¢ed 12 eruptive centres which were activeduring two phases of volcanism (Figs. 2 and 3).Age constraints were provided by a limited num-ber of biotite and amphibole K^Ar dates [28,29].The earlier, Pliocene volcanic episode was domi-

Fig. 2. Simpli¢ed geological map of Aegina Island (modi¢ed from [28]), showing the distribution of rocks of the ¢rst (Pliocene)and second (Pleistocene) volcanic episodes and the location of palaeomagnetic sampling sites. Note that details of lithologiessampled and of source volcanic edi¢ces are given in Table 2.

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A. Morris / Earth and Planetary Science Letters 176 (2000) 91^105 93

nated by eruption of dacites, with minor andesiticdacites and rhyodacitic pyroclastic rocks (Fig. 3).The second volcanic episode was dominated byandesite £ows, following a rhyodacite precursor(Fig. 3). The exact timing of this second phaseof activity is di¤cult to establish. It is consideredto be either entirely Pleistocene in age [28] or torepresent a more restricted phase of Upper Plio-cene activity [27].

The only previous systematic palaeomagneticstudy of the calcâlkaline series of Aegina [27]attempted to use palaeomagnetic polarity as aguide to stratigraphic correlation. Pe-Piper et al.[27] collected 26 hand samples distributedthroughout nine units de¢ned on the basis of ¢eldpetrological characteristics. Stepwise alternating¢eld (AF) demagnetisation was used to identifythe magnetic polarity of each sample, but the de-magnetisation characteristics and directions of re-manence components with their associated statis-tical parameters were not reported, beyond thegeneral comment that remanences were close toeither the present geomagnetic ¢eld direction orits antipode. No consideration was given to as-sessment of the e¡ects of palaeosecular variationand potential post-magnetisation tilting ofsampled units on in situ magnetisations. Using

the radiometric ages available at the time [29]and two additional KÂr whole-rock dates, Pe-Piper et al. [27] used their magnetic polaritydata to suggest the following constraints on tim-ing of volcanism: (i) the second (andesitic) epi-sode was assigned to the Upper Pliocene part ofthe Matuyama polarity epoch on the basis of re-versed polarities in all younger samples and a sin-gle whole-rock KÂr age of 2.1 þ 0.1 Ma on asample from the Oros eruptive centre (see Fig.2); (ii) £ow units of the older, Pliocene episoderecord both normal and reversed polarities andwere assigned to the Gilbert and Gauss polarityepochs (although the possibility of a Matuyamaage for some normally magnetised units could notbe excluded).

3. Sampling and laboratory procedures

Samples for this study were collected from 13sites distributed throughout the volcanic succes-sion (Fig. 2) which were selected on the basis ofaccess, freshness of material and presence of £owstructures. Three sites were collected in Pleisto-cene andesitic units which lacked clear £ow surfa-ces in order to examine the e¡ectiveness of AMS

Fig. 3. Generalised lithostratigraphy of the Aegina volcanic pile, based on data from Dietrich et al. [28]. Black boxes show thestratigraphic levels of sampled £ows. White ovals show KÂr ages quoted by Dietrich et al. [28].

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in de¢ning primary igneous fabrics. Elsewhere,site-scale £ow orientations were recorded as aver-ages of a minimum of ¢ve determinations of dipand strike of £ow surfaces. Locality-scale average£ow azimuths mapped by Dietrich et al. [28] werealso con¢rmed in the ¢eld where possible. Theestimated error in determination of £ow azimuthsis þ 25³. Seven^10 samples were drilled in situ persite using a standard rock drill and were orientedwith both magnetic and sun compasses. Sampleswere collected from 3^9 consecutive £ows at eachsite, and one or more standard palaeomagneticspecimens were prepared from each orientedcore sample.

Bulk susceptibility and AMS determinationswere performed for all specimens prior to demag-netisation experiments using Minisep and KLY-3S (Agico) anisotropy meters. Magnetic rema-nence measurements were made using a Molspin£uxgate spinner magnetometer. Both thermal andAF stepwise demagnetisation techniques were em-ployed, using a MMTD1 furnace and Molspintumbling AF demagnetiser, respectively. Eachspecimen was subjected to 11^14 demagnetisationsteps up to either a maximum temperature of575³C or a peak ¢eld of 100 mT. Characteristiccomponents of magnetisation were found usingorthogonal vector plots and principal componentanalysis [31] and site mean remanence directionscomputed using Fisherian statistics. Isothermalremanent magnetisation (IRM) acquisition andthermal and AF demagnetisation responses ofnatural remanent magnetisations (NRMs) wereused to characterise the magnetic mineralspresent.

4. Results and interpretation

4.1. Magnetic mineralogy

IRM acquisition curves rise steeply and gener-ally reach saturation by ¢elds of 0.2^0.3 T (Fig.4), although several specimens (e.g. EN0509 andEN0704B; Fig. 4) show a minor gradual rise inisothermal remanence up to the maximum applied¢eld of 800 mT. In all cases, histograms of therate of change of IRM acquisition suggest that

low coercivity minerals are dominant. Unblockingtemperatures of NRMs are predominantly be-tween 525 and 575³C. These data are thereforeconsistent with the presence of magnetite/Ti-poor titanomagnetite within these rocks. Thesephases may have evolved from more Ti-rich pri-mary titanomagnetite phases by deuteric oxida-tion during initial cooling, although pure primarymagnetite phases are commonly found in silicicigneous rocks [32].

Median destructive ¢elds determined during AFdemagnetisation range from 12 to 40 mT, with amean value of 28 mT, suggestive of the presenceof ¢ne-grained magnetite. Comparison of IRMacquisition curves with those of sized magnetitegrains [33] indicates a predominance of pseudo-single domain grains in the 1.0^4.0 mm range.Physical grain sizes may be larger than this, how-ever, if primary titanomagnetite phases have beensub-divided into ferrimagnetic magnetite-rich andparamagnetic ilmenite-rich domains during deu-teric oxidation.

4.2. AMS

The magnetic fabric of the sampled £ows wasdescribed by the AMS. This re£ects the preferredorientation of grains, grain distributions and/orthe crystal lattices of minerals which contributeto the magnetic susceptibility of a rock. AMScorresponds to a second order tensor and can berepresented by an ellipsoid speci¢ed by the orien-tation and magnitude of its principal axes (K1, K2

and K3, being the maximum, intermediate andminimum susceptibility axes, respectively). TheAMS of a rock may result from contributionsfrom diamagnetic, paramagnetic, antiferromag-netic and ferrimagnetic minerals (see [34] for areview).

The mean susceptibilities at a site level arelisted in Table 1 and vary from 3.8U1033 to61.2U1033 SI. These data suggest magnetite con-centrations of 0.1^2.0% by volume. At these con-centrations, the contribution of high susceptibilitymagnetite/titanomagnetite grains will dominatethe AMS characteristics of the rocks. K1 axesare aligned along the long axes of non-equantmagnetite grains or along the axes of linear chains

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of interacting equant grains, usually resulting incorrelation of specimen AMS lineations with pet-rofabric x-axes (e.g. £ow lineations). K3 axes lienormal to petrofabric foliations. Comparison ofmagnetic fabrics with macroscopic petrofabric el-ements (£ow surfaces; local and average £ow azi-muths) can therefore provide insights into lava£ow dynamics and morphology. This relationshipbetween principal susceptibility axes and petrofa-bric elements is, however, dependent on magneticdomain state and only holds for pseudo-singleand multi-domain grains.

The oblateness or prolateness of individualspecimen AMS ellipsoids is described by theshape parameter T, with 06T91.0 for oblate el-lipsoids and 31.09T6 0 for prolate ellipsoids.

The majority of specimens exhibit oblate AMSellipsoids with a maximum T value of 0.92 (Fig.5), although at nine sites, several specimens withprolate ellipsoids occur (minimum T value of30.73). Strength of anisotropy is described bythe corrected anisotropy degree, PJ [35], andranges from 1.03 to 1.75 for the individual speci-mens studied here (Fig. 5), with a modal value of1.08 (i.e. 8% anisotropy). There is no preferredrelationship between PJ and T, and no correlationbetween PJ and the mean susceptibility, indicatingthat the degree of anisotropy is not dependent onthe ferrimagnetic concentration.

At a site level, clustering of K1 and K3 axesde¢nes the magnetic lineation and the pole tothe magnetic foliation, respectively. Oblate fabrics

Fig. 4. Typical IRM acquisition curves, demonstrating a dominance of low coercivity magnetic phases. The characteristics ofthese curves combined with unblocking temperature and median destructive ¢eld data derived from demagnetisation experimentsare consistent with presence of pseudo-single domain magnetite/Ti-poor titanomagnetite in these rocks.

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are characterised by clusters of K3 axes orthogo-nal to great circle girdle distributions of K1 andK2 axes, whereas prolate fabrics are characterisedby clusters of K1 axes orthogonal to girdle distri-butions of K2 and K3 axes. In triaxial fabrics, thethree principal susceptibility axes form distinctgroups. The orientation tensor technique de-scribed by Woodcock [36] is adopted here to spec-ify the mean directions and distributions of K1

and K3 axes at a site level (Table 1). For each

set of principal axes, the eigenvectors [v1, v2, v3]and normalised eigenvalues [S1, S2, S3] of the ori-entation tensor are calculated. The eigenvector v1

is an estimate of the distribution mean direction,whereas the eigenvalues are used to de¢ne twoparameters which specify the shape of the distri-bution around this mean direction [36] :

KW � �ln �S1=S2�=ln �S2=S3�� and C � ln �S1=S3�

KW = 1 indicates a distribution with equal girdleand cluster tendencies, 09KW 6 1 indicates a gir-dle distribution and 16KW9r characterises aclustered distribution (note that in the originalnotation of Woodcock [36], this parameter wasgiven the symbol K and has been renamed KW

here to avoid confusion with the standard nota-tion for susceptibility). The parameter C is a mea-sure of the strength of the preferred orientation,with low values (6 1) indicating a random distri-bution and high values (approximately s 4) indi-cating a strong preferred orientation.

Orientations of the principal AMS axes arecompared to local £ow surfaces represented bygreat circles and their corresponding poles in thestereoplots of Fig. 6. At a site level, minimum(K3) axes form strong clusters (KW s 1.0;Cs 3.0; Table 1) which are coaxial to the poles

Table 1AMS results, average £ow direction and £ow surface data

Site Kgeom Average £owdirection

Max axis Pole to £owsurface

Min axis

(U1033 SI) v1 C KW v1 C KW

EN01 17.75 205 309/21 2.29 12.17 154/64 143/69 5.69 0.23EN02 14.87 285 295/06 3.77 0.73 186/63 192/69 4.28 13.58EN03 20.58 027 213/38 4.59 1.08 090/44 078/43 6.25 1.03EN04 7.78 020 277/11 4.26 0.91 ^ 023/56 4.19 9.64EN05 13.12 317 061/08 4.97 0.12 228/79 271/84 5.10 7.15EN06 9.74 301 291/04 6.20 14.28 130/80 038/78 6.07 3.11EN07 3.77 280 086/05 2.08 0.48 257/71 206/72 2.73 0.46EN08 11.37 104 111/05 3.39 0.78 ^ 220/70 4.49 0.74EN09 17.54 103 346/24 3.94 0.90 ^ 114/52 5.62 1.82EN10 18.48 158 131/57 3.44 0.91 351/32 001/21 3.70 4.76EN11 15.44 342 327/68 3.28 5.93 151/24 151/22 3.60 12.99EN12 61.19 140 218/34 5.74 0.45 347/46 347/44 5.69 8.30EN13 47.07 125 073/15 6.15 0.32 303/69 311/62 6.40 4.34

Kgeom = geometric (log) mean of specimen susceptibilities; v1 = azimuth and plunge of maximum eigenvector of orientation tensorfor speci¢ed susceptibility axis; C, KW : see text for explanation; Pole to £ow surface = azimuth and plunge of pole to £ow sur-faces observed in the ¢eld.

Fig. 5. Bi-plot of AMS factors for individual specimens. Ob-late AMS ellipsoids dominate the Aegina lava £ows with amodal anisotropy of 8% (PJ = 1.08). Most sites also containsome specimens with prolate ellipsoids, although site levelfabrics are consistently oblate.

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to the bounding surfaces of sampled £ows (Fig. 6;Table 1), with the exception of site EN07 whereK3 axes are more scattered. Maximum (K1) andintermediate (K2) ellipsoid axes lie within the local£ow planes, forming either two distinct clustersindicating overall triaxial fabrics at a site level(e.g. EN06; Fig. 6) or girdle distributions indica-tive of oblate fabrics (e.g. EN13; Fig. 6). Aniso-

tropy of IRM (AIRM) ellipsoids determined for arestricted number of specimens are coaxial withAMS ellipsoids. Together with the close correla-tion between magnetic and petrofabrics, this indi-cates that the sampled £ows are characterised by`normal' magnetic fabrics that are strongly con-trolled by alignment of ferrimagnetic mineralsduring primary magmatic £ow. Similar AMS el-

Fig. 6. Stereoplots of AMS ellipsoid principal axes, together with planes and poles of site-scale £ow surfaces (where observed)and mapped average £ow directions ([28] and author's ¢eld data). K1 axes either show no distinct clustering or form groupingsparallel or orthogonal to average £ow directions.

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lipsoid distributions are observed at three siteswhere primary foliations were not visible in the¢eld (EN04, 08 and 09). AMS data may be usedwith con¢dence in these cases to de¢ne £ow ori-entations in order to assess the degree of tiltingwhich has a¡ected the sites (see palaeomagneticresults below).

The distributions of K1 axes vary from girdleswith KW 6 0.5 (EN07, 12 and 13), through transi-tional girdle/cluster distributions with KWW1.0(EN02, 03, 04, 08, 09 and 10), to near uniaxialclusters with KW s 5.0 (EN01, 06 and 11). A KW

value of 0.12 at site EN05 provides a poor de-scription of the distribution of K1 axes and re-£ects the e¡ect of interchanges of K1 and K2

axes in several specimens (Fig. 6).

The AMS fabrics of the Aegina £ows show aclear relationship with £ow surfaces measured at asite-scale in all cases, with K3 axes clustering sub-parallel to poles to £ows (Fig. 6). This re£ectsarrangement of oblate magnetite grains parallelto £ow planes and/or containment of the longaxes of prolate grains within these planes. In ad-dition, where magnetic lineations de¢ned by sig-ni¢cant groupings of K1 axes occur, they exhibitsystematic parallel or orthogonal relationships toaverage £ow directions. The former may re£ectthe in£uences of laminar £ow and simple shearduring £ow, whereas alignment of K1 axes orthog-onal to average £ow directions may result fromrolling of non-equant magnetite grains or fromlocalised £ow orthogonal to the overall £ow di-

Fig. 7. Typical results of AF and thermal demagnetisation experiments, showing stable directions of magnetisation following re-moval of minor viscous components.

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rection resulting from over-topping or collapse ofbounding lava levees. In the case of sites EN10and 11, the steepness of local £ow surfaces and K1

axes may re£ect the e¡ects of signi¢cant sub-£owpalaeorelief, £ow folding, or (more likely) prox-imity to eruptive centres generating steeply in-clined upwards £ow during formation of a centraldacitic dome.

4.3. Palaeomagnetic results

The mean NRM intensity determined from allspecimens is 1.71 A m31. Both thermal and AFdemagnetisation isolated single, stable compo-nents of magnetisation. Minor viscous compo-nents were removed by ¢elds of less than 15 mTand temperatures of less than 250³C. Typical ex-amples of demagnetisation behaviour are shownin Fig. 7. Both demagnetisation techniques provedequally e¡ective in identifying stable components

of magnetisation, and the majority of sampleswere subjected to AF treatment.

In situ site mean directions of magnetisation arelisted in Table 2. The majority of sites recordnegative inclinations with southward directed de-clinations consistent with magnetisations acquiredduring reversed polarity chrons, with three sitesdisplaying positive inclinations. An appropriateexpected direction of magnetisation for theseyoung (6 5 Ma) rocks is the geocentric axial di-polar ¢eld direction, which is Dec = 000³,Inc = 57³ for the latitude of Aegina Island. Theremanence at eight sites is within 20³ of this ex-pected direction (or its antipode) and at a furthersite (EN03) is within 32³ of this direction. Thesevariations are consistent with the dispersion ex-pected from palaeosecular variation for this lati-tude [32]. Remanences at the remaining four sites(EN01, 02, 10 and 13) are discussed separatelybelow, and are excluded from the treatment of

Table 2Palaeomagnetic results

Site Description Age n In situ K A95 Flow surfaces Untilted

(Ma) Dec Inc Strike Dip Dec Inc

EN03 Oros and Lazarides typehypersthene andesite

6 1.6 8 208.9 332.5 301.9 3.2 180 46 172.1 341.8


6 1.6 8 198.6 343.5 288.1 3.3 *113 34 188.5 377.2


6 1.6 8 176.7 341.1 305.7 3.2 318 11 183.0 333.7


6 1.6 8 182.8 340.0 745.1 2.0 220 10 177.1 333.5

EN09 Oros type andesite 6 1.6 7 196.9 352.9 218.7 4.1 *204 38 161.7 336.0EN08 Oros type basaltic andesite 6 1.6 7 176.7 338.3 114.8 5.7 *310 20 184.3 322.8EN07 Kakoperato type rhyodacite 6 1.6 9 167.5 352.4 85.6 5.6 347 19 190.3 348.4EN01 Megali Kori¢ type biotite-

hornblende dacite3.8^4.2 8 341.8 357.7 102.9 5.5 244 26 8.9 382.7

EN10 Megali Kori¢ type biotite-hornblende dacite

3.8^4.2 9 97.5 55.3 144.6 4.3 081 58 136.2 17.9

EN11 Megali Kori¢ type biotite-hornblende dacite

3.8^4.2 9 1.3 60.8 55.1 7.0 241 66 345.0 31.2

EN12 Kokkinovrahos type biotite-hornblende dacite

3.8^4.2 10 192.0 373.0 80.2 5.4 077 44 332.7 360.8

EN13 Kokkinovrahos type biotite-hornblende dacite

3.8^4.2 8 5.4 364.7 241.6 3.6 033 21 339.7 350.6

EN02 Skotini type andesitic dacite 4.4^4.2 8 301.4 52.3 113.2 5.2 276 27 323.2 35.8

Description = source vent and lithology determined by Dietrich et al. [28]; Age = K^Ar and stratigraphic ages used by Dietrich etal. [28]; n = number of specimens; Dec = declination; Inc = inclination; K = Fisher precision parameter; A95 = semi-angle of 95%cone of con¢dence; £ow surfaces = orientation of £ow surfaces (strike anticlockwise from dip direction; * = magnetic foliation de-¢ned by AMS data); untilted magnetisation directions are derived by restoring £ow surfaces to horizontal (see text for discussionof age of remanence).

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the age of magnetisation and the e¡ect of tiltingwhich follows.

The most common approach adopted to estab-lish the age of magnetisation of sampled units isthe palaeomagnetic tilt test [37^39], which deter-mines the timing of remanence acquisition relativeto deformation (tilting). This involves restorationof units to a palaeohorizontal reference frame byrotation around strike-parallel horizontal axes.Table 2 lists the tilt-corrected site mean directionsof magnetisation from the £ows sampled in thisstudy. Tilt corrections are based on observed insitu £ow orientations, with AMS magnetic folia-tion planes used as proxies for £ow planes atthree sites (see above) by calculating best ¢ttinggreat circles using eigenvector analysis. The over-all mean directions and statistics for the nine siteswith in situ remanences within 32³ of the referencedipolar direction (and following inversion of thenormal polarity direction at site EN11) are:

In situ : Dec � 187:1�; Inc � 348:9�; K � 27:1;

A95 � 10:1�

Tilt-corrected : Dec � 178:2�; Inc � 344:10�;

K � 5:9; A95 � 23:1�

The increase in dispersion following full tilt cor-rection constitutes a negative tilt test which is sig-ni¢cant at the 95% con¢dence level [38], whichmight imply that these rocks have been remagne-tised. This is unlikely since this young volcanicsuccession has not experienced any obviouspost-eruption episodes of burial, thermal eventsor secondary alteration. Moreover, the full tiltcorrection approach makes the critical assump-tion that £ow orientations represent palaeohori-zontal surfaces. It is highly likely, however, thatlavas were erupted down signi¢cant palaeoslopes.Field-measured foliations therefore potentially in-corporate components of initial dip. In particular,steep foliations in dacites at sites EN10 and 11may re£ect steep palaeorelief on a local scale,£ow folding or proximity to a central dome [40].Foliations at these sites clearly do not represent

palaeohorizontal surfaces as the in situ site meanremanence inclination at site EN11 is within 6³ ofthe expected value of 57³.

An estimate of the degree of post-eruption tilt-ing experienced at the sampled sites can be as-sessed by rotating magnetisation vectors aroundhorizontal axes by incrementally applying stand-ard about-strike corrections using the observed insitu £ow orientations or their AMS proxies. Thisyields a maximum Fisherian precision parameter,K, of 27.9 following progressive untilting by 8%,with a corresponding mean remanence directionof Dec = 186.3³, Inc =348.8³ and A95 = 10.0³. Inaddition, the progressive tilt test formulation ofWatson and Enkin [41] allows estimation of the95% con¢dence interval for the optimum degreeof untilting. A circular S.D. of 10³ on the deter-mination of poles to the lava £ow surfaces hasbeen used in the parametric calculation in orderto take into account potential errors in ¢eld struc-tural measurements. One thousand resampling tri-als then give the following statistics :

Fig. 8. VGPs and 95% con¢dence ellipses derived from thein situ site mean directions of Table 2. VGPs at sites EN01,02, 10 and 13 lie at lower latitudes than 60³S indicating thatthese sites record transitional ¢eld directions. (Note that anti-podes of the normal polarity VGP at site EN11 and of thetransitional ¢eld VGPs at sites EN02 and 10 are shown toallow plotting of all VGPs on a single hemisphere).

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Optimum untilting (50 percentile) = 7.4%Lower 95% con¢dence limit (2.5 percentile) =0.1%Upper 95% con¢dence limit (97.5 percentile) =15.4%

These analyses suggest that the observed magnet-isations were acquired with £ows close to theirpresent orientations. The limited amount ofpost-magnetisation tilting implied by these datais consistent with the generally low dips observedwithin coeval Pliocene sedimentary sequences ex-posed on the island. Also, the calculated meandirection of magnetisation at 8% untilting is sta-tistically indistinguishable from the antipode ofthe dipolar reference direction expected at the lat-itude of Aegina (Dec = 180³, Inc =357). The dis-tribution of virtual geomagnetic poles (VGPs)

from these units around geographic north (Fig.8) is consistent with scatter associated with secularvariation of the geomagnetic ¢eld.

The remaining four sites, EN01, 02, 10 and 13,have in situ directions which are clearly discord-ant with respect to those observed at other sitesand lie at 64³, 33³, 50³ and 58³, respectively, awayfrom the reference direction. This may be poten-tially explained by: (i) volcano^tectonic disrup-tion relative to other sites (e.g. slumping); (ii) iso-thermal remagnetisation by lightning strikes; (iii)de£ection of remanence vectors by strong mag-netic anisotropy; and (iv) changes in the geomag-netic ¢eld ambient during magnetisation relativeto other sites. Flow orientations at these sites arewithin the range of orientations observed at othersites. Additionally, they occur in close proximity

Fig. 9. Relationship of sampled £ow units to chrons of the geomagnetic polarity timescale within the framework of existing ra-diometric and stratigraphic age constraints (see text for discussion).

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to sites with concordant magnetisation directionswith no obvious structural discontinuities presentbetween magnetically concordant and discordantsites. Isothermal remanences due to lightningstrikes characteristically result in anomalouslyhigh intensities of magnetisation. NRM intensitiesat the discordant sites are, however, consistentwith those observed at concordant sites. AIRMexperiments suggest that the natural remanencesin these rocks can only be de£ected by a maxi-mum of 2³ from the ambient magnetic ¢eld direc-tion at the time of remanence acquisition. Tecton-ic, isothermal (lightning strike) and anisotropicorigins for the anomalous magnetisation direc-tions at sites EN01, 02, 10 and 13, therefore, ap-pear highly unlikely. Instead, the magnetisationsrecorded at these sites are interpreted as thermor-emanences acquired in transitional ¢elds duringgeomagnetic polarity reversals. The VGPs calcu-lated from these sites lie at latitudes below 60³S(Fig. 8), consistent with the commonly used cri-teria for identi¢cation of transitional magnetisa-tions [42].

Fig. 9 illustrates the relationship between thegeomagnetic polarity timescale [43] and the polar-ity of each palaeomagnetic site. The oldest unitsampled is the andesitic dacite of site EN02,which has a radiometric age of 4.2^4.4 Ma [28]and a transitional ¢eld direction. These data sug-gest formation of these £ows during the transitionfrom reversed polarity chron C3.2R to normalpolarity chron C3.2N (i.e. 4.26 Ma). The biotite-hornblende dacites of sites EN01 and EN10^13are constrained stratigraphically to have formedduring a short time interval of ca. 0.3 Ma aftereruption of the andesitic dacite of EN02 but priorto the deposition of extensive volcaniclastic £owsdated at 3.87 þ 0.05 Ma [28,29]. This time intervalincludes two polarity transitions separatingchrons C3.2N, C3.1R and C3.1N. The strati-graphic age of these rocks is, therefore, consistentwith the presence of normal, reversed and transi-tional ¢eld directions at these sites.

The magnetic data also help to further con-strain the timing of the second volcanic episodeon Aegina. All sites within the younger sequenceare of reversed polarity. The basaltic andesites ofsite EN08 occur at the base of the volcanic pile

associated with the Oros eruptive centres, andcorrespond to the part of the sequence dated at2.1 þ 0.1 Ma by Pe-Piper et al. [27]. The reversedpolarity of this site is consistent with this age andcon¢rms formation of these rocks during chronC2R (Fig. 9). The precursor phase to the youngervolcanic episode is represented by the rhyodacitesof site EN07. This unit is therefore assigned to theearly part of chron C2R, indicating an absoluteage of 2.2^2.45 Ma. Finally, the ¢ve sites (EN03^06, 09) located in that part of the Oros sequenceabove site EN08 are most likely to have formedentirely during chron C1R, although the andesitesof site EN09 may potentially have formed duringthe latest part of chron C2R.

5. Discussion and wider context

Flow-parallel maximum principal AMS axeshave widely been reported in lava (e.g. [5,44])and pyroclastic £ows (e.g. [2,45]), suggestingthat K1 axes may be used as proxies for £owdirection. This approach is particularly attractivebecause the sensitivity of AMS measurements toweakly developed petrofabrics would allow deter-mination of £ow directions in visually isotropicrocks. The magnetic fabrics of dacitic and ande-sitic lava £ows on Aegina are clearly controlled byprimary magmatic £ow, with overall oblate AMSfabrics coaxial to £ow surfaces. Inverse magneticfabrics are not observed at any site, consistentwith a ferrimagnetic mineralogy dominated bypseudo-single domain magnetite/titanomagnetitegrains. The orientations of maximum principalsusceptibility axes cannot be correlated uniquelywith ¢eld-determined £ow azimuths, however, be-cause of the presence of both £ow-parallel and£ow-perpendicular K1 axes. These variations re-£ect the possible in£uence of a variety of £owdynamic and morphological factors (e.g. local pa-laeorelief ; simple shear operating across the £owpro¢le; laminar £ow). Such variability in the re-lationship between principal AMS axes has alsobeen recognised in a number of other volcanicenvironments ([4,6,7] and references therein).Thus, even without the complicating e¡ects ofdomain state on AMS ellipsoids [30], the orienta-

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tions of K1 axes cannot be used as a proxy forlava £ow directions without a priori knowledge ofaverage £ow azimuths determined from ¢eld in-dicators or from petrological fabric measurements(e.g. Universal stage measurements of phenocrystorientations). AMS £ow proxies do appear morepromising in ignimbrite deposits, however, whereK1 axes are generally aligned with source directionand are source-ward plunging [45].

Palaeomagnetic analyses of the Aegina lava£ows indicate that they record spot readings ofthe geomagnetic ¢eld in its normal, reversed andtransitional ¢eld states. Observed variations inmagnetisation directions are attributed purely togeomagnetic ¢eld variations, with the scatter ofdirections at non-transitional ¢eld sites beingwithin the limits of palaeosecular variation. Thisindicates that vertical axis rotations in the vicinityof Aegina must have been minimal over the last4.4 Ma, in contrast to signi¢cant rotation of co-eval extrusive sequences previously analysed onMilos to the east [11].

Finally, the magnetic polarity of sampled £ows,combined with existing radiometric and strati-graphic age controls, places further constraintson the timing of volcanic activity in the westernpart of the South Aegean arc. In particular, themost recent volcanic products from the centralOros eruptive centre are assigned to the Pleisto-cene part of the Matuyama polarity epoch. This isconsistent with the stratigraphic age estimate ofDietrich et al. [28] but di¡ers from the earliermagnetic age estimate of Pe-Piper et al. [27].The Aegina volcanic centre has, therefore, beeninactive for at least the last 720 000 yr, in contrastto the historical activity of centres further to theeast (e.g. Santorini [46]).

Acknowledgements

I would like to thank Fotini Liakopoulou forassistance in the ¢eld and Mark Anderson, Dar-ren Randall, Stuart Scott and Don Tarling foruseful discussions. The paper bene¢ted from crit-ical reviews by Bill MacDonald and CharonDuermeijer. Funding for ¢eldwork was providedby the Natural Environment Research Council

(Grant No. GR9/621). The remanence data wereanalysed using PC programs developed by RandyEnkin.[RV]

References

[1] A. Morris, M. Anderson, First palaeomagnetic resultsfrom the Cycladic Massif, Greece, and their implicationsfor Miocene extension directions and tectonic models inthe Aegean, Earth Planet. Sci. Lett. 142 (1996) 397^408.

[2] M.D. Knight, G.P.L. Walker, B.B. Ellwood, J. Diehl,Stratigraphy, paleomagnetism, and magnetic fabric ofthe Toba tu¡s: constraints on the sources and eruptivestyles, J. Geophys. Res. 91 (1986) 10355^10382.

[3] W.D. MacDonald, B.B. Ellwood, Anisotropy of magneticsusceptibility: sedimentological, igneous, and structural-tectonic applications, Rev. Geophys. 25 (1987) 905^909.

[4] E. Can¬on-Tapia, G.P.L. Walker, E. Herrero-Bervera,Magnetic fabric and £ow direction in basaltic Pahoehoelava of Xitle Volcano, Mexico, J. Volcan. Geotherm. Res.65 (1995) 249^263.

[5] J. Urrutia-Fucugauchi, Palaeomagnetic study of the Xitle-Pedregal de San Angel lava £ow, southern Basin of Mex-ico, Phys. Earth Planet. Int. 97 (1996) 177^196.

[6] E. Can¬on-Tapia, G.P.L. Walker, E. Herrero-Bervera, Theinternal structure of lava £ows - insights from AMS mea-surements II: Hawaiian pahoehoe, toothpaste lava and`a'-a, J. Volcan. Geotherm. Res. 76 (1997) 19^46.

[7] B. Cagnoli, D.H. Tarling, The reliability of anisotropy ofmagnetic susceptibility (AMS) data as £ow direction in-dicators in friable base surge and ignimbrite deposits:Italian examples, J. Volcan. Geotherm. Res. 75 (1997)309^320.

[8] J. Jackson, Active tectonics of the Aegean region, Ann.Rev. Earth Planet. Sci. 22 (1994) 239^271.

[9] C. Kissel, C. Laj, The Tertiary geodynamical evolution ofthe Aegean arc a palaeomagnetic reconstruction, Tecto-nophysics 146 (1988) 183^201.

[10] C. Kissel, C. Laj, A. Poisson and K. Simeakis, A patternof block rotations in central Aegea, in: C. Kissel and C.Laj (Eds.), Palaeomagnetic Rotations and ContinentalDeformation, Kluwer Academic Publishers, Dordrecht,1989, pp. 115^129.

[11] D. Kondopoulou and S. Pavlides, Palaeomagnetic andneotectonic evidence for di¡erent deformation patternsin the south Aegean volcanic arc: the case of Melos is-land, Proc. of I.E.S.C.A., Izmir, 1990, pp. 210^223.

[12] M. Westphal, D. Kondopoulou, Paleomagnetism of Mio-cene volcanics from Lemnos Island (Northern Aegean)implications for block rotations in the vicinity of theNorth Aegean Trough, Ann. Tecton. 7 (1993) 142^149.

[13] A. Morris, Rotational deformation during Palaeogenethrusting and basin closure in eastern central Greece: pa-laeomagnetic evidence from Mesozoic carbonates, Geo-phys. J. Int. 121 (1995) 827^847.

EPSL 5358 2-2-00


[14] D. Kondopoulou, A. Atzemoglou and S. Pavlides, Palae-omagnetism as a tool for testing geodynamic models inthe north Aegean: convergences, controversies and a fur-ther hypothesis, in: A. Morris and D. H. Tarling (Eds.),Palaeomagnetism and Tectonics of the Mediterranean Re-gion, Spec. Pub. Geol. Soc. Lond. 105, 1996, pp. 277^287.

[15] S. Dimitriadis, D. Kondopoulou, A. Atzemoglou, Dextralrotations and tectonomagmatic evolution of the southernRhodope and adjacent regions (Greece), Tectonophysics299 (1998) 159^173.

[16] D. Avigad, G. Baer, A. Heimann, Block rotations andcontinental extension in the central Aegean Sea: palae-omagnetic and structural evidence from Tinos and Myko-nos (Cyclades, Greece), Earth Planet. Sci. Lett. 157 (1998)23^40.

[17] A. Morris and M.W. Anderson, Comment on `Block ro-tations and continental extension in the central AegeanSea: palaeomagnetic and structural evidence from Tinosand Mykonos (Cyclades, Greece)` by D. Avigad, G. Baerand A. Heimann, Earth Planet. Sci. Lett. (1999) (in press).

[18] C.E. Duermeijer, W. Krijgsman, C.G. Langereis, J.H.TenVeen, Post-early Messinian counterclockwise rotationson Crete implications for Late Miocene to Recent kine-matics of the southern Hellenic arc, Tectonophysics 298(1998) 177^189.

[19] C.E. Duermeijer, C.G. Langereis, Late Neogene geody-namic evolution of the Aegean arc: an integrated paleo-magnetic approach, J. Conf. Abs. 4 (1999) 76.

[20] M. Fytikas, F. Innocenti, P. Manetti, R. Mazzouli, A.Peccerillo and L. Villari, Tertiary to Quaternary evolutionof volcanism in the Aegean region, in: J.E. Dixon andA.H.F. Robertson (Eds.), The Geological Evolution ofthe Eastern Mediterranean, Spec. Pub. Geol. Soc. Lond.17, 1984, pp. 687^699.

[21] T. Doutsos, D.J.W. Piper, Listric faulting, sedimentationand morphologic evolution of the eastern Corinth rift:¢rst stages in continental rifting, Bull. Geol. Soc. Am.102 (1990) 812^829.

[22] C. Perissoratis, The Santorini volcanic complex and itsrelation to the stratigraphy and structure of the Aegeanarc, Greece, Mar. Geol. 128 (1996) 37^58.

[23] G. Pe-Piper, K. Hatzipanagiotou, The Pliocene volcanicrocks of Crommyonia, western Greece and their implica-tions for the early evolution of the South Aegean arc,Geol. Mag. 134 (1997) 55^66.

[24] R. Van Leyden, Der Vulkanismus des Golfes von Aeginaund seine Beziehungen zur Tektonik, Publ. stift. Vulka-ninst. I. Friedlander 1, Zurich, 1940, pp. 1^151.

[25] G.I. Livaditis, Geological and geomorphological observa-tions on Aegina Island, Ph.D. thesis, University ofAthens, 1974, 64 pp.

[26] G.G. Pe, Petrology and geochemistry of volcanic rocks ofAegina, Greece, Bull. Volcan. 37 (1973) 491^514.

[27] G. Pe-Piper, D.J.W. Piper, P.H. Reynolds, Paleomagneticstratigraphy and radiometric dating of the Pliocene vol-canic rocks of Aegina, Greece, Bull. Volcan. 46 (1983)1^7.

[28] V. Dietrich, P. Gaitanakis, I. Mercolli and R. Oberha«nsli,Geological map of Greece, Aegina Island, Stiftung Vul-kaninstitut Immanuel Friedlander, Zurich, 1991.

[29] P. Mu«ller, H. Kreutzer, W. Harre, Radiometric dating oftwo extrusives from a Lower Pliocene marine section onAegina Island, Greece, Newsl. Stratigr. 8 (1979) 70^78.

[30] A. Stephenson, S. Sadikun, D.K. Potter, A theoreticaland experimental comparison of the anisotropies of mag-netic susceptibility and remanence in rocks and minerals,Geophys. J. R. Astr. Soc. 84 (1986) 185^200.

[31] J.L. Kirschvink, The least-squares line and plane and theanalysis of palaeomagnetic data, Geophys. J. R. Astr.Soc. 62 (1980) 699^718.

[32] R.F. Butler, Paleomagnetism: Magnetic Domains to Geo-logic Terranes, Blackwell Scienti¢c Publications, Boston,MA, 1992, 319 pp.

[33] R. Thompson, Modelling magnetization data using SIM-PLEX, Phys. Earth Planet. Int. 42 (1986) 113^127.

[34] D.H. Tarling and F. Hrouda, Magnetic Anisotropy,Chapman and Hall, London, 1993, 217 pp.

[35] V. Jelinek, Characterization of the magnetic fabric ofrocks, Tectonophysics 79 (1981) 63^67.

[36] N.H. Woodcock, Speci¢cation of fabric shapes using aneigenvalue method, Geol. Soc. Am. Bull. 88 (1977) 1231^1236.

[37] J.W. Graham, The stability and signi¢cance of magnetismin sedimentary rocks, J. Geophys. Res. 54 (1949) 131^167.

[38] M.W. McElhinny, Statistical signi¢cance of the fold testin palaeomagnetism, Geophys. J. R. Astr. Soc. 8 (1964)338^340.

[39] P.L. McFadden, D.L. Jones, The fold test in palaeomag-netism, Geophys. J. R. Astr. Soc. 67 (1981) 53^58.

[40] R.A.F. Cas and J.V. Wright, Volcanic Successions: Mod-ern and Ancient, Allen and Unwin, London, 1987, 528pp.

[41] G.S. Watson, R.J. Enkin, The fold test in paleomagnetismas a parameter-estimation problem, Geophys. Res. Lett.20 (1993) 2135^2137.

[42] M. Fuller, I. Williams, K.A. Ho¡man, Paleomagnetic re-cords of geomagnetic ¢eld reversals and the morphologyof the transitional ¢elds, Rev. Geophys. Space Phys. 17(1979) 179^203.

[43] W.B. Harland, R.L. Armstrong, A.V. Cox, L.E. Craig,A.G. Smith and D.G. Smith, A Geologic Time Scale,Cambridge University Press, 1989, 263 pp.

[44] W.D. MacDonald, H.C. Palmer, A. Hayatsu, Egan RangeVolcanic Complex, Nevada: geochronology, paleomag-netism and magnetic fabrics, Phys. Earth Planet. Int. 74(1992) 109^126.

[45] H.C. Palmer, W.D. MacDonald, Anisotropy of magneticsusceptibility in relation to source vents of ignimbritesempirical observations, Tectonophysics 307 (1999) 207^218.

[46] T.H. Druitt, R.A. Mellors, D.M. Pyle, R.S.J.S. Parks,Explosive volcanism on Santorini, Greece, Geol. Mag.126 (1989) 95^126.

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