The magnetic fabric of metasediments in a detachment shear zone: the example of Tinos Island (Greece)

Tectonophysics 321 (2000) 219–236 www.elsevier.com/locate/tecto

The magnetic fabric of metasediments in a detachment shear zone: the example of Tinos Island (Greece) C. Aubourg a, *, R. Hebert a, L. Jolivet b, G. Cartayrade a a ESA CNRS 7072, Department des Sciences de la Terre. University of Cergy Pontoise, 8, Le Campus, 95031 Cergy cedex, France b ESA CNRS 7072, Laboratoire de Tectonique, Case 129, 4 Place Jussieu, 75252 Paris cedex 05, France Received 13 January 1999; accepted for publication 31 January 2000

Abstract We present a magnetic fabric study (12 sites) in strongly deformed metasediments (micaschists and calcschists) from a well-exposed detachment shear zone at Tinos Island (Cyclades, Greece). A gradient of deformation is observed from the southwest side to the northeast side of Tinos. The detachment shear zone is contemporaneous with a greenschist retrogression episode (ca. 25–20 Ma). High-pressure (HP) (45 Ma) metasediments are preserved in the southwest side of Tinos. We compared the anisotropy of magnetic susceptibility (AMS, 158 samples) to the anisotropy of isothermal remanent magnetization (AIRM, 40 samples). AMS is primarily dominated by phyllosilicates whilst AIRM reflects the contribution of mixture of ferromagnetic grains (magnetite, hematite, goethite and pyrrhotite). Interestingly, we observe the appearance of goethite (aFeO · OH ) is observed when approaching the detachment area. The orientation of the AMS and AIRM axes shows a general consistency. On the other hand, AIRM is more prolate and anisotropic than AMS. We document a close agreement between the AMS magnetic foliation and the structural cleavage that has been measured from the core sample. The magnetic lineation is generally parallel to the stretching lineation observed on the field. However, composite magnetic lineations suggest well-preserved petrofabrics contemporaneous to the 45 Ma HP episode. An increase in the AMS foliation anisotropy parameter is observed in micaschists when deformation increases. Unfortunately, no quantitative estimate of finite deformation is possible using AMS due to the dominant role of phyllosilicates and the complex ferromagnetic mineralogy. © 2000 Elsevier Science B.V. All rights reserved. Keywords: detachment fault; ductile deformation; magnetic fabric; metasediments; Tinos

1. Introduction Magnetic fabric mirrors the preferred orientation of crystallographic lattices and shape of a large variety of magnetic minerals (Hrouda, 1982). It is visualized by an ellipsoid with the principal axes labeled K >K >K . The anisotropy of low1 2 3 field magnetic susceptibility (AMS) measures all * Corresponding author. E-mail address: [email protected] (C. Aubourg)

magnetic contributions (i.e. diamagnetic, paramagnetic and ferromagnetic minerals) whereas other techniques such as the anisotropy of remanent magnetization (ARM ) (Jackson, 1991) or high field magnetic susceptibility anisotropy (Bergmu¨ller and Heller, 1996), respectively, discriminate the ferromagnetic and paramagnetic contributions. Unfortunately, these two analytical methods are time consuming and therefore not as widely used such as AMS. The directions of magnetic fabric axes (K >K >K ) are closely related 1 2 3

0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 4 0- 1 9 51 ( 0 0 ) 0 00 4 9 -4

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to the petrofabric axes (l >l >l ; strain axes) 1 2 3 (Borradaile and Henry, 1997). Two magnetic fabric elements are distinguished: the magnetic foliation (plane K –K ) and the magnetic lineation 1 2 (K ) when they are statistically defined. In 1 addition, the magnitude of these axes is expected to be proportional to the magnitude of finite strain (Borradaile, 1991; Rochette et al., 1992). In metamorphic rocks, the agreement between petrofabric and magnetic fabric was stated early (Hrouda, 1982). The magnetic foliation is generally parallel to the cleavage and the magnetic lineation is parallel to either the stretching or the crenulation zonal axis of minerals. Recently, Borradaile et al. (1998) proposed the comparison of the petrofabrics of phyllosilicates ( X-ray goniometry analysis), AMS and anisotropy of anhysteretic remanent magnetization (AARM ) in Archean anorthosite. They found a significant obliquity between magnetic and petrographic foliation planes and thus they have suggested a temporal chronology for explaining this geometrical relationship. Phyllosilicates fabric acquisition is prior to AMS (phyllosilicates+ferromagnetics grains), which is also prior to AARM (fabrics of ferromagnetics exclusively). Thus, the angular deviation provides a sense of shear, which can be particularly useful in weakly deformed metamorphic rocks. Obliquity between magnetic foliation and structural foliation in metasediments has been described by Rathore (1985). Aranguren et al. (1996) discussed the obliquity of magnetic and structural foliations in mylonites and made the conclusion of a competition between the S and C planes. Beside the good qualitative agreement between the magnetic fabric and petrofabric, the authors investigated also the relationship between the magnitude of strain and magnetic fabric. A semiquantitative agreement with strain and magnetic fabric was discussed by Hrouda (1982). This author showed that the degree of anisotropy (P=K /K ) increases continuously from 1.05 in 1 3 weakly deformed sedimentary rocks up to 1.2 in gneisses. In strongly deformed rocks such as mylonites, P gain values are close to 3 (Goldstein, 1980; Housen et al., 1995). In granodiorite where the paramagnetic contribution dominates AMS,

Aranguren et al. (1996) documented a nice gradient from undeformed granodiorite (P=1.03) to mylonite (P=1.09). Note that P is significantly lesser when paramagnetic dominates AMS (P<1.1). A direct comparison between markers of finite strain and the magnetic fabric axis has also been made ( Kligfield et al., 1981). Tarling and Hrouda (1993) compiled together such a correlation and concluded in an empirical law of:

A B A B l a K 1 = 1 l K 3 3

where a varies from 0.959 to 1.107. These authors credited the variability of a to the differences in the minerals that carry the anisotropy and in the deformation mechanism. However, some authors cautioned the use of such a law because the anisotropy of a single magnetic mineral has a finite value and as the strain increases the anisotropy of magnetic fabric saturates (Borradaile and Mothersill, 1984; Lamarche and Rochette, 1987). It is now accepted that this empirical law works only for a limited degree of strain and for the same magnetic mineralogy (Borradaile and Henry, 1997). The importance of a precise knowledge of magnetic mineralogy in the interpretation of AMS is being increasingly recognised (Rochette et al., 1992). The magnitude of magnetic anisotropy parameters has two principal origins: (1) the preferred orientation and distribution of magnetic grains (shape and crystal lattices); and (2) the intrinsic magnetic anisotropy of minerals. However, it is often difficult to obtain a description of the magnetic mineralogy with the desired accuracy (content, size and distribution). To compare AMS parameters to a finite strain, it was proposed to study the maximum intrinsic anisotropy parameters obtained from rock powder (Aubourg et al., 1996). In order to test the possible role of the magnetic mineralogy, Borradaile (1987) and Rochette (1988a,b) proposed checking the relationship between the mean susceptibility K [K =(K +K +K )/3] and P. A positive relam m 1 2 3 tionship between P and K is the sign of a possible m dependence of magnetic fabric on the magnetic

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mineralogy. Such a relationship between K and m P is observed within the Parry Sound mylonites (Housen et al., 1995) even if it remains questionable. Indeed, Housen et al. (1995) noticed an increase in magnetite content from 0.01% in mylonite up to 1–2% in ultramylonites. The authors do actually not discuss whether this drastic increase in magnetite content has any consequences concerning the magnitude of P. High P values (up to 2) can be explained alternatively by the enhancement of the anisotropy caused by distribution (Stephenson, 1994). In this study, we propose a magnetic fabric investigation of strongly deformed metasediments in the context of a detachment zone. To measure the magnetic fabric, we use two techniques: AMS and AIRM. We will show a close agreement between petrofabric and magnetic fabrics. However, the predominant role of phyllosilicates on the AMS bracketed the magnitude of the AMS parameters and thus limited quantitative application, which was the initial purpose of this study.

2. Geological setting and sampling The island of Tinos (Cyclades, Greece) forms a NW–SE-trending elongated structure (~28 km length, ~12 km width) made of three main lithological units (Melidonis, 1980) ( Fig. 1): (1) an Upper ophiolitic Unit which is made of metabasites, mainly metagabbros and serpentinites; (2) a Lower Unit, also called Cycladic blueschist Unit, mainly composed of interlayered metapelites, metabasites and metacarbonates derived from a volcano-sedimentary sequence; and (3) a composite pluton made of a calcalkaline affinity medium grained monzogranite (Melidonis, 1980) and a S-type granite cropping out as a NE– SW elongated in the eastern part of the island. The Upper and Lower Units recorded very different metamorphic histories as shown by their petrological and radiometric data. The Upper Unit has been metamorphosed under LP–HT ( low pressure–high temperature) conditions at ca. 70 Ma (Patzak et al., 1994) whilst the Lower Unit underwent regional polymetamorphism. Firstly, a HP–LT (high pressure–low temperature) meta-

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morphism producing eclogite and blueschist parageneses (12–15 kbar and 450–550°C ) at ca. 45 Ma (Bro¨cker et al., 1993) and secondly, an intense greenschist facies metamorphism (5–7 kbar and 400–500°C ) taking place at 25 to 20 Ma (Altherr et al., 1982; Bro¨cker et al., 1993) which largely overprinted the first event. Though this low-grade retrogression is widespread all over the Lower Unit, it is worth noticing that HP relics are better preserved in the southwestern part of the island than in the northeastern part where they are rarely very well-preserved ( Fig. 1) (Patriat, 1996; Jolivet and Patriat, 1999). These two metamorphic units were eventually intruded by granitic intrusions at ca. 19 to 14 Ma (Altherr et al., 1982) which developed a contact metamorphism in the country-rocks. Noteworthy is the fact that the Upper Unit was not affected by the 45 Ma Cenozoic metamorphism. The island of Tinos is located in the Aegean domain (Greece) which has undergone postorogenic extension since the early Miocene. As a result, Tinos is interpreted as a metamorphic core complex (Gautier and Brun, 1994; Jolivet et al., 1994; Jolivet and Patriat, 1999) which major structural features are as follows (Fig. 1): (1) An asymmetrical dome of foliation with a steeper dip on the southwestern side. (2) The contact between Lower and Upper Units is a typical detachment fault (Gautier and Brun, 1994; Jolivet and Patriat, 1999). It juxtaposes a HP relics-bearing unit with a LP metamorphic unit. (3) An intense ductile deformation primarily observed within the Lower Unit. This generated a L–S tectonic fabric, that is, primarily a NW–SEtrending foliation forming a dome structure and a NE-trending stretching lineation. This deformation occurred under greenschist facies metamorphic conditions at ca. 25–20 Ma. A similar planar fabric is also observed in the granitic intrusion as well as in the Upper Unit but only just close to the detachment. (4) Ductile deformation shows an heterogeneous pattern at the scale of the island-scale. The distribution map of kinematic criteria shows that deformation is rather coaxial on the southwest side of the island, whilst non-coaxial flow dominates on the northeast side (Patriat, 1996; Jolivet

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Fig. 1. Synthetic map and cross-section of Tinos Island.

and Patriat, 1999). In this latter domain, shear criteria always indicate a top-to-the-NE sense of motion. (5) A deformation gradient increasing to the NE, that is, towards the detachment fault separat-

ing the Lower and Upper Units, is observed (Patriat, 1996). (6) Late brittle faulting consistent with the general extensional regime. As a whole, the deformation pattern of Tinos

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is primarily due to post-orogenic extension which allowed a continuum of extensional strain (from ductile to brittle) and exhumation of metamorphic rocks to be recorded. We sampled a total of 158 cores (10.8 cm3) covering the retromorphic transition between blueschist and greenschist facies metamorphism (Fig. 1). Ten sites (2T–11T ) are localized along a road transect from Isternia to Panormos where the gradient is particularly well exposed and documented (Patriat, 1996; Jolivet and Patriat, 1999). Site 12T is situated in Kolimpithra Bay where fine exposures of mylonites are present and site 1T is close to Tinos city.

3. Petrography and microstructures The studied metasediments are micaschists and calcschists which, according to their modal compositions ( Table 1), may derive from more or less carbonated pelites and semi-pelites. Most of the samples are Grt–Bt or Bt-bearing micaschists that contain Qtz+WM (white mica)±carbonates (Cal and/or Fe_Carb)+Opq+Gra(graphite)+Czo+ accessory minerals (±Trm±Spn±Ap) [international mineral abbreviations from Kretz (1983)]. In addition to these primary HP–LT metamorphic parageneses (Bro¨cker et al., 1993; Patriat, 1996; Jolivet and Patriat, 1999), a greenschist facies overprint is widely observed. Initially it is the development of typical greenschist facies mineralogical assemblage Ab+Chl+Ep in some samples ( Table 1). There are secondly late transformations which are: (1) the more or less complete replacement of garnet and biotite by chlorite (Fig. 2A), (2) the weathering of opaque minerals; and (3) the replacement of Fe_Carb by calcite+ Fe-oxides. These latter are being concentrated within calcite cleavage planes (Fig. 2B). Among the studied rocks, samples 2T, 4T and 8T must be distinguished because of their mineralogical composition dominated by carbonates (respectively 35%, ~65% and 80% of volumetric proportion; see Table 1). They all contain quartz and phyllosilicates in addition to accessory minerals. These calcschists also

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underwent the greenschist facies overprint (development of the greenschist facies mineralogical assemblage Ab+Chl+Ep in sample 2T; retrogression of Grt into Chl in sample 4T; and replacement of Fe_Carb by Cal+Fe-oxides in all three samples). All the samples show an intense deformation. They are fine-grained L–S tectonites displaying granolepidoblastic textures with a pronounced foliation defined by the shape fabric of the phyllosilicates (white micas, biotite, chlorite) and clinozoisite which, in some cases also outlined stretching lineation. Three types of porphyroblasts may be observed. Relictual garnets (samples 1T, 3T, 4T and 7T ) show evidence for rotational deformation. Indeed, many porphyroblasts (1– 2.5 mm in size) show a curved internal schistosity (S ) made of graphite and quartz inclusions i ( Fig. 2A) which is not continuous with the foliation of the matrix (S ). The relationship is consise tent with a syn-kinematic growth of Grt during HP–LT 45 Ma metamorphic episode. Albite porphyroblasts (samples 1T, 2T, 6T, 9T, 12T ) display also a sigmoidal S pattern but it is coni tinuous with S and rotated with respect to S e e ( Fig. 2C ). This supports a syntectonic growth for albite during greenschist facies metamorphism (25 to 20 Ma) and rotational deformation. The third type of porphyroblast is made of iron-rich carbonates. They have a relictual appearance and show, as previously described extensive retrogression ( Fig. 2B). Special attention is paid to opaque minerals. Regarding their volumetric amounts (traces), they are considered as accessory minerals except in samples 8T and 9T where they represent ca. 1% of the volume of the rocks. Finally, it is worth noting, in this study, the occurrence in minor amounts of tourmaline in two samples (7T and 8T ) as this silicate possesses an inverse magnetic fabric (Rochette et al., 1992). From a structural point of view, several microstructures are observed in thin sections in addition to foliation, mineral stretching lineation and porphyroblast-deformation relationships. They are, in order of abundance: (1) shear band cleavages filled by chlorite (CMS; Fig. 2D) (2) mica-fish (Fig. 2E);

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Table 1 Petrographical and microstructural details of the studied samples [mineral abbreviation fromKretz (1983)]; includes rock type and modal composition (estimated volumetric proportions), nature and relative proportions of ferromagnetic grains deduced from threeaxes thermal demagnetization of composite IRM, and additional microscopic observations (GSf=greenschist facies) Sample ref. Rock type Minerals Qtz (quartz) W.M (white mica) Bt (biotite) Chl II (chlorite) Grt (garnet) Ab (albite) Cal (calcite) Fe-Carb (Fe-carbonate) Epi (epidote) Czo (clinozoisite) Opq (opaque mineral ) Gra (graphite) Accessory minerals

1T Micaschist

2T Calcschist

3T Micaschist

4T Calcschist

5T Metapsammite

6T Micaschist

20 12 1 10 3 30 5

15 20 2 10

25 25

10 15

75 20 4

5 10

5 5

40 50 2 4

15 12 1< 4 1% 3

55 8 1<

2 15 1% 1 Spn (sphene)

10 35 3 1 1% 3 Spn

2

1% 1

1< 1% 1% Fe-oxides

1y 1

Ferromagnetic minerals: relative proportions nent magnetization Mag (magnetite) a Hem (hematite) a Po (pyrrhotite) b Gt (goethite) c

estimated from three-axes stepwise thermal demagnetization of composite isothermal remac a a a

a a c a

a

Retrogression

GSf overprint, GSf assemblage, Opq weathered, BtChl II

GSf overprint, Fe-Carb Cal+Fe-oxides

GSf overprint, GrtChl II, Fe-CarbCal +Fe-oxides

micafish; Chl–Qtz–Cal shear bands; microfolding

CMS

CMS; a symetrical P-shadows around Grt

Microstructures

Other details

GSf overprint, GSf assemblage, Grt pseudomorphoses, BtChl II CMS; S i in porphyroblasts

c

a c b a

a c GSf overprint, GSf assemblage, unidentified pseudomorphoses completely replaced by Chl

mica-fish

CMS

Opq minerals altered showing a red aureole

(3) symetrical or asymetrical pressure-shadows around Grt or Ab porphyroblasts (e.g Fig. 2C ); and (4) microboudinage.

Senses of shear, deduced from these microstructures, along our cross-section are consistent with macro-scale structures and former works (Patriat, 1996; Jolivet and Patriat, 1999).

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C. Aubourg et al. / Tectonophysics 321 (2000) 219–236 Table 1 (continued ) Sample ref. Rock type

Minerals Qtz (quartz) W.M (white mica) Bt (biotite) Chl II (chlorite) Grt (garnet) Ab (albite) Cal (calcite) Fe-Carb (Fe-carbonate) Epi (epidote) Czo (clinozoisite) Opq (opaque mineral ) Gra (graphite) Accessory minerals

7T Micaschist

8T Calcschistsubmarble

9T Micaschist

10T Micaschist

11T Micaschist

12T Micaschist

45 35

7 10

55 25 4 1

45 10 5 20

25 30 3 15

45 30 5 2

1 10

15

4 20

1 8

4 1 5 Traces 1 Trm (tourmaline)

Ferromagnetic minerals: relative proportions nent magnetization Mag (magnetite) a Hem (hematite) Po (Pyrrhotite) c Gt (goethite)

60 20

1 1< Trm (1–2%)

1 2 1< 2< Spn

1< 1 2–3

1 1< 1 Spn, Apa (apatite)

15

1< 1 Fe-ox

estimated from three-axes stepwise thermal demagnetization of composite isothermal remaa a c

b a a b

GSf overprint, Chl II (20%)

GSf overprint, Fe-CarbCal+ Fe-oxides

CMS; sygmoides

CMS; sygmoides; micro-boudinage

GSf overprint, BtChl II, Fe-CarbCal+ Fe-oxides P-shadows around Ab porphyroblast; CMS; sygmolides Very small opq (20–50 mm)

b b a

b b b

b a b

GSf overprint, GSf assemblage, BtChl II CMS; mica-fish

Retrogression

GSf overprint, Fe-CarbCal +Fe-oxides

GSf overprint, Fe-CarbCal +Fe-oxides

Microstructures

shear bands; P-shadows around Grt Growthzoning in Trm with graphite inclusions

CMS; mica-fish

Other details

1

Euhedral Opq

Large opaque minerals (up to 500 mm) with internal schistosity S i

Most intense deformation

a Low relative occurrence. b Medium relative occurrence. c High relative occurrence.

4. Methodology Magnetic mineralogy investigation and two types of magnetic fabric measurements have been carried out. All measurements, except hysteresis curves, were performed at the Laboratory of

Magnetic Anisotropy ( University of CergyPontoise, France). We studied on selected paired samples hysteresis curves and stepwise demagnetization of composite isothermal magnetization (Lowrie, 1990). Hysteresis curves were obtained with an home-

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designed inductometer (St Maur Geomagnetism Laboratory). The maximum field available is 1.5 T and the sensitivity is ca. 10−5 A m−1. The volume of the sample is ca. 6 cm3 and one measurement takes 20 min. On the second sample, we applied successively DC magnetic fields of 0.1, 0.6 and 1.2 T on the X, Y, and Z-axis of the core, respectively. Thermal demagnetization was performed with a shield furnace (J. Shaw) and remanence was measured with a spinner magnetometer (JR5, Agico). Two methods were used to measure the magnetic fabric: AMS and AIRM. AMS was measured according to the Jelinek procedure (Jelinek, 1977) with an impedance bridge Kly-2 (Agico) which has a sensitivity of 0.05 mSI. Mean tensorial statistics (Jelinek, 1978) were applied. Tensorial statistics provided mean AMS axes with their two axis of confident ellipses. AIRM was measured according to the Jelinek procedure (Jelinek, 1993) procedure. We selected at least three representative samples per site. Six independent positions provide the second rank tensor K ij. For each position, a 20 mT DC magr netic field (Pum-1, Agico) is applied successively upward and downward. The remanence was therefore measured therefore twice. This allows one to distinguish precisely the residual imparted isothermal remanent magnetization for each position. The complete measure of AIRM for one sample takes ca. 45 min. AIRM measurements were reproducible. In particular, we did not find any evidence of artifacts as documented by Tauxe et al. (1990). AMS and AIRM data are plotted in equal areas downward stereo-projection. For clarity, we plotted only the maximum and minimum axis of each tensor. Anisotropy parameters L=K /K , F=K / 1 2 2 K , P=K /K are used. L and F respectively mirror 3 1 3 the stretching and the flattening of the ellipsoid. The shape of the magnetic fabric ellipsoid is described with the T parameter (T=

227

2 ln F/ln P−1). It is oblate (prolate) when T>0 (T<0). To compare magnetic fabrics with petrofabric elements, we systematically measured accurately (within one degree) systematically the cleavage of each core (S-foliation) using a home designed goniometer (Robion et al., 1995).

5. Magnetic mineralogy In this study, 87% of the samples have a mean magnetic susceptibility K <700 mSI (44% m <300 mSI ). These low susceptibilities indicate a minor ferromagnetic input (Rochette, 1987a). In order to constrain the ferromagnetic contribution, we measured the hysteresis curves of seven samples. Paramagnetic behavior is essentially observed when K <600 mSI. From high-field magm netic susceptibility K , we derived the ferromaghf netic contribution K (K =K −K ). We present f f m hf in Fig. 3A the relative proportion of K versus f K . The pattern suggests that ferromagnetic contrim bution as calculated above is minor (<50%) when K is <700 mSI. Thus, for ca. 90% of samples m from this study, the paramagnetic contribution is dominant (>50%). We show in Fig. 3B the mean magnetic susceptibility versus the corrected degree of anisotropy. No clear relationship is observed suggesting that magnetic mineralogy does not control the magnitude of AMS. However, a rough linear trend is suggested for calcschists, and possibly site 7T. We present typical diagrams of three-axes stepwise demagnetization of IRM along the Isternia– Panormos transect ( Fig. 4). We distinguish both coercivity and unblocking temperature behaviors. Lowrie (1990) and Lehman et al. (1996) discuss the use of composite stepwise demagnetization. Soft to medium coercivities with maximum unblocking temperatures at 580°C correspond to

Fig. 2. Microphotograph and associated schematic representation of microstructures observed within studied samples. (A) Pseudomorph of garnet completely replaced by chlorite. Note that the internal schistosity (S ) which is mainly outlined by quartz i inclusions and, in a lesser extent by graphite inclusions, is not continuous with the schistosity of the matrix (external schistosity S ; e site 1T ). (B) Relictual porphyroblast of Fe-carbonate destabilized into an association of calcite+Fe-oxides, which are concentrated along cleavage planes (site 7T ). (C ) Sheared porphyroblast of albite with S (inclusions of graphite) and asymetrical calcite-bearing i pressure-shadows (site 9T ). (D) Example of C–S microstructures (site 2T ). ( E ) Example of micafish (site 5T ).

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Fig. 3. (A) Ferromagnetic contribution K in % versus K f m derived from hysteresis curve analysis [%K =100× f (K −K )/K ; K is the high-field susceptibility]. From these m HF m HF two diagrams, we deduce a large contribution of phyllosilicates (paramagnetic) to AMS. (B) Corrected degree of anisotropy P∞ versus the mean susceptibility K with their standard error bars. m In both diagrams, open (grey) circles correspond to micaschists (calcschists). P∞=exp(E2[(g −g )2+(g −g )2+(g −g )2], 1 m 2 m 3 m g =ln K , g =(g +g +g )/3. i i m 1 2 3

magnetite (Fig. 4A). Medium to medium coercivities with blocking temperature up to 680°C are hematite components (Fig. 4A,C and D). We interpret the sharp decrease of soft and medium components between 300 and 350°C as being the signature of pyrrhotite ( Fig. 4B) since iron sulfide is commonly encountered in metamorphic rocks (Robion et al., 1995). The sharp decrease in the high coercivity component between 100 and 150°C is typical of goethite. We summarized the relative occurrence of these minerals in Table 1. First, it appears that ferromagnetic mineralogy is largely

composite, with the equal participation of all coercivities. Second, a rough evolution can be drawn. The coexistence of hematite and magnetite is observed where blueschist facies parageneses are best preserved (site 2T ). The pyrrhotite is apparently well expressed in the blueschist to greenschist transition (sites 3T, 4T, 7T ). Goethite is identified in the most retrograde samples (sites 1T, 6T, 8T, 9T, 10T, 12T ). Note that sites 6T– 12T are close to the detachment. Petrographic and magnetic investigation revealed the composite nature of matrix (phyllosilicates) and ferromagnetics. We discuss the intrinsic AMS magnetic anisotropy of these minerals based on recent data (Rochette et al., 1992; Borradaile and Henry, 1997). Note that no intrinsic AIRM data are available for the ferromagnetic grains. The intrinsic anisotropy of phyllosilicates (P <1.4) is comparable to those of natural magnei tites (P <1.2) (Borradaile et al., 1987) or goethite, i but it is two or three orders of magnitude less than those of hematite (P <100) or pyrrhotite i (P <1000). The shape anisotropy is essentially i planar (F =P ) for phyllosilicates, hematite or pyri i rhotite and variable for magnetite. Aubourg et al. (1996) showed that the maximum anisotropy parameters for a phyllosilicate-bearing rock are L<1.12 and F<1.4. Amphibole and feldspars have linear anisotropy but they weakly account for AMS. Note that single domain ferromagnetic grains, goethite and tourmaline present inverse properties of AMS (interchange of K and K ) 1 3 (Rochette et al., 1992). This peculiar behavior can decrease significantly the magnitude of anisotropy parameters, as discussed by Lehman et al. (1996) who documented abnormal sub-isotropic magnetic fabrics due to the occurrence of goethite.

6. Magnetic fabric results We measured 164 and 40 samples for AMS and AIRM, respectively. Scalar and directional magnetic data are shown in Table 2. AMS ellipsoids are essentially oblate ( Fig. 5A) except for some samples of sites 12T and 8T. Some samples from site 12T are characterized by high magnetite contents (about 0.2% in volume when assuming the initial susceptibility of magne-

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Fig. 4. Examples of three-axes stepwise thermal demagnetization of composite isothermal remanent magnetization (Lowrie, 1990). Hard, 1.2 T; medium, 0.6 T; soft, 0.1 T.

tite at 1 SI for a rock mean susceptibility K =16 400 mSI ). The maximum F and L m ASM ASM values are <1.2 and 1.08, respectively (apart from samples from site 12T ). In contrast, AIRM ellipsoid are more triaxial ( Fig. 5B) and 36% of the samples exhibit a prolate shape. F <2.4 and AIRM L <1.8 are larger than their AMS homologues. AIRM One can observe a poor relationship between the magnitude of the degree of anisotropy P and AMS P (Fig. 5C ). AIRM AMS magnetic fabrics are well defined (Fig. 6, Table 2) except for site 8T. The magnetic foliation (perpendicular to the grouping of K ) is similar to 3 the S-foliation measured either on the field (Fig. 6) or with a goniometer (Table 2). The magnetic linea-

tions (grouping of K ) are fairly parallel to the 1 mineral stretching lineation. Note that two significant behaviors are unusual: the composite magnetic lineations EW and NE observed at site 3T and the apparent interchange of K and K at site 8T. 1 3 Well-defined AIRM magnetic fabric is generally observed except at sites 3T and 11T ( Fig. 6). In these two sites, the measurement of individual samples showed large errors (confidence angles >25°). This explains the poor results obtained. For the other sites, the correspondence between AMS and AIRM fabrics is relevant except for sites 2T and 8T. At site 2T, AMS and AIRM magnetic lineations are significantly different. AIRM magnetic lineation is more parallel to the stretched

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Table 2 AMS and AIRM resultsa Site

n

AMS K

1T 2T 3T 4T 5T 6T 7T 8T 9T 10T 11T 12T

16 10 10 10 12 11 10 10 23 11 16 19

m

355 381 451 445 141 312 368 70 329 174 255 272

AIRM sK L m

F

56 34 99 226 29 40 81 10 47 23 121 53

1.062 0.24 22/22 1.128 0.85 273/19 1.052 0.42 244/10 1.128 0.68 235/14 1.042 0.74 195/7 1.083 0.62 247/14 1.109 0.46 235/14 1.007 −0.01 123/48 1.097 0.73 37/18 1.113 0.66 61/3 1.109 0.67 225/18 1.078 0.38 32/4

1.033 1.010 1.017 1.023 1.006 1.018 1.039 1.007 1.015 1.022 1.013 1.040

T

K 1

K

3

2660/47 72/70 11/73 55/76 88/67 104/73 77/75 309/42 240/71 167/79 337/50 225/85

Total 158

E

12

3 33 27 12 15 26 17 72 6 6 11 5

E

23

21 8 6 7 5 21 4 34 3 11 5 7

E

32

6 12 8 9 7 10 5 20 5 6 6 3

S-foliation

n

K

AIRM

L AIRM

F AIRM

T

Pole

3 3 3 3 3 3 3 3 3 3 3 7

257 251 1568 12350 249 215 215 3530 560 525 1618 728

1.204 1.225 1.106 1.364 1.084 1.179 1.164 1.186 1.405 1.193 1.229 1.235

1.109 1.298 1.267 1.147 1.188 1.198 1.295 1.130 1.516 1.365 1.488 1.270

−0.28 0.28 0.26 −0.37 0.32 0.09 0.21 −0.18 0.07 0.27 0.14 0.05

262/61 7 71/70 6 355/70 11 51/81 9 95/78 6 95/79 7 51/80 8 210/61 5 206/75 8 155/82 6 341/46 6 162/79 5

a

95

40

a AMS results. n, Number of samples, K =(K +K +K )/3, mean susceptibility in mSI; sK , standard deviation at 95% level; m 1 2 3 m L=K /K , lineation parameter; F=K /K , foliation parameter; T=2[ ln F/ln(LF )]−1, shape parameter; −1≤T≤0, prolate; 1≥T≥0, 1 2 2 3 oblate. Geographic coordinates (declination, inclination) of AMS axes K and K ; E , E , E , half confidence angles at 95% level 1 3 12 23 32 of AMS axes (E describes the half angle of the ellipse of confidence of direction K within the plane K –K ); AIRM scalar results: 12 1 1 2 n, number of samples; K , susceptibility of isothermal remanence acquired at 20 mT (dimensionless: mSI ); L , AIRM AIRM F and T have the same definitions as their AMS homologues. S-foliation, pole of foliation in the low-hemisphere measured AIRM AIRM on the sample with a goniometer. a , Fischer dispersion at 95% level. 95

minerals. Interestingly, the same two trends are also distinguishable for AMS magnetic lineations at site 3T. At site 8T, AIRM K and K are 1 3 opposite to their AMS homologues. Although the AMS at site 8T is poorly defined ( Table 2), the fair grouping of K at a right angle of the structural 1 foliation as well as the low magnitude of AMS parameters ( Fig. 6, Table 1) suggests an abnormal magnetic fabric (Stephenson et al., 1986). The interchange of AMS and AIRM axes can be explained by an inverse AMS due to magnetic mineralogy perturbation (Jackson, 1991). Whether paramagnetic or ferromagnetic grains are responsible of the inverse AMS fabric remains questionable. The magnetic mineralogy investigation at site 8T revealed that matrix dominates AMS (K /K =0.62). In these calcschists, a possible canp m didate to explain inverse magnetic fabric can be the tourmaline or siderite ( Fe-rich carbonates) ( Ellwood et al., 1986). Goethite (Lehman et al., 1996) and fine ferromagnetic grains (Rochette, 1988) are also possible candidates.

7. Discussion Several aspects are discussed in this section: (1) the significance of magnetic mineralogy in terms of relationship with magnetic fabric and occurrence; (2) the qualitative correspondence between structural data and magnetic fabric elements; and (3) the semi-quantitative evolution between AMS and AIRM parameters and the gradient of deformation close to the detachment.

7.1. The significance of magnetic mineralogy The metapelites of Tinos are characterized by the governing role of the matrix for the AMS and a composite ferromagnetic mineralogy for AIRM. Let us distinguished successively the role of magnetic mineralogy for AMS and AIRM. Afterwards, we will discuss the occurrence of goethite and pyrrhotite.

C. Aubourg et al. / Tectonophysics 321 (2000) 219–236

Fig. 5. Flinn type diagram. Lineation (L) versus foliation (F ) parameters. (A) AMS, (B) AIRM. Shape of magnetic fabric is prolate (oblate) when L>F (L
7.1.1. Magnetic mineralogy and AMS We found that paramagnetic phyllosilicates in the metasediments control AMS when magnetic susceptibility K is below 700 mSI. The governing m role of phyllosilicates has two consequences: (1) AMS parameters are rather low compared to those documented for mylonites. For example, in the study of Housen et al. (1995), the highest anisotropy parameters are P<1.3 when K <700 mSI while P<3 when magnetite controls m AMS. In the present study, AMS parameters are comparable to those documented by Aranguren et al. (1996) in paramagnetic mylonite. (2) The shape of AMS ellipsoid is strongly oblate (T>0.5) throughout the Isternia–Panormos

231

transect in spite of a spectacular macroscopic stretching lineation within the different lithologies close to the detachment. In contrast, Housen et al. (1995) found rather a prolate shape of AMS for magnetite-bearing ultramylonites. It appears therefore that phyllosilicates are not good candidates for tracing the development of linear anisotropy in a detachment zone. Another striking perturbation due to magnetic mineralogy is the inverse AMS magnetic fabric recorded at site 8T. It results from the apparent interchange of K with K and also with large a 1 3 decrease in AMS parameters. In this regard, the occurrence of inverse AMS minerals in this study such as goethite, siderite or tourmaline can have some of the consequences as emphasized by Lehman et al. (1996). Although these minerals play a minor role in the AMS (except at site 8T ), one can expect a slight decrease in AMS parameters. In contrast, pyrrhotite has very high intrinsic anisotropy and is likely to enhance AMS parameters by its sole contribution (Aubourg et al., 1996). With sites 4T and 7T, where pyrrhotite is the main ferromagnetic carrier ( Table 1), we observe a marginal correlation between the degree of anisotropy P∞ and mean susceptibility K ( Fig. 3B). This m suggests that pyrrhotite can control the anisotropy magnitude. 7.1.2. Magnetic mineralogy and AIRM The meaning of AIRM can be obscured by the composite source of ferromagnetic grains of different shapes, coercivities and intrinsic anisotropy. It is self-evident that all these parameters are not constrained in our study and it is not the purpose of this paper to discuss these complications. In this study, we observed a large variability of anisotropy parameters, and rather a better consistency of the direction of the axes. 7.1.3. The occurrence of goethite and pyrrhotite The presence of goethite near the detachment should be correlated with evidence for fluid advection within the detachment zone. When approaching the detachment from below in the lower unit, deformation becomes more and more intense and a transition in time and space from ductile to brittle regime is observed. This transition

Fig. 6. AMS and AIRM magnetic fabrics are plotted in the equi-areal lower hemisphere. Also plotted are the foliation and stretching direction measured in the field. Locations of the sites are projected along the Isternia–Panormos transect. The grey diagrams are calcschists.

232 C. Aubourg et al. / Tectonophysics 321 (2000) 219–236

C. Aubourg et al. / Tectonophysics 321 (2000) 219–236

is accompanied with the formation of thick cataclastic layer immediately below the brittle detachment (Patriat, 1996). This breccia is rich in quartz and talc and reworks both the lower and upper units. A large quantity of veins is seen in the reddish breccia. Those veins are filled with quartz, carbonates, iron-carbonates and ironhydroxides. The same material is often also observed along shear bands further down in the lower unit. The magnetic mineralogy suggests that hematite is more systematically replaced by goethite when approaching the detachment. This observation supports the observation that hydrous fluids have migrated from the detachment zone within the lower unit. The relationship between the occurrence of pyrrhotite and the development of greenschist metamorphism has been fairly well documented in metapelites from the Western Alps (Rochette, 1987b) and in the French Ardennes (Robion et al., 1995). Pyrrhotite is also detected in greenschist clastic rocks (Borradaile and Dehls, 1993). Close to Tinos Island, Morris and Anderson (1996) found evidence of pyrrhotite in the Mykonos Miocene granodiorite. At Tinos, pyrrhotite is not recognized in the well-preserved HP metasediments facies from Isternia (site 2T ). Thus, we believe that pyrrhotite is rather a secondary mineral occurring during the exhumation of HP rocks, that is, during greenschist facies metamorphism. 7.2. The comparison of magnetic fabrics with petrofabric directional elements The comparison between magnetic fabrics and petrofabric reveals a fair agreement as expected. We examine this degree of agreement with foliation and lineation successively. We plot for this purpose the AMS compilation of sites along the transect between Isternia (2T–7T ) and Panormos (9T– 11T ) in a synthetic sketch (Fig. 7). All AMS K are remarkably close to being 3 vertical after tilting S-foliation to the horizontal (Fig. 7), suggesting that magnetic foliations are closely parallel to S-foliation. This is particularly true when approaching the detachment [i.e. for the eastern sites (9T–11T )] close to the detachment. However, one can note a small deviation of K (5–10°) for sites further west to the detachment. 3

233

Whether this deviation is significant remains debatable. This deviation can reflect a competition between S and C planes as reported by Aranguren et al. (1996). However, the obliquity between S and C planes is apparently similar in all sites. Another possibility is the occurrence of subfabrics. Borradaile and Dehls (1993) discussed the origin of subfabrics, petrofabrics of quartz and felspars, AMS of phyllosilicates and AIRM of magnetite. They found small but significant deviations between the axes of subfabrics and they argued for the late development of phyllosilicates and magnetite fabrics within respect to the quartz and felsdpar fabrics. It is therefore possible that the slight deviation of K with respect to the S-plane 3 corresponds to the development of a cryptic foliation plane, westward-dipping, presumably related to a top-to-the-E sense of shear. Close to the detachment, subfabrics are probably more coaxial so that no significant deviation of K is observed. 3 The trends of AMS magnetic lineations correspondence closely to the mineral stretching regional pattern ( Figs. 7 and 1). The same trend of AMS magnetic lineations is documented in subisotropic granodiorite from Mykonos Island (Morris and Anderson, 1996). The composite magnetic EW and NE lineations observed on the well preserved blueschist facies metasediments of sites 2T and 3T provide additional information. The EW trend is documented at Syros Island (Gautier and Brun, 1994; Jolivet and Patriat, 1999) where preserved blueschists and eclogites crop out. We speculate that EW trends found in the southern part of Tinos Island are some relics of HP fabric (45 Ma) that were not completely overprinted during the greenschist stage. At site 2T, AMS and AIRM magnetic lineations trend NNE and NE. This suggests that ferromagnetic fabrics post-dated the phyllosilicates fabric. This relic of magnetic lineation from metamorphism at 45 Ma in the HP metasediments supports that these rocks are relatively preserved from pervasive non-coaxial deformation. 7.3. Magnitude of anisotropy parameters with respect to the gradient of deformation Comparison of the shape of AMS and AIRM magnetic fabrics showed that: (1) AMS is oblate

234

C. Aubourg et al. / Tectonophysics 321 (2000) 219–236

Fig. 7. Sketch showing the geometry of the detachment along the Isternia–Panormos transect and AMS data in the S-foliation coordinates (tilting S-foliation horizontally). Note the slight eastward obliquity of the magnetic foliation plane relative to the S-foliation for sites 2T–7T. Also shown are the semi-quantitative evolution of the foliation parameter F along the Isternia– AMS Panormos transect. Abnormally low or high values can be explained by rocks types (calcschsits) and mineralogical disturbances (pyrrhotite at site 7T ).

while AIRM is rather prolate; and (2) that the magnitude of AIRM parameters is larger. Borradaile and Dehls (1993) obtained similar results. The absence of a prolate shape in AMS

may be due to the governing role of platy phyllosilicates. In addition, the predominant role of phyllosilicates obscured the increase of linear anisotropy as it must be expected close to the detachment.

C. Aubourg et al. / Tectonophysics 321 (2000) 219–236

235

However, we found a significant increase of the AMS foliation parameter (F ) from Isternia to AMS Panormos (Fig. 7). Abnormal F values are AMS observed in calschists (sites 2T, 4T and 8T ). This can be interpreted to be the occurrence of strong anisotropic minerals such as pyrrhotite (site 4T ) or due to inverse magnetic fabric (site 8T ) as previously discussed. This positive correlation suggests the enhancement of planar anisotropy towards the detachment. The coexistence of HP– LT and later LP–LT ductile deformations, as well as complex magnetic mineralogy are probably the principal causes of the absence of a clear gradient, as shown in the field.

of the flattening towards the detachment zone. This suggests that non-coaxial deformation related to intense shear gradually disappears southwards and is relatively limited geographically (in the order of 1 K ). This is in good agreement with m well preserved blueschist relics exposed close to the sites with a lower AMS magnitude. To pursue this investigation, we believe that rocks with only one dominant ferromagnetic mineral are the best candidates to trace quantitatively this deformation gradient. With this aim, the metabasite rocks, which have only a few percents of magnetite, may constitute a better lithology for such a study.

8. Conclusion

Acknowledgements

In a detachment shear zone where rocks are strongly deformed, the magnetic fabric constitutes an original approach to qualitatively and quantitatively describe the petrofabrics. In the spectacular detachment that crops out at Tinos, we document a fine agreement between metamorphic foliations and magnetic foliations on one hand, and between stretched minerals and magnetic lineations on the other hand. However, one can expect more from magnetic fabric investigation, and particularly, the use of anisotropy parameters to describe the shape of the petrofabric and to trace the deformation gradient. As emphasized by several authors (e.g. Tarling and Hrouda, 1993), the use of magnetic fabric as a strain gauge is restricted to rocks with simple magnetic mineralogy, say well-calibrated ferromagnetic or paramagnetic grains. In this regard, the metasediments from Tinos present quite simple magnetic mineralogy largely dominated by phyllosilicates. However, phyllosilicates (essentially biotite, muscovite and chlorite) have rather weak finite AMS planar anisotropy so that the AMS magnitude enhancement is self-limited. In addition, the AMS magnitude can sometimes be disturbed at different fabrics due to different families of ferromagnetic grains. Despite all these limitations, the AMS magnitude of metapelites (calcschists give more scattered anisotropy parameters) support a regular increase

This study benefited from discussion with J.P. Guezou and P. Robion. The reviewers L. Sagnotti and J. Tubia improved greatly the quality of this manuscript.

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