ELSEVIER Earth and Planetary Science Letters 171 (1999) 511-512

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Discussion

Comment on "Block rotations and continental extension in the central Aegean Sea: palaeomagnetic and structural evidence from Tinos and

Mykonos (Cyclades, Greece)" by D. Avigad et al.

Antony Morris *, Mark W. Anderson

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

Received 8 February 1999; accepted 6 July 1999

Avigad et al. [1] provide important new palaeo- magnetic data from the Island of Tinos in the Cy- cladic metamorphic core complex of the central Aegean which place additional constraints on the Neogene extensional and rotational tectonic evolu- tion of this complex region. Samples collected from ten dacitic dykes and three sites in the Tinos mon- zogranite pluton of Miocene age yield an in situ remanence direction of D = 028 °, I = 42 °, which when compared to the reference 'pole position' of D = 005 °, I = 42 ° suggests that significant clockwise rotation (23 ° ± 16 °) and no tilting has affected these sites. These data are combined with structural data on the orientation of a series of barite-filled ten- sion gashes on the adjacent Island of Mykonos and with our previously published palaeomagnetic data from an 11 Ma granodiorite exposed over much of Mykonos [2]. The similarity in structural orientation of the sub-vertical dacitic dykes sampled on Tinos [1] and the Mykonos barite veins is taken as evi- dence that tectonic tilting of both islands pre-dated dyke emplacement and vein formation, and that the whole assemblage of granodiorite pluton, dykes and veins then underwent a 23 ° clockwise vertical-axis rotation. The new data presented by Avigad et al. [1] therefore helps to resolve the order of components of

*Corresponding author. Tel.: 4-44-1752-233120; Fax: +44- 1752-233117; E-mail: amorris@plymouth.ac.uk

tilting and rotation required to produce the observed magnetic remanence data set, providing that the key assumption that dykes and veins were originally ver- tical is correct.

We welcome the new aspects of this study, par- ticularly the structural data on barite vein orientation which aids interpretation of our data from Mykonos [2]. Several points in the analysis and discussion presented by Avigad et al. [1] require clarification, however, since the combined data set [1,2] is likely to form a point of reference for future structural, seismological and palaeomagnetic research in this region.

Avigad et al. [1] refer to a tilt-corrected "refer- ence pole position" from the Kimi region of Evvia which they incorrectly attribute to Kissel and Laj [3] and which is in fact a reference direction. We feel it is important that the derivation of this reference direction is clearly explained for future reference. As noted by us [2], the use of reference directions derived from the apparent polar wander paths of the major plates is inappropriate given the widespread occurrence of anomalously low inclinations recorded by Tertiary rocks in the Aegean region. We there- fore proposed [2] the use of a composite reference direction for Miocene units of the Cyclades, which combines an expected declination of 005 ° derived from the African apparent polar wander path with an expected tilt-corrected inclination of 42 ° derived

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512 A. Morris, M.W. Anderson/Earth and Planetary Science Letters 171 (1999) 511-512

from Miocene units near Kimi, Evvia [4,5]. Kissel et al.'s [4] data from Kimi are derived from Miocene andesites which lack an internal palaeohorizontal and are structurally corrected using the orientation of coeval sedimentary rocks exposed along the nearby coastal section. The robustness of the Kimi incli- nation data therefore depends critically on results from these sedimentary rocks reported by Morris [5], which pass a palaeomagnetic fold test and which show identical tilt-corrected inclinations to the an- desires.

In the analysis of our palaeomagnetic data from Mykonos and Naxos [2], the difficulty of palaeo- horizontal control was overcome by adopting a net tectonic rotation viewpoint [6], wherein the in situ remanence data were compared to the tilt-corrected reference direction to determine potential rotation histories. We were careful to stress, and again reiter- ate, that an infinite number of net tectonic rotations are capable of restoring the direction of magneti- sation to its primary orientation in the absence of palaeohorizontal markers. However, it is informa- tive to consider plausible sequential components of tilting and rotation (or vice versa) by incorporating information on the orientation of the major block- bounding structures into the analysis. This allows derivation of two end-member scenarios for the de- formation path on Mykonos, namely a 22 ° clockwise vertical-axis rotation followed by 27 ° of tilting to the southwest or 37 ° of tilting followed by a 13 ° clockwise rotation [2]. Avigad et al. [1] incorrectly state that we argued that the first of these scenar- ios was most likely. In fact, we emphasised that these end-members represent only two possible de- compositions of the net tectonic rotation and were careful to restrict discussion to the sense of rotation of Mykonos and not the absolute angle of rotation, since "the magnitudes of these rotations lack ab- solute constraints" [2]. In the light of the inferred relationship between the orientation of dykes on Tinos and barite veins on Mykonos noted by Avigad et al. [1], it is now perhaps possible to discount a rotation-tilt sequence. An apparent discrepancy be- tween estimated rotation angles of 13 ° [2] and 23 ° [1]

in the alternative tilt-rotation end-member scenario favoured by Avigad et al. is purely a geometrical ef- fect resulting from different estimates of the average strike of the bounding faults (165 ° [2] and 150 ° [1]).

As noted by Avigad et al. [1] in their concluding remarks, a significant new implication of their data and analysis is that the well-documented Pliocene to Recent extension in the Cycladic area of the Aegean domain has been accommodated almost purely by vertical-axis rotation of fault blocks. This is clearly a hypothesis which will provoke renewed interest in the kinematic evolution of the Cyclades and which highlights the need for integration of palaeomag- netic, structural, seismological and reflection seis- mic data in future syntheses. These should aim to (i) evaluate the extent of the rigid block that has remained apparently untilted for the last 12 Ma, and (ii) provide independent confirmation that off- shore structures, which dominate the Aegean crustal framework, display an appropriate history over the last 12 Ma. [AC]

References

[1] D. Avigad, G. Baer, A. Heimann, Block rotations and conti- nental extension in the central Aegean Sea: palaeomagnetic and structural evidence from Tinos and Mykonos (Cyclades, Greece), Earth Planet. Sci. Lett. 157 (1998) 23-40.

[2] A. Morris, M. Anderson, First palaeomagnetic results from the Cycladic Massif, Greece, and their implications for Miocene extension directions and tectonic models in the Aegean, Earth Planet. Sci. Lett. 142 (1996) 397-408.

[3] C. Kissel, C. Laj, The Tertiary geodynamical evolution of the Aegean arc: a palaeomagnetic reconstruction, Tectono- physics 146 (1988) 183-201.

[4] C. Kissel, C. Laj, A. Poisson, K. Simeakis, A pattern of block rotations in central Aegea, in: C. Kissel, C. Laj (Eds.), Palaeomagnetic Rotations and Continental Deforma- tion, Kluwer, Dordrecht, 1989, pp. 115-129.

[5] A. Morris, Rotational deformation during Palaeogene thrust- ing and basin closure in eastern central Greece: palaeomag- netic evidence from Mesozoic carbonates, Geophys. J. Int. 121 (1995) 827-847.

[6] W.D. MacDonald, Net tectonic rotation, apparent tectonic rotation, and the structural tilt correction in palaeomagnetic studies, J. Geophys. Res. 85 (1980) 3659-3669.