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Naxos - Neotectonics and the non-metamorphic nappe

Carla Hillebrand RWTH Aachen University

Field Course "Naxos" - Group B (7 days)

1 Introduction  Naxos is the largest island of the Attic Cycladic Massif located in the south Aegean Sea, which is part of the East Mediterranean Sea. Geologically it can be divided into four units (Fig. 1): a migmatite dome; a metamorphic core complex of marbles, pelitic rocks, metabauxites and metavolcanics; a granodiorite intruded in the metamorphic core complex and non-metamorphic allochthonous sediments, also called non-metamorphic nappe (Urai et al., 1990). The neotectonic evolution of both, Naxos and the Aegean Sea has been investigated intensively. The neotectonic era began to evolve after the last profound tectonic change. The

early Alpine compressional phase 50 Ma ago was followed by a phase of intensive N-S Miocene crustal extensional tectonics (Urai et al., 1990). This extensional tectonics is responsible for the widespread “back-arc” extension in the Aegean Sea, “evolved in a geological setting behind the Hellenic convergent pate boundary” (Fig. 2, Hejl et al., 2003). This “back-arc” extension started in the Early-Middle Miocene and led to the updoming of various metamorphic core complexes.

All mentioned units above have been influenced by crustal-scale extension and experienced either ductile or brittle deformation. In present times, the tectonic setting of the Aegean Sea can be described as a land-locked basin, which moves along the Hellenic subduction zone (Urai et al., 1990).

Fig.1: Simplified geological map of Naxos showing lithologies and the detachment fault (modified after Cao et al, 2013)

Field  Course  “Naxos”   Naxos  –  Neotectonics  and  the  non-­‐metamorphic  nappe  

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The “non-metamorphic nappe” (Fig. 1), which only consists of non-metamorphic sedimentary deposits, was emplaced circa 9,5 Ma ago (Urai et al., 1990). This was also a result of the widespread extension in the Aegean Sea.

2 Neotectonic evolution After an intense phase of crustal thickening, which led to the formation of the Hellenides 45 Ma ago (Jolivet & Patriat, 1999), extension resulted in the formation of ductile shear zones, low angle normal faults and not only exhumation of metamorphic core complexes but also in the intrusion of the granodiorite (Hejl. et al., 2003). The main force behind this N-S extensional phase is subduction retreat during the subduction of the African Plate beneath the Apulian-Anatolian microplate in the Hellenic subduction zone (Fig. 2, I.S. Buick, 1991). The recent movement rates (fig.2) of the Hellenic trench are 3.3cmy-1 southwards and 2.1cmy-1 for the Anatolian westward movement (Phillippon et al., 2014). The roll back of the Aegean slab resulted in crustal thinning in the back arc region (13 +- 5 Ma), which led to the rapid uplift of lower crustal rocks. In this context, these lower crustal crystalline rocks were brought into contact with upper crustal units along a major shallow dipping shear zone between 12-13 and 3 Ma ago (Urai et al. 1990, I.S. Buick, 1991). The

Fig.  2:  Overview  over  the  geological  setting  of  the  neotectonic  on  Naxos  showing  the  N-­‐S  stretching  lineations  in  the  metamorphic  core  complex  indication  the  extension  direction,  (modified  after  Jolivet  et  al.  2014)  

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sense of shear is upper plate moving north (Urai et al., 1990). Lister et al. (1984) first proposed upper plate moving south, but this was proved false by various scientists. Lineation in the metamorphic core complex also confirms the top-to-the north sense of shear (Fig 2). The crystalline rocks of the lower unit show rapid cooling ages with cooling rated ranging between 50 and 130°C/Ma caused by tectonic exhumation in the course of the crustal extension (Hejl et al., 2003). These crystalline rocks experienced ductile deformation during the phase of extension. The metamorphic event M2b, which occurred synchronously to the back-arc extension (16Ma), was responsible for the overprint of the compressional metamorphic M1 event in the rocks of the metamorphic core complex (Urai et al.,1990). In the upper unit brittle crustal extension resulted in the development of tilted horsts and half-grabens in which Miocene non-metamorphic sediments were deposited (Fig. 3). The contact between the lower and this upper unit is a major north dipping detachment fault, with a rather low dip angle of 35° (Gautier & Brun, 1993 & Hejl et a., 2003). The strong N-S crustal-scale extension is proven by the occurrence of low-angle normal faults in the Miocene sediments close to the detachment fault (Fig. 3). These hanging-wall normal faults cut the sedimentary deposits in various directions but generally dip to the North or West, subparallel to the underlying detachment fault (John & Howard, 1995). The shear zone was active between 20 and 9 Ma and is correlated with the onset of the extension (Urai et al., 1990). The last movements documented on Naxos are the deformations

of the boundaries of the granodiorite at

brittle-ductile transitions and the

associated emplacement of the

non-metamorphic nappe 9.5 Ma ago (Urai et al., 1990).

Fig. 3: (A) Example showing brittle deformation in the sediment deposits close to the detachment fault, equal-area lower hemisphere projections show poles to faults in the sedimentary unit (modified after John and Howard, 1995) (B) schematic cross-section of Naxos-Paros showing detachment fault between brittle and ductile deformation and low-angel normal faults in sediments (modified after Gautier and Brun 1994a,b)  

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3 The non-metamorphic nappe The non-metamorphic nappe, also called “upper unit” in some literature is located in the northwest of Naxos, situated between and in contact with the granodiorite in the West and micaschist and marbles (metamorphic core complex) in the East (Figs. 1 & 4). It consists of ophiolites and predominately Miocene non-metamorphic sediments (G. Roesler, 1978). These sediments were deposited on an ophiolite nappe syn-extensional in half grabens, which evolved during crustal stretching and opened in between normal faults. The contact between the “upper unit” and the lower unit is a normal sense, low-angle detachment fault dipping to the north.

The brittle deformation in the upper unit can be interpreted as a result from the above mentioned crustal-scale extensional phase. The ophiolite nappe’s juxtaposition occurred as an extensional allochthon during the widespread extensional phase close to the updoming metamorphic core complex (Kuhlemann et al., 2004). In general only small remnant of this sedimentary succession are preserved. These unmetamorphosed sediments were deposited onto the deformed crystalline rocks along the low angle, normal sense detachment fault. Figure 4 also shows the sediment cover in the East of Naxos deposited above the Moutsounas shear zone. These sediments have Miocene and Pliocene ages but do not belong to the non-metamorphic nappe and are considered separately.

3.1 Composition of the non-metamorphic nappe The non-metamorphic nappe consists of ophiolites and predominately Miocene non-metamorphic sediments. These sediments can have marine, fluvial or continental origin. The ophiolite succession is strongly disturbed, so only limited remnants can be found (Fig. 1). Serpentinite, Basalt and Radiolarite are part of this ophiolite succession (Kuhlemann et al. 2004). The marine sequence represents shales, marls, sandstones and conglomerates and has a maximum thickness of 200m (Hejl et al, 2003). These marine sediments are associated with ophiolites and are locally in contact with the crystalline basement (G. Roesler, 1978). The

Fig. 4: E-W profile from Naxos from Fig. 1 (A-A’) showing the location of the non-metamorphic nappe in contact with the granodiorite in the West and the metamorphic core complex in the East (modified after Cao et al. 2013)

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conglomerates contain pebbles of ophiolites and nonmetamorphed sediments, but also pebbles of the crystalline basement can be found (G. Roesler, 1978). Kuhlemann et al. (2004) intensively investigated the Miocene siliciclastic sediments deposited on the ophiolite nappe. These sediments have been derived from an uplifting mountainous region in the South (Kuhlemann et al. 2004).

3.2 Neotectonics in the non-metamorphic nappe The non-metamorphic nappe and its Miocene syn-extensional sedimentary cover tectonically overlies the detachment fault, which marks the contact between the metamorphic core complex and the granodiorite in the West (Kuhlemann et al., 2004). The contact to the West is characterized by mylonitization of the granodiorite (Fig. 5). A wide ultracataclastic zone marks the contact to the metamorphic core complex in the East. These evidences for deformation along the tectonic contact is a proof for the final juxtaposition of the ophiolite nappe under brittle conditions up to temperatures below 300°C (Kuhlemann et al., 2004). Normal faults in conglomerates close to the detachment fault are evidence for a syn-extensional environment because these conglomerates experienced still brittle deformation. Respective to the deposition age, the sediments must have been deposited before the exhumation of the metamorphic core complex because of the absence of metamorphic compounds from the metamorphic complex in the Miocene sediments (Kuhlemann et al., 2004). The emplacement of this non-metamorphic nappe is suggested to happen 9.5 Ma ago (Urai et al. 1990 & I.S. Buick, 1991). Figure 5 shows the evolution of the juxtaposition of the non-metamorphic nappe (here called ophiolite nappe) with its sedimentary cover during Middle Miocene times. Originally this ophiolite nappe was situated farther south from today’s location but moved northwards along the low-angle normal fault with time. The final juxtaposition of this nappe took place about 10-9 Ma ago (I. S. Buick, 1991) and was placed in contact with the granodiorite and the metamorphic core complex. This figure shows also the simultaneous appearance of many different processes in Miocene times, like the uplifting of the metamorphic core complex, the intrusion of the granodiorite and the juxtaposition of the non-metamorphic nappe.

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4 Summary After a phase of compressional tectonics in the Aegean Sea, the neotectonic evolution in the Aegean Sea and on Naxos was affected by an era of intensive N-S crustal-scale Miocene extension with crustal thinning, back-arc basin formation and rapid uplift of lower crustal rocks (Urai et al., 1990). This all happened in a geological setting north of the Hellenic subduction zone, associated with subduction roll-back. The first evidence for the extension occurred 25 Ma ago (Gautier & Brun, 1993), the back-arc extension began 13 Ma ago (I. S. Buick, 1991). The sense of shear of the Cycladic shear zone was upper plate moving north

Fig. 5: schematic profile of the evolution of crustal deformation since 16 Ma (Middle Miocene) and possible depositional location of the Miocene sediments (modified after Kuhlemann et al 2004)  

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and resulted in formation of normal faults in the sediment deposits of the non-metamorphic nappe. Last evidence for movements were recognized at the boundary of the granodiorite at the brittle-ductile transition and were connected with the emplacement of the non-metamorphic nappe 9.5 Ma ago (Urai et al. 1990).

5 References [1] BUICK, I. S. (1991). The late Alpine evolution of an extensional shear zone, Naxos, Greece, Journal of Geological Society London, vol. 148, pages 93-103 [2] CAO, S., NEUBAUER, F., BERNROIDER, M., LIU, J. (2013). The lateral boundary of a metamorphic core complex: The Moutsounas shear zone on Naxos, Cyclades, Greece, Journal of Structural Geology, vol. 54, pages 103–128 [3] GAUTIER, P., BRUN, J.-P. (1993). Structure and kinematics of upper extensional detachment on Naxos and Padros (Cyclades Islands, Greece), Tectonics, vol. 12, no. 5, pages 1180-1194, October 1993 [4] HEJL, E., RIEDL, H., SOULAKELLIS, N., VAN DEN HAUTE, P., WEINGARTNER, H. (2003). Young Neogene tectonics and relief development on the Aegean island of Naxos, Paros and Ios (Cyclades, Greece), Mitteilungen der Österreichischen Geographischen Gesellschaft, pages 105-127 [5] JOHN, B. E., HOWARD, K. A. (1995). Rapid extension recorded by cooling-age patterns and brittle deformation, Naxos, Greece, Journal of Geophysical Research, vol.100, pages 9969-9979 [6] JOLIVET, L., FACCENNA, C., HUET, B., LABROUSSE, L., LE POURHIET, L., LACOMBE, O., LECOMTE, E., BUROV, E., DENÈLE, Y., BRUN, J.-P., PHILIPPON, M., PAUL, A., SALAÜN, G., KARABULUT, H., PIROMALLO, C., MONIÉ, P., GUEYDAN, F., ARAL I. OKAY, OBERHÄNSL, R., POURTEAU, A., AUGIER, R., GADENNE, L., DRIUSSI, O. (2013). Aegean Tectonics: Strain localisation, slab tearing and trench retreat, Tectonophysics, vol. 597-598, pages 1-33 [7] JOLIVET, L., PATRIAT, M. (1999). Ductile extension and the formation oft the Aegean Sea, Geological Society London, Special Publication, vol.156, pages 427-456 [8] KUHLEMANN, J., FRISCH, W., DUNKL, I., KAZMER, M., SCHMIEDL, G. (2004). Miocene siliciclastic deposits of Naxos Island: geodynamic and environmental implication for the evolution of the southern Aegean Sea (Greece), BERNET, M. & SPIEGEL, C. (eds) Detrital Thermochronology. Provenance Analysis, Exhumation and Landscape Evolution of Mountain Belts, Geological Society of America Special Papers, 378, pages 51–65.

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[9] PHILIPPON, M., BRUN, J.-P., GUEYDAN, F., SOKOUTIS, D. (2014). The interaction between Aegean back-arc extension and Anatolia escape since Middle Miocene, Tectonophysics, available online 2014 [10] ROESLER G. (1978). Relics of non-metamorphic sediments on Central Aegean islands. Alps, Apennines, Hellenides. (eds Closs, H., Roeder, D., Schmidt, K.), IUCD. Pages 480-481 [11] URAI, J. L., SCHUILING, R. D., JANSEN, J. B. (1990). Alpine deformation on Naxos (Greece), Geological Society London, Special Publication, vol. 54, pages 509-522