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COMPOSITION, STRUCTURE AND EVOLUTION
OF THE METAMORPHIC CORE COMPLEX
AND ITS DETACHMENT FAULT OF NAXOS, GREECE
Bernhard Schuck
RWTH Aachen University
Field Course Naxos, Summer Term 2014 - Group B
Draft version August 31, 2014
ABSTRACT
Naxos is the largest island of the Cycladic archipelago. It formed in the back - arc of the Hellenic
subduction system due to large - scale regional extensional tectonics as a result of the retreat of the
African slab, which reworked a nappe pile originating from the earlier Alpine orogeny. A metamorphic
core complex, covering mainly the complete island, consists of a large elongated structural dome, with
a core of migmatitic gneiss, surrounded by metasediments. Non - metamorphic rocks can only be
found in a few spots. A large, shallow - dipping normal detachment fault separates the metamorphic
units from the non - metamorphic ones. The extension - related unroofing of the nappe stack caused
the exhumation of the deep - seated metamorphic units by isostatic rebound.
1. INTRODUCTION
The N - S extension of the Aegean Sea accounts for the
formation of several metamorphic core complexes in the
Cycladic archipelago, which is in back - arc position of the
Hellenic trench. This back - arc extension is associated
with the roll - back of the Hellenic subduction system to
the SW (Cao et al. 2013, Kruckenberg et al. 2011).
The Cycladic archipelago consists of about 30 islands,
with Naxos being the largest one (fig. 1). It is part of
the Attic-Cycladic Massif, which has been a↵ected by at
least two alpine regional tectono - metamorphic events.
On Naxos this led to the formation of a metamorphic
core complex, which is expressed as an elongated structural dome covering the whole island. The metamorphic
rocks record a phase of subduction, which has been associated with orogenic crustal thickening as result of the
Africa - Eurasia convergence and a succeeding phase of
extensional collapse owed to the retreat of the African
slab (Urai et al. 1990, Kruckenberg et al. 2011).
This paper covers the structure and the evolution of
the metamorphic core complex of Naxos. Further it evaluates the relationship between the core complex and the
associated detachment fault during upflit of the deep
crustal metamorphic rocks, which are exhumed on the
island.
2. METAMORPHIC CORE COMPLEXES
The term metamorphic core complex (MCC) refers
to metamorphic cores, gneissic and often plutonic rocks
originating in the deep crust, which have been brought
up to the surface by large extensional shear zones and
low - angle detachment faults (Jolivet et al. 2009, Cao
et al. 2013). The metamorphic rocks often show indicators of strong ductile deformation, developed under high
temperature conditions (Gautier et al. 1993). They
are separated from overlying non - metamorphic rocks by
shear zones usually located within the brittle - ductile
transition zone, topped by a shallow dipping large normal
fault. Usually lineations attest for strong deformation
and the sense of shear (Jolivet et al. 1994, Jolivet
et al.2009). Examples for MCCs can be found in the
Basin and Range province, U.S., in Scandinavia and the
Aegean Sea (Jolivet et al. 2009).
3. THE METAMORPHIC CORE COMPLEX OF NAXOS
3.1. Composition of the MCC
METAMORPHIC UNITS
In Naxos, the basement of the metamorphic units of
the MCC comprises a core of a pre - Alpine gneiss dome
of probably Variscan age (approx. 370 Ma), which underwent subsequent migmatization resulting in a wide
range of melt fractions (Gautier et al. 1993, Cao et
al. 2013, Kruckenberg et al. 2011). This foliated
migmatite complex is mantled by a complex consisting of alternating metasediments, coarse - grained marbles with Triassic algae, especially in the eastern part,
and fine - grained siliciclastics, (amphibolitic) schists and
metabauxites as well as discontinuous layers of ultrabasic, tu↵aceous rocks (figs. 1 and 3). Additionally,
metamorphic isogrades form a nearly concentric zonation
around the core, showing an evolution from high (Silimanite, close to the migmatite core) to low (Corundum
in the SE of the island) metamorphic grades (Gautier
et al. 1993, Cao et al. 2013, Urai et al. 1990, Schenk
et al. 2005, Kruckenberg et al. 2011).
SL - fabrics are formed by the migmatitic foliation and
a dominating, NNE trending lineation, which is associated with elongated minerals in schists and micaschists.
Dip angles of the NNE or SSW plunging lineation are
shallow (< 30 ) but can be steeper (up to 45 ) in the
vicinity of the migmatite core (Cao et al. 2013, Urai et
al. 1990, Schenk et al. 2005).
Within the MCC there are three generations of folds,
a↵ecting each other, resulting in structures, which are
folded along an E - W axis. Both, foliation and lineation,
form an elongated structural dome, which warps slightly
over the migmatite complex (Urai et al. 1990).
NON - METAMORPHIC UNITS
The overlying non - metamorphic units, which are in
tectonic contact with the migmatite complex and the
2
Figure 1. (A) Overview map of the geological setting. The Attic - Cycladic Massif (ACM) is in back - arc position of the Hellenic Trench.
(B) Geological map of Naxos. The metamorphic core complex of Naxos consists of a migmatite complex, which is mantled by metasediments.
Metamorphic isogrades show an evolution from high to low metamorphic grades. Red line: see fig. 3 for more details. (C) Cross sections
from W to E (A - A’) and S to N (B - B’). Note the high strain zone in the centre of the section from W to E (figure slightly modified
according to Kruckenberg et al. 2011).
metasediments, are neritic limestones from the late
Lower Cretaceous and strongly disrupted ophiolites, consisting of serpentinite and basalt with radiularite. Additionally, there are clastic sediments from the Miocene
and Pliocene. The Miocene units are considered to
have been deposited before uplift and exhumation of
the MCC. These hanging - wall units are exposed only
along the eastern and north - western margin of the MCC
(Gautier et al. 1993, Cao et al. 2013, Urai et al.
1990).
In the western part of Naxos there is an I - type granodiorite batholith, which intruded the MCC in the
Miocene and has intrusive contacts with the marble and
schist sequences (Cao et al. 2013, Urai et al. 1990).
3.2. The Detachment Fault
In MCCs a large, low - angle normal detachment separates the metamorphic units from the non - metamorphic
ones. The major normal - sense ductile detachment fault
of Naxos, which is located along the northern and western margin of the island (fig. 2), dips to the north with
a relatively shallow angle of about 35 (Gautier et al.
1993, Cao et al. 2013).
Veins associated with the granodiorite show normal
o↵sets (Gautier et al. 1993). Progressive development of di↵erent metamorphic stages and deformation
during exhumation produces sequences of meso - and microstructures within the detachment, attesting the transition from ductile to brittle conditions (Cao et al. 2013,
Urai, et al. 1990). Ultramylonites as well as pseudo-
Figure 2. Simplified geological map of Naxos. The metamorphic
units of the metamorphic core complex are dominating the island
and are present in the central an eastern part. In the west there
is an I - type granodiorite, which intruded in the Miocene. The
non - metamorphic units, mainly neritic limestones and ophiolites,
are located along the north - western and the eastern margin of the
island. Metamorphic and non - metamorphic units are separated by
a large, shallow - dipping, normal - sense ductile detachment fault,
which is exposed along the northern and western margin as well
as a small spot on the eastern side of the island (figure modified
according to Jolivet et al. 2009).
tachylites, which formed about 10 Ma ago, and cataclasites can all be found in the vicinity of the northern and western parts of the detachment fault. Pseudotachylites and Cataclasites are the result of brittle shear
movement, which overprinted ultramylonites, structures
formed within the ductile shear zone (Cao et al. 2013).
3
Additionally, field observations of cataclasites and pseudotachylites indicate transport of the upper plate to N
(Urai et al. 1990). These observations are evidence
for ongoing tectonic activity under conditions of declining temperature until nearly surficial conditions were
reached (Cao et al. 2013, Urai et al. 1990). Deformation of the margin of the granodiorite at brittle - ductile
transition conditions and the associated emplacement of
the non - metamorphic nappe are the latest indicators for
active shear deformation (Urai et al. 1990). Consequently, the regional - scale shear zone has had to be active at least from the time of migmatization up to at
least until the emplacement of the pluton (Gautier et
al. 1993, Jolivet et al. 2009).
Brichau et al. 2006 concluded that the Naxos detachment created an o↵set of about 50 km between 16 Ma and
8 Ma.
3.3. Structure of the MCC
Migmatite cores (partially molten and magmatic crust)
are common features of MCCs. The structure of the elongated dome with migmatitic foliation of Naxos reveals
complex flow patterns of an anatectic crust beneath an
extensional detachment system. Whereas the migmatitic
foliation at the contact between the migmatite dome and
the surrounding metasediments is concordant, it becomes
highly variable in the migmatite core, showing concentric
patterns. These patterns reveal that the migmatite dome
is set up by three second - order domes (fig. 4). These
subdomes (a southern one, a central one and a northern
one) are arranged in elongate, en echelon compartments,
which are separated by a high strain zone and pinched
synforms (Kruckenberg et al. 2011).
Figure 4.
The pattern of the migmatitic foliation of the
migmatite core can be used to interpret the set up of the dome. The
foliation reveals complex flow patterns of anatectic crust, which resulted in the formation of three subdomes. The southern subdome
and the central one are separated by a high strain zone, the central subdome and the northern one by pinched synforms (modified
according to Kruckenberg et al. 2011).
Figure 3. Detailed map of the migmatite core of the metamorphic core complex of Naxos. The migmatite is mantled by
coarse - grained marbles with Triassic algae, amphibolitic schists
and high grade schists as well as discontinuous layers of ultrabasics, tu↵aceaous rocks. In addition, the three subdomes can be
recognized, however, not very pronounced (after Kruckenberg et
al. 2011).
The subdomes are characterized by a steeply dipping
migmatitic foliation. Along the northern edge of the central subdome it is overturned and southward dipping.
The boundary between the central and the northern subdome is defined by pinched synforms, whose axial traces
dip to the S. The northern subdome is characterized
by mesoscale folds with WNW - ESE striking fold axis.
The elongated high strain zone, which extends for more
than one kilometer, is N - S oriented and made of intensely deformed, steeply dipping, leucogranitic gneisses
and migmatites, a↵ected by solid - state deformation. It
separates the southern subdome from the central one
(Kruckenberg et al. 2011).
These (sub)domal structures are the result of melt present deformation. Additionally, pinched synforms as
well as curved lineation trajectories point to partition-
4
ing of buoyancy driven flow during dome evolution. The
pattern of the lineation suggests bulk flow within the
migmatites to have been perpendicular to the long axis
of the dome, which means normal to the direction of
the regional extension and the top - to - the -North shear
related to the overlying detachment. Consequently all
these structures have had to be a↵ected by shearing
during dome development and exhumation of the MCC
(Kruckenberg et al. 2011).
By looking at the geometric features of the dome,
Kruckenberg et al. 2011 interpret its formation as
a result of buoyancy (isostasy) - driven flow of partially
molten crust. Reasons for the significant role of buoyancy - driven flow are the existence of subdomes and
pinched synforms as well as the highly variable fabric.
Kruckenberg et al. 2011 explain the formation of
subdomes by gravitational instabilities, which resulted
from diapirism and / or overturning of the crust, containing high melt fractions. Upper crustal extension
and coeval deep crust contraction triggered this converging and upwelling flow of anatectic crust, which
has been additionally a↵ected by the top - to - the -North
sense of shear associated with the detachment kinematics
(Kruckenberg et al. 2011).
4. EVOLUTION OF THE NAXOS MCC
The formation of the MCC of Naxos is the result of two
tectono - metamorphic events: the Alpine orogeny in the
Eocene, followed by post - orogenic back - arc extensional
tectonics originating in the retreat (roll - back) to the SW
of the Hellenic trench during the Early Miocene (fig. 5)
(Jolivet et al. 2009, Cao et al. 2013, Urai et al. 1990,
Schenk et al. 2005).
Eurasia convergence. It led to the closure of the Mesozoic
Pindos Ocean and large scale thrusting, creating compressional structures, especially a thickened crust, generated by the evolution of a nappe pile (Jolivet et al.
2009, Cao et al. 2013) This first event ended approximately 50 - 40 Ma ago (Schenk et al. 2005). The relicts
of the related Meso - Hellenic high pressure, low temperature (HPLT) metamorphic event (further refered to as
M1 ), are mainly blueschist facies minerals. They are preserved in the SE of Naxos (Kruckenberg et al. 2011,
Urai et al. 1990).
Surficial extension as response to the roll - back of the
Hellenic trench during the Early Miocene caused reworking of the nappe stack by extensional flat - laying detachments, which had a velocity of approximately 1.8 9.5 mm a 1 , and rapid exhumation of the metamorphic
crust as well as the emplacement of melts due to an elevated high heat flow (Jolivet et al. 1994, Gautier et
al. 1993, Cao et al. 2013 Urai et al. 1990). The related
processes of crustal thinning are complex and involve
boudinage and asymmetric shear of incompetent layers
(Jolivet et al. 1994). The second phase of metamorphism (M2 ), the Hellenic Barrovian - Type medium pressure, medium temperature (MPMT) metamorphism affected the area between 23 and 16 Ma (Cao et al. 2013).
It overprinted most of the relicts of M1 and was characterized by near isothermal decompression and rapid
exhumantion as well as evolution of greenschist to upper
amphibolitic facies along the detachment. Succeeding localized HTLP metamorphism related to the development
of thermal (sub)domes and migmatite formation by partial crustal anatexis overprinted M2 remnants at 20 - 15
Ma ago (Cao et al. 2013, Urai et al. 1990, Kruckenberg et al. 2011). Ongoing top - to - the - N shearing
along the detachment led to folding of the migmatic dome
(fig. 6) (Kruckenberg et al. 2011).
The main stage of updoming as isostatic respone to unroofing of the nappe stack with associated crustal thinning and exhumation of the metamorphic core is considered to have taken place between 14 Ma and 11 Ma,
simultaneously to the exhumation and intrusion of the
granitoid batholith in the centre of the dome (Cao et al.
2013)
5. CONCLUSION
Figure 5. Sketch illustrating the formation of metamorphic core
complexes in the Aegean Sea. The nappe pile resulting from the
Africa - Eurasia convergence is reworked by flat - lying detachments
as response to the retreat of the African slab. Black arrows: sense
of movement along faults. Colour scheme of the nappes: Asteroussia (light blue), Gavrovo (medium blue), Ida - Phyllites (dark
blue) (for more information on nappe tectonics in the Aegean Sea
see Jolivet et al. 1993 and references therein. Figure modified
according to Jolivet et al. 1993 and references therein).
The Alpine orogeny originated in the northward subduction of the African slab as a result of the Africa -
The metamorphic core complex of Naxos is an elongated, structural dome, which is made of three subdomes separated by a high - strain - zone and pinched synforms. The core complex consists of a migmatite core
mantled by metasediments (mainly marble and schists).
Non - metamorphic units, limestones, ophiolites and clastic sediments, have only been preserved in a few outcrops
in the east and north - west of the core complex. A large,
shallow - dipping normal detachment as a result of extensional tectonics is located along the northern and western
margin of Naxos and shows top - to - the - North sense of
shear. Meso - and microstructures (e.g. ultramylonites
and cataclasites) attest shear deformation being active
under ductile as well as brittle conditions.
Metamorphism on Naxos is related to two tectono metamorphic events during the Alpine orogeny, which
created a nappe pile, and the succeeding large - scale regional extension, reworking the compressional structures.
Extensional tectonics are related to the roll - back of the
5
Figure 6. Three dimensional model of the metamorphic core complex, with a special focus on the migmatitic core. The model shows
the structural set up of the migmatite complex and the interaction of tectonic and magmatic processes, which led to the formation of the
complex (Kruckenberg et al. 2011).
Hellenic subduction system. As isostatic response to the
extensional unroofing, anatectic deep - crustal material
has been brought up to the surface and exhumed, now
forming the main part of Naxos.
6. REFERENCES
[1] Brichau, S., Ring, U., Ketcham, R. A., Carter,
A., Stockli, D. & Brunel, M. (2006): Constraining the
long - term evolution of the slip rate for a major extensional fault system in the central Aegean, Greece, using
thermochronology. – Earth and Planetary Science Letters 241: 293 - 306; Amsterdam.
B. (1994): Exhumation of deep crustal metamorphic
rocks and crustal extension in arc and back - arc regions.
– Lithos 33: 3 - 30; Amsterdam.
[5] Jolivet, L., Faccenna, C. & Piromallo, C.
(2009): From mantle to crust: Stretching the Mediterranean. – Earth and Planetary Science Letters 285:
198 – 209; Amsterdam.
[6] Kruckenberg, S. C., Vanderhaeghe, O., Fere,
E. C., Teyssier, C. & Whitney, D. L. (2011): Flow
of partially molten crust and the internal dynamics of a
migmatite dome, Naxos, Greece. – Tectonics 30: 1 - 24;
Washington, DC.
[2] Cao, A., 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 54: 103 128; Amsterdam.
[7] Schenk, O., Urai, J. L. & Evans B. (2005): The
e↵ect of water on recrystallization behavior and grain
boundary morphology in calcite observations of natural
marble mylonites. – Journal of Structural Geology 27:
1856 - 1872; Amsterdam.
[3] Gautier, P., Brun J. - P. & Jolivet, L. (1993):
Structure and kinematics of Upper Cenozoic extensional detachment on Naxos and Paros (Cyclades islands,
Greece). – Tectonics 12: 1180 - 1194; Washington, DC.
[8] Urai, J.L., Schuiling, R.D. & Jansen, J. B. H.
(1990): Alpine deformation on Naxos (Greece). In:
Knipe, R. J., Rutter, E. H. (Eds.), Deformation Mechanisms, Rheology and Tectonics. – Geological Society
Special Publications 54: 509 - 522; London.
[4] Jolivet, L., Daniel, J. M., Truffert, C. & Goff,