Detachment Faulting and the Metamorphic Core Complex on Naxos, Greece

EWGENIJ KOSSI ● NAXOS: DETACHMENT FAULTING AND METAMORPHIC CORE COMPLEX ● NAXOS FIELD COURSE 2014

DETACHMENT FAULTING AND THE METAMORPHIC CORE COMPLEX ON NAXOS, GREECE

EWGENIJ KOSSI RWTH Aachen University Field Course: Naxos 2014 - Group A

Abstract

Naxos is part of the exhumed metamorphic belt in the Aegean Sea and features a distinguished geology, comprising structural records of at least Eocene-present tectonic events on a broad variety of scales. Meso-scaled structures as the Naxos-Paros extensional fault systems and the metamorphic core complex on Naxos reveal a complicated history of extensional tectonics in the back-arc basin that has led to the exhumation of high-grade metamorphic rocks, juxtaposing them against significantly younger material of Miocene age. The exact driving mechanisms for the MCC uplift are still debated, but recent research indicates that the MMC’s uplift was caused by a combination of buoyancy- and isostasy-driven flow during phases of Barrovian metamorphism in an extensional regime, with upper-plate-to-the-north sense of shear.

1. Introduction northern border of the Africa-Eurasia subduction/collision zone. Due to the proximity to this Naxos, an island located on the Aegean Sea zone the Aegean Sea Plate has undergone Plate, a Eurasian microplate representing the

Figure 1. (A) Overview map of the geological setting of the Cyclades. (B) Geological map of Naxos showing the distribution of the Upper unit (Hanging Wall Rocks) well as the Cycladic blueshist unit (Foot Wall Rocks). (C) N-S and W-E oriented cross- sections of the MCC (KRUCKENBERG et al. 2011).

EWGENIJ KOSSI ● NAXOS: DETACHMENT FAULTING AND METAMORPHIC CORE COMPLEX ● NAXOS FIELD COURSE 2014 phases of compactional as well as extensional mon tectonic unit in the study region is the Cy- tectonics. The extensional phases have steadily cladic blueshist unit of the lower plate (LISTER et stretched open the central Aegean Sea, opening al. 1984). Three nappe units are comprised in it: a backarc basin and creating numerous detach- (1) a mélange-like unit of ophiolitic rocks underlain ment fault zones that are not entirely understood by (2) a post-Carboniferous shelf sequence, and yet. Another feature enabled by extension is the (3) a Carboniferous basement unit (BRICHAU et al. exhumed metamorphic belt comprising the central 2006). Juxtaposed against the blueshist unit, Aegean islands, including Naxos. As far as current mostly non-metamorphic sediments of Miocene theory goes, movement of deeper crustal rocks age (deposition occurred before the MCC uplift) along these detachment faults enabled the uplift comprise the Upper plate’s unit (BRICHAU et al. of high grade metamorphic rocks that formed dur- 2006; CAO et al. 2013; LISTER et al. 1984). North ing Cretaceous-Eocene time, known as metamor- and west of Naxos, the brittle part of the Naxos- phic core complexes (MCC), to the surface Paros extensional fault system (NPEFS) marks (KRUCKENBERG et al. 2011; URAI et al. 1990). De- the boundary between the Cycladic blueshist unit spite the fact that some details might have been and Upper non-metamorphic unit, e.g. lower and misinterpreted initially, LISTER et al. (1984) pre- upper plates (BRICHAU et al. 2006). It is assumed sented a simple but brilliant model that explained that this detachment fault system’s motion of the the relationship of extensional tectonics, detach- upper plate moving to the north enabled the uplift ment faults and metamorphic core complexes in of the Naxos MCC to the surface (GAUTIER et al. the Aegean region. 1993; KRUCKENBERG et al. 2011; LISTER et al. 1984; URAI et al. 1990). 2. Geological Evolution and Geology of the Naxos MCC The Cycladic blueshist unit has witnessed different metamorphic events initiating with a high- Naxos is located in the Cycladic-Attic massif pressure compaction event in the Eocene at ca. that is comprised of subsequently formed meta- 50 - 55 Ma (M1), followed by extensional green- morphic belts over a volcanic arc. The most com- shist to amphibolite overprinting beginning at

~20 - 25 Ma (M2a) (BRICHAU et al. 2006; URAI et al.

Figure 2. Exhumation history of the metamorphic basement found on Naxos based on metamorphic and isotopic measure- ments (LISTER et al. 1984).

EWGENIJ KOSSI ● NAXOS: DETACHMENT FAULTING AND METAMORPHIC CORE COMPLEX ● NAXOS FIELD COURSE 2014

1990) and a more localized high-T-low-P meta- border of the Cycladic-Attic massif (BRICHAU et al. morphism of Barrovian character, culminating at 2006).

~16 Ma (M2b) (URAI et al. 1990). Thermal domes Regarding the islands geology, Naxos is one of in the region are considered to have formed during two island that mostly consist of the blueshist unit, M2b deformation (URAI et al. 1990) and most struc- which is also overprinted by amphibolite-meta- tures nowadays active in the Aegean region are morphism conditions that reached anatetic condi- considered remnants of the M2 deformation tions of 670 ± 50°C and 5 - 7 kbar in the Miocene events (KOUKOUVELAS & KOKKALAS 2003). (BRICHAU et al. 2006). Being onion shaped, the In the Miocene, the Cyclades became part of rock units and isograds on Naxos show typical

Figure 3. Sketch of LISTER et al. (1984) initially proposed model. Note that the South Cyclades shear zone is meanwhile believed to be dipping northwards, opposed to the southwards dipping depiction. the magmatic arc of the southward retreating sub- characteristics of an elongated mantled gneiss duction zone, accordingly magmatic rocks in the dome structure with increasing metamorphic range between 5 Ma and 12 Ma and intrusive grade towards the center (see Figure 1) (BRICHAU granites dating 10 Ma to 15 Ma can be found et al. 2006; CAO et al. 2013; URAI et al. 1990). (BRICHAU et al. 2006). Granodiorites intruding the Around the dome, sequences of foliated marbles footwall of the NPEFS west of Naxos date at and shists are present in abundance while some ~12 Ma and are considered synkinematic, evident synkinematic granites can be found north of the by the presence of pseudotachylites (BRICHAU et dome structure and ophiolites W-SW (CAO et al. al. 2006). Today the retreating volcanic arc can be 2013). At its core the structural dome consists of found further southwards and marks the southern migmatites which can be subdivided into solid dominated metatexites, characterized by

EWGENIJ KOSSI ● NAXOS: DETACHMENT FAULTING AND METAMORPHIC CORE COMPLEX ● NAXOS FIELD COURSE 2014 gneisses and shists with continuous foliation and magma dominated diatexites which are mainly comprised of granite (KRUCKENBERG et al. 2011). To the west, where the aforementioned granodio- rite intrudes the dome, a zone of contact metamorphism rocks accordingly can be found (GAUTIER et al. 1993). Consistent with the location at a shear zone, mylonites as well as pseudotachylites are locally present, as well as a variety of deformation structures of different scales and orientations, generally supporting the “upper plate moving north” hypothesis (URAI et al. 1990). A brief sum- mary of exhumation history is provided in Figure 2.

3. Detachment Faults in Extensional Backarc Basins Initially it must be pointed out that the term “detachment” poses a difficulty due to its ambiguous usage. In this paper detachment fault refers to the post-orogenic extension system in the Aegean backarc region, as proposed by (JOLIVET et al. 2009).

Starting in the Late Oligocene, the extension of the Aegean region was mainly due to slab retreat at the African-Eurasian subduction zone (JOLIVET et al. 2009). This timeframe corresponds to the

Barrovian type overprint (M2b) of M1 and M2 structures (BRICHAU et al. 2006; LISTER et al. 1984; URAI et al. 1990). It is proposed by LISTER et al. (1984) that a (at least) 1 - 2 km thick shear zone in the crust must exist, with the “higher levels mov- Figure 4. Largescale structure inside the Naxos MCC dome ing south” (LISTER et al. 1984). URAI et al. (1991) (KRUCKENBERG et al. 2011). instead propose that “the upper plate is moving GAUTIER et al. 1993). Comprised in this detach- north”, while accepting LISTER et al.’s (1984) ment are traces of the brittle-ductile transition model otherwise. Meanwhile, URAI et al.’s (1991) phase, evident by the presence of ductile fault- proposition has been independently proven for rocks, e.g. ultramylonites, which have been sub- other localities of the Aegean region (JOLIVET et sequently deformed in the brittle domain during al. 2009). uplift (CAO et al. 2013). LISTER et al.’s (1984) initial model is depicted in . In the upper crustal levels, the crustal shear- Figure 3 zone translates into a brittle normal fault, dipping Earlier research has proven that detachment north at a shallow angle of ~35° (CAO et al. 2013; faults can have slip rates in the range of ~1 Km Myr-1 to >20 Km Myr-1, movement rates present in 4

EWGENIJ KOSSI ● NAXOS: DETACHMENT FAULTING AND METAMORPHIC CORE COMPLEX ● NAXOS FIELD COURSE 2014 the Aegean region average at 15 Km Myr-1 with lateral flow is affected by thinning of the upper single fault systems reaching ~5 Km Myr-1 crust (REY et al. 2011). In the deep crust, the flow (BRICHAU et al. 2006). For the NPEFS slip rates of changes from verging lateral flow to a convergent ~6-8 Km Myr-1 have been estimated for the upward flow. This motion coupled with changes in timeframe of the brittle-ductile transition buoyancy of ductile material also causes contrac- (8 – 16 Ma) while a total offset of at least 50 Km tions in the crust (KRUCKENBERG et al. 2011). Ac- has been achieved (BRICHAU et al. 2006). cording to KRUCKENBERG as well as REY et al. (2011), the buoyancy forces can either manifest 4. Formation of the Cordilleran Type Met- as diapirism, density-driven convection, or as as- amorphic Core Complex on Naxos sumed for the Naxos MCC, a combination of both As previously mentioned, Naxos features an mechanisms. exhumed MCC with migmatite at its core (BRICHAU 5. Conclusion et al. 2006; KRUCKENBERG et al. 2011; URAI et al. 1990). The MCC’s migmatic core is dominated by Naxos and its surrounding area feature a com- diatexites while metatexites are scarcely found plex geology that opens a window to remnants of (KRUCKENBERG et al. 2011). Structurally, the distinguishable ancient tectonic events and their dome can be further subdivided into three sub- resulting present-day active structures. The most domes and a pinched synform, divided by a high- prominent feature of the study region is a Cordil- strain zone oriented in N - S direction leran type MCC, uplifted due to buoyancy- and (KRUCKENBERG et al. 2011; REY et al. 2011). isostasy driven flow along a shallow-dipping de- These structures are shown in Figure 4. Models tachment fault system with upper-plate-to-the- preferred in the recent past suggest that the north sense of motion. Featuring evidence for the Naxos MCC is a double-dome structure originat- brittle-ductile transition zone, this fault zone bears ing from a combination of buoyancy- as well as great importance for the general understanding of isostasy-driven flow processes (KRUCKENBERG et extensional tectonics in back-arc regions. al. 2011; REY et al. 2011). Despite that there are On Naxos, the dome’s shape manifests as general difficulties in distinguishing metamorphic three subdomes divided by a N-S oriented high- domes with different origins (e.g. fold-dominated, strain zone surrounded by a sequence of alternat- detachment-dominated or gravity dominated ing marbles and shists, giving room for debate domes) (REY et al. 2011), KRUCKENBERG et al. about its origin. At the NPEFS, a detachment fault (2011) make a good point for ruling out the possi- system found north and west of Naxos and initi- bility of the Naxos MCC being a fold-dominated ated by extensional tectonics, non-metamorphic dome. KRUCKENBERG et al. (2011) suggest that units of Miocene age are juxtaposed against the the Naxos dome structure might have been gen- high-grade metamorphic Cycladic blueshist unit. erated by E - W contraction coupled with conver- The brittle part of this detachment fault is assumed gent flow and upwelling of migmatite, triggered by to descend into a ductile shear zone of crustal regional “upper plate moving north” (URAI et al. scale, dipping northwards at a shallow angle of 1990) detachment activity. Additionally, REY et al. 35°. (2011) propose that viscous collision in the flow channel can explain the double-dome structure of the Naxos MCC while KRUCKENBERG et al. (2011) explains the presence of multiple domes with the presence of melts during shearing. It is concluded that the pressure gradient responsible for deep 5

EWGENIJ KOSSI ● NAXOS: DETACHMENT FAULTING AND METAMORPHIC CORE COMPLEX ● NAXOS FIELD COURSE 2014

References

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. In: Earth and Planetary Science Letters, 241, 293-306.

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. In: Journal of Structural Geology, 54, 103-128.

GAUTIER, P., BRUN, J.-P. & JOLIVET, L. (1993): Structure and kinematics of upper cenozoic extensional detachment on naxos and paros (Cyclades Islands, Greece). In: Tectonics, 12, 1180-1194.

JOLIVET, L., FACCENNA, C. & PIROMALLO, C. (2009): From mantle to crust: Stretching the Mediterranean. In: Earth and Planetary Science Letters, 285, 198-209.

KOUKOUVELAS, I. K. & KOKKALAS, S. (2003): Emplacement of the Miocene west Naxos pluton (Aegean Sea, Greece): a structural study. In: Geological Magazine, 140, 45-61.

KRUCKENBERG, S. C., VANDERHAEGHE, O., FERRÉ, E. C., TEYSSIER, C. & WHITNEY, D. L. (2011): Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece. In: Tectonics, 30, n/a-n/a.

LISTER, G. S., BANGA, G. & FEENSTRA, A. (1984): Metamorphic core complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece. In: Geology, 12, 221-225.

REY, P. F., TEYSSIER, C., KRUCKENBERG, S. C. & WHITNEY, D. L. (2011): Viscous collision in channel explains double domes in metamorphic core complexes. In: Geology, 39, 387-390.

URAI, J. L., SCHUILING, R. D. & JANSEN, J. B. H. (1990): Alpine deformation on Naxos (Greece). In: Geological Society, London, Special Publications, 54, 509-522.