DETACHMENT FAULTING AND THE METAMORPHIC CORE COMPLEX ON NAXOS, GREECE

Transcription

1 DETACHMENT FAULTING AND THE METAMORPHIC CORE COMPLEX ON NAXOS, GREECE EWGENIJ KOSSI RWTH Aachen University Field Course: Naxos 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 Naxos, an island located on the Aegean Sea Plate, a Eurasian microplate representing the northern border of the Africa-Eurasia subduction/collision zone. Due to the proximity to this zone the Aegean Sea Plate has undergone 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 crosssections of the MCC (KRUCKENBERG et al. 2011). 1

2 phases of compactional as well as extensional tectonics. The extensional phases have steadily stretched open the central Aegean Sea, opening a backarc basin and creating numerous detachment fault zones that are not entirely understood yet. Another feature enabled by extension is the exhumed metamorphic belt comprising the central Aegean islands, including Naxos. As far as current theory goes, movement of deeper crustal rocks along these detachment faults enabled the uplift of high grade metamorphic rocks that formed during Cretaceous-Eocene time, known as metamorphic core complexes (MCC), to the surface (KRUCKENBERG et al. 2011; URAI et al. 1990). Despite the fact that some details might have been misinterpreted initially, LISTER et al. (1984) presented a simple but brilliant model that explained the relationship of extensional tectonics, detachment faults and metamorphic core complexes in the Aegean region. 2. Geological Evolution and Geology of the Naxos MCC Naxos is located in the Cycladic-Attic massif that is comprised of subsequently formed metamorphic belts over a volcanic arc. The most common tectonic unit in the study region is the Cycladic blueshist unit of the lower plate (LISTER et al. 1984). Three nappe units are comprised in it: (1) a mélange-like unit of ophiolitic rocks underlain by (2) a post-carboniferous shelf sequence, and (3) a Carboniferous basement unit (BRICHAU et al. 2006). Juxtaposed against the blueshist unit, mostly non-metamorphic sediments of Miocene age (deposition occurred before the MCC uplift) comprise the Upper plate s unit (BRICHAU et al. 2006; CAO et al. 2013; LISTER et al. 1984). North and west of Naxos, the brittle part of the Naxos- Paros extensional fault system (NPEFS) marks the boundary between the Cycladic blueshist unit and Upper non-metamorphic unit, e.g. lower and upper plates (BRICHAU et al. 2006). It is assumed that this detachment fault system s motion of the upper plate moving to the north enabled the uplift of the Naxos MCC to the surface (GAUTIER et al. 1993; KRUCKENBERG et al. 2011; LISTER et al. 1984; URAI et al. 1990). The Cycladic blueshist unit has witnessed different metamorphic events initiating with a highpressure compaction event in the Eocene at ca Ma (M 1), followed by extensional greenshist to amphibolite overprinting beginning at ~20-25 Ma (M 2a) (BRICHAU et al. 2006; URAI et al. Figure 2. Exhumation history of the metamorphic basement found on Naxos based on metamorphic and isotopic measurements (LISTER et al. 1984). 2

3 1990) and a more localized high-t-low-p metamorphism of Barrovian character, culminating at ~16 Ma (M 2b) (URAI et al. 1990). Thermal domes in the region are considered to have formed during M 2b deformation (URAI et al. 1990) and most structures nowadays active in the Aegean region are considered remnants of the M 2 deformation events (KOUKOUVELAS & KOKKALAS 2003). In the Miocene, the Cyclades became part of border of the Cycladic-Attic massif (BRICHAU et al. 2006). Regarding the islands geology, Naxos is one of two island that mostly consist of the blueshist unit, which is also overprinted by amphibolite-metamorphism conditions that reached anatetic conditions of 670 ± 50 C and 5-7 kbar in the Miocene (BRICHAU et al. 2006). Being onion shaped, the 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 subduction zone, accordingly magmatic rocks in the range between 5 Ma and 12 Ma and intrusive granites dating 10 Ma to 15 Ma can be found (BRICHAU et al. 2006). Granodiorites intruding the footwall of the NPEFS west of Naxos date at ~12 Ma and are considered synkinematic, evident by the presence of pseudotachylites (BRICHAU et al. 2006). Today the retreating volcanic arc can be found further southwards and marks the southern characteristics of an elongated mantled gneiss dome structure with increasing metamorphic grade towards the center (see Figure 1) (BRICHAU et al. 2006; CAO et al. 2013; URAI et al. 1990). Around the dome, sequences of foliated marbles and shists are present in abundance while some synkinematic granites can be found north of the dome structure and ophiolites W-SW (CAO et al. 2013). At its core the structural dome consists of migmatites which can be subdivided into solid dominated metatexites, characterized by 3

4 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 granodiorite 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 summary of exhumation history is provided in Figure 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 (M 2b) of M 1 and M 2 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 moving south (LISTER et al. 1984). URAI et al. (1991) instead propose that the upper plate is moving north, while accepting LISTER et al. s (1984) model otherwise. Meanwhile, URAI et al. s (1991) proposition has been independently proven for other localities of the Aegean region (JOLIVET et al. 2009). In the upper crustal levels, the crustal shearzone translates into a brittle normal fault, dipping north at a shallow angle of ~35 (CAO et al. 2013; Figure 4. Largescale structure inside the Naxos MCC dome (KRUCKENBERG et al. 2011). GAUTIER et al. 1993). Comprised in this detachment are traces of the brittle-ductile transition phase, evident by the presence of ductile faultrocks, e.g. ultramylonites, which have been subsequently deformed in the brittle domain during uplift (CAO et al. 2013). LISTER et al. s (1984) initial model is depicted in Figure 3. Earlier research has proven that detachment faults can have slip rates in the range of ~1 Km Myr -1 to >20 Km Myr -1, movement rates present in 4

5 the Aegean region average at 15 Km Myr -1 with single fault systems reaching ~5 Km Myr -1 (BRICHAU et al. 2006). For the NPEFS slip rates of ~6-8 Km Myr -1 have been estimated for the timeframe of the brittle-ductile transition (8 16 Ma) while a total offset of at least 50 Km has been achieved (BRICHAU et al. 2006). 4. Formation of the Cordilleran Type Metamorphic Core Complex on Naxos As previously mentioned, Naxos features an exhumed MCC with migmatite at its core (BRICHAU et al. 2006; KRUCKENBERG et al. 2011; URAI et al. 1990). The MCC s migmatic core is dominated by diatexites while metatexites are scarcely found (KRUCKENBERG et al. 2011). Structurally, the dome can be further subdivided into three subdomes and a pinched synform, divided by a highstrain zone oriented in N - S direction (KRUCKENBERG et al. 2011; REY et al. 2011). These structures are shown in Figure 4. Models preferred in the recent past suggest that the Naxos MCC is a double-dome structure originating from a combination of buoyancy- as well as isostasy-driven flow processes (KRUCKENBERG et al. 2011; REY et al. 2011). Despite that there are general difficulties in distinguishing metamorphic domes with different origins (e.g. fold-dominated, detachment-dominated or gravity dominated domes) (REY et al. 2011), KRUCKENBERG et al. (2011) make a good point for ruling out the possibility of the Naxos MCC being a fold-dominated dome. KRUCKENBERG et al. (2011) suggest that the Naxos dome structure might have been generated by E - W contraction coupled with convergent flow and upwelling of migmatite, triggered by regional upper plate moving north (URAI et al. 1990) detachment activity. Additionally, REY et al. (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 lateral flow is affected by thinning of the upper crust (REY et al. 2011). In the deep crust, the flow changes from verging lateral flow to a convergent upward flow. This motion coupled with changes in buoyancy of ductile material also causes contractions in the crust (KRUCKENBERG et al. 2011). According to KRUCKENBERG as well as REY et al. (2011), the buoyancy forces can either manifest as diapirism, density-driven convection, or as assumed for the Naxos MCC, a combination of both mechanisms. 5. Conclusion Naxos and its surrounding area feature a complex geology that opens a window to remnants of distinguishable ancient tectonic events and their resulting present-day active structures. The most prominent feature of the study region is a Cordilleran type MCC, uplifted due to buoyancy- and isostasy driven flow along a shallow-dipping detachment fault system with upper-plate-to-thenorth sense of motion. Featuring evidence for the brittle-ductile transition zone, this fault zone bears great importance for the general understanding of extensional tectonics in back-arc regions. On Naxos, the dome s shape manifests as three subdomes divided by a N-S oriented highstrain zone surrounded by a sequence of alternating marbles and shists, giving room for debate about its origin. At the NPEFS, a detachment fault system found north and west of Naxos and initiated by extensional tectonics, non-metamorphic units of Miocene age are juxtaposed against the high-grade metamorphic Cycladic blueshist unit. The brittle part of this detachment fault is assumed to descend into a ductile shear zone of crustal scale, dipping northwards at a shallow angle of 35. 5

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