The outcrops and panoramas described during this field trip illustrate part of the geology of Naxos as retrieved from published data and a multidisciplinary study conducted by the authors of this guide with the help of other colleagues. As a result of this study, we propose a model for the structural and metamorphic evolution of Naxos rocks and also for the contemporaneous evolution of sedimentary basins in the frame of the geodynamic evolution of the Aegean crustal accretionary wedge.

Structural evolution of a crustal wedge, from accretion to collapse

As seen in the field trip, the three lithologic-metamorphic units of Naxos display contrasted structural styles. This in part reflects diachronous deformation of the various units but also probably suggests mechanical decoupling among the different structural levels represented by these units.

The oldest structures preserved in Naxos are probably the foliation of the marble and schist sequence underlined by blueschists mineral paragenesis at the southern tip of the island. This structure probably records deformation near the pressure peak during burial of these rocks following the subducting slab. This foliation is partially transposed with asymmetric folds indicating a northward vergence to the south of the island. It was unfortunately not possible to see these strutures in this field trip. Folds exposed near Filoti affect the same marble and schist sequence but are structurally below. At this level, fold axes are transposed according to the NW-SE stretching lineation associated with the development of sheath folds. The mineral paragenesis associated with these structures indicates a greenschist facies overprint in a medium pressure/medium temperature environement. We interpret these structures as reflecting increasing transposition of the early HP/LT fabric following thermal relaxation during vertical shortening associated with top-to-the-northwest shearing. Sheath folds are concentrated within the middle unit at the transition between the marble-dominated sequence to the schist-dominated sequence. This might represent a decollement layer within the middle unit. Upright folding of the foliation within the middle unit records a roughly NE-SW shortening direction in part coeval with the top-to-the-northwest shearing resulting in a constrictional finite strain ellipsoid. This constriction might be locally due to the development of the migmatite dome or to flow of the marble and schists around it. However, it appears to be a general feature at the scale of the Aegean domain and we will further discuss the significance of this feature below.

Deformation of the migmatites underlined by a synmimatitic foliation indicates that it occurs while the rocks were partially molten. The synmigmatitic foliation delineates the elliptical shape migmatite dome which indicates that the dome formation is coeval with the presence of melt. Subdomes display complex shapes that are interpreted to reflect the combination of vertical upwelling of the migmatites owing to their buoyancy partially transposed according to top-to-the-northwest shearing. The subdomes are preferentially elongated parallel to the first order dome axis and NW-SE trending vertical synmigmatitic foliations are dominant. This might indicate that development of the migmatite dome was in part coeval to NE-SW shortening.

In contrast to the simple view of a single detachment separating the upper and middle unit, our work shows a complex network of structures associated with the progressive development of detachments systems and migration of the “brittle-ductile” transition as rocks are exhumed. Part of the high-angle faults and cataclastic shear zones affecting the upper unit are rooting into a decollement horizon. In the middle unit, ductile fabrics grade into mylonitic zones. All of these structures are cross-cut by high-angle normal and strike-slip faults. These cross-cutting relationships suggest a propagation of the localized zone of deformation in space and time in response to themigration of the brittle-ductile transition during exhumation and cooling of metamorphic rocks. Landslides are surface witnesses of this tectonic activity.

Metamorphic evolution of a crustal wedge, from accretion to collapse

We interpret the age and metamorphic gradient from greenschist facies to amphibolite facies metamorphic grade exposed in the island of Naxos as representing progressive transposition of an earlier HP/LT fabric during thermal relaxation of the Aegean crustal accretionary wedge. U/Pb ages of 50 Ma obtained on zircon rims from the migmatites that are similar to Argon ages obtained in the blueschists relics suggests that all these rocks were buried in the Eocene.

Further detailed work is needed to determine metamorphic gaps between the middle and upper units and within the middle unit across the decollement horizon marked by the sheath folds.

Magma emplacement and detachment activity

To the south of the island, the granodiorite is intrusive into marbles and schists of the middle unit and yields U/Pb ages on zircon, argon ages on hornblende and biotite, and FT ages on zircon and apatite ranging between 9 and 12 Ma. By that time, the middle unit already cooled below300- 200°C according to argon and fission track thermochronology and the migmatite dome was already developed according to U/Pb geochronology on zircon. This implies that part of the exhumation history of Naxos was already achieved before the emplacement of the granodiorite. Accordingly, the emplacement of the granodiorite represents a short time interval during the exhumation history. The concentration of cataclasites around and cross-cutting the granodiorite suggest that it favored localized deformation probably because it represented a local source of heat and thus a sharp rehological boundary at the time of emplacement. The spatial relationships between cataclasites and faults suggest that at the time of emplacement, the granodiorite represented a ductile/brittle transition and that shortly after, as it rapidely cooled down the brittle/ductile transition jumped deeper in the crustal section.

Fluid circulations during MCC formation

Preliminary results on the fluid circulation record in Naxos rocks allow the distinction between two crustal reservoirs. A shallow reservoir in which meteoric and basin fluids percolate and a deep reservoir containing fluids released during metamorphic reactions and magma cristallization. The transition between these reservoirs corresponds to the brittle-ductile transition which position is controlled by the thermal evolution of the crust. During crustal thinning and the development of MCC, metamorphic rocks are exhumed across the brittle-ductile transition. The distribution of fluid inclusions according to mineral textures and mircostructures of these rocks as well as the chemical composition of the various fluids records this transition from a deep to shallow reservoir. P, T, t reconstruction of these fluid inclusions indicates that this transition from a deep ductile crustal reservoir to shallow brittle reservoir is also associated with a change in the thermal regime as rocks are affected by rapid cooling as they cross the brittle-ductile transition.

Sedimentary record of exhumation and topographic evolution during MCC formation

Naxos sedimentary basins are filled mainly by coarse silicoclastic deposits that record progressive exhumation of the metamorphic units. Sedimentologic features in Naxos but also in Paros suggest:

(1) Oligocene shallow marine to continental deposits registered mainly the erosion of the underlying ophiolitic nappe with some evidences of pebbles originated from non-metamorphic nappes that are not outcropping in the Cyclades. These deposits indicate relatively poorly developed relief at the transition from thickening to thinning of the crustal accretionary wedge.

(2) Early Miocene subsidence started from reef deposits to deeper marine turibidites, recording local underwater slope instabilities (olistolithes) due to synsedimentary fault activity. These marine series, while progressively shallowing, display the increasing appearance of pebbles from the Upper metamorphic Unit. This subsidence trend is almost coeval with the thermal pulse in the Naxos migmatitic dome.

(3) Travertine, lacustrine then detrital continental deposits during Serravalian to Tortonian times registered the exhumation of the Upper than Middle metamorphic Unit. Strong relief associated to kilometer-scale landslide and olistostromic deposits characterized the active tectonic border on the western side of the Naxos dome while more quiet environments with lacustrine limestones characterized more soft landscapes with karstification of the exhuming marbles. This global change to a regional uplift and continental environments seems coeval with the granodiorite setting between Naxos and Paros. Strong co-seismic activity during this period is revealed by these landslides, pseudotachylites in the granodiorite and seismites in the sediments.

(4) From Late Miocene to possibly Pliocene times, the exhumation of the migmatites was sealed by a fluvial erosional surface while the distinct exhumation of the granodiorite kept going on in a context of active extensional and transtensional faulting. The progressive regional uplift or eustatic change following this surface is revealed by the incision of the following fluvial sequences which recorded the erosion of the migmatitic Lower Unit. These periods are coeval with siliceous fluid migration and dacitic volcanism around 9 My. The tectonic exhumation of the granodiorite was achieved after this event as revealed by the last fluvial deposits containing pebbles of the granodiorite.