Polycyclic alpine orogeny in the Rhodope metamorphic complex: the record in migmatites from the Nestos shear zone (N. Greece)

Abstract

-The Rhodope Metamorphic Complex (RMC) is a high-grade crystalline massif located at the northern margin of the Aegean region. Numerous scenarios have been proposed for the evolution of the RMC during Alpine times. A debated issue is whether there has been a single protracted orogenic cycle since around the mid-Mesozoic or whether Alpine orogeny involved distinct episodes of subduction and crustal accretion. We describe a key outcrop located on the Nestos Shear Zone (NSZ), a major NNE-dipping top-to-SW shear zone characterized by an inverted metamorphic sequence. Structural and petrological data document the existence of two anatectic events. The first event, best preserved in decametric structural lenses, is pre-kinematic with respect to top-to-SW shearing and involved high-temperature "dry" melting. Zircon and monazite LA-ICPMS U-Th-Pb data indicate that this event occurred at ∼140 Ma. The second event is syn-kinematic with respect to top-to-SW shearing and involved lower-temperature water-assisted melting. Zircon and rutile LA-ICPMS U-Pb data indicate that this second event occurred at ∼40 Ma. During ongoing top-to-SW shearing and as late as ∼36 Ma, the rocks from the outcrop were at higher temperatures than the peak temperatures experienced by lower levels of the NSZ. This confirms the existence of the inverted metamorphic sequence and demonstrates that the NSZ was a major thrust at 36-40 Ma. The ∼100 Myr time laps between the two anatectic events encompasses the period from ∼115 to ∼70 Ma characterized by a gap in the geochronological record on the scale of the RMC (the Eastern Rhodope excluded). This ∼45 Myr gap likely reflects a period of tectonic quiescence between the mid-Mesozoic orogen and the Cenozoic one, attesting for polycyclic Alpine orogeny in the RMC. Unlike assumed in several geodynamic scenarios, the Alpine evolution of the RMC did not consist of a single orogenic cycle of Mesozoic age followed by Cenozoic crustal-scale extension triggered by mantle delamination. Polycyclic orogeny has resulted in a two-loop P-T-t path for the hangingwall unit of the NSZ. The Cenozoic P-T paths of this unit and the footwall unit merged while both units were being exhumed, a feature attributed to synthrusting extensional spreading of the main mass of the hangingwall unit above the NSZ.Keywords: Aegean / syn-deformation migmatization / U-Pb dating / LA-ICPMS / inverted metamorphism / synmetamorphic thrusting Résumé -Orogenèse alpine polycyclique dans le complexe métamorphique du Rhodope : enregistrement dans les migmatites de la zone de cisaillement du Nestos (Grèce). Le Rhodope est un complexe métamorphique de haut grade situé à la frange Nord de la région égéenne. De nombreux scénarios ont été proposés pour son évolution durant la période alpine. Une question débattue est de déterminer s'il n'y a eu qu'un seul cycle orogénique de longue durée depuis le milieu du Mésozoïque ou bien si l'orogenèse alpine a impliqué des épisodes distincts de subduction-accrétion crustale. Nous décrivons un affleurement-...

Petrochronology and hygrochronology of tectono-metamorphic events

“…Crystalline complexes occasionally feature rocks having experienced polycyclic metamorphism during several successive orogenies and/or other geodynamic settings. Polycyclism needs to be distinguished from continuous successive stages of a P-T path during a single orogenic cycle (Gautier et al 2017). The possible identification of several stages of deformation or successive metamorphic parageneses may only reflect the complex evolution of a single orogen.…”

Core Complex Segmentation in North Aegean, A Dynamic View

“…This tectonic contact is often interpreted as a major thrust zone of Alpine age as it put older on younger or more metamorphic on less metamorphic (e.g., Kilias & Mountrakis, ; Koukouvelas & Doutsos, ; Burg et al, ; see review in Burg, ). However, because the tectonic history of this contact is long and complicated, involving reactivation during the Tertiary, some authors call it in more neutral ways, for example, “Nestos Fault” by Dinter () or “Nestos Shear Zone” by Nagel et al () or Gautier et al (). Before SRCC exhumation the Southern and Northern Rhodopia (Figures a and c) constituted a single continental block below which Pelagonia has been subducted (Brun & Sokoutis, ; Kydonakis, Brun, Sokoutis, & Gueydan, ).…”

“…The Pangeon‐type unit, located in the vicinity of The Nestos Thrust, and the base of the Rhodope Metamorphic Complex both display evidence of ductile deformation of late Eocene age (Bosse et al, ; Gautier et al, ; Nagel et al, ). However, both thrusting and extension have the same top‐to‐SW sense of shear leading to controversial interpretations.…”

“…To the north of the Nestos the NRCC domes (MCC 1 in Figure b), whose exhumation makes the Nestos Thrust footwall to reappear at surface, expose strongly deformed and partially molten gneisses (Burg, ; Gautier et al, ; Schenker et al, ) indicating that, during Tertiary extension, the crust underlying the Rhodope Metamorphic Complex was ductile. As mentioned above, the Nestos Thrust dip is not original but due to the upward bending of the brittle crust (i.e., hanging wall of the Nestos Thrust) during the development of the SRCC (MCC 2 in Figure b).…”

“…Because the width of the NRCC domes in the direction of stretching is ranging between 30 and 50 km, the minimum displacement of the Nestos Thrust on top of the ductile crust must have been of at least of 30 km, taking into account the upward bending of the Nestos Thrust. Therefore, the late Eocene age of SW‐directed shear in the Nestos Thrust zone (Gautier et al, ) simply resulted from this rather large extensional displacement with top‐to‐SW sense of shear at the BD interface. In other terms, the Nestos shear zone that was the base of an Eocene age southwestward thrust wedge was still affected by a top to‐SW sense of shear but extensional in late Eocene, as clearly identified by Nagel et al ().…”