The exposed base of a collapsing wedge: The Nestos Shear Zone (Rhodope Metamorphic Province, Greece)

Abstract

[1] The Nestos Shear Zone (NSZ) in the Rhodope Metamorphic Province is a major high-strain zone between two metamorphic terranes. Microdiamond-bearing ultrahigh-pressure (UHP) rocks occur in the NSZ which was therefore interpreted as a suture zone where subduction and exhumation of these rocks and terrain accretion occurred during the Mesozoic. Our petrological study of samples from the lower part of the NSZ, together with monazite dating of a microdiamond-bearing garnet schist, structural observations, already published results from the upper part, and other published timing constraints, results in a fundamentally different picture: the NSZ is the base of an Eocene age southwestward thrust wedge which included not only the structurally higher parts of the Rhodope Metamorphic Province but also the entire Internal Hellenides. The UHP rocks, for the peak pressure of which we derive an age of ∼200 Ma by monazite dating, are unrelated to the tectonic processes in the NSZ and probably represent slivers of a higher tectonic unit captured by thrusting along the NSZ. Pressure decrease in the footwall and regional extension and basin formation in the hanging wall during the activity of the NSZ show that the overlying thrust wedge was collapsing in late Eocene times. We hypothesize that the transition from subduction to continental collision may start with a pronounced accretion event when the first detachment horizon forms in the middle crust of the downgoing plate. Such an event could trigger slab retreat, an extensional collapse of the internal wedge, and subsequent magmatism.

Tourmalinite Veins and Breccias From the Symvolon-Kavala Pluton, Northern Greece: Petrogenetic Preliminary Results

“…The Rhodope Metamorphic Province (RMP) is a polymetamorphic geological complex located in northeast Greece to southwest Bulgaria, in the hinterland of the Hellenic Subduction System (Burg et al, 1990;Bonev et al, 2006;Krenn et al, 2010;Jahn-Awe et al, 2010;Nagel et al, 2011). RMP consists of three allochthon tectonic units (Lower, Middle and Upper) and is bordered by Servomacedonian Massif in the west and Circum-Rhodope Belt in the east (e.g.…”

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

“…1), a major ductile shear zone with top-to-SW kinematics that has received much attention since the 1990s. Contrasting opinions exist as for the origin of top-to-SW shearing along the NSZ, which could reflect either thrusting (e.g., Kilias and Mountrakis, 1990;Barr et al, 1999;Krohe and Mposkos, 2002) or extensional shearing (Nagel et al, 2011). Determining the origin and age of shearing along the NSZ is important considering the apparent conflict that exists between scenarios favoring crustal-scale extension across the RMC since 40 Ma or more (e.g., Kilias et al, 1999;Brun and Sokoutis, 2007;Jolivet and Brun, 2010;Burg, 2011;van Hinsbergen and Schmid, 2012) and studies suggesting ongoing thrusting along the NSZ as late as 37 Ma (Dinter, 1998;Krohe and Mposkos, 2002) or 33 Ma (Bosse et al, 2009;Gautier et al, 2010) and/or eclogite facies metamorphism in the RMC as late as 42-45 Ma (Liati and Gebauer, 1999;Liati et al, 2011;Kirchenbaur et al, 2012).…”

“…Since the late 1990s, numerous scenarios have been proposed for the tectonic evolution of the RMC during Alpine times. A primary difference between these scenarios is whether there has been a single protracted orogenic cycle since around the midMesozoic (Ricou et al, 1998;Barr et al, 1999;Papanikolaou and Kranis, 2004;Turpaud and Reischmann, 2010;Krenn et al, 2010;Burg, 2011;Kydonakis et al, 2016) or whether Alpine orogeny involved distinct episodes of subduction and crustal accretion (Liati, 2005;Mposkos et al, 2010;Liati et al, 2011Liati et al, , 2016Nagel et al, 2011;Kirchenbaur et al, 2012;Froitzheim et al, 2014;Wawrzenitz et al, 2015). The difference between the two views mainly stems from different interpretations of the available geochronological data (e.g., Burg, 2011;Liati et al, 2011).…”

“…The two shear zones were initially regarded as distinct, synthetic thrust zones (Ricou et al, 1998). Because they display roughly the same petro-geochronological record (e.g., Bosse et al, 2009;Didier et al, 2014), the NSZ and CSZ have been reinterpreted as two occurrences of the same shear zone (Turpaud and Reischmann, 2010;Nagel et al, 2011;Burg, 2011). With respect to the NSZ, Papanikolaou and Panagopoulos (1981) initially defined a lower unit and an upper unit, later named the Pangaeon Unit (PU) and Sidironero Unit (SU).…”

“…The PU is equivalent to the "Thracia Terrane" of Turpaud and Reischmann (2010), the "Lower Terrane" of Burg (2011), and the "Lower Allochthon" of Nagel et al (2011). The SU is equivalent to the "Rhodope Terrane" of Turpaud and Reischmann (2010) and the "Middle Allochthon" of Nagel et al (2011). A large part of the SU consists of migmatites and granitic plutons derived from the anatectic melts.…”