Lateral growth of the continental crust through deep level subduction–accretion: a re-evaluation of central Greek Rhodope

Abstract

The Rhodope Massif of northern Greece and southern Bulgaria has been variably interpreted as a fragment of pre-Alpine (possibly Hercynian or Precambrian age) continental crust that was actively involved in European margin convergent tectonics during the Mesozoic and Tertiary. Alternatively we propose that, for central Greek Rhodope, the lithological associations and tectonothermal history are consistent with progressive south-directed Mesozoic crustal growth through deep level subduction–accretion, a process which involved the near continuous transferral of material of oceanic and continental margin affinity, from footwall to hanging wall within a north-directed Palaeotethyan subduction system. Central Greek Rhodope comprises a sequence of schists, gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing. Extensive occurrences of marble are associated with amphibolites, variably retrogressed eclogites, meta-gabbros and ultramafic rocks. Quartzo-feldspathic gneisses are abundant throughout the area, and are associated with psammitic meta-greywacke sequences, pelites and quartzites. The presence of intercalated marble slivers and thin manganiferous and iron-rich siliceous bands is significant. The various components exposed in the subduction–accretion complex were first underthrust northwards to deep crustal levels, subjected to high pressure, eclogite facies metamorphism and underplated to the base of the overriding Eurasian plate. This latter accretion event was characterised by intense southwest-directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting. It was accompanied by migmatisation and intracrustal melt generation, and followed by subduction related magmatism. Deep level subduction–accretion leads to southward growth of the continental margin, which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north. Subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of Eurasia. This new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre-existing continental crust. If so, then the importance of deep level subduction–accretion as a mechanism of continental crustal growth has hitherto been underestimated.