Small-scale high-temperature metamorphic core-complexes: From nappe stacking to orogen parallel extension and subsequent thrusting in the Megina dome of North Dobrogea orogen

Abstract

Orogenic extension has been demonstrated to generate lower crustal exhumation observed in metamorphic core complexes. Observations and modelling have shown that orogen parallel extensional domes are challenging to constrain due to the superposition with coeval or subsequent geodynamic processes, such as orogenic thrusting, magmatism, or the evolution of subducted slabs. We utilise outcrop and micro scale observations to structurally quantify the original nappe stack burial and extensional exhumation structures of one such dome in the Măcin unit of the European North Dobrogea orogen. The results demonstrate an initial top to NE nappe stacking associated with the previously defined Late Carboniferous – Permian burial metamorphism, which affected a mafic protolith and its sedimentary cover. This burial was followed by Triassic extensional exhumation, retrograde metamorphism, magmatism and top to SE ductile shearing, which created a newly identified major detachment and orogen-parallel extensional dome. The kinematic study is furthermore complemented by a low-temperature fission track thermochronology performed to discriminate the extensional exhumation from the effects of subsequent thrusting. The thermochronology demonstrates that the Triassic extensional dome was subsequently affected by a Jurassic–Early Cretaceous exhumation, which is a novel quantification of the previously defined Cimmerian thrusting. Our results infer a new interpretation of the Măcin unit as a Palaeotethys ophiolitic suture zone, which contains a typical Hercynian metamorphic core-complex of relatively small dimensions. When compared with other similar larger structures observed elsewhere, our results show that small-scale orogen-parallel extensional domes can also develop at high temperatures during or after the main stage of nappe stacking metamorphism.