Abstract
The Aegean domain is a well-suited place to study the formation of metamorphic core complex (MCC) and to investigate the role of syn-tectonic granites on their development. In the northern Cyclades, the Mykonos–Delos–Rhenia MCC is characterized by the intrusion of a kilometer-scale Late Miocene pluton of I-type granitoids within a migmatitic gneiss dome. New combined AMS (anisotropy of magnetic susceptibility) and microstructural studies on the Mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers. The Mykonos granitoids form a laccolith-like intrusion with a N70°E long axis. The laccolith is strongly asymmetric with an outlying root zone to the SW and a major body mainly developed to the NE. The laccolith construction is due to successive pulses of more or less differentiated magma that intruded the Cycladic Blueschist Unit. The attitude of stretching markers suggests an important (about 60°) vertical-axis local rotation phenomenon in the cycladic upper crust during the exhumation of the Mykonos MCC. Structural data suggest a four-stage evolution of the Mykonos MCC: (i) a first stage characterized by flat shearing toward the N–NE and by the formation of a domal structure in migmatitic paragneisses with multi-scale generation of folds with axes either perpendicular or parallel to the regional stretching, as a result of the interplay between regional N20°E-directed extension and EW shortening; (ii) a second stage marked by the emplacement of the Mykonos laccolith at 13.5 ± 0.3 Ma at the top of the migmatitic paragneisses; (iii) the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions; and (iv) the late stage marked by an acceleration of the exhumation of the Mykonos MCC. This exhumation was accommodated by important rotations of upper crustal blocks. During the end of the exhumation processes, around 10 Ma, deformation localized at the top of the laccolith in semi-ductile conditions and then in brittle conditions in the major detachment plane. Our study shows that the Cycladic plutonism event had no role on the initiation of the MCC. However, the geometry of the Mykonos intrusion supports that the magmas are “sucked” into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the MCC development. This acceleration was marked by a very fast exhumation of the laccolith after its emplacement.
Highlights
Metamorphic core complexes (MCC) or extensional gneiss domes are crustal-scale structures that develop during extension of a thickened and thermally mature crust ([Coney and Harms, 1984], [Lister and Davis, 1989], [Malavieille, 1993] and [Whitney et al, 2004])
We present a study of the evolution of the ductile deformation in a 2 km-thick crustal crosssection within the granitoids and migmatites of the Mykonos MCC beneath the Nord Cycladic Detachment System (Jolivet et al, 2010)
Because the unblocking temperature of pyrrhotite and the temperature of the brittle–ductile transition are very close, we suggest that the local rotation recorded in the Mykonos MCC by the stretching markers was not recorded by mineral remanence
Summary
Metamorphic core complexes (MCC) or extensional gneiss domes are crustal-scale structures that develop during extension of a thickened and thermally mature crust ([Coney and Harms, 1984], [Lister and Davis, 1989], [Malavieille, 1993] and [Whitney et al, 2004]). Some models use a pre-existing fault in the brittle upper crust to develop a MCC ([Buck, 1993], [Koyi and Skelton, 2001], [Lavier et al, 1999] and [Rey et al, 2009]) In these models, the formation of the dome is a consequence of the displacement along the detachment. An alternative view is to consider the detachment not as the primary cause for the formation of the MCC but instead as a consequence In this case, numerical and anologue modeling emphasize the importance of pre-existing viscosity–density anomalies such as a granite or a migmatitic body below the brittle–ductile transition ([Brun et al, 1994], [Tirel et al, 2004], [Tirel et al, 2008] and [Vanderhaeghe and Teyssier, 2001]).
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