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1 Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
2 Current address: Dublin Institute for Advanced Studies, Geophysics Section, 5 Merrion Square, Dublin 2, Ireland
3 Université Rennes 1, Géosciences Rennes UMR 6118 CNRS, Campus de Beaulieu, 35042 Rennes, France
4 Paleomagnetic Laboratory ‘Fort Hoofddijk’, Faculty of Geosciences, Department of Earth Sciences, Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands
*Corresponding author (e-mail: celine{at}cp.dias.ie)
The mechanics of metamorphic core complex (MCC) development and the associated process of lower crustal flow have been the topic of several modelling studies. The model setup usually includes a local heterogeneity forcing deformation to localize at a given site, enabling only one MCC to develop. This paper presents numerical lithospheric-scale experiments in which deformation is not a priori localized in a specific place, in order to examine whether multiple MCCs could develop during extension, at which conditions, and how. Configurations with either a single MCC or several far-distant MCCs aligned in the section parallel to extension are obtained for a relatively wide range of initial conditions, the only firm requirement being that the lower crust and the sub-Moho mantle both have very low strengths. In contrast, only a narrow range of conditions leads to the development of closely spaced MCCs. In this case, the MCCs interfere with one another (the domes are partly superimposed or/and share a shear zone in common) and develop in sequence. This configuration is compared with the Cyclades archipelago, where closely spaced chains of MCCs have been described in the literature. A review of available data on the islands documents a good agreement with the experiments in terms of final depth of the Moho, geometry and kinematic pattern of the MCCs, and timing of exhumation of the metamorphic rocks. Based on this agreement, we tentatively deduce from the numerical results some of the conditions that prevailed at the initiation of, and during, post-orogenic MCC-type extension in the Cyclades. The most likely initial thickness of the crust is between 40 and 44 km. A thermal lithospheric thickness of only c. 60 km is also likely, which might be a condition at the onset of extension or may have been obtained during early stages of extension while the lithosphere was warmed up. Either a backarc subduction setting or a process of mantle delamination may account for this situation. The numerical results also suggest a boundary velocity of 2.0–2.3 cm/a, which should basically reflect the rate at which the South Hellenic subduction zone retreated. Considering c. 500 km as an upper bound for the amount of retreat balanced by Aegean extension and assuming that this retreat mostly occurred during MCC-type extension in the Cyclades, we find that the boundary velocity could have been as high as 2.1 cm/a if MCC-type extension lasted 24 Ma, starting at c. 30 Ma and finishing at c. 6 Ma, as suggested by available geochronological data. A velocity of 2.1 cm/a agrees well with the numerical results.
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