TECTONIC DENUDATION AT THE ROOF OF THE RH ODOPIAN-MACEDONIAN METAMORPHIC CORE - THE GABROV-DOL DUCTILE NORMAL-FAULT (BULGARIA)

The Rhodopian-Macedonian high grade metamorphic core is controversiall y interpreted either as a pre-Alpine continental block within the Alpi ne chains or as a system of Alpine synmetamorphic nappe units formed a t the active margin of Eurasia against the Cretaceous Tethyan ocean. W hile recent studies on the major, Rhodopian, part of the metamorphic c ore give evidences for a Cretaceous age under the transgressive Eocene -Oligocene cover, the western, Macedonian, part is said to show strati graphic evidences for an Archean age. The western boundary area of the metamorphic core shows three superposed groups, here named I, II and III from base to top: high grade metamorphic rocks, mainly gneiss and amphibolites; low grade metamorphic rocks, mainly metadiabases and met adiorites; fossiliferous Palaeozoic to Triassic sedimentary rocks, bea ring the features of a Hercynian orogeny. The present study is focused on the contact between groups I and II, formerly interpreted as a str atigraphic contact. Analysis of the Gabrov Dol type section and of add itional sections covering 90 km of the length of the contact, shows th at the layered rocks which crop out along the contact and were previou sly interpreted as detritic metasediments are actually mylonites which derive mostly from the overlying metabasites. Consequently, these sec tions do not give evidence for a pre-Alpine age of the gneiss and amph ibolites. Subject to verification along the whole western limb of the gneiss core, the Eocene-Oligocene beds appear as the oldest sedimentar y cover post-dating the gneiss and the mylonitic contact. This mylonit ic shear zone of hectometrical thickness was formed under greenschist facies conditions which affected at the same time the groups I and II. The associated foliation is parallel to the shear zone and shear-sens e criteria consistently indicate a top to north-west displacement, awa y from the metamorphic core. Adding the drastic reduction of thickness which affects the rocks of intermediate metamorphic type between I an d II, the Gabrov Dol fault appears as a major detachment fault during tectonic denudation which determined the present-day roof-boundary of the high grade metamorphic core. Our observations sustain the Alpine i nterpretation: terrains gathered at the subduction boundary should hav e been engulfed during the Cretacous under the Hercynian Europe, repre sented here by the group III, then exhumed at the beginning of the Cen ozoic.