Low-temperature constraints on the Alpine thermal evolution of the central parts of the Sredna Gora Zone, Bulgaria

ブルガリア・Sredna Gora帯中部におけるアルプス造山期の熱的発展の低温領域における制約

Balkanska, E.*; Georgiev, S.*; Kounov, A.*; Anti, M.*; 田上 高広*; 末岡 茂   ; Wijbrans, J.*; Peytcheva, I.*

Balkanska, E.*; Georgiev, S.*; Kounov, A.*; Anti, M.*; Tagami, Takahiro*; Sueoka, Shigeru; Wijbrans, J.*; Peytcheva, I.*

The central parts of the Sredna Gora Zone in Bulgaria have experienced a complex Alpine tectonic evolution. The main tectonic and thermal events since the end of the Triassic are the Late Jurassic-Early Cretaceous (Early Alpine) and Late Cretaceous-Paleogene (Late Alpine) compression/transpression separated by Late Cretaceous island-arc magmatism and intra/back-arc extension and basin formation. During the Cenozoic, the area was mainly under post-orogenic extension. Here, we present the first apatite and zircon fission-track results and new muscovite and biotite Ar/Ar age analysis on late Carboniferous-Permian granitoids together with Late Cretaceous volcanic and subvolcanic rocks and Late Maastrichtian-Danian conglomerates from the Panagyurishte basin, which allowed us to reveal the Alpine thermal and tectonic evolution of the Central Sredna Gora Zone. Our new results disclosed the existence of several thermal and cooling episodes related to different tectonic and magmatic events in the studied area. The 40Ar/39Ar dating of samples from the metamorphic basement constrain the thermal peak of an Early Alpine thermal event at about 139-137 Ma at temperatures between 300 and 400C. Through the apatite FT dating and thermal modelling, the time of a Late Alpine (post-Danian) event was constrained at 65-55 Ma, during which the tectonically buried sediments of the Panagyurishte basin attained temperature of 120C. The Cenozoic cooling and exhumation of the Central Sredna Gora Zone metamorphic basement was related to post-orogenic extension and denudation which took place probably at two stages during middle Eocene to Oligocene.