Detrital supply from subduction/accretion complexes to the Eocene–Oligocene post-collisional southern Thrace Basin (NW Turkey and NE Greece)

Abstract

The Thrace Basin is a large, mostly Eocene–Oligocene post-collisional sedimentary basin which developed following the closure of the Vardar–İzmir–Ankara oceanic domain (latest Cretaceous–Paleocene). Sandstone petrologic data (framework and heavy-mineral analyses) and the synthesis of preexisting and new sedimentological observations along representative stratigraphic sections show that the basin fill of the southern Thrace Basin was mostly derived from the İzmir–Ankara and Biga (?Intra-Pontide) subduction/accretion complexes to the south. Proximal facies consistently show northward paleocurrents whereas most paleocurrent indicators measured downcurrent point to an eastward paleoflow, likely the result of the deflection of primary gravity flows originated along the southern margin of the basin. Detrital contributions from the Rhodopian basement complex to the west are virtually absent within the southern Thrace Basin fill. Conversely, Rhodopes-derived, Eocene proximal facies in northeastern Greece are characterized by a series of coarse-grained fan-deltas prograding eastward and likely feeding the basin–plain turbidites of the depocentral portion of the Thrace basin, now concealed in the subsurface to the north of our study area. Arenites of the southern Thrace Basin are mostly lithic arkoses and arkosic litharenites. Provenance from the İzmir–Ankara and Biga suture zones to the south is characterized by ophiolitic, granitoid/gneissic, low-grade metamorphic, and extrabasinal carbonate rock fragments, as well as by picotite and glaucophane. The application of detailed petrographic observations for discriminating paleo- vs. neovolcanic and penecontemporaneous vs. noncoeval terrigenous sands lead to a substantial revision of the geodynamic interpretation of the Thrace Basin, formerly considered a forearc basin. A significant penecontemporaneous volcanic component is common in the Upper Eocene–Lower Oligocene section and can be related to extensive post-collisional volcanism following the closure of the Vardar–İzmir–Ankara ocean. The coexistence of pure neovolcanic layers (crystal tuffs and cinerites) and hybrid arenites rich in penecontemporaneous carbonate grains with sands derived from a continental basement and ophiolitic suites indicates the presence of episutural basins where shallow-water carbonates were deposited on top of the exhuming subduction–accretion prism. These carbonates were mixed with penecontemporaneous neovolcanic and terrigeneous components and redeposited in deeper marine environments.