The balkan thrust wedge and foreland basin of eastern Bulgaria: Structural and stratigraphic development

H. D. Sinclair*, S. G. Juranov, G. Georgiev, P. Byrne, N. P. Mountney

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The Balkan Mountains of Bulgaria run east-west and outcrop along the north-south-running Black Sea coastline. Immediately north of the Balkan thrust front is the Kamchia Depression, which has been interpreted to represent the North Balkan foreland basin. To the south is the Srednogorie Zone, comprising Cretaceous calc-alkaline volcanics representing a remnant volcanic arc. A north-south structural cross section can be generated by the integration of coastal exposures, with deeper level constraint from onshore and offshore seismic data. In this section, the Balkans comprise two large synclines bounded by major faults. This folding and thrusting detached at a horizon within the Jurassic succession at ∼5 km depth. Section restoration across the Balkans from the remnants of the volcanic arc in the south to the Balkan thrust front in the north gives a minimum of 18 km of shortening. Seismic stratigraphy indicates two periods of shortening across the Balkans. Initially, deep-seated normal faults that offset the Triassic were reactivated as reverse faults at the end of the Cretaceous. The sea-floor topography generated during this compression was subsequently draped by Paleocene and lower and middle Eocene strata. At end-middle Eocene times, thin-skinned thrusts propagated into the basin, initiating the main Balkan structures. The termination of shortening is recorded by the blanketing of thrust-related topography at end-Oligocene times. Therefore, the >18 km of shortening took place from early Paleocene to end-Oligocene times; this indicates a time-averaged rate of shortening of ∼0.5 km/m.y. The sedimentary fill of the Kamchia Depression is intimately linked to the growth of the Balkan Mountain belt. At end Cretaceous/early Tertiary times, it was characterized by emergence in the north linked to the deep-level fault reactivation, and deepening in the south to bathyal depths, where calci-turbidites accumulated. During the early Eocene times, siliciclastics were deposited in the south and center of the basin, and the northern margin experienced a marine transgression; this is thought to be related to the load-induced subsidence of the southern margin of the Moesian Platform by the Balkan Mountains. By the middle Eocene, immediately prior to thrust encroachment into the basin, the northern margin underwent mass wasting in the form of debris flows, slumps, and gravity glide sheets. At a similar time, micropaleontological indicators suggest reduced oceanic circulation, possibly linked to physical isolation of the Black Sea region by growing mountain belts to the south. Subsidence analysis offshore indicates accelerated tectonic subsidence during middle and upper Eocene times, thus strengthening the proposed link between early thrusting and deepening of the basin. The structural, sedimentological, and subsidence history of this basin strongly supports its interpretation as the north Balkan retro-arc foreland basin. Rates of tectonic subsidence (>0.05 km/m.y.) and crustal shortening (

Original languageEnglish
Pages (from-to)91-114
Number of pages24
JournalAAPG Memoir
Issue number68
Publication statusPublished - 1997

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