Heterogeny of the Subthrust Structures in Front of a Thin-Skinned Thrust Belt
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БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2012“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2012” Heterogeny of the subthrust structures in front of a thin-skinned thrust belt – an example from the East Fore-Balkan Хетерогенност на поднавлачните структури пред гънково-навлачен пояс със срязване на покривката – един пример от Източния Предбалкан Hristo Kiselinov1, Dian Vangelov2, Polina Andreeva1, Victoriya Vangelova2 Христо Киселинов1, Диан Вангелов2, Полина Андреева1, Виктория Вангелова2 1 Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia; E-mail: [email protected] 2 Sofia University “St. Kliment Ohridski”, 1504 Sofia, Bulgaria Key words: subthrust tectonics, thin-skinned thrust belt, Eastern Fore-balkan, Eastern Bulgaria. Introduction leogene limestones, conglomerates, sandstones and clays, separated by the Asparuhovska syncline. This The investigated area (Fig. 1) occupies the eastern- part of the zone is characterizesd by well prominent most part of the Fore-Balkan Zone (Centralbalkan boundaries – “Chudnite Skali” Dislocation (Kockel, –Fore-Balkan in sense of Ivanov, 1988), which is 1927) to the South with the East Balkan zone and the characterised by folded Early Cretaceous turbiditic so called Preslav-Shkorpilovtsi fault zone (Georgiev, sequences in the cores of Predzhanska and Asparu- Monahov, 1979), and to the North with the Moesian hovska anticlines, outlined by Late Cretaceous-Pa- platform. Fig. 1. Geological sketch map for the investigated area: 1, Quaternary; 2, Neogene; 3, Paleocene; 4, Eocene; 5, Upper Cretaceous–Lower Paleocene; 6, Kamchia Fm (Lower Cretaceous); 7, Ticha Fm (Lower Cretaceous); 8, East Balkan Zone (Triassic–Lower-Middle Jurassic); 9, fault; 10, normal fault; 11, thrust fault 109 New field results Conclusion Newly acquired field data do not support the published The preliminary field, analyzed well data, seismic and ones, including the Geological Map of Bulgaria in Scale DEM data, orogenic events, facies analysis and basin 1:100 000, Sheets: Provadia (Milanova, Cheshitev, evolution suggest: 1992), Varna and Zlatni Pyassatsi (Cheshitev et al., – Deformation is result of subthrust tectonics in 1994), Aytos (Kanchev, 1995a) and Dolni Chiflik front of a thin-skinned thrust fold belt (similar to the (Kanchev, 1995b). In the summarized information East Balkan belt characteristics) with flat/ramp ge- (Kara gjuleva, 1971) the zone composition is character- ometry of the master fault and sinistral transpression ized only by folding without any faults. On opposite, mechanism of shortening. in the Geological Map of Bulgaria are shown numer- – The facies distribution and traces of older struc- ous faults but in general missing in the field, suggested, tural events, indicates serious impact of the Early with different orientation and position, at the same time Alpine tectonics during Late Alpine time along the prominent shearings were not documented. southern boundary of the zone east of Luda Kamchia The field data suggest also revising of the zone north- river valley. ern boundary in the studied area because to it can not be – The deformation along the northern zone mar- traced in the field. The existing fault structures do not gin is significantly affected by salt tectonics and this, show such characteristics like in Preslav and Shkorpilovtsi together with other structural data allows to replace segments, where they are well investigated and recogniz- or revise the boundary in this area. Nevertheless able. At the same time, south of Provadia are established that salt diapirs are with different ages (Permian in brittle-ductile shear zone, which can be interpreted as tip Preslav anticline and Late Triassic in Provadia), the of the thrust (subthrust) deformation, related to the Late halokinesis is most likely related to the Late Alpine Alpine phace and affected by salt tectonics. Along the deformations. northern rim of the Royak plateau can be suggested an- – Different levels of detachment are one of the rea- other fault plain on the base of higher hypsometric level sons for the heterogenic deformation, combined with of older rocks, than the younger, outcropped northward sinistral transpression regime, lateral/oblique ramps and digital elevation model (DEM) of the relief. Along and thick turbidite sequences in the zone basement. the southern rim of the same plateau were documented series of echelon faults, dissected by lateral or oblique Acknowledgements: This study is a contribution ramps. These structures developed on the Moesian plat- to Project DMU 03/64, financed by the National form, exclude “platform” characteristics of the plateau Science Fund. area, considering existing low-angle and possible blind shearings. The “Provadia” fault and the faults along the References Royak plateau SW rim are parallel to the fault system de- veloped in front of the Preslav anticline and in such case Cheshitev, G., V. Milanova, N. Popov, E. Kojumdgieva. 1994. can be suggested a displacement of the zone boundary Explanatory Note of the Geological Map of Bulgaria in Scale 1:100 000. Map Sheet Varna and Zlatni Pyassatsi. northward, traced along an echelon faults system with Sofia, Geology and Geophysics Corp., CGMR, 75 p. (in NW–SE orientation, connected by faulted relay ramps. Bulgarian). The described low-amplitude folding of the area Georgiev, G., I. Monahov. 1979. New data about the boundary was not confirmed as well. Most of the folds are fault between the Moesian plate and the Fore-Balkan in North- predestinated. More than 15 locally developed folds east Bulgaria. – Ann. Univ. Sofia, Fac. Geol and Geogr., with different characteristics and orientation were rec- 71, 1–geol., 367–378 (in Bulgarian). Ivanov, Z. 1988. Aperçu general sur l’évolution géologique ognized. The described Predzhanska, Asparuhovska, et structurale du massif des Rhodopes dans lae cadre des Gradistenska and Lopishnenska anticlines and Asparu- Balkanides. – Bull. Soc. Géol. France, 8, IV, 2, 227–240. hovska, Arkovnenska synclines were not recognized Kanchev, I. 1995a. Explanatory Note to the Geological Map and need to be seriously revised. The “olistoliths” in of Bulgaria in Scale 1:100 000. Map Sheet Aitos. Sofia, front of Yankovo ramp-anticline are in fact outliers. Geology and Geophysics Corp., CGMR, 111 p. (in Bul- The shape of this part of the zone needs to be ana- garian). lyzed also, due to its wideness in the Preslav segment Kanchev, I. 1995b. Explanatory Note to the Geological Map of Bulgaria in Scale 1:100 000. Map Sheet Dolni Chiflik. (40–45 km), shortened to 4–5 km along the Armera Sofia, Geology and Geophysics Corp., CGMR, 113 p. (in river valley. The concentration of deformations along Bulgarian). the northern boundary in the Preslav segment and along Karagjuleva, J. 1971. The Preslav anticlinorium. – In: Bonchev, the southern one east of the Luda Kamchia river, indi- Е. (Ed.). Tectonics of the Fore-Balkan. Sofia, Publishing cates different distribution of the stress fields, detach- House of the Bulgarian Academy of Sciences, 333–421 (in ment level, scale, style and intensity of deformation in Bulgarian). the frame of the zone. One of the possible explanations Kockel, C. 1927. Die Grundzüge des Baues von Bulgarien. – Ztschr. Dtsch. geol. Ges., 79, 8–10, 192–193. is the salient geometry of the East Balkans thrust front, Milanova, V., G. Cheshitev. 1992. Explanatory Note to the which allowed to separate the studied area in to a west- Geological Map of Bulgaria in Scale 1:100 000. Map Sheet ern “wide” and less deformed part and eastern “narrow Provadia. Sofia, Geology and Geophysics Corp., CGMR, one”, with more and reoriented structures. 54 p. (in Bulgarian). 110.