Deep Seismic Profile in the Rhodopes (S
Total Page：16
File Type：pdf, Size：1020Kb
БЪЛГАРСКО ГЕОЛОГИЧЕСКО ДРУЖЕСТВО, Национална конференция с международно участие „ГЕОНАУКИ 2017“ BULGARIAN GEOLOGICAL SOCIETY, National Conference with international participation “GEOSCIENCES 2017” Deep seismic profile in the Rhodopes (S. Bulgaria) and its conceptual significance Дълбочинен сеизмичен профил в Родопите (Ю. България) и неговото концептуално значение Khrischo Khrischev1, Stefan Shanov1, Stefka Pristavova2, Yotzo Yanev1 Хрисчо Хрисчев1, Стефaн Шанов1, Стефкa Приставова2, Йоцо Янев1 1 Bulgarian Geological Society; Acad. G. Bonchev str., bl. 24, 1113 Sofia; E-mails: [email protected]; [email protected] 2 University of Mining and Geology “St Ivan Rilski”, 1 Prof. B. Kamenov str., 1700 Sofia; E-mail: [email protected] Kеywords: deep seismic profile, оbduction, Rhodopes. The most informative profile from deep seismic pro- testify to the sub-oceanic character of the crust. Four filing in the Rhodopes (Velev, 1996) is geologically plates are distinguished based on the patterns of seis- reinterpreted after complete processing and trans- mic reflections, as well as inner “discordances” and formation of time scale to metric one (Khrischev, surfaces of shearing, plus one upper plate with more Shanov, 2017). Thus, it is closer to the real struc- local distribution. tural pattern defining its conceptual significance. Surface outcrops, and the resulting dating of them The profile covers a small part of the Central have 4 plates (1st, 3rd, 4th, and 5th). The first three Rhodopes (east of Ardino), mainly crossing the of them are represented by high-grade metamorphites Eastern Rhodopes in East-West direction (Fig. 1). with radioisotopic ages of the protolites from Late The two tectonic units are highly distinguished by Proterozoic to Late Paleozoic: 1st plate, Arda mel- their structure and by the nature of the Earth’s crust ange; 3rd plate, Byala Reka and Startsevo litho-tec- – continental or sub-oceanic, respectively, which tonic units; 4th plate, Krumovitsa litho-tectonic unit are clearly separated in the seismic record by the (Geological Map of Bulgaria, scale 1:50 000). The top degree of stratification and reflection of boundaries. plate is of anhimetamorphic volcanogenic sedimen- Both units are related by obduction (Kozhuharova, tary rocks of Triassic–Jurassic age. 1984; Velev, 1996). The plates 1, 2 and 3 have a community in their Interpretation. The Central Rhodopes, with a configuration, representing a common obduction phe- wide development of the granite-gneisses, have sig- nomenon. The lowest of them has the largest thickness nificant seismic transparency, marking the granitiza- (up to 7 km) and a complex internal structure with du- tion (mainly Hercynian according to the radioisotope rable sub-horizontal shear zones. Plate 4th is extreme- data – Geological Map of Bulgaria, scale 1:50 000 and ly different in position (horizontal, spatially sustained references therein). In the eastern direction, reflection and expressively discontinuous), thickness (several surfaces become more precise and expressive, indi- times less, ~1 km) and specific seismic record, testi- cating the probable framework of granite protoliths. fying to a complex rock structure (undefined reflec- Their convergence and wedging to the east suggests tion sites). This is a Later Alpine thrusting structure belonging to a continental margin. with a South thrusting direction (Geological Map of In the Eastern Rhodopes, the Earth’s crust beneath Bulgaria, scale 1:50 000). the Paleogene cover is dominated by metamorphic Other features identified by the seismic record are: rocks and has a complex internal structure. Almost • The nature of the obduction boundary – oblique, across its 20 km thickness above the Lower Crust, it in a small stretch steep (here with diffraction on both shows a distinct stratification at a high degree of re- sides), displaced by a vertical fault zone. flection and spatial sustained reflection slabs, and in • Seismic transparency of the lower crust with un- some places with disharmony features plus internal clear boundary with the upper, layered crust. wedging, sometimes resembling progradational struc- • Reflection of Mohorovicic boundary with an el- tures or the effect of oblique shearing zones. These evation of 32 km in the central part up to 29 km in the features, as well as the rocks in the surface outcrops, eastern one. 79 Fig. 1. Geological interpretation of the deep seismic profile IR-c along the line Ardino-Popsko • Clearly shaped Momchilgrad Depression, filled is related to the Mesozoic development of Tethys with thin-layered Paleogene rocks up to 1.5 km thick. (Stampfli, Hochard, 2009) – the complete closure • Intrusive bodies (Paleogene and Upper Creta- of the Küre ocean provokes the collision of Sakaria- ceous), clearly outlined in the seismic record accord- Strandzha with the Rhodopes in the western direction ing to its transparency. during Kimmeridgian. Thus, Gočev’s idea (in: Bokov • A system of lateral Late Alpine sub-vertical and et al., 1993) for the thrusting of the Eastern on the more oblique faults, indicating the extension of the Central Rhodopes along Ardino zone as a part of the Earth’s crust and associated Paleogene magmatism. so-called Strandzhidi was revived. • The vertical fault zone with a width of 6 km, located in the area of the most expressive on the References Bulgarian territory regional gradient of the gravity field, and obstructing the confident correlation of the Bokov, P., P. Gočev, R. Ognyanov. 1993. Tectonic position, western part of the profile with the superficial geo- hydrocarbon exploration and future potential of Bulgaria. – Geologica Balc., 23, 3, 3–34. logy, due to the wide development of diffractions. Khristchev, Kh., S. Shanov. 2017. Geological interpretation Discussion. The data from the deep seismic pro- of the regional deep seismic reflection profile in Eastern file confirm the conceptions that the Rhodopes are Rhodope Mountain (South Bulgaria). – In: Extended a complex structure of thrusting, lately submitted to Abstracts of the 9th Congress of the Balkan Geophysical an extension with an active magmatic manifestation. Society. Antalya, EAGE EarthDoc. These data illustrate the obduction relations between Kozhuharova, E. 1984. The origin and structural position of the continental margin of the Central Rhodopes and the serpentinized ultrabasites of the Precambrian ophiolite association in the Rhodope massif. І. Geological position the sub-ocean crust of the Eastern Rhodopes, with and composition of the ophiolite association. – Geologica latter thrusting to the South. In the two of the well- Balc., 14, 4, 9–36 (in Russian with an English abstract). known geodynamic hypotheses concerning Tethys re- Stampfli, G. M., C. Hochard. 2009. Plate tectonics of the Alpine gion possible explanations of the time and the events realm. – Geological Society London, Special Publications, that caused this obduction can be found. According 327, 89–111. to the first one (Stampfli et al., 2002), the obduction Stampfli, G. M., J. F. von Raumer, G. D. Borel. 2002. Paleozoic could be associated with an inter-terranean collision evolution of pre-Variscan terranes: From Gondwana to the Variscan collision. – Geological Society of America, in transcurrent equatorial movements after the accre- Special Paper, 364, 263–280. tion of the Peri-Gondwanan Hunic terrane (European Velev, A. 1996. Deep seismic profiling of the Earth crust along segment), to the passive edge of Laurasia during the the regional profile Ardino-Ivaylovgrad. – Bulg. Geophys. Variscan time. The second hypothesis, more probable, J., 22, 2, 91–107 (in Bulgarian). 80.