Deep seismic profile in the Rhodopes (S. Bulgaria) and its … · 2017-12-11 · 80 • Clearly...
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79 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 Khrischev 1 , Stefan Shanov 1 , Stefka Pristavova 2 , Yotzo Yanev 1 Хрисчо Хрисчев 1 , Стефaн Шанов 1 , Стефкa Приставова 2 , Йоцо Янев 1 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- filing in the Rhodopes (Velev, 1996) is geologically reinterpreted after complete processing and trans- formation of time scale to metric one (Khrischev, Shanov, 2017). Thus, it is closer to the real struc- tural pattern defining its conceptual significance. The profile covers a small part of the Central Rhodopes (east of Ardino), mainly crossing the Eastern Rhodopes in East-West direction (Fig. 1). The two tectonic units are highly distinguished by their structure and by the nature of the Earth’s crust – continental or sub-oceanic, respectively, which are clearly separated in the seismic record by the degree of stratification and reflection of boundaries. Both units are related by obduction (Kozhuharova, 1984; Velev, 1996). Interpretation. The Central Rhodopes, with a wide development of the granite-gneisses, have sig- nificant seismic transparency, marking the granitiza- tion (mainly Hercynian according to the radioisotope data – Geological Map of Bulgaria, scale 1:50 000 and references therein). In the eastern direction, reflection surfaces become more precise and expressive, indi- cating the probable framework of granite protoliths. Their convergence and wedging to the east suggests belonging to a continental margin. In the Eastern Rhodopes, the Earth’s crust beneath the Paleogene cover is dominated by metamorphic rocks and has a complex internal structure. Almost across its 20 km thickness above the Lower Crust, it shows a distinct stratification at a high degree of re- flection and spatial sustained reflection slabs, and in some places with disharmony features plus internal wedging, sometimes resembling progradational struc- tures or the effect of oblique shearing zones. These features, as well as the rocks in the surface outcrops, testify to the sub-oceanic character of the crust. Four plates are distinguished based on the patterns of seis- mic reflections, as well as inner “discordances” and surfaces of shearing, plus one upper plate with more local distribution. Surface outcrops, and the resulting dating of them have 4 plates (1st, 3rd, 4th, and 5th). The first three of them are represented by high-grade metamorphites with radioisotopic ages of the protolites from Late Proterozoic to Late Paleozoic: 1st plate, Arda mel- ange; 3rd plate, Byala Reka and Startsevo litho-tec- tonic units; 4th plate, Krumovitsa litho-tectonic unit (Geological Map of Bulgaria, scale 1:50 000). The top plate is of anhimetamorphic volcanogenic sedimen- tary rocks of Triassic–Jurassic age. The plates 1, 2 and 3 have a community in their configuration, representing a common obduction phe- nomenon. The lowest of them has the largest thickness (up to 7 km) and a complex internal structure with du- rable sub-horizontal shear zones. Plate 4th is extreme- ly different in position (horizontal, spatially sustained and expressively discontinuous), thickness (several times less, ~1 km) and specific seismic record, testi- fying to a complex rock structure (undefined reflec- tion sites). This is a Later Alpine thrusting structure with a South thrusting direction (Geological Map of Bulgaria, scale 1:50 000). Other features identified by the seismic record are: • The nature of the obduction boundary – oblique, in a small stretch steep (here with diffraction on both sides), displaced by a vertical fault zone. • Seismic transparency of the lower crust with un- clear boundary with the upper, layered crust. • Reflection of Mohorovicic boundary with an el- evation of 32 km in the central part up to 29 km in the eastern one.
Transcript of Deep seismic profile in the Rhodopes (S. Bulgaria) and its … · 2017-12-11 · 80 • Clearly...
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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 значениеKhrischo 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] 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-filing in the Rhodopes (Velev, 1996) is geologically reinterpreted after complete processing and trans-formation of time scale to metric one (Khrischev, Shanov, 2017). Thus, it is closer to the real struc-tural pattern defining its conceptual significance. The profile covers a small part of the Central Rhodopes (east of Ardino), mainly crossing the Eastern Rhodopes in East-West direction (Fig. 1). The two tectonic units are highly distinguished by their structure and by the nature of the Earth’s crust – continental or sub-oceanic, respectively, which are clearly separated in the seismic record by the degree of stratification and reflection of boundaries. Both units are related by obduction (Kozhuharova, 1984; Velev, 1996).
Interpretation. The Central Rhodopes, with a wide development of the granite-gneisses, have sig-nificant seismic transparency, marking the granitiza-tion (mainly Hercynian according to the radioisotope data – Geological Map of Bulgaria, scale 1:50 000 and references therein). In the eastern direction, reflection surfaces become more precise and expressive, indi-cating the probable framework of granite protoliths. Their convergence and wedging to the east suggests belonging to a continental margin.
In the Eastern Rhodopes, the Earth’s crust beneath the Paleogene cover is dominated by metamorphic rocks and has a complex internal structure. Almost across its 20 km thickness above the Lower Crust, it shows a distinct stratification at a high degree of re-flection and spatial sustained reflection slabs, and in some places with disharmony features plus internal wedging, sometimes resembling progradational struc-tures or the effect of oblique shearing zones. These features, as well as the rocks in the surface outcrops,
testify to the sub-oceanic character of the crust. Four plates are distinguished based on the patterns of seis-mic reflections, as well as inner “discordances” and surfaces of shearing, plus one upper plate with more local distribution.
Surface outcrops, and the resulting dating of them have 4 plates (1st, 3rd, 4th, and 5th). The first three of them are represented by high-grade metamorphites with radioisotopic ages of the protolites from Late Proterozoic to Late Paleozoic: 1st plate, Arda mel-ange; 3rd plate, Byala Reka and Startsevo litho-tec-tonic units; 4th plate, Krumovitsa litho-tectonic unit (Geological Map of Bulgaria, scale 1:50 000). The top plate is of anhimetamorphic volcanogenic sedimen-tary rocks of Triassic–Jurassic age.
The plates 1, 2 and 3 have a community in their configuration, representing a common obduction phe-nomenon. The lowest of them has the largest thickness (up to 7 km) and a complex internal structure with du-rable sub-horizontal shear zones. Plate 4th is extreme-ly different in position (horizontal, spatially sustained and expressively discontinuous), thickness (several times less, ~1 km) and specific seismic record, testi-fying to a complex rock structure (undefined reflec-tion sites). This is a Later Alpine thrusting structure with a South thrusting direction (Geological Map of Bulgaria, scale 1:50 000).
Other features identified by the seismic record are:• The nature of the obduction boundary – oblique,
in a small stretch steep (here with diffraction on both sides), displaced by a vertical fault zone.
• Seismic transparency of the lower crust with un-clear boundary with the upper, layered crust.
• Reflection of Mohorovicic boundary with an el-evation of 32 km in the central part up to 29 km in the eastern one.
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• Clearly shaped Momchilgrad Depression, filled with thin-layered Paleogene rocks up to 1.5 km thick.
• Intrusive bodies (Paleogene and Upper Creta-ceous), clearly outlined in the seismic record accord-ing to its transparency.
• A system of lateral Late Alpine sub-vertical and more oblique faults, indicating the extension of the Earth’s crust and associated Paleogene magmatism.
• The vertical fault zone with a width of 6 km, located in the area of the most expressive on the Bulgarian territory regional gradient of the gravity field, and obstructing the confident correlation of the western part of the profile with the superficial geo-logy, due to the wide development of diffractions.
Discussion. The data from the deep seismic pro-file confirm the conceptions that the Rhodopes are a complex structure of thrusting, lately submitted to an extension with an active magmatic manifestation. These data illustrate the obduction relations between the continental margin of the Central Rhodopes and the sub-ocean crust of the Eastern Rhodopes, with latter thrusting to the South. In the two of the well-known geodynamic hypotheses concerning Tethys re-gion possible explanations of the time and the events that caused this obduction can be found. According to the first one (Stampfli et al., 2002), the obduction could be associated with an inter-terranean collision in transcurrent equatorial movements after the accre-tion of the Peri-Gondwanan Hunic terrane (European segment), to the passive edge of Laurasia during the Variscan time. The second hypothesis, more probable,
is related to the Mesozoic development of Tethys (Stampfli, Hochard, 2009) – the complete closure of the Küre ocean provokes the collision of Sakaria-Strandzha with the Rhodopes in the western direction during Kimmeridgian. Thus, Gočev’s idea (in: Bokov et al., 1993) for the thrusting of the Eastern on the Central Rhodopes along Ardino zone as a part of the so-called Strandzhidi was revived.
ReferencesBokov, P., P. Gočev, R. Ognyanov. 1993. Tectonic position,
hydrocarbon exploration and future potential of Bulgaria. – Geologica Balc., 23, 3, 3–34.
Khristchev, Kh., S. Shanov. 2017. Geological interpretation of the regional deep seismic reflection profile in Eastern Rhodope Mountain (South Bulgaria). – In: Extended Abstracts of the 9th Congress of the Balkan Geophysical Society. Antalya, EAGE EarthDoc.
Kozhuharova, E. 1984. The origin and structural position of the serpentinized ultrabasites of the Precambrian ophiolite association in the Rhodope massif. І. Geological position and composition of the ophiolite association. – Geologica Balc., 14, 4, 9–36 (in Russian with an English abstract).
Stampfli, G. M., C. Hochard. 2009. Plate tectonics of the Alpine realm. – Geological Society London, Special Publications, 327, 89–111.
Stampfli, G. M., J. F. von Raumer, G. D. Borel. 2002. Paleozoic evolution of pre-Variscan terranes: From Gondwana to the Variscan collision. – Geological Society of America, Special Paper, 364, 263–280.
Velev, A. 1996. Deep seismic profiling of the Earth crust along the regional profile Ardino-Ivaylovgrad. – Bulg. Geophys. J., 22, 2, 91–107 (in Bulgarian).
Fig. 1. Geological interpretation of the deep seismic profile IR-c along the line Ardino-Popsko
