Field trip B26

Bulgarian part

Melnik Popovi livadi checkpoint Koulata/Promahon

Leader: Ivan Zagorchev

From Melnik the itinerary crosses the Neogene of the Sandanski graben,and enters in the Pirin horst the Precambrian metamorphics of the Rhodopian Supergroup. At the highest point of the pass (Popovi Livadi) granites of the Palaeogene Teshovo pluton crop out. A full section of the Rhodopian Supergroup is exposed along the road and is briefly described hereafter. The section is in inverted position(upside down), and from the Popovi livadi pass the itinerary follows downward in the succession (although upward in the geometric section).

Stop 5.1. Contact of the Palaeogene Teshovo pluton with the Precambrian Dobrostan Marble Formation
The Dobrostan Marble Formation consists of gray or white, massive or banded calcite marbles locally interlayered with gray fine-grained dolomite marbles, hornblende schists, biotite schists and quartzo-feldspathic gneisses. In the schists and impure marbles, a prominent mineral lineation parallel to synmetamorphic fold hinges is observed. In the road cutting, in very rare cases, the observations reveal that there are two generations of synmetamorphic mineral lineations and fold hinges. The earlier lineation and fold hinges are deformed (folded) and/or obliterated by the second fold generation. The second generation lineation and hinges are usually situated at a very small angle (5 - 10o) to the first generation, and very rarely,at high(50 - 80o) angles when the relations become obvious.The Palaeogene Teshovo granitoid pluton is built up of biotite or hornblende-biotite equigranular (locally with porphyric feldspar) fine- to medium-grained granites to granodiorites.The road cutting displays complex interrelations between the pluton and the marbles. The intrusive contacts often follow normal faults, and locally suggest a subsidence of the pluton host rocks into the magma chamber.Subparallel apophyses intrude the marbles along the foliation planes. The overall impression is that of extension conditions during the pluton intrusion.

Comparative sections of the Rhodopian Supergroup
(lithostratigraphic subdivision after Kozhoukharov,1984)

be-Belashtitsa Formation;do-Dobrostan Marble Formation; lu-Loukovitsa Varied Formation;ba-Bachkovo Leptitoid Formation; bo-Boykovo Formation; vc-Vucha Varied Formation; bg-Bogoutevo Formation;ce-Chepelare Varied Formation

The pre-Palaeogene structure of the Pirin unit has a complex and polydeformational character.The following sequence of events has been described (Zagorchev, 1994): (1) Neoproterozoic(?) deposition of a thick (8 10 km) sequence of flyschoid island-arc deposits with basic volcanism (Chepelare, Bogoutevo and Vucha Formation), a mostly psammitic sequence (Boykovo and Bachkovo Formation), another varied sequence of psammites,pelites, marls and limestones (Loukovitsa Formation) and thick limestones and dolomites (Dobrostan Formation); (2) intense folding in Cadomian(?) times with formation of two synmetamorphic fold generations; the first generation F1 consists of isoclinal folds, and the second one (F2), by tight to isoclinal folds; a single planar structure is observed, and it is a foliation of a probable multiphase development (composite foliation).The hinges of the isoclinal F1 folds and the mineral lineation L1 trend nearly N-S, and plunge at 5 20o South.

Mostly in the marbles of the Dobrostan Formation of the Orelek syncline, the foliation and the F1 hinges are deformed by F2 folds. The principal folds (Petrovo anticline and Orelek syncline) have been formed during the second phase. (3) Intrusion of the Spanchevo granitoid pluton (Hercynian?) approximately in the core of the Petrovo anticline;(4) Compression and formation of a superimposed schistosity in the Spanchevo granitoids, roughly parallel to the older composite foliation; this event probably coincided with the Mid-Cretaceous thrusting (Strymon thrust) of the Ograzhden over the Pirin unit; (5) Deformation of the older structures(composite foliation included) into the Belyovo conical fold; (6) Intrusion of the Late Cretaceous Dautov and Bezbog plutons during an extensional event coinciding with the extension within the Srednogorie zone North of the Morava-Rhodope superunit; (7) Extension and deposition of thick Palaeocene? Middle Eocene? terrigenous sediments in the southern parts and around the Pirin unit; (8) Folding;(9) New Palaeogene extension, deposition of thick volcanic and sedimentary deposits in depressions around the unit, and Late Eocene Early Oligocene intrusion of granitoids in the Pirin unit; (10) Late Oligocene deposition of coal-bearing sediments (Egerian fluviolacustrine system Pernik Bobovdol Brezhani); (11) Thrusting of the Pirin horst over the Brezhani graben related to the transpression along the Strouma fault belt; (12) Extension(after Early- to Late Miocene planation) and rifting with formation of the neotectonic horsts and grabens, and of the new Strouma/Strymon fluviolacustrine system.

Schematic tectonic map of the Pirin unit

Stop 5.6. Granitoids of the Spanchevo pluton
The Spanchevo granite pluton is intruded into the rocks of the Bogoutevo and Vucha Formation fromthe core of the Petrovo anticline. The southern part of the anticline has now an eastern to SE vergency.The whole structure, the Spanchevo granites and their planar structures included, has been deformed with the formation of the Belyovo conical fold.The Spanchevo granitoid pluton is built up of biotite and two-mica coarse-grained or porphyric granites to granodiorites. The heterogeneous internal structure is demonstrated by the frequent occurrence of schlieren and melanocratic enclaves. In the road cutting, the granites exhibit also a clear superimposed schistosity (strike 160o/35oSW;lineation 200o/22o) that intersects also the numerous pegmatite and aplite dykes within the granitoids. The pegmatites and aplites are often folded. The superimposed schistosity intersects the dykes, and is roughly parallel to the axial planes. Late normal faults strike 115-120o/60-65oNNE.

Stop 5.7. Gorno Spanchevo neotectonic fault
East of the village of Gorno-Spanchevo, the West-Pirin fault zone is represented by a low-angle (25 - 30o)normal fault that displaces the folded and mylonitized gneisses and amphibolites(with pegmatite veins) of the Vucha Formation (footwall) from the conglomerate of the Kalimantsi Formation (hanging wall).The displacement occurred in latest Pliocene or Eopleistocene time, since the Kalimantsi Formation belongs to the time interval from Pontian (or latest Maeotian) to Romanian.

The folded rocks of the Vucha Formation exhibit several fold generations(recumbent folds included), and a last compressive event with mylonites and east-vergent movement. This structure is intersected and displaced by numerous minor normal faults dipping South-West at angles from 25-30o(parallel to the master fault) to 60-70o.The master (Gorno-Spanchevo) fault is a normal fault with very narrow gauge along the fault surface.The Kalimantsi Formation is represented by the typical conglomerate built up predominantly of granite pebbles coming from the Palaeogene Central-Pirin and Teshovo plutons.Towards South and South-East, a large Eopleistocene pediment (mountain step) gently-dipping towards West is beautifully exposed. It truncates the metamorphic rocks of the Pirin horst, the tilted conglomerate beds of the Kalimantsi Formation and the bounding Gorno-Spanchovo low-angle fault, and is partly covered by proluvial and alluvial Eopleistocene sediments.

Author: Ivan Zagorchev

Geological Institute of the Bulgarian Academy of Sciences
Author: Ivan Zagorchev
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