Rhodope Geodynamic Hazards,
Late Alpine tectonics and Neotectonics
International Conference - May 2001

Tectonics of the Pirin horst

    The Pirin horst is one of the most spectacular geological structures in Bulgaria. It is a neotectonic (Neogene - Quaternary) block structure that has inherited elements of Precambrian, Palaeozoic, late Mesozoic and Palaeogene structures (Fig. 1).

Figure 1. Simplified tectonic map of the Pirin horst and the surrounding structures
(Zagorchev, 1994, 1995)

Comparison of the sections of the Rhodopian Supergroup in the type area (Central Rhodope),
Pirin and Pangaion (Zagorchev, 1994, 1995) (Fig. 2).

Fig. 2
be - Belashtitsa Formation (biotite schists, calcareous schists, etc.) = C (Menikio Schists); do - Dobrostan Marble Formation = D (Bos Dag Marbles); lu - Loukovitsa Formation (schists, marbles, amphibolites, basic metavolcanics and metatuffs, etc.) = E (Nympheas "Series"); ba - Bachkovo Formation (quartzo-feldspathic gneisses); bo - Boykovo Formation (banded leucocratic biotite and two-mica gneisses); vc - Vucha Formation (varied alternation of gneisses, schists, amphibolites, marbles, etc.); F - Nevrokopi "Series" (probably equivalent to ba and bo); bg - Bogutevo Formation (biotite and two-mica gneisses and migmatites with amphibolite rootless bodies); ce - Chepelare Formation (varied alternation of gneisses, schists, amphibolites, marbles, etc.);

    In the pre-Late Cretaceous structure, most of the Pirin Mountain and the Slavyanka and Kroupnik Mountains formed a distinct Pirin Unit. It is characterized with the following features:
    - wide occurrence of Precambrian amphibolite-facies metamorphics, correlatable with the Rhodopian Supergroup (Fig. 2);
    - complicated polydeformational (see PHOTOGALLERY) synmetamorphic and pre-granitic fold structure (Fig. 3);
    - intrusion of the Palaeozoic Spanchevo granite pluton that underwent later superimposed deformations (schistosity, lineations) ans was deformed by the Belyovo conical synform;
    - thrusting of the Ograzhden over the Pirin unit in Mid Cretaceous times along the Strimon thrust;
    - intrusion of the Late Cretaceous Dautov (North Pirin) and Bezbog granite plutons; the North Pirin pluton intruded also the allochthon of the Ograzhden unit;
    - prolonged uplift of the Pirin Mts., erosion, denudation and karstification in earliest Palaeogene times.
    The Palaeozoic granitoids belong to the group of the Southbulgarian granitoids determined as Carboniferous in the Sredna-gora block. In the Pirin unit, they are represented by the Spanchevo, Kroupnik and Dobrinishte plutons. Often, they are affected by a superimposed schistosity, displaced by the Mid Cretaceous thrusting, and are later intruded by Alpine granite bodies.

Fig. 3. Schematic bloc-diagramme for the pre-granitic structure of the northern parts of the Pirin unit (Zagorchev, 1994, 1995)

    The Strimon thrust (Ograzhden, vs. Serbo-Macedonian unit over Pirin-Pangaion unit) has been first described by Kockel and Walther in 1965. It is covered with depositional contact by the sediments of the Neogene Sandanski graben. North of the latter, the Mid-Cretaceous Alikochov and Breznitsa thrusts are the prolongations of the Strimon thrust. They have a NE vergency, cross-cut the granitoids of the Kroupnik pluton, and are cross-cut by the granitoids of the Late Cretaceous Daoutov (Kresna) pluton. An offspring of the latter (the small Breznitsa pluton) is entirely intruded into the Ograzhdenian Supergroup of the allochthon.
    At the end of Cretaceous times, the Pirin unit was characterized by extremely thickened continental crust. Increased heat flow generated through anatexis acid magmas of two generations: the Late Cretaceous and the Palaeogene (Late Eocene - Early Oligocene) ones. In the southern parts of the Pirin unit, active faulting in Palaeocene? - Middle Eocene times resulted in the formation of more than 1500 m thick coarse (breccia, calcarenites, limestone, sandstone) sedimentary rocks (Paril Formation). They are intruded by the Palaeogene Teshovo granite pluton. The Teshovo, Central-Pirin, Goleshevo and Lehovo
granite plutons (Bulgaria) and Vrondou pliton (Greece) are the deep correlates of the volcanic massifs situated within and coeval to the Priabonian - Lower Oligocene sedimentary formations along the Mesta valley.
    Problems of the neotectonic structure are described in the webpage Neogene low-angle normal faults in the Sandanski graben.

Harkovska, A. 1983. Spatial and temporal relations between volcanic activity and sedimentation in the stratified Palaeogene from the central parts of the Mesta graben (SW Bulgaria). - Geologica Balcanica, 13, 2; 3-30.
Zagorcev, I. 1994. Structure and tectonic evolution of the Pirin-Pangaion Structural Zone (Rhodope Massif, southern Bulgaria and northern Greece). - Geological Journal, 29; 241-268.
Zagorchev, I. 1995. Pre-Paleogene Alpine tectonics in Southwestern Bulgaria. - Geologica Balcanica, 25, 5-6; 91-112.
Zagorchev, I. 1995. Pirin. Geological Guidebook. Academic Publishing House _Marin Drinov_, Sofia; 70 pp.
Zagorchev, I. 1998. Pre-Priabonian Palaeogene formations in Southwestern Bulgaria and Northern Greece: stratigraphy and tectonic implications. - Geological Magazine, 135, 1, 101-119.
Zagorchev, I. 1998. Rhodope controversies. - Episodes, 21, 3; 159-168.
Zagorchev, I., Moorbath, S., Lilov, P.1987. Radiogeochronological data on the Alpine magmatism in the western parts of the Rhodope massif. - Geologica Balcanica, 17, 2; 59-71 (in Russian).

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