Late Alpine tectonics and Neotectonics
International Conference - May 2001
Fig. 1. Tectonic map for the pre-Palaeogene structure of the Krayshte area (Zagorchev, 1996)
The geological structure of the Krayshte area exhibits superposition of structural elements of different ages interwoven in a
very complicated jigsaw puzzle. Elements of Precambrian and Palaeozoic age are included in Alpine structural units. The principal compression
phase occurred in Mid Cretaceous times when due to continent collision the Morava and Strouma superunits have been folded and thrusted. New intense
folding and thrusting occurred in Late Cretaceous times as an imprint of similar processes in the Srednogorie volcanic island arc.
The Late Cretaceous complex arc of the Srednogorie zone was bordered to the South by the Morava-Rhodope (Macedonian-Rhodope) region (superunit) of thickened continental crust that consisted mostly of two Mid-Cretaceous superunits: the autochthonous to parautochthonous Strouma Superunit and the allochthonous Morava Superunit. The Morava Superunit consists of several thrust sheets (units) each of them containing a different set of pre-Alpine formations.
Fig. 2. Schematic section through the pre-Palaeogene tectonic units in SW Bulgaria and schematic columnar sections for the principal tectonic units (Zagorchev, 1996). Color legend for cross section based on tectonic units and is different from the color legend (based on standard chronostratigraphic legend) of columnar sections.
Pre-Alpine type sections. The comparison of
the pre-Alpine (and usually, pre-Permian) basement in the Alpine tectonic
units reveals the presence of several section types. Their succinct characteristics
are as follows:
1) Rhodope type: superpositional position of the Precambrian Rhodopian Supergroup above the Ograzhdenian (Prerhodopian) Supergroup; both are intruded by Precambrian and Hercynian granitoid plutons;
2) Vlahina type: transgressive covering of the Ograzhdenian Supergroup by the Vendian - Early Palaeozoic Frolosh Formation; the latter is built up of metadiabases and tuffs with few metaterrigenous rocks that have suffered greenschist-facies metamorphism, and have been intruded by early Palaeozoic gabbro, diorite, granodiorite and plagiogranite: Strouma diorite formation, Lyutskan igneous complex;
3) West-Balkan type: Vendian? ocean-floor ophiolites covered by early Palaeozoic Berkovitsa and Dulgidel Groups that represented island-arc associations;
4) Kouchay type: diabase-phyllitoid complex of Vendian? - Early Palaeozoic age, covered by a full marine sequence of Ordovician, Silurian and Devonian rocks, ending with the Upper Devonian - Lower Carboniferous flysch; very low-grade to greenschist-facies metamorphism; intruding pre-Permian basic and granitoid rocks;
5) Vlasina type: amphibolite-facies metamorphic rocks covered by metapsammites, metapelites, marbles and metaphosphorites of Ordovician age;
6) Eleshnitsa type: Precambrian? amphibolite-facies metamorphic rocks are covered by metadiabases, metatuffs and metaterrigeneous rocks of Vendian - Early Palaeozoic age, and by graphite-bearing micaschists, quartzous schists, quartzites and limestones with recently proven Silurian to Early Devonian age.
These different pre-Alpine sections occur now in different Alpine tectonic units. Alpine thrusts bounding the Alpine tectonic units separate pre-Alpine sections that have belonged to different Palaeozoic sedimentation zones situated at considerable distances one from the other. The present mutual position may be a result of plate motions and accretion of different terranes. Possible horizontal movements could be evaluated by provenance data for pebbles in the Alpine conglomerates.
The characteristics of the pre-Palaeogene tectonic units distinguished in Southwest Bulgaria and the adjacent areas of East Serbia may be summarized as follows:
Srednogorie Superunit. The basement of the western parts of the Srednogorie Superunit is of the Kouchay type, and south-east of Pernik and Sofia, of the Vlahina type. If the two types belonged to different Palaeozoic terranes, their accretion had occurred before Permian time as far as Permian and Lower Triassic terrigenous deposits sealed these sections and their contacts. The central parts of the Srednogorie Superunit (Timok Unit; Viskyar-Vitosha synclinorium) are characterized by normal Triassic and Jurassic sections of Balkanide (peri-Tethyan) types (mostly continental and shallow-water marine deposits) covered unconformably by Turonian continental and marine deposits, and sealed by the Senonian volcano- sedimentary complex. Within the southern margin of the Srednogorie Superunit, volcanic rocks are absent in the Senonian sedimentary formations. The pre-Upper Cretaceous sections in the units defined (Kouchay-Svrlig = Ljubash; Golo-burdo; Sichevachka; Suva-planina) possess significant differences. Thus, Lower to Middle Jurassic transgressive formations cover in the Ljubash Unit a deeply eroded (down to the Permian or Lower Triassic) section whereas in the Golo-burdo Unit they seal the Upper Triassic red beds of the Moesian Group. Upper Cretaceous sedimentary formations have an insignificant presence in these two units whereas they are widespread in the Sicevacka and Melovete units. Lower Cretaceous formations of Urgonian facies are typical of the Suva-planina Unit being almost absent in the other units of the southern Srednogorie margin.
Strouma Superunit. The Strouma Superunit is subdivided into two units. The northern, Luznnitsa-Trun Unit consists of a pre-Alpine section of Vlahina type, locally covered by Permian red beds. The Alpine section is built of Triassic sedimentary formations of Balkanide (peri-Tethyan) type unconformably covered by Jurassic sedimentary formations. Two sedimentation zones are distinguished: the eastern (Svetlya) sedimentation zone with peri-Tethyan signature, and relics (sealed by thrusts of the Morava Superunit) of a western (Treklyano) sedimentation zone with abundant deep-marine shales and radiolarites. Both sections are covered unconformably by the Tithonian - Middle Berriasian flysch of the Nish-Trojan flysch trough. The boundary between the northern and the southern unit runs along the Poletintsi-Skrino fold-thrust zone that consists of highly strained structures of polydeformational character and different kinematics in the phases recognized. The first thrusting occurred in Late Triassic times and was characterized with a southern vergency. Afterwards, the southern (Osogovo-Vlahina) unit has been uplifted, and has not been covered anymore by Mesozoic sedimentation basins. Along the Poletintsi-Skrino fold-thrust zone, intense deformations have occurred partly in conditions of increased temperature, and some structures contain Permian and Triassic sedimentary formations metamorphosed in greenschist-facies conditions. The Osogovo-Vlahina Unit has also a basement of Vlahina type, covered in the Bosilegrad area (East Serbia) by low-grade Ordovician sandstones. The basement has been deeply eroded and covered by Permian and Triassic sedimentary formations.
Morava Superunit. All units of this superunit (Figure 1) are now devoid of Alpine cover, and are distinguished on the basis of their different pre-Alpine types sections. The last thrusting occurred in Mid-Cretaceous time. However, no evidence exists about the time of eventual thrusting and/or accretion of different terranes or subterranes in late Palaeozoic times.
The comparison of the Alpine sections in the different tectonic units is of utmost importance for the recognition of eventual Alpine terranes accreted during the Alpine orogenies. The principal compression events occurred in late Triassic, Mid-Cretaceous and Late Cretaceous times, and the last compression event recorded had place in earliest Neogene time.
At the end of Cretaceous and beginning of Palaeogene times, the whole area has been deeply eroded again. After weathering (partially, in lateritic conditions), the area was subject in Late Eocene times of normal faulting and formation of lacustrine graben basement interrelated by river valleys. A marine ingression in latest Eocene - earliest Oligocene times almost coincided with the peak of intermediate to acid volcanic activity. A regression, with formation of coal-forming lacustrine basins occurred in Late Oligocene times, and was followed by intense folding and thrusting in earliest Miocene times. The problems of the Palaeogene and Neogene evolution are considered in a special web page.
Zagorchev, I. 1994. Alpine thrust and fault tectonics in South Bulgaria.
- Bull. Geol. Soc. Greece, 30, 2; 487-496.
Zagorchev, I. 1995. Pre-Paleogene Alpine tectonics in Southwestern Bulgaria. - Geologica Balcanica, 25, 5-6; 91-112.
Zagorchev, I. 1996. Pre-Palaeogene Alpine tectonic units and terranes in the border area of SW Bulgaria and Yugoslavia.
- in: Knezevic, V., Krstic, B., eds. Terranes of Serbia. Belgrade; 81-86.
Zagorchev, I. 1998. Rhodope controversies. - Episodes, 21, 3; 159-168.
Zagorchev, I. 1998. Pre-Triassic sections and units in West Bulgaria. - in: Newsletter No 6,IGCP Project 276,
Geol. Soc. of Greece, Athens, Special Publication No 3; 42-53.
During the first day of the field trip we will cross a part of the Golo-burdo Unit of the Srednogorie Superunit, will compare different Jurassic facies in the Louzhnitsa-Trun Unit of the Strouma Superunit, and will observe some of the most prominent Mid Cretaceous thrusts of the Morava over the Strouma superunit, and some thrusts of the Poletintsi-Skrino fold-thrust zone with Permian and Triassic low-grade to greenschist-facies rocks.