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

    Southwest Bulgaria is characterized by a complex tectonic structure that represents a result of superposition of several tectonic cycles.

Fig. 1. Sketch map for the pre-Palaeogene tectonic units in Sw Bulgaria
(Zagorchev, 1995)

    The superunits distinguished are often characterized by different  sequences of pre-Alpine formations.
In some of the superunits, the pre-Alpine section is covered by Triassic (or Upper Permian) and younger sequences whereas in others the Alpine part of the section begins directly with Palaeogene or Neogene formations.
    The amphibolite-facies metamorphic complexes in the region studied are referred to the Precambrian (Kozhoukharov et al., 1988). Two supergroups have been distinguished, with an assumed primary discordant position of the younger, Rhodopian Supergroup over the older, Ograzhdenian (or Prerhodopian) Supergroup.
    The Ograzhdenian Supergroup is built up mostly of biotite or two-mica migmatites and gneisses as well as of amphibolites (mostly metabasic rocks) and serpentinites (metaultrabasites). It is a polyphase (polymetamorphic and polydeformational) complex intruded by younger (Palaeozoic and Mesozoic) igneous rocks.
    The Rhodopian Supergroup is widespread in the Rhodope Superunit but is absent in the other superunits due to deep Early Palaeozoic denudation. It consists of metasedimentary and metavolcano-sedimentary sequences subjected to amphibolite-facies regional metamorphism before the intrusion of Hercynian granitoids. Proterozoic (mostly Riphean) microphytofossils have been reported from the metasediments of the Rhodopian Supergroup almost in all tectonic units. Marbles are widespread, especially in the top parts (Dobrostan Formation) of the section.
    Diabase-phyllitoid formations (Frolosh Formation and equivalents) of supposed Vendian - Lower Palaeozoic age cover directly the Ograzhdenian basement in the Strouma Superunit. They consist of diabases and their tuffs, and lesser amounts of pelitic and psammitic rocks, all metamorphosed under greenschist-facies conditions. Their regional metamorphism is superimposed as diaphthoresis over the underlying amphibolite-facies rocks.
    The Palaeozoic section in the Morava Superunit consists of low-grade or very low-grade metasedimentary formations with palaeontologically proven Ordovician, Silurian and Devonian age. Palaeozoic rocks of the same age are present also in the basement of the Lyubash Unit.
    The Palaeozoic igneous rocks belong to several igneous complexes. The Strouma diorites (Strouma diorite "formation") occur mainly in the Strouma Superunit. They were formed after the main phase of greenschist-facies metamorphism of the diabase-phyllitoid complex (Frolosh Formation in the Strouma Superunit), and before the Permian (covered with inconformable depositional contact by the Permian and Triassic). In the Bosilegrad area, granitoids of the Strouma diorite formation are covered also with unconformable depositional contact by fossil-bearing Ordovician sandstones. An age of c. 540 Ma has been determined recently for the Strouma diorites.
    The sections of Rhodope type (Ograzhdenian and Rhodopian Supergroup in superposition) are intruded by granitoid plutons of another igneous complex ("Southbulgarian granitoids"). It  consists of three main phases  respectively at ca. 340 Ma BP (I-type granitoids) and 320-300 and 270-245 Ma (S-type granitoids) formed  during Hercynian collision.
    The pre-Alpine rock units have been usually subjected to Alpine deformations that produced superimposed planar and linear structures (when in greenschist-facies or amphibolite-facies conditions) or discrete fault (thrusts, strike-slip faults or normal faults) surfaces with associated fractures and minor faults. The degree of superimposed deformation differs considerably in the different tectonic units, and usually varies across the zones or their boundaries.

Principal Alpine events
    The principal post-Permian tectonic events have been dated in SW Bulgaria on the basis of biostratigraphic determination of the age of the youngest displaced (deformed) and the oldest sealing (undeformed) sedimentary formations.
    The Early Triassic sedimentation began with the deposition of mature quartz oligomictic conglomerates and sandstones. They sealed the late Hercynian structures, and developed transgressively into deposition of carbonate marine formations in the time interval Spathian - Norian, with a following regressive sedimentation with red bed deposition in conditions of extension and block movements from Carnian (locally) to late Norian times. The compressive event at the end of the Triassic and the beginning of the Jurassic formed mostly open folds, and locally, isoclinal folds and thrusts.
    The Jurassic transgression lasted from Pliensbachian to the Bajocian times, and the marine sedimentation gradually covered considerable areas north of the Poletintsi-Skrino fold and thrust zone. The sedimentation continued throughout the whole Jurassic, with formation of the Nish-Troyan flysch trough in the late Jurassic time. The flysch trough was superimposed discordantly over different depth zones of the former basin, and was situated between the uplifted area to the South (mainly the Rhodope Massif), and the carbonate platform, to the North. The following compressive events resulted in folding followed by differential uplift and denudation. During the next compressive episode, large-scale northeast-verging thrust sheets from the Morava Zone (Superunit) covered the folded and eroded Strouma Zone (Superunit). The episodes described occurred in post-Valanginian and pre-Turonian times. The two superunits were further folded together during a post-thrusting episode.
    Marker events are lacking in the Rhodope Superunit in the whole time interval between the Hercynian (Carboniferous - Permian) Southbulgarian granitoids and the Late Cretaceous and Palaeogene granitoids and Palaeocene - Oligocene sedimentary and volcano-sedimentary formations. Mid Cretaceous thrusts can be proven by the presence of intersecting Late Cretaceous granitoids. Late Cretaceous thrusts cannot be distinguished from the Mid Cretaceous thrusts in the cases when both are sealed by Palaeogene formations.
    The comparison between the four superunits and their thrust structures shows that the present pattern of WNW-ESE-elongated units is mostly due to the Late Cretaceous formation of the Srednogorie Zone (a volcanic island arc and back-arc through) and the thickened crust of the Morava-Rhodope Zone south of it, which both have been further reworked by late Cretaceous and later thrusting and normal faulting.

Pre-Palaeogene tectonic units
The pre-Palaeogene tectonic units have been formed or reworked and thrusted in different times. As already mentioned, most of the bounding thrusts were formed in Mid Cretaceous or Late Cretaceous times. The upper age limit is indicated by their sealing by Palaeogene (Palaeocene - Middle Eocene or Upper Eocene - Lower Oligocene) sedimentary and volcano-sedimentary formations, or cross-cutting by Late Cretaceous (about 85 Ma) granitoids or Palaeogene (c. 34 Ma) granitoids and subvolcanic dacites to andesites.
    The similarities between some of the pre-Palaeogene sections of the tectonic units show that they belong to four (Morava, Strouma, Rhodope and Srednogorie) superunits. The affiliation of the Ograzhden Unit is not yet certain because of similarities with the sections of the Morava, Strouma and Rhodope superunits.
    The Srednogorie Superunit is situated mostly to the north of the region studied. In Late Cretaceous times, it participated in a volcanic island arc with thinned crust, volcanic activity with several different trends, and intrusive activity ranging from gabbros through monzonite to granodiorite, the magma being of upper mantle or lower depleted crust origin. In its southern margin, Late Cretaceous sedimentary sequences devoid of volcanic products are present in the Lyubash, Melovete and Golo-bardo units. The pre-Late Cretaceous sections of the units differ both in the composition of the pre-Alpine basement, and in the depth of the pre-Late Cretaceous erosion and the features of the Jurassic formations present.
    The area to the south of the Srednogorie Superunit represented in Late Cretaceous times a Morava-Rhodope Zone with generally thickened crust formed by accreted (in the broadest sense of the word) units in Mid Cretaceous time. It consists of the allochthonous Morava Superunit and Ograzhden Unit, and the parautochthonous to autochthonous Strouma and Rhodope Superunits.
    The Morava Superunit is built up of Palaeozoic and pre-Palaeozoic formations. They were uplifted and deeply eroded in Late Jurassic to Late Cretaceous times, during north-eastward subduction of the Vardar ocean beneath the thickened crust of the Morava-Rhodope Zone, and the European continent.
    The Ograzhden Unit consists of migmatites of the Ograzhdenian Supergroup, deeply eroded and covered unconformably by the Vendian - Lower Palaeozoic Frolosh Diabase-Phyllitoid Formation, and the Palaeozoic volcano-sedimentary Kadiytsa Formation.
 The Strouma Superunit  is characterized by a deeply eroded pre-Permian basement, covered by Upper Permian and Triassic formations. The pre-Permian basement consists of Ograzhdenian migmatites unconformably covered by the diabase-phyllitoid Frolosh Formation. The following sequence of Alpine events can be outlined: 1) Early Triassic (Spathian) transgression over deeply eroded pre-Permian basement and Permian red beds; 2) Deposition of carbonate formations in a shallow sea during Spathian to Norian times; 3) Late Triassic (Norian - Rhaetian) uplift and erosion of horsts, with supply of Triassic carbonate pebbles into the residual basins with deposition of red beds; 4) Late Triassic gentle folding, with intense isoclinal folding and Southwest-vergent thrusting along the Poletintsi-Skrino thrust-fold zone; 5) Late Triassic and Early Jurassic uplift of the region, with continuous uplift of the southern, Osogovo-Vlahina Unit during the whole Jurassic and Cretaceous, and Early to Middle Jurassic transgression in the northern, Louzhnitsa-Tran Unit; 6) Coverage of the Louzhnitsa-Tran Unit by different bathymetric zones of the Jurassic basin, with a gradual deepening of the basin from the Bajocian to the Oxfordian, and oblique superposition of the Nish-Troyan flysch trough; 7) Beginning subduction at the north-eastern margin of the Vardar ocean, with uplift of the Morava Superunit, in the Late Jurassic; 8) Folding of the Strouma Superunit (and namely, of the Luzhnitsa-Tran Unit) in Early Cretaceous time (post-Valanginian); 9) uplift and erosion; 10) Mid Cretaceous Northeast-vergent thrusting of the Morava Superunit (zone) over the Strouma Superunit; 11) Late Cretaceous(?) refolding of the whole assemblage (Strouma Superunit + overthrusted Morava Superunit); 12) Late Cretaceous(?) thrusting (or re-thrusting) of some parts of the superunit; 13) uplift and erosion; 14) sealing of the pre-Palaeogene structure by Palaeogene (Upper Eocene - Lower Oligocene and Middle Oligocene - Lower Miocene) sedimentary and volcano-sedimentary formations during differential subsidence (grabens) and uplift (horsts).
    The Rhodope Superunit is built up of the rocks of the Ograzhdenian ("Prerhodopian") Supergroup covered by the Rhodopian Supergroup. They are intruded by the Hercynian Southbulgarian granitoids. The correlations between the sections in the units show that the groups and formations of the Rhodopian Supergroup are fully correlatable all over the Rhodope Superunit. Although the boundaries between the units are usually thrust surfaces, the relative displacements reach few kilometres, as shown by the displacements of older markers (e.g., contacts of the Hercynian granitoid plutons). The following sequence of Alpine events can be outlined:
  1) Mid Cretaceous folding and formation of superimposed schistosity and intersection lineation, mostly in amphibolite-facies (in the deeper parts) and greenschist-facies conditions, during the coeval thrusting of the Ograzhden Unit over the Pirin Unit;
  2) Late Cretaceous intrusion of granitoids or monzonites;
  3) Late Cretaceous thrusting, mostly in greenschist-facies conditions;
  4) Late Cretaceous uplift and erosion;
  5) sealing of the pre-Palaeogene structures by Palaeogene (Palaeocene - Middle Eocene, and Upper Eocene - Lower Oligocene) sedimentary and volcano-sedimentary formations, with thrusting in late Middle Eocene, and Late Oligocene - Early Miocene times.
The boundaries between the tectonic units are usually thrust surfaces or complex tectonic zones. One of the most complicated example is the Poletintsi-Skrino fold-thrust zone. The first movements along this zone are expressed as southwest-verging isoclinal folds with numerous imbrications within the Permian and Triassic formations and their basement (Strouma diorites). These movements occurred most probably in Late Triassic time. After that, the southern (Osogovo-Vlahina) Unit has been exhumed, and was a dry land during the whole Jurassic. In Mid Cretaceous time, the whole structure suffered northeast-verging thrusting (the major thrusting episode of the Morava Superunit over the Strouma Superunit), and the new thrusts (Skrino Thrust) deformed the older southwest-verging fold and thrust structures. In Late Cretaceous times, the complex nappe structure (Morava thrust sheets over the Strouma Superunit, with the internal thrusts within the latter) has been deformed by dome folding. Later on, new southwest-verging thrusting affected the complex structure of the Poletintsi thrust and the underlying Dragovishtitsa thrust (footwall horse), the southwest vergence being proven by observations on shear-sense indicators in the footwall. The older thrust sheets have been transported in piggy-back fashion. The last movements recorded are again northeast-verging thrusts that affect the Upper Eocene - Lower Oligocene marine deposits (which sealed the whole pre-dating structure, northeast-verging Late Triassic thrust surfaces and related folds).
 The thrusts of the Morava Superunit over the Struma Superunit are of Mid- Cretaceous age. They affect the whole post-Valanginian fold structure of the Struma Superunit. The complicated thrust pattern includes the Morava thrust sheets with several internal thrusts with imbrications (leading imbricate fans) and second-order folds, and "rabotage thrusts" (footwall horses) formed as trailing imbricate fans and further reworked by continuous translation, duplexes (often using anisotropies of bedding, bedding-parallel lamination, and cleavage), detachment surfaces in most competent layers, in inverted limbs of tight to isoclinal folds, or as trailing imbricate fans. Sectors of some of the thrust surfaces have been intruded (Fig. 6) by subvolcanic sill-like bodies of trachyandesite to trachydacite. In some cases, thrust surfaces have been reactivated as normal faults in Palaeogene times.
    Some of the thrust surfaces in the Rhodope Superunit have been obliterated by Alpine (Late Cretaceous and/or Palaeogene) granitoid bodies. Thus, the boundary between the West-Rila and Mousala Units has been obliterated by young granitoids of Late Cretaceous, or even of Palaeogene age.
    The Palaeogene and early Neogene tectonics, although predominantly extensional, formed also some thrust structures (or reactivated older ones) in short compressional phases, or in result of gravity phenomena.
    The complex assemblage of pre-Palaeogene tectonic units and superunits, and Palaeogene and Neogene block structures (grabens and horsts, often complicated by thrusts and intrusive bodies and lava flows) is situated obliquely and discordantly to the pre-Late Cretaceous sedimentation environments and structures.

Fig. 2. Sketch map for the Palaeogene and Neogene geology of SW Bulgaria (Zagorchev, 1995)

References
Boyanov, I., Dabovski, C., Gochev, P., Kostadinov, V., Tzankov, Tz., Zagorchev, I. 1989. A new view on the Alpine evolution of Bulgaria. - Geologica Rhodopica, 1; 107-121.
Kozhoukharov, D., Kozhoukharova, E., Papanikolaou, D. 1988. Precambrian in the Rhodope Massif. - In: Zoubek, V. (ed.). Precambrian in Younger Fold Belts. Wiley & Sons, Chichester; 723-778.
Zagorcev, I. S. 1992. Neotectonic development of the Struma (Kraistid) Lineament, southwest Bulgaria and northern Greece. - Geological Magazine, 129, 2; 197-222.
Zagorchev, I. 1995. Pre-Palaeogene Alpine tectonics in Southwestern Bulgaria. - Geologica Balcanica, 25, 5-6; 91-112.
Zagorchev, I. 1998. Rhodope controversies. - Episodes, 21, 3; 159-168.

    The field trip in SW Bulgaria aims to demonstrate most of the outstanding features of the geology of this part of Bulgaria. Samples of some outcrops may be seen in the PHOTOGALLERIES linked to the corresponding pages.