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

Classical extensional structures are moderately steep (45-60^o) normal faults formed by a stress field with subvertical maximum principal compression and subhorizontal maximum principal extension. Fault surfaces may be of planar or bended shapes. The kinematic mechanism may be irrotational or rotational (Wernicke, Burchfiel, 1982). During continuous rotational deformation, fault planes rotate and the angle of deep decreases to less than 45^o. Low-angle normal faults may be formed also as bended listric faults, or through transformation (new movements in a different stress field) of previously existing low-angle compressional structures (thrusts, imbrications). In cases of considerable rheological contrast between crystalline basement and sedimentary cover, shear stresses concentrate along ductile levels, along the contrast boundary or at low angles to them. Thus, interformational or/and intra-formational glides and shear zones may be formed, and in extensional environments, they resemble low-angle normal faults. When of considerable dimensions, they are known as decollements or detachments. Movements along detachments may exhume metamorphic core complexes (Lister, Davies, 1989).

Fig. 1. Schematic presentation of the rift and exhumation models

The regional and global tectonic processes that control extensional environments on the continents are mostly: the extensional collapse of orogens (Dewey, 1988), movements along detachments with exhumation of metamorphic core complexes, rifting (Fig. 1), and transtensional movements with formation of pull-apart basins. In all cases mentioned, low-angle normal faults and/or interformational and intraformational shear zones and glides can be formed.

Fig. 2. Simplified tectonic map of the Pirin horst and Sandanski graben (Zagorchev, 1995)

First data about the presence of low-angle normal faults in Bulgaria have been found and published in 1965-1971 (publications cited by Zagorchev, 1992). In the central and southern parts of the Neogene Sandanski graben (Zagorchev, 1992), low-angle fault planes from the West Pirin fault zone (Fig. 2) vary in dip from 55 to 20^o towards south-west, with a gradual flattening to the south. At the village of Lyubovka, the master fault splits in two low-angle faults: the Melnik fault that displaces the whole Neogene section of the graben filling, and the Spanchevo fault (Fig. 3) that separates the mylonitized metamorphics of the Rhodopian Supergroup (Pirin horst) from the Pontian - Romanian Kalimantsi Formation. Both faults are truncated and sealed by the Eopleistocene peneplain and its cover gravels.

Dinter, Royden (1993) and Socoutis et al. (1994) tried to explain the low-angle normal faults of the West Pirin fault zone as a continuation of a huge low-angle regional Strimon detachment. In their interpretation, a Rhodope metamorphic core complex - within the Pirin-Pangaion unit had been subject of Palaeogene regional metamorphism, and was exhumed in Middle Miocene times. These ideas contradict the well-known evidence as, e.g.,
 1) the Rhodopian Supergroup in the Pirin-Pangaion unit contains Precambrian acritarchs, and is cross-cut by granitoids not only of Palaeogene but also, of Palaeozoic and Late Cretaceous age;
 2) marbles of the Rhodopian Supergroup are covered by non-metamorphic pre-Priabonian Palaeogene sedimentary rocks;
 3) low-angle normal faults of the West Pirin fault zone intersect and displace the whole Neogene (Badenian to Romanian inclusive) section of the  Sandanski graben, i.e., the last movements along the low-angle faults are dated as latest Pliocene;
  4) Neogene deposits of similar facies and age crop out at both sides of the Pirin horst thus pointing at almost identical environments in the Sandanski and Gotse Delchev grabens that have been controlled by the uplift velocity of the horst during the whole Neogene.

    The formation of decollements and detachments at or near the contacts between basement and cover is a well-known mechanism, and these structures have much in common with inter- and intraformational shears and glides. However, the formation and evolution of such structures with considerable to enormous scale and displacements that could lead to uplift and exhumation of metamorphic rocks from the lower parts of the Earths crust, proved to be mechanically possible only in cases of extremely rare coincidence of different geodynamic factors (Westaway, 1999). All the evidence published shows that such large-scale detachments accompanied by exhumation of metamorphic core complexes have not been proven neither in Bulgaria nor on the Balkan Peninsula. Palaeogene and Neogene sedimentation processes have been controlled (Fig. 4) by the extensional collapse of the Late Cretaceous (Srednogorie) and Palaeogene (Balkanides) orogens and of their plateau with thickened continental crust (Rhodope massif) as well as by isostatic vertical movements, etc. In Neogene times, these processes have been accompanied or/and replaced by rifting of neotectonic swells and in transtensional conditions
(along the Strouma and Maritsa lineaments), opening of small pull-apart basins, as well as the development of listric fault systems (Tzankov et al., 1996).

Dewey, J. 1988. Extensional collapse of orogens. - Tectonics, 7, 6; 1123-1139.
Dinter, D., Royden, L. 1993. Late Cenozoic extension in north-eastern Greece: Strymon valley detachment system and Rhodope metamorphic core complex. - Geology, 21; 45-48.
Lister, G., Davis, G. 1989. The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region. _ J. Struct. Geol., 11; 65-94.
Shipkova, K., Ivanov, Z. 2000. The Djerman detachment fault - an effect of the Late Tertiary extension in the North-west part of the Rhodope Massif. C.-r. Acad. Bulg. Sci., 53, 2.
Socoutis, D., Brun, J.-P., Van den Driessche, J., Pavlides, S. 1994. A major Oligo-Miocene detachment in southern Rhodope controlling North Aegean extension. J. Geol. Soc., London, 150; 243-246.
Tzankov, Tz., Angelova, D., Nakov, R., Burchfiel, B.C., Royden, L.H. 1996. The Sub-Balkan graben system of central Bulgaria. - Basin Research, 8; 125-142.
Wernicke, B., Burchfiel, B. 1982. Modes of extensional tectonics. J. Struct. Geol., 4, 2; 105-115.
Westaway, R. 1999. The mechanical feasibility of low-angle normal faults. Tectonophysics, 308; 407-443.
Zagorchev, I. 1992. Neotectonic development of the Struma (Kraistid) Lineament, southwest Bulgaria and northern Greece. - Geological Magazine, 129, 2; 197-222.
Zagorchev, I., Goranov, A., Vulkov, V., Boyanov, I. 2000. Palaeogene sediments in the Padala graben, northwestern Rila Mountain, Bulgaria. C.-r. Acad. Bulg. Sci., 53, 6

   The field trip during the International Conference will demonstrate some of the crucial outcrops along the Strouma valley and within the Pirin horst.
We intend to demonstrate: - the site of the Kroupnik Earthquake (4.04.1904; M = 7.8; more than 1300 foreshocks and aftershocks within the period 1901 - 1911) at the outcrops of the Kroupnik normal fault, and the Kroupnik Monitoring Site (for measuring the contemporary horizontal and vertical movements along the fault surface, and movements of fault-induced landslides within the Neogene filling of the Simitli graben);
- the neotectonic panorama of the West-Pirin fault zone, with demonstration of the Neogene filling of the Sandanski graben, correlation of sediments and planation surfaces, etc.;
- polymetamorphic rocks of the Ograzhdenian Supergroup of the Ograzhden tectonic unit;
- polymetamorphic rocks of the Rhodopian Supergroup, foliated Palaeozoic granitoids, and Late Cretaceous and Palaeogene granitoids (Fig. 2). in the Pirin tectonic unit;
- outcrops of the Gorno-Spanchovo low-angle normal fault (Fig. 3) of the West-Pirin fault zone;
- coarse pre-Priabonian Palaeogene breccia and conglomerate of the Paril Formation over the marbles of the Precambrian Rhodopian Supergroup; and many other geological features.