Historical Constructions, P.B. Lourenço, P. Roca (Eds.), Guimarães, 2001 913 Neotectonic, geodynamic and ecological problems in the region of the “Madara horseman” monument, Madara plateau (NE Bulgaria) Dora Angelova Geological Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria ABSTRACT: The region of the “Madara horseman” (an engraved into the rocks relief dating back to the early Middle Ages, which is under the protection of UNESCO) is threatened by the enhanced contemporary rock-fall/landslide phenomena, strong erosion and destruction of engineering equipment in its vicinity. The region is characterized by high potential seismicity and an incomplete geodynamic process. The problem with the protection of environment is of crucial importance, since the material history of Bulgaria is preserved at this place starting from primitive men (from the stone-Copper Age – 4000 BC) till now. The protection of the unique “Madara horseman” monument and the more than 200 monk cells found in the rocks and dating back to the late Middle Ages, depends to a great extent on the good understanding of the geodynamic development of the area. 1 INTRODUCTION The investigated region falls within the northern part of the Madara plateau (Fig. 1 A). It is an indivisible part of the Provadia plateau. A relief dating back to the early Middle Ages had been cut into the rocks (Fig. 2). This monument is under the protection of UNESCO and belongs to the world cultural heritage. Other important monuments of primitive man culture had been discovered and preserved in the region (from the Stone-Copper Age – 4000 BC). The problem with their protection is an especially up-to-date one. The contemporary morphodynamic processes (gravitation, weathering, erosion and others) have been rather enhanced during the recent years. This has brought to the forth the necessity of evaluating the global, regional and local ecological hazard. Considerable number of investigations and activities on the stabilization of the rock massif has been performed. The Madara plateau is built of alternating thick-layered, with middle- to coarse grain-size and gravel calciferous sandstone, sandy limestone, detritus limestone, small-piece conglomerates of the Cenomanian-Lower Turonian age, belonging to the Madara Sandy-limestone Formation (Yolkichev, 1988). The described lithological complex lies with an erosion boundary on the Barremian clayey marls of the Gorna Oryahovitsa Formation (Nikolov, 1969). The established thickness of the Madara Sandy-limestone Formation varies in a wide interval (from 2-3 to 60- 110 m) within the strata type range. Thick deluvial and rock-fall/landslide Quaternary deposits and formations, covered by loess unconsolidated materials, are observed in the foot of the plateau, their thickness being considerable - from 60 to 140 m. The considered region falls within the Fore-Balkan morphostructural zone (Boncev et al., 1957) and the Madara plateau is regarded in the range of the northern wing of the Provadia syncline. The average slopes of the layers are 5-7° and very seldom - 10° in south-southeast direction. Tzankov (1930) established shallow dome-like folds in the Upper-Cretaceous sediments and a number of faults, the biggest one being to the southeast of the Kyulevcha village (a length of 2.5 km, direction of 90° and amplitude of the southern block subsidence of about 100 m). 914 Historical Constructions Hadjiev et al. (1977) established several faults with sub-parallel, diagonal and oblique direction according to the layer surfaces in the region of the “Madara horseman” monument. The main fissuration system has a north-south direction. Figure 1 : A. situation map of the region of the “Madara horseman” monument: 1 – rock slopes and single gravitation blocks; 2 – the “Madara horseman” monument; 3 – fortress walls and architectural complexes and monuments from Roman, old-Bulgarian and Meieval culture; 4 – gravitation caves and precipice caves; 5 – paleoseismic dislocation; 6-7 – tourist routes: 6 – stairs, 7 – alleys; 8 – horizontals; 9 – park. B. A model of the Madara plateau formation (according to the sample model of Montenat et al., 1991): a – prior to the Quaternary; b – beginning of the Quaternary; c – Middle Pleistocene; d – contemporary stage. The established outlevelling resulted mainly from the brittle deformations caused by a complex of factors: the subsequent tectogenesis during the Quaternary and the plastic lithogenic deformations expressed in the formation of a river-valley network, the structural edge, domes, banks, steps. As a result, the Madara plateau was disintegrated in single small blocks of different dynamics. The most important ones are situated in the northern most part of the massif (Kyulevcha, Madara and Kaspichan blocks), since the “Madara horseman” monument is engraved there on a scale, marking the Quaternary tectonic development of the slope. It is entirely separated from the “relatively” stable massif of the plateau by two parallel to the rock massif D. Angelova 915 wide cracks, opened in their upper part. The first crack is pinching and is filled with clay, while the second one is wider (0.5-0.8 m) and is partially filled with blocks which play the role of wedges and make the dynamic state (equilibrium) of the block more complicated (Angelova, 1995). The small gravitation “Malka” cave is situated in the lower part of this crack. It represents a primitive dwelling with cultural remains from the Stone-Copper Age - 4000 BC, (Popov, 1934). Figure 2 : The famous relief image (the oldest rock relief) of the Madara horseman was cut into the torn by the earthquake rock plate during the 9th century. The historic monument is under the protection of UNESCO and is included in the World Cultural Heritage list. Remains from primitive men, Romans, old-Bulgarians and the first Bulgarian Kingdom are found in this place. It is also a natural geological- geomorphological phenomenon. 2 NEOTECTONIC AND GEODYNAMIC PROBLEMS In a geodynamic aspect the investigated region is situated in the contact zone between two lithosphere plates and is characterized by the specificity of their development. The passive continental periphery with its sub-aqueous part of the Euxino-Caspian basin was situated here in the beginning of the Miocene. The tectonic movements were fluctuating with small amplitude, confirmed by the coastal line changes. After the manifestations of the Athian tectonic phase the formation of the Provadia plateau as a positive morphostructural unit was started at the boundary of the accumulative and destructive part of the Danubian Plain and the Stara Planina mountain. The formation of the vast initial denudation flattening began. The subsequent stage consisted in the general and differentiated vertical rising at the Pliocene-Quaternary boundary, including the initial erosion incision and rearrangement of the Provadiiska and Kamchia rivers. The horizontal compressive movements were at about 145º. The structural edges of the plateau were formed and the separation of the Madara block from the vast Provadia plateau and the valley pediments with thick alluvial-proluvial sediments exceeding 18 m, took place. During the Lower Pleistocene the region was subjected to total rising and deepening of the river network. The plateau was formed as a result of these movements and the erosion along the fault structures. The total tectonic activity was from 200 to 310 m. 916 Historical Constructions The initial surface was broken apart under the influence of the initial tectonic impulse and the formation of the foot of the plateau was started. This surface was separated in the northern board of the plateau by the young valley of the Madara river and by landslide/rock-fall processes with a characteristic step in the slopes in the direction towards the interior of the plateau due to the action of the erosion and subsequent tectonic slow and impulse movements. During the Middle Pleistocene the region in the vicinity of the monument and the area to the north and northeast of the Kyulevcha and Madara villages were subjected to two impulse movements correlating in time with the formation of the third and fourth over-flood terraces, causing the formation of the two landslide terraces. The “Madara” paleoseismic dislocations were formed as a result of the earthquake events, their known length being 1.0-1.5 km in northwest-southeast direction. Both dislocations were of the seismic-gravitation type. Open cracks originated in the core massif and the shifted torn and slid masses to the north and north-west reached a height of up to 1 km from the initial 100-140 m in the rare part of the slope, where the seismogenic fault was situated. The width of the opened due to the earthquake events crack, marking the already torn scale from the time of its formation till now, reached 0.80 m in its upper part and was more than 1.50 m in the lower part where the “Malka” cave was located. The crack exhibits a trend to increasing under the influence of gravitation. According to the classifications this phenomenon could take place when Io = 10-11, M ³ 7-8 and the depth of the focus was from 2 to 10 km (Fig. 3 and Fig. 4). Figure 3 : The “Madara” paleoseismic dislocation. The Holocene also started with numerous differentiated vertical and horizontal movements which led to different rising and rolling of the structural edges, banks, deformation of the built relief forms and hence to changes in the slope inclination and transition to a new dynamic state of the slope process, accompanied by rock-falls, unsticking, adjustment of the river-valley network along the fault structures, block disintegration and enhancement of the weathering processes along the strongly developed fissure network. The region of the “Madara horseman” monument was very mobile in a neotectonic aspect. It fell within the second (Madara) block with maximal values of vertical rising of 250-300 m/km2 (Fig. 5), i.e.