ELEMENTS OF THE GEOLOGIC STRUCTURE OF THE BELASITSA-OGRAZHDEN BLOCK ACCORDING TO GEOPHYSICAL DATA

SUMMARY In the territory of Bulgaria, to the Belasitsa-Ograzhden block as a part of the Serbian-Macedonian block, are related parts of the Belasitsa Mountains, the Ograzhden Mountains and the Malashevska Planina Mountains. The block is limited to the east by the Strouma fault system, to the north and northeast - by the KadiytsaBreznitsa fault zone, to the south - by the Middle-Mesta fault zone which controls the south edge of the Belasitsa horst in Greece. To the west the block continues in Macedonia, where it is bounded by the Sasa-Tоranitsa fault zone. A negative background characterizes the observed gravitational field of the territory under study. The Belasitsa-Ograzhden block is bounded by two very well pronounced gravity transitions. It is limited to the east by the Strouma gravity transition, tracing the western edge of the Strouma rift structure. This transition is striking S35oE and has a stable horizontal gradient of 8-10 mGal/km along its entire length of about 30 km. The block northern border is the Breznitsa gravity transition B-B, that is almost perpendicular to the Strouma one. The studied area gravitational field is characterised by a regional trend positive to the south, starting from the Breznitsa gravity transition towards the two opened to Greece anomalies. The presented field distribution rose-diagram (excluding the areas of the Strouma and the Breznitsa gravity transitions) shows three dominant directions : a subequatorial one, one striking N(45o-60o)E and one striking S35oE. These three directions correlate well with the concept of development of longitudinal and transverse fault deformations along the rift structures periphery. The local field characteristics are analyzed according to the residual fields derived from the regional transforms and according to the variation anomalies compiled from the center-point and ring method of Griffin using a circle of radius R = 5 km. The observed geomagnetic field of the studied territory is characterised by negative background values within the limits of -100 and -150 nT. On this background two anomalies can be separateded. A pronounced positive anomaly is very obvious in the northern part. A negative anomaly is present in the southwestern part of the map. It has an intensity of about -300 nT, and remains opened to the south. The subequatorial direction (90-110o) is predominating on the presented rose-diagram. The two other directions observed are one striking roughly S45oE and one striking roughly N45oE. The compound analysis and systematization of the studied geophysical fields and other characteristics is the reason for some basic conclusions. The Strouma gravity transition is pronounced without ambiguity in all regional and local gravitational fields. The transition characteristics in the regional maps, which are reflecting the depth interval 3-10 km, are showing that it dips west. In the local fields maps, this gradient zone is tracing the Ograzhden fault limiting the Strouma fault zone to the west The fault-block tectonics have predominant influence in the depth interval 3-10 km. The Lebnitsa, the Southern-Ograzhden, the Podgoritsa, the Kolarovo and the Petrich gravity transitions are forming the block division of the studied territory. The Northern-Igralishte and the Klyuch negative anomalies are mapping relatively deep granitoid plutons. The geophysical fields main characteristics in the depth interval down to about 3 km are mapping the near-surface disposition of plutons, reflected by the Northern-Igralishte, the Igralishte, the Nikoudin and the Klyuch negative anomalies. The areas having increased thickness of the metamorphites are separated by positive anomalies. The Stroumeshnitsa graben and especially the areas having increased thickness of the decompressed sedimentary rocks that are filling it are mapped very well by negative anomalies. In the studied near-surface interval, gradient transitions are tracing faults of two main systems - the Lebnitsa system and the Stroumeshnitsa one. The analysis of the main elements of the geologic-geophysical structure in the Belasitsa-Ograzhden block is showing that the studied block has been developing under the direct influence of the rift processes in the regional Strouma rift structure. These processes and their performance are reflected mainly in fault deformations transverse and oblique to the Strouma rift system. In the Bulgarian territory the Belasitsa-Ograzhden block has been developing under the direct influence of the rift processes in the regional Strouma rift structure. These processes and their performance are reflected mainly in fault deformations transverse and oblique to the Strouma rift system, which are successfully mapped by well-pronounced gravity transitions.