| Popis: |
Lake Ohrid is located at the border between the Republic of Macedonia and the Republic of Albania and is situated in an active tectonic region of the Balkanides. Several phases of deformation affected the area since the Tertiary which is today mainly controlled by the influence of the Northern Hellenic Trench and the North Anatolian Fault Zone. This results in zonation into a compressional coastal domain, separated by a narrow zone of transition to the extensional domain further east in which the Neogene basins formed. The Lake Ohrid Basin forms one of the most active seismic zones in Albania/Macedonia, which is documented by several moderate earthquakes in the last few centuries and a few major earthquakes during historical times. This seismic activity has created a seismic landscape with a variety of morphological features that are preserved in the surroundings. Earth- quake focal mechanisms show active N-S normal faulting with horst and graben structures in a basin and range like environment. This study provides insight into landscape formation and the landforming processes that the Ohrid Basin has been exposed to since the Quaternary. A multidisciplinary approach with a combination of different geoscientific methods (e.g. structural and palaeostress analysis, evaluation of the historical and instrumental seismicity, shallow geophysics, remote sensing) was chosen. According to this a workflow is proposed for neotectonic studies in the Mediterranean and areas with similar climatic conditions. The main objectives were: to determine the palaeostress fields which controlled the evolution of the area; to gain information on fault orientation, fault geometry and spatial distribution of fault scarps; to determine the grade of activity in the Lake Ohrid Basin; and to define active/less active areas experiencing tectonic deformation. The palaeostress analysis revealed major shifts in the geodynamic setting, which can be described by three successive periods of basin development: (1) an orogenic phase with (i) NE-SW compression in Cretaceous-Paleogene, (ii) NE-SW extension in Late Eocene- Oligocene, and (iii) NE-SW shortening and strike slip movement in Oligocene-Miocene; (2) a transtensional phase with NW-SE extension and strike-slip movement in Mid-Miocene; and (3) E-W extension from Late Miocene to present. Sedimentological studies with data from shallow drillings and geophysical investigations provided insight into the Holocene coastal evolution of the lake and allowed two main geomorphological systems to be determined. The plains north and south of the lake are dominated by clastic input related to climate variations and uplift/erosion. In contrast, the steep western and eastern margins are controlled by recent tectonics and normal faulting. Geomorphological studies show that the lithological zonation of the basin causes an inhomogeneous morphological surface expression within the influence of the same stressfield. The main trend of the normal faults is N-S and therefore a graben structure has formed. The average geometry of the fault scarps measured in the field is an upper slope that dips at 22°, beneath this the scarp’s free face dip angle ranges between 42° and 85° and a lower slope that matches the upper slope angle at 22° in most cases. The mean constructed fault height is 39.5 m for the west coast, 29.3 m for the east coast, and 17.6 m for the north. Fault lengths vary between 10 and 20 km and are expected to have the potential for earthquakes between M 6.5 - 7.0. The values of calculated slip rates inferring a post-glacial development of the exposed fault scarps range between 0.28 and 3.25 mm/a for the last 18 ka. These values far exceed well defined slip rates in comparable conditions. This leads to the concept of “stop and go” faults where the older outer faults slip every time a younger fault evolves. This therefore creates a higher relief. The fault scarps are also influenced by gravitational forces which cause the highly fractured rocks to react to seismic events, or large offshore mass wasting processes, and slip downslope. Mainly, the west coast is dominated by mass wasting processes, while the east coast is highly segmented with tilted blocks of basement. This study has illustrated that by combining different fieldwork techniques our knowledge of the seismicity and tectonic evolution of an area can be significantly advanced. |
