Lindhorst, Katja (2012) Sedimentary and Neotectonic History of Lake Ohrid (Albania/Macedonia): Acquisition and interpretation of new hydro-acoustic and seismic data. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 187 pp.

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Abstract

Ancient Lake Ohrid an oligotrophic lake is probably the oldest existing lake in Europe. It is located on the Balkan Peninsula within the Dinaride-Albanide-Hellenide mountain belt, and is often referred to as a hotspot of endemic biodiversity. This study sheds light on the tectonic and sedimentary evolution of Lake Ohrid based on newly-acquired hydro-acoustic and seismic data sets. It testifies the importance of Lake Ohrid as a valuable archive susceptible to provide a continuous sediment record within the scope of the International Continental Drilling Program (ICDP) campaign scheduled for September/October 2012. Interpretation of multichannel seismic cross sections and bathymetric data reveals that Lake Ohrid formed in two main deformation phases: (1) a transtensional phase resulted in an opening of a pull-apart basin probably in Late Miocene; and (2) an extensional phase since the Pliocene to recent that led to the present geometry of Lake Ohrid. After initial opening during phase 1 a symmetrical graben formed, bounded by the Central Basin faults that acted as master faults The early stage geometry of the basin has a typical rhomboidal shape restricted by two sets of major normal faults, namely Lini and Kaneo (NE-SW), Piskupat and Pescani (NW-SE) Faults. The location of the basin initiation coincides with the greatest depth of the acoustic basement; nowadays a thick succession yof undisturbed sediments is found in this area. Neotectonic activity since the Pliocene takes place along the roughly N-S directed Eastern and Western Major Boundary Normal faults that are partly exposed at the floor of the lake. Numerous faults are present in the northern area offsetting syn-tectonic sediments, thus confirming the hypothesis that Lake Ohrid Basin is still experiencing extension. Furthermore, the active Gorenci sinistral strike slip Fault is most likely responsible for the opening of the Struga Graben detectable at the lake floor as a NE-SW directed elongated graben structure. Seismic stratigraphic interpretation revealed a regular depositional pattern of fluvial deposits overlying the pre-rift basement, the uppermost unit shows deep lacustrine sediments, which form the majority of the entire sedimentary infill; such a sedimentary succession is comparable to a tripartite sediment succession described for rift lakes indicating the different stages of basin evolution. A major outcome of this study is the establishment of a chrono-stratigraphic scheme developed for undisturbed lacustrine sediments back to an age of 430 kyr indicating that these sediments document glacial and interglacial cycles back to Marine Isotope Stage (MIS) 12. A refined calculation on the basis of our new geological data set revealed a limnological age of at least 2 Myr for Lake Ohrid. Stacked clinoforms indentified on seismic lines in the southern area indicate significantly lower lake levels prior to MIS 6. This period is also characterized by a progressive rise of water level with intermittent stillstands since its existence as water-filled body, which might have favored expansion of endemic species within Lake Ohrid. Significant lake level fluctuations with prominent lowstands of ca. 60 and 35 m below the present water level occurred during MIS 6 and 5, respectively. The effect of these lowstands on the biodiversity along most of the coasts of the lake is probably negligible, due to only small changes in lake surface area, coastline, and habitat. In contrast, the biodiversity in shallow-lacustrine areas was more severely affected due to disconnection of sublacustrine springs from the main water body. Other interesting features in Lake Ohrid are mass movement deposits that are widespread within the basin and have been mapped at different stratigraphic levels of the basin. A cluster of slide deposits around Magic Mountain (a prominent basement high in the central basin) indicates a relationship between mass movement events and the activation of faults that bound the intrabasinal high. In general slides are present adjacent to major fault structures such as the Lini fault, the Eastern and Western Major Boundary faults, and the Udenisht fault in the south suggesting that they are seismically triggered. The Udenisht slide complex in the southwestern part of the lake is by far the largest mass failure event within the basin. The slide deposits cover an area of ~27 km², are up to 50 m thick, and sum up to a volume of ~0.11 km³. First age estimations suggest that the Udenisht Slide is less than 1,500 years old. Although the volume of the Udenisht slide is well within the range of landslide volumes capable to trigger tsunamis, detailed analysis of our data suggests that no major tsunami was triggered. In contrast, subsurface and morphological indications for lake floor instabilities along the western margin outline potential rotational slumping with a high risk of tsunami generation in the future.

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