Sedimentary and tectonic evolution of Lake Ohrid (Macedonia/Albania)

Lake Ohrid, located on the Balkan Peninsula within the Dinaride-Albanide-Hellenide mountain belt, is a tectonically active graben within the South Balkan Extensional Regime (SBER). Interpretation of multichannel seismic cross sections and bathymetric data reveals that Lake Ohrid formed during two main phases of deformation: (1) a transtensional phase which opened a pull-apart basin, and (2) an extensional phase which led to the present geometry of Lake Ohrid. After the initial opening, a symmetrical graben formed during the Late Miocene, bounded by major normal faults on each side in a pull-apart type basin. The early-stage geometry of the basin has a typical rhomboidal shape restricted by two sets of major normal faults. Thick undisturbed sediments are present today at the site where the acoustic basement is deepest, illustrating that Lake Ohrid is a potential target for drilling a long and continuous sediment core for studying environmental changes within the Mediterranean region. Neotectonic activity since the Pliocene takes place along the roughly N-S-striking Eastern and Western Major Boundary Normal Faults that are partly exposed at the present lake floor. The tectono-sedimentary structure of the basin is divided into three main seismic units overlying the acoustic basement associated with fluvial deposits and lacustrine sediments. A seismic facies analysis reveals a prominent cyclic pattern of high- and low-amplitude reflectors. We correlate this facies cyclicity with vegetation changes within the surrounding area that are associated with glacial/interglacial cycles. A clear correlation is possible back to ca. 450kyrs. Extrapolation of average sedimentation rates for the above mentioned period results in age estimate of ca. 2Myrs for the oldest sediments in Lake Ohrid.