New laser ablation-inductive coupled plasma-mass spectrometry U-Pb analyses on oscillatory-zoned zircon imply Early Miocene crystallization (18.64 ± 0.11 Ma) of the Pohorje pluton at the southeastern margin of the Eastern Alps (northern slovenia). Inherited zircon cores indicate two crustal sources: a late Variscan magmatic population (~270-290 Ma), and an early Neoproterozoic one (850-900 Ma) with juvenile Hf isotope composition close to that of depleted mantle. Initial εHf of Miocene zircon points to an additional, more juvenile source component of the Miocene magma, which could be either a juvenile Phanerozoic crust or the Miocene mantle. the new U-Pb isotope age of the Pohorje pluton seriously questions its attribution to the Oligocene age 'Periadriatic' intrusions. the new data imply a temporal coincidence with 19-15 Ma magmatism in the Pannonian basin system, more specifically in the styrian basin. K-Ar mineral-and whole rock ages from the pluton itself and cogenetic shallow intrusive dacitic rocks (~18-16 Ma), as well as zircon fission track data (17.7-15.6 Ma), gave late Early to early Middle Miocene ages, indicating rapid cooling of the pluton within about 3 Million years. Medium-grade Austroalpine metamorphics north and south of the pluton were reheated and subsequently cooled together. Outcrop-and micro scale structures record deformation of the Pohorje pluton and few related mafic and dacitic dykes under greenschist facies conditions. Part of the solidstate fabrics indicate E-W oriented stretching and vertical thinning, while steeply dipping foliation and NW-sE trending lineation are also present. the E-W oriented lineation is parallel to the direction of subsequent brittle extension, which resulted in normal faulting and tilting of the earlier ductile fabric at around the Early / Middle Miocene boundary; normal faulting was combined with strike-slip faulting. renewed N-s compression may be related to late Miocene to Quaternary dextral faulting in the area. the documented syn-cooling extensional structures and part of the strike-slip faults can be interpreted as being related to lateral extrusion of the Eastern Alps and/or to back-arc rifting in the Pannonian basin.

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UvodPotreba po razumevanju narave in njenih zakonitosti, med katere spadata tudi dinamika in kinematika težnostnih pobo~nih premikov, je v zadnjem ~asu vse ve~ja. Študij le-teh je namenjen predvsem zavarovanju prebivalcev in lastnine pred katastrofalnimi pobo~nimi dogodki. Neposredna povezava med spreminjajo~o se klimo in pobo~nimi procesi se odraža v pogostej{ih naravnih nesre~ah, kar nakazuje potrebo po ve~ji previdnosti pri poseganju ~loveka v ob~utljivo naravno okolje.Drobirski tokovi so le ena oblika gravitacijskih pobo~nih premikanj. V svetu so pogost pojav in nemalokrat povzro~ajo tudi nesre~e s ~love{kimi žrtvami (NAKAGAWA et al., 2001 Izvle~ekNa vr{aju Koro{ka Bela smo v dveh raziskovalnih izkopih opravili popise sedimentov in litolo{ke analize klastov z namenom {tudija geneze vr{aja in ugotavljanja izvora materiala. Vr{aj sestavlja ve~ zaporednih plasti diamiktov in sorodnih subaerskih sedimentov, ki so posledica masnih tokov, v nekaterih primerih zagotovo drobirskih tokov. Material v glavnem izvira iz karbonskih in permskih klasti~nih in deloma karbonatnih kamnin, ki so na primarnih mestih tektonsko poru{ene. V diamiktnih plasteh se pojavljajo tudi prodniki drugih kamnin iz zaledja vr{aja, ki so jih drobirski tokovi na svoji poti erodirali. Potok Bela s svojim zaledjem tudi v prihodnje predstavlja potencialno nevarnost za nastanek drobirskih tokov. AbstractThe Koro{ka Bela alluvial fan deposits were investigated to determine the genesis of the fan and the source area of sediments. The alluvial fan is composed of a sequence of diamicton layers, and related subaeric sediments that were deposited by multiple mass flow events, in some cases certainly by debris flows. The predominant sources of sediments are tectonically deformed clastic and partly carbonate Carbonifferous and Permian rocks. In diamictons also pebbles of other rocks from the hinterland are present. These were eroded from the channel of Bela during the mass flow events. We estimate the future debris flow hazard along Bela stream as high.V okviru ciljnega raziskovalnega projekta »Ocena ogroženosti zaradi delovanja drobirskih tokov« so bili na obmo~ju Karavank v letih 2006 in 2007 raziskani sedimenti {tirih vr{ajev ter njihova zaledja, med njimi tudi vr{aj potoka Bela, na katerem smo izkopali dva raziskovalna izkopa. Sedimenti v izkopih so bili podrobno sedimentolo{ko popisani, vzorci sedimentov litolo{ko analizirani, zaledje potoka Bela pa je bilo geolo{ko pregledano.Ugotovili smo, da vr{aj potoka Bela kot sedimentno telo ni samo posledica kontinuirane fluvialne sedimentacije, pa~ pa tudi in predvsem posledica nenadnih katastrofi~nih tokov pobo~nih sedimentov. Izvor materiala za posamezne gravitacijske dogodke je vezan predvsem na mesta z manj odpornimi kamninami oziroma na mesta, kjer so kamnine tektonsko poru{ene. Heterogena geolo{ka zgradba zaledja dopu{~a tudi možnost razli~nih izvornih obmo~ij. Sedimenti vr{aja so lahko tudi presedimentirani, kar v primeru potoka Bele pomeni, da je izvorni material ledeni-

S U M M A R YThe Periadriatic lineament is a 700-km-long, first-order tectonic boundary in the Alps, along which several Tertiary intrusions are located that were emplaced while tectonic processes were active. The Pohorje pluton is one of the Periadriatic magmatic suites and comprises a pluton and a dacite stock that were emplaced at depths of about 10 and 2 km, respectively. Oriented samples were collected from 33 sites in the magmatic, and three in the surrounding metamorphic rocks and subjected them to standard palaeomagnetic and magnetic susceptibility anisotropy processing. There is extreme variation in susceptibility (although the magnetic mineral are always magnetite) and in its anisotropy within the pluton. Lowest values are in the mafic members (in the gabbro, the oldest and in some lamprophyres, the youngest member of the plutonic suite), while the tonalite (granodiorite) exhibits extreme variation, far exceeding those observed for other Periadriatic plutons. The 'abnormality' of the Pohorje pluton is attributed to the complex history that involves assimilation, fractional crystallization, stratification in the magma chamber, incorporation of wall rocks, variable degree of K-metasomatism and hydrothermal alteration. Despite the above-described heterogenities and sometimes very high degree of susceptibility anisotropy (much higher that in the metamorphic samples) in the acid member, the palaeomagnetic directions observed for the pluton define three clear-cut groups. The directions show no dependence on the degree of magnetic susceptibility anisotropy. In the order of crystallization, they exhibit decreasing degree of clockwise declination deviation from north. The dacite stock is characterized by counter-clockwise rotated declinations. As the Pohorje pluton was uplifted very fast within the right-lateral Periadriatic shear zone system, it is interpreted that the palaeomagnetic directions observed for the pluton are evidence for a clockwise rotation during exhumation. The palaeomagnetic directions for the dacite, which formed after the climax of the external forces, fits the regional pattern of counter-clockwise rotation observed in the Neogene sediments around the Pohorje pluton. This shows that by the time of the dacite intrusion, the Pohorje pluton was already incorporated into the 'block' that was driven by counter-clockwise rotated Adriatic microplate.

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