The Maliac Ocean: the origin of the Tethyan Hellenic ophiolites

International audienceThe Hellenides, part of the Alpine orogeny in Greece, are rich in ophiolitic units. These ophiolites and associated units emplaced during Jurassic obduction, testify for the existence of one, or several, Tethyan oceanic realms. The paleogeography of these oceanic areas has not been precisely described. However, all the authors now agree on the presence of a main Triassic-Jurassic ocean on the eastern side of the Pelagonian zone (Vardar Domain). We consider that this Maliac Ocean is the most important ocean in Greece and Albania. Here, we limit the detailed description of the Maliac Ocean to the pre-convergence period of approximately 70 Ma between the Middle Triassic rifting to the Middle Jurassic convergence period. A quick overview on the destiny of the different parts of the Maliac Ocean during the convergence period is also proposed. The studied exposures allow to reconstruct: (1) the Middle to Late Triassic Maliac oceanic lithosphere, corresponding to the early spreading activity at a Mid-Oceanic Ridge; (2) the Western Maliac Margin, widely exposed in the Othris and Argolis areas; (3) the Eastern-Maliac Margin in the eastern Vardar domain (Peonias and Paikon zones). We established the following main characteristics of the Maliac Ocean: (1) the Middle Triassic rifting marked by a rapid subsidence and volcanism seems to be short-lived (few My); (2) the Maliac Lithosphere is only represented by Middle to Late Triassic units, especially the Fourka unit, composed of WPB-OIB and MORB pillow-lavas, locally covered by a pelagic Middle Triassic to Middle Jurassic sedimentary cover; (3) the Western Margin is the most complete and our data allow to distinguish a proximal and a deeper distal margin; (4) the evolution of the Eastern Margin (Peonias and Paikon series) is similar to that of the W-Margin, except for its Jurassic terrigenous sediments, while the proximal W-Margin was dominated by calcarenites; (5) we show that the W- and E-margins are not Volcanic Passive Margins; and (6) during the Middle Jurassic convergence period, the Eastern Margin became an active margin and both margins were affected by obduction processes

Volcanic Lake Sediments as Sensitive Archives of Climate and Environmental Change

In efforts to understand the natural variability of the Earth climate system and the potential for future climate and environmental (e.g., biodiversity) changes, palaeodata play a key role by extending the baseline of environmental and climatic observations. Lake sediments, and particularly sediment archives of volcanic lakes, help to decipher natural climate variability at seasonal to millennial scales, and help identifying causal mechanisms. Their importance includes their potential to provide precise and accurate inter-archive correlations (e.g., based on tephrochronology) and to record cyclicity and high frequency climate signals. We present a few examples of commonly used techniques and proxy-records to investigate past climatic variability and its influence to the history of the lakes and of their biota. This paper is rather a presentation of potentials and limits of palaeolimnological and limnogeological research on crater lakes, than a pervasive review of palaeolimnological studies on crater lakes. We show the importance of seismic stratigraphy for the selection of coring sites, and discuss problems in core chronology. Then we give examples of physical and chemical proxies, including magnetism, micro-facies and oxygen and carbon stable isotopes from crater lake deposits mainly located in central and southern Europe. Finally, we present the use of air-transported (pollens) and lacustrine biological remains. The continuing need to develop new approaches and methods stimulated us to mention, as an example, the potential of the studies of subsurface biosphere, and the effects of microbiological metabolism on mineral diagenesis in sediments