TY - THES

T1 - A contribution of Middle – Late Jurassic Mélanges in the Northern Calcareous Alps (Raucherschober/Schafkogel) and the Inner Western Carpathians (Jaklovce) to the reconstruction of the Triassic – Jurassic passive and active Neo-Tethys distal continental margin

AU - Drvoderic, Sebastian

N1 - embargoed until null

PY - 2021

Y1 - 2021

N2 - Mélange analysis can contribute fundamentally to solve open questions of the geodynamic history and paleogeographic reconstructions of mountain ranges. In this study, the reconstruction of the Middle Triassic – Middle Jurassic passive and Middle – early Late Jurassic active margin of the Western-Tethys realm is in focus. Two mélanges 1) the ophiolitic Meliata Mélange (Jaklovce) in the Inner Western Carpathians and 2) the Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel) are studied. This provides new insights to solve further questions about the timing and the tectonic structure of the obduction of the Neo-Tethys ophiolites. The re-investigation of a well-known ophiolite block with its sedimentary cover rocks (Steinmann-Trinity) in the ophiolitic mélange in the Jaklovce area (Meliata unit of the Inner Western Carpathians) sheds new light on the depositional area of these sediments. Volcanic ashes gradually turn into siliceous clays and up to more carbonatic sedimentary rocks. In cases total silicification of these rocks makes it even hard to obtain meaningful biostratigraphic data. Nevertheless, radiolarian ages from the Ladinian to the Carnian clearly defining an orientation of the oceanic sediments as an overturned block. Alteration of the basalt blocks clearly show recrystallisation of chlorite, actinolite and epidote. Excess of iron is compensated by the formation of magnetite in the process of ocean floor metamorphism. Further, a so far unknown Hallstatt mélange below the Late Jurassic Plassen Limestone, north of the Mürzalpen nappe and west of the Hengstpass (Northern Calcareous Alps) contains ophiolitic material of the former Neo-Tethys Ocean together with Late Triassic-Early Jurassic open-marine carbonates deriving from the outer shelf (Hallstatt Limestone facies belt). These blocks rest within a Middle to early Late Jurassic radiolaritic – argillaceous matrix. Therefore, the previous assumption of the investigated area, forming a Cretaceous thrust complex cannot be confirmed. Relics of ophiolitic material show calc-alkaline volcanic arc affinity, defining the rock as the product of an intra oceanic subduction and the formation of an early arc during stacking of the oceanic crust. In this case, a Middle Jurassic onset of ophiolite obduction is favoured instead of a Late Jurassic to Early Cretaceous one. Furthermore, the interpretation of an obducting ophiolitic nappe stack instead of a single ophiolite sheet is preferred. At a time of decreasing tectonic activity in the Late Jurassic, the mélange basin was sealed by the Plassen Limestone, showing a shallowing-upward trend.

AB - Mélange analysis can contribute fundamentally to solve open questions of the geodynamic history and paleogeographic reconstructions of mountain ranges. In this study, the reconstruction of the Middle Triassic – Middle Jurassic passive and Middle – early Late Jurassic active margin of the Western-Tethys realm is in focus. Two mélanges 1) the ophiolitic Meliata Mélange (Jaklovce) in the Inner Western Carpathians and 2) the Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel) are studied. This provides new insights to solve further questions about the timing and the tectonic structure of the obduction of the Neo-Tethys ophiolites. The re-investigation of a well-known ophiolite block with its sedimentary cover rocks (Steinmann-Trinity) in the ophiolitic mélange in the Jaklovce area (Meliata unit of the Inner Western Carpathians) sheds new light on the depositional area of these sediments. Volcanic ashes gradually turn into siliceous clays and up to more carbonatic sedimentary rocks. In cases total silicification of these rocks makes it even hard to obtain meaningful biostratigraphic data. Nevertheless, radiolarian ages from the Ladinian to the Carnian clearly defining an orientation of the oceanic sediments as an overturned block. Alteration of the basalt blocks clearly show recrystallisation of chlorite, actinolite and epidote. Excess of iron is compensated by the formation of magnetite in the process of ocean floor metamorphism. Further, a so far unknown Hallstatt mélange below the Late Jurassic Plassen Limestone, north of the Mürzalpen nappe and west of the Hengstpass (Northern Calcareous Alps) contains ophiolitic material of the former Neo-Tethys Ocean together with Late Triassic-Early Jurassic open-marine carbonates deriving from the outer shelf (Hallstatt Limestone facies belt). These blocks rest within a Middle to early Late Jurassic radiolaritic – argillaceous matrix. Therefore, the previous assumption of the investigated area, forming a Cretaceous thrust complex cannot be confirmed. Relics of ophiolitic material show calc-alkaline volcanic arc affinity, defining the rock as the product of an intra oceanic subduction and the formation of an early arc during stacking of the oceanic crust. In this case, a Middle Jurassic onset of ophiolite obduction is favoured instead of a Late Jurassic to Early Cretaceous one. Furthermore, the interpretation of an obducting ophiolitic nappe stack instead of a single ophiolite sheet is preferred. At a time of decreasing tectonic activity in the Late Jurassic, the mélange basin was sealed by the Plassen Limestone, showing a shallowing-upward trend.

KW - Mélange

KW - Neo-Tethys Ozean

KW - Ophiolit

KW - Nördliche Kalkalpen

KW - Westkarpaten

KW - Steinmann-Trinität

KW - Radiolarien

KW - Mélange

KW - Neo-Tethys Ocean

KW - Ophiolite

KW - Northern Calcareous Alps

KW - Western Carpathians

KW - Steinmann-Trinity

KW - Radiolarian

M3 - Master's Thesis

ER -