Paleomagnetic reconstruction of geodynamic events and climate changes in the Eastern Alpine Miocene.

Research project P 13738 Geodynamic events in the Eastern Alpine Miocene Robert SCHOLGER 11.10.1999 Tectonic and geodynamic processes during the Miocene lead to substantial changes of the marine environments and ecosystems in the Eastern Alpine region by changing basin geometries, subsidence, water depth and salinity, input of terrigenous material, and climate. In order to investigate the link of distinct changes of fossil communities with geodynamic events in the Eastern Alpine Miocene, we propose a paleomagnetic study of Neogene Austria. This project is part of a proposed multi-disciplinary project cluster (Changes in Eastern Alpine Miocene Ecosystems and their Geodynamic Control) that integrates paleoecological and biostratigraphical studies of the marine and terrestrial phases of the Eastern Alpine Miocene development, as well as paleoenvironmental proxy data. The reconstruction of geodynamic processes requires exact dating which will be performed by well-founded biostratigraphic data in connection with, stable isotope data and magnetic results. The magnetic measurements would aim at providing information about the geodynamic development of the Eastern Alpine Neogene basins in terms of rotation and latitudinal movements throughout the Miocene. Furthermore, magnetic measurements will improve stratigraphic age determinations where the polarity patterns of the primary magnetizations provide a magnetostratigraphic zonation. In addition to a detailed sedimentologic description of every outcrop under investigation, measurements of the anisoptopy of the magnetic susceptibility will help to describe the sediment texture in terms of paleocurrent directions and foliation planes. The paleomagnetic investigations should also allow for a possibility to correlate the biostratigraphic results from observations of marine and continental deposits. At the same time, using the synergies of the joint, research group, we propose to study different methods for paleoclimate estimation in selected stratigraphic sections from marine as well as from continental deposits. In order to determine the applicability of magnetic parameters as a cost effective and quick tool for paleoclimate estimation, we plan to investigate a limited number of well dated stratigraphic sections and correlate magnetic climate proxies with more conservative parameters, e.g. the study of stable isototpe (oxygen, carbon) composition of skeletal material. We propose to establish a systematic and comprehensive paleomagnetic record of various sedimentary sections from the Molasse zone and the Vienna-, Styrian- and Lavanttal basins, which should enable us to construct a quantitatively backed paleogeographic model of the development of the Eastern Alpine Miocene. Integration of paleoecological and stable isotope data with data on tectonics will allow a serious conclusion on the degree of dependence of ecosystem changes from the geodynamic development.

A systematic and comprehensive paleomagnetic database from the Molasse zone and the Vienna-, Styrian- and Lavanttal basins was established, which should enable to construct a quantitatively backed paleogeographic model of the development of the Eastern Alpine Miocene and provide means to improve the stratigraphic age determinations of various sedimentary sections. In order to investigate the link of distinct changes of fossil communities with geodynamic events in the Eastern Alpine Miocene, we studied Neogene basins in Austria by means of paleomagnetic methods. The magnetic measurements provided new insights in the geodynamic development of the Eastern Alpine Neogene basins in terms of rotation and latitudinal movements throughout the Miocene. Furthermore, the magnetic measurements improved stratigraphic age determinations where the polarity patterns of the primary magnetizations provided a magnetostratigraphic zonation. All investigated sites have been studied by means of other, independent stratigraphic methods at the same time. The success of the research project was made possible by continuos interdisciplinary co-operation with partners from Austrian Universities, the Natural History Museum Vienna and the Geological Survey of Austria. The new results from basins located North, East and South of the Eastern Alps show a general trend for synsedimentary counterclockwise rotations of 20 degrees with respect to the Eurasian paleopoles during the Karpatian and Badenian stages. A second period of counterclockwise rotation occurs during the Pannonian stage, while the uppermost Miocene results are in accordance with the Eurasian geodynamic reconstruction. Such a pattern of rotations cannot be explained be individual movements of the subbasins. At the same time, rotations in a clockwise sense are observed in the Eastern and south-eastern parts of the study area. The new geodynamic achievements are of great importance for the understanding of the geological processes that lead to the formation of ore deposits related with Cretaceous to Neogene mineralisations in the Alpine-Carpathian Province. The magnetostratigraphic results provide means for a new, refined stratigraphic zonation of the Neogene Basins with direct relevance for the hydrocarbon exploration in Austria. Results from the research project were presented in international and national scientific meetings. Further international co-operation projects are planned to investigate the nature and exact positions of the boundaries between stable Europe and the lithospheric element (terrane, microplate) that underwent a joint counterclockwise rotation during and after the Miocene.