International Geologiical Congress - Oslo 2008

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GDP-04 Palaeozoic-Mesozoic earth geography: Palaeomagnetic, faunal and facies constraints

 

High resolution global plate tectonics snenarios for the Paleozoic and Mesozoic

 

Gérard Stampfli, Université de Lausanne (Switzerland)
Cyril Hochard, Université de Lausanne (Switzerland)
 

 

To construct reliable palinspastic models of the Earth major geological elements, plate tectonics constraints must be taken into consideration, besides other elements such as faunal distribution or paleomagnetic data. Plate tectonic concepts allow to assess the kinematics of displacements of terranes and continents, keeping in mind that these continental entities are part of larger plates. For most continents/terranes, thousand km scale transport can be demonstrated, and used to construct paleogeographic models. These models usually consist in simple continental drift, and therefore, are poorly constrained in term of plate velocities.

In such models it is not unusual to find velocities over 50 to 70 cm/y (usually based on paleomagnetic data) which are not acceptable in term of plate tectonics, mainly when they involve large continents like Gondwana. Plate tectonic concepts have been systematically applied to our global palinspastic models moving away from pure continental drift, not constrained by plate limits, to produce a model which finally is more and more self-constrained. In this approach inter-dependant reconstructions are created from the past to the present. Except during collisions, plates are moved step by step, as single rigid entities. Lithospheric plates are constructed by adding/removing oceanic material (symbolized by synthetic isochrones) to major continents and terranes. In the last years we changed our tools and moved into GIS softwares and built a geodynamic database to support the reconstructions, and the model was, and still is, extended to the whole globe, spanning the earth history from 600 Ma to 20 Ma, with average steps of 15 Ma.
The building up of this new approach is presented in a companion presentation by Hochard and Stampfli. Our high precision reconstructions allow to assess the building of the Variscan cycle plate tectonics scenario, derived from a relatively well constrained model for the Caledonian cycle. The Variscan scenario is complex, and involved large ribbon like microcontinents detachment from Gondwana since the Silurian and their accretion to Laurasia/north China. This process did not stopped with the late Paleozoic collision of Gondwana with Laurasia and the making of Pangea. The Cimmerian cycle is the continuation of the Variscan one, but affected only the Tethyan realm. This is due to the Late Triassic closure of the Paleotethys in this area. During the Permian and Triassic, back-arc oceans opened in the active margin of Eurasia bordering Paleotethys, and many of these only closed in the Cretaceous and even during the final Alpine collisional event. New back-arc basins appeared also during the closure of Neotethys, the major Tethyan ocean that finally replaced Paleotethys.

 

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