Objective
The most important Phanerozoic biological crisis was the Permian-Triassic boundary which occurred 250 million years ago. During this P-Tr crisis, 90 % of species in the oceans, 70 % of land vertebrates families and a similar proportion of plants disappeared. For the similar, but more popular Cretaceous-Tertiary crisis, several scenarios have been proposed: sudden death due to collision with a meteorite, or a still catastrophic process due to climate modifications caused by the Deccan traps volcanism or continuous, progressive biological changes. For the Permian-Triassic boundary, the evidence of a collision with a meteorite is very tenuous and most people think that the main factor was the huge Siberian traps volcanism (5.106 km{3}).
Different questions arise about this crisis and the answers to these questions need precise comparisons of different Permo-Triassic sections from different regions: in the Alpine belt (e.g. Austria, Greece), in the south of the NIS (Turkmenistan, Tadjikistan, Kazakhstan) and in central Siberia. Previous studies of the P-Tr boundary and others, like the K-T boundary, have shown that, geologically, disappearance and recovery are quick events, lasting probably less than 1 million years. Flood basalts volcanism is also a rather short and brutal event. Such time lengths are shorter than the present confidence intervals of good absolute geochemical ages. Magnetostratigraphy, recording the succession of the Earth's magnetic field polarity changes independently of local conditions, provides worldwide synchronous time markers. This tool will be used together with sedimentological, palaeontological and geochemical techniques.
The first type of results concern two main stratigraphic and magnetostratigraphic markers which are looked for, being the beginning of the first Illawara reversals after the long Permian reversed superchron before the end of the Permian (middle Tatarian) and the biostratigraphic Permian-Triassic boundary. Sections with good magnetostratigraphic sequences showing the beginning of the Late Permian Illawara reversals and the first Lower Triassic ones will then help to correlate distant places and events in the Alpine-Tethyan belt, on the southern margin of stable Eurasia, on the Russian platform and in Siberia where a clear magnetostratigraphy is already available. The thickness of the different polarity zones of different places will be used to derive relative ages and sedimentation rates, and to find sedimentation gaps in these regions.
The second type of results concern the deformations of the southern margin of Eurasia and the convergence with the Alpine-Tethyan belt. These will be obtained by comparing well defined palaeomagnetic direction and poles with similar results from the Alps, stable Eurasia, Turkmenistan and Tadjikistan. Inclination differences or anomalies will show palaeolatitudes and northward movements. Declination differences will show rotations.
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67084 Strasbourg
France