Paléomagnetisme de la formation des 'grès-à-Voltzia' (Buntsandstein supérieur) du Massif des Vosges (France)

Abstract

The results of a paleomagnetic study of 99 samples (156 specimens) from 6 quarries spread over an area of 6000 km2 in the Lower Triassic Voltzia sandstone formation from the Vosges (France) are presented. More precisely, this paper is limited to a detailed statistical study of magnetization directions at different stages of physical treatment of the magnetized material: exposure in null field for long time periods, up to 18 months; attack with moderate alternative fields (600 Oe (4.8 × 104 A/m) eff. maximum), then strong fields (1600 Oe (12.7 × 104 A/m) eff.); thermal treatment to 670 °C.Here is a summary of the results:(1) Strong alternative field attack or moderate thermal treatment (≤400 °C) do not result in the acquisition of parasitic magnetization, however weak, of preferred orientation, but seem to cause a random scatter of magnetization directions.(2) Thermal treatment seems preferable to alternative field attack to completely destroy secondary magnetization, ubiquitous and intense, the direction of which is close to the present terrestrial field direction.(3) Primary magnetization usually shows inverse polarity. The coordinates of the corresponding virtual geomagnetic pole are: 43.1° N, 145.7° E; Dm = 4.9°; Dp = 2.5°.(4) This study allowed the testing of the validity of secondary magnetization cancellation method by vectorial calculus techniques using the two primary magnetization polarities obtained after long exposure in null field. [Journal Translation]