The apparent polar wander path of the European plate in Upper Triassic-Lower Jurassic times and the Liassic intraplate fracturing of Pangaea: new palaeomagnetic constraints from NW France and SW Germany

Abstract

In order to constrain the European plate motions during Triassic-Lower Jurassic times, new palaeomagnetic data were acquired from various Triassic-Liassic sediments of northeastern France and southwestern Germany. A Carnian palaeopole 49°N/131°E, K = 193, A95 = 7°, N = 4 was obtained for Keuper marls with consistent directions of normal and reversed polarities. Rhaetian sandstones, also with both polarities, and Carnian dolomites with a magnetization of probable secondary origin, provide a mean pole 51°N/112°E, K = 109, A95 = 6°, N = 6, more westerly than that given above. The Lower Hettangian, sampled in limestones of the Xeuilley quarry near Nancy shows a N-R-N polarity sequence with well-defined anti-parallel directions. The mean Hettangian pole is 55°N/100°E, K = 123, A95 = 11°, N = 3. Induan sandstones from the northern Vosges show two distinct overprints, one consistent with the Ladinian-Carnian magnetizations already observed in the same region, the second consistent with the recalculated mean Lower Jurassic pole of Europe. Five other sites with different rock types also exhibit late overprints with Lower Jurassic directions. The mean pole calculated from these overprints is 74°N/124°E, K = 125, A95 = 6°, N = 6. The revised apparent polar wander path (APWP) presented here shows that for the mid-Permian to the earliest Jurassic the palaeopoles of Europe stayed around 50° latitude, while the longitudes drifted from 185°E to 100°E. The previously established Upper Triassic-Lower Jurassic loop of the APWP is shifted westwards by about 25°. The revised APWP involves the following plate motions. After the late Alleghanian tectonic phase, which marked the end of the Variscan orogeny, central western Europe rotated counterclockwise during about 70 Myr, up to the earliest Jurassic. The south-north drift in latitude by 35° (nearly 4000 km) remained regular from 320 to 205 Ma, with a constant rate of 3.4cm yr-1. Around 205 Ma, this motion ceased, probably because of a collision somewhere in the north of Eurasia. Then, in the Lower Liassic, the plate underwent a rapid counterclockwise rotation with a Eulerian pole located in northeastern Asia, synchronous with fracturing in the Atlantic and the Neotethys areas. It is proposed that the intraplate fracturing of Pangaea was initiated by the rapid change of the megacontinent motion at the end of the Trias.