Implications of palaeomagnetic data from the Tortworth Silurian inlier (southern Britain) to palaeogeography and Variscan tectonism

Abstract

Palaeomagnetic data are presented from early Silurian (Upper Llandovery) lavas from the Tortworth Silurian inlier of south-west England. Two magnetization components are recognized on the basis of contrasting demagnetization characteristics. A lower unblocking-temperature component (<350°C) is oriented parallel to the Earth's present magnetic field at the sampling site ( N = 46, declination = 352°, inclination = 69°, k = 52.1, Α 95 = 2.9°). A higher unblocking-temperature component (350–600°C) is inferred to be primary on the basis of a stratigraphy-parallel reversal pattern (combined tilt-corrected mean: N = 42, declination = 056°, inclination = -30°, k = 22.6, Α 95 = 4.7°). Geological implications of these new data are as follows. (1) the calculated Upper Llandovery ( c. 430 Ma) palaeolatitude for the site (16°S) is consistent with tectonic models invoking pre-Wenlock closure of the Iapetus Ocean across Britain. (2) the declination of the primary magnetization indicates substantial clockwise rotation within this sector of the Variscan thrust belt. Rotation is most likely to be linked to early Variscan northward thrust transport with components of dextral transpression. the amount of clockwise rotation is approximately 85° in the south (Mendips) and decreases to 33-63° in the north (Tortworth). (3) the polarity of the primary magnetization (Lower Trap lava: reversely polarized; Upper Trap lava: normally polarized is consistent with that expected from a preliminary analysis of the Silurian magnetic-field reversal pattern.