Palaeomagnetism of the (early devonian) lower old red sandstones of South Wales: implications to variscan overprinting and differential regional rotations

Abstract

Component structures of magnetic remanence in the Old Red Sandstones of South Wales are related to tectonic history by detailed study of three contrasting areas: (1) remote from, (2) bordering, and (3) within the Variscan orogenic belt. A uniform southerly shallow-directed component dominates remanence in each sector. In area 1, where it is considered most representative of the regional field, this magnetisation has a mean D I = 191.1/− 8.0° (151 samples in situ, α 95 = 2°) equivalent to a pole position at 341.1°E, 41.2°S. Fold tests show that it was acquired at an intermediate stage of orogenesis (and during the Carboniferous-Permian Reversed Superchron). It is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south. Remote from the orogenic front, a higher blocking temperature dual-polarity remanence of probable Lower Devonian age has sporadically survived ( D I = 246/38° , 27 samples tilt adjusted, α 95 = 7.2°) corresponding to a pole position at 297°E, 3°N. This remanence is entirely replaced close to the frontal zone of the orogen. Within the orogen, low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum. This region was deformed into upright folds before the magnetising event. Following remanence acquisition, continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs. Structures in the south were already more deformed than those in the north at the time of magnetisation, implying that the causative fluid front moved northwards with time. The palaeomagnetic vectors are rotated clockwise to the west in South Wales and, in part, record the effects of regional dextral transpression; the largest rotation however, is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain.