Origin of the secondary magnetization of the old red sandstones of the Anglo-Welsh cuvette

The geometry of the NRM vectors of the steeply folded Devonian Old Red Sandstone strata in Pembrokeshire is suggested to be the result of a secondary magnetization acquired during the Hercynian orogeny while both limbs of the fold made an angle of 30° with the horizontal. Magnetite has been identified as the main carrier of the secondary magnetization. The weak but thermally stable component of primary magnetization resides in hematite grains, which were formed by oxidation from the magnetite. It is suggested that the secondary magnetization occurred during burial and deformation as a result of the elevation of the temperature to perhaps 200°C. The relaxation time of the magnetite was reduced enough for it to be remagnetized, but the hematite was not affected. This mechanism has been termed viscous PTRM.