A palaeomagnetic investigation of the Malvernian and Old Radnor Precambrian,Welsh Borderlands

Alternating field and thermal demagnetization of igneous rocks of the Malvern Hills identifies a number of magnetite-held components which are characterized by a high blocking temperature (M2) component D = 283°, I = 47°, and lower blocking temperature (M3) component D = 269°, I = ?43° which is of complex origin or more than one age. Two subordinate components are (M1) D = 7°, I = 56° and (M4) D = 174°, I = 51° in later dolerites. A pervasive hematite-held remanence with a mean D = 186°, I = ?5° is linked to Hercynian palaeofield directions and the uplift/folding of the Malvernian axis. The similarity of the magnetization directions in the Stanner–Hanter (702 Ma) and Malvernian (681 Ma) rocks suggests that folding of the Palaeozoic rocks in the Malvern Hills was achieved by upthrust of the basement and involved little folding of the latter. The Old Radnor sediments possess a post-folding remanence D = 117°, I = ?13° of probable Cambrian age and a subordinate remanence which may be Hercynian in age. The late Precambrian–Cambrian palaeomagnetic record (ca. 700–500 Ma) of England and Wales is compared with data from the Armorican Massif. Although the apparent polar wander (a.p.w.) paths are widely dissimilar prior to 550 Ma, the two regions had similar latitudes and went through similar palaeolatitudinal movements throughout this interval. The palaeomagnetic data support models involving tectonic rotations but little closure across this part of the Hercynian Belt.