Preliminary palaeomagnetic results on Oligocene–early Miocene mafic dykes from southern Spain

Abstract

A preliminary palaeomagnetic study has been carried out on Oligocene–early Miocene mafic dykes intruding the Maláguide and Alpujárride complexes of the Internal Betic Zone (southern Spain). The aims of the study were to ascertain if relative perpendicular orientations (NE–SW and NW–SE) shown by different dyke families are of primary or secondary origin and to find out if palaeomagnetic rotations can be detected in that area. Ten sites with dykes showing both orientations were sampled. In addition to palaeomagnetic analysis, susceptibility-versus-temperature curves of several representative samples were measured and ore microscopic studies were carried out.Palaeomagnetic behaviour of samples was characterised by low intensities of magnetisation and alterations occurring during the demagnetisation procedure, and often also by overlapping of palaeomagnetic components. Besides an initial viscous component, two different palaeomagnetic components, named M and H were recognised. Component M is carried by pyrrothite and magnetite, and seems to record a primary palaeomagnetic direction. Component H is difficult to isolate and seems to be related to the presence of magnetite and hematite.Nine of ten studied sites offer consistent palaeomagnetic results, although in two cases statistical parameters are relatively weak. Strong clockwise rotations (approximately 90–140°) are observed. These rotations are in accordance with other block rotations observed in the Internal and External Zones of the central and western Betics, although their magnitudes are much stronger. On the other hand, both dyke families show similar rotations, independently of their original direction. Thus, their relative perpendicular orientations seem to be of primary, intrusive origin.